UNIT-1 ELECTRIC DRIVES

1.1 Type of Electric Drives

1. Explain briefly about an electric drive with block diagram.
Answer
Electric Drive:
The process of conversion of electrical energy to mechanical energy using machine equipment
(like prime mover) to provide electric control is called as an electric drive. The machine
equipments may be a petrol engine, hydraulic motor or turbine. The electric drive is employed in
fans, trains, machine tools, automation purpose etc.,
Parts of an electric drive

 Source
 Power modulator
 Motor
 Load
 Control unit
The block diagram of an electric drive is shown in figure.

Figure: Block Diagram of an Electric Drive

Source
The source used for different drives is available in different types of supply and levels of voltage.
Usually, the supply fed for the drives with high power motors is of 3Phase AC supply, but for
few drives such as traction 1-ph AC supply is used though the requirement of power is high, due
to economy issues. Medium and Low power motors like space and aircraft applications are fed
with 400V, 50 Hz supply such that high power to weight ratio is achieved, and for higher motors,
high voltage rating i.e., 3.3 kV, 6.6 Kv, 11 kV is required.
DC Supply is fed to the few of the drives like underground traction systems, where the clearance
between live conductor and earth is low. A D.C voltage of 500-700 V is supplied to them and
also the drives like fork, lifts, trucks and milk vans are battery operated drives. The voltage level
varies according to the size with the range 6V, 12 V, 24 V, 48 V and 110 V DC supply.
Power modulator
The power modulator performs various functions which are as follows.
a. The power modulator receives the electrical power from source and then modulates the
power into a desired form of energy which is required to the motor. The desired speed
torque characteristics can be obtained by the load.
b. It is possible to convert any type of sources of energy into a different form of energy to
which the motor is suitable. For example, if the source is supplied with DC voltage and
the motor is employed with synchronous type, then the power modulator performs its
operation i.e., conversion of DC voltage into a variable AC voltage.
c. During transient operation, excess flow of source current and motor current is blocked by
modulator as it causes voltage dip or overload.
Motor
There are enormous motors employed for drives that are DC motors which include Shunt, series,
compound and permanent magnet and AC motors which include Induction motors (Squirrel
cage, wound type and linear type), synchronous motor (wound field and permanent magnet),
brushless dc motor, stepper motor, switched reluctance motor.
For constant speed drives induction and synchronous motors are used and for variable speed
drives DC motors are used. Now-a-days AC motors are also being employed for variable speed
drives due to development of semi conductor converters like thyristor, power transistor, GTO,
IGBT etc.,
Load
For an electric drive the loads may be in different forms such as washing machine, trains, drills,
machine tools etc.,
Control unit
The control unit is designed according to use of power modulator i.e., when control of an
interlocking and phase sequence are required to control then switching circuits are used. In the
same manner, semiconductor converters are also used where firing circuits are employed.
2. What are the advantages and disadvantages of an electric drive?

Answer:
The advantages and disadvantages of an electric drive are listed below.

Advantages
1. Electric drive has long life.
2. Storage and transportation of fuel is not required.
3. Due to the absence of fuel and fumes, it pollution free.
4. Space occupied by the drives is less.
 5. Efficiency of the drive is high compared to other drives.
6. It is possible to start and stop the motor conveniently by using various speed control
methods.
7. The maintenance required is less hence the maintenance cost is less.
8. It is possible to apply full load when the motor begins as it starts immediately.
9. The use of electric drive is economical as it uses electricity which is less cost compared
to fuel drives.
Disadvantages
1. Electric drive cannot be operated when there is a supply failure.
2. It cannot be used for longer distances where there is no supply of electricity.
3. There is a chance of accidents due to faults or short circuits or leakage from conductors

3. Differentiate AC and DC drives

4. Explain the types of electric drives.

 OR
Classify various electric drives.
OR
Classify Different types of electric drives.
Answer
Electric drives are divided into three types.
1. Group drive
2. Individual drive
3. Multi-motor drive
1. Group drive
In a group drive, single electric motor is used to drive a group of active machines with the help of
line shaft. It is also named as line shaft drive and as it is connected with multi stepped belts and
pulleys the speed of the machine varies. Figure 1 shows the group drive system.

Figure 1: Group Drive System

Advantages
.
A. Initial cost is less
compared to
individual drive.
B. Overload capacity is high.
C. Able to turn off the operation of a group drive as there is a possibility of sequence of
continuity operation followed by machines.
D. At or near rated load, the powerfactor and efficiency of the drive is higher.
E. Maintenance cost of the drive is low hence maintenance cost is less.
F. Space required for the drive is less.

Disadvantages
A. Noise pollution at operating site is high.
B. Due to energy transmitting mechanism there is a power loss.
C. Reliability of the system s less as the whole operating equipment become standstill when
any fault occurs on driving motor.
D. Speed control and energy transformation through pulleys is inefficient.
E. Results in poor power factor and less efficiency when all machines does not work with
shaft together.
2. Individual drive
In an individual drive, single electric motor is used to drive single machine with the help of
pulleys, gears etc., The operation of an individual drive is smooth and safe. This type of an
individual drives are applicable to constant speed operations like textile industry and paper mills.
Few other examples for this type of drives are electric hand tools, single spindle drilling
machines, and metal mechanism tools. Figure 2 shows the Individual drive system.
 Figure 2: Individual Drive System
Advantages

A. As a single motor drives each machine, it is possible to operate or disconnect the system
at our desire.
B. When a fault occurs in the system, the machine get shutdown or disconnect quickly as
each machine operator has full control of operation over it.
C. Machines can be installed at flexibly locations
D. Disconnection of any machine during faulty condition affects the other systems
operation.
E. No maintenance is required for line shafts, bearings and pulleys.
F. Appearance of machine is clean and safe as the control unit and motor connections of
machine are in built.
Disadvantages
A. Requires high initial cost for large factories.
B. Not employed for heavily operated or driving machines such as paper mills.
3. Multi-motor Drive
In a multi-motor drive, various electric motors are used for operating different parts of machine
of driven mechanism in production unit. This is applicable in travelling cranes where three electric
motors are used which performs three different operations such as hoisting, long travel motion
operation and cross travel motion operation. Figure 3 shows the multi-motor drive system.

Figure 3: Multi-motor Drive System

Advantages
A. Multi-motors are employed for automation in production units where the productivity of
various organizations have improved along with reliably and safety of operation.
B. Reduction in complexity of system.
Disadvantages
A. Requires high capital cost as various motors are required to operate.

1.2 Choice of Motor.

5.What are the factors governing selections of motor?
Answer
Basically an electric motor is a reliable operated machine. It requires maintenance and for this a
selection or choice of motor is being made. Hence few factors are taken into consideration for
this which are as follows.

1. Nature of Supply: It is to be noticed that whether the supply is available for the motor to be
used either an A.C or rectified A.C or pure D.C.
2. Nature of the drive: It is to be noticed that whether the usage of drive to operate the motor is
either a group or individual or multi-motor type.
3. Nature of Load: There are few points to be noted in this criteria which are as follows.
a. Whether the load requires high starting torque or low starting torque
b. Whether the load is proportional to speed or remain constant
c. Whether the inertia of load is high or low. In case if it is high, then requires long starting
time.
4. Motor Electrical Characteristics
a. Starting characteristics
b. Running characteristics
c. Speed control
d. Braking characteristics
5. Mechanical Considerations
a. Type of enclosures
b. Type of bearing
c. Transmission of Drive
d. Noise level
6. Size and rating of Motor
a. It is to be noticed that whether pull out torque and overload capacity are sufficient or not
b. It is to be observed the type of load cycle i.e., whether it is continuous or intermittent or
variable.
7. Types of Cost
a. Capital cost: The cost which is required during initial stage of machine i.e., cost of
purchase and erection.
b. Running cost: The cost which is required during operation of the electric drive i.e.,
Maintenance cost, fuel cost.

6.Explain Electrical Characteristics in brief.

Answer

Once the load is known, then proper selection of motor can be done. Hence in order to select the
motor, the electrical characteristics for different kinds of motor to be known which are as follow.

a. Speed-torque characteristics
It is required to determine speed-current and torque-current characteristics so as to
determine speed-torque characteristics
b. Speed control of the motor
The following points are necessary to consider in case of speed control of motor.

 If the speed of the motor increases the back emf of armature motor increases i.e., N α Eb
N α V - Ia * Ra … (1)
  If the speed of the motor increases the field flux per pole decreases i.e.,

1
Nα
ϕ
… (2)
Hence mathematically, it can be written as,
Eb
Nα ϕ

From equations 1 and 2 , we get,

N α V - Ia * Ra /ϕ

 The developed torque, T is proportional to product of armature current and flux per pole.
Mathematically it can be written as,
T α Ia ϕ … (3)

Figure shows speed-torque characteristics with armature current.

1.3 Starting and Running Characteristics

 1.4 Speed Control

Discuss the motoring and braking operations of an electric drive in both

directions, forward and reverse taking hoist as an example.
Discuss in general the various speed control methods adopted for motors.
OR
With neat diagrams, describe the following methods of speed control for dc traction
motors. Point out the merits in each case:
i. Series parallel control
ii. Field control by flux weakening ( For Answer Refer only topic Speed Control Of Series
Motor )

Back emf Eb of a DC motor is nothing but the induced emf in armature conductors due to
rotation of the armature in magnetic field. Thus, the magnitude of Eb can be given by EMF
equation of a DC generator.
Eb = PØNZ/60A
(where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature conductors, A =
parallel paths)

Eb can also be given as,

Eb = V- IaRa
thus, from the above equations
N = Eb 60A/PØZ

but, for a DC motor A, P and Z are constants

Therefore, N ∝ K Eb/Ø          (where, K=constant)

Speed Control Of Shunt Motor

The classification of speed control methods for a DC shunt motor are similar to those of a DC
series motor. These two methods are:

Armature Control Methods

Field Control Methods

Armature controlled DC shunt motor can be performed in two ways:

 Armature Resistance Control

 Armature Voltage Control

Armature Resistance Control

In armature resistance control a variable resistance is added to the armature circuit. Field is
directly connected across the supply so flux is not changed due to variation of series resistance.
This is applied for DC shunt motor. This method is used in printing press, cranes, hoists where
speeds lower than rated is used for a short period only.

Armature Voltage Control

This method of speed control needs a variable source of voltage separated from the source
supplying the field current. This method avoids disadvantages of poor speed regulation and low
efficiency of armature-resistance control methods.
The basic adjustable armature voltage control method of speed d control is accomplished by
means of an adjustable voltage generator is called Ward Leonard System.

Ward Leonard System

This method is used to supply variable voltage to the motor. A d.c. generator G is connected
mechanically with the prime mover which rotates the generator at a constant speed as shown in
Figure.

The field winding of d.c. generator is connected to a constant voltage a.c. supply line through a
field regulator and reversing switch.

The d.c. motor is fed from the generator G and its field winding is connected directly to a
constant d.c. supply line.

The voltage of generator fed to the motor can be varied from zero to maximum value. By
reversing switch the direction of rotation of motor can also be changed.

This system is initially very costly but is suited where un limited speed control and reversing of
direction is required.

 Flux/ Field Control Method

This shows the speed of a dc motor is directly proportional to the back emf and inversely
proportional to the flux per pole.
It is already explained above that the speed of a dc motor is inversely proportional to the flux
per pole. Thus by decreasing the flux, speed can be increased and vice versa.
To control the flux, a rheostat is added in series with the field winding, as shown in the circuit
diagram. Adding more resistance in series with the field winding will increase the speed as it
decreases the flux. In shunt motors, as field current is relatively very small, I sh2R loss is small.
Therefore, this method is quite efficient. Though speed can be increased above the rated value by
reducing flux with this method, it puts a limit to maximum speed as weakening of field flux
beyond a limit will adversely affect the commutation.

Speed Control Of Series Motor

1. Flux Control Method

 Field diverter: A variable resistance is connected parallel to the series field as shown in fig
(a). This variable resistor is called as a diverter, as the desired amount of current can be
diverted through this resistor and, hence, current through field coil can be decreased. Thus,
flux can be decreased to the desired amount and speed can be increased.
 Armature diverter: Diverter is connected across the armature as shown in fig (b).
For a given constant load torque, if armature current is reduced then the flux must increase,
 as Ta ∝ ØIa
This will result in an increase in current taken from the supply and hence flux Ø will
increase and subsequently speed of the motor will decrease.
 Tapped field control: As shown in fig (c) field coil is tapped dividing number of turns.
Thus we can select different value of Ø by selecting different number of turns.
 Paralleling field coils: In this method, several speeds can be obtained by regrouping coils
as shown in fig (d).

2. Variable Resistance In Series With Armature

By introducing resistance in series with the armature, voltage across the armature can be
reduced. And, hence, speed reduces in proportion with it.

3. Series-Parallel Control
This system is widely used in electric traction, where two or more mechanically coupled series
motors are employed. For low speeds, the motors are connected in series, and for higher speeds,
the motors are connected in parallel.
When in series, the motors have the same current passing through them, although voltage across
each motor is divided. When in parallel, the voltage across each motor is same although the
current gets divided.

Note: Numerical Problems on Speed control of DC motors are attached with

other PDF

Problems on Speed control of DC motors

 1.5 Temperature Rise

Explain the factors that affect the final temperature rise of a motor on load.
Derive the expressions for Heating and cooling curves of Motor

OR

Derive the expression for temperature rise

 1.6 Particular applications of Electric Drives
Q. Give with reasons for the type of application for which the following motors are best
suited,
(a) D.C shunt motor
(b) D.C series motor
(c) Synchronous motor
(d) 3-phase induction motor.
Answer:
(a) D.C shunt motor
A D.C shunt motor is nearly a constant speed motorand the torque varies directly as the current.
The starting torque produced is normal and the maintenance cost of a D.C shunt motor is
average.
For this type of motor, a continuous speed range of 4:1 is maximum.
The applications for which this type of motor is best suited are for driving constant speed line
shafts, lathes, vacuum cleaners, pressure blowers, constant head centrifugal pumps, compressors,
reciprocating pumps, fans, wood-working machines such as circular saws, wood planners,
laundry washing machines, milling machines, passenger elevators, continuous conveyors,
grinders, polishers, small printing presses, paper making machines, metal cutting machines etc.
(b) D.C Series Motor
A D.C series motor possesses high starting torque i.e., 3 times the full-load torque with twice
full-load current at start.
The torque of a D.C series motor varies linearly as the square of the current. It is a variable speed
motor and at no-load condition, the speed is dangerously high.
The speed of a D.C series motor can be altered with the maximum speed range of3:1. The speed
is self adjustable and the maintenance coil is medium.
The application for which a D.C series motor is best suited is heavy duty application such as
electric railways, rolling mills, metallurgical works, mine hoists, continuous conveyors, cranes,
valve operations etc.
(c) Synchronous motor
Synchronous motors are constant speed motors at varying load. It does not have self starting
torque and also speed of synchronous motors cannot be controlled.
The maintenance cost of synchronous motors is medium and they have the capability of working
over a wide range of power factor both lagging and leading.
The applications for which a synchronous motor is best suited are for voltage control at the end
of transmission line, for improving power factor like capacitors in large power systems in which
considerable lagging power factor equipment is installed.
The applications also include main drives of controlling continuous rolling mills, medium and
large drives in the paper and cement industries, for driving continuously operating equipment at
constant speed such as air compressors, motor generator sets etc.
(d) 3-phase induction motor
Induction motors are of two types, namely,
(i) Slip ring induction motor and
(ii) Squirrel cage induction motor.
(i) Slip Ring Induction Motor
Slip ring induction motor is nearly a constant speed motor and has a high overload capacity. The
starting torque of a slipring induction motor is very high about twice the full-load torque with a
starting current of twice the full-load current.
Speed control of this type of motor can be affected as to 50-75% and the operating power factor
is about 0.75 to 0.9 lagging. The maintenance cost is low.
The applications where in slip ring induction motors are best suited are for industrial loads of
high power factor where high starting torque and speed control are required such as driving line
shafts, lifts, pumps, generators, winding machines, smoke exhausters etc.
(ii) Squirrel Cage Induction Motor
The starting torque of a squirrel cage induction motor is low and it is nearly a constant speed
motor.
It has high overload capacity and the speed control cannot be achieved.
The applications for which squirrel cage induction motor best suited are compressors, line shafts
where some starting torque is to be developed but load torque remains fairly steady.

19. Explain the applications of industrial drives.

Travelling cranes: Both AC and DC drives are used for the different operations in a crane. The
preferred drives on consideration of economy end utility are indicated below:

ROLLING MILL DRIVES: The following types of drives are used for rolling mills: (i) DC
motors, (ii) AC slip ring motors with speed control
The DC motors, because, of their inherent characteristics, are best suited for the rolling mills.
Speed control is effected either through Ward Leonard system or by grid controlled mercury arc
rectifiers. AC slip ring motors are suitable for roughing and re-rolling mills where very precise
speed control is not required Their efficiency is low because of the power wasted in the rotor
resistance. There is also abrupt rise in motor speed when the material leaves the rolling stands.
KILN DRIVES

Call for a starting torque of about 250% in addition to the speed control feature. The commonly
used drives are: (i) Slip ring induction motor (ii) Three phase shunt wound commutator motor
(iii) Cascade controlled AC motor (iv) Ward Leonard controlled DC motor (v) DC motor with
transformer step switch control.
TEXTILE INDUSTRY: The textile industry requires special types of drives for (i) weaving,
and (ii) spinning. (i) Applications of Industrial Drive > Weaving: The motors used in weaving
mills must have good cooling capacity to keep their temperatures within limits in the presence of
large power losses. The rating of the motors and the cooling facility must be properly selected,
because these motors are used in conditions where high moisture content is present along with
lot cage induction motors with high rotor resistance, totally enclosed, fan cooled and having high
temperature insulation are used to drive looms. For light fabrics like cotton, silk, nylon etc. small
motors of less than 1 hp may be sufficient. For heavy fabrics such as wool, the rating of these
motors may be 2-3 hps. These motors are normally run at 750-1000 rpm.
Spinning: The spinning mills use one of the following three types of drives: (i) A 4-pole or 6-
poIe squirrel cage induction motor, (ii) A pole amplitude 4/6 or 6/8 poles induction motor, (iii)
Two separate motors to be runs at 1500/1000 or 1000/750 rpm. But whatever may be the types of
motor used, the motor must be started with controlled torque.
PAPER INDUSTRY: In a paper industry, the drives are required for (i) Pulp making, and (ii)
Paper making. In the pulp making process, the logs of wood are either ground in mechanical
grinders or else they are chemically treated with alkalis and simultaneously beaten up to turn
them into soft pulp. In the mechanical method of pulp making, the electrical power requirement
is very high because the wood is hard. Since the mechanical grinders operate at a constant speed
of about 200-300 rpm,the motors can be started on no load. Thus synchronous motors are used
for these drives. These motors normally run at 1000-1500 rpm and gears are used to reduce the
speed to 200-300 rpm. In the chemical method of pulp making, the logs of wood are
continuously beaten by the beaters at the time of treatment with alkali. The power requirement of
the beater motors is less than those of grinder motors but these motors require high starting
torque. Therefore, slip ring induction motors with gears are used to drive these beaters at about
150-200 rpm.
BELT CONVEYRS: Normal starting current, high starting torque (double cage) squirrel cage
motors with direct-on-line starters are used for conveyor drives because they have often to start
with full load.
Compressors: Wound rotor induction motors and synchronous motors are generally used on
large size machines. Squirrel cage motors are used for small compressors only.
Blowers-fans: The squirrel cage induction motors and synchronous motors are used for driving
blowers and fans.
Pumps: Centrifugal pumps are driven by squirrel-cage induction motors or synchronous motors.
Reduced voltage starters can be used because of low starting torque requirements.
Machine tools: Squirrel cage motors are normally used for them.
Jaw crushers: Belted slip ring induction motor is almost invariably used as, very often, the
motor has to start against heavy load or a stuck crusher. Pipe ventilated motors should be used so
that supply of cool and clean ventilating air can be ensured even industry dusty atmosphere.

1.7 Types of Industrial Loads - Continuous, Intermittent and

Variable loads

How do you classify industrial loads? Explain briefly.

Explain classes of duty cycle with diagrams
. Draw the speed–torque characteristics, giving examples with justification, of the
following types loads:
i. Constant torque
ii. Constant power
 1.8 Load Equalization
8. Write short note on Load equalization.
OR
What is load equalization? Briefly explain how it is carried out.
OR
What is meant by load equalization? Derive the expression for instantaneous motor
torque.
 The temperature rise of a motor after a continuous run on full load is 40ºC.The heating
time constant is 100minutes. How long can the motor be run at twice the continuously
rated output without overheating? Efficiency of motor is maximum on full load.
250V DC shunt motor with constant field excitation drives a load whose torque varies as the
square of the speed. The armature current is 20A, when the motor is running at 500rpm. Find
the speed of the motor when running with a 25Ω resistor connected in series with the armature.
Neglect motor losses.

A 220V D.C series motor takes an armature current of 20A at 800rpm. Calculate the speed
if a diverter 0.4Ω is connected in parallel with series field and the motor current remains
20A. Armature resistance 0.5Ω, series field resistance 0.2Ω.

Discuss the characteristics and suitability of single phase AC series motors used as
drivers.
Give the equation for developed torque in D.C motor and show the variation of
armature torque with armature current.