Vsd3

TECHNICAL
REPORT
NO. 1

SECOND
EDITION VARIABLE SPEED DRIVES
AND MOTORS
A GAMBICA/REMA TECHNICAL REPORT

Motor Insulation Voltage Stresses
Under PWM Inverter Operation

ASSOCIATION FOR ROTATING ELECTRICAL
INSTRUMENTATION, CONTROL, AUTOMATION MACHINES ASSOCIATION

Developed by the joint GAMBICA/REMA Working Group

Foreword Scope
This Technical Report has been produced to This report provides information on the basic
meet a demand for an authoritative guide on operating principles of PWM inverters with an
good practice in the application of motors on emphasis on the type using IGBT switching
PWM inverter supplies with respect to voltage devices and the interface with the motor. It deals
stresses. It is the result of a study carried out by with transmission line effects in the supply cables
GAMBICA and REMA taking note of well between the inverter and the motor and the
established fundamental theory, technical papers impact of voltage peaks at the motor terminals.
and carrying out specific investigations.The Advice is given on the capability of motors to
information given, while it can be applied to withstand inverter supplies and when
motors and inverters in general, is specific to preventative measures to control peak voltage
current generation products of member and/or excessive capacitive currents should be
companies. taken.
The Technical Report principally considers Supplies considered are low voltage up to 690V
voltages developed by the voltage source PWM a.c.
inverter, supply line effects and motor insulation
systems. It supplements IEC 60034-17: 1998
(Guide for the application of cage induction
motors fed from converters) which provides
additional information on other important
aspects including: torque de-rating, additional
losses, noise and maximum safe operating speed.
This report covers motors and inverters installed
with a separate cabling system connecting the
components together, it is not wholly applicable
to drives integrated into a motor design, generally
now available up to 7.5kW - see section 7.2.
This information supplements the
GAMBICA/REMA Technical Guide. (See 10a)
The references together with a useful list of
further reading are included in sections 9 and 10
of this report.

(i)

Summary
x Motor winding insulation experiences x For supply voltages less than or equal to
higher voltage stresses when used with 500V a.c., most standard motors are
an inverter than when connected immune to these higher stresses.
directly to the a.c. mains supply. x For supply voltages over 500V a.c., a
x The higher stresses are dependent on motor with an enhanced winding
the motor cable length and are caused by insulation system is required.
the interaction of the fast rising voltage Alternatively, additional components can
pulses of the drive and transmission line be added to limit the voltage stresses to
effects in the cable. acceptable levels.

Motor Selection
For nominal supply voltages less than or equal to 500V a.c.

Select a standard motor from REMA manufacturers. No further considerations are necessary for
PWM inverters.
For motors from other suppliers, where the motor cable length exceeds 10m, the pulse
withstand capability should be established by reference to the supplier.The permitted
voltage/rise-time characteristic should equal or exceed curve A shown in Figure 17.

For nominal supply voltages greater than 500V and up to 690V a.c.

Select a motor with an enhanced insulation system available from REMA manufacturers.The
additional cost of the enhanced insulation is typically 10-20% of the standard motor cost.
Motors with enhanced insulation systems may be slightly de-rated compared to standard
motors. No further insulation considerations are necessary for PWM inverters.
For motors from other suppliers, the pulse withstand capability should be established by
reference to the supplier.The permitted voltage/rise-time characteristic should equal or exceed
curve B shown in Figure 17.

Notes:
i) Motor frame selection must be appropriate for the application and duty.
ii) Insulation requirements may be affected by the application specific notes in section 7.
iii) The pulse withstand requirements of curves A and B in Figure 17 both exceed the minimum capability
specified in IEC 60034-17:1998, which is also shown in Figure 17.

(ii)

Contents
1. INTRODUCTION 1
2. PRINCIPLES OF PWM DRIVES 2
2.1 General 2
2.2 Terminal voltage 3
2.3 Winding voltage 6
3. THE INDUCTION MOTOR AND INSULATION SYSTEMS 7
3.1 General
3.2 Standard motor insulation systems 7
3.3 Enhanced motor insulation systems 9
4. INSULATION BEHAVIOUR 9
5. PRACTICAL INSULATION REQUIREMENTS 11
6. ALTERNATIVE PREVENTATIVE METHODS
6.1 General 12
6.2 Output reactors 12
6.3 Voltage limiting filter (dv/dt filter) 12
6.4 Sinusoidal filter 12
6.5 Motor termination unit 13
6.6 Relative characteristics of preventative measures 13
6.7 Cost comparisons for preventative measures 14
7. APPLICATION SPECIFIC NOTES 14
7.1 Cable capacitance effects 14
7.2 Combined inverter/motor solutions 14
7.3 Applications with frequent or continuous braking duties 14
7.4 Active front end (sinusoidal rectifier) considerations 15
7.5 Drives with special control strategies 15
7.6 Explosive atmosphere approval 15
7.7 Applications with ‘very long’ cable lengths 15
8. CONCLUSIONS 15
9. REFERENCES 16
10. FURTHER READING 16

(iii)

1. INTRODUCTION
Variable speed a.c. drives as illustrated below are winding insulation failures which appear to have
used in ever-increasing numbers because of their been caused by use with a PWM drive [1].
well-known benefits for energy efficiency and Many of the failures reported have been in
flexible control of processes and machinery using systems using a.c. supply voltages in the region of
low-cost maintenance-free a.c. motors.Virtually 525V to 575V and above, which are most widely
all a.c. drives use power switching techniques and used in North America and South Africa.There is
generate high rates of change of voltage. little evidence of such effects occurring with
Most modern a.c. drives use voltage-source standard European LV supplies.The number of
PWM inverters with very fast-switching power failures reported is small, but sufficient to cause
semiconductor devices such as Insulated Gate concern in some application areas. As a result
Bipolar Transistors (IGBT). some users and consultants may be specifying
The fast-changing voltage generated by such a counter measures which are costly and are often
drive causes some increase in the voltage stress unnecessary.This report aims to give a clear
on the motor winding insulation - although this explanation of the effects involved and
report will show that in most cases the additional straightforward advice on what precautions are
stress is well within its capability. For some years required to avoid problems.
there have been occasional reports of motor

1

2. PRINCIPLES OF PWM DRIVES
DC Link Voltage
Output Voltage

Ud 1.35 x UN

Diodes
L IGBT Transistors
3 Ph Supply UN

DC Link
U
C
Ud V Motor
W PE
Optional
DC chokes Protective Earth Conductor
L

External
ground
bonding
The incoming 3 phase supply is rectified
by the diode bridge arrangement to
provide an approximately constant DC
voltage source. Motor Current

The IGBT transistors are controlled to give a stream
of variable width pulses with amplitude Ud to the motor.
The control algorithm ensures that a "sine weighted"
PWM waveform is achieved. All pulse edges are unipolar.

The DC link is used to decouple the input
and output. The large capacitance provides
some smoothing of the voltage along with
a reservoir of energy.

Figure 1: Essential elements of a PWM inverter drive

2.1 General
Voltage source PWM inverter drives are the most deliberately reduced switching frequency in
common type of low voltage inverter drives Figure 2 below.
which are currently in use.The process of
obtaining the required frequency involves
converting the incoming alternating voltage to DC U
by means of a rectifier, smoothing the DC in an Voltage
intermediate DC link with capacitive energy +Ud

storage, then inverting back to an alternating
current. Standard texts [2] provide detailed
Time
explanations, but Figure 1 above illustrates the
basic principles:
The pulsed output voltage is applied to the motor Current

and the resultant current, modified by the
-Ud
significant motor inductance, consists mainly of the
fundamental sinewave at the required operating
frequency with a superimposed low magnitude
ripple component based on the switching
frequency. Both voltage and current over one Figure 2.: PWM inverter output voltage and current
cycle are illustrated in simplified form with waveforms.

2

Drive designers generally aim to use the highest contributary to steep wavefronts.The values
practical switching frequency, since this has a listed in Table 1 illustrate the relative values of
variety of benefits including reducing the audible pulse rise time compared to the power
noise from the motor. Faster power switching frequency and switching frequency period.
devices give shorter rise-times, which are

Frequency (Hz) Period/time
Power frequency 50 20ms
Pulse switching frequency 3000 333µs
Pulse rise-time - 100ns

Table 1:Typical frequencies and times

2.2 Terminal Voltage

The PWM pulse rise-times are so short that their where L (Henries) and C (Farads) are the
propagation along the motor cable to the motor inductance and capacitance per metre,
can change the pulse shape and may produce a respectively.The velocity of propagation of a
voltage overshoot.The cable can be considered pulse in a typical PVC-insulated cable is about
as a transmission line, i.e. a long string of 1.7 x 108m/s (i.e. in 100ns the pulse travels only
distributed series/parallel connected, inductor- 17m). It varies little over the variety of cable
capacitor sections as shown in Figure 3. For types in general use, since it is determined
simplicity only one phase is represented. mainly by the permittivity of the internal
At each pulse edge, the drive has to charge the insulating material.
inductance and capacitance of the cable, so a The essential features of how a pulse propagates
pulse of energy is delivered into the cable. along the motor cable are illustrated in Figure 4
Transmission line theory shows that the pulse and Figure 5 on the following pages. More
travels at a speed equal to detailed analysis is given in reference [1].
( 1 ) m/s
√LC

Inverter Cable
L1 L2 L3 Ln

U C1 C2 C3 Cn Motor

Figure 3: Distributed inductance and capacitance of cable

3

(a) Time t=tr (i.e. at the end of the rise-
time of the pulse)
Each pulse represents one ‘edge’ in the PWM
tr is the pulse waveform.
rise-time
The pulse enters the drive end of the cable at time
+Ud Voltage
t=0 and rises to Ud in time tr. In this idealised
DRIVE example tr is smaller than the cable propagation
MOTOR
time tp, corresponding to a case where the cable
tr
length exceeds about 30m.
Voltage pulse
leaving drive

(b) Time t=tr+tp (i.e. after one cable
+2Ud
tp is the time of propagation propagation time)
of the pulse along the line
length
The pulse travels from the drive along the cable to
+Ud
the motor. When it reaches the motor it is reflected,
because the motor’s high frequency impedance is
tr + tp tr + tp higher than that of the cable. This causes the voltage
to rise towards twice its original peak voltage. The
voltage can be represented as having two
Voltage pulse
reflected at motor components, the forward pulse and the reflected
pulse, each having magnitude +Ud.

(c) Time t=2tr+2tp (i.e. after two cable
+2Ud propagation times)
The reflected pulse returns to the drive and because
+Ud the drive’s impedance is very low, the pulse is
reflected in a negative sense. This reflection does not
2tr + 2tp 2tr + 2tp appear in the voltage waveform at the drive
terminals because the drive clamps the voltage to
Reflected Equivalent Ud. A negative current pulse results, which is
current reflected voltage
Ud transformed into a negative voltage pulse as it
Returned pulse returns along the cable.
reflected at drive

+2Ud (d) Time t=2tr+3tp (i.e. after three cable
propagation times)
+Ud The second reflection, which returns from the drive in
the reverse polarity, is also reflected as in stage (b)
2tr + 3tp 2tr + 3tp and is doubled at the motor. It counteracts the
original motor voltage increase. If the cable is short
Second reflection so that 2tp is less than tr, the voltage never reaches
cancels the first
2Ud. However, with a longer cable as illustrated here,
the reflection arrives too late to reduce the peak.

Figure 4: Idealised pulse propagation in motor cable

4

In the idealised case the reflections would cause
U
the voltage to oscillate indefinitely. In practice, the Completion of
voltage rise-time is increased due to high first reflection
t2 + tp
frequency losses in the cable, and the waveforms
become rather rounded and less clear-cut than
the idealised waveforms illustrated in Figure 4.
Also due to high frequency losses, the peak
voltage oscillations over one pulse cycle
Ud
progressively decay and stabilise at the DC link
voltage. Figure 5 shows the waveform with 42m
of cable and derives its main features from the
process described in Figure 4.
0

Motor peak voltage is therefore a function of Pulse first arrives Completion of
third reflection
at motor t=tp
both cable length and rise time. For example, t = 2tr +3tp

with 20m of cable with a velocity of 1.7 x
t
108m/s, any pulse with a rise-time less than
235ns can be expected to increase by nearly Figure 5: Features of a typical pulse waveform
100%. Figure 6 shows some typical measured at the motor terminals - cable length 42m
voltage waveforms (based on a 460V test supply)
which show this effect in practice. Even with 4m
of cable some overshoot is apparent. With 42m
the overshoot is virtually 100%.

U U U

PEAK VOLTAGE 664 V PEAK VOLTAGE 824 V PEAK VOLTAGE 1300 V

200 V / div 200 V / div 400 V / div
400 ns / div 400 ns / div 2 s / div

t t t
a) Cable Length = 0.5m b) Cable Length = 4m c) Cable Length = 42m
(Note scale changes)
Figure 6: Motor terminal voltage waveforms for
varying cable lengths

The pulse rise-time is an important factor in the application of each method to the same sample
study of these effects. Since the pulses which waveform whereby it can be seen that the IEC
arrive at the motor terminals are not a method gives a value of approximately twice that
trapezoidal shape, there is no self-evident calculated using the NEMA definition. All values in
definition for the rise-time. Unfortunately the two this report are given in accordance with the IEC
standard bodies, IEC and NEMA, have chosen to method as defined in IEC60034-17 1998.
use different definitions. Figure 7 illustrates the

5

U
U U

90% peak

Ud Ud Ud
90% ultimate value

10% peak 10% ultimate value
t t

a) IEC 60034-17 1998 b) NEMA MGI part 30:1998
Figure 7: Different definitions of rise time
2.3 Winding Voltage
The voltage overshoot has little effect on the coil ends.This effect progressively decays to a
main motor insulation systems between phases uniform voltage distribution in subsequent coils
and from phase to earth, which are designed to due to high frequency inductive and capacitive
withstand large over voltages. However, because losses.
of its short rise-time the voltage overshoot also Dependent on motor and winding parameters
stresses the insulation between turns, and (e.g. motor rating, type of winding, number of
especially between randomly touching turns, size of coil, turn propagation time etc) and
conductors within a coil or between coil ends. the time between reflected peaks in the incident
The front edge of the voltage pulse with its terminal voltage, the voltage appearing between
succession of voltage peaks (Figure 5) will travel turns or randomly within a coil may briefly reach
around the motor winding as it does along the between 30% and 90% of the incident peak
motor cable, with a measurable propagation voltage. Figure 9 shows the possible variations in
time. Figure 8 illustrates how this may result in a first coil voltage plotted against the rise time of
large proportion of the pulse appearing between the incident peak voltage at the motor terminals.
turns, at random points within a coil or between With a sinusoidal supply voltage (uniformly
distributed), the coil ends only experience a
fraction of the phase voltage, as determined by
the number of coils. With a variable speed drive
Voltage pulse at first turn Delayed voltage pulse through therefore, there can be a considerable increase in
turns of coil
Up the voltage stress within a coil.
Ud
T=Propagation Time

U=0 time T
1
) peak

Motor
Terminal
First Coil Voltage (per unit of U

Pulse 0.8
Voltage
1st Coil
Turns
T time 0.6
Voltage between Variations due to
adjacent conductors motor ratings, winding parameters,
0.4
motor cable matching and drive configuration
0.2
STEADY STATE VALUE
Figure 8: Propagation of a voltage pulse through
0
motor windings 0 1 2

Incident Voltage Rise Time us

Figure 9: First coil voltage distribution against
incident voltage rise time

6

3. THE INDUCTION MOTOR AND
INSULATION SYSTEMS
3.1 General
The development of the squirrel cage induction series of standards. These cover aspects of
motor, with its associated insulation system, has performance, starting characteristics, thermal
generally been for sinusoidal supplies. Its design is classifications, mechanical protection, safety,
well proven and inherently robust leading to long insulation level by dielectric test etc.
reliable service with minimum maintenance. Developments in the materials and varnishes used
Practical life of insulation systems, and hence in motor insulation systems have improved the
motor life, can be many years with ultimate failure thermal, mechanical and dielectric characteristics
likely to be through thermal and mechanical considerably beyond the minimum requirements
degradation of the insulating materials, not by of those standards and overall, the standard
short-term direct electrical breakdown. induction motor is well able to withstand the
Requirements for motors with standard supplies voltage waveforms encountered with the majority
are established internationally in the IEC 60034 of inverter drives.

3.2 Standard Motor Insulation Systems
For low voltage motors up to 690V, there are
two main types of winding, broadly classed as
random and form. Lower power motors are
generally random wound, i.e. with coils in which
the turns of round section wire are randomly
located in the coil forming process as illustrated
in Figure 10. For larger powers, form windings are
often utilised where the pre-formed coils are
layered up uniformly - usually with rectangular
section conductors. Coils for both types of
windings are shown in Figure 11, with typical slot
cross sections for random and form windings
depicted in Figure 12.

Figure 10: Random coil forming

7

Random Form

Figure 11: Random and form wound coils

Slot wedge
Slot wedge Slot closure
Slot closure Slot liner

Conductors Slot separator
Slot separator
Rectangular section
copper wrapped with
Slot liner polyester/mica tape

Figure 12:Typical slot cross-section for Random winding and Form winding

The essential elements of both random and form wound insulation systems consist of:
q Phase to earth insulation - slot liner and closure.
q Phase to phase insulation - slot separator and end-winding.
q Inter-turn insulation - slot and end-winding.
q Impregnating varnish - slot and end-winding

Typical phase to earth and phase to phase wrapped film in the case of rectangular form
insulation will be polyester film/meta aramid paper wound turns. Figure 13 shows the insulation of a
composites with inter-turn insulation provided by random winding in an intermediate stage of
multi-layer polyester/polyamide-imide enamel on manufacture and Figure 14 a partially wound form
the conductor or alternatively mica/polyester winding.

Figure 13: Partially wound stator core with Figure 14: Partially wound stator core with
random winding form winding

8

Impregnating the winding, typically with class F or H
polyester resin, provides mechanical strength with
overall electrical insulation and resistance to
environmental contamination. Figure 15 shows a
completed random wiring.

Figure 15: Completed random winding

3.3 Enhanced Motor Insulation Systems

To withstand the higher stresses on supplies special winding wire.This is completed by a
greater than 500V and up to 690V, an enhanced multiple impregnation regime. In the case of form
random wound motor insulation system will winding, standard windings, having mica/polyester
involve further reinforcement of slot liners, slot wrapped conductors, would meet enhanced
closures, slot separators, inter-phase barriers, end- insulation requirements.
winding bracing, etc, and possibly the use of

4. INSULATION BEHAVIOUR
There are three suggested possibilities for failure mechanism in the inter-turn insulation
insulation damage:- occurs.The failure mechanism is a complex
q Breakdown between coil and stator core phenomenon called partial discharge (PD).
➯ Normally not a problem as slot liners PD is a low energy discharge that occurs when
are used both the following conditions apply:
q Phase to phase failure - in the slots or q The peak value of the applied voltage is
end-windings lower than the actual breakdown voltage of
➯ Normally not a problem as motors use the insulation system
inter-phase barriers (or are form-wound) q The local electric field intensity that is created
q Inter-turn failure between adjacent in a void or cavity is sufficient to exceed the
conductors in the stator winding breakdown strength in air (Partial Discharge
➯ The most probable cause of failure due to Inception Voltage)
the non-uniform distribution of voltage When subject to continuous partial discharges, the
along the stator windings, associated with insulation system progressively degrades,
the short rise times of the incident voltage prematurely ageing the insulation material.The
pulses as described in Section 2.With form ageing process results from an erosion of the
wound motors, this is a less significant insulation material, reducing its thickness at the
problem because the turns are evenly discharge sites until its breakdown voltage
distributed within the slot. capability is reduced to below the level of the
Depending upon the homogeneity of the stator applied voltage peak, at this stage insulation
winding impregnation, there may be voids in the breakdown occurs.
impregnating resin. It is in such voids that the Recent investigations, particularly at Dresden

9

University [3] have produced relationships for probability of PD occurring, both plotted against
model insulation systems between the applied the pulse amplitude of the applied voltage.
peak voltage, rise times, the probability of PD and PD inception voltage is influenced by temperature.
the insulation lifetime.The results are shown in Temperature increases occur from the normal
Figure 16 for a reference temperature of 20˚C losses in the motor, compounded to some extent
with a typical standard induction motor insulation by the losses associated with the high frequency
system that is rated for operation with a nominal nature of the applied voltage pulse. Reports [3]
supply voltage up to 500V a.c. on an inverter indicate that an increase in temperature of 80K
supply. may reduce the PD inception voltage by
approximately 10%. In circumstances
where partial discharges are occurring
kV
this reduction in the inception voltage
4.5
will result in an acceleration of the
4.0
ageing of the insulation system.
3.5
If the motor insulation system is
3.0 operated such that the applied peak
voltage is less than the PD inception
2.0
voltage, or at a voltage where the
Pulse Amplitude

1.75 Unipolar
pulses probability of PD is low and the
1.5
number of pulses to breakdown
1.25 exceeds 1012 in Figure 16, then no
1.0
Bipolar
reduction in lifetime is expected.
0.8
pulses In relating the unipolar curve of Figure
16 to standard insulation windings, it is
0.6
6 7 8 9 10 11 12 13 -3
necessary to apply correction factors
10 10 10 10 10 10 10 10 10 1 10 50 90 99 %

(a) Number of Pulses to Breakdown (b) Probability of PDs Occuring
for temperature and the first coil
voltage (Figure 9). For the normal
Figure 16: Relationship between peak Voltage, class B temperature rise (80K), these
factors effectively equalise each other indicating a
insulation lifetime and PD probability
permissible terminal voltage peak of 1.3kV with a
rise time of 0.1µs for a projected lifetime of 1013
Figure 16(a) which is based on the Dresden pulses.This equates to the practical situation of
results, shows the cumulative number of pulses standard motors from REMA manufacturers and
(0.1µs rise time, 5µs duration) that the insulation supply voltages up to and including 500V a.c. (see
should survive whilst Figure 16(b) shows the curve A in Figure 17).

Bipolar Switching Methods
Bipolar switching, where the polarity of the inverter output pulses may be reversed in
successive switching operations (thereby producing alternating pulses), may be found in
inverters which use hysteresis switching unless inhibited in the implementing technology.
In these cases, a conventional PWM modulator is not used, and the output may offer different
switching patterns. In these control schemes it is possible to generate pulses which change
from +Ud to -Ud in one transition unless inhibited in the design implementation. Note that it
is theoretically possible to obtain peak voltage reflections, as described in Figure 4, of up to
4Ud. (See also section 7.5)
If frequent polarity reversals occur the stress on insulation is increased and thus the predicted
failure time is decreased as can be seen in Fig 16.

10

5. PRACTICAL INSULATION
REQUIREMENTS
IEC (60034-17:1998) and NEMA (MG1 part 31:1998) when inverter fed. In both cases, these curves are not
have both revised their respective standards giving now representative of present day practice - see Figure
withstand characteristics for motor insulation systems 17 below.

Example of test results with
2.4 steel wire armour cable at the
Curve B - enhanced insulation or form wound
2.2 lengths indicated:
2.0
NEMA MG1 pt31:1998, 600V a.c. (Note ii)
415V a.c.measurements
Peak Voltage (kV)

1.8
1.6 Curve A - standard insulation
690V a.c.measurements
1.4
NEMA MG1 pt31:1998, 400V a.c. (Note ii) Effects of increasing motor
1.2 rating
1.0 IEC 60034-17:1998
30m
50m 100m Pulse Withstand Curves
0.8 20m
0.6 5m ≤ 500V a.c. (Curve A)
0.4
0.2 > 500-690V a.c. (Curve B)
0
IEC 60034 -17:1998
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Voltage Pulse Rise Time (µs) NEMA MG1 pt 31:1998

Figure 17: Limit curves of admissible motor terminal peak voltage for a.c. motors up to and including 500V
a.c. (Curve A) and from >500V a.c. to 690 a.c. (Curve B).
Notes:
i) Motor pulse withstand requirements on 400/415V supply generally exceed the minimum capability
specified in IEC 60034-17.
ii) The pulse withstand requirements detailed in MG1 part 31:1998 for definite purpose inverter fed motors
(Vpeak = 3.1 * Vrated for NEMA rise times ≥ 0.1µs and Vrated ≤ 600V a.c. ) are not adequate for all cases of
modern PWM inverter operation.
iii) Pulse rise times shown are normalised in accordance with the IEC 60034-17 definition.
iv) These curves are based on the practical experience of GAMBICA and REMA members.
The example measurements shown are typical of voltages at the motor terminals that are outside
lower power motor and are for illustrative the IEC60034-17:1998 profile, even when
purposes only as the actual peak voltages are operating from a 400/415V a.c. supply. Curve A
dependent on a series of factors including - indicates that REMA motor manufacturers
motor rating, winding configuration, connection produce, as standard, motors whose capability
details (star or delta) and cable type/size. substantially exceeds the requirements of
However, the test results plotted in Figure 17 IEC60034-17:1998, and the enhanced insulation
illustrate in principle the effect of lengthening the systems developed by REMA manufacturers
motor cable.The rise time increases steadily with exceeds the NEMA curve requirements and
increasing length, whilst the peak voltage comfortably meet the 690V peak voltage
overshoot tends to reduce after a peak at about requirements.
50m.The voltage stress on the motor therefore Figure 17 can be used in discussions with non
usually declines above quite moderate cable REMA motor manufacturers to indicate the peak
lengths (except in the special case of very long voltage/rise-time withstand profile that is required
motor cables described in Section 7.7). for reliable operation (Curve A or B depending on
From the test results given in Figure 17 above, it supply voltage) and to ensure that the expected
can be seen that standard PWM drives with cable operating life is achieved. An alternative motor
lengths of 20m or more can produce peak supplier should be asked to confirm this capability.

11

6. ALTERNATIVE PREVENTATIVE
METHODS
6.1 General
In applications where it is not feasible to employ 792V - acceptable to most standard motors.
motors which meet the withstand capability Normally, the output reactor is mounted within
achieved with standard or enhanced insulation the inverter cabinet and of course leads to the
given in Figure 17 curve A or B respectively, some acceptance of extra space, cost and reduced
form of alternative solution is required. Examples efficiency (less than approx. 0.5%). Output reactors
where these alternative solutions may be required can also be used to compensate for cable charging
include: currents (balances cable capacitance) and may be
q Undefined motor characteristics used for motor cable lengths up to many hundred
metres on larger drives (check technical data).
q Retrofit application of VSDs to ‘old’ motors
q Motors with inadequate pulse withstand 6.3 Voltage Limiting Filter (dv/dt Filter)
capabilities
In the above cases, some form of motor terminal In this case, a design consisting of capacitors,
voltage modification technique is necessary.These inductors and diodes or resistors may be used to
techniques essentially involve placing additional limit the dv/dt to typically less than 500V/µs (to
apparatus between the motor and the inverter to comply with the 1992 version of IEC34-17),
limit the rate of rise of the pulse, reduce the drastically reducing both the amplitude and the
reflection coefficient and thereby reduce the peak rate of rise of the peak voltage. In the example
voltage level. Some of the devices are also used shown in Figure 18(b), the peak voltage is reduced
to compensate for large capacitive cable charging to 684V with a dv/dt of 40V/µs. Such filters allow
currents.These techniques may be summarised as the use of most motors without problem and are
follows: therefore recommended if the data of a motor is
q Output Reactors unknown (e.g. in the case of a retrofit), particularly
on higher voltage supplies (›500V). Increased
q Output dv/dt Filters losses of 0.5 - 1.0% must be accommodated.
q Sinusoidal Filters
q Motor Termination Units 6.4 Sinusoidal Filter
These solutions should be correctly matched to A special design of low pass filter allows the high
the application and the basic characteristics are as frequency currents to be shunted away.This
described below. resulting waveform at the motor becomes
sinusoidal, the voltage and current are, for one
6.2 Output Reactors cycle of the waveform, as shown in Figure 18(c).
These are specially designed reactors which can These types of filters are the most expensive and
accommodate the PWM waveform without also have other limitations.They prevent the
causing undue reactor heating and can also motor voltage from exceeding 90% of the supply
provide the necessary inductance values over the voltage (thereby de-rating the inverter).They also
frequency spectrum needed.They are used to will not be suitable for applications that require
reduce the dv/dt and peak voltage. However, care high dynamic performance. However, they have
is needed as reactors can theoretically extend the the following additional advantages:
duration of overshoot if incorrectly selected - q Reduced motor noise
particular care is needed with ferrite core q Reduced motor losses
reactors. In the case shown in Figure 18(a), which
compares to Figure 6(c) (42m case), the addition q Simplifies hazardous area motor certification
of the reactor has extended the rise time to q Allows use of standard motors and long motor
around 5µs and reduced the peak voltage to cables (eliminates capacitive charging currents)

12

6.5 Motor Termination Unit would be compromised thereby subjecting the
motor to the high transients without any warning.
Some manufacturers produce series Some users [4] have reported potential difficulties
resistive/capacitive filters which may be locally in matching the inverter current rating to the
connected at the motor terminals (usually as an motor rating to obtain the inherent l2t protection
extra box mounted near to the motor).The fast facilities available on many drives (presumably due
rising incident pulse sees the capacitor as a short to the terminator capacitive charging current).
circuit and the resistive element is temporarily Termination units must not be used with motors
connected across the end of the cable. If this designed for use in explosive atmospheres
resistor approximates to the characteristic (‘Ex’ motors - see Section 7.6).
impedence of the cable, overvoltages will not
occur. As the capacitor charges, the current 6.6 Relative Characteristics of
through the circuit reduces - therefore the losses Preventative Measures
in the resistor are limited to the rising edge
duration.Typically, these filters add around 0.5 - The relative motor terminal voltage characteristics
1.0% losses. [5] of the preventative measures discussed above
For example illustrated in Figure 18(d) the peak are shown in Figure 18 and should be compared
voltage is now only 800V with a rise time of 2µs with the earlier Figure 6(c).
which should be suitable for most motors. The mitigation methods below are specific to the
To date, these devices have not been popular. motor/inverter installation and the actual selection
One concern is that the parallel connection should therefore be advised by the drive supplier.

Power Circuit Power Circuit
Outline Outline
single output pulse single output pulse
792 V
684 V

200 V / div 200 V / div
10µs / div 10µs / div

Figure 18(a) Output Inductor 3% Figure 18(b) Output dv/dt Filter

Power Circuit Power Circuit
Outline Outline
waveform single output pulse
344 V
Output Frequency = 30 Hz

800 V

100 V / div 400 V / div
5ms / div 3µs / div

Figure 18(c) Sinusoidal Filter Figure 18(d) Motor Termination Unit
Note scale changes

Figure 18: Relative characteristics of alternative preventative measures.

13

6.7 Cost Comparisons for Preventative Measures

In considering the relative merits of the
competing solutions, the issue of costs should also
be considered.The table below gives some
indications.

TYPICAL RELATIVE COSTS - DRIVES AND PREVENTATIVE MEASURES
(MOTOR = 100%)

PREVENTATIVE MEASURE
RATING DRIVE OUTPUT OUTPUT dv/dt SINUSOIDAL MOTOR
INDUCTOR FILTER FILTER TERMINATION UNIT

2.2kW, 415V 350% 75% 440% 330% 170%

75kW, 415V 220% 15% 100% 150% 10%

250kW, 415V 120% 5% 65% 110% 3%

160kW, 690V 150% 15% 40% N/A 4%

250kW, 690V 140% 15% 40% N/A 2%

500kW, 690V 140% 15% 35% N/A 1%

POWER CIRCUIT
OUTLINE
l
put pulse
Table 2: Additional preventative methods and costs

7. APPLICATION SPECIFIC NOTES
7.2 Combined Inverter/Motor Solutions
7.1 Cable Capacitance Effects

In addition to the peak voltage effects, total motor In the mid 1990’s manufacturers introduced the
cable length should be considered in the context combined motor inverter topologies in which the
of instantaneous current peaks. At each inverter inverter is integrally mounted within the motor
output pulse, the distributed cable capacitance (C1 enclosure, typically in the terminal box or
- Cn in Figure 3) must be charged and discharged. sometimes as an extension to the motor casing.
For small motors with ‘long’ cables, the The very short cable length between the inverter
cable charging currents may be of the output connections and the motor windings limits
same order as the motor rated current! the reflections and therefore the peak voltage
Cable charging currents may cause nuisance problems do not exist.Taken in the context of the
inverter overcurrent tripping.This leads to additional benefits of simplified installation,
recommendations for each inverter frame size reduced EMC problems and lower overall costs,
regarding the maximum cable lengths for both this solution is well suited to lower power
shielded (braided or armoured) or unshielded applications and is now rapidly gaining market
cables and may vary from 10m on very small acceptance.
drives to above 250m on high power drives.
Mitigation measures such as additional reactors, 7.3 Applications With Frequent or
transformers or filters may be used to extend the Continuous Braking Duties
maximum cable length - refer to the technical
catalogues of the particular inverter type. For applications such as powered unwind stands
on web handling machines, the motor may spend
a large part of its operating time in the braking
mode.The braking energy is transferred through

14

flywheel diodes back on to the intermediate DC 7.5 Drive Switching Strategies
link, thereby giving a 15-20% increase in the DC
link voltage (and also therefore the peak motor As described in Section 4, some drive types use
voltage).The effect is similar to increasing the control strategies which could allow bipolar
voltage supply by up to 20%; this should be taken switching. These systems could theoretically
into consideration - e.g. treat a 400V application double the motor peak voltage stress.
as if it was supplied with 480V (therefore a
GAMBICA inverter manufacturers implement
standard motor would still be suitable).
control mechanisms to inhibit bipolar switching.
7.4 Active Front End (Sinusoidal
7.6 Explosive Atmosphere Approval
Rectifier) Considerations
The application of inverters to ‘Ex’ motors may
For drives with PWM active front ends invalidate the hazardous area certification - refer
(regenerative and/or unity power factor), special to the motor manufacturer.
considerations may be required. As a function of
the operation of active front end drives, the DC
link voltage is continuously between 10-15%
7.7 Applications With ‘Very Long’ Cable
higher than for standard inverters.The effect is Lengths
similar to increasing the supply voltage by up to
The definition of ‘very long’ depends on the drive
15%; this should be taken into consideration - e.g.
rating and type, and may vary between 250m for
treat a 480V application as if it was supplied with
lower power drives and 500m for higher power
550V (therefore enhanced insulation or other
ratings - refer to manufacturers technical
preventative measure is now required). Refer to
documentation. For these applications, new
the inverter supplier for further guidance
factors which could influence the voltage stress,
are introduced and the drive supplier should be
consulted.

8. CONCLUSIONS
• A combination of fast switching transistors and ‘long’ motor cables can cause peak voltages up
to twice the DC link voltage (2.7 times the supply voltage) due to transmission line effects. In
extreme cases, this high peak voltage and the uneven voltage distribution in the motor windings
can cause a low energy partial discharge between turns of the first coil. Partial discharge can
cause premature ageing effects of the winding insulation system until failure occurs.

• By selecting the correct motor, or by the use of appropriate preventative measures, damaging
partial discharge can be avoided, thereby ensuring the maximum intended motor lifetime is
achieved.

Following the recommendations described in this report will ensure that
motor insulation life is not compromised.

15

9. REFERENCES
[1] Persson E, “Transient Effects in Application of PWM inverters to Induction Motors”
IEEE-IAS 28.1095-1101, 1992

[2] Mohan, Undeland & Robbins, “Power Electronic Converters, Application & Design”
Wiley 1989

[3] Kaufhold M, Borner G, Eberhardt M, Speck J, “Failure Mechanisms of the Interturn Insulation of Low
Voltage Machines Fed by Pulse Controlled Inverters”
IEEE Electrical Insulation Magazine Vol 12, No. 5, 1996

[4] Doherty, K G, “Investigation of Voltage Reflections Associated with PWM Inverter Installations”
IEE Submission 1996

[5] Finlayson P T, “Output Filter Considerations for PWM Inverter Drives With Induction Motors”
IEEE, Industry Application Magazine Jan 1998

10.FURTHER READING
[a] GAMBICA/REMA, “Motor Insulation & PWM Inverter Drives” Shortform Guide 1999

[b] IEC60034-17:1998 “Rotating electrical machines - cage induction motors when fed from
converters - application guide”

[c] NEMA MG1-1998: Motors and Generators - part 30 - “Application considerations for constant
speed motors used on a sinusoidal bus with harmonic content and general purpose motors used
with adjustable-voltage or adjustable-frequency controls or both”

[d] NEMA MG1-1998: Motors and Generators-part 31 “DEFINITE - PURPOSE INVERTER -
FED POLYPHASE MOTORS”

16

LIST OF MEMBERS
GE Power Controls Limited Rockwell Automation Ltd EFACEC (UK) Ltd
GAMBICA MEMBERS East Lancashire Road Pitfield, Kiln Farm Unit 12, Riverside Industrial Park
- Variable Speed Drives Liverpool L10 5HB Milton Keynes, Bucks MK11 3DR Ipswich, Suffolk IP2 8JX
Tel: +44 (0)151 524 1122 Tel: +44 (0)1908 838800 Tel: +44 (0)1473 602352
Fax: +44 (0)151 524 6282 Fax: +44 (0)1908 839699 Fax +44 (0)1473 602194
ABB Automation Ltd Email: sales@gepc.ge.com Email: emccourt@ra.rockwell.com
9 The Towers, Wilmslow Road Web: www.gepowercontrols.com Web: www.automation.rockwell.com Essex International Ltd
Didsbury, Manchester M20 2AB Ellis Ashton Street
Tel: +44 (0)161 445 5555 HID Ltd / Hitachi Schneider Electric Ltd Huyton Business Park
Fax: +44 (0)161 448 1089 Shuttleworth Close University of Warwick Science Park Liverpool L36 6BW
Email: drives.sales@gb.abb.com Gapton Hall Ind Estate, Great Yarmouth Sir William Lyons Road Tel: +44 (0)151 443 6000
Web: www.abb.co.uk Norfolk NR31 0NQ Coventry CV4 7EZ Fax: +44 (0)151 443 6025
Tel: +44 (0)1493 442525 Tel: +44 (0)2476 416255 Web: www.superioressex.com
ALSTOM Power Conversion Ltd Fax: +44 (0)1493 442323
Boughton Road, Rugby Fax: +44 (0)2476 417517 Invensys Brook Crompton
Email: sales@hid.co.uk Web: www.schneider.co.uk
Warwickshire CV21 1BU Web: www.hid.co.uk St Thomas Road, Huddersfield
Tel: +44 (0)1788 563625 West Yorkshire HD1 3LJ
Fax: +44 (0)1788 563780 Hill Graham Controls Ltd Siemens plc Tel: +44 (0)1484 422150
Email: Lincoln Road, Cressex Automation & Drives Fax: +44 (0)1484 557201
general.drives@powerconv.alstom.com High Wycombe, Bucks HP12 3RB Sir William Siemens House Web: www.brookcrompton.com
Web: www.alstom.com Tel: +44 (0)1494 440121 Princess Road
Fax: +44 (0)1494 438810 Manchester M20 2UR Morley Electric Motors Co Ltd
Baldor UK Ltd Email: hillgraham@compuserve.com Tel: +44 (0)161 446 6400 Bradford Road, Leeds
Mint Motion Centre Web: www.hillgraham.com Fax: +44 (0)161 446 5471 West Yorkshire LS28 6QA
6 Bristol Distribution Park Email: info@plcman.siemens.co.uk Tel: +44 (0)113 257 1734
Hawkley Drive, Bristol BS32 0BF IMO Precision Controls Ltd Web: www.siemens-industry.co.uk Fax: +44 (0)113 257 0751
Tel: +44 (0)1454 850000 100 North Circular Road Email: sales@morleymotors.com
Fax: +44 (0)1454 859001 Staples Corner, London NW2 7JP Yaskawa Electric Europe GmbH Web: www.morleymotors.com
Email: sales@baldor.co.uk Tel: +44 (0)208 452 6444 Units 2/3 Centurion Court
Web: www.baldor.co.uk Fax: +44 (0)208 450 2274 Brick Close, Kiln Farm Peebles Electrical Machines Ltd
Milton Keynes, Bucks MK11 3JA Wood Road, Rosyth
Claude Lyons Ltd Email: imo@imopc.com Dunfermline, Fife KY11 2EA
Web: www.imopc.com Tel: +44 (0)1908 565874
Brook Road, Waltham Cross Fax: +44 (0)1908 565891 Tel: +44 (0)1383 422001
Hertfordshire EN8 7LR Web: www.yaskawa.co.uk Fax: +44 (0)1383 422285
Tel: +44 (0)1992 768888 Lenze Ltd
Fax: +44 (0)1992 788000 Caxton Road SEM Ltd
Email: info@claudelyons.co.uk Bedford MK41 0HT Faraday Way, Orpington
Web: www.claudelyons.co.uk Tel: +44 (0)1234 321200 Kent BR5 3QT
Fax: +44 (0)1234 261815 REMA MEMBERS Tel: +44 (0)1689 884700
Control Techniques UK Email: sales@lenze.co.uk - Motors Fax: +44 (0)1689 884884
Stafford Park 4 Web: www.lenze.co.uk Web: www.sem.co.uk
Telford TF3 3BA
Tel: +44 (0)1952 213700 Mitsubishi Electric Europe B.V. ALSTOM Electrical Machines Ltd Siemens plc
Fax: +44 (0)1952 213701 Automation Systems Division Leicester Road, Rugby Automation and Drives
Email: ctdirect@compuserve.com Travellers Lane, Hatfield Warwickshire CV21 1BD Sir William Siemens House
Web: www.controltechniques.com Herts AL10 8XB Tel: +44 (0)1788 542121 Princess Road
Tel: +44 (0)1707 276100 Fax: +44 (0)1788 541280 Manchester M20 2UR
Danfoss Ltd Fax: +44 (0)1707 278695 Web: www.powerconv.alstom.com Tel: +44 (0)161 446 5330
Perivale Industrial Park Web: www.industrial.meuk.co.uk Fax: +44 (0)161 446 5395
Horsenden Lane South Email: info@plcman.siemens.co.uk
Greenford, Middlesex UB6 7QE Moeller Electric Ltd ALSTOM Power Ltd Web: www.siemens-industry.co.uk
Tel: +44 (0)870 241 7200 PO Box 35, Gatehouse Close Lichfield Road
Fax: +44 (0)870 241 7150 Aylesbury, Bucks HP19 8DH Stafford ST17 4LN Teco Electric Europe Limited
Email: uk.drives.sales@danfoss.com Tel: +44 (0)1296 393322 Tel: +44 (0)1785 223221 Marshall Stevens Way
Web: www.danfoss.co.uk Fax: +44 (0)1296 421854 Fax: +44 (0)1785 274176 Trafford Park
Email: support@moeller.co.uk Manchester M17 1PP
Eaton Ltd, Cutler-Hammer Web: www.moeller.co.uk Baldor UK (Ltd) Tel: +44(0)161 877 8025
Carina, Sunrise Parkway Mint Motion Centre Fax: +44(0)161 877 8030
Linford Wood, Milton Keynes Omron Electronics UK Ltd 6 Bristol Distribution Park
Bucks MK14 6NR 1 Apsley Way, Staples Corner Hawkley Drive
Tel: +44 (0)1908 541600 London NW2 7HF Bristol BS32 0BF
Fax: +44 (0)1908 660527 Tel: +44 (0)20 8450 4646 Tel: +44 (0)1454 859000
Email: Ch-help-uk@eaton.com Fax: +44 (0)20 8450 8087 Fax +44 (0)1454 859001
Web: www.ch.cutler- Email: oeeuk_sales@eu.omron.com Email: sales@baldor.co.uk
hammer.com/global/uk Web: www.omron.co.uk web: www.baldor.co.uk

THE GAMBICA ASSOCIATION LIMITED ROTATING ELECTRICAL MACHINES ASSOCIATION
WESTMINSTER TOWER WESTMINSTER TOWER
3 ALBERT EMBANKMENT 3 ALBERT EMBANKMENT
LONDON SE1 7SW LONDON SE1 7SL
Tel: +44 (0)20 7793 3050 Tel: +44 (0)20 7793 3041
Fax: +44 (0)20 7793 7635 Fax: +44 (0)20 7582 8020
Email: assoc@gambica.org.uk Email: rema@beama.org.uk
Web: www.gambica.org.uk Web: www.rema.uk.com

GAMBICA is the Association for Instrumentation, Control, Automation and has a group profile for suppliers of
Variable Speed Drives.
REMA is the Rotating Electrical Machines Association representing manufacturers of rotating electrical machines,
other than turbine type machines, traction motors or machines for the use in aircraft.
The greatest care has been taken to ensure the accuracy of the information contained in this guide, but no liability
can be accepted by GAMBICA, REMA or their members, for errors of any kind.
Always refer to your Drive and Motor Suppliers if in doubt about correct matching.

©Gambica/Rema 2001 2/01

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