CHAPTER 10

Electromagnetic C o m p a t i b i l i t y (EMC)

@ 1 INTRODUCTION 213

@ 2 REGULATIONS AND STANDARDS 214

@ 3 EMC BEHAVIOUR OF VARIABLE-SPEED DRIVES 215

@ 4 INSTALLATION RULES 217

@ 5 THEORETICAL BACKGROUND 220

®iN 6 ADDITIONAL GUIDANCE ON CABLE SCREENING

FOR SENSITIVE CIRCUITS 223

INTRODUCTION

GENERAL electromagnetic interference, and in compliance with rele-

vant regulations. Of necessity, only general guidelines have
The purpose of this Chapter is to set out the necessary con- been provided, but since real installations have a wide
siderations for system designers and others when incor- variety of detailed requirements, explanation of the under-
porating electronic variable-speed drives into complete lying principles is given to allow the designer to cope with
machines and systems without encountering problems with specific situations.
214 General
INTRODUCTION:

PRINCIPLES OF EMC which correspond to radiofrequencies beginning at about

100 kHz, and low-frequency effects. Broadly speaking, low-
All electrical equipment generates some degree of electro- frequency effects operate only by electrical conduction,
magnetic emission as a side effect of its operation. It also whereas high-frequency effects may be induced and operate
has the potential to be affected by incident electromagnetic at a distance without a physical connection. Of course there
energy. Equipment using radiocommunication contains is no precise dividing line between the two, and the larger the
intentional emitters and sensitive receivers. geometry of the system, the lower the frequency at which
induction becomes effective. However, this division is
The basic principle of EMC is that electromagnetic emission helpful in understanding the principles.
of electrical equipment, whether intentional or unintentional,
must not exceed the immunity of associated equipment. This
means that controls must be in place on both emission and
immunity. Given the variety and uncertainty of effects and EMC REGULATIONS
situations, some margin of safety must be provided between
these two factors. Regulations exist throughout the world to control intentional
and unintentional electromagnetic emission, in order to pre-
Although all equipment exhibits some degree of emission
vent interference with communications services. The author-
and susceptibility, the limiting factors in most common
ities generally have the power to close down any equipment
environments tend to be related to radio equipment, with its
which interferes with such services.
powerful transmitters and sensitive receivers. Therefore, the
majority of EMC standards are related to the requirements of Many countries have regulations requiring consumer and
radiocommunications systems. other equipment to be tested or certified to meet these
requirements - for example, the FCC rules in the USA and
In principle, EMC covers phenomena over an unlimited
the C-tick system in Australia. The EMC directive of the
range of frequencies and wavelengths. The EU EMC
European Union is unusual in requiting immunity as well as
directive limits itself to a range of 0 to 400 GHz. This range
emission to be certified.
is so wide that a perplexing number of different effects can
occur, and there is a risk that all electrical phenomena It is not possible in a short section to explain all of these
become included in the scope of EMC. This is unhelpful. regulations. Most emission regulations are based on inter-
With current industrial electronic techniques, no significant national standards produced by CISPR, and the three basic
effects occur above 2 GHz. Below 2 GHz, it is convenient to standards CISPR11, CISPR14 and CISPR22 underlie most
separate out effects very crudely into high-frequency effects, other emission standards.

2 REGULATIONS A N D STANDARDS

REGULATIONS regulations 1992 no. 2372). Precise interpretation of this in

the realm of industrial products such as variable-speed drive
The underlying principle of all EMC regulations is modules has caused difficulty, since it is clear that some
that equipment should not cause interference to other drive modules are used as virtually self-contained units,
equipment, and especially to communications systems. In whereas others are built in to other end-user equipment. The
addition, in many countries there is a requirement that module cannot meaningfully be tested without its associated
equipment must be certified in some way to show that it motor, cables and other peripherals. Large fixed installations
meets specific technical standards, which are generally may contain numerous drives and other electronic products,
accepted as being sufficient to show that it is unlikely to and cannot practically be tested against standards which
cause interference. were primarily intended for compact freestanding consumer
products.
Equipment standards are primarily written to specify test
methods and emission limits for self-contained products The European Commission published further guidelines
such as electrical consumer goods and office equipment to the application of the EMC directive in 1997, which
which are basically free-standing units, even if they have the clarify the intention for CE marking under the EMC direc-
capability to interconnect with peripherals and networks. tive. It applies to complete equipment and also to sub-
The EU EMC directive currently applies to 'a product with assemblies which are intended for direct use or installation
an intrinsic function intended for the end user' (UK EMC by the end user, but not to equipment which is exclusively
 Chapter 10.3 215

for incorporation by a manufacturer into another product. Emission standards work by specifying a limit curve for the
These guidelines have no formal legal impact, although they emission as a function of frequency. A measuring receiver
are influential. The EMC directive is being revised during is used with a coupling unit and antenna to measure voltage
2000-2001 under the SLIM initiative, and will in future or electric field. The receiver is a standardised calibrated
incorporate the main thrust of the guidelines. device, which simulates a conventional radio receiver.
Most drive manufacturers have chosen to test their prod- Immunity standards are rather diverse because of the many
ucts in representative arrangements, against harmonised different electromagnetic phenomena which can cause inter-
European standards, and to apply the CE mark. It is, how- ference. The main phenomena tested for are:
ever, equally valid to offer a drive module without CE
marking under the EMC directive (although it will normally • electrostatic discharge (human body discharge)
carry the CE mark under the low-voltage directive - elec- • radiofrequency field (radio transmitter)
trical safety), provided it is stipulated that it is intended • fast transient burst (electric spark effect)
solely for incorporation into other equipment, and that the • surge (lightning induced)
installer takes responsibility for the EMC compliance of the
end product. The purchaser should expect to be provided There are very many more tests available; those listed are the
with comprehensive EMC data on the module, and clear required tests under the CENELEC generic standards.
guidelines on how it should be installed. The EMC com-
The most important standards for drive applications are the
pliance of the end product cannot be taken for granted even
following:
where all of the constituent parts are CE marked under the
EMC directive or otherwise shown to meet relevant stan-
dards. There is always the possibility of summation of IEC61800-3 and
emissions, or other kinds of interaction. However, it may be EN61800-3 - power drive systems(contains emission and
possible to obtain certification for the end product through immunity requirements)
the technical construction file route on the basis of con- EN50081-2 - generic emission standard for the
formity of the subassemblies to specific standards. In prac-
tical terms, if the end product is a fixed installation where the industrial environment
legal requirement is no more than to meet the essential EN50082-2 - Genetic immunity standard for the
requirements of the EMC directive, that the equipment industrial environment (to be replaced by
should neither cause nor suffer from electromagnetic inter- IEC and EN61000-6-2)
ference, then a combination of compliant subassemblies is
most unlikely to cause interference and therefore very likely
The product standard IEC61800-3 applies in principle to
to meet the requirements.
variable-speed drive modules where they are sold as end
products. There are however many cases where the drive
STANDARDS will be incorporated into an end product which is not in itself
a power drive system and is more likely to fall into the scope
Standards with worldwide acceptance are produced by the of the generic standards. In this case it is the generic stan-
International Electrotechnical Commission (IEC). Standards dards which are of interest. The permitted levels are gen-
for application under the EU EMC Directive are European erally similar, except that IEC61800-3 defines a special
harmonised standards (EN) produced by CENELEC. Every environment where the low-voltage supply network is
effort is made to keep these two families of standards in line, dedicated to nonresidential power users, in which case
and most of them have the same number and identical relaxed emission limits apply. This can permit useful
technical requirements. There are some exceptions. economies in input filters.

3 EMC BEHAVlOUR OF VARIABLE-SPEED DRIVES

IMMUNITY Control Techniques' drives meet them without any special

precautions such as screened signal wires or filters except for
Most drives can be expected to meet the immunity require- particularly fast-responding inputs such as data links and
ments of the CENELEC generic standard EN50082-2. incremental encoder ports.
216 EMC BEHAVIOUROF VARIABLE-SPEEDDRIVES: Immunity

Table 10.1 Interference effects of extreme circumstances

Situation Effect Cure

Very inductive D.C. loads such as spurious drive trip fit suppression to brake coil, or
electromagnetic brakes, without when brake released move wiring away from drive wiring
suppression, and with wiring or applied
running parallel to drive control wiring
High radiofrequency field from drive malfunction when provide RF screening, or move
powerful radio transmitter transmitter operates to a location further from the
(e.g. adjacent to aircraft nose) transmitter antenna
Severe lightning surges due to drive trip or damage from provide additional high-level surge
exposed low-voltage power lines overvoltage suppression upstream of drive

The standard sets levels corresponding to a reasonably harsh The drive itself is not an important source of direct emis-
industrial environment. However, there are some occasions sion, because its dimensions are much less than a half
where actual levels exceed the standard levels, and inter- wavelength over the relevant frequency range. There may be
ference may result. Specific situations that have been strong electric and magnetic fields close to the drive hous-
encountered are given in Table 10.1. ing, but they diminish rapidly, by an inverse cube law, with
increased distance from the drive. However, the wiring
connected to the drive can be widespread and is likely to be
long enough to form an effective antenna for the frequencies
LOW-FREQUENCY EMISSION generated by the drive.

Drives generate supply frequency harmonics in the same The power output connections of a drive carry the highest
way as any equipment with a rectifier input stage. Supply
level of high-frequency voltage. They can be a powerful
harmonics are discussed in detail in Chapter 9. Harmonics source of electromagnetic emission. Since the cable con-
generated by an individual drive are most unlikely to cause necting the drive to the motor is a dedicated part of the
interference, but they are cumulative so that an installa-
installation, its route can be controlled to avoid sensitive
tion containing a high proportion of drive loads may cause circuits, and it can be screened. Provided the screen is
difficulties. connected correctly at both ends, emission from this route is
then minimised.
Apart from supply harmonics, emission also occurs as
a result of the switching of the power output stage over
The power input connections of a drive carry a high-
a wide range of frequencies which are harmonics of the
frequency potential which is mainly caused by the current
basic switching f r e q u e n c y - that is, size times the supply
flowing from the drive output terminals to earth through
frequency for a six-pulse D.C. drive, and the PWM carrier
the capacitance of the motor cable and motor windings to
frequency for a PWM drive. This covers a range extend-
earth. Although the voltage level here is rather lower than at
ing from 300 Hz, for D.C. drives, up to many MHz for A.C.
the output, control measures may be needed because these
drives. Unwanted electromagnetic coupling is relatively
terminals are connected to the widespread mains supply
unusual at frequencies below about 100 kHz. Few stand-
network. Most commonly a radiofrequency filter of some
ards set limits in that range, and interference problems are
kind is installed here.
unusual.
The control terminals of the drive carry some high-
frequency potential because of stray capacitance coupling
HIGH-FREQUENCY EMISSION within the drive. This is usually of no consequence, but
screening of control wires may be required for conformity
The power stage of a variable-speed drive is a potentially with some emission standards.
powerful source of electromagnetic emission (noise)
because of the high voltage and current which is subject to Figure 10.1 summarises the main emission routes for high-
rapid switching. Thyristors are relatively slow switching frequency emission. Note that the current paths are in the
devices, and this limits the extent of the emission spectrum common mode, i.e. the current flows in the power con-
to about 1 MHz; with IGBTs the spectrum may extend to ductors and returns through the earth. Series-mode paths are
about 50 MHz; If attention is not paid to installation guide- relatively unimportant in high-frequency EMC.
lines then interference is likely to occur in the 100 kHz to
10 MHz range where emission is strongest. Since the return currents in the common mode all flow in the
earth wiring, earthing details are particularly important for
This frequency range is lower than that associated with good EMC. Much of the installation detail is involved with
personal computers and other IT equipment, which tend controlling the earth return paths and minimising mutual
to cause direct radiated emission associated with the inter- inductances in the earth system, which cause unwanted
nal microprocessor clock and fast digital logic circuits. coupling.
 C h a p t e r 10.4 217

direct emission

I,.
power line
current
TIT motor

/, I

power line potential

!
!
zapacl[anc;e
earth current earth current of windings
to frame

parasitic earth potential

,..n
Vl

stray earth
current

Figure 10.1 High-frequency emission routes

4 INSTALLATION RULES

EMC RISK ASSESSMENT Proximity sensors using high-frequency techniques, such

as capacitance proximity sensors.
When a drive is to be installed, a cost-effective approach to
EMC is to initially assess the risk of interference problems
arising. This is in addition to considering any legal constraints BASIC RULES
on emission levels. Most industrial electronic instrumenta-
tion and data-processing equipment has good immunity, and For installations where it is known that no particularly
can operate with drives with only modest precautions to sensitive equipment is located nearby, and where no specific
control emission. Some specific types of equipment have emission limits are in force, some simple rules can be
been found to be susceptible to interference from drives. The applied to minimise the risk of interference caused by a
list shows some product families which call for special drive. The aspects requiring attention are as follows.
attention:
Segregation
• Any equipment which uses radiocommunication at fre-
The drive supply and output cables must be segregated
quencies up to about 30 MHz. (Note that this includes
from cables carrying small signals. Crossing at right angles
AM broadcast and short-wave radio, but not FM, TV or
is permitted, but no significant parallel runs should be
modern communications services which operate at much
allowed and cables should not share cable trays, trunking or
higher frequencies.)
conduits unless they are screened and the screens correctly
• Analogue instrumentation using very low signal levels,
terminated.
such as thermocouples, resistance sensors, strain gauges
and pH sensors. A practical rule of thumb has been found to be" no parallel
• Other analogue instrumentation using higher levels (e.g. run to exceed 1 m in length if spacing is less than 300 mm.
0-10 V or 4 - 2 0 m A ) - only if very high resolution is
required or cable runs are long. Control of Return Paths, Minimising
• Wideband/fast-responding analogue circuits such as Loop Areas
audio or video systems (most industrial control systems
are intentionally slow acting and therefore less suscep- The power cables should include their corresponding earth
tible to high-frequency disturbance). wire, which should be connected at both ends. This mini-
• Digital data links, only if the screening is impaired, or mises the area of the loop comprising power conductors
not correctly terminated, or if there are unscreened runs and earth return, which is primarily responsible for high-
such as rail pick-up systems. frequency emission.
218 INSTALLATIONRULES: Basic Rules

Earthing installation, then more attention must be given to the

allocation and arrangement of earth connections. The
The main drive power circuit earth loop carries a high level concept of separate power earth and signal earth has
of radiofrequency current. As well as minimising its area as been discredited in EMC circles recently, but is valid in
described above, these earth wires should not be shared with widely spread installations where a good equipotential
any signal-carrying functions. There are two possible earth structure is not available.
methods for minimising shared earthing problems, depend-
ing on the nature of the installation: Figure 10.2 illustrates how two earthed circuits in a system
may have different noise potentials. Local circuits earthed to
(i) Multiple earthing to a g r o u n d p l a n e - if the installa- either point will work correctly (circuits 1 and 2), but if a
tion comprises a large mass of metallic structure then single circuit is earthed at both points then it will experience
this can be used to provide a ground plane. All circuit a noise potential which might cause disturbance (circuit 3).
items requiring earth are connected immediately to the
The solution is to nominate one earth point as the signal
metal structure by short conductors with large cross-
earth and use it as the sole reference point for shared signal
sectional area, preferably fiat, or by direct metal-to-
circuits, as illustrated in Figure 10.3. This prevents the
metal assembly. Screened cables have their screens
creation of loops for the noise current. The disadvantage is
clamped directly to the structure at both ends. Safety
that this situation is difficult to manage in a large complex
earth connections are still provided by copper wire
installation, and sneak paths can easily arise which cause
where required by safety regulations, but this is in
addition to the EMC ground plane. problems and are difficult to trace. Alternatively, circuit 3
must be designed to be able to accept a high earth potential
(ii) Dedicated earth points, earth segregation - if no difference, for example by using optical isolation between
earthed metallic structure exists throughout the the circuits associated with the motor and the drive.

motor

earth potential

earth current

--"--local local"
earth 1 stray earth earth 2
current

circuit 1 circuit 2
| L,,, ,,
!

Figure 10.2 Earth potentials and their effect on signal circuits

motor

earth potential
,,

earth current

signal local
earth earth
.... ,. . . . .

circuit 1
iu ........

!
t
,~,circuit 3
,,, ! ~
i circuit2
- ! ' '']
J
Figure 10.3 Use of single signal earth
 Chapter 10.4 219

SIMPLE PRECAUTIONS AND FIXES full precautions must be observed. The drive installation
guide should give these precautions in full detail
There are some simple techniques, which can be used to for specific drives. The following outlines the essential
reduce high-frequency emission from a drive at modest principles:
cost. These techniques should preferably be applied in
conjunction with the basic rules given above, but they may (i) A suitable input filter must be fitted. The filter spe-
also be useful as a retrospective cure for an interference cified by the drive manufacturer should be used, and
problem. any limits on motor cable length or capacitance and on
The single most effective measure that can be taken is to fit PWM switching frequency adhered to. Many filters
capacitors between the power input lines and earth, as illu- which are not specifically designed for this application
strated in Figure 10.4. This forms a simple RFI filter, giving have very little benefit when used with a drive.
a reduction of typically 30 dB in overall emission into the (ii) The filter must be mounted on the same metal plate as
supply network, sufficient to cure most practical problems the drive, and make direct metal-to-metal contact, to
unless exceptionally sensitive equipment is involved. minimise stray inductance. Any paint or passivation
Emission from the motor cable is not affected by this mea- coating must be removed to ensure contact. A back
sure, so strict cable segregation must still be observed. The plate of galvanised steel, or other corrosion-resistant
capacitors must be safety types with voltage rating suited to bare metal, is strongly recommended.
the supply voltage with respect to earth. Earth leakage cur- (iii) The motor cable must be screened. A copper braid
rent will be high, so a fixed earth connection must be pro- screen with 100 per cent coverage works best, but steel
vided. The values shown represent a compromise between wire armour is also very effective, and steel braid is
effectiveness at lower frequencies and earth leakage current. adequate.
Values in the range 100 nF to 2.2 laF can be used. (iv) The motor cable screen must be terminated to the
drive heat sink or mounting plate, and to the motor
The length of the motor cable affects emission into the frame, by a very low inductance arrangement. A gland
power line, because of its capacitance to earth. If the motor giving 360 ° contact is ideal, a clamp is also effective
cable length exceeds about 50 m then it is strongly recom- and a very short pigtail is usually tolerable but the
mended that these capacitors are fitted as a minimum drive instructions must be adhered to.
precaution. (v) The input connections to the filter must be segregated
A further measure, which reduces emission into both supply from the drive itself, the motor cable and any other
and motor circuits, is to fit a ferrite ring around the output power connections to the drive.
cable power conductors, also illustrated in Figure 10.4. The (vi) Interruptions to the motor cable should be avoided if
ring fits around the power cores but not the earth, and is possible. If they are unavoidable then the screen
most effective if the conductors pass through the ring three connections should be made with glands or clamps to
times (a single pass is shown, for clarity). The ferrite should an earthed metal plate or bar to give a minimum
be a manganese-zinc power grade. Care must be taken to inductance between screens. The unscreened wires
allow for the temperature rise of the ferrite, which is a should be kept as short as possible, and run close to the
function of motor cable length; the surface temperature can earthed plate. Figure 10.5 illustrates an example where
reach 100°C. an isolator switch has been incorporated.

With some drives, the control wiring needs to be screened

FULL PRECAUTIONS with the screen clamped to the heat sink or back plate. The
installation instructions should be adhered to in this res-
If there is known sensitive equipment in the vicinity of pect. Omitting this is unlikely to cause interference prob-
the drive or its connections (see list in Chapter 10.4), or if lems, but may cause standard limits for radiated emission to
it is necessary to meet specific emission standards, then be exceeded.

ferrite / 3 turns
rin-" f (1 shown
A L1 .....u for clarity)
T A
L2 drive
I T _
l i T vv !iii!ii
T _1- T
a x 4 7 0 nF
capacitors

Figure 10.4 Some low-cost emission reduction measures

220 THEORETICALBACKGROUND" Emission M o d e s

isolator

from the to the

drive motor

coupling bar or plate

m

(if required)

Figure 10.5 Managing interruptions to motor cable

5 THEORETICAL BACKGROUND

EMISSION MODES of 16 V with duration lOOns in the 1 m of wire. Whether

this causes interference with associated circuits depends on
Although the digital control circuits, switch-mode power their design, but certainly a 16 V, 100 ns pulse is sufficient to
supplies and other fast switching circuits in a modern digital cause a serious error in a digital circuit or a fast acting
drive can all contribute to radiofrequency emission, their analogue one.
suppression is a matter for the drive designer and suitable
Figure 10.1 shows the main emission paths. Because of its
internal measures can keep such emission under control. It is
high voltage, the motor cable is the main potential source of
the main power stage, especially the inverter of a PWM
emission and will be an effective transmitting aerial at a
drive, which is an exceptionally strong source of emission
frequency where its length is an odd number of quarter
because the fast changing PWM output is connected directly
wavelengths. For example, a 20 m cable will be particularly
to the external environment (i.e. the motor and motor cable).
effective at about 3.75MHz and also at 11.25MHz and
This is also the reason for the installation details having a
18.75 MHz. This will be modified somewhat by the presence
major effect on the overall EMC behaviour.
of the motor and by the distance of the cable from sur-
Figure 10.6 shows the main circuit elements of an A.C. rounding earthed objects. In order to prevent this emission,
inverter variable-speed drive. The output PWM waveform the cable is usually screened.
has fast changing pulse edges with typical rise times of the
Figure 10.1 also shows how the high-frequency voltage in
order of 50-100ns, containing significant energy up to
the motor and cable causes current to flow into the earth,
about 30 MHz. This voltage is present both between output
because of their capacitance. The capacitance of a motor
phases and also as a common-mode voltage between phases
winding to its flame may be in the range 1 nF to 100nF,
and earth. It is the common-mode voltage which is pri-
depending on its rating, and the capacitance from the
marily responsible for emission effects, because it results
cable power cores to earth is generally between 100pF
in high-frequency current flowing to earth through the
and 500pF per metre. These values are insignificant
stray capacitances of the motor windings to the motor
in normal sinusoidal supply applications, but cause sig-
flame, and the motor cable power cores to the earth core
nificant current pulses at the edges of the PWM voltage
and/or screen.
wave. The current returns through a variety of paths,
High-frequency current causes unexpected voltage drops in which are difficult to control. In particular, current may
wiring because of the wiring self inductance. The sig- find its way from the motor flame back to the supply
nificance of this can be illustrated by a simple example. A through any part of the machinery, and if it passes through
1 m length of wire has a typical inductance of about 0.8 laH. earth wires in sensitive measuring circuits it may disturb
The output current from a drive to charge the stray capaci- them. Also, a major return route to the drive is through the
tance of a motor winding would be typically 2 A peak with a supply wiring, so any equipment sharing the supply may be
rise time of 100 ns. This current would cause a voltage pulse disturbed.
 Chapter 10.5 221

from to
supply motor
L1 U
L2 v
L3 w

)...s
I
I
I
I
I

/ E

!
I
I
I
,
'/
s
/ %
~j
\
I

I
!
I
l/

i I

" rise time /

Figure 10.6 Main elements of an A.C. inverter drive

return current
circulates in filter
no emission from
screened cable
screened /
cable ~/ _/ "N~

\
power line
current returns /.--.,J J
current alongscr~~ _. L
minimised
stray earth
current minimised
Figure 10.7 The effect of an RFI filter and screened motor cable

Figure 10.7 shows the effect of using a screened motor cable the D.C. smoothing capacitor in the drive. The filter provides
and an input filter. Fields emitted from the motor cable are some series-mode attenuation to control this.
suppressed by the screen. It is essential that both ends of the
screen are correctly connected to earth at the motor and the
PRINCIPLES OF INPUT FILTERS
drive, in order that the magnetic field cancellation property
of the cable gives its benefit. The screened cable also mini-
Figure 10.8 shows the circuit of a typical input filter. The
mises the earth current flowing from the motor frame into
capacitors between lines provide the series-mode atte-
the machinery structure, because of its mutual inductance
nuation, and the capacitors to earth and the inductance
effect. This subject is generally not well understood outside
provide the common-mode attenuation. The inductance is
the EMC profession, and the reader is referred to Chapter
constructed as a common-mode component, which is not
10.6 for a fuller explanation.
magnetised by the main power current, therefore minimising
The input filter provides a low-impedance path from the its physical dimensions. It uses a high permeability core,
earth to the drive input lines, so that the high-frequency which can accept only a very limited unbalance (common-
current returning from the motor cable screen has an easy mode) current.
local return route and does not flow into the power network.
Filters for drives are carefully optimised for the applica-
The primary role of the filter is to suppress common-mode tion. The drive presents an exceptionally low impedance
high-frequency emission from the drive. There is also some source to the filter, which means that conventional general-
series-mode emission because of the nonzero impedance of purpose filters may have little benefit. The usual method of
222 THEORETICALBACKGROUND"Principles of Input Filters

O
L1 LI'

L2
i .o A
w
i L2'

supply °O drive

L3 L3'

TT T TT T1
w
T !
E 71. E
!

Figure 10.8 Basic input filter

specifying a filter is in terms of its insertion loss in a test surrounds a three-phase set then the magnetic field is only
set up with 50 f~ source and load impedance. An alternative caused by the common-mode current, and saturation is
test attempts to be more realistic by using 0.1 f~ source and avoided. The manganese-zinc ferrite exhibits high loss in the
100 f~ load. Neither of these tests correctly represents a drive 1-10 MHz frequency range where motor cable resonance
application, and neither can be used as any more than a very occurs, and this gives useful damping of the resonance and a
rough guide to the suitability of a filter. substantial reduction in the peak current. The loss in the
ferrite does cause a temperature rise, and with long motor
cables the temperature of the ferrite rises until its losses
SCREENED MOTOR CABLES
stabilise, close to the Curie temperature.
The screening capability of screened cable is generally
measured by the parameter ZT, the transfer impedance per FILTER EARTH LEAKAGE CURRENT
unit length. In an ideal cable, any current flowing in the
internal circuit produces no voltage between the ends of the Because of the low source impedance presented by the drive,
cable screen, and conversely current flowing in the screen suitable filters generally have unusually high values of
from an external source produces no voltage in the inner capacitance between lines and earth. This results in a leak-
circuit. These two aspects minimise the emission from the age current to earth exceeding the 3.5 mA which is generally
cable and the immunity of inner signal circuits to external accepted as permissible for equipment which derives its
disturbance, respectively. In practice, the resistance of the safety earth through a flexible connection and/or plug/
screen, its imperfect coverage and other details cause a socket. Most filters require the provision of a permanent
departure from the ideal and a nonzero value of ZT. fixed earth connection with sufficient dimensions to make
the risk of fracture negligible. Alternative versions with low
The transfer impedance is not however the only factor
leakage current may be available, which will have more
involved. The cable exhibits strong internal resonances,
severe restrictions on the permissible motor cable length.
which cause high currents to flow internally. The current is
limited by the natural damping caused by electrical losses in
the cable. Steel sheaths have a higher resistance and there-
fore give better damping than copper sheaths. Steel gives an
FILTER MAGNETIC SATURATION
inferior transfer impedance to copper, but the two factors
With long motor cables the common-mode current in the
largely cancel so that a steel wire armoured cable gives no
filter rises to a level where the high-permeability core of the
greater emission with a drive than a good quality copper
filter inductance becomes magnetically saturated. The filter
braided screened cable.
then becomes largely ineffective. Filters for drive applica-
tions therefore have limits on motor cable length.
FERRITE RING SUPPRESSORS The capacitance of the cable determines the additional cur-
rent loading on the drive and the filter. Screened cables
The use of a ferrite ring as an output suppressor was intro- with an insulating jacket between the inner cores and
duced in Chapter 10.4. The ferrite ring introduces impe- the screen present a tolerable capacitance. Some cables
dance at radiofrequencies into the circuit which it surrounds, have the screen directly wrapped around the inner cores,
thereby reducing the current. Because of its high perme- which causes abnormally high capacitance and therefore
ability the ring will not work if it surrounds a conductor reduces the permissible cable length. This also applies to
carrying power current, due to magnetic saturation, but if it mineral insulated copper-clad cables.
 Chapter 10.6 223

6 A D D I T I O N A L GUIDANCE ON CABLE
SCREENING FOR SENSITIVE CIRCUITS

The subject of signal circuit cable screening is often mis-

understood and it is quite common for such circuits to be E (inner and outer)
-.q

incorrectly installed. This applies to critical signal circuits

for drives, such as analogue speed references and posi-
tion feedback encoders, and also to circuits in other equip-
ment in the same installation as the drive. This section
outlines the principles, in order to assist readers in avoid-
ing and troubleshooting EMC problems in complete
installations. magnetic field

Figure 10.9 Magnetic-field induction in screened cable

CABLE SCREENING ACTION

Correctly used, a cable screen provides protection against

E + IR (outer)
both electric and magnetic fields, i.e. against disturbance • ....

from both induced current and induced voltage. E (inner)

Electric-field screening is relatively easy to understand. The
screen forms an equipotential surface connected to earth,
which drains away incident charge and prevents current
from being induced in the inner conductor. / R

Magnetic-field screening is more subtle. An incident alter- Figure 10.10 Resistance coupling in screened cable
nating magnetic field, which corresponds to a potential
difference between the cable ends, causes EMF to be
induced in both the screen and the inner conductor. Because
the screen totally surrounds the inner conductor, any mag-
netic field linking the screen also links the inner conductor,
so an identical EMF is induced in the inner. The voltage ,
I
,
I
_L_
differential between the inner and the screen is then zero. OV(1) I I 0 V (2)
I I
This is illustrated in Figure 10.9. I I
optional ~,
....
~ , optional
earth -- earth
In order for this benefit to be realised, it is essential that the
screen be connected at both ends. Although high-frequency Figure 10.11 Correct screened cable connection for high-
engineers routinely observe this practice, it is common in frequency screening effect
industrial control applications for the screen to be left
unconnected at one end. The reason for this is to prevent the
screen from creating an earth loop, or an altemative earth
path for power-frequency current.
frequencies exceed the cable cut-off frequency, the screen
The problem of the earth loop is specifically a low- should be connected at both ends.
frequency effect. If the impedance of the cable screen is
predominantly resistance, as is the case at low frequencies,
then any unwanted current flowing in the screen causes a CABLE SCREEN CONNECTIONS
voltage drop which appears in series with the wanted signal.
This is illustrated in Figure 10.10. The conclusion of this is that for all but low-frequency
interference, the screen should be used as the return path for
At higher frequencies the cable screen impedance is pre- data, as shown in Figure 10.11. Whether the screen is con-
dominantly inductive. Then the mutual inductance effect nected to earth at each end, or to the equipment metalwork,
takes over from resistance, and the voltage induced in the is less important than that it be connected to the circuit
internal circuit falls. Further, the skin effect in the screen common terminals. The recommendations of the equipment
causes the extemal current to flow in the outer surface so manufacturer should be followed. It is usual to clamp the
that the mutual resistance between inner and outer circuits screen to a metallic structural part because this gives the
falls. The net result is that at high frequencies the cable least parasitic common inductance in the connection. A
screen is highly effective. A cut-off frequency is defined at pigtail causes a loss of screening benefit, but a short-tail (up
the point where the injected voltage is 3 dB less than at D.C., to 20 ram) may be acceptable for drive applications where
and is typically in the order of 1-10 kHz. Where disturbing screening is not critical.
224 ADDITIONAL GUIDANCE ON CABLE SCREENING" C a b l e S c r e e n C o n n e c t i o n s

The screened cable should ideally not be interrupted

throughout its run. If intermediate terminal arrangements are
included with pigtails for the screen connections, every
pigtail will contribute an additional injection of electrical
noise into the signal circuit. If interruptions are inevitable, earth 1 _.
i capacitor
+ beed resistor

either a suitable connector with surrounding screening shell

should be used or a low-inductance bar or plate should be I earth 2
used for the screen connection as in Figure 10.5. Suitable Figure 10.13 Use of capacitor for high-frequency earthing
hardware is available from suppliers of terminal blocks. while blocking power-fault current

Low-frequency interference associated with earth loops is

not important for digital data networks, digital encoder
signals or similar arrangements using large, coarsely quan-
tised signals. It is an issue with analogue circuits if the
bandwidth is wide enough for errors to be significant at
the relevant frequencies, which are primarily the 50/60 Hz signal
I
power-line frequency. Many industrial control systems
have much lower bandwidths than this and are not affected =OV
I
by power frequency disturbance. Servodrives however do I
respond at power-line frequency, and can suffer from noise 0 V to other circuits
I
I
and vibration as a result of power frequency pick up. The I

cable screen should not be used as the signal return con- I optional
•-- earth
ductor in this case. The correct solution for wideband sys-
tems is to use a differential input. Analogue differential
inputs give very good rejection of moderate levels of
common-mode voltage at power-line frequency. This signal
rejection falls off with increasing frequency, but then the
I ~OV
screening effect of the cable takes over. The combination of I I
I I
differential connection and correct cable screening gives I I
good immunity over the entire frequency range. A typical optional
I
i
I
I
arrangement is illustrated in Figure 10.12. earth optional
"- earth
There are electrical safety issues associated with earthing (note-multiple earthing may cause
disturbance at power frequency)
decisions. A galvanically isolated port with the screen con-
nected only to the isolated common rail prevents low-
frequency circulating current, but carries the risk that a fault
elsewhere might make it electrically live and a hazard to •• +- signal
maintenance staff. Cable screens should be earthed in at least
I ~ov
one place for every disconnectable length, to prevent a I !
length becoming isolated and live. This approach is used in I I
I !
the Interbus industrial data network, where each link is !
optional , !
earthed at one end and isolated at the other. earth ' , optional
-- earth
Earthing at both ends carries the risk that an electrical fault
might cause excessive power current to flow in the cable
screen and overheat the cable. This is only a realistic risk in
large-scale plant where earth impedances limit power-fault D /
current levels. The correct solution is to provide a parallel I
power earth cable rated for the prospective fault current. An 5\
alternative is to provide galvanic isolation, although this
OV

differential optional I , optional

. amplifier earth --" --" earth
input
Figure 10.14 Preferred connection methods
_1__ ' ' _L
I I a l o w - s p e e d d i g i t a l c i r c u i t - no special
0 V (1) i i 0 V (2)
I I precautions, o p e n w i r i n g
I I b low-bandwidth l o w - p r e c i s i o n analogue
optional , , optional
earth
. . . .

-- earth c i r c u i t - screened
c wide bandwidth and/or high-precision
Figure 10.12 Correct screened cable connection for analogue c i r c u i t - screened, differential
high-frequency screening and d w i d e b a n d w i d t h d i g i t a l data
low-frequency interference rejection c i r c u i t - screened, differential
 Chapter 10.6 225

carries the risk of a transiently high touch potential at the etc. The capacitor must be rated at the mains voltage. It is
isolated end during a fault. usual to provide a parallel bleed resistor to prevent accu-
mulated static charge. Figure 10.13 illustrates this capacitor
Some galvanically isolated inputs include a capacitor to
arrangement.
earth, which provides a high-frequency return but blocks
power-frequency fault current. This is actually a require-
ment of certain bus systems. In principle such a capacitor RECOMMENDED CABLE ARRANGEMENTS
should not be necessary, but it may be required to ensure
immunity of the isolated input to very fast transients, or to Figure 10.14 summarises the recommended connection
suppress radiofrequency emission from microprocessors methods for several cases.