CHAPTER 12

Applications

m 1 TYPICAL LOAD CHARACTERISTICS AND RATINGS 259

m 2 TECHNIQUES COMMON TO MANY APPLICATIONS 264

/ 3 APPLICATION PRINCIPLES/EXAMPLES 288

It is not practical to describe all possible applications and/or applications are covered in varying degrees of technical
characteristics for electrical variable-speed drives. This depth. This chapter is not intended as a summary of what is
chapter aims to provide an insight into some of the possi- possible, rather a sample of what has been done and the
bilities/opportunities. Typical characteristics are covered fundamental applications techniques which have been
and techniques applied in many different applications are applied.
described. Then, a large number of examples of actual

TYPICAL LOAD CHARACTERISTICS AND RATINGS

In order to successfully select and apply the optimum drive driven. The following listings of common loads could prove
system, it is necessary to understand the essential features of useful when selecting a drive.
both the alternative drive technologies and the load to be
260 TYPICALLOAD CHARACTERISTICSAND RATINGS: Metals Industries

METALS INDUSTRIES
See Table 12.1.

Table 12.1 Typical load characteristics and ratings for the metals industry

Drive duty Rating range Comments/drive type

Rolling mill up to 1000s of kW high-impact torque loading, constant kW speed range, steel works specification and
difficult environment; closed loop induction motor drives and D.C. drives.
Strip mill up to 100s of kW normal torque loading (150% maximum) constant kW speed range, steel works
specification and difficult environment; closed loop induction motor drives and D.C. drives.
Slitters and 50 to 150 kW in metal finishers' plant, environment and specification easier - normally IP23 enclosure,
perforators forced ventilation with filter is acceptable; drive often integrated with wind/unwind
stand drives; closed loop induction motor drives and D.C. drives.
Wind/unwind up to 200 kW constant kW rating over build-up range; regenerative braking with four-quadrant
operation; steel works specification and difficult environment; in metal finishers' plant
easier conditions apply as above; closed loop induction motor drives and D.C. drives.
Tube mill up to 300 kW constant kW rating over pipe diameter range; usually four-quadrant with regenerative
braking; environment can be difficult with oil spray present; closed loop induction motor
drives and D.C. drives.
Cast tube 20 to 50 kW high values of acceleration and deceleration torque required; four-quadrant
spinner regenerative, typically four to six speeds required: clean, spray, fill, spin 1,
spin 2; difficult environment; single pipe vent or box-enclosed motor with filtered
air supply; closed loop induction motor drives and D.C. drives.
Machine tool up to 150 kW mostly flange mounting and timer belt drive to 30 kW gearbox coupled above; always field
spindle 5 to 30 kW typical range control, reversible, four-quadrant drive; often with encoder for spindle orientation;
forced-vent with filter; often coaxial fan unit to 60 kW; closed loop induction motor drives;
at low powers permanent magnet servo drives and at very high powers D.C. drives are used.
Machine tool up to 200 kW foot or flange-mounting gearbox otherwise as above; permanent magnet servo drives;
table 20 to 75 kW at higher powers closed loop induction motor drives and D.C. drives are used.
typical
Wire drawing 5 to 75 kW constant kW speed range with individual motor field control on multiblock drives with
single controller; progressive speed increase between heads as wire diameter reduces and
speed increases; dancer arm tension control between heads and tension-controlled winder
take off; forced-vent motors require filter against wire end entry; four-quadrant acceleration/
deceleration; closed loop induction motor drives or at higher powers D.C. drives.

PLASTICS
See Table 12.2.

Table 12.2 Typical load characteristics and ratings for plastics

Drive duty Rating range Comments/drive type

Extruder 5 to 400 kW constant torque drive with high torque required to start a stiff extruder screw; environment
can be difficult with plastic particle and fume risk; single pipe vent of motor is advisable;
closed loop induction motor drives and D.C. drives; open loop induction motor drives may
be used in some applications.
Sheet line reeler 1.5 to 15 kW constant kW drive over reel build-up ratio but often sized as constant torque drive;
braking usually mechanical; environment can be difficult; TEFC IP55 used for
low kW ratings; closed loop induction motor drives and D.C. drives.

RUBBER
See Table 12.3.

Table 12.3 Typical load characteristics and ratings for rubber

Drive duty Rating range Comments/drive type

Banbury mixer up to 1000 kW very heavy peak duty, duty cycle rated typically: 250% full load torque for 10 s,
150% for 20 s, 100% for 120 s, 10% for 30 s repeating continuously; difficult environment
with particle rubber and carbon black, single and double pipe vent is usual with some CACA
and CACW motors used; check through Banbury manufacturers' drive specification - safety
environment; D.C. drives predominate but closed loop induction motor drives growing in use.
Callender up to 500 kW environment as above, with easy duty, constant torque, but 200% dynamic braking; check
through manufacturers' drive specification - safety involvement; closed loop induction
motor drives and D.C. drives.
 Chapter 12.1 261

CHEMICAL
See Table 12.4.

Table 12.4 Typical load characteristics and rating for chemical industries

Drive duty Rating range Comments/drive type

Mixer up to 150 kW generally constant torque but could be rising torque requirement with increasing mix stiffness;
often explosion proof or hazardous location enclosure requirement; environment can be
difficult; closed loop induction motor drives and D.C. drives.
Extruder up to 100 kW usually constant torque often explosion proof or hazardous location enclosure requirement;
environment can be difficult; closed loop induction motor drives and D.C. drives.
Stirrers and agitators up to 400 kW high reliability requirement to avoid loss of mix through drive shutdown; often rising torque with
mix stiffness; energy saving of importance as process can occupy many days; drive often outdoor
mounting with CACA weatherproof enclosed motor; in particularly exposed positions motor and
gearbox should have additional protection; closed loop induction motor drives and D.C. drives.

MATERIALS HANDLING
See Table 12.5.

Table 12.5 Typical load characteristics and ratings for materials handling

Drive duty Rating range Comments/drive type

Conveyor 0.5 to 20 kW cascading of multiple drives can be a requirement with progressive speed increase or
synchronised drives; constant torque application with dynamic or regenerative braking;
open loop induction motor drives predominate unless synchronisation/close coordination
required where closed loop induction motor drives are used.
Automated as above usually three-axis systems, constant torque four-quadrant 150% full-load torque at
warehousing starting to duty cycle rating; closed loop induction motor drives predominate; D.C. drives
can be used and in less demanding applications open loop A.C. is used.

LIFT, HOIST AND CRANE

See Table 12.6.

Table 12.6 Typical load characteristics and ratings for lifts, hoists and cranes

Drive duty Rating range Comments/drive type

Lift and hoist 30 to 100 kW four-quadrant with 200% full-load torque at starting; starting duty 90-200 starts/hour;
smooth, quiet response very important; closed loop induction motor drives are widely used;
D.C. drives are used; open loop drives are applied to non safety critical applications and
installations where smooth ride quality is not critical.
Crane 3 to 75 kW four-quadrant high torque requirement; sometimes weatherproof enclosure; operating
duty cycle requires evaluation; as for Lift and Hoist.

CONCRETE PIPE MANUFACTURE

See Table 12.7.

Table 12.7 Typical load characteristics and ratings for concrete pipe manufacture

Drive duty Rating range Comments/drive type

Pipe spinner 10 to 100 kW D.C. motor needs particular protection against water, cement and vibration; four-quadrant
multispeed drive requirement with regenerative braking; duty cycle requires evaluation;
closed loop induction motor drives can also be used.
262 TYPICALLOAD CHARACTERISTICSAND RATINGS: Fans and Blowers

FANS AND BLOWERS

See Table 12.8.

Table 12.8 Typical load characteristics and ratings for fans and blowers

Drive duty Rating range Comments/drive type

Axial flow fan 0.5 to 40 kW as cage motors specially adapted for air stream use with impeller on motor shaft;
existing motors will be retained under inverter control and may require slight derating or
slightly reduced maximum speed; inverse cube law relationship between fan kW load and
speed; noise falls as the fifth power of fan speed; open loop induction motor drives are
most widely used.
Centrifugal fan 0.5 to 500 kW cube law kW/speed relationship extends acceleration time on large fans to moderate
starting current requirement; power saving important on large fans as is top speed fan
noise; open loop induction motor drives are most widely used.
Rootes-type blowers 3 to 200 kW positive displacement blowers are constant torque and kW loading is linear with speed into
a fixed system resistance; load pulsates heavily; Rootes blowers are noisy but easily
started; power saving can be important; D.C. drives and closed loop induction motor
drives predominate; open loop induction motor drives can be used with care.

PUMPS
See Table 12.9.

Table 12.9 Typical load characteristics and ratings for pumps

Drive duty Rating range Comments/drive type

Centrifugal pumps 0.5 to >500 kW early cage motors with A and E class insulation require care over winding temperature
under inverter control; power saving on large drives important; open loop induction
motor drives predominate.

PAPER AND TISSUE

See Table 12.10.

Table 12.10 Typical load characteristics and ratings for paper and tissue manufacture

Drive duty Rating range Comments/drive type

Paper machine up to 500 kW environment difficult with water, steam and paper pulp present; pipe vent motors
and pumps common; often nonstandard A.C. and D.C. motors; usually closely coordinated drives
in a paper line; closed loop induction motor drives and D.C. drives.
Winders and reelers 5 to 100 kW constant kW range over build-up range; four-quadrant operation with regenerative
braking; IP23 motor enclosure with filter is common; closed loop induction motor
drives and D.C. drives.

PRINTING
See Table 12.11

Table 12.11 Typical load characteristics and ratings for printing

Drive duty Rating range Comments/drive type

Printing press up to 200 kW some special coaxial motor designs for series connection on line; field weakening for
wide speed range; four-quadrant with slow ramp acceleration and inch/crawl control
plus emergency stop; pipe vent where ink fumes may be a hazard; closed loop
induction motor drives and D.C. drives.
Folders, unwind and up to 100 kW often integrated in printing line drive with press drive and unwind stand drive under
rewind stands master control; otherwise as above; closed loop induction motor drives and
D.C. drives.
 Chapter 12.1 263

PACKAGING
See Table 12.12.

Table 12.12 Typical load characteristics and ratings for the packaging industry
Drive duty Rating range Comments/drive type

Boxing, stamping, up to 75 kW mostly four-quadrant with slow ramp acceleration with inch/crawl and E/stop; often
folding, wrapping integrated line control; P.M. servo drives are widely used in precision packaging
machines; closed loop induction motor drives and some D.C. drives are also used.

ENGINEERING INDUSTRIES
See Table 12.13.

Table 12.13 Typical load characteristics and ratings for engineering industries

Drive duty Rating range Comments/drive type

Test rigs of many types up to > 15 MW test rig drives require careful engineering; often high speed with fast response,
accurate speed and torque measurement, usually four-quadrant with field weakening
control; engine test rigs require special knowledge of throttle control drive/absorb
changeover and power measurement; drive control/monitoring particularly important;
closed loop induction motor drives and D.C. drives; P.M. servo drives are also used for
precision applications.

WIRE AND CABLE

See Table 12.14.

Table 12.14 Typical load characteristics and ratings for wire and cable industries

Drive duty Rating range Comments/drive type

Bunchers and stranders 10 to 150 kW generally multiple drives with cage or bow, capstan plus take-up drives under
integrated control; constant torque except take up with four-quadrant acceleration/
deceleration with inch/crawl/E-stop controls; motors require filter protection
against metal dust entry; closed loop induction motor drives and D.C. drives.
Capstan 5 to 100 kW as above.
Take-up and unwind stands 5 to 50 kW as above but constant kW over build-up ratio.
Extruders 5 to 150 kW see extruders under plastics industry but control is often integrated in cable line drives.
Armourers 10 to 150 kW as buncher/strander drive above.
Caterpillars 1.5 to 30 kW constant torque duty and low kW rating in view of low haul-off cable speeds; often
integrated in cable line drives; motor protection generally no problem; closed loop
induction motor drives and D.C. drives.

HYDRAULICS
See Table 12.15.

Table 12.15 Typical load characteristics and ratings for hydraulics

Drive duty Rating range Comments/drive type

Pump and motor test rigs up to 250 kW hydraulic fluid is a contamination risk; pipe vent often used; generally
constant torque to medium/high speeds with four-quadrant drive; speed
torque and power measurement often required with full drive
monitoring on endurance rigs; closed loop induction motor drives
and D.C. drives.
264 TYPICALLOAD CHARACTERISTICSAND RATINGS: Electric Motors and Alternators

ELECTRIC MOTORS AND ALTERNATORS

See Table 12.16.

Table 12.16 Typical load characteristics and ratings for electric motors and alternators

Drive duty Rating range Comments/drive type

A.C. and D.C. motors/generators/ up to 15 > MW all rotating electrical machine manufacturers have elaborate test-bed
alternators test-bed rigs rigs, supplying their own rotating machines and obtaining control
systems from the drives industry to their own requirements; closed loop
induction motor drives and D.C. drives.

TEXTILES
See Table 12.17.

Table 12.17 Typical load characteristics and ratings for textiles

Drive duty Rating range Comments/drive type

Ring frame machines, up to 150 kW difficult environment in which IP55 enclosure has become a standard; ring frame
carding machines, Schrage and D.C. thyristor drive; use pipe ventilation; all drives constant torque
looms four-quadrant for speed modulation (ring frame) or best speed-holding accuracy
with slow ramp acceleration/deceleration on carding drives; special A.C. cage loom
motors are high-torque, high-slip designs; today A.C. inverter drives predominate
since their characteristics are particularly suitable.

FOODS, BISCUITS AND CONFECTION

See Table 12.18.

Table 12.18 Typical load characteristics and ratings for foods, biscuits and confection

Drive duty Rating range Comments/drive type

Extruder 5 to 400 kW hose-proof motors for plant cleaning; continuous production requiring high
levels of reliability, control and monitoring; otherwise as plastics industry extruders.
Mixer 5 to 150 kW as above and see chemical industry mixer drive.
Conveyors 0.5 to 120 kW as above and see materials handling industry conveyor drive.

2 TECHNIQUES C O M M O N TO M A N Y APPLICATIONS

SPECIAL D.C. LOADS Traction Motor Field Control

The following are a few applications of D.C. thyristor drives Traction motors, such as those used in railway locomotives,
in which the connected load is not the armature of a D.C. are invariably of series-wound construction. This gives high
motor. The Mentor drive is used in the following examples, starting torque, since the armature current passes through the
since its ease of configuration makes it very suitable for field windings resulting in maximum flux under heavy load
unorthodox applications. conditions.
 Chapter 12.2 265

It is possible to duplicate this characteristic in a separately- which sodium hypochlorite is manufactured from brine.
excited motor by controlling its field current by a thyristor Deposition in such processes takes place at a rate which
drive configured as a current regulator, the reference being is proportional to current, and therefore the converter is
derived from the motor's armature current through either configured as a current controller.
a shunt or a D.C. current transformer (DCCT).
Electric Heating and Temperature
The advantages of this technique include increased motor
output (since the resistance of the field windings is not
Control
connected in series with the armature) and the facility to set A thyristor drive may be used for heating applications either
maximum and minimum limits of field current, thus pre- in open-loop or closed-loop control configuration, and the
venting saturation of the magnetic circuit and improving application suits a D.C. drive and certain A.C. soft starts,
performance under light and overhauling load conditions, which are phase controlled.
e.g. downhill running.
Most heating elements consist of wire-wound or grid-type
The speed amplifier needs to be used as a buffer amplifier by resistances supported on ceramic formers. When cold, the
reducing its gain to unity. This allows the ramps, speed limits resistance of such elements is low in comparison with that at
and lower current limits to be used to control rate of change, normal operating temperatures, and if connected directly to
minimum and maximum current, respectively. A flywheel the mains supply a heavy current would flow, possibly
diode needs to be connected across the output terminals to causing localised overheating, or hot spots, which may
provide a path for circulation of current (the load being in- reduce the life of the element. Therefore, the current limit is
ductive) although sometimes this is omitted in order to enable used to set an upper limit to the output current of the con-
the drive to force field current down rapidly. The omission of verter to give controlled warm up from the cold condition.
the flywheel diode also makes field reversal possible without Actual current, and therefore the rate at which heat is pro-
the use of contactors, should this requirement exist. duced, is set by a potentiometer and adjustments are made by
the operator according to the final temperatures to be
Battery Charging attained. Open-loop control would use an instrument con-
nected to a thermocouple in contact with the process
The charging of secondary cells (e.g. lead-acid or nickel-iron material to indicate actual temperature to the operator.
accumulator batteries) is an application which calls for the
control of current. The charging current is proportional to the Such a system gives poor control of temperature, since it
area of the plates within a cell, multiplied by the number relies on the operator to monitor actual temperature and
of cells connected in parallel. The Mentor drive range is make the necessary adjustments. Better control can be
suitable for charging currents up to 1850 A, covering the obtained by controlling the heater-on contact by a thermo-
majority of secondary standby power-supply systems. stat. In this case, the current reference potentiometer deter-
mines the rate of rise of temperature, but the actual
The voltage required for charging is proportional to the temperature reached is controlled by the thermostat, which
number of cells connected in series, the charging voltage per opens at the set temperature and switches off the drive,
cell reaching a maximum when fully charged. switching it on again as the temperature falls below the set
For this application a D.C. drive needs to be configured as a point, Figure 12.1.
simple current regulator, the reference input being config- True closed-loop control of temperature requires a feedback
ured to control the current reference. signal proportional to temperature. This might be provided
The current limit protects both drive and battery against
overcurrent if a fully discharged battery is connected.
If a contactor is used the charge contact might be an early-
break auxiliary, ensuring that the contactor is not required to
F-
break current when it opens.
A more refined battery charging system might use special
(D
application software to reduce the charging current when a c~
E
predetermined voltage is reached or, after a period of time,
give a trickle charge facility suited to such applications as >
time, t
uninterruptible power supplies and standby supplies for
communications or medical equipment. Such an application
programme might be written by the user, or purchased as a
package. The programme could combine battery terminal
voltage sensing with an adjustable delay before switching or
ramping between two (preset) current levels, protecting the
battery against overcharging which could result in damage
through loss of electrolyte or overheating.

Electrolytic Processes time, t

Examples of electrolytic processes include electroplating, Figure 12.1 Thermostat control showing temperature
refining of copper and other metals, and chlorination cells in and current with respect to time
266 TECHNIQUES COMMON TO MANY APPLICATIONS:Special D.C. Loads

by a thermocouple amplifier or other temperature sensor problems. The exact speed ratios must be designed into the
giving a linear 0-10 V output over the operating range. Other mechanism as speed changing online is difficult, although it
signal ranges can be accommodated, using programmable can be achieved using taper pulleys or variable-ratio gear-
offsets and scale factors, but linearity is important. In such a boxes. This is a physical operation, not to be achieved at the
case, the outer control loop is configured as a temperature flick of a switch or under computer control.
loop. By comparison with the thermostat control described
The introduction of electronic variable-speed drives has
above, which controls temperature by regulating the on/off
permitted the removal of gearboxes and mechanical speed
time or duty cycle of the heaters at a constant current setting,
variators in many applications. The speed-holding capability
the closed-loop system continuously regulates the current
of the modem motor drive system is such that in some
supplied to the heaters, sensing actual temperature and
applications the units can simply be run in tandem, with
giving smoother and more precise control.
manual speed trimming. This does demand, unavoidably, the
constant attention of an operator to monitor material build up
or starvation between sections of a process.
DIGITAL SLAVING
To enable more accurate speed following, the speed refer-
General ence for a follower can be derived from the previous section
in the system. This can be done either by using a common
Modem manufacturing industry depends on high efficiency, speed reference or a tachometer-derived signal, Figure 12.2.
high accuracy and high output. This has prompted a positive
move towards continuous process production in many This system works well for noncritical processes but suffers
industries. Where continuous processes already exist, com- from the same problems as any analogue system, i.e. drift
panies are looking for additional benefit from their invest- and poor accuracy and repeatability. Position following is
ment by exploiting capital equipment to optimum effect and not possible with this technique.
by minimising running costs. To enable position following, a digital system is normally
A continuous process can range from two machines operating required although resolvers can be used and such systems
together to several hundred in an integrated process system, have been designed for retrofitting to analogue drives. The
as in the case of steel mills. Simple or complex, strict coor- principle is simple. An encoder is placed on the master drive
dination between workstations of a system is essential to shaft with a second encoder on the slave shaft. While the two
ensure optimum throughput and quality, and to reduce online encoders are rotating in unison there is no difference in the
storage requirements. As the speed of the process increases counts and therefore zero position error and no adjustment is
and the quality of product becomes paramount the need to made. If a position error appears, the speed of the slave
match process speeds accurately becomes obvious. drive is adjusted accordingly to bring the slave back into
position.
Many industries today have a requirement to run a variety of
materials of different characteristics on a single process line. The principle is similar in the fully digital slaving system.
Such process lines must be configured or adjusted to accom-
modate each change of material, sometimes on a regular Principle of Digital Speed/Position
basis. The ease of adjustment is a major consideration in Following
continuous process design.
In any following system, there are always at least two
Standards of precision in the control of speed, and in the components: the master and the slave(s).
following of speed or position of one motor by another, have
The master is the component to be followed. This can be a
been dramatically raised by the introduction of digital con-
motor shaft, axle, spindle or a wheel rotating, or a linearly
trol. Such precision offers new opportunities for the process
moving material or track.
engineer seeking throughput and quality. Digital control has
opened up wider opportunities still. The compatibility of The slave is the component that will follow the movement of
digital controls with programmable controllers and compu- the master. This can either be positional or speed following
ters, and the convenience of communication, has made by a selected ratio.
possible centralised control, remote control, reconfiguration
by menu, data logging and even simulation of process
changes for verification before implementation.

Drive Slaving Techniques

The principle of drives following each other is not new -
many systems are available to achieve this. Possibly the
oldest method of controlling many machines with fixed
relative speeds is the line shaft. This method has been used
since the days of the steam engine to transmit power,
occasionally to whole factories, along a single axle and to
distribute it to process machines by belts or gearboxes.
1
w
master
reference
II
speed trim
The major disadvantage of this method is the physical size
of the mechanisms, associated losses and maintenance Figure 12.2 Analogue master-slave control
 Chapter 12.2 267

The movement of the master component is monitored using Nonrigid lock is the mode usually termed as true speed
an incremental encoder, as shown in Figure 12.3. The following. The master encoder is used as the speed refer-
incremental encoder produces a square-wave output at a ence, which the second drive will follow. A ratio of speeds
frequency proportional to the speed of rotation. The number can be set very precisely. If the slave drive speed should vary
of pulses per revolution (p.p.r.) is determined by the level for any reason (due to load influence, for example) the
of accuracy required, the speed of rotation and the fre- controller will compensate by bringing the slave drive
quency response of the input circuitry of the following precisely back to the master speed.
system.
This form of speed following is similar to the use of
For bidirectional applications a second output channel is a nontoothed belt system, where the speeds are synchronous
required. This again is a square wave but phase shifted by although the relative positions may vary.
90 ° from the first, often described as being in quadrature. By
Rigid lock is true position following. The master encoder is
utilising a decoding circuit to detect the rising and falling
used as the position reference for the slave drive. As the
edges of each waveform, the direction of rotation can be
master moves the slave will maintain a relative angular
determined.
position with a precisely adjustable ratio. In the case of the
This master encoder signal now becomes the speed reference slave drive varying in speed, the controller will adjust the
(for a single or multiple slave drive) in the case of nonrigid speed to bring the shafts back to the same relative positions.
lock, and a position reference in the case of rigid lock. The
This form of control is analogous to a mechanically locked
encoder is obviously a vital item in the system, therefore
system with, for example, timing belts, chains or gearboxes.
the choice of encoder and coupling is extremely important.
It must be borne in mind that under normal circumstances a
The following points should be considered when making a
mechanically locked system will never lose synchronism
choice:
whereas an electronically locked system may momentarily
1 The degree of protection (IP rating) of the encoder lose synchronism if step changes of load occur.
should be at least the same as that of the motor.
A typical digital-lock control scheme is shown in Figure 12.5.
2 Encoders are relatively delicate instruments and there- The principle of the control is as follows. Both the master
fore should be housed either within the motor or in a and slave encoder signals are fed directly into hardware
robust housing. counters to give maximum frequency capability. The two
3 For hazardous environments, intrinsically safe or other signals can be digitally scaled to allow for maximum
Ex-protected and certified encoders only should be speed calibration. A fixed multiplier can also be introduced
used. to increase the count value when using a low-resolution
encoder.
In common with most rotary transducers, care must be
observed in the choice of coupling to ensure that the encoder As a rule, the greater the number of pulses per revolution the
accurately follows the master. A coupling which allows smoother the operation of the control loop, since the loop
slack movement, or which twists or oscillates at high speed, variable will more closely approximate to a continuous
can cause severe instability in the system. function. This theoretically means that the input frequency
should be as high as possible. In practice, this is limited by

The Digital Speed/Position Controller

The control function for digital slaving is incorporated in the
slave drive. To enable the slave drive to carry out the digital
lock function, appropriate hardware and software is "0
required; this may be standard in the drive (e.g. Unidrive and {3.
(/)
Mentor).
Digital slaving can take two forms: nonrigid and rigid lock,
Figure 12.4.
a time, t
Qr.p.m.

I nQr.p.m.

master ~ b time, t
Figure 12.4 Digital slaving characteristics
slave
a nonrigid lock
Figure 12.3 Digital master-slave control b rigid lock
268 TECHNIQUESCOMMONTO MANY APPLICATIONS"Digital Slaving

selectfixed
speed master control signals
master reference slaveratio to slavedrive
encoder~ ---~ ~.I X - - % ~-I x "-~~T
(reference)"" J' f l , velocity
I I masterencoder ' feed
scaling 'forward
precision i
i

velocity [ fixedreference [ i

reference
enable position position
Ithumbwheelswitchesl ) position error error
loop register
t / li~nit
relative . ,lP gainl .
inch :position
reference ',control
enable digit 'velocity
slave feedback 'correction
x 10000
feedback 0 - [
encoder "~~1 -IXl ;I ' 4 '~ enable digital velocity
"12621
slave
I
slave encodercount
encoder
scaling
slaving feedback

Figure 12.5 Simplified position-loop diagram

the response of the encoder photodetectors and of the programmable parameters, therefore the system can be
transmission-line buffers. easily set up or controlled from a remote source such as a
computer or programmable controller.
The master encoder count is then scaled by the desired
speed ratio. This can typically be done by either serial
communications or thumb wheels. The resultant signal is the
speed reference for the slave drive, termed the velocity
LOAD SHARING
feedforward reference. This would give some degree of
speed following, but not of sufficient accuracy for position
General
following.
There are many instances in industrial variable-speed drive
The velocity feedforward signal from the master encoder is
applications where multiple motors sharing load provide the
compared with the velocity feedback signal from the slave
optimum or only solution.
encoder to give a position error. This error appears as a
count, which is accumulated in the position-error register. For example, in high-power, high-speed test-rig applica-
The register value is multiplied by a proportional term and tions, two coaxial-coupled D.C. motors often provide the
limited before being added into the slave drive speed best solution. In such cases mechanical considerations may
reference as an unramped speed trim. The position loop is prohibit the manufacture of a single, large, high-power,
constantly active in the case of position following (rigid high-speed motor. However, two motors of half the power,
lock). In speed following (nonrigid lock) the position loop mechanically coupled together and arranged to share the
only becomes active when the slave drive is typically within load precisely, gives the same result in terms of torque,
0.8 per cent of the correct speed. This gives high-accuracy speed and power but without the manufacturing, operational
speed following. or even financial considerations presented by a single
motor.
By adding or subtracting from the master encoder derived
velocity signal it is possible to adjust the relative position Similarly in mines and quarries, huge conveyor systems used
of the master to the slave. This is commonly termed the for conveying the raw material from the mine to the pro-
relative-inch function, which can be used to create or relieve cessing plant can extend for thousands of metres, and a
tension in a locked system. By feeding the relative inch from distributed multiple motor drive system arranged to share the
a dancer arm or tension transducer it is possible to maintain load equally is the only practical solution. In this instance,
exact values of tension within the system. because of the environment, inverter-controlled A.C. induc-
tion motors are usually chosen for their robust construction
Digital slaving of drives offers the following advantages:
and low maintenance requirements.
1 Accuracy and repeatability- long-term speedholding
Mechanical arrangements can vary considerably. In princi-
accuracy is better than 0.01 per cent. Digital setting of
ple the motors are physically coupled together and therefore
parameters gives absolute repeatability.
compelled to rotate at the same speed, so it is normally only
2 Low maintenance - the elimination of mechanical cou- necessary to control the speed of one motor (the master) and
plings and gearboxes significantly reduces maintenance ensure by connection or other control strategy that the other
requirements. motors share the electrical load equally.
3 Programmability and flexibility - the ratios and The following discussion covers various methods of load
characteristics of the system are held as software sharing in D.C. and A.C. drive systems.
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270 TECHNIQUES COMMON TO MANY APPLICATIONS: Load Sharing

D.C. MOTORS CONNECTED IN SERIES CURRENT-SLAVED CONVERTERS

Figure 12.8 shows a typical arrangement for load sharing Figure 12.9 shows a typical load-sharing arrangement using
between motors connected in series. The master motor M is two current-slaved converters. Each motor is independently
arranged in a conventional speed control loop with feedback fed from its own converter. The master motor M is arranged
from a shaft-mounted tachogenerator. Any deviation from in a conventional speed control loop with feedback from a
the set speed will result in the motor armature voltage and shaft-mounted tachogenerator. Any deviation from the set
current being adjusted to restore the control loop to a state of speed results in the motor armature voltage and current
equilibrium. The slave motor S, which is connected in series being adjusted to correct the speed and restore the loop to
with the master motor, inherently shares the same armature a state of equilibrium. The slave converter operates in
current and is also compelled by the mechanical coupling to current control from the same current demand signal as the
rotate at the exact speed of the master. master converter, and so inherently shares the current.
Assuming identical motors, the load will be shared exactly
Both master and slave motor fields are also connected in
when Vmlm = Vsls. By careful setting up of the slave con-
series so that the field currents, which determine the mag-
verter it is possible to ensure that V m - Vs and perfect load
netic field strength, are always the same independent of
balance is achieved. Any imbalance of armature voltage can
supply fluctuations and field temperature change during
be trimmed out by resistors connected in each motor field
warm up.
circuit.
Assuming identical motors, the load is shared equally when
This method of load sharing between motors is the most
the product of armature voltage and armature current of each
flexible arrangement, in that different characteristics can
motor is the same, as before:
easily be accommodated. In fact motors of quite different
V,nL, = V,I, design and output rating can be slaved in this way and
arranged to share the load in proportion to their respective
But for series-connected motors, Im= Is, so the measure of ratings. The load currents can be trimmed proportionally by
how closely the motors are sharing the load is determined by adjustment of the individual converter burden resistors, and
the degree of balance of the individual armature voltages. If the armature voltages trimmed by field current adjustment.
the motor characteristics are closely matched, a close degree
Its advantages are:
of balance will be achieved.
1 Very flexible and simple to set up.
Advantages are:
2 Multiple motor schemes can easily be accommodated.
1 It is not so important to have closely-matched char-
acteristics. (However, the closer the match the better the 3 Each motor is individually overload protected by its own
degree of load sharing.) converter.
2 Simple to set up and maintain balance. 4 Motors of quite different design and output can easily be
accommodated.
3 Multiple motors are easily accommodated.
5 In a multiple motor system, the loss of a single converter
4 All motors can be overload-protected by the converter.
or motor does not necessarily mean that production stops.
The disadvantage is that motor armature and field voltages
The sole disadvantage is the cost, which tends to be high, but
are determined by the number of motors in series. Motors
may in some applications be acceptable.
may have to be specially designed, increasing delivery lead
times.
A

. r - -

Ac ( [ I
supp,y - Ac I
//J supp,y

, (

field rectifier ~i" [] --T

/ k--J-
\ S-field
rectifier

M-field
set speed .C. supply i, rectifier
A.C. supply
Figure 12.8 Load sharing by mechanically-coupled D.C. set speed
motors connected in series and fed from a
common converter Figure 12.9 Load sharing by current slaving of converters
 Chapter 12.2 271

A.C. Inverter-fed Systems circulating between the two motors. Also the system can be
designed so that in the event of the loss of an inverter or
PARALLEL-CONNECTED A.C. CAGE MOTORS motor, the process can continue, albeit at a reduced output.
Figure 12.10 shows a typical arrangement of two A.C. The advantages are:
induction motors fed from a single variable-frequency
1 Simple to apply, as load sharing is inherent in the motor
inverter. For accurate load sharing the motors must be
characteristics.
identical, with identical torque v e r s u s speed characteristics.
Since the motors are mechanically coupled they are com- 2 In the event of the loss of an inverter or motor the process
pelled to turn at identical speeds and of course are both fed can continue.
from a common three-phase supply. In the zero to full-load
3 Suited to multimotor installations.
torque range, the torque developed by each motor is
approximately proportional to the slip. 4 Motors can be overload protected from their respective
inverters.
Since both motors have identical characteristics and are
operating with identical slip then load sharing is exact. In Disadvantages are:
practice slight differences in torque v e r s u s slip character-
1 Motor characteristics must be closely matched for
istics which occur even between identical motors introduce
good load sharing. Cannot use motors from different
errors, but these are usually quite small - less than 1 per cent.
manufacturers.
This method of load sharing is not suitable between motors
2 System cost.
of different manufacture as the torque-speed characteristics
will differ too widely to produce accurate load sharing.
CURRENT-SLAVED INVERTERS
The advantages:
Figure 12.12 shows a typical arrangement of two A.C.
1 Simple to apply, as load sharing is inherent in the motor induction motors mechanically coupled and fed from inde-
characteristic. pendent variable-frequency inverters. The slave inverter
operates in current control, current slaved from the master
2 Suited to multimotor installations.
inverter in order to load share. A signal proportional to load
Disadvantages are: on the master drive is fed to the current control input of the
1 In the event of loss of the inverter or motor, the process
stops.
variable-
2 Motor characteristics must be closely matched for frequency
good load sharing. Cannot use motors from different inverters

!/;/I
manufacturers. III
/// "f
3 Motors must have individual overload protection.
freq. ref

FREQUENCY-SLAVED INVERTERS

Figure 12.11 shows a typical arrangement of two A.C.

induction motors mechanically coupled and fed from inde-
pendent variable-frequency inverters. The slave inverter is
frequency slaved from the master in order to load share. The
set speed
frequency of the master drive is fed to the frequency control
input on the slave drive. In this way both inverters operate at Figure 12.11 Load sharing by frequency slaving of
identical frequencies. Electrically, this is the same as oper- inverters
ating two motors in parallel from a common supply, but
has the advantage of excluding the possibility of currents
variable-
frequency
variable- inverters
frequency
II/ ..
inverter III

///
/// y-/ /// _
/// • .
O/L1

current ref
n ref

I
set speed
set speed
Figure 12.10 Load sharing by mechanically-coupled A.C.
cage motors fed from a common converter Figure 12.12 Load sharing by current slaving of inverters
272 TECHNIQUESCOMMONTO MANY APPLICATIONS:Load Sharing

slave inverter. In this way both motors operate with the same Frequency Control of A.C. Induction
current and share the load. Where motor characteristics
Motors
differ, as they do between motors with the same number of
poles and power output rating but from different manufac- The operational speed of the classical A.C. induction motor
turers, this is the only practical solution. Load-sharing is restricted primarily by the frequency of the connected
accuracy would be in the order of 10 per cent, so the motors supply voltage.
should be derated by about the same amount. For example,
This is shown by the relationship:
two 50 kW motors would be a suitable choice for a total
load requirement of 90 kW.
n = ( f x 60)/p
Advantages are:
where n - speed in min-1, f = frequency of supply in Hz
1 Simple to apply.
and p - number of pairs of poles.
2 Motors do not need to be closely matched; similarly- For a two-pole machine connected to a 400 Hz supply this
rated motors from different manufacturers can be used. would result in a synchronous running speed of:
3 Suited to multimotor installations.
4 In the event of the loss of an inverter or motor the process n- (400 x 60)/1
can continue. = 24 000 min-1
5 Motors can be overload protected by their respective
inverters. Varying the frequency of motor supply voltage is therefore
the essential process to obtain control of motor speed. It
Disadvantages are the system cost, and slightly less accurate should be noted that both increases and decreases of motor
load sharing. speed are achievable in this way. Where speed increases are
required, the major constraints are mechanical and arise
from motor design, construction and degree of balance.

HIGH-FREQUENCY INVERTERS In addition to variation of frequency, the motor supply

voltage must also be controlled in order to achieve and
General maintain the correct magnitude of flux within the magnetic
circuit of the motor.
International pressure of competition is forcing welcome
developments among designers and builders of production Normally, the voltage is linearly increased with frequency
machines of all types. This is leading inevitably to faster and this leads to a term commonly used in the design of
processing and machining of many materials. Additionally, high-frequency motors and their control inverters known as
adoption of high-speed machining techniques often gives a volts per Hertz (V/Hz or V/f) ratio. V/Hz ratio defines, for a
surface finish far superior to traditional rough-cut and second- specific motor, the per-unit value of supply voltage for the
finish techniques, to the point where further machining and per-unit value of supply frequency.
polishing is rendered unnecessary. For example, a 400 V motor designed to run on a 200 Hz
Machines for processing man-made fibres, for example, are supply has a V/Hz ratio of."
classed by their throughput speed, usually measured in
metres/minute, with the faster machine producing the higher 400/200 - 2 V/Hz
figure of merit.
The motor designer will have chosen a suitable ratio at an
Indeed, many processes are essentially impractical unless early point in the magnetic design of the machine and both
high surface cutting speeds are available. Typical amongst the magnetic calculations and the final performance of the
these would be" machine will depend greatly on the availability of the correct
glass engraving up to 12 000 min- 1 V/Hz supply source.

diamond polishing up to 12 000 min -1 The control inverter must be configured to comply with the
motor V/Hz ratio, since any deviation will affect the general
internal grinding (barazon/diamond) up to 90 000 min -1
system performance.
semiconductor wafer slitting saw up to 60 000 min -1
Increasing the V/Hz ratio (overvolting) will give a higher
Traditional approaches to attainment of high speed have motor torque availability but at the cost of much higher
been appropriate to a limited range of applications and motor operating temperatures, acoustic noise and possibly
would include the use of universal motors, usually of small torque pulsations which can be superimposed on the rota-
power, air-powered motors and step-up belt and pulley tional moment of the motor shaft and eventually show as
arrangements. patterning on the workpiece.
The A.C. induction cage motor, with its simple construction Conversely, a lower than design point V/Hz ratio (under-
and freedom from running contact parts (other than bear- volting) will result in reduced torque availability (motor
ings), such as brushes and commutators, offers itself as an torque is proportional to the square of the terminal voltage)
ideal solution to this problem if suitably designed for the which may result in inadequate torque for the duty. It must
rotational duty, and if a cost-effective source of variable be said that the motor will run both cooler and quieter in this
frequency and voltage can be provided. condition.
 C h a p t e r 12.2 273

> 400
>

0
>
~ .~ e 0 0 -
o O
0 J
E E~
I I
J
50 100 50 87
motor frequency, Hz motor frequency, Hz

2.6-/
power

1.5-
0

torque 0
0
E J
0 sb0 3o'00 > 1480 2560
motor speed, min-1 motor speed, min-1

Figure 12.13 Characteristics of standard A.C motors up to Figure 12.14 Use of motor star and delta configuration to
and above standard frequency operate at increased frequency and power
a V/Hz characteristic for high-speed a voltage supply to 200 V, four-pole, 1.5 kW
operation motor from inverter
b motor torque and power characteristic b additional kW operating 50Hz motor at
87Hz
A consequence of the theoretical V/Hz motor requirement is
that two approaches to operation in the high-speed region are
available. The use of a standard inverter operating from a 380/415 V
supply allows, by suitable V/Hz configuration within the
Figure 12.13 illustrates how a 50 Hz motor can be operated
control electronics, the maximum output voltage to be
at higher speeds without electrical modification, and shows
reached at a frequency somewhat above the normal 50 Hz.
the resultant torque and kW characteristic that may be
The voltage supplied to the motor at 50 Hz will be 220 V,
expected.
which is correct for the chosen motor connection and will
As the speed demand to the controlling inverter is increased allow the motor to develop rated torque and kW at the rated
from zero, both the voltage and frequency are increased on a speed.
linear basis until the output frequency reaches approxi-
As the frequency is increased above 50 Hz the voltage will
mately 50 Hz. At this point a normally configured inverter
also increase until the line supply value is reached; this
will reach an output voltage approximately equal to that of
happens at approximately 87 Hz on a 400 V supply. Speci-
the incoming line supply, Figure 12.13a.
fically, the motor is now running at 74 per cent above its
Increasing the speed demand to the inverter further will rated speed, the V/Hz ratio has been maintained and so full
continue to raise the output frequency although the voltage rated torque is available. Overall then, the motor is capable
supplied to the motor cannot increase, being limited to the of delivering 74 per cent more power than the 50Hz
level of the incoming line supply. The motor will increase in nameplate figure by virtue of the higher achieved running
speed responding to the rising frequency. However, the speed. Figure 12.14 illustrates this point.
available shaft torque will fall away as the square of the
effective voltage reduction. For example, a 10 per cent For example, in the case of a 1.5kW, 50Hz, four-pole
increase in frequency above 50 Hz would normally require machine with a rated speed of 1480 min,- 1 the power would
a motor supply voltage increase of 10 per cent to maintain increase to a theoretical 1.5 x N3 = 2 . 6 k W developed at
design torque; this 10 per cent deficiency in voltage will 1 4 8 0 x N 3 - - 2 5 6 0 m i n -1. In practice, frictional losses,
result in a 19 per cent shortfall in torque. magnetic losses and additional power absorbed by the
cooling fan would detract from this level, but a substantial
In this region the motor is essentially a constant-kW device, benefit can be obtained by this method nevertheless.
since reducing torque while increasing speed results in a
constant-power characteristic, Figure 12.13b. It is important that the mechanical constraints of the motor
and the driven system must not be overlooked.
Clearly, mechanical limitations such as bearing perfor-
mance, must be borne in mind when operating a standard
induction motor above its rated speed.
High-frequency Purpose-Designed
Standard induction motors of 7.5 kW or less commonly have Motors
windings arranged in a six-wire configuration to allow
connection for dual voltages such as 220 V in delta config- The vast majority of motors used in true high-frequency
uration and 400 V in star configuration. This makes it pos- applications are specifically designed for the purpose.
sible to develop extra kW above the nameplate rating of the Motors rated up to > 180 000 min-1 are available, along with
motor by a combination of voltage and frequency control. appropriate inverters with a >3000 Hz capability.
274 TECHNIQUES COMMON TO MANY APPLICATIONS: High-Frequency Inverters

Motors designed for such speeds are normally of a slim torque characteristic right across the speed range, i.e. kW
construction in order to minimise the centrifugal forces and increases with speed.
rotor inertia, offering a better dynamic response. Special
The inverter must be designed to operate in controlled output
bearings are invariably employed and range from fairly
voltage mode (normally employing pulse-width modulation
standard deep-groove type up to 12000min -1, oil-mist
(PWM) techniques) not just up to 50 Hz or so but to 300 Hz
lubrication types up to 60 000 min -1 and air-beating or gas-
or even 1000 Hz and beyond, depending on the motor design
type beatings for even higher rotational speeds.
detail.
Thermal considerations are also significant to the motor
Synthesis of voltage waveforms to ensure good A.C. motor
design, since the motor generally has a small physical size
phase current waveforms is complex and demands high
for its power rating and may run hot. Depending on the
processing capability and speeds within the control elec-
thermal reserve in the design, cooling may be surface only or
tronics of the inverter. This difficulty increases as the motor
extend to systems where air is drawn through the body of the
frequency rises and a compromise is usually made between
motor; this, however, can invade the protection integrity of
the quality of the motor phase current waveforms and
the motor. In the extreme, motors may be water cooled and
the cost.
fitted with elaborate water jackets and feed systems.
This is an important point since any degradation in wave-
High-frequency motors are most often designed to offer a
form brings with it the risk of modulation patterning on the
constant torque characteristic across the full operational
workpiece despite the flywheel effect of the rotating mass at
speed range and to avoid the constant kW/field-weakening
relatively high speeds.
region referred to earlier. Consequently, the motor winding
is designed and wound for the highest operating voltage to To assist motor braking, and to guard against inverter
coincide with the maximum design frequency. In other nuisance overvoltage tripping when the motor produces
words, a constant V/Hz ratio is used across the speed range, regenerative energy in its decelerating mode, it is normal
Figure 12.15. practice to equip the inverter with a dynamic braking
system.

High-Frequency Inverters
High-Frequency Applications
The need for inverters to operate with very high-speed
motors affects the design of the inverter in most cases and Woodworking machinery is a traditional area where high-
this has led to the introduction of specialised inverters; speed cutting and finishing is essential to produce the basis
however it must be said that general-purpose inverters of of a satisfactory final product.
more recent design are extremely versatile in their V/Hz Traditionally, normal induction motors driving through a
adjustment range. step-up belt drive system have been employed. However,
For applications where full output voltage is to be achieved the problems associated with belt maintenance, and the
at 50 Hz, followed by constant output voltage as the fre- desire for even higher throughput speeds and improved
quency continues to rise, additional inverter design con- surface finish, have led to the adoption of high-frequency
siderations are minimal, although control stability may be an motors. These offer the additional benefit of compactness
issue with some loads. However, as mentioned previously, in the cutting head area where space is often at a premium.
most true high frequency motors operate with a constant Normal speeds reached are in the range of 12000-
18 000 min -~.
Modem woodworking machines have to accommodate many
Vmaxz shapes and profiles, and as a consequence employ numeric
I : ~ L_" control (NC) systems to ensure the flexibility required.
>

0 "-
f Inverters lend themselves readily to control by NC, and by
using such techniques as load monitoring the type or quality
"6E of wood can be evaluated automatically in order to set the
Eo
optimum cutting speed.
300
Tool changing is also a feature of these machines, and the
motor frequency, Hz
NC can stipulate the speed and the torque available from the
motor to suit the chosen tool.
(1)~'-"kWma
Profiling and curvature machines are quite common in the
O
woodworking industry. Multiple tools are also used, and
{3.
L these would include such functions as grooving cutters,
0 facing drills and a circular saw for parting and slotting.
E
These are often high speed although they normally run
18000
b motor speed, min-1 at differing speeds. For example the grooving cutter may
run at 300Hz (18000min -1) the facing drill at 120Hz
Figure 12.15 Characteristics of high-frequency purpose- (7200rain -1) and the saw at 75 Hz (4500 min-1). Only one
designed motors motor is used at any one time and this offers the possi-
a high-frequency VIHz characteristic bility of employing just one inverter but incorporating a
b high-frequency motor kW characteristic suitably interlocked changeover system to connect each
 C h a p t e r 12.2 275

/ )
dancer

I po~er I I power ]
LY~
= r.p.m. integral
(clamped at
low speed)
limit

=diameter
proportional

line
speed
= linear speed

Figure 12.16 Speed-controlled centre wind with dancer

speed

Pi~~P~,
~erlgI e I po~erl
= torque

IA'X'I A
B set
tension

B
I
limit line
speed
=diameter

Figure 12.17 Torque-controlled centre wind (open loop)

motor in turn to the inverter. At the same time the V/Hz CENTRE WINDERS
ratio of the inverter is adjusted to preprogrammed values
which suit each individual motor. Clearly, cost and space General
savings are attractive. During noncutting periods the con-
nected motor is ramped down under full control into a low- Centre-wind tension control systems fall into two categories;
speed condition, drastically reducing machine noise but they can be either speed controlled or torque controlled.
being ready to accelerate back to working speed within Either way the objective is to maintain material tension
seconds. throughout the full diameter range and over the machine
276 TECHNIQUES
COMMONTO MANY APPLICATIONS:Centre Winders

linear speed range at a controlled value commensurate with Systems using dancer feedback whether operating in speed
obtaining a satisfactory rewound roll of finished product. or torque mode pose a somewhat more difficult problem if
taper is to be achieved. Dancer systems rely on the pre-
The following observations are intended to help in the
loading of the dancer mechanism to set the material tension,
choice of control philosophy and drive configuration to be
to achieve taper this preloading must be modified according
used for centre-wind applications.
to diameter changes. Solutions are possible where dancer
loading is pneumatic using, for example, electric to pneu-
Speed or Torque Control matic (E to P) transducers controlled via an analogue output
Speed control systems are only practical where direct from the drive diameter calculating software.
feedback of material tension from a dancing roll mechanism
Taper may be required to start from the central core diameter
is available. Load cell tension feedback is not normally
or may be introduced at some diameter threshold. The slope
practical for speed-controlled winders as no material storage
of the reduction in tension should be adjustable and will
is provided to allow for control system span.
normally be set by the operator. Taper is usually required for
Torque control systems can be used with or without direct material with a smooth surface where there is a possibility of
tension feedback; the tension feedback for torque control the outer layers slipping over the inner layers, usually
systems may be derived from either a dancer mechanism or resulting in the rewound roll telescoping. Materials needing
load cell. high degrees of taper include coated paper and some plastic
films.
The selection of speed or torque control is usually decided
by the machine manufacturer, who will have had previous
experience of the machine and the material being wound; Constant Torque and Field Weakening
however some background information is always useful to
The decision concerning the use of field weakening
the drive engineer.
depends to a large extent on the power requirements of the
Torque control solutions are normally adopted when the application. For lower-power applications constant torque
material to be wound is nonextensible, e.g. paper, steel, motors used in conjunction with oversized converters are
nonferrous metals. However, care should be exercised the normal solution as the control strategy and set up is
where no tension feedback is to be used. It is essential that simple. D.C. constant-torque solutions have the dis-
mechanical transmission losses be kept to the absolute advantage that they operate at very poor power factor and
minimum. Low-tension applications where tension powers draw comparatively high A.C. currents, as they must pro-
are comparable to transmission losses should not be duce maximum torque at minimum speed. This means that
attempted without some form of direct tension feedback. the motor and converter be sized using the build-up ratio
Where tension powers are high compared to transmission multiplied by the tension power, possibly resulting in an
losses simple predictive torque control systems with no expensive power converter.
overriding tension feedback can be completely satisfactory.
By using the constant-power characteristic of the motor the
Extensible materials such as certain types of plastic and converter size can be reduced; on D.C. systems the A.C.
polyester films, or machine configurations where non- current reduces accordingly and the power factor is improved
extensible material is to be drawn from a catenary e.g. a as the motor operates at its optimum voltage through the
looping pit, should normally be approached with a speed- diameter range. This solution should be considered where
control solution in mind. Because speed-controlled systems tension powers above about 50 kW are to be provided.
always rely on a tension feedback signal they are less sen-
sitive to the problems experienced with predictive torquen As an example, a system requiring 20 kW of tension power
control systems where inertia and transmission losses can over a 5:1 diameter range with no taper using a constant
cause tension disturbances; however, the transmission losses torque motor will require a 100 kW motor and a 100 kW
and inertia effects should still be kept to the minimum. The converter to match. This would be economically acceptable
dancer error operates on the drive in speed mode causing any but for D.C. systems it is important to consider the likely
tension disturbances due to torque changes within the load level of the A.C. currents and the low power factors when the
system to be very quickly compensated for by the effect of drive is operating at maximum diameter.
the speed regulator. Where large inertias are involved it may Obviously, if the tension power is increased to 200 kW over
still be necessary to provide some form of compensation in 5:1 diameter range the constant torque solution becomes
order to reduce the demands on the tension control loop. totally unacceptable as both the motor and controller
The diagrams below show the basic control configuration for wouldeffectively be sized at 1000 kW. A 330 kW controller
speed and torque-controlled centre-wind systems. with 3:1 field controller would be a far more effective
solution, costing less and reducing the current required at
maximum diameter to one third that of the constant-torque
Taper Tension solution.

Some materials require the tension to be reduced as the Modem D.C. motors can normally only be operated over
rewound diameter increases; this is termed taper tension. 3:1 or 3.5:1 range of diameter by field weakening, any
Taper tension may be implemented on open-loop torque- larger ratio must be provided by the constant-torque range of
controlled systems and closed-loop torque-controlled systems the motor and the converter oversized accordingly. Some
using load-cell measurement quite simply, by modifying the older motors encountered on refits may have 4:1 or even
system tension set point as the diameter increases. 5:1 attainable by field weakening.
 Chapter 12.2 277

Constant-power solutions using field weakening on A.C. application is involved then the speed and power require-
systems are also feasible. ment at base speed should also be quoted.

When offering constant-power solutions it is always neces- Any additional power required to compensate for transmis-
sary to check with the motor supplier; these are not standard sion losses and peaks for acceleration should be added to the
windings and should be selected by the motor designer. above result. Winding heavier gauges of metal requires
additional power to form the metal around the periphery of
In the first example above, the motor, A.C. or D.C., would be the coil. Tighter bends occur at smaller diameters and the
specified as 100 kW at 1500 min- 1. bending effort therefore reduces with increasing diameter
In the second example above, the D.C. motor could be unlike the tension torque which increases with diameter.
specified as: This means that the drive benefits from a balancing of these
two torques but the control system may need to calculate the
200/200/200kW individual torque components in order to achieve accurate
control over the tension.
300/500/1500 min -1
270/450/450V
Inertia Compensation
The converter would need to be rated for 333 kW, i.e.
During line speed changes energy must be supplied to
(200 x 450)/270.
or removed from the rotating masses of the rewind
An A.C. option for the second example above would pos- mechanism. The amount of energy transferred depends
sibly be: upon the inertia of the total system and the rate of change
of speed. All rewinds and unwinds can be considered as
200/200/200kW having two inertia elements, one of fixed inertia made up
of the motor and transmission components together with
300/600/1500 min -1
the core or spindle onto which the material is wound. The
20/40/100 Hz other is the inertia of the wound material, which obviously
207/415/415V varies from zero to a maximum value as the diameter
increases.
The converter would need to be rated for 500kW, i.e. An estimate of acceleration torque referred to tension torque
(200 × 100)/40. at various diameters will give an indication of the degree of
The speeds and voltages selected are for example only. tension disturbance to be expected during speed changes.
Ideally, the motor speed should be as close to the required Systems with low inertia or slow rates of acceleration,
spindle speed as is practical; this reduces errors in tension where the acceleration torque is small compared to the
due to transmission losses and the effect of motor rotor tension torque will need very little or no compensation.
inertia. High-speed winders with rapid acceleration and high inertia,
where acceleration torque can be equal to or greater than
the tension torque will obviously need precise inertia
Power Requirements for
compensation.
Centre-Driven Winders
Deriving rate of change signals for use in the inertia com-
Winder motors should always be sized from knowledge of pensation calculation can be troublesome. The most satis-
the required winding tension and line speed: factory system is to use an S-ramp to set the acceleration
characteristic of the line speed controller. The S-ramp
winding tension power (kW) =
should be configured to provide not only a speed reference
line speed (m min -1) x total tension pull (N) value but also an acceleration rate signal; this can be used in
60 000 conjunction with the inertia values to produce a torque
feedforward for the winder drive to compensate for inertia
If constant torque control is to be employed then the motor
effects.
and converter should be rated:
Simple systems with acceleration torque, which are rela-
motor/converter (kW) =
tively constant throughout the diameter range, can use block
winding tension power (kW) × maximum diameter (m) compensation, basically switching the compensation torque
minimum diameter (m) in and out on the result of a simple acceleration/deceleration
detection system. More sophisticated arrangements may
line speed (mmin -1) x gear ratio need some shaping of the torque related to diameter but can
motor speed (min -1) =
7r x diameter (m) still rely on an acceleration switch.
High-performance systems should always use S-ramp
This will ensure that the drive can produce the torque
acceleration of the line speed controller with accurate speed
required at maximum diameter and the speed required at
and trapezoidal rate of change of speed signals fed forward
minimum diameter.
to the winder control software. Systems have been installed
Constant-power applications are best specified by stating and commissioned running up to 2200mmin -1 with
the power and speed requirements at both ends of the dia- acceleration rates of 20 m min- ~per second using both A.C.
meter range. If a combined constant-power/constant-torque and D.C. drive technology.
278 TECHNIQUES
COMMON TO MANY APPLICATIONS: Centre Winders

Loss Compensation the operating range by field weakening above the motor
base speed.
Transmission systems should be selected to be as loss free
as possible; worm gear boxes should be avoided. Ideally, Accuracies on simple predictive systems of between 3-5
motor speeds should match winder spindle speeds with no per cent of maximum tension are attainable with improve-
requirement for speed reduction but on smaller machines ment to 1-2 per cent using tension feedback. Due to the
this is normally not practical. Toothed belt or low-loss large predictable torque range available from the motor, wide
inline gear boxes should be specified. Some discussion with ranges of tension and diameter range product are practical.
the machine manufacturer may be necessary here as not all Typical operating parameters for standard packages are:
machine builders realise the effect that the mechanical tension range 1:10
transmission system can have on winder drive performance.
diameter range 1:10
Losses are usually considered to split into two components, material speed 1:20
basic stiction which is considered to be constant throughout
the speed range and viscous friction where the loss increases Such a system requires a sensitive method of speed feedback
with speed. Small values of both components are relatively as the overall speed range of the rewind motor is 200:1;
simple to compensate for but often complications arise if the D.C. tachogenerator or a high-pulse count encoder are
losses are comparable to the tension component. Any var- essential to ensure performance at the low speed.
iation in actual loss due to temperature changes in the The performance can easily be assessed by logging motor
lubricants, general running in of machinery etc. can cause current and voltage throughout the diameter range and
substantial disturbances to the tension. The most satisfactory checking for a constant power product, corrections being
solution here is to use direct measurement of tension with made for any changes in line speed or taper tension effects
closed-loop tension control. during the test.
If this is not possible then some form of autocompensation The above performance figures assume minimal transmis-
should be considered where the machine losses are measured sion losses.
and stored by the control system and used to provide the
compensation during production runs. These solutions are A.C. FLUX-VECTORDRIVE
hardly ever totally satisfactory. Ideally for satisfactory open-
loop tension control the torque required to overcome trans- It is important to note that centre-wind control with an A.C.
mission losses should be no greater than 10 per cent of the drive is only possible with a high-performance drive, such as
minimum tension torque. Unidrive, capable of precise control of torque. Operation
above base speed is possible with a suggested limit of
2.5/4: 1; above this torque linearity deteriorates somewhat.
Flux Compensation
The torque produced by a motor is determined by the pro- Typical operating parameters for standard packages are:
duct of its active current and its flux. When the motor runs tension range 1:10
above base speed the relationship between active current and
diameter range 1:5
actual torque changes due to the reduction of flux. High-
performance flux vector A.C. drives such as the Unidrive material speed 1:20
have a torque loop which automatically corrects for this Such a system requires a sensitive method of speed feedback,
thereby ensuring that the actual torque delivered matches the as the overall speed range of the rewind motor is 100: 1.
torque demanded.
The above performance figures assume minimal transmis-
D.C. drives however do not typically provide this facility, sion losses.
therefore when using D.C. motors in torque-control appli-
cations above base speed it is necessary to modify the SERVODRIVES
relationship between torque demand and armature current
demand to compensate for the change in motor performance. Brushless A.C. motors provide a more accurate means of
This must form an integral part of the winder control soft- controlling torque than do asynchronous counterparts but no
ware package. A simple algorithm to achieve this is: if field-weakening range is available.
speed > base speed then: Typical operating parameters for standard packages are:
torque x speed
current = tension range 1 : 10
base speed
diameter range 1:10
else:
material speed 1:20
current = torque

Drive Selection- Limiting Parameters SECTIONAL DRIVE SYSTEMS

D.C. DRIVE General
A D.C. drive has been the traditional solution for all winder Sectional drive systems may be characterised as a multidrive
applications for many years. It has the advantage of a linear application where the operation of a drive is determined by
motor torque characteristic with the possibility of extending the process itself or by the drives around it. In other words,
 Chapter 12.2 279

a sectional controller is essentially a speed regulator in a drive speed to the modified reference and pass that modified
multidrive coordinated process line. In this type of appli- reference along to one or more upstream or downstream
cation, typically seen in film or paper production web lines, sections.
a section is usually a roller rotated by a drive and motor. The
term web refers to the material linking the rolls which The reference may be optimised by local feedback devices
constitute the machine. Setting the speed of this roller is not such as a dancer arm, load cell, or pressure sensor which
as simple as it seems. All the rollers in a web machine run at indicates tension. The program allows a tension set point to
various ratios of one another; these ratios are dependent on be compared to the tension feedback and the error signal is
process temperature, humidity, roller size, gearing, tension, applied to a proportional/integral/derivative (PID) algo-
trim inputs etc. As the machine accelerates from zero line rithm. The PID error output can be used to modify the ratio
speed to maximum line speed, the relative ratios of the or simply adjust the reference directly. The tension set point
sections must be maintained or the web will break. Many and feedback signals can be any parameter on the section's
sections will require local fine tuning of the ratio or section drive or in any attached inputs/outputs, or can be any
speed to account for tension adjustments (the operator might parameter elsewhere on the network.
observe that the material is wrinkling and reduce the tension, The new section reference, based on a ratio and a tension
for example). When a local adjustment is made, the effect of adjustment, can be propagated to up to six destination sec-
this adjustment must be propagated to the sections which tions. A master section would typically propagate the
follow. reference to the upstream and downstream sections. A slave
There are two ways to implement a sectional controller. One section would typically propagate the reference to a single
way is to use a single computational platform to work out all upstream or downstream section. However, any section
the required section speeds and transmit them periodically to can propagate the reference to six possible destinations,
the respective sections. This has the advantage of a single thus permitting a parallel path topology to be created.
controller doing all the work. The disadvantage is heavy Figure 12.18 shows some of these process line topologies.
network traffic flowing into the single controller, since the
controller has to know what the other sections are doing and Reference passing over the network is handled by a high-
read all the I/O devices used for adjustment. speed fieldbus; in the example CTNet is used. This incor-
porates a cyclic data feature, which is a preprogrammed
The second way is to position intelligence at each section so transfer scheduled to occur on a periodic basis. A section
that the ratio and trim adjustments are done locally and the expects its incoming reference to arrive via a fast cyclic data
results are passed only to the upstream or downstream sec- channel and be deposited into PLC register _S00%. A high-
tions that need them. In this scenario some control loops are speed clock task within the Unidrive coprocessor reads the
implemented locally utilising feedback available from the incoming reference, multiplies it by the selected ratio, makes
local I/O on the drives controlling each section. This reduces an adjustment for tension error and creates a final section
the load on the network, which is then only called upon to reference. This final reference is used to set the local drive
transfer slower outer-loop references less sensitive to var- speed and is also deposited into PLC register _R00% for
iations in the deterministic response. Although the dynamic eventual transmission to other drives via cyclic data. Six
requirements of the network are less, flexible peer-to-peer possible destinations can be specified. The references and all
communication is needed to allow data to be shared easily set-up parameters are 32-bit integers in times 1000 format
between distributed processes. but the calculations are all done in floating point, so there is
A distributed architecture has some advantages over a cen- no loss of precision.
tralised structure: a powerful controller necessary to cope
In a coordinated process line, all drive ramps should be
with large levels of computation for the whole system is
disabled. The ramp algorithm in the coprocessor program is
expensive and the software will invariably be unwieldy and
relied upon to protect the drives. One and only one section
difficult to maintain. Also the I/O, which most drives pro-
(the master) will have its ramp algorithm enabled. All the
vide as standard, can be utilised very efficiently by a local
other slave sections will have their ramp algorithms dis-
process without loading the network. Clearly, it may be
abled. This permits the slaves to follow the master's ramp
difficult to partition and distribute some systems and a
exactly, a crucial element of a coordinated process line.
centralised approach is best (e.g. the interpolation functions
of a multiaxis CNC machine). Changes to the ratio are also protected by an onboard ramp
The method described here, as an example, is completely system. This ramp system is normally enabled for all
distributed. It is based upon each section of the line running sections and any abrupt ratio change is slew-rate limited by
a local application program. The particular program described the ratio ramp and this effect is propagated to all follower
has features built in which permit it to act as a line master or sections.
a line slave, selection being via selectable parameters to The sectional controller also uses a cyclic data channel for
configure the operation. Network traffic can be heavy, but it start/stop operations. Usually, the master section will
is node-to-node, not a blizzard of messages going to a single broadcast the start command: this means that each slave
node as in the star or centralised configuration mentioned drive will see the start command at the same instant. How-
above. It is based upon a system employing Unidrive drives ever, to stop the coordinated process line, the master section
each with an applications coprocessor and a high-speed first commands all drives to regulate to zero speed, follow-
fieldbus interface fitted. ing the master's ramp. When the master determines that zero
The primary purpose of the sectional controller is to accept speed has been attained, only then will the stop command be
an incoming speed reference, multiply it by a ratio, set its broadcast.
280 TECHNIQUESCOMMON TO MANY APPLICATIONS" Sectional Drive Systems

-- Sl S2 S3 S4 S5 S6
I _ tl tl ~I ~1

master slave slave slave slave slave

section # 1 section # 2 section # 3 section # 4 section # 5 section # 6

Sl = line s p e e d * RATIO1

re S2 - Sl * RATIO2
eed S3 = S2 * R A T I O 3
S4 = S3 * R A T I O 4
$ 5 = S4 * R A T I O 5
S6 = S5 * R A T I O 6
PC

Sl S2 S3 S3 S4 S5 S6
il , tl I,I
slave slave master slave slave slave
section # 1 section # 2 section # 3 section # 4 •section # 5 section # 6

Sl = S2 * R A T I O 1
line S2 = S3 * R A T I O 2
speed S3 = line s p e e d * R A T I O 3
S4 - S3 * R A T I O 4
S5 - S4 * R A T I O 5
S6 = S5 * R A T I O 6

Sl $2 S3 S4 S5 S6
I ,I,! ~1,, " t i tl t
master slave slave I slave slave slave
section # 1 section # 2 section # 3 I section # 4 section # 5 section # 6
I
Sl = line s p e e d * RATIO1
line S7 S8 S9
S2 = S l * R A T I O 2
speed ~ I tl t
S3 = S2 * R A T I O 3
slave slave slave
S 4 = S3 * R A T I O 4
section # 7 section # 8 •section # 9
S5 - S4 * R A T I O 5
S6 - S5 * R A T I O 6

S7 = S 3 * R A T I O 7
Sl0 Sll S12
S8 = S7 * R A T I O 8
I,I ~,1
S9 - S8 * R A T I O 9
slave slave slave
S l 0 = S3 * R A T I O 1 0
section # 10 section # 11 section # 12
Sll - Sl0 * RATIO11
S12 = Sll * RATIO12

Figure 12.18 Coordinated process line topologies

a cascaded line w i t h master at o n e e n d
b cascaded line w i t h master in m i d d l e
c cascaded line w i t h parallel branches

includes a scale and offset, allowing a Unidrive analogue

The sectional controller can also operate in a manual mode
input and speed potentiometer to be set up for line speed
where the reference is not derived from incoming cyclic
control.
data, but rather a number of other sources. In manual mode,
one of six preset speeds can be selected, as well as an Each possible reference input has its own acceleration and
analogue speed, which can be any parameter on the local deceleration rate that the user can specify. Two ratios are
drive or any other drive on the network. The analogue speed provided, switched by a digital bit parameter. The ratios and
 Chapter 12.2 281

the reference are clamped by respective maximum and modify the reference. The equation implemented permits the
minimum limits. tension trim to affect the reference in two ways: adjustment
dependent on line speed and adjustment not dependent on
The tension PID adjustment allows any parameter on the
line speed. The constants kl and k2 permit selection of either
local drive or any drive on the network to be used as the
method or a mix of both. In the sectional controller, the
source of the tension set point and the tension feedback.
following equation is the heart of the system:
Tension set-point and feedback signals have a scale and
offset available to permit use of drive or remote input/output reference -
(I/O) analogue channels. The PID algorithm has a number of ((float(_SO0%)/lO00.) x ratio)+
adjustment parameters allowing this system to be tuned to
((float(_SO0%)/1000.) x ratio ×
the customer's requirements.
trim error x kl) + (trim error x k2)
The tension set point includes an increase/decrease function.
This is a bit parameter which advances or retards the tension As can be seen from the equation above, setting k l and k2 to
set point much like a motorised potentiometer system. The zero simplifies the reference calculation to a basic ratio.
longer the tension increase/decrease button is held down, the Constant kl (_P08%) adds in the tension trim adjustment,
faster it increments the set-point value. but makes it dependent on line speed; that is, the trim
adjustment has more effect at higher line speeds. Likewise,
A slack take-up/let-out feature allows any bit parameter on
constant k2 (_P09%) adds in the trim adjustment but makes
the local drive or any other drive on the network to add or
it not dependent on the line speed.
subtract a fixed value from the reference, as long as the
button is held down. This allows any web material lying on Sometimes a threading procedure will leave slack in the web
the floor due to a threading procedure to be quickly taken up. material or the operators want to create some slack at a
section to make adjustments; for these situations, a slack
All parameters configuring the sectional controller are
take-up/let-out register (_P39%) is added to the reference
nonvolatile. They exist in the coprocessor's _Pxx% and
to permit web material to be taken up or let out. This is
_Qxx% PLC register sets which can be saved to onboard
controlled by a slack take-up function.
flash memory at the user's discretion or at power down.
Figure 12.19 illustrates the reference ratio operation.
The sectional controller program is programmed to work
with either an A.C. Unidrive or a D.C. Mentor drive. By
AUTOMATIC MODE- SELECTINGTHE RATIO
reading virtual parameter #90.10 at start up, the type of drive
can be determined. This detection of drive type is used The sectional controller provides two ratios, selectable by a
throughout the program to select the proper precision bit parameter. Since the two ratios are adjustable parameters,
reference parameter and start/stop parameter. In fact, the accessible over the network, a SCADA system or network-
only line of code that must be changed is the #DRIVE compatible keypad can change these ratios at any time.
UNIDRIVE header line.
Each ratio has individual maximum and minimum limits and
these limits can be disabled, if desired.
Theory of Operation The ratio is protected by its own ramp system. This prevents
This section provides all the operational details of the sec- an arbitrary change in ratio from tripping the drive or any
tional controller. The diagrams show the flow of the data follower drives linked to it. The ratio ramp should always be
through the system and identify the parameters which the enabled.
user can adjust.
The ratios and the limits are all programmed in an integer
AUTOMATIC MODE- THE RATIO OPERATION times 1000 format. For example, a ratio of .98 would be
entered as 980 and a ratio of 1.527 would be entered as 1527.
The heart of the sectional controller is the ratioing operation.
In this operation, the incoming reference is deposited by the Figure 12.20 illustrates the ratio selection process.
cyclic data system into PLC register _S00%. This is a 32-bit
integer register, so the reference speed is expressed in times
AUTOMATIC, MANUAL AND
1000 format. This allows specification of a reference to a
MANUAl/BYPASS OPERATION
precision of three decimal places.
For example, 1468.015 m i n - 1 would be expressed as the Sectional controllers usually operate in three modes: auto-
integer quantity 1468015. It will be converted by the appli- matic, manual and manual/bypass.
cations program into floating point and all subsequent cal- Automatic mode accepts a reference from the fast cyclic data
culations will be done in floating point. Only when the final channel and applies the ratio, tension trim and optionally the
reference needs to be propagated to another slave drive will ramping operations before setting the drive speed and pro-
it be reconverted back into the integer times 1000 format. pagating that speed to other destinations.
In the simplest case, the sectional controller does a simple Manual mode allows the user to supply the reference, from a
operation such as: variety of sources, and no ratio operations are done. Manual
reference - (float(_S00%)/1000.) × ratio mode still permits propagation of the references via cyclic
data to other drives. This would typically be used in a master
To permit the use of a tension trim adjustment, the above section where the user wants to employ a speed potenti-
equation is extended to allow the tension trim component to ometer for the line speed.
282 TECHNIQUES
COMMONTO MANY APPLICATIONS:Sectional Drive Systems

!1adjust.,,
elo teP91%
nsiOn trim error k2_ trim error

] multiply
trim error * k2

K1

r+
kl trim error
multiply

reference * ratio * trim error * kl

d

Ilinc°miCTNet
no pt
reference
_S00%
J multiply
ref_ratio
reference * ratio
i

ratio

post_pid_ref . ~ reference

slack take-up adjust

Figure 12.19 Application of the ratio, tension trim and slack take-up adjustments

Manual/bypass mode applies a simple ratio to the incoming slave sections are followers, they follow the master's speed
cyclic data reference and passes it on to the destination and thereby follow the master's ramp system. Acceleration
sections, thereby isolating the drive from the line. Manual/ and deceleration rates are tailored to the specific reference
bypass mode would be popular during start up where the selected. For manual mode, there is usually a separate
installer wants to rotate a roller for testing purposes. acceleration and deceleration rate for each reference source.
The sectional controller also includes a maximum and
SLEW RATE CONTROL minimum clamp; this is engaged on each and every section
Typically, in a coordinated process line, only one agent to prevent runaway.
controls the slew rate of all drives. Usually, this is the master
section. DISTRIBUTION OF REFERENCES
The drive's internal ramping system is turned off and we rely The final reference, which is the incoming reference mod-
on the master section to ramp the speed up and down. The ified by the ratio, the tension adjustment, the ramping system
 Chapter 12.2 283

ratio selector II]lrati°l 0] - max

P 0 9lim
%it ] I[ rati°_P11%
1 max limit
_P05%
Ilratio 0 Tin limit I1 ratio 1 Tin limit
_P 10% I _P 12% I

ratio #0 ratio 0 ratio limiter

u P03% ratio
v

ratio #1 ratio 1 1
A

_P04% W

Ira,io_P06%
rampenab'ell
II ratioaccel rate
II _P07% ~" ---hi,li'""ratio_p08O
decel rate
o I[
_

ratio accel/decel
target _ratio function
ratio Ib,
w v

-~'0
0

Figure 12.20 Selection of ratio

and the high and low limits, needs to be distributed to this method. A broadcast message is one that every node receives
drive as a speed command and communicated to the other simultaneously; this allows all drives to turn on or shut off
drives which should follow it. within milliseconds of each other.
It is typical to allow the destination of reference data to be Starting and stopping a line is not however as simple as
changed by simply a parameter setting. This allows the user broadcasting a stop command - this would easily break the
great flexibility in the control of the line which may be web. In a coordinated process line, the stop command should
required for different materials or gauges of materials which actually cause a sequence of events to occur. First, the stop
are to be processed. command must be held in abeyance until the sections have
regulated to zero speed. If the start/stop control is in the
ACQUISITION OF TENSION SET POINT master section, as it should be, the sectional controller's start/
stop sequencer will force a zero line reference. This will
The reference may be trimmed by a tension error signal. The cause the line to begin ramping towards zero. The start/stop
tension error is the difference between the tension set point sequencer will monitor the section speed until it reaches zero
and the tension feedback. and then and only then will the stop command be broadcast.
The tension set point can be assigned to any parameter on
the drive or attached remote I/O or it may be fetched via SYSTEM FAIL-SAFE OPERATION
CTNet from any parameter on any other drive or I/O on the The sectional controller needs to provide a fail-safe system
network.
to ensure proper shutdown in emergency situations. A
coordinated process line should be shut down if any drive
ACQUISITION OF TENSION FEEDBACK trips or loses network communications. In this situation, all
The tension feedback normally comes from a load cell or remaining drives should be commanded to ramp quickly to
dancer; these can be connected to an unused Unidrive ana- zero and then stop.
logue input or a remote I/O. In any case, any parameter on Communications loss is detected by the buddy station
this drive may be selected as well as any parameter on any method. In a distributed application like this sectional con-
other drive or I/O on the network. troller, each section may transmit its node ID to a buddy
station via a fast cyclic data channel. If a communications
START/STOP OPERATIONS loss or a trip occurs, the section must do two things:

Start/stop commands are normally propagated over the 1 Ramp quickly to zero speed then stop.
network via a fast cyclic data channel using a broadcast 2 Signal the other drives to ramp quickly to zero then stop.
284 TECHNIQUES COMMON TO MANY APPLICATIONS: Sectional Drive Systems

To fail-safe stop the drive, the normal start-stop logic source node to a contiguous block of _Sxx% registers on
is used to ramp to zero speed then stop. The only differ- the destination node at either the slow or fast cyclic
ence is that special fail-safe acceleration and deceleration update rate.
rates are engaged. The drive must be reset to clear this
For the sectional controller example, we use the cyclic data
condition.
editor to create nine dummy cyclic links for each section.
These links are nonfunctional at system start up. The sec-
Using an IEC61131-3 Programming Tool tional controller program allows an outside agent, such as a
to Configure a Sectional Drive Line SCADA system, to reprogram each cyclic link to do some-
Graphical programming tools such as Control Techniques' thing specific. Figure 12.23 shows the completed dummy
SYPT (system programming tool) are well suited for use on links as set up by the cyclic data editor.
sectional applications. From a single workstation the drive As an example, assume node 12 is the master section that
system engineer can edit, compile, download and debug must transmit its reference to two adjacent nodes (node 11
every section over the CTNet token-ring network. The watch and node 13), broadcast the start/stop command, and send a
window is network cognisant and can display variables and heartbeat to node 13. Using the SYPT watch window's
parameters dynamically from several different sections command area, the following commands would reprogram
simultaneously. the cyclic links to achieve this, illustrated in Figure 12.24.
Figure 12.21 shows a typical SYPT configuration screen for
a six-drive system.
ENERGY SAVING
The configuration screen shows the nodes on the CTNet
network and the cyclic data links that are defined between General
nodes. In offline mode, the insert/cyclic data menu option
The application of variable-speed drives within almost
allows the designer to create cyclic links between one node
any process will provide improved control and thereby
and others. Figure 12.22 shows the dialog box which permits
offers the means to optimise quality, production rate and
specification of a cyclic data link between two nodes.
energy. The detailed calculation of the total energy impact
When a source node and a destination node have been which variable-speed control can have on an entire facility
selected, as shown above, a special cyclic data editor will is complex. It is however helpful to highlight the major
appear (Figure 12.22) that will permit the user to enter the area of energy saving opportunity in respect to the applica-
specifics of the intended transfer. Remember that a cyclic tion of variable-speed drives, namely loads with a variable
link transfers a contiguous block of_Rxx% registers on the torque/speed characteristic. Centrifugal fans and pumps

m ,m ~ I |
................~ ' . ~ ................~......... ~ ...................' ~ " ~ ...................~ - - ~ .....................
~ : ~ " ~ ....................
~............~....................
~'i~ ...................
~.................. i~::,I
i'.
..............................
~ ...................................
~ ...................................
L~L~" .....................................
C~L.~ .........................................
i~ISSI~L~ i ~i~~i ~i

!......!.L
.. ..iii!.................................. !...!-!.!...!.!ii!i!I!...!!.i.!.i!.!.i!t.!!-i!.i.!!i!....!..!!i!.i!.!!!i!i!. i.....!...,i!!!!....!.!i!.!.i!.!i!.,!!i!!!!!!!!L.!i!i!i!iiii
i!.=......!.!.!!!!L~:i

Figure 12.21 SYPT configuration screen

Figure 12.22 Specifying a cyclic link

 Chapter 12.2 285

.................................::::: ......................... :: ........................... ............................................... ............................................................................................................................................................................. ................................................................................................................................ ::i

ii

Figure 12.23 Creating nine dummy cyclic/inks

,, ~ . . . . . . . . . . . . ",. : ~ :.: ..: " . . . . . . N ~ ... :~ ...... • . . . . . . . . . ~:~y::~%~N~:.. ~. . ". ~ , ~ N .

~iL.Pgl.Y.{MA~ER]I = I 1 # reference p r o p a g a t i o n node ID l o t cy~c|lc channel $| .<OK>

~PCJ2~MASTER]i node ID lor cynic channel ~Z .<:OK) " 1;3 ~t reterence propagation .......
.!_Pg3~MASTEI~! node ID for cyclic channel #3 <:OK> =O tt re:terence propagation iiiiiii!i,
~ii~.IIJ4~MASTER]I =0
r~ode ID for ~ l i c c h ~ n e l #4 <:OK) ~ reference propagation ;iii!i!i!iii:.
iiL~S~MASTER) r~ode I D for cyclic channel ~ <OK:> ~-0 tr reference propagation ii~ii!i~i!i!i!
ii~Pgfi:Y,~MASTER;)--0 tt fete:fence propagatlon r~ode I D tor cyclic channel it5 <OK) iii;iiii!i!!
~.P02/~.MASTEI~) = I tt MASTERISLAVE select for cyclic channel 87 <OK.:> ii!i!i!i!il;i
~i~P9I..~MASTE I~) - 1:3 /t tall-safe node to receive the heart beat for cyclic channel #R (OK) .i.i...!
;i:2:iii::iiiii~
;!I_P00NMASTER) = I tt ~rce re.programming of all cyclic Iin~ ¢OK) ~i!i!i!;Ji;~!

Figure 12.24 Dynamically changing the cyclic links using watch window

typify this category with their cubic characteristic i.e. power characteristic of a real water pump with the following
absorbed (shaft power required) is proportional to the cube specification:
of the speed. Reducing speed by 50 per cent on such a load
therefore reduces the shaft power requirement to rated flow - 10.6 m 3 min- 1
0.53 - 0 . 1 2 5 i.e. an 87.5 per cent reduction. Compare this head = 37.5 m
with mechanical alternatives to flow control which may give
a 20 per cent reduction in shaft power requirement, and it is rated speed - 1480 min- 1
easy to see why so many drive systems are readily justified
on the basis of energy saving-capital expenditure payback Figure 12.25 shows the head/flow relationship for constant
periods of less than six months are not unusual. speed. The curve of efficiency is also included and is seen to
peak at a slightly lower flow than the rated flow of the pump.
In order to fully appreciate how such dramatic energy sav- Efficiency at rated flow is 83 per cent so the pump shaft
ings can be made it is necessary to review the basic operating power requirement is given by:
characteristics of specific loads and compare the effective-
ness of flow control using variable-speed drives as compared
shaft power (kW)
with throttling control using valves, vanes etc.
= (head (m) × acceleration due to gravity (N/kg)

× flow (ms-1))/efficiency
Centrifugal Pumps
= (37.5 × 9.81 × (10.6/60))/0.83
Figure 12.25 shows the characteristic curve of a typical
centrifugal machine. It is based upon the performance = 78.3 kW
286 TECHNIQUES COMMON TO MANY APPLICATIONS" E n e r g y Saving

100
90
80
70
60
head (rn)
50
efficiency (%)
40
30
20
10
0 I I I I I I

0 2 4 6 8 10 12
flow, m3 per min

Figure 12.25 Characteristic flow curve of a typical centrifugal machine (pump, fan or compressor)

60 /

50

40
head at 1480 min-1 (m)
30 head at 1210min -1 (m)
head at 960 min-1 (m)
20

10

| | | | | .

0 2 4 6 8 10 12
flow, m3 per min

Figure 12.26 Characteristic flow curves of a typical centrifugal pump operating at different speeds

100
90
80
70
60
flow (%) I
50 head/shaft torque (%)
40 power (%)
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
speed %

Figure 12.27 Characteristic relationship between flow, head~torque and power in a centrifugal pump operating at
different speeds

If the pump speed is reduced from the rated speed of head is proportional to (n2/nl)2
1480min -1, a similarly shaped curve exists relating the
power is proportional to (n2/n1)3
head/flow characteristic of the pump at any particular speed.
Figure 12.26 shows a family of curves generated for speeds It follows that in respect of the shaft:
over the range 1480 to 960 min- 1 (65 per cent). shaft torque is proportional to (n2/nl) 2
The head/flow curves for any speed can be readily estimated
Figure 12.27 shows these relationships graphically.
from the constant/rated speed characteristic. The following
relationships apply: For pumping applications, the characteristics of the system
into which pumping takes place must be considered and the
if the rated speed is n] and the reduced speed is n2, then:
back pressure of the pump consists of two components:
speed ratio is (n2/nl)
• static head
flow is proportional to (nz/nl) • friction losses
 Chapter 12.2 287

60

50

40 head at 1480min -1 (m)

head at 1210min -1 (m)
30
head at 960 min -1 (m)
20 system demand

10

0 2 4 6 8 10 12
flow, m 3 per min

Figure 12.28 Characteristic flow curves of a centrifugal pump operating at different speeds with system demand
superimposed

45
40
35
30
25 head at 1210min -1 (m)
20 system demand
15
10
5
0 I I I I I I )

0 2 4 6 8 10 12
flow, m 3 per min

Figure 12.29 Characteristic flow curve of a centrifugal pump operating at reduced speed with the system demand
superimposed

Figure 12.28 shows a system demand curve superimposed The pump shaft power requirement is given as:
on the variable speed curves of Figure 12.26.
shaft power (kW)
Static head is seen to be about 22 m and represents a total lift
from the pump inlet to elevation of the point of discharge. = (head (m) x acceleration due to gravity
Friction losses are a function of pipe diameter, pipe length,
(N/kg) x flow (ms-'))/efficiency
inlet losses, pipe C factor, specific gravity of the liquid, type
and number of bends, fittings, reducers, valves etc. and are = (28 x 9.81 x (6.8/60))/0.785
approximately proportional to the square of the flow. = 39.7 kW
From Figure 12.28 it can be seen that at the maximum (rated)
speed of 1480 m i n - 1 the system demand curve intersects the If, instead of using variable-speed control, a mechanical
pump curve at 11 m 3 min- 1., this represents 100 per cent flow. throttle were used, the system curve would be modified as
shown in Figure 12.30.
Consider the condition where flow is to be reduced to 6.8 m 3
min,- 1 a reduction of 40 per cent. The pump speed required The operating conditions for the pump are now:
to operate at this point on the demand corresponds to the
pump characteristic drawn for operation at 1410min-~. head - 45 m
This is shown on Figure 12.29. flow - 6.8 m3min -1

The system back pressure at this point is 28 m including efficiency - 81%

friction losses of about 23 m:
The pump shaft power requirement is given as:
n2/nl- 1210/1480
= 0.82 shaft power(KW)
With this 18 per cent reduction in shaft speed the effi- = (head (m) x acceleration due to gravity (N/kg)
ciency of the pump would typically fall by the order of
3 per cent. Figure 12.25 shows the efficiency at a flow x flow (ms-l))/efficiency
rate of 6.8 m3min - ~ to be 81 per cent so the efficiency at = (45 x 9.81 x (6.8/60))/0.81
the reduced speed can be assumed to be 0.81 x 0 . 9 7 -
78.5%. = 61.8 kW
288 TECHNIQUES COMMON TO MANY APPLICATIONS" E n e r g y Saving

60 /

50-
head at 1480min -1 (m)
40

30 system demand
(without throttle)
20- system demand
(with throttle)
10-

0 I I I I I I I

0 2 4 6 8 10 12
flow, m 3 per min

Figure 12.30 Characteristic flow curve of a centrifugal pump operating at fixed speed with the system demand with
throttling superimposed

The excess head dropped across the valve therefore repre- The power requirements in the above example, at the
sents the energy loss due to throttling. reduced flow, can be restated as:
The energy saving by using a variable-speed drive in mains power for variable
this example, at that duty point, is (61.8-39.7)kW = speed solution = 39.7/(0.97 x 0.92) --44.5 kW
22.1 k W - 35.7 per cent. mains power for the
This example does assume that the pump selection and valve throttling solution = 61.8/0.94 = 65.7 kW
sizing have been optimised for the application. The power saving is 21.2 kW/32.2 per cent.
It needs to be noted that even at the design flow rate, some
excess head must exist across the valve for it to be able to Centrifugal Fans and Compressors
control flow. Typically this would be 5-10 per cent of the Centrifugal fans obey the same basic characteristics as cen-
rated pump head. trifugal pumps and the shape of their pressure/volume curves
So far only the pump has been considered. In order that is therefore similar to those of pumps. The calculations and
overall losses are included in the calculation, i.e. from the the rationale for variable-speed operation are the same.
electrical supply to the pump output, it is necessary to take The pressure/volume characteristic for centrifugal com-
into account the efficiency of the drive system. For the fixed- pressors, although generally similar to that for the pump,
speed motor rated in 75 kW, the rated efficiency could be differs in that a pressure peak occurs between low flow and
expected to be 94 per cent. For a modem A.C. variable- rated flow. The range of flow reduction cannot therefore
speed drive, the converter efficiency can be conservatively extend beyond the limit where surging takes place. Good
assumed to be 97 per cent. The motor under variable fre- flow control can be achieved with variable-speed operation
quency control could be conservatively assumed to lose two over typically a 2:1 speed range, although care must be
points of efficiency. taken to avoid surge problems.

3 APPLICATION PRINCIPLES/EXAMPLES

The applications in which variable-speed drives are applied variable-speed drives offer from the user's perspective. In
are many and extremely varied. The reasons for using many cases it can be seen that not only is the core perfor-
variable-speed drives are almost as diverse. This section mance of the drive critical, but also the auxiliary features.
brings together a broad cross section of applications. In some The capability and flexibility of the soft logic program-
cases technical details behind the application of the drives mable applications module features heavily in many of the
are given; in others emphasis is given to the benefits that application examples.
 Chapter 12.3 289

CRANES AND HOISTS Requirements typically include:

• slewing control on a cargo crane
General • grab control on the hold-and-close drive
The use of variable-speed drives on cranes and hoists is very • synchronisation on two or more motors
widespread. Historically the characteristics of slip-ring • load-dependent speed control on the hoist movement
induction motors with resistance and later phase control in
the rotor circuit were very popular. Today the use of A.C. By utilising an applications module it is possible to provide
cage induction motors predominates, with closed-loop all the movement-related control in the drive itself. There-
flux-vector control used for precise control, and where fore, in most instances, there is no need for an additional
performance is not important open-loop control is applied. PLC-a major cost saving!

There is a substantial market for retrofitting of the drives

systems on cranes since the electrical systems invariably Slewing Control
have a shorter life span than the very substantial mechanical
On many conventional cranes, the slewing movement is
parts. There comes a point where drives maintenance on
undertaken with slip-ring motors. The slip-ring motor, in
cranes becomes uneconomic and downtime is seriously
combination with rotor resistors, meets the crane driver's
affecting the operation of the factory or dockyard.
needs in most instances. There is good motor torque control
for acceleration and deceleration and it is possible to coast
Planning an Installation when the controller is moved to zero. However, this method
In most retrofit applications with state-of-the-art inverter of control is very poor at low speeds, with sudden steps in
control systems, there are several issues: torque between resistor steps, wasting a lot of energy, and
the system requires very regular and intensive maintenance.
• the need to install the equipment in the existing
environment When replacing the slip-ring motors with a modem dynamic
• the need to create the same characteristics on the drive system, the results can be very disappointing. Figure
movements as before with the desired feel on the 12.31 shows the behaviour of a conventional speed-
controller controlled drive system that makes it difficult to control the
• the maximum capacity of the crane swaying of the load.
• application of the present safety rules - these are To counter this effect it is possible to design a program to run
constantly being revised and tightened on the internal applications module that gives the crane
With regard to the maximum capacity of the crane, this is driver optimal control over the swaying load, without the
only valid if all of the controls for the crane movements are need for a PLC.
tuned in such a way that the driver is able to control the load Such a control system would typically provide the driver
in an easy and safe manner. with control over both the speed and the motor torque. Speed
When designing a drives system for a retrofit - or, indeed, control is important for accurate positioning at low speed. It
for a new crane - it is essential that the driver's needs are also provides compensation for the wind forces on the load.
fully considered. The ability to build in particular control Torque control is crucial for controlling the sway. In this
characteristics is a very important feature. Some drive way, the driver is always able to anticipate the movement of
manufacturers have available programmable applica- the load and compensate for it. By bringing the controller
tion modules for both D.C. and A.C. drives, to customise the back to zero, the movement is effectively coasting, which
performance and features of the drive to exactly meet the gives a major damping effect on the sway of the load
requirements of the user. (Figure 12.32).

13 0

O,

time, s

Figure 12.31 Comparison of desired speed of the load against the actuality, with the load speed finally being totally out of
control, for a speed-controlled slewing drive
290 APPLICATIONPRINCIPLES/EXAMPLES: Cranes a n d Hoists

1.2

1.C

0.8

0.6
"0
(D
o_ 0.4

0.2

time, s

Figure 12.32 Using integrated software, the driver is back in control and is easily able to control the load (typical slewing
control)

Figure 12.33 190 tonne overhead crane upgrade (courtesy ScanRope A/S)

Crane Refurbishment for a Norwegian "It increases the unit weight of rope coils we can supply to
Steel Wire Rope Maker over 200 tonnes," he explains, "which is why this crane
upgrade is so important to our company. We have placed
A major rebuild of an overhead crane (Figure 12.33), great emphasis on getting everything right - the steelwork,
upgrading it from 125 tonnes to a theoretical maximum 190 the motor and most important of all, the drives systems."
tonnes rating, at ScanRope A/S of Tonsberg in Norway
opened the way for the company to break into new market The ASEA overhead crane is situated in the despatch
areas in the offshore oil industry, according to development area and extends outside onto the nearby dockside for direct
manager, Nils Martin Teien. ship loading. Originally installed in 1976, the crane was
 Chapter 12.3 291

substantially overengineered, allowing the upgrade to almost The advantages of the parallel twelve-pulse system over the
double its rated capacity. Nevertheless, some reinforcing of six-pulse are:
the mechanical part of the crane, designed by mechanical/
• reduction of electrical interference on the electrical
civil engineering consultant Finn Strom, has been necessary,
as well as new motors and drives. supply
• reduction in audible noise from the motor and thus
The 200 kW A.C. hoist motor is controlled by two 120 kW reflection into the lift shaft
variable-speed drives, load sharing and operating in flux • reduced harmonics in the line currents
vector mode (giving full torque at standstill) with feedback • less ripple current thus less torque variation (producing
from a shaft-mounted encoder. A plug-in applications module better lift ride performance)
provides simple, smooth directional control. In addition
a 6150Nm disk brake was installed, which is applied Today, the D.C. lift market is declining and D.C. systems are
automatically whenever the hoist is at rest. mostly used as refurbishment units. The decline of the D.C.
market has been due to the developments in A.C. motor and
The short and long travel drives, 7.5 kW and 30 kW, res- in particular A.C. flux vector controlled drives.
pectively, are also fitted with intelligent application modules
and operate in open-loop V/F mode, the latter controlling The lift market can be split into three basic types of control
four 6 kW brake motors in synchronism. system:
A further function of the application modules, with addi- 1 Hydraulic - these are fitted in low-rise buildings and are
tional I/O plug-in modules on the drives, has been to limit based on the extension and retraction of a hydraulic ram
the position of the crane as it travels out onto the dockside. fitted to the bottom of the lift car. The lift speeds vary,
Although the crane can lift to its full rating across its entire typically in the range 0.5 to 1.25 m s -1.
width within the factory, as it travels outside and further onto
2 Geared- these are fitted on low to intermediate-sized
the dockside, where the gantry is pivoted to allow boats to
buildings and are based on a motor gear arrangement (the
come alongside, the permitted area of travel is limited to a
gearing used being of helical, planetary or worm type)
narrower span in the centre of the track.
with lift speeds in the range from 1.0 to 2.5 m s-1. The
Reduced loadings are permitted in defined distances to either speed of these systems is limited by the losses / audible
side of the centre line. Information from limit switches, noise created in the gearbox at higher speeds.
combined with load data, is computed in the application
3 G e a r l e s s - lift systems of this type are generally found in
modules to ensure that operation is always within safety
taller buildings with lift speeds in the range of 2.5 to
limits.
1 0 m s - 1 for passenger elevators and even faster for
The control cabinets, which also included main supply goods elevators. However, with the latest development
distribution, transformers, RFI filters, radio transmitters and in technology in motor design and A.C. drives (less
overload protection relays, were installed on the crane's power stage losses), gearless machines running at 1 m/s
bridge. As well as a hand-held remote radio controller, with can now be found.
stepless control on all three axes, an additional controller
was installed in the existing driver's cab as a back-up The main components of a lift system are:
option. 1 Variable-speed drives.

The crane has been certified at 190 tonnes plus 10 per cent. Lift controller- the lift controller's main functions can
In the future, with the assistance of a new 60 tonne crane to be split into the following sections:
be running on the same track, it will be possible to increase a Handling car/landing calls - a separate group con-
loads to a full 250 tonnes. troller is often used to control the car/landing calls
when more than one lift is used in a building.
ScanRope A/S has the world's largest facility for the pro-
b Learn and store floor positions - floor sensors are
duction of spiral strand mooring lines for deep sea applica-
set up at each landing level. The lift is moved at a
tions. It is already one of the largest manufacturers of wire
controlled speed from the bottom floor to the top
ropes for the offshore oil industry, with the main product for
floor. Positional information is read from an incre-
mooring applications being six-strand wire rope tructions
mental encoder as the lift passes by the floor plates,
manufactured in units weighing up to 140 tonnes. This
which indicate floor level. The position is then
represents steel wire rope with a breaking strain of 1700
stored in memory. This positional storing is con-
tonnes and a weight of 95 kg per metre.
tinued until the top floor of the building is reached.
c To generate lift speed patterns (16 bit and 32 bit)
ELEVATORS AND LIFTS (these speed patterns reference the drive speed loop).
d Calculate lift position.
Lift System Description e Provide general control/safety functions.
f Motor brake control.
Until recently lift systems operated using motor generator
sets and Ward-Leonard multimotor control. With the devel- Safety g e a r - a mechanical safety device that is posi-
opment of drive technology, motor generator sets were tioned underneath the car and works in conjunction
replaced with D.C. motor and drive converters incorpora- with the governor. It is activated if the speed of the lift
ting six-pulse and twelve-pulse systems (i.e. two drives in exceeds the contract lift speed by a fixed percentage set
parallel or series configuration). at the commissioning stages of the lift. The function of
292 APPLICATIONPRINCIPLES/EXAMPLES:Elevators and Lifts

the safety gear is to bring the lift to a halt in overspeed direct-to-floor positioning and this is now more widely used
conditions. in new lift systems. The system works on real-time pattern
generation using third-order positioning algorithms. This
4 Counterweight - a counterbalance weight that is ap-
allows optimum profiles to be generated depending on the
proximately the weight of the lift car plus 40 per cent to
shaft distance to be covered, giving optimum flight times
50 per cent of the lift carrying capacity. Long-term
and passenger comfort.
studies of people traffic in lift cars have shown that the
car on average is mostly 40 to 50 per cent full on most
of the lift journeys. With this factor in mind, from the
Load Weighing Devices
above it can be seen that having a counterweight In some lift applications load cells are used in the control
reduces the amount of power required to run the lift. system. This helps in removing drop back when the lift
5 Governor - a pulley that links via a rope to the lift car brake, and then lift patterns, are released. It does this by
and runs at the same speed as the lift. The pulley has a adding a torque feedforward term into the torque loop of the
configuration of masses on board which move out in drive that is proportional to the load in the car. The motor is
proportion to the lift speed due to centrifugal force. If thus preexcited with the torque before the brake is released,
the lift should overspeed by a fixed percentage the improving zero speed holding. However, with the improved
masses come out enough to trip the safety gear. speed/current loops in today's drives this is becoming a thing
of the past.
6 Buffers - hydraulically-filled rams that act as dampers
to the lift and counterweight should the lift reach the Figure 12.37 shows a simplified lift electrical system.
extreme limits of the shaft. There is a buffer for the
counterweight and one for the lift car. METALS AND METAL FORMING
7 Positional e n c o d e r - sends positional information back
The use of variable-speed drives in the metals industry is
to the lift controller.
widespread. The processing of metal is a classical sectional
8 Floor sensors - sensors in the shaft that show the true process, with closely coordinated speeds, tension control etc.
floor level. Winders and unwinders are common. It is interesting,
9 Motor/brake. therefore, to consider a somewhat more unusual application
relating to metal forming.
10 Shaft peripherals - shaft limits are positioned at the
extremes of the lift shaft as electrical safety checks to
Winding, Crimping and Precise Cutting
the controller.
A complex drives task, involving the winding and crimping
11 Selector - shows the lift floor position. The selector
of stainless steel strip and square cutting of lengths of tube to
increments and decrements as the lift passes the floor
precise tolerances, has been achieved using variable-speed
sensors in the lift shaft.
drives. Using an applications module within a drive module,
Key mechanical components are shown in Figure 12.34. it was possible to simplify the design and eliminate the need
for additional, costly, PLC control.
The lift controller's main function is to generate optimum
speed profiles for every lift journey with minimum floor to The machine, designed and built by Senior Precision in
floor times, floor level accuracy typically better than 3 mm Oldbury, Birmingham, UK for group company United
and, most importantly, passenger comfort. Flexible of Merthyr Tydfil, manufactures flexible steel
exhaust hose, used on trucks (Figure 12.38).

Speed Profile Generation The machine is much simpler than others that perform a
similar function. Although a PLC is used to provide the basic
As mentioned previously, the speed profile generation is sequencing, all the real control loops are in the drives. This
normally done in the lift controller but can be incorporated gives the designer and user much more flexibility. The
within the drive if an applications module is available. system can easily be reprogrammed if required and extra
Different profiles are generated dependent on different floor functions can be added in software.
distances travelled (Figure 12.35).
Fitted between the turbocharger and the main exhaust
Jerk is the rate of change of acceleration and can be opti-
system, the function of the hose, apart from being gas tight,
mised in the profile on set up to give smooth take off of the
is to take up the offset and misalignment of pipe work and to
lift from floor level. This is a very important factor in lift
reduce transmitted vibration. To this end, the exhaust hose
control. The different jerk acceleration rates can be modified
has to perform to demanding extension and compression
on the run by the controller, or the lift variables can be
criteria at a specified force, with movement being equal
entered as fixed values on less complex systems.
either side of the mid point.
In earlier systems the profile was slightly modified from the
Flat stainless steel strip is fed into preforming rollers,
above with the final sections of the slowdown dropping to a
wrapped around a mandrel, crimped into a four-fold labyr-
slower speed called the creep speed before aiming into the
inth, which allows sidewards movement without gas leak-
floor position (Figure 12.36).
age, and emerges as a helically-wrapped tube, which is then
Sensors in the lift shaft indicate to the controller when to square cut using a plasma source, removed and spot welded.
start slowing down and also when to start different sections Tubes in a range of sections from 50 to 200 mm diameters
of the profile. However, nowadays the trend is towards and up to 800 mm long can be manufactured on this machine.
 C h a p t e r 12.3 293

spring thrust bearing

load cell

car

,..... .... ! ! II damping springs

hydraulic buffer

counter roll

drive with traction heave

position reference and

monitoring

emergency limit switch

(15 cm above final position)
................................! 10 ropes

11 compensation ropes

12 car frame

13 mechanical safety catch

14 counter weight (car

weight plus 50% nominal
load)

15 compensation ropes

16 emergency limit switch

(15 cm under final position)

17 counter roll
compensation rope

Figure 12.34 Components of a lift system

1st floor down)

2nd floor
-0
3rd floor "0
~creep speed sensor 2
C).
own)

,v distance
distance

Figure 12.35 Lift speed profiles for different journeys Figure 12.36 Typical velocity profile

7.5 kW drives operated, in closed-loop speed-control mode, A PLC provides overall control of the sequencing, the hy-
power the former and the mandrel motors, and a 0.75 kW draulic pressure control on the former and mandrel and is
drive, complete with PM servomotor, provides the servo connected to all three drives via CTNet fieldbus. The motors
function for the lead screw-mounted plasma torch, which for the former and the mandrel both have shaft-fitted encoders
follows the tube to ensure a perfectly square cut. for speed reference, and the servomotor has a resolver fitted.
294 APPLICATIONPRINCIPLES/EXAMPLES"Metals and Metal Forming

floor 30 I
RS-485

RS-485
i - - - -

',
|
cubicle
Unidrivew/UD70 ~ PC or
desktop

floor 29 I
RS-232
RS-485 encoder
/// power I
disl~layand group
floor 28 I con|rol to other lift
| . . . . . . . . . . .
windingdrum ~ ~ , , ge~bbr°ke~
RS-485 11 I1' fl'ux vector motor
indicator

-up/
RS-485 down
floor 2 I
trailing
floor 1 RS-485 c~ble floor selector
call station with F panel car/lift
home limit I;
ground
r//////////////A (home position)

Figure 12.37 Simplified block diagram of typical lift electrical system

Figure 12.38 Machine for the manufacture of flexible steel hose (courtesy Senior Precision)
 Chapter 12.3 295

The encoder speed reference from the former drive motor is A feed roll, under the control of a 7.5 kW servodrive and
retransmitted via a dedicated plug-in module to the mandrel 142 frame servomotor with resolver feedback, is position
drive, which in turn retransmits its speed reference to the controlled by a standalone motion controller to an accuracy
servodrive for the plasma torch. The servo's resolver signal of 0.03 mm.
is also fed back to the former drive. An application module
The system feeds the strip through and selects a combination
on the mandrel drive incorporates software to ensure the
of two hydraulic punches and a shear at a demanded speed
precise following of the former drive and provides the
of up to 120 operations per minute and giving a maximum
interface with the servodrive. Thus the whole system is
acceleration time to full speed of 0.7 seconds.
digitally locked together.
Open-loop drives and motors were used for the pinch roll
Programming for the cutting is contained in an applications
drive on an output conveyor, which runs slightly faster than
module on the drive for the plasma torch. Knowing the
line speed to ensure that material is quickly cleared out of
diameter of the product and the feed speed of the strip
the way.
determines the rate of growth of the tube. A signal initiates
the plasma cutting and the servodrive moves the gun during The control desk and integral cubicle incorporates the con-
the cutting to keep pace with the tube. This on-the-fly cutting trolling PLC, keypad and screen. There are two modes
ensures a perfectly flat end section. of operation: manual jog control, where the operator can
control any part of the line separately, and fully automatic
United Flexible is part of Senior Flexonics, which is a highly
control, where the system will cycle for a preset number of
diversified world-wide manufacturer and distributor of cor-
batch counts on pressing line start; the looping pit fills and
rugated and tubular products and is considered a leader in
the punches, conveyors etc. are started automatically.
each of the markets it serves - automotive, aerospace, OEM
technologies and industrial. At the Merthyr Tydfil factory in The first punch punches a pattern of holes and the strip is
Mid Glamorgan, the company manufactures a wide range of moved a set distance, confirmed by two sensors (one looks
flexible metal hose assemblies and the Compoflex flexible for a hole, the other for material) under the second punch
composite hose for cargo transfers. which then completes the punch pattern and forms the
material. Without confirmation from the sensors, the second
Roll Feed Line press will not start. The material is then cut to length and is
fed to the outfeed conveyor.
A new roll feed line installed at Color Steels in South Wales
combines a maximum speed of 30 m min - 1 with a pitch feed The system is very versatile and can be reprogrammed for a
accuracy of 0.03 mm (Figure 12.39). wide range of products within the maximum component size
limits. The complete control system, interfacing with A.C.
The system comprises a hydraulic decoiler which feeds into
and D.C. drives, hydraulic controls for the presses and shear
an 11 kW D.C. leveller drive. The strip is fed into a two-
and the hydraulic pump control, is housed in the main
metre deep looping pit with four photocell sensors which
control desk 1600 mm wide. Data entry includes cut lengths,
feed back to the drive, which has an applications module with
line speed, acceleration and batch quantities.
PID loop control software, to control the size of the loop. The
objective is to run the leveller drive at the optimum speed to Color Steels distributes the widest range of precoated steels
keep a constant size of loop. in the UK. State of the art technology is used in the various

Figure 12.39 Roll feed line (courtesy Color Steels)

296 APPLICATION PRINCIPLES/EXAMPLES: Metals and Metal Forming

slitting, decoiling and shearing processes. Swift conversion PLC and transmitted to the additional processor in each
from master strip to sheets and blanks is achieved within a Mentor drive. Thus all stand drives are constantly and
single site situated at Blackvein Industrial Estate, Cross simultaneously updated, thereby avoiding tension/compres-
Keys, in Gwent. sion during rolling.
This improvement in dynamic response has had the effect of
WIRE AND CABLE MANUFACTURE improving rod quality, both in finish and in a more con-
sistent rolling pattern and dimensions. Product rejects have
Four-Quadrant D.C. Drives for a Bar Mill been greatly reduced and production volume has been
substantially increased.
Steel rods, 150mm in diameter and 9 to 10m long, are
heated in the bar mill furnace and conveyed to the finishing
mills. For a particular product requirement, the speed Wire-drawing Machine
through the mill stands is determined by the finishing dia-
The wire drawing process typically reduces wire diameter
meter required. The subsequent speed of each stand is then
from 12 to 1 mm over up to 14 tungsten carbide dies. As the
cascade controlled by the master finishing stand.
diameter reduces, the speeds of successive blocks drives
The system for the finishing line, covering mill stands 10 to have to increase to handle the increased material length and
15, comprises a main D.C. drive suite, with six D.C. drives, to maintain a constant tension between each stage.
the finishing line PLC and auxiliary controls. Four of the
Between each block there is a dancer arm which takes up the
D.C. drives are for 375 kW motors and two are for 750 kW
slack wire. From each of these a noncontact proximity detector
motors (Figure 12.40).
sends an analogue signal to the application module of the
All of the D.C. drives are fully regenerative/four quadrant, following drive. Using a program with a PID-type function,
which gives good speed control on both acceleration and the drive speeds up or slows with the objective of keeping
deceleration. The drive system allows future development to the dancer arm in its central position. In practice, the control is
serial parameter control from a host computer, running a such that, after the start-up, during which the software works
product recipe suite of software. out the sizes of the dies in use, the dancer arms have minimal
movement from the optimum central positions. The speed ratio
Before, if the drives overshot their required speed, it often between drives is daisy chained from the master drive - the
took 10-15 seconds to drop back, giving an overshoot on the final block drive ofthe sequence - and each ofthe slave drives.
line. Drives were constantly speeding up and slowing down. This gives optimum speed following which prevents wire
Coasting to a halt could take up to five minutes and reversing breakage even during a fast stop.
the line necessitated reversing the field.
With the dancer arms running in central positions, more trim
Now the line holds perfect speed control, with additional is available before reaching limit stops, so there is less dis-
feedback from scanners monitoring the loop height between ruption to production and far fewer incidents of wire
stands, and cascade control from stand 15 which acts as breakages. At the first limit in either direction, the drive
master. Calculations concerning the motor speed refer- accelerates or stops ramping up and at either second limit,
ence for each drive are carried out continuously in the the machine comes to a controlled stop.

Figure 12.40 Bar mill finishing line (courtesy Allied Steel and Wire)
 Chapter 12.3 297

Drives can be set to allow ramping up from standstill at a at Dartford; high-strength galvanised wires and strands are
preset ratio, with the speed of the fastest drive being limited used on the Tsing Ma and Kap Shui Mun bridges on the
to allow slower drives on earlier blocks to ramp up in the Hong Kong airport approach route.
correct ratio, without exceeding their current limits. Any
drive approaching maximum speed can hold the speed of the
PAPER MANUFACTURING
other drives thus preventing wire breakage due to incorrect
drafts (die sizes). During threading of a new wire, each new
General
block becomes the speed master, the master pilot block
moving down the machine automatically (Figure 12.41). Since first invented by the Chinese, paper has always been
made in basically the same way. Paper is manufactured by
D.C. drives have been installed for the spoolers (30 kW) and
pulping wood or similar cellulose material with water to
rotors for the formers (56 kW).
produce a stock. In the simplest process, this stock is laid on
Each control desk features an operator screen with bar a gauze base to allow the water to drain. More water is then
graphs showing the current of each drive, the facility to extracted by squeezing, and finally the paper is dried by
calculate machine efficiency based on set length and speed evaporation.
and automatic slow down during gun or former changeover.
Whereas paper was originally made sheet by sheet, a modem
The sophisticated monitoring system gives the operator a
paper-making machine combines all these processes to
comprehensive view of machine conditions.
produce paper at continuous speeds of up to 1000 or 2000
A PC with custom software, placed in the operator's desk, metres per minute with web widths up to five metres. These
allows improved operator feedback and system monitoring. machines often run under computer supervision with many
This shows the operator the normal operation mode, speed, interactive control loops to achieve the required production
current (torque) and dancer arm position of each block, schedules and to maintain consistent quality. A very import-
allowing the operator to trim each block between viable ant part of such a machine is its electrical drive system,
limits with real feedback. which may consist of up to a dozen main motors and many
smaller helper motors, with a total installed capacity of
Other key aspects include system status icons and time to run
several hundred kilowatts.
calculator. Maintenance is incorporated into the system,
with user-explicit alarm captions and a 12-monthly logging
Sectional Drives
system. Further maintenance features allow an automatic
gateway for engineers to use software tools to monitor the The speed of each section of the machine must be accurately
drives and the system's performance while the line is in controlled to maintain the correct intersection speed differ-
production. These tools assist the operator in being able to entials, or draws, to cater for the changes in characteristic of
quickly identify faults using onboard diagnostics such as the product as it makes its way from the wet end of the
guards up, dancers back etc. process through the press section and drying sections to be
reeled at the end of the machine ready for the next process.
Bridon International is one of the world's leading manu-
facturers of steel wire and one of the few companies which The shunt-wound D.C. motor has been the traditional choice
has the capability to manufacture large structural strands. for driving paper machines for many years; it has good
These are used, for example, on the Queen Elizabeth bridge speed-holding characteristics, a high torque availability at

~:i~!~~,~I:: : .....

Figure 12.41 Wire-drawing machine (courtesy Bridon International)

298 APPLICATIONPRINCIPLES/EXAMPLES:Paper Manufacturing

very low speeds - essential when starting high inertia sec- section and usually rationalising around two or three frame
tions of the machine - and proven reliability. When com- sizes of motor to reduce the number of spare parts to be
bined with a modem thyristor power converter it provides an carried. Each motor drives its respective section via a speed-
efficient and reliable means of obtaining the degree of reducing gearbox which is carefully chosen with the arduous
control and response which is essential to the paper-making 24-hour, 7-day operating requirements in mind. For smooth-
process. ness of transmission, helical gears are usually specified.
Often, a restricted range of different gearbox sizes is used
A.C. drives, now available in capacities of 600 kW and
with interchangeable wheels and pinions - again to reduce
above, are, however, making significant inroads into the
the cost of spare parts stock.
paper industry. They give the benefits of lower motor costs
and low maintenance and are now more often the preferred Each section motor is fitted with a high-accuracy, tem-
choice for new machines, although D.C. will still have a perature-compensated tachogenerator or high-grade digital
good market for some years yet, particularly for conversions encoder to provide accurate speed feedback to the section
of existing machines already utilising D.C. motors. power controller.
Both provide an efficient and reliable means of obtaining the The motors must operate under very adverse conditions:
degree of control and response which is essential to the very wet at the wire section, and hot and humid at the dryers.
paper-making process. D.C. motors are therefore often ventilated by a clean air
Early machines were fitted with a line shaft driven by a large supply ducted from outside the machine room, air-flow
D.C. shunt motor. Cone pulleys were provided to adjust switches normally being provided to monitor the flow of air
inter-section speeds, and clutches allowed sections to be through each motor and to shut the equipment down if the air
stopped and started as required. Modem paper machine drive supply fails. In contrast, because of their inherently high
systems are sectional, a motor and controller being provided degree of protection, A.C. motors can be used without any
for each section of the machine, as shown schematically in special provisions.
Figure 12.42. Typical motor sizes for the medium-sized
machine making paper at 300 metres per minute and a width Loads and Load Sharing
of three metres are:
The maximum power that may be transmitted into a section
wire section 100 kW
(i.e. to drive the felt or the wire) by any particular roll is
first press 50 kW
limited by the arc of wrap of the medium around the roll and
second press 50 kW
the coefficient of friction between the roll and the driven
first dryer 50 kW
medium. Where this power is less than the total power
second dryer 50 kW
required to drive the section, additional power is provided by
third dryer 50 kW
helper motors driving other rolls in the section.
calender 100 kW
reel 50 kW Typical examples of such arrangements are the wire-turning
roll driven by a helper motor to assist the couch roll motor,
making a total installed capacity of 500 kW.
as shown in Figure 12.43, or the multiple drive points of
Selecting the motor sizes for a sectional paper machine drive a machine felt section. These helper motors are usually tied
involves checking the normal running load (NRL) and mechanically to the main section speed-controlled motor by
recommended drive capacity (RDC) calculated for each the wire or felt which passes around the section.

other
sections-
presses,
wire section(master) first presssection dryersetc. reel section

NI i N -1 (

cEA >
+4
SEA + i tension

l draw
~increase
l speed for
tension
control

masterspeed
reference
Figure 12.42 Sectional paper machine drive layout
 Chapter 12.3 299

L suction boxes . ~ new reelshell

wire ~ underwire ~~ , ~ normal n shell
positioreelas
(~- ~"~ UI1UU ~/~suction -\ .
~""couch up
roll
r ~ wire
turning

[I U r°''

t ) Figure 12.44 Arrangement of reel section

While the threading process is going on, the section motor

speeds must be held constant in spite of rapid load changes

),
as the paper passes through the machine and as its width
increases. Any slight variation in speed at this time will
CEA cause the fragile web of paper to break.
The paper reel is a surface-driven system, Figure 12.44. The
reel shell with the paper winding on to it is pressed against
! the reel drum. When the reel is complete a new shell is
l lowered on top of the reel drum and the paper is transferred
from the filled shell to the new shell allowing paper to be
SEA/~ reeled continuously. The reel section drive normally oper-
ates in speed control while threading and during reel
changes, but tension control is normally used while reeling.

Control and Instrumentation

The basic elements of a paper machine sectional drive
masterspeedreference
control system normally comprise a set of A.C. or D.C.
Figure 12.43 Section motor driving wire-turning roll has power converters, each housed in its own cabinet and sup-
tachogenerator fitted for speed control. plied through individual main isolators, motor contactors,
Section couch roll driven by helper motor excitation and control circuits to provide inch, crawl and run
under current control functions. Each section has its own stop, crawl and run
operator interface. Electrical power is normally distributed
On earlier systems, the helper motor can therefore be con- to the cabinets by an enclosed three-phase A.C. busbar
nected in parallel, sharing a percentage of the section load system for D.C. drives or a D.C. system fed from a common
with the section motor, and using a common converter; in rectifier for A.C. drives.
more modem solutions, the two motors may be controlled
The traditional instrumentation and controls for the operator
by independent converters arranged to operate in torque
are now commonly integrated on touch screens, which allow
control, as in Figure 12.43.
customisation for touch inputs, plant displays, push-button
A paper machine operates over a very narrow speed range, machine adjustment etc., all incorporated in one small
probably never more than 3:1, but low speeds are required box. Serial links to drives cut down on both wiring and
for fitting and running in the wire and felts and when installation costs and simplify maintenance.
cleaning. Unlike other types of process machine, the paper is As the paper travels along the machine, passing through
threaded through the machine at normal paper-making various pressing and drying sections, it undergoes changes in
speed, each section being brought in as the web finds its way length. The section speeds must be adjusted to allow for this.
through the machine. The stock is fed onto the wire and
The intersection speed differentials thus created (known as
recycled through large broke pits, situated below the wire
draw) may vary between zero and +5 per cent of maximum
section, until its consistency is correct. A thin tail of paper is
speed. All the drive sections receive a speed reference signal
then picked off the wire and threaded through the machine
from a master speed reference unit. This signal is generated
section by section. When the paper tail reaches the reel
as a digital value and is transmitted to the drives over a high-
section and is running between the reel drum and the empty
speed local area network.
reel shell its width is gradually increased at the wire until a
full width is passing right through the machine. At this stage Individual section speeds are trimmed with respect to the
the paper is wrapped around the reel shell and reeling master reference to obtain the required draws. Normally, one
commences. section is tied solidly to the master reference with no draw
300 APPLICATIONPRINCIPLES/EXAMPLES:Paper Manufacturing

adjustment. Draws may be individually and separately new drives allow many possibilities to be explored, such as
adjusted, or cascade adjustment may be provided whereby speed and draw menu storage and immediate set up by
an adjustment to one section is automatically passed on to all product code. When used in conjunction with a modem
succeeding sections, so obviating the necessity for the paper-machine process control computer, such programmes
operator to reset all draws following an adjustment at one allow very quick and efficient changes in product grade, and
section. improved start-up times with less time required for the
machine to settle in to the new grade. Management reporting
A slack take-up system operated from a push button on each
on drive performance can now be provided relatively easily
section is normally provided, for all except the master sec-
via the serial communications port of the drive. Assistance to
tion, to allow the operators to pull out any slack in the tail
the electrical maintenance departments in fault location and
as it is fed from section to section while threading up.
correction can be provided by recording and storing, in the
Arranging for all the drives to follow a single master refer- drive memory, the historical trends of various important
ence signal ensures that no velocity lags occur between drive parameters prior to a shutdown condition.
sections, as would be the case if each drive took its reference
from the preceding section; it also ensures that slight
instability on one section is not transferred to succeeding
Winder Drives
sections. Once the paper has been wound into reels on the paper
machine, it must be slit into widths to suit the printing presses
The master section is normally the wire section, as this is the
or other converting machinery for which it is intended, and
first section, and all draws may be expressed with reference
rewound onto disposable cores in known lengths. This pro-
to it. This arrangement is particularly useful where cascaded
cess is carried out on a slitter rewinder, which normally
draw systems are required, as all draws can then be
consists of an unwind stand (into which the reel is loaded as
considered to be positive.
it comes from the paper machine), a set of driven slitter
The rate of change of the master reference is usually chosen knives and a rewind system.
to be very slow, thus limiting the rate at which changes in
overall machine speed may be made. This ensures good The rewind usually comprises two drums, mounted side by
speed tracking between sections which may have very dif- side, upon which the slit and rewound roll of paper sits. The
ferent torque to inertia ratios. It also ensures that the process distance between the two drum centres is only slightly
control side of the machine can keep up with machine speed greater than their individual diameters so ensuring that the
changes and is able to maintain the correct flow of stock to rewound roll is supported by both drums. A rider roll which
the wire section and the correct amount of steam feed to the sits on top of the rewound roll is arranged to rise in a vertical
dryer section. slide as the diameter of the rewound roll increases.

Individual section drives are completely independent as The pay-off stand is controlled by a braking generator and
regards starting and stopping. The rates of acceleration is centre driven, whereas the rewind drums and rider roll
applied to the various sections, when running up to the preset provide a surface drive to the rewound roll.
master reference speed, should be limited according to the Each motor is provided with its own A.C. or D.C. power
maximum power which can be mechanically transmitted controller; the pay off normally runs in tension control and
into the section. The couch roll and forward drive rolls, the two drums run in a master and slave configuration in
which are the main drive rolls on the wire section, must not speed control, the second drum load sharing with the first.
be allowed to accelerate so rapidly that they lose traction and The rider roll also runs under torque control, its control
slip inside the wire. strategy depending upon the particular winder type.
Two possible solutions are used here: first, to accelerate A two-drum winder of this type will wind paper at speeds up
under reduced torque limit and switch to a higher value once to 2000 m m - 1 and handle reels from the paper machine up
running speed is attained; alternatively, to use a ramp gen- to 2.5 m diameter and 5 or 6 m wide. The rewound rolls will
erator to control the rate of acceleration of the section up to be from 1 to 1.5 m in diameter, necessitating several stops in
running speed. This is then disabled to avoid the introduction the course of rewinding one parent reel.
of any additional velocity lag during overall changes in
The winder is therefore designed to run at two or three
machine speed.
times the speed of the paper machine in order for it to
Some sections have large inertia-to-torque ratios, the dryers keep up with the production rate. An installation of this
in particular, which consist of a dozen or more 1.6 m (5 ft) type is a very large capital investment and it is not acceptable
cylinders to each section. Accelerating these up to speed for the paper machine to be held back by undercapacity of
from start requires substantial overload capacity: at least two the winder.
to two and a half times full load torque should be catered for.
Control of the rewind drum motors is fairly straightforward;
Older machines may also have plain bearings, requiting
the front drum (the one nearest the unwind stand) runs under
break away torque of up to four or five times normal running
speed control and sets the speed of the rewinding process.
load. These factors should always be given careful con-
The rear drum runs under torque control, its load being
sideration when planning to update an old drive system,
determined as an adjustable percentage of the front drum
particularly if the original drive was a Ward-Leonard system
load; a load-trimming adjustment is provided for the
with essentially no limit on the current available.
operator to make adjustments. The torque differential so set
With the advent of microprocessor-controlled drive systems, between the two drums is used to control the tension, which
paper machine drives have become more sophisticated. The is wound into the roll and therefore determines its hardness.
 Chapter 12.3 301

Most modem winders employ profiling of this hardness Typical parameters for a medium-size winder are:
by automatic adjustment of the load sharing in relation to
the rewound roll diameter. This is easily measured by a speed 2000 m min-]
transducer attached to the rider roll. parent reel diameter 1.7 m
reel shell diameter 0.33 m
Additional hardness control may also be provided by paper sheet width 4.25 m
adjusting the rider roll torque. Both these current-controlled
sheet tension 9 g m m - 1 width
motors normally have some form of speed override to pre-
vent excessive overspeed when they are not in contact with This gives a parent reel weight of about 6000 kg and a total
the rewound roll.
sheet tension of 382kg, resulting in a tension power of
The slitter knives consist of rotating discs with ground faces. 93 kW to be absorbed by the brake generator. The variation
They can be moved laterally across the machine to provide in stored energy of the expiring parent reel and the reel shell
different widths and are usually individually motorised and as the diameter reduces may be seen from Table 12.19.
provided with plugs and multiposition sockets to allow This clearly shows the effect of the fixed inertia component
selection to suit the job on the winder at the time. The slitter provided by the reel shell and brake generator armature, and
knives must run at a speed slightly in excess of the paper speed the varying inertia component of the parent reel as it is
to provide a cut. But if this speed difference is too great, unwound and reduces in weight and diameter.
excessive knife wear will occur; if too small, the knives will
not cut. The slitter motors are normally supplied from a The total stored energy reduces as the reel diameter reduces
common variable-frequency inverter which is referenced but increases at lower diameters as the rotational speed
from the drum drive but given a fixed positive bias to ensure increases.
the correct positive differential in speed over the paper. While accelerating and decelerating, the torque in the
The unwind brake generator at the payoff operates as a braking generator must be adjusted to compensate for the
constant-power braking system to maintain constant tension stored energy values outlined. This is a technique known as
in the paper fed from the unwind. Constant tension in this inertia compensation. The actual change in torque demand
part of the machine is essential to ensure correct operation of must be predicted, bearing in mind the total stored energy of
the slitter knives. Any variation will cause interleaving of the the unwind and field flux level of the motor at the operating
slit drums at the rewind. diameter and the rate of change in speed.

The unwind brake generator will normally be required to Figures 12.45 and 12.46 show the relationship between
control tension over a diameter range of about four or five to inertia compensation torque-producing current and diameter
one and also deal with a range of sheet tensions which for the values listed in Table 12.19, taking the effect of
depends on the variety of grades to be handled. A typical motor excitation into account.
tension range may be five to one, resulting in a torque
regulating range of 25: 1. Table 12.19 Comparison of stored energy

Diameter (m) Reel (kWs) Shell (kWs) Total (kWs)

Brake Generator Power and Energy
1.52 366 20 386
The size of the brake generator is relatively simple to decide: 1.27 241 29 270
it must provide only the tension power required by the paper. 1.00 68 46 114
This may be calculated from the winding speed and max- 0.76 58 84 141
imum permissible sheet tension. Calculations should also be 0.60 180 180
made to check on the overload capacity required to accel-
erate and decelerate the unwind with a large reel of paper
present and, if necessary, the brake generator should be
increased in power to cater for this. This is particularly )
min diameter max
important where low tensions are to be used with materials
such as newsprint, where the acceleration power may be
substantial in relation to the tension power requirement.
current
In making these calculations, it is convenient to split the -50
components of the machine into fixed and varying inertia
components and calculate their stored energies in kilowatt
seconds (kWs). This allows each part of the machine to be
tension current in unwind
assessed separately throughout the diameter range; summing -100
the results gives the total stored energy, from which an
estimation of the acceleration power requirements can
readily be made.

It is common practice for the motors on a winder of this type -150 -

to be arranged for direct drive. This is of particular impor-

tance on the unwind, as it reduces frictional losses in the
system to a minimum and makes control of sheet tension Figure 12.45 Effect of inertia compensation on unwind
more accurate. brake generator- decelerating
302 APPLICATION PRINCIPLES/EXAMPLES: Paper M a n u f a c t u r i n g

I
rotating them into position to perform a splice as the old reel
+50
inertia compensation expires, thereby maintaining continuous processing. Nip
current rolls pull the paper from the unwind. There are usually one
or two coating heads, depending on whether both sides of the
paper are to be coated, together with their respective drying
min diameter max sections which may be ovens or heated cylinders or a
combination of both. Hold-back rolls control the paper
t.-
before it enters the rewinder which again, like the unwind,
L.-
may be arranged for continuous operation with rotating
-50 turrets and reel changing on the run.
total current
,,.~ln unwind As the paper passes through the machine it undergoes con-
siderable changes in moisture content and hence the tension
tension current levels between the various sections vary considerably.
-100 Dancer rolls are usually fitted between important sections to
allow the state of the paper to be controlled more readily.

Each section is provided with its own motor, which must be

Figure 12.46 Effect of inertia compensation on unwind accurately controlled to maintain correct speed or tension.
brake generator- accelerating The powers involved in a coating machine are considerably
less than those on a paper-making machine or winder, and
the control problems are somewhat different.
Unwind Brake Generator Control
The payoff must control sheet tension accurately through a
The drive control system for an unwind brake generator is
four or five to one diameter range, but it must also accelerate
quite complex. The system must be capable of operating
a new reel up to match the speed of the expiring reel to
under speed control during threading of the winder and
enable a flying splice to be successfully completed. Once the
under tension control once the machine has been threaded
splice has been made the payoff must smoothly and quickly
with paper and put into the normal operating condition.
transfer to tension control which is maintained until the reel
During threading, the unwind will be set to follow the same is finished, at which point it must bring the empty reel shell
speed reference as the rewind drums. But, as the rewind is quickly to rest after the next splice transfers control back to
surface driven and the unwind is centre driven, this speed the other payoff.
reference must be adjusted to allow for the diameter of the
roll of paper on the unwind to ensure a match of peripheral The speed-controlled sections must track speed accurately
speeds between the two parts of the machine. To allow the throughout the full speed range of the machine, which may
operators to manoeuvre the paper while threading, some be 50 or 100 to one. Trims from dancer rolls must also be
adjustment of the difference in these two speeds must be incorporated into some of the section speed controllers. The
made available with the additional possibility of driving the dancer roll control loops may have gain-adjusting circuits to
unwind forwards or backwards to pay out or take up paper increase their effect as the machine speed increases.
loops. The twin rewinds are normally centre driven, each with its
Once the drive is selected to tension control, calculations of own motor and, like the payoffs, must accelerate a new reel
required tension torque and acceleration torques are made core up to speed in readiness for a changeover at speed when
and the results of these calculations used to predict the the previous reel becomes full, changing to tension control
torque setting of the motor. This open-loop tension control is once the splice is complete.
used while preparing the winder. Once the machine is
Owing to the mechanical arrangement of the payoff and
operating satisfactorily under open-loop tension control at
rewind, with concentric drive shafts which pass the power
low speed, the tension loop is normally closed by taking a
through the centre of the rotating stands, considerable
feedback signal from a tension-measuring load cell and
attention must be paid to providing adequate compensation
using this signal via an auxiliary PI control loop to trim the
for frictional losses within the tension control systems. Both
predicted torque level applied to the motor. Normally, the
static and dynamic friction compensation circuits should be
drive designer will endeavour to achieve as accurate a torque
incorporated into the controller together with inertia com-
prediction system as is possible, so reducing the amount of
pensation. Also, the paper surface, once coated, becomes
control necessary from the load cell loop.
very smooth. This results in telescoping of the rewound
reels unless the tension is carefully monitored and con-
Coating Machines trolled. Usually, coated papers require some degree of taper
tension to prevent telescoping and the rewind tension
As the name implies, coating machines coat the paper. They
control system must invariably include this feature. The
are used to impart a special finish which cannot be done on
tension taper control reduces the web tension as the dia-
the paper machine itself. Coated paper products include
meter increases by some preset ratio under control of the
high-gloss magazine papers, NCR paper and other such
operator. As with a paper machine, each section is provided
products.
with its own motor and drive which all follow a master
A coating machine consists of an unwind stand often speed reference signal and are controlled from the master
arranged to handle two reels, supporting them on turrets and line run and stop controls.
 Chapter 12.3 303

The machine is threaded with paper at crawl speed and the

unwind and rewind set to tension control. The machine is
then accelerated up to operational speed and the drive
system must maintain control over the paper throughout
this process.

Paper-slitting Machine
Denaeyer Papier is one of the few companies in Belgium still
producing its own paper. Using a unique process, with no
addition of chemicals, the company converts raw pulp
into thin, hard-surface backing paper simply through the
application of heat and high pressure.
End users take delivery in small coils ready cut to the
required width. This is where the paper-slitting machine
comes in. The customer wanted an increased performance,
within the mechanical limitations of the existing machine,
and improved reliability, with particular emphasis on precise
torque control to prevent ripping of the very thin paper.
The paper is supplied on reels of a maximum diameter of
two metres. The unwind reel is controlled by a 93 kW D.C.
motor under the control of a four-quadrant D.C. drive. To
accommodate the line-speed variation of 0-1000 m/min and
the reel diameter which varies from 2 m down to 32 cm,
a field controller is fitted to allow the motor to go into
overspeed with field weakening.
The paper is guided by rolls to the slitter section, where the
rotating knives are computer set to give the required widths. Figure 12.47 Paper-slitting machine (courtesy of Denaeyer
The knives are controlled at line speed by an 11 kW asyn- Papier)
chronous motor with an open-loop drive. After the slitting
process, each strip is then wound to the correct length, under It is crucial that there is no shock start. The winder roll is
tension, onto cardboard cores. Two D.C. motors, 32 and held at zero and the unwinder rolls are run in reverse to
75kW, both controlled by D.C. drives, provide the close stretch the paper. As the correct torque (current) is detected
torque control for the final winding process (Figure 12.47). by the D.C. drive, both winder rolls are started and the
speed is run up very gently. The winder is running under
D.C. drives with application modules, complete with stan-
speed control, the unwinder under torque mode with speed
dardised centre wind software, were chosen for the torque-
override.
critical D.C. motors. The unwind drive calculates and adjusts
the unwind motor armature and field current to maintain Paper Board Machine
constant tension. This software also provides inertia com-
pensation since the inertia of a loaded reel is significantly At the start of the board-making process white chemical
different from that of an empty one. This prevents tension processed pulp is poured onto moving wire and the water
errors during speed changes. vacuumed off to form the first paper layer. This is repeated
another four times as the paper moves through each section
All drives are controlled from a cubicle near to the machine of the machine, each section adding another layer of paper
which contains the start and stop functions, the potenti- until the product leaves the fifth section as the formed five-
ometers which define line speed, start tension, end tension ply board. More moisture is removed as the board then
etc. and indicators. passes through four separate presses which ensure that it is
When the machine is started, all the drives are running in flat, compact and of uniform thickness. Product thickness for
speed control so that the operators can guide the paper from the machine varies between 450 and 750 microns.
the unwind reel through the rolls and knives to the winder Two dryer sections remove the last of the moisture from the
rolls. This is achieved at very low line speed and, with a top and underside as the board is pressed between two sets of
starting diameter of 2 m, it was necessary to provide feed- large steam-filled cylinders. Its speed through the dryers is
back for the drives with incremental encoders to minimise critical to prevent bowing which causes the plys to separate.
the influence of noise.
The board then passes around a large, rotating glazing
When everything is ready to start, the line speed is set - with cylinder which acts like a steam iron, giving the top surface
a maximum of 1 0 0 0 m m i n - 1 - and the start and end of the board a highly polished finished. The rotation speed of
tensions are also set. This is to define the tension profile of the cylinder varies from 150 m i n - 1 to 225 m i n - 1, depend-
a wound and slitted reel; i.e. the tension is higher at the ing on the finish quality required for a specific product.
beginning and decreases as the diameter increases. The rest
is automatic. The diameter, necessary for the calculations, is Next, the board goes through the size press where it is coated
provided by an ultrasonic device. with starch to prepare the finished surface for printing.
304 APPLICATIONPRINCIPLES/EXAMPLES:Paper Manufacturing

used to generate the master speed reference for the system.

This reference signal is transmitted with high precision,
1 part in 65 000, to the first machine drive. Specially written
software running on the drive's onboard application module
modifies this reference to insert draw, presents it to the
digital lock speed-control software in that drive and then
passes it back to the master drive for transmission on to the
next drive down the line where the exercise is repeated.
This approach provided an extremely accurate speed/draw
processing system at minimal cost, completely eliminating
the need for a programmable controller (PLC) or master
processor. Further, just a single, screened, twisted-pair cable
was needed to transmit the reference between sections.
The software in each of the drives enables both progressive
Figure 12.48 Paper board machine (courtesy Iggesunds and independent adjustment of draw at any point along the
Paperboard)
machine. Certain sections are also fitted with load cell ten-
sion measurement and this signal is used in conjunction with
It then goes through a final drying section and on to the the Mentor's own PID (proportional integral derivative)
coating section. Here it is calendered, coated and dried twice software to control tension at critical points in the machine.
before going through a third and final calender. The finished
All the drives on the machine are continually interrogated by
board is then reeled onto large drums ready for slitting,
a PC to provide the operators with machine set up and
rewinding or sheet stacking and shipping to the customer.
performance information. The actual speed, set draw and
Operators can wind the drums to a diameter for reel orders or
percentage load of each drive is displayed on a colour
to weight for sheet orders. Whichever method is used the
monitor, together with a fault monitor screen and full paper-
reeled drums leaving the machine can have a maximum
break history analysis with graphical trending.
diameter of 5 ft and/or a maximum weight of five tonnes,
depending on the end product (Figure 12.48). To maintain sufficiently high data collection rates to ensure
a meaningful analysis, the PC is fitted with two intelligent
The machine's new drive system uses D.C. drives in the
serial communications boards which give a total of eight
range 420-825 A on the main sections - 18 in total. The
RS-485 communications lines running asynchronously in
moving wire on each of the five initial ply-laying sections is
parallel enabling the entire machine to be scanned at half-
controlled by an 825 A drive. There are also drives on each
second intervals.
of the four presses, each of the three dryers, the size press,
the three calender sections, the paperboard reel up and the As a maintenance aid the system also includes a drive
threader drive which threads the tail of the paper into the software package which gives access to all the machine
dryer on start up. drives through the intelligent communications boards on
the PC.
Other drives on the machine include: two 4 kW flux vector
A.C. drives on the applicator rolls in the two coating sec-
tions; four, 2.2 kW flux-vector A.C. drives on the paper lead BUILDING MATERIALS
rolls which help pull the paper on its vertical journey through
the second coating dryer; a 75kW open-loop A.C. drive Brick-handling Line
driving less critical lead roll motors connected in parallel.
A major new brick-making plant, for Ibstock Building
Open-loop A.C. drives are also used for all the auxiliaries Products Ltd, features 37 A.C. drives for numerous appli-
including various fans and pumps along the machine and cations on two brick-handling lines. The drives, ranging
controlling the speed of three brush sections which polish the from 1.1 to 18.5 kW, may be operated in sensorless vector
board after coating. These are fitted into a multimotor con- (open-loop) or flux-vector (closed-loop) mode simply by
trol panel and controlled from a process control system. selecting a parameter without the need for extra hardware
At the machine's wet end the pouting of the pulp onto the (Figure 12.49).
wire is critical to the stiffness and strength of the final The brick-making process starts with clay being prepared to
paperboard; the speed of the wire therefore needs to be the right particle size, mixed and extruded to set column
matched accurately to the speed of the pulp flow and the lengths in two rows. A brick cutter cuts the wet bricks to size
drives need to keep their speed holding between machine and they move from one conveyor 90 ° onto a transfer con-
sections or paper breaks. Under normal operation the veyor which is moving slightly faster to create a gap between
machine lays at the same number of tonnes/hour, so for the bricks, and are then picked up by pallets to one of two
thinner board the machine is speeded up and v i c e v e r s a . The combilifts. (One fills a dryer car from the top down to half
speed-holding accuracy along the rest of the machine is also way - the other completes the bottom half.) Six rows make
critical to end product quality. a shelf and each dryer car has a total storage capacity of 18
The original drive-to-drive serial communications of the shelves.
D.C. drive were modified to enable full cascading along the The conveyors are controlled by 1.1 kW drives running in
machine. A drive, which is not associated with any motor, is open loop, at a fixed ratio. The combilifts are driven by
 Chapter 12.3 305

Figure 12.49 Drives for a brick handling line (courtesy Ibstock Building Products Ltd)

5.5 kW drives in closed-loop vector drive, receiving signals financial savings that the cost of the project was recovered in
from fibre-optic positioning encoders. just seven working days!
The dry bricks are conveyed to an elevator working under The replacement of old D.C. drives with modem A.C. drives
closed-loop vector control from 18.5 kW drives, where they and A.C. motors, coupled with a complete reevaluation of
are picked up, sixteen rows at a time, and placed onto a the software control of a tile manufacturing plant has seen an
sixteen-row chain conveyor for transport through the tunnel improvement in production availability from 40 to nearly
kiln at a throughput of 27 000 bricks per hour. 90 per cent, with a corresponding increase in profitability.
The chain conveyor features pin devices which interrupt
The concrete tile mixture is extruded onto an alloy mould or
brick movement to create predetermined gaps. A star
pallet that is fed, by conveyor, into an automatic racking
wheel which turns bricks through 90 ° and a hydraulically-
elevator system. The conveyor runs underneath the rack
controlled inverter to turn bricks by 180 ° to create particular
elevator and the pallets are loaded into crates from the front
types of stacking in patterns of 11 over 4, 10 over 4, face-to-
and back, with a pusher mechanism allowing loading four
face (for facing bricks), flat setting, single edge or double-
deep. Each completed column is indexed across to the next
face setting etc. for optimum firing conditions. Bricks are
until the completed crate of five columns, i.e. a total of 660
then conveyed through the kiln, restacked and shrink
tiles, is loaded. Following curing, these are then fed back
wrapped for transport.
into the system, by a similar deracking mechanism, for
Throughout this process, the drives provide precise open- separation from the pallet and collated for packing
loop control of each of the conveyor drive motors. The (Figure 12.50).
facility to change programs quickly gives great flexibility
The operation of the racking system is crucial to the whole
and the capability of changing settings for different products
of the plant's output capability. Any stoppages directly
on the same line.
affect the plant efficiency. Poor repeatability of the existing
D.C. drives during the loading of the crate was causing
Roofing-Tile Manufacturing Plant frequent pusher jams and damage to the product. In addition,
At Redland Roofing Systems in Westerham, a complete to prevent tiles sticking together during the curing process, a
upgrade of the drives systems has resulted in such large gap of 15 mm has to be maintained between the products.
306 APPLICATIONPRINCIPLES/EXAMPLES:Building Materials

Figure 12.50 Drives controlling a tile production line (courtesy Redland Roofing Systems)

Again, because of the inconsistencies in the push, caused by The drive applications module on the elevator drives enables
speed variations in tum caused by supply voltage fluctua- the intelligence to be at drives level, eliminating the need for
tions, it was proving impossible to maintain this crucial gap PLC system control. This has simplified both the design and
causing a loss in product. operation of the system and means that, if necessary, a drive
can be changed very quickly. The applications module is
Under the control of a PLC, 1.5 kW drives, with start/stop plugged into the new drive and off it goes.
feedback signals, control the brake motors for the rack
Tiles, and therefore pallets, are counted in and counted out
elevators for both loading and unloading. 0.75 kW drives,
to ensure that there is a balanced flow in the system.
with applications modules, complete with position-control
Finally, a closed-loop drive gives control of the output
programs, drive closed-loop vector motors to control the
conveyor.
front and back pushers, giving a precise push to tiles each
and every time, maintaining the crucial gap between them. All of the drives were retrofitted into existing cubicles
This repeatable tile alignment and position accuracy alone because they were much smaller than the existing drives, and
has been responsible for a dramatic fall in spoilt product. the motors used were standard induction motors. A system
 Chapter 12.3 307

of resolvers monitors the speed of the input and output TEXTILES

conveyors and the racking, the difference being fed to the
drives which corrects the error to zero to give precise syn- Fabric-dyeing Machine
chronisation so that tile gaps are maintained. All drives have
very smooth acceleration and deceleration ramps to ensure Sclavos, founded in 1948, established a worldwide market
that the wet and easy-to-fracture products are handled as for its dyeing machines, with its Diplo ballooning-type
gently as possible. machine and the next generation twin-soft-flow Apollon
range introduced in 1990. In 1995, Sclavos introduced
The purpose of the collator, which was originally fitted with a revolutionary new concept in rope dyeing - the Venus
two brake motors on the elevator, is to lift and stack tiles into system (Figure 12.51).
blocks of 50, assembling nine blocks of 50 to form a 450-tile The Sclavos Venus system uses a new process called
module for shipping. A stripper pulls the carry rails apart and AquaChron, which allows the whole process of prescouring,
drops the tile onto a carriage, stacking the tiles vertically. bleaching, dyeing, rinsing and softening to be carried out
The synchronisation between the collator and stripper is without ever stopping the fabric circulation, creating an
critical and any out-of-phase causes jams and damaged tiles. optimum environment for achieving the highest possible
Indexing of the elevator rails used to use a proximity switch fabric quality. It gives the dyer total control over all of the
which itself had an error of 6 to 7 mm and this was com- process parameters of heating, cooling, fabric movement and
pounded by the old brake motor drives. With such a system it water management.
was impossible to get perfect synchronisation and a techni-
cian had to be on site around the clock just to keep the This is achieved with four inverter drives - to control pump
machine running. It was not uncommon to have a two day speed, for liquid flow, for the folding of fabric inside the
downtime on the machine. machine, for pump control of chemicals supply and for
fabric movement.
However, once the brake motors had been replaced by The net result is a 40 per cent reduction in water con-
standard induction motors controlled by 2.2 kW closed-loop sumption, a process time reduction of 35 per cent and a
drives, again with plug-in application modules running productivity improvement of a massive 54 per cent. In head-
special application software, accuracy was no longer an to-head tests against the best of the competition, based on the
issue. The motors are sized to allow a feed rate of 140 tiles dyeing of 540 kg of cotton, Venus with AquaChron was over
per minute maximum index rate, which means that the drive an hour faster, used over 26 000 litres less water and 165 kg
accelerates and decelerates the 300 kg elevator load up to 2.2 less steam, with a power consumption less than one quarter
times a second. that of the other machine on test.

Figure 12.51 Fabric-dyeing machine (courtesy Sclavos S.A.)

308 APPLICATION
PRINCIPLES/EXAMPLES:Textiles

Figure 12.52 Quilting machine (courtesy TEC Multineedle)

So, with pressure on profit margins increasing day by day, However, it is when the more detailed technical specifica-
with client demands for even higher fabric quality and the tions are examined that the parallels with machine tools
pressing environmental need for reducing water consump- becomes apparent-a cycle time of 50ms, very precise
tion, it is hardly surprising that only a short time after its movements to tolerances of +0.0005 mm, loads of 14.50
launch, over 200 Venus machines are already in operation Newtons plus the ability to use CAD/CAM techniques for
worldwide. the programming of intricate patterns.
The changeover from traditional D.C. to A.C. drive tech- In machine quilting, even a small error becomes very
nology also gives customer benefits such as lower overall noticeable, since two halves of a pattern will not then match
maintenance and water-tight motors with sealed bearings: a up. A correction factor has therefore been incorporated into
definite advantage in working conditions which include the programming so that an adjustment can be made while
water, steam and chemicals. the machine is running.
Each Sclavos machine is supplied with up to five drives, The CAD program runs on a motion control card built into
depending on its size. All are used in open-loop control, in a PC, which determines the X-Y movements, the pitch, the
communication with the system controller, operating under stitch size and running speed of the machine. Designs can be
software designed by Sclavos. previewed on the screen and common designs can be stored for
future use. A number of basic patterns are incorporated as
Quilting Machine standard.

TEC Multineedle, a leading manufacturer of specialist The drives have a 4-10 V differential input and a simulated
quilting machines, is applying techniques more common to quadrature encoder output fed back to the PC giving a
machine tools than textiles with the results that its machines resolution of one in 16 384. Typically, the machine runs at
are becoming faster, more versatile and more reliable than 600 stitches (min-1) per minute which, depending on the
other machines on the market (Figure 12.52). complexity of the pattern, allows a material throughput of up
to 200 metres per hour.
Each Multineedle quilting machine, designed and manu-
factured by the Textile Enterprise Corporation at their
PLASTICS EXTRUSION
Heywood factory in Lancashire, is produced to customer
specification. Material widths from 167mm to 3340mm
General
can be accommodated, with two or three needle bars, one
inch needle spacing and 132, 176, 220 or 264 needles on The concept of a single-screw extruder as a pump of tubular
each bar. design has been in existence for several thousand years.
 Chapter 12.3 309

However, ram extruders to manufacture lead pipe were first Possible remedies are to run the extruder slower, increase the
used in the early 19th century. Towards the end of the 19th pressure, cool the screw and lower the metering zone tem-
century the demands of the rubber processing industry perature to achieve a bite limit. Bite limit is where the melt
rationalised design to a machine of a type which was the reaches the correct consistency, and is not too soft (when the
basis of modem extruders. The extrusion of PVC in 1925, screw would fail to drive the melt).
and later on polythene, marked the commencement of The process of extrusion has certain basic requirements,
modem extrusion technology and by the late 1940s a which are:
machine of today's basic layout had arrived.
The extruder should be capable of processing polymer at
The thermoplastic extrusion industry experienced rapid high and consistent output rates. To aid this, a drive with
growth in the 1950s, and since that time the development of a high degree of torque/speed stability is required.
new polymers, the development of new extrusion technol-
ogy and almost total penetration of many product areas has The polymer produced should be within an acceptable
ensured an upward trend in the industry. melt temperature range, and the temperature should
not vary. A good quality temperature controller with
Extrusion processes involving the forcing of molten thermo- proportional, integral and differential (PID) control will
plastic through a die to form a continuous product are used to enhance temperature control.
make a wide range of products such as given in Table 12.20.
The pressure developed in the extruder should be con-
Also, with the use of additional machinery, other products
sistent. This requires a combination of good control of
can be manufactured such as those in Table 12.21.
process variables and speed. The use of a process
Secondary products are manufactured by extrusion, such as controller and a carefully considered choice of variable-
packaging bags from film and thermoformed articles from speed drive will aid consistent production quality.
sheet.
The polymer should be sufficiently well mixed and not
One of the main problems in extrusion is surging. This contain any low-temperature volatiles which would spoil
usually appears as a fluctuating extruder output, giving a product appearance. Mixing is assisted by the screw
variable product geometry. It is caused by inadequate design, both with single screws and twin screws.
mixing, melt instability, feed temperature effects, and solid
The optimising of screw designs has made lasting changes in
bed break-up variations. The result is dimensional variations
extrusion technology. These developments have included
and take-off problems, e.g. drift.
many features from those listed in Tables 12.22 and 12.23.

Table 12.20 Products made by extrusion Table 12.23 Twin-screw design features

Film tubular or flat Type Characteristic feature

Sheet rolls and cut sheet, laminated and coextruded,
embossed Nonintermeshed
Profiles window frames, gaskets twin screws
Pipe rigid pressure pipe, gas pipe Intermeshed contra-rotating
twin screws
Contra-rotating twin
screws, self-wiping type
Table 12.21 Products made using additional machinery Modular twin screws interchangeable screw components
for different formulations
Coated substrates paper, aluminium foil Conical twin screws better feed of dry blend; better
Insulated materials wire and cable bearing support
Filaments and strapping uniaxially-orientated for ropes,
textiles, sacks
Table 12.24 Typical extruder performance ranges

Nominal Screw Maximum Power Dynamic load

Table 12.22 Single-screw design features torque diameter screw speed drive of thrust
(kPm) (mm) (min-1) (kW) bearing (MPa)
Type Characteristic feature
80 45 105-322 7.5-18.5 47.5
Rubber screw reducing root diameter, 160 45, 50, 60 84-280 11-30 47.5/81.5
decreasing pitch 220 60 110-278 23-45 81.5
Plastic screw reducing root diameter, 350 60, 70, 72-275 23-60 81.5/118
constant pitch 80, 90
Metering screw zoned screw, with feed, 700 70, 80, 90, 64-230 37-110 118/153
compression 100, 120
and metering zones 1300 90, 100, 110, 58-207 68-189 153/224
Two-stage, or double two metering screws connected 120, 150
metering screw by a decompression vent 3000 120, 1 4 0 , 45-146 110-280 224/375
Mixing and barrier sections e.g. pins and Madelock heads 150, 160,
Solid-channel/melt- various designs of barrier 2O0
channel screws flight screw 7000 180, 2 0 0 , 32-100 230-600 585/810
grooved feed sections 220, 250
310 APPLICATION
PRINCIPLES/EXAMPLES:Plastics Extrusion

.--~ pitch I'*" r-- overall diameter

vr-- feed depth ~ metering
depth

-~ feed zone ~ ~ compression zone-~ meteringzone _~

Figure 12.53 Principal zones of typical feed screw

Other machine and process technology developments have The melted polymer is transferred to the pumping or
given large gains in output and improvements in cost/per- metering zone, which is shallow and of constant depth. This
formance ratio. Such developments include improvements in zone delivers the polymer melt at a uniform flow rate,
the very critical area of compound formulation to hold composition and temperature.
polymers to specific tolerances.
The length of the metering section must be such as to allow
There is a trend towards open modular design, enabling it to achieve the melt while the compression zone develops
components to be changed without stripping down the sufficient pressure to pump the melt through the die at a
machine, leading to greater operational efficiency and main- constant output unaffected by small fluctuations in pressure.
tenance economy. In addition to this, some machine manu- It is clear that the design of the screw fixes certain para-
facturers offer, for any given application, the option of meters at the design stage, but the operator can improve
selecting the torque, screw diameter, screw speed range, axial mixing and melting in several ways:
pressure and drive power most appropriate for their series of
1 Reduction of screw speed. This will reduce the channel
machines. Table 12.24 shows a typical range of figures.
length required for melting to be completed; the obvious
Basic Extruder Components drawback is reduced production rate.
2 Increase of pressure at screw exit. This can be done by
As with most industries, extrusion has its own technical
reducing die temperature, or by using a finer screen
terms, although different terms are not always consistently
pack. Temperature within the polymer would increase.
used in the industry. At the end of the screw is the adapter
region and filter pack and the head and die region. Beyond 3 Use of screw cooling. This delays the break up of the
this are the various take-off units, which may be devices polymer solid bed. Output is reduced, but this can
such as accumulators, conveyors, cutters, winders or others usually be overcome by increasing the screw speed.
not relevant to this discussion. This has some benefits in the melt in terms of mixing,
but power consumption is increased.
In some of the downstream equipment, variable-speed drives
are used to match up to the extruder speed or, for example, to
introduce a specific ratio required for the finished product
from its extruded size to the finished size. Stretching and Overall Extruder Performance
heating produces orientation of the polymers and makes
the end product much stronger. For extruder performance to be assessed, some knowledge
of the die characteristics is required. A die may be char-
Regardless of the physical design of the machine used, the acterised by a pressure drop and a flow rate and therefore the
extrusion process is broken down into four basic steps: output/pressure characteristics should be used with those of
• solid feeding the screw to achieve an overall picture of the performance.
• melting The limits to output are summarised in Figure 12.54.
• mixing
• melt conveying
Energy Considerations
These operations are identified by the zones of the screw,
Figure 12.53. The feed zone is a zone of constant feed depth Most operations of polymer processing involve the heating
and its function is to preheat the bulk material before it is and cooling of solid or liquid polymers. Usually the net
conveyed by screw rotation to the next zone. The tempera- energy change from solid granules to the extrudate is very
ture rise is produced by heaters around the barrel, and also by small compared with the energy which flows into and out of
frictional heat generated by the granules travelling along the the polymer, as shown in Figure 12.55.
barrel. The compression zone is the part of the screw where
On the whole, there is not much that can be done to affect
the root diameter is gradually increased while pitch and
the basic form of this diagram once extrusion is chosen as
overall diameter remain constant, thus reducing the volume.
the means of process. Nevertheless, a number of points are
This accelerates heat generation and therefore melting, and a
noteworthy:
change in the bulk density of the polymer also occurs; as
more granules melt, any air trapped in the barrel is squeezed 1 The cost of different forms of energy; in particular,
out through the hopper. directed energy (electrical and mechanical) is obtained
 C h a p t e r 12.3 311

temperature I
overheating = degradation I
~ " " :--!!7 cooling capacity limited sizing I

I power [~ __~ .m-" _~ ~ " ' ~ ~ , o

[Ihiohd'e
IowgearboxratingHlX._:___p
not enough kW

esistanoel X ~~,X" ~~'X

T X 'Ill
melting pumping take off
heat transfer / flightwear speed limits
heater capacity | die resistance inspection
melt solids separation | screw geometry packing
screw geometry
1

feed ] mixing nonmachine limits

particle form (low densitY)l mixing devices products storage
particle surface (lubricity) I channel depth shipping
barrel temperature I screw temperature sales order input rate
barrel surface (grooved) I screw too short safety
screw geometry ] head pressure
vertical pressure

Figure 12.54 Factors imposing output limitations

heating of As the screw diameter transmits the power to the material, its
granules diameter governs the power of the drive, therefore:
melting power (at max speed) = 2D 2"5
of
t-
eD crystalline where D is the screw diameter.
C
¢.-~ polymer
ii
"0
"O function Minimum power required (for 80 per cent drive efficiency):
cu of
B },
heating Pmin = Y x Qw x Cp(T1 - T2)
and melt
Where Pmi,= minimum power, Qw=output by weight,
stage
Cp=specific heat (average, between 7'1 and T2), Y=
22.2 x 10 -4 (S1 units) or 5.3 x 10 -4 (Imperial British
units), T] = melt temperature and T2 = feed temperature.
>.,
loss in
t-
die Some typical Cp values of common polymers are:
region
C
c "~
.m nylon 0.65
E PVC 0.85
I,_

HDPE 0.85

Approximate losses of energy in a typical small extruder are

shown in Figure 12.56. As can be seen, some saving could be
made by effective lagging and by precise application and
cooling of effective control of electrical heating.
extrudate

Figure 12.55 Energy considerations

Motors and Controls
Motors for plastics extrusion applications have historically
been either slip-ring motors, where speed variation is
from thermal energy and is relatively expensive. The
obtained by varying the effective rotor resistance, or Schrage
mode of heating to be used needs to be chosen with care.
motors with rotating multiple tings of brushes producing
The capital cost of providing the needs of A, Figure a transformer effect to increase and decrease speed. The
12.55, have to be matched by the cost of enabling the rotation of the brush rings was usually done by a pony motor.
energy to be removed, B; i.e. cooling air or water and This type of motive power is expensive to maintain, quite
associated installations. Therefore a saving in A will lead apart from its high initial cost.
to equipment and services saving in B also.
More recently, extruder screws have usually been driven by
In extrusion, the rejected energy offers little potential for D.C. motors controlled by thyristor converters. The use of
useful recovery, being largely to air or water. the thyristor converter offers important benefits, as follows:

Figure 12.55 shows that for the net input (A) to the 1 Power consumption becomes a function of applied load,
polymer, heat losses to the machine and then to atmo- thus creating a power saving as compared with the old
sphere offer some potential for savings. A.C. drives.
312 APPLICATIONPRINCIPLES/EXAMPLES: Plastics Extrusion

15 kW input 6.6 kW

drive ~ heaters

barrel

3 kW 3.3 kW 2.4 kW 12.9 kW

losses radiated convected polymer
to die

Figure 12.56 Typical extruder energy balance

2 Extremely high resolution of speed holding is possible

by using a tachometer feedback from the motor. This
helps when a constant throughput is required from the
extruder.
3 As the screen becomes progressively obstructed, the
pressure increases in the barrel causing an increase in the
torque required to drive the extruder. A converter com-
pensates automatically, to aid in achieving a constant
throughput.
Figure 12.57 Animal feed hammermill (courtesy
4 For extruders that need to run at different speeds for O. Bouman B.V.)
different products, the speed is easily varied by the
use of a potentiometer or increase/decrease push buttons. The hammermills are used to reduce the size of particles in
each animal feed recipe to a specified size. Such is the ser-
5 Modem D.C. drives have serial communication ports to
vice and flexibility offered by Bouman that an individual
connect with computers and other drives. Computers
farmer can order feed tailor made to his own specification.
enable automatic control of integrated systems and, as
Up to 26 ingredients are used - peas, soya, malt, maize,
more extruder manufacturers move towards continuous
tapioca, soya milk and coconut husk being just a few - and
production lines, fully integrated controls become
these arrive daily from around the globe.
essential. There is the further advantage that drive para-
meters can be changed on line. The communication ports After a mix is made, fines are separated out and particles
can also be used for data collection. This is a valuable larger than specification are fed in batches to a hammermill.
benefit in view of the increasing application of ISO 9000, Each mill has eight rows of 22 hammers, which are rotated at
which requires that a record of production characteristics up to 3000 m i n - 1 _ typically for four minutes for a 2000 kg
should be kept on a data logger or other such device. batch. Meshes of between 3 and 12mm fitted around the
diameter determine final particle size. 400 varieties of pro-
A.C. drives are increasingly being used as modem high-
cessed feed are extruded to the required pellet or nut size
performance systems and are able to provide the required
needed for chickens, pigs, cattle, horses etc.
high starting torque.
Traditionally, fixed-speed motors have been used. Direct
online starting gave a starting current typically of 800 A and
FOOD it took a full minute to run up to speed. Further, with a fixed
speed there were often problems making the product fine
Control of Hammermills in Animal Feed enough. Worst of all, because of the inertia, these mills take
Production 15 minutes to stop; downtime was considerable.
One of Holland's leading suppliers of animal feed, The solution offered for speed control was straightforward
O. Bouman B.V. of Andel, has found major cost benefits in a with an open-loop inverter, but the braking required a little
switch to a variable-speed drive for one of its two ham- more thought. Conventional braking resistors were out of the
mermills (Figure 12.57). question because of the potential hazard of dust explosion,
so it was agreed to turn a problem into a benefit by feeding
The ability to be able to select running speed has improved
braking energy through a regeneration unit back into the
product quality - and the considerable time savings in
plant power supply.
stopping and restarting the high-inertia machine achieved
with a regeneration unit have cut downtime considerably and Now, the hammermill runs up to speed in just 18 seconds,
led to greatly reduced energy costs. with supply current limited to 250 A, a major benefit since
 Chapter 12.3 313

no peak power charges are incurred. Even more significant is cubic metres per hour circulation requirement was provided
that the stopping time has been cut from 15 minutes to just by a fixed-speed 160 kW motor-driven pump.
30 seconds!
After the main computer operation was transferred to another
site 2 years previously, the two-storey computer block was
demolished, effectively reducing the site demand for chilled
HVAC water for air conditioning by 40 per cent. Nevertheless, with
2500 staff, hundreds of personal computers, printers and other
Air Conditioning for Driver and Vehicle electrical equipment, plus solar gain, there was still a con-
Licensing Agency siderable cooling load but there was clearly no longer any
The running costs of a chiller pump have been slashed by need to maintain all the chilling units on line.
over £1000 per month since the installation of variable- As part of a constant drive to reduce electrical power con-
speed drives at the Driver and Vehicle Licensing Agency sumption the DVLA property manager agreed to isolate two
(DVLA) in Swansea (Figure 12.58). of the chiller units which in effect reduced the circulation
The 160 kW motor driving the chilled water pump for the pump requirement. The possibility of reducing the power
air-conditioning system at the DVLA provides chilled water requirement of the chilled-water circulating pump was then
for both the 18-storey (heavily glazed and air conditioned) evaluated.
D-Block input centre and the five storey output (dispatch) The choice was either to change the motor or to introduce an
centre, C-Block. Some 2500 people work at the Agency, inverter drive to replace the star/delta starter for the pump.
which has responsibility for the licensing of drivers in Great The decision was made to do the latter, as it gives maximum
Britain, and the registration and licensing of vehicles and the flexibility to increase or reduce the load as required.
collection and enforcement of Vehicle Excise Duty (VED)
in the United Kingdom. This involves the collection of over The inverter drive chosen was designed specifically to meet
£4 billion in VED and the maintenance of 36 million driver the requirements of a fan/pump load. The drive features
records and 25 million licensed vehicle records. dynamic voltage/frequency control which automatically
optimises the voltage to the required load. The result is a
The five main centrifugal chilling units installed in the substantial energy saving, as the power is matched to the
power centre serving the site are connected in parallel and in actual need. Incorporated within the drives panel is a soft-
order to ensure a constant flow of 200 cubic metres per hour start bypass facility to ensure the system has a back up in
through each chiller, each unit is provided with a full case of emergency.
balanced bypass system when not in use. The total 1000
Following installation of the drive, the power consumption
of the pump fell from almost 160 kW to under 50 kW.

The savings are between 100 and 115 kWh for the 12/15
hours a day that the chilled-water system is running. Despite
the fact that it is only running for around seven months of the
year, the savings, which can be directly attributed to the
drives, mean a payback of around 18 months.

Air-handling Units at Oxford Brookes

University Students' Union
Two variable-speed drives, installed on air-handling units
for an Oxford students' union, are saving 10 to 15 per cent
of energy costs and, at the same time, improving the unit's
performance (Figure 12.59).

The students' union at Oxford Brookes University has a

main hall which is used for all manner of functions. It fea-
tures an air-handling system which has two 25 kW two-
speed motors for extraction and the supply of fresh air,
respectively.

Previously, the only control was by star/delta starters and

manual switching between full speed, at 25 kW, and low
speed at 7 kW. During a major function, there can be 3000
students in the students' union, at other times the number
varies considerably.

The temperature and the air quality (carbon dioxide levels)

are monitored by detectors in the hall and fed back to the
drives via 0-10 V signals. The drives, which are also con-
Figure 12.58 Air-conditioning control panels (courtesy nected into the building management system, now provide
DVLA) a continuous adjustment to fan speed. This gives a much
314 APPLICATION
PRINCIPLES/EXAMPLES:HVAC

Figure 12.59 Air-handling units at students" union (courtesy Oxford Brookes University)

improved level of temperature and air-quality control, with form of 12-bit parallel words. All stand speeds are simul-
the added bonus of a reduction in fuel bills in the area of 10 taneously updated - this avoids tension/compression during
to 15 per cent, giving a payback of under three years. speed changes.
The drives have a simple menu structure specifically If the speed of any section is adjusted using the desk-
designed to meet the needs of the fan and pump market. mounted cascaded speed trim potentiometers, the PLC
Dynamic voltage/frequency control means that the motor calculates the new speed of all upstream drives and simul-
speeds are always matched to the requirements of the air- taneously updates them. Individual speed trims for the
handling system. The result is a substantial energy saving. drives, catering for errors in initial calibration, are available
to the operators from wall-mounted stations near the mill
STEEL stands. The speed of the master stand is set by the operator,
using a preset potentiometer.
Anshin Steel Ltd of Shah Alam, Malaysia, needed a proven
and reliable drives and controls system for its hot rolling
combination merchant bar and rod mill. The solution
Auxiliary Drives
provided is described below. The roughing mills are both 1000 HP A.C. motors running at
constant speed. The conveyor systems for the roughing mills
Main Mill Drives are equipped with D.C. motors and are controlled by D.C.
The main mill motors use D.C. drives rated at 1850 A D.C., controllers. These conveyors are manually controlled via
600 V (Figure 12.60). joysticks and are required to accelerate rapidly up to speed to
guarantee entry of the bar into the roughers. The pinch roll
Localised control boxes enable the operator to run/jog drive at the entry to stand 2 and the exit from the disc shear
reverse/stop the stands and to lock off the power if work are both D.C. drives running at bar speed (Figure 12.61).
needs to be done on the stands themselves. The mill cannot
be started from the main pulpit, but the cascaded speed The disc shear, after the final stand, is designed to cut the bar
control is carried out from this position. to presettable lengths which are multiples of the cut length at
the cold shear and can be accommodated within the cooling
The menus for the range of products intended to be rolled are bed itself. The speed of the bar leaving the finishing stand,
stored in a PC located in the main pulpit. When a particular and its instantaneous position, is determined by measuring
product is selected for rolling, the information concerning the elapsed time when the nose of the bar passes two hot
the cross-sectional area of the billet and the mean roll metal detectors at a known distance apart, situated after the
diameter of each stand is downloaded to a PLC. finishing stand.
The control system is based on the principle of constant
Using this information the bar can be cut precisely to pro-
volume rolling and cascaded speed control whereby the
duce the required preset length for the cooling bed. The disc
speed of each stand is related to the master finishing stand.
shear itself is driven by three D.C. drives - one for each disc
Calculations concerning the motor speed reference for each blade and one to orientate the shear mechanism itself. The
drive are carried out continuously in the PLC and trans- run-in conveyor to the cooling bed is driven by 32 D.C.
mitted to the applications module in the D.C. drives in the motors connected in parallel across the Mentor controller.
 Chapter 12.3 315

main panel

lkV ( ~ ~ computer
lkV °' I " I" B×12.BCT L ne, downloaded

T4
speed
references
I
L machine
interface
I
menu
menustorage

cascadedspeedtrims (4-20) ma

I ...........
° Perat°il;r:!!is~i~iQ:itr-°}
!!n de~:sk .i]]1~~ (~ (~ (~ (~ (~ (~

remotecontrolstations

individualspeedtrims
Figure 12.60 Layout of the main mill D.C. drives on the combination merchant bar and rod mill of Anshin Steel

hydraulic pusher
U B
roughing mill 8 1000 HP furnace
30t/hr
(~ (~ 2 3 4 5 6 7 8 9 10 11

Psheart 500 500 850 900 shear 900

HP HP HP HP HP

12 13
kick off
"-,,,,,. t,^

900 disc run-in '~'". . . . . . "

HP shear conveyor .J>~'~....
- I r ~
J~f ............ I I packing
~ line-
run-out conveyor cold
shear

Figure 12.61 Overall mill layout from reheat furnace to packing line at Anshin Steel

The kick-off shaft is driven by five 35 HP D.C. motors dis- The cold shear is a brake/clutch unit with a flywheel driven
tributed along the shaft and controlled by five four-quadrant by a 50 HP slip-ring induction motor. Three more D.C.
drives which load share. This enables the kick off to con- conveyor drives carry the cut bars down to the packing line
tinue to operate with only three drives in the event of failure. which is driven by four open-loop inverter drives and con-
Slow down and stopping is by regenerative braking with the trolled by a PLC mounted in the control desk. In total, there
brake operating as a holding brake only. The slow down and are 45 D.C. drives and six A.C. inverters controlling more
stop position is from the cam box driven from the shaft. than 100 motors, and this mill has been operating reliably
since installation.
The 80-metre-long cooling bed operates in a similar manner
to the kick off; it is driven by five 50 HP D.C. motors con-
trolled from five four-quadrant D.C. drives which accurately
control the slow down and stopping. CHEMICAL
Shuffle bars, driven by five 15 HP D.C. motors controlled by Enamel Painting of Fluorescent Tubes
single-quadrant Mentors carry the bars from the cooling bed
to the run-out conveyor. This in turn is driven by three 30 HP At Demaglass in Harworth near Doncaster, the upgrading of
drives operated manually from a joystick controller at the a semiautomatic twin-head spraying machine, used for the
cold shear desk. Three more D.C. conveyor drives carry the enamel painting of fluorescent tubes, has increased pro-
cut bars down to the packing line which is driven by four duction output by 50 per cent, reduced paint consumption by
open-loop inverter drives and controlled by a PLC mounted 60 per cent and improved the appearance of the finished
in the control desk. product (Figure 12.62).
316 APPLICATION
PRINCIPLES/EXAMPLES:Chemical

of a twin-head brushing station to clean the tops of the tubes

post spraying.
The most significant improvement has been the rebuilding of
the spray-gun reciprocating mechanism. The system is based
on a single-axis drive for raising and lowering the 10 kg twin
enamel spray heads. These are driven by a one-metre-long
ballscrew with a pitch of 15 mm and a preload of 1 Nm. A
servodrive, with a door-mounted MMI, provides the control
of a servomotor. Operation is synchronised to the speed of
the feed conveyor, with a manual push button input at start
up providing the datum position.
The drive is fitted with a plug-in applications module, which
was specifically programmed for the task and a plug-in
option for a second encoder input. Feedback from an enco-
der on the motor driving the chain conveyor provides a
reference for the synchronous operation of the spraying
cycle and an encoder mounted on the servomotor provides
information on the position of the spray head. Limit switches
at the top and bottom of the ballscrew prevent overtravel.
Further drives, each with plug-in encoder modules, were
fitted to provide improved control of the chain conveyor for
the tube-spraying unit and also for the baking unit chain drive.
Both quality and output have improved dramatically. Paint
consumption has been cut by 60 per cent, saving some
£30000 over a year on this alone. The output has been
increased from 22 to 32 tubes per minute - nearly a 50 per
Figure 12.62 Fluorescent-tube spray machine (courtesy
cent increase - and the reject rate has also been cut by at
Demaglass)
least a third. The enamel coating is now very even and the
brush heads ensure that the tubes have a good sharp edge on
This has been achieved by converting the existing chain, the enamelling. On top of this, one person can now control
gear and wire-driven paint spraying mechanism to servo- the spraying operation as the need for fitting and removing
drive control, giving a greatly increased accuracy, better end caps has now been eliminated.
throughput and improved reliability.
The operator uses the operator keypad to set tube lengths of
Previously, 25.5 mm soda glass tubes for spraying were 267, 305,330 and 368 ram, with the unenamelled sections at
loaded manually onto moving spindles on the spraying unit each end of the tubes adjustable to 40 or 60 mm. From the
and blanking caps placed on the top of the tubes. The keypad the operator also sets spray count and the software
spindles progressed past the spray booth, where the tubes converts this to a motor speed. The speed of the baking
were sprayed two at a time by two vertically reciprocating conveyor is also synchronised to the feed conveyor speed.
spray guns. The spray guns also followed the rotating glass
Demaglass' Harworth factory is one of the most modem
tubes as they passed. After spraying, the blanking caps had
in Europe, using ribbon machine technology for glass
to be removed and the tubes transferred manually onto
bulbs, including incandescent lamp shells and Christmas
moving spindles in the baking unit, the blanking caps being
tree omament bulbs. The factory also produces lead glass,
placed in a tumbler to remove the overspray. With the labour
pharmaceutical glass, two-ply glass and fluorescent tubing
and paint accounting for nearly half of the cost of the fin-
on state-of-the-art production lines.
ished tube, it was important to look for ways of making the
process more accurate, to reduce overspraying, and improve
overall efficiency in order to increase the productivity of the
plant.
MARINE APPLICATIONS
The plant output was restricted because of the limits of the Cable Laying
mechanical reciprocating mechanism and also the speed of
A new generation of cable-laying ships to meet the demands
manual loading with the need to put on the blanking caps. In
of the rapidly expanding fibre-optic submarine cable-laying
addition, 100 per cent inspection was needed because of the
market has specific requirements for the controlled feeding
variable quality of the spray finish.
of cable that can only be met with digital drive technology.
There were also frequent problems with the mechanical con-
Linear cable engines have difficulty in handling very small
trol because of wear in the chains and sprockets, backlash,
diameter submarine fibre-optic cable. However, such cable
stretching of the wire rope and wasted paint from overspray.
can be handled with a cable drum engine. The critical factor
The enamel spraying unit has now been completely refur- is the ability of the drives to limit the cable tension to prevent
bished, with improvements to the spray guns and the elim- cable damage. In conventional systems, it can take a starting
ination of the need for fitting blanking caps by the addition tension of 1.7 tonnes to start the cable drum moving, so the
 Chapter 12.3 317

objective is a frictionless system, with very precise low tension control application software. The drives work in master
tension control. - slave mode. In the event of the master controller failing, one
of the other two is assigned as master. For maintenance pur-
On the Cable & Wireless stern working vessel, shown in
poses and in an emergency, the drum will run on one drive
Figure 12.63, Dowty has supplied two cable lines - a cable
alone for laying duties (all three are required for cable retrie-
drum engine with D.C. motor drive, with a maximum pull of
val). The system is designed for maximum drive availability at
40 tonnes (for cable retrieving duties) together with a four-
all times.
tonne/four-wheel pair haul-off/hold-back unit, plus a 21-
wheel pair linear cable engine for conventional cable laying.
The drives are designed to operate in four distinct modes.
The cable drum (Figure 12.64) engine has three 85kW D.C. During the initial paying out of cable, the drum has to haul the
motors which drive a ring gear around the drum. These are cable out of the cable tank and push it over the stem. The drives
controlled by D.C. drives, with field controllers and applica- run in speed control mode until there is sufficient weight of
tions modules programmed with customised centre-wind cable in the sea for it to exert a positive pull on the drum. At

Figure 12.63 Cable-laying ships (courtesy Dowty Aerospace)

Figure 12.64 Cable drum engine (courtesy Dowty Aerospace)

318 APPLICATION PRINCIPLES/EXAMPLES" M a r i n e Applications

this point, the drives switch to tension control and begin to 18-man diving system combine to enable the vessel to
regenerate power, which is fed into the ship's electrical system. undertake a wide range of subsea activities. These include
Although the regenerated power is not massive in relation to pipe laying using the reeled pipelay system, an advanced,
the total generated power on board, it is nevertheless sig- technology-led system designed to load and lay large
nificant and will produce some fuel savings. quantities of rigid and flexible pipelines, umbilicals or
cables in shallow or deep water.
In this mode the tension controller, taking its signal from a
load cell, can be used to limit the current to the set maximum The rigid pipelay system covers pipe sizes from 6 to
for the laying of skinny cable. 12 inches, with the tower angle variable between 30 ° and
vertical, and a reel capacity from 12 000m (12 inch pipe) to
A further operating mode is for picking up cable, when the 29 000 m (6 inch pipe) at a lay/spooling rate of up to 900 m
drives are again operating using speed control. per hour.
The operator has controls for speed and direction, tension The drives are fitted on the 1250 tonne product storage reel,
control, selection for speed control or torque control (down to undertaking a role which conventionally would have been a
250 kg) and drive selection. Thus the operator can drive the hydraulic application, bringing the benefits of the avail-
drum engine from zero up to maximum speed (12 km/hr) in ability of full torque at all speeds and standstill for the
either pick-up or pay-out mode, with the tension (current control of a massive 18.3 metre reel for the feeding of a rigid
limit) set if required. Alternatively, the operator can choose to steel pipe.
let the drive be overhauled by the weight of the cable and the
ship's movement, after an initial nudge in the payout direc- The pipe is loaded in 500-metre lengths which are welded
tion to cause the brakes to release. The outboard cable speed together on shore and wound onto the reel. The system com-
and tension would then be controlled by the drum engine prises three 75 kW flux-vector drives, complete with appli-
tension controller, with the drives operating in regen mode. cations modules with software to provide load sharing and a
master-slave changeover facility, each driving an 80 kW
A.C. drives have now been introduced into this application induction motor, the shafts of which are mechanically coupled.
with excellent results.
The units work in master-slave configuration, the master
Pipe Laying unit running in speed control and the other two in torque
control, with a filter smoothing the current to give smooth
Closed-loop flux-vector A.C. drives were chosen for a rigid load sharing. Effectively, all three work as one big flux-
steel pipe-laying system on the Norlifl, a pipe-laying vessel vector drive acting on a peripheral chain drive.
owned by McDermott Subsea Contractors Ltd, which is
laying pipelines 190 km west of the Shetland Islands in over Although the three drives are set up to share the load, if
500 metres of water. one (or more) should fail, the system must continue to run
with minimal disruption. Under normal conditions, drive 1
The MSV Norlift (Figure 12.65) is a multipurpose monohull is master, drives 2 and 3 acting as slaves. An applications
construction vessel, which provides a large, stable work module on each drive monitors a digital input on that
platform for subsea construction. Her flexible and rigid drive. The status of this input determines whether the
reeled pipe-laying capability, dynamic positioning and an drive is a master or slave. The signal is derived from the

~ :i ~ i i

r ~i~ •ii!~¸ ~,~,~

,i~i~i ~::i~)?ii?ii!~:~iii!!:

Figure 12.65 MSV Norlift- pipe laying ship (courtesy McDermott Subsea Contractors Ltd)
 Chapter 12.3 319

drive-healthy relay and drive override inputs; if drive 1 is advantages of using electronic flux-vector drives instead of
not healthy, or is overridden, drive 2 becomes master - if the conventional solution.
this is not healthy, drive 3 is then master. In the event of a
Baricon Systems is a private company which has established
change of status, the relevant applications module recon-
its reputation in providing high-integrity systems for the
figures its drive as master, setting up the parameters in
demanding world of the North Sea oil industry, but is now
milliseconds, and takes control. This happens so fast and
having considerable success in other offshore contracts and
so smoothly that there is no discernible change, or jolt, in
also in the design and support of original equipment for
the feed rate.
manufacturers.
In an extreme situation, one drive alone is capable of running
the reel feed. Control of Lock Gates and Sluices
The drives provide full torque at standstill. The system is Renovation projects both at Antwerp docks and at the
designed so that the reel brakes can only be released when important Albertcanal in Belgium have resulted in the suc-
the drives are enabled and therefore producing torque and cessful installation of variable-speed drives for the control of
the drives remain enabled until a signal is received to say lock gates and sluices (Figure 12.66).
that the brakes have come on.
The Albertcanal is considered to be one of the most
During normal laying operations, the drives running in important waterways in Belgium, being the only connection
torque control are in regenerating mode, producing a con- between Flanders, with the ports of Antwerp and Zeebruges,
siderable amount of energy which has to be dissipated. Two and Wallonia. This makes it one of the main routes for
large water-cooled resistors are used for this purpose. The international water traffic through to the waterways of
torque limit on each of the drives limits the current, effec- Germany and France.
tively also limiting the speed and thereby providing braking,
The Albertcanal lock system at Wijngem comprises a three-
to stop the pull of the heavy pipe causing overspeed.
stage sluice, one hydraulically controlled, two electrical.
One further requirement was for simple fault finding by the Because of increasing maintenance costs (the eight elec-
operators. It is critical that any problem is sorted out as trically-operated sluice gates were controlled by D.C. slip-
quickly as possible by operating staff while the vessel is at ring motors constantly running at full speed) the Belgium
sea. Therefore, full diagnostics and simple instructions on govemment decided to modemise the whole installation and
what to do ensure that any situation can be resolved in contracts were awarded in 1993.
seconds rather than minutes.
The requirement was for precise and smooth low-speed
An indication of the confidence that Baricon Systems Ltd control (+14 m i n - 1) of each of the 11 kW motors with full
has in the drives is the serious consequences should the torque during the opening of each sluice. This is to prevent
drives fail or lose torque. The process of pipe laying is a turbulence, which can cause damage to ships in the lock. At
continuous one which cannot be stopped once started with- a defined point, all four sluices open fully to allow more
out incurring substantial technical difficulties and costs. rapid filling of the lock. D.C. drives were selected for this
Since the installation, the system has worked perfectly, duty and eight 45 A regenerative drives were installed. All
proving the reliability of the drives and demonstrating the drives are under the overall control of a PLC.

~ i ~'] ~ i i:!ii i ~¸i!i~¸~:i:!~i~!~:i!i:~i

~ ~ i ~i:ii:i i

• ~:i i¸ ~: ! • • ~ i I~'~II~,~! ~I~ :i! ~~i~ I~

Figure 12.66 Albertcanal

320 APPLICATION PRINCIPLES/EXAMPLES:Marine Applications

The provision of digital speed references for the drives because the system has to cope with continuous registration
proved problematical because cables had to run some 500 errors and the print quantities are much larger. Despite these
metres adjacent to the power cables, with resulting noise difficulties APS Engineering has engineered a system which
interference. The problem was solved by installing 150 V enables operators to restore registration and web tension in
tachogenerator feedback devices with a specifically designed real time, either locally, via individual manual stations, or
noise filter which effectively eliminated the interference. remotely via a centralised touch screen controller. The
benefits of this to the user are greater throughput, increased
In the actual harbour of Antwerp, at the Sluice Royale, a
operator flexibility and reduced costs as a result of less
retrofitted flux vector drive, rated at 75 kW, has provided the
material wastage and reduced setting time.
solution for the control of a massive lock gate. The client
wanted to keep the existing ten-pole slip-ring A.C. motor, APS's control system is unique in that it combines the
which has a base speed of 600 min-1. Key requirements benefits of real-time, totally digital control in a package
were for full torque at zero speed and the ability to switch which employs A.C. induction motors, rather than more
from speed to torque control (achieved by the fitting of an expensive servomotors, to provide the shaftless control of
application module). multiple stations. The motors are able to achieve levels of
high-speed accuracy and resolution previously attainable
On closing, the gate drive is run at full speed to within 20 cm
only by D.C. machines.
of the wall, where it slows to 10 per cent speed and switches
to torque control. As the gate hits the wall, the drive goes This is possible due to a combination of bespoke software
into current limit but does not trip out. Torque is progres- written by APS and the use of drives fitted with the plug-in
sively decreased, to relieve the tension on the drive chain, to application module. Each of the eleven flux-vector drives
reduce stretching, and at zero torque the motor brake is used on the finishing line has a compact application module
applied. A special encoder with 1280 p.p.r, was provided to fitted.
give the accuracy of feedback for the drive.
Equipped with 32-bit RISC processors, and an onboard
single-axis position controller, which can be synchronised to
PRINTING speed or encoder tasks, the modules eliminate the require-
ment for costly PLCs or other standalone controllers. They
Real-time Registration and Shaftless interface to APS's black box via a fast RS-485 duplex net-
Web Tensioning Control work and are instrumental in the system achieving pre-
viously unattainable levels of performance in real-time
A unique digital system, for real-time registration and registration and web tensioning control.
shaftless web tensioning control, has recently been installed
The implementation would have been impossible without
and commissioned by APS Engineering on an eleven-station
the application module. Its use allowed the design of a
offiine print finishing system, for direct mail specialists The
completely digital system. The module is extremely fast and
Lettershop Group of Leeds (Figure 12.67).
allows access to virtual drive parameters, such as shaft
Offiine finishing adds individual names and addresses position and position of secondary encoders, without which
to previously printed rolls in a high-speed operation. it could not have achieved the required system flexibility and
The process is more difficult than for an online application response.

Figure 12.67 Offline print finishing system (courtesy APS Engineering)

 Chapter 12.3 321

One of the major ways in which the APS system is PC interface with modem. The latter allows remote loading
helping The Lettershop Group to reduce waste, and hence of new software, and diagnostic monitoring from anywhere
cost, is by ensuring that the finishing line does not have to in the world, via a laptop and mobile phone.
slow down for splicing and then speed back up to get into
At the application level, the black box presently commu-
registration when rolls are changed. Now expensive splicers
nicates with the drive application modules at 38 400 baud,
can be dispensed with, because synchronisation is achieved
although there are plans to expand this quite soon to five
from one roll to the next with minimal wastage - usually
megabits/s using the CTNet fieldbus. The control program is
with no more than a maximum of ten copies lost. This
application specific and very fast.
compares extremely favourably with conventional systems
employing splicers, where slow-down and ramp-up times of
up to two minutes are quite common and thousands of copies Offset Printing Presses
can be wasted as a result. KBA-Planeta of Dresden is one of the world's leading
suppliers of high-speed printing presses, from small
APS's newly developed Beacon System means that the
machines for the jobbing printer up to the huge web presses
Lettershop Group's operators are aware at any given moment
seen in major newspaper giants.
in the offiine process of the status of registration. This
information is conveyed by means of three lamps. Green These are modular inline machines with a common drive
means that the tool is in register, yellow means that a mark from one shaft. Generally, each colour printing stage requires
has been located and the system is moving into register and an additional 15 kW over the base load of 15 kW, so for a
red means that no mark has been located. six-colour press the main drive is rated at 105 kW.

When the operator sees a red lamp he can adjust the regis- Typical of the printing machines is the general-purpose
tration locally, at five individual stations (perforator, die sheet-fed offset press, the RAPIDA 104, ideal for jobbing
cutter, wet flap gluer, portable gluer and rotary cutter), via work and packaging printing in the 720 × 1040 mm format
APS's single-user registration control units (SUC 100Rs), all class, and the larger RAPIDA 130-162 range, which can
of which are interfaced to the drive application modules. handle sheet sizes of up to 1200 × 162 mm. Both machines
Alternatively, if more than one station requires registration, feature a high level of automation, high production rates and
this is undertaken centrally via the master shaftless regis- low wastage (Figure 12.68).
tration system. The registration system communicates to the
In both cases, the impression cylinders and transfer drums
drive-mounted application modules via a touch-screen inter-
run without play in precision antifriction bearings and are
face and APS's black box. This recommended set up yields
driven by a continuous gear train from the main drive shaft
the maximum flexibility for the user, but at a competi-
driven by a single D.C. drive and D.C. motor.
tive price.
Overall control of each press is by PLC, with binary signals
Similar facilities are provided for tension control. The turner being decoded by an applications module on the Mentor
bar, plough folder (three-off) puller and ribbon shifter sta- drive. Some 25 different functions have to be carried out by
tions in the line are equipped with APS's single-user tension the drive, with the applications module being programmed
control units. These provide individual web tensioning, once for:
again via the drive application modules. However, if more
than one station needs tensioning, this is effected centrally • jogging mode, in both directions, to aid setting up and
from the touch-screen controller. maintenance
• special limited jogging mode to enable cleaning with
In addition to its links with the registration system and maximum safety
APS's black box, the user-friendly touch screen also has a • decrease/increase

Figure 12.68 Offset printing press (courtesy KBA-Planeta)

322 APPLICATION PRINCIPLES/EXAMPLES:Printing

• positioning the lift stops with great precision, just before the crucial
• crawl point at which he would have been crushed.
as well as emergency stop, torque monitoring and alarms. Engineering stunts like these, and others such as the specta-
The software in the application module is a derivative of the cular collapse of an eight-ton ceiling in The Mummy, are
standard spindle orientation package. There are two distinct all in a day's work for Unusual Rigging. Like the majority of
modes of operation. One is for start/jogging with encoder Unusual's custom automation solutions, they were achieved
feedback and the second, a speed loop under tachogenerator using flexible variable-speed drives with applications
control for normal running. modules.

The KBA RAPIDA 104 range, with its unit construction, is Unusual Rigging's growing reputation for providing fast,
available for printing up to eight printing couples, double- well engineered solutions was responsible for the company
size impression cylinders and transfer drums, at an output of being asked to help overcome problems with the rooftop
up to 15 000 sheets per hour. The method of sheet transfer jump in Tomorrow Never Dies. Historically, such leaps have
ensures a flat, smooth sheet run and excellent print quality. A been achieved using air descenders, but these are slow and
high level of automation ensures short make-ready times and laborious and cause long delays between takes.
low wastage.
In an attempt to overcome these problems, an alternative
The large format KBA RAPIDA 130-162 range also system using two steel-wire winches linked to harnesses was
achieves a maximum printing performance of 15 000 sheets tried. The reason for using winches was that they can rewind
per hour in straight printing and up to 12 000 sheets per hour quickly to allow for multiple takes. Unfortunately, in this
when perfecting. The high level of automation, with auto- case, the acceleration of the drum was so fast that there were
matic plate changing, automatic pile changing and automatic problems in getting the cable to come off without loops
presetting of almost all machine components and auxiliary developing. Unusual was called in to solve the problem, and
systems, guarantees practically the same short make-ready given two to three weeks to design, manufacture and ship a
times as for smaller machines. The standard configurations system to Bangkok in Thailand.
span two to eight colour versions, with a wide range of
options being available. The system that the company devised comprised a take-off
motor and pinch roller, controlled by a drive with an onboard
applications module. The applications module was used
STAGE SCENERY- FILM AND THEATRE
because it is extremely fast and allows access to virtual drive
parameters, such as the position of secondary encoders,
James Bond Film Stunts
without which the required system performance could not
When James Bond and his female companion jump off a have been achieved. The pinch roller has its own encoder
32-storey building in the film Tomorrow Never Dies the pair linked to the applications module, which also receives speed
grab an advertising banner which rips and peels progres- data from the winch encoder. By comparing the two streams
sively, slowing their descent. They plummet a mere 15 floors of speed data the drive/applications module, in combination
and live to fight another day (Figure 12.69). When Tom with the pinch roller, ensures that tension in the cable never
Cruise is suspended beneath a lift in Mission Impossible goes slack.

Figure 12.69 Death-defying jumps (courtesy Unusual Rigging)

 Chapter 12.3 323

With Unusual's system fitted the cable was unwound from underlying theme. What resulted was an exhibit which
the drum at 4.5 metres/second with no looping problems dominated press and television coverage of the Hanover
whatsoever. Moreover, the system was able to control an Messe where it was exhibited, played on Children's televi-
accelerated drop of 15 stories and stop, repeatedly, at the sion and played for the Queen by special invitation
exact point required for the start of the next scene. (Figure 12.70).
The drives were of course Unidrives. The vision was also
Controlling Acoustics provided by Control Techniques - 'We'd like you to build us
Unusual Rigging has been responsible for installing a an orchestra for our exhibition stand at Hanover Messe' was
movable acoustic ceiling in the auditorium of the new the request made to Unusual Rigging.
Milton Keynes theatre, and for rigging and controlling the The idea of drives creating music was not new. At an earlier
animatronic moth which was the subject of so much interest exhibition, Control Techniques Engineers had played 'God
in the stage revival of Doctor Dolittle. Once again, drives save the Queen' on a D.C. drive/motor with the aid of an
feature heavily in both projects. applications module. The note or tone was based mainly on
The acoustic ceiling, weighing 25 tonnes, has been sus- the speed of the motor. The faster the motor rotated, the
pended on 22 hanging points in the roof of the theatre. Wires higher the pitch of the note.
from the hanging points are hung over driving drums, each The brief for the orchestra was to build nine motors and
controlled via a drive with an onboard applications module. drives in three sections of three. The sections were defined
The benefit of using the applications module is that Unusual by the mode of operation of the Unidrive:
avoided the costs of a PLC and its programming limitations.
1 A.C. servo
The positioning programme for the ceiling is actually in the
2 A.C. closed-loop flux vector
applications module at the front end of the drive, which
3 A.C. open-loop
makes for faster operation and better response all round. The
flexibility of the overall control program, which was written A five-metre diameter circular area was allocated for the
by Unusual using a soft logic programming and configura- exhibit.
tion tool, means that the applications module can actually
The first problem was how to create good quality music. In
learn positions from a simple dial-in keyboard and display.
collaboration with the University of York, it was agreed that
An additional benefit is that the control software com-
it was practical for the pitch of the note to be determined by
municates with all the existing firmware in the theatre.
the rotational speed of the motor. However, the idea of using
its inherent noise was seen as low quality and, frankly, bad
Exhibition Focal Point- The Control publicity as many applications, including stage scenery,
Techniques' Orchestra require the motors to be as quiet as possible. The idea of
mounting something such as a siren to the motor shaft was
Trade exhibitions are in many respects a stage, and an eye-
discussed but discounted as its scope was very limited.
catching exhibit can be memorable and technically chal-
lenging. With the launch of Unidrive in 1995 Control In the end, for reasons of versatility and simplicity, a rotating
Techniques was looking for something with great visual disk with an electronic pickup to produce the basic electrical
impact, but with the high performance of the product as the waveform at the required frequency was used (Figure 12.71).

Figure 12.70 The Control Techniques" orchestra

324 APPLICATIONPRINCIPLES/EXAMPLES:Stage Scenery- Film and Theatre

i
. . . .

ii
~:i!i!i:ii~: ii!i~
. . .~iiiiiiiiiiiiiiiii~i!~:
.. ::~/

Figure 12.71 Toothed wheel and electronic pickup

This signal was then processed and fed to a loudspeaker - a To reinforce the connection between motor speed and
principle reminiscent of the Hammond tone wheel organ. pitch of the note, each musician would be equipped with a
rope light thermometer-type indicator, calibrated in semi-
The requirement for the music was wide ranging - it could be
tones, as well as displaying the speed numerically on the
anything from classical to pop, from rock and roll to film
face of the drive. To make the orchestra visually capti-
themes. Before starting on the music arrangements it was
vating there also had to be some animation. If there was
necessary to establish the musical characteristics of the
too much it would detract from the drives. In the end, a
instruments and see what limitations existed. The three main
liberal sprinkling of moving hands and tapping feet,
issues were accuracy (getting the notes in tune), stability (will
swaying heads and revolving bow ties was incorporated.
there be any unwanted vibrato) and dynamic response (getting
The musicians were arranged on a 4.5 metre tiered
quickly from one note to the next). The results of the early
podium, with a small stage area at the front for a human
trials exceeded all expectations. Accuracy was not an issue.
performer or demonstrator.
Even the open-loop drives were accurate to one or two min- 1.
Stability was good and dynamic response was amazing. It was So the technology and hardware were established but control
agreed to settle on a working range of two-and-a-half octaves still had to be considered. In the end, a PC-based MIDI
for each instrument. The open-loop drives would play bass system was used. The PC contained a Music Quest MQX-
with a top note of middle C, the closed loop flux-vector drives 32M MIDI interface card with built in timecode facility. The
would be tenor with a range an octave above the basses and the musical arrangements were commissioned in MIDI format
servomotors treble, an octave above that. and transferred directly. Nine microprocessor units were
Having established the technology of the orchestra, what built, one for each musician, to first filter out and act upon
was it going to look like? The orchestra had to look like an the locally relevant MIDI commands. Second, they had to
orchestra, and from the early days of creation the key con- process the continuous square-wave output of the optical
stituents were in place. The Unidrive itself was to be the pickup produced by the spinning toothed disk on the end of
head, below that the torso would be a box containing the the motor shaft. The unit sent a speed demand signal to the
loudspeaker and any other electronics. In front of the torso Unidrive via a high-speed serial link.
was a motor representing the instrument being played. Arms
and legs were made of anglepoise light fittings and there The processing comprised chopping up the signal into
would be a variety of hairstyles fashioned from cables and a individual notes and adding 'a bit of colour'. This avoided a
selection of bow ties and necklaces (Figure 12.72). Stylophone type of sound.
 Chapter 12.3 325

Figure 12.72 The conductor and a musician

The choice of the music catered for all tastes: Beethoven's The band had three 5.6 m high, 3 m wide television screens
Ode to Joy, Mozart's Eine Kleine Nachtmusik, Handel's specially built to enable thousands of fans who packed each
Water Music. Then there were a few Beatles songs, as well as venue to see what was happening on stage. Each screen
big band swing numbers such as the Pink Panther and New weighed six tonnes. The centre screen was fixed, but the left
York, New York and some military brass band arrangements. and right screens were driven by winches, controlled by
As the original exhibition was in Germany a number of flux-vector drives. The two outer screens moved horizon-
Bavarian umpah tunes were included for good measure. tally from the sides of the stage to the edge of the centre,
fixed screen where all three locked together to form a single,
To complete the stand a conductor was hired. His speciality
massive 9 m wide television.
was to be able to lean his body forwards about 45 ° , anchored
to bolts in the floor- who says trade exhibitions cannot be fun! The drive's accuracy enabled the screens, moving at speeds
of approximately 0.5 m s -1, to be positioned within a milli-
The orchestra still performs today, on special occasions such
metre of their target point; critical for numbers which require
as carol concerts at local schools.
the screens to join together as one. Position sensors on all
three screens relayed the necessary information via the
Rock Concert show's control computer back to the drive via its serial
communications link.
Six 7.5 kW flux-vector drives moved the spectacular high-
tech scenery on the 1992 world tour of rock group Genesis Adding to the show's special effects was a group of four
(Figure 12.73). moving lighting pods, two on each side of the stage. These
326 APPLICATIONPRINCIPLES/EXAMPLES"Stage Scenery- Film and Theatre

Figure 12.73 Stage set for Genesis (courtesy hit and run music)

Figure 12.74 The Lovers" Duet (courtesy The Millennium Experience)

moved along two, 25 metre long tensioned steel wire ropes during slow numbers to 1 m s - 1 for rapid positioning before
which ran at an angle of 45 ° from floor level, one at each end the start of a new song.
of the set, to the top of 18 metre high masts on opposite sides
of the stage, crossing centre stage above the band. Each
lighting pod, weighing half a tonne and carrying six rotating
Millennium Dome Aerial Ballet
lights and special-effect plates and mirrors, was moving The haunting image of two sylph-like creatures hanging by
using a cable-driven winch controlled by a flux-vector drive. silver threads while performing an aerial pas-de-deux (the
The lighting pods travelled along the tensioned cables at Lovers' Duet) 44 metres above a crowded arena is one that is
speeds ranging from an almost imperceptible 0.1 m s - 1 uniquely memorable (Figure 12.74). It is a spectacle which
 Chapter 12.3 327

owes much to the skill of the performers themselves. rapid rate of 2 m s - 1., two vertical (for hoists) traversing at
However, the aerial ballet is a marriage of art and technol- an even faster rate of 3 m s - ~ and a rotational function for
ogy. It would not be possible without the five-axis servo and the turntable.
SCADA system.
All of the five axes are controlled over the CTNet fieldbus
The Millennium Dome is a unique building and posed its
system using the combination of five servodrives and
own set of problems. For example: the sheer height of the
servomotors. The drives are coordinated via a motion con-
Dome meant that any servo control system would have to be
troller. This unit has the capability to control and interpolate
situated over almost 50 metres in the air, close to the roof-
between the motions of the axes.
mounted hoists and winches that it controls. In contrast, its
associated supervisory SCADA would, by necessity, be at The link from the floor-mounted console to the roof-
floor level. mounted turntable is also via CTNet, operating at a baud rate
Much more difficult in system terms was the requirement to of 1.25 Mbits/s. Interestingly, the final connection to the
accommodate the circular profiles within the Lovers' Duet turntable is via slip rings. This arrangement has worked
routine. This entailed mounting the panel, which enclosed perfectly since the system was commissioned.
the servo and axis control system, on a circular turntable
The overall control program for the aerial Lovers' Duet is
suspended from the Dome roof. Of course, with the table in
housed in the control console.
motion the process of ensuring continuous signal integrity
between the roof-mounted servo system and the floor- Also fundamental to the control package, and included in the
mounted SCADA is that much more difficult. floor-mounted control console, is a touch-screen computer
and keyboard running the SCADA program. It operates in
In addition to the physical and signal logistical problems,
conjunction with two joy sticks on the console and provides
the overall system performance demands were pretty exact-
great flexibility to create new routines. Operators can draw
ing too. The varied nature of routines in the aerial ballet
pictures using the software and enter function routines.
sequence meant that five axes of fast movement were
These are then sent via the CTNet driver to the motion
required, with many sequences needing complex interpola-
controller and servodrives, enabling new routines to be
tion between axes. The system specification also called for a
performed. The powerful facilities of the SCADA software
manual teach function to operate in conjunction with two
enable it to record all the five axes of fully interpolated
joysticks. This was to enable the performers to experiment
motion, and also to sample profiles executed using the joy-
with new moves and, if they liked them, integrate them into
sticks and reconstitute cues that are stored. This flexibility is
their performance.
all important because it means that if the operators see
The system includes a broad range of products. Its five axes something they like, it can be easily integrated into the
comprise two horizontal (for winches), traversing at the overall routine.