DRAFT FOR DEVELOPMENT DD IEC/TS

60034-17:2006

Rotating electrical
machines —
Part 17: Cage induction motors when
fed from converters — Application
guide

ICS 29.160

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 DD IEC/TS 60034-17:2006

National foreword

This Draft for Development reproduces verbatim IEC/TS 60034-17:2006. It

supersedes DD IEC/TS 60034-17:2002 which is withdrawn.
This publication is not to be regarded as a British Standard.
It is being issued in the Draft for Development series of publications and is of
a provisional nature. It should be applied on this provisional basis, so that
information and experience of its practical application can be obtained.
A review of this Draft for Development will be carried out not later than 2 years
after its publication.
Notification of the start of the review period, with a request for the submission
of comments from users of this Draft for Development, will be made in an
announcement in the appropriate issue of Update Standards. According to the
replies received, the responsible BSI Committee will judge whether the Draft
for Development can be converted into a British Standard or what other action
should be taken.
Observations which it is felt should receive attention before the official call for
comments will be welcomed. These should be sent to the Secretary of
BSI Technical Committee PEL/2, Rotating electrical machines, at British
Standards House, 389 Chiswick High Road, London W4 4AL, giving the
document reference and clause number and proposing, where possible, an
appropriate revision of the text.
A list of organizations represented on this committee can be obtained on
request to its secretary.
Cross-references
The British Standards which implement international or European
publications referred to in this document may be found in the BSI Catalogue
under the section entitled “International Standards Correspondence Index”, or
by using the “Search” facility of the BSI Electronic Catalogue or of British
Standards Online.
This publication does not purport to include all the necessary provisions of a
contract. Users are responsible for its correct application.

Summary of pages
This document comprises a front cover, an inside front cover, the IEC/TS title
page, pages 2 to 19 and a back cover.
The BSI copyright notice displayed in this document indicates when the
document was last issued.

This Draft for Development Amendments issued since publication

was published under the
authority of the Standards
Policy and Strategy Committee Amd. No. Date Comments
on 31 July 2006

© BSI 2006

ISBN 0 580 48663 X

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 DD IEC/TS 60034-17:2006

TECHNICAL IEC
SPECIFICATION TS 60034-17
Fourth edition
2006-05
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Rotating electrical machines –

Part 17:
Cage induction motors when fed
from converters – Application guide

Reference number
CEI/IEC/TS 60034-17:2006

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 DD IEC/TS 60034-17:2006
60034-17 –2–

CONTENTS

INTRODUCTION ..................................................................................................................... 3

1 Scope ............................................................................................................................... 4
2 Normative references........................................................................................................ 4
3 Characteristics of the motor .............................................................................................. 4
4 Frequency spectrum of voltage and/or currents ................................................................. 5
5 Losses caused by harmonics ............................................................................................7
6 Torque derating during converter operation ..................................................................... 10
7 Oscillating torques .......................................................................................................... 11
8 Magnetically excited noise .............................................................................................. 12
9 Service life of the insulation system ................................................................................ 13
10 Bearing currents ............................................................................................................. 15
11 Installation (cabling, grounding, bonding) ........................................................................ 18
12 Maximum safe operating speed....................................................................................... 18
13 Power factor correction ................................................................................................... 19

Figure 1 – Waveform of phase current i phase in delta connection for current source
converter supply (idealized example) .......................................................................................5
Figure 2 – Waveform of line-to-line voltage u LL for voltage source converter supply with
switching frequency f s = 30 × f 1 (example) ................................................................................6
Figure 3 – Example for the dependence of the motor losses caused by harmonics P h,
related to the losses P f1 at operating frequency f 1 , on the switching frequency f s in case
of voltage source converter supply ..........................................................................................7
Figure 4 – Influence of converter supply on the losses of a cage induction motor
(frame size 315 M, design N) with rated values of torque and speed ........................................9
Figure 5 – Fundamental voltage U 1 as a function of operating frequency f 1 ............................ 10
Figure 6 – Torque derating factor for cage induction motors of design N, IC 0141 (self-
circulating cooling) for current source converter supply as a function of operating
frequency f 1 (example) .......................................................................................................... 11
Figure 7 – Limiting curve of admissible impulse voltage Û LL (including voltage reflection
and damping) at the motor terminals as a function of the rise time t r ...................................... 14
Figure 8 – Definition of the peak rise time t r of the voltage at the motor terminals.................. 14
Figure 9 – Ring flux including shaft voltage and resulting circulating current i circ ................... 15
Figure 10 – Common mode circuit model and bearing voltage u brg ......................................... 17

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 –3– DD IEC/TS 60034-17:2006

INTRODUCTION

The performance characteristics and operating data for drives with converter-fed cage induc-
tion motors are influenced by the complete drive system, comprising supply system, converter,
induction motor, mechanical shafting and control equipment. Each of these components exists
in numerous technical variations. Any values quoted in this technical specification are thus
indicative only.

In view of the complex technical interrelations within the system and the variety of operating
conditions, it is beyond the scope and object of this technical specification to specify numerical
or limiting values for all the quantities which are of importance for the design of the drive.

To an increasing extent, it is practice that drives consist of components produced by different

manufacturers. The object of this technical specification is to explain and quantify, as far as
possible, the criteria for the selection of components and their influence on the performance
characteristics of the drive.

The technical specification deals with motors within the scope of IEC 60034-12, i.e. low-voltage
series-fabricated three-phase cage induction motors, which are designed originally for mains
supply, covering the design N or design H requirements. Motors which are specifically designed
for converter supply are covered by IEC 60034-25.

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 DD IEC/TS 60034-17:2006
60034-17 –4–

ROTATING ELECTRICAL MACHINES –

Part 17: Cage induction motors when fed from converters –

Application guide

1 Scope

This technical specification deals with the steady-state operation of cage induction motors
within the scope of IEC 60034-12, when fed from converters. It covers the operation over the
whole speed setting range, but does not deal with starting or transient phenomena.

Only indirect type converters are dealt with. This type comprises converters with impressed
direct current in the intermediate circuit (current source converters) and converters with
impressed d.c. voltage (voltage source converters), either of the block type or the pulse
controlled type, without restriction on pulse number, pulse width or switching frequency. For the
purposes of this technical specification, a converter may include any type of electronic
switching device, for example transistors (bipolar or MOSfet), IGBTs, thyristors, GTO-
thyristors, etc. with analog or digital control electronics.

2 Normative references

The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.

IEC 60034-1, Rotating electrical machines – Part 1: Rating and performance

IEC 60034-12, Rotating electrical machines – Part 12: Starting performance of single-speed
three-phase cage induction motors

IEC 60034-25, Rotating electrical machines – Part 25: Guide for the design and performance of
cage induction motors specifically designed for converter supply

3 Characteristics of the motor

The output current of a current source converter passes through the stator winding of the motor
during the commutating period. Therefore, a knowledge of the motor equivalent circuit is
important for the design of the commutating circuits.

In the case of voltage source converters, a knowledge of the motor equivalent circuit is
normally not important for the design of the commutating circuit, but the harmonic impedances
of the motor greatly influence the losses caused by harmonics.

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 –5– DD IEC/TS 60034-17:2006

The above conditions are relevant for the basic operation capability of the drive. If details are
required of the additional torques (in particular oscillating torques) and of the additional losses,
which occur during converter operation, then a knowledge of the equivalent circuit parameters
of the motor covering the harmonic spectrum will be necessary.

Due to the existing design variants of cage induction motors with design N (e.g. copper deep-
bar rotors and aluminium double-cage rotors are used) and due to the wide frequency range of
the most important harmonics (band width from 0 kHz up to 30 kHz), a generally valid motor
equivalent circuit cannot be specified. As a rule, it is not admissible to use the quantities from
the equivalent circuit for steady-state operation at power frequency (e.g. with leakage
inductances for normal running) in order to calculate torques and losses due to harmonics.
The motor manufacturer may provide appropriate values of the equivalent circuit only if the
frequency spectrum of currents and/or voltages generated by the converter is known.

4 Frequency spectrum of voltage and/or currents

With respect to the necessary torque derating and to the oscillating torques excited by
harmonics, it is important to know the frequency spectra of motor voltages (in case of voltage
source converters) or motor currents (in case of current source converters).

Figure 1 shows the typical waveform of the motor phase current in the case of a current
source converter drive. The produced harmonics are of the order n = 5; 7; 11; 13... The
relative harmonic content is influenced by the commutating time interval which may differ
in different drives.

iphase

60° 120° 180° 240° 300° 360° ωt

IEC 784/06

Figure 1 – Waveform of phase current i phase in delta connection

for current source converter supply (idealized example)

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 DD IEC/TS 60034-17:2006
60034-17 –6–

Figure 2 shows the typical waveform of the motor line-to-line voltage for operation with a
voltage source converter with pulse width modulation (PWM converter).

uLL

240° 300° 360°

60° 120° 180° ωt

IEC 785/06

Figure 2 – Waveform of line-to-line voltage u LL for voltage source

converter supply with switching frequency f s = 30 × f 1 (example)

In the case of voltage source converters a variety of modulation types is in use. Hence it is not
possible to make global statements on the effects of the harmonics. For definite statements,
the harmonic content of the converter output voltage must be known and its consequences on
the motor shall be studied.

Converters using carrier modulation, together with synchronised and asynchronous pulse
patterns, as applied in many cases, produce the frequencies:

f = k s × f s ± k1 × f 1

where k s = 1, 2, 3,... and k 1 = 1, 2, 4, 5, 7... are multiplying factors of the switching frequency
f s and of the operating frequency f 1 , respectively. The formula is valid also in the case of
converters with space-phasor modulation.

Converters with carrierless modulation, where no pre-determined switching frequency is

existent, are also in practical use. In this case, the frequency spectrum of the output voltage is
characterised by broadband random noise without spikes at specific frequencies.

With pulse controlled converters, the content of harmonics with low frequencies can be kept
low, while the dominant harmonics (which are near the switching frequency) will occur at
relatively large frequency values, not having much effect due to the motor winding inductances.

In 7.2.1 of IEC 60034-1 the permissible harmonic content of the supply voltage of cage
induction motors is expressed by one single numerical value called the harmonic voltage factor
(HVF). However, this factor is not applicable for converter power supplies.

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 –7– DD IEC/TS 60034-17:2006

5 Losses caused by harmonics

Harmonics of voltage and current in a cage induction motor supplied from a converter cause
additional iron and winding losses in the stator and the rotor.

In the case of motors supplied by voltage source converters, the additional iron losses cannot
be neglected. They depend on the amplitudes of the phase voltage harmonics, but they are
nearly independent on its frequency.

The harmonic currents, which are responsible for the winding losses, are limited by the leakage
reactances. Although the harmonic currents are small, the winding losses cannot be ignored
because of the current displacement (skin effect) due to the high frequencies. This statement
applies to both form-wound and random-wound windings. Rotors with pronounced current
displacement (skin effect) are especially sensitive to these losses.

It is verified by many tests, that the total value of the additional losses caused by harmonics
does not depend on load; they decrease with increasing switching frequency (see Figure 3).
This effect is caused by the small additional winding losses at high switching frequencies.

Ph
Pf1

0,4

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0,3

0,2

0,1 2

0
0 1 2 3 4 5 6 7 8 9 10 11 fs kHz
IEC 786/06

1 = Total harmonic losses

2 = Harmonic winding losses
3 = Harmonic iron losses

Figure 3 – Example for the dependence of the motor losses caused by harmonics P h,
related to the losses P f1 at operating frequency f 1 , on the switching
frequency f s in case of voltage source converter supply

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 DD IEC/TS 60034-17:2006
60034-17 –8–

In the case of motors supplied by current-source converters, the additional iron losses are
nearly negligible with the exception of the so-called commutation losses. The fast change of
the leakage fluxes during the commutation interval generates eddy currents in the teeth of
stator and rotor. There are no commutation losses in the case of operation from voltage source
converters because the commutation currents do not flow through the motor windings.

The additional rotor winding losses play an important role due to the relative high amplitudes of
the harmonic currents of low frequency.

There is no simple method to calculate the additional losses, and no general statement can be
made about their value. Their dependence upon the different physical quantities is very
complex. Also, there is a great variety both of converters (e.g. current and voltage source
converters with different switching frequencies and pulse patterns) and of motors (e.g. kind of
winding, slot geometry, specific iron loss). The quality of core manufacture is also an important
feature.

The columns in Figure 4 show, as an example, the calculated loss composition of a specific
motor (frame size 315 M; design N) when supplied both from different converters with different
harmonic content and from a sinusoidal supply. The example illustrates the relative importance
of the different types of losses for the converter systems most widely used today. The
comparison cannot be transferred to other converter-fed cage induction motors and other types
of converters (with different modulation schemes and pulse frequencies). To facilitate
comparison in Figure 4, the fundamental voltages and currents during converter operation are
assumed to be the same as under rated conditions.

According to Figure 4, the harmonic losses are higher for supply by current source converters
than by voltage source converters. The difference diminishes at partial load, because the
harmonic losses are constant for voltage source converter supply, but the harmonic losses
increase with load for current source converter supply.

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 –9– DD IEC/TS 60034-17:2006

J - 10 %
I-1% I - 0,5 %

H-1% H – 7,5 %
G - 10 % G – 4,5 %
F-3%
E-6% E-6% E-6% F - 2,5 %
D-2% D-2% D-2%

C - 25 % C - 25 % C - 25 %

B - 26 %
B - 26 % B - 26 %

A - 41 % A - 41 % A - 41 %

1 2 3

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≈

100 % 125 % 115 % 5

95,3 % 94,2 % 94,6 % 6

IEC 787/06

Losses caused by fundamental frequency Losses caused by harmonics

E – Frictional losses J – Commutation losses
D – Additional load losses I – Additional load losses
C – Iron losses H – Iron losses
B – Rotor winding losses G – Rotor winding losses
A – Stator winding losses F – Stator winding losses

1 = Sinusoidal voltage 2 = Current source converter

3 = Voltage source converter with carrier modulation 4 = Time dependence of the impressed quantity
(switching frequency ≈ 3 kHz)
5 = Losses 6 = Efficiency

Figure 4 – Influence of converter supply on the losses

of a cage induction motor (frame size 315 M, design N)
with rated values of torque and speed

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 DD IEC/TS 60034-17:2006
60034-17 – 10 –

6 Torque derating during converter operation

When the motor is supplied from a converter at the motor rated frequency, the available torque
is usually less than the rated torque on a sinusoidal voltage supply due to increased
temperature rise (harmonic losses). An additional reason for the reduction may be the voltage
drop of the converter. Maintaining of the rated torque may reduce insulation service-life.

The full-line curve in Figure 5 refers to a converter producing approximately the same
fundamental motor flux as at sinusoidal supply. The motor manufacturer can determine the
temperature rise for this operating point if the harmonic spectrum of the converter is
known. The temperature rise depends on the individual motor design and the type of
cooling (e.g. IC 01 or IC 0141). W hen determining the derating factor, the thermal reserve
of the particular motor is important. Taking all these influences into account, the derating
factor at rated frequency typically ranges from 0,8 to 1,0.

U1/UN

1,0

0,8

0,6

0,4

0,2

0
0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
f1/fN
IEC 788/06

Figure 5 – Fundamental voltage U 1 as a function of operating frequency f 1

(see clause 6)

Frequently, in practice, the converter rating does not imply that the fundamental flux at rated
frequency is the same as on sinusoidal voltage. The consequence is an additional torque
deviation, the values of which depend on the individual parameters.

Within the speed setting range below the synchronous speed at motor rated frequency,
applying a constant ratio U 1 /f 1 leads to a constant pull-out torque only if the stator winding
resistance is negligible in comparison with the motor reactances. To compensate for the effect
of the motor stator resistance, some converter controls are designed to have a characteristic in
accordance with the dashed line in Figure 5. At low speeds, higher torques are generated than
in the absence of such a compensation.

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 – 11 – DD IEC/TS 60034-17:2006

Above the 1,0 p.u. operation point of voltage and frequency in Figure 5, the converter output
voltage is generally constant as the frequency increases (field weakening range). In the
event of this occurring within the frequency operating range, then the derating factor will
change with a rapid reduction in torque capability similar to the characteristic shown in
Figure 6 above f 1 /f N = 1,0.

Figure 6 shows an example of a derating curve for a typical motor supplied by a current source
converter. The curve of a voltage source converter drive is of similar shape. Such a curve can
be declared by the motor manufacturer, if the harmonic spectrum and the voltage-frequency
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characteristics of the converter are known. With respect to the different cooling (IC 01 or
IC 411) and ventilation methods (self-circulation cooling or independent cooling) it is not
possible to produce a curve which applies to all cases. In general, however, motors supplied
from pulse-controlled converters (switching frequency in the kHz range) require smaller torque
reductions than those supplied from block converters. The derating is normally reduced as the
switching frequency increases.

T1/TN

1,0

0,8

0,6

0,4

0,2

0
0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
f1/fN
IEC 789/06

Figure 6 – Torque derating factor for cage induction motors of design N, IC 0141
(self-circulating cooling) for current source converter supply as a function
of operating frequency f 1 (example)

7 Oscillating torques

The asynchronous (time-constant) torques generated by harmonics have little effect on the
operation of the drive. However, this does not apply to the oscillating torques, which produce
torsional vibrations in the mechanical system.

In the case of three-phase induction motors supplied from current source converters in a six
pulse circuit, the oscillating torques with 6 and 12 times the operational frequency (f 1 ) are of
practical importance; their amplitudes are in the order of 15 % (frequency 6 × f 1 ) and 5 %
(frequency 12 × f 1 ) of the rated torque. In addition, oscillating torques are excited by harmonics
which are based on the ripples of the d.c. current in the intermediate circuit; these torques are
of the frequency 6(f 1 – f p ) and 12(f 1 – f p ), where f p is the power frequency of the mains. A
careful calculation of the critical torsional speeds is advisable, particularly for drives with
transmission elements which are only slightly damped. In some applications, skipping of a
small band of operating frequencies is unavoidable.

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 DD IEC/TS 60034-17:2006
60034-17 – 12 –

In drives with pulse-controlled converters, the frequencies of the dominant oscillating torques
are determined by the switching frequency while their amplitudes depend on the pulse width.
Thus, the oscillating torque amplitudes may be as high as 15 %, provided that the switching
frequency exceeds 10 times the fundamental frequency, which is usually the case in today’s
converters. With higher switching frequencies (in the order of 21 × f 1 ) the oscillating torques of
frequencies 6 × f 1 and 12 × f 1 are practically negligible, provided a suitable pulse pattern is
applied (e.g. modulation with a sinusoidal reference wave or space-phasor modulation).
Additionally, oscillating torques of twice the switching frequency are generated. These,
however, do not exert detrimental effects on the drive system since their frequency is far above
the critical mechanical frequencies.

8 Magnetically excited noise

The magnetic noise of induction motors is essentially caused by waves of tensile stress acting
in radial direction on the stator bore. These so-called Maxwell forces are excited by the
interaction of the various magnetic fields in the air-gap. The tensile stress is characterised by
its amplitude, frequency and mode. As the amplitudes are small, the tensile stress results in
disturbing tones only when frequency and mode of a specific wave coincides with the frequency
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and mode of a natural frequency of the stator core.

In the case of sinusoidal supply voltages, the magnetic noise is caused by the spatial
harmonics of the air-gap field. The aim of a professional design is to avoid resonances at the
rated operating conditions of the motor. But, because of the large variety of contributing spatial
harmonic fields, audible magnetic noise is unavoidable at specific speeds, when the motor is
operated at constant flux over a wide speed setting range, even when the supply voltage is
sinusoidal. Skipping of a small frequency band is frequently used to avoid a too high noise
emission at the associated speed.

The statements given above are valid also when a motor is supplied from a converter. But, in
this case, the magnetic fields produced by the time harmonics are superimposed. With respect
to considerable magnetic noise, it is sufficient to consider the interaction of the fundamental
air-gap fields (number of pole pairs p) of the operating frequency and the different harmonics.
Therefore, the additionally generated waves of tensile stress are of the modes r = 0 and r = 2p.
The natural frequencies of these modes depend on the size and the design of the motor. For 2-
pole and 4-pole motors, the resonance frequencies of modes r = 0 and r = 2p can be grouped
approximately as follows:

shaft height < 200 mm: f 0,r=0 > 4 500 Hz, f 0,r=2 > 800 Hz, f 0,r=4 > 4 000 Hz
shaft height > 280 mm: f 0,r=0 < 3 000 Hz, f 0,r=2 < 500 Hz, f 0,r=4 < 2 500 Hz

When supplying a motor by a current source converter, additional magnetically excited tones
are generated by the harmonic currents. The amplitude of each harmonic is inversely
proportional to its order. The frequencies of the noise exciting forces are apart from the natural
frequencies of the active parts of the stator. Thus, the level of the noise increments is in the
same range during motor operation at constant flux and constant currents. The increment is
influenced only little by the control devices of the converter or the design of the motor.
According to experience, the A-weighted noise level increases in the range of 1 dB to 6 dB for
operation up to rated frequency compared with operation on a sinusoidal supply at rated
voltage and rated frequency. The upper limit of the increase is applicable to motors with a low
noise level during operation on a sinusoidal voltage.

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 – 13 – DD IEC/TS 60034-17:2006

When supplying a motor by a voltage source converter, the frequencies of the important
harmonics are close to the switching frequency of the converter or multiples of it. Depending on
the switching frequency and the control scheme of the converter, it is much more likely that the
natural frequency of the stator core for r = 0 or r = 2p is met than in the case of supply from a
current source converter. When the switching frequency is changeable, this may have
tremendous influence on the generation of magnetic noise. In addition, the type of modulation
influences the amplitude of specific harmonics and may also have reasonable effect on the
emission of magnetic noise. Converters with a carrierless or random PWM control normally
cause a lower increase of noise than converters with a fixed carrier frequency. Therefore, the
noise level increase due to the converter supply varies more widely compared with operation at
a sinusoidal supply of rated voltage and rated frequency as for current source converter
supplied motors. According to experience, the increase at constant flux is likely to be in the
range 1 dB to 15 dB.

9 Service life of the insulation system

The insulation system of converter fed motors is subject to higher dielectric stresses than in
the case of sinusoidal power supply.

In the case of supply from slow switching current source converters (usually equipped with
thyristors), peaks occur in the motor voltage during the commutation interval, which stress the
main and interturn insulation. The commutation peaks normally do not endanger the insulation
system because their rise time is relatively long and the repetition rate is relatively low.

In the case of supply from fast-switching voltage source converters (equipped for instance with
IGBT semiconductors), voltage gradients may significantly stress the interturn insulation,
particularly that of the entrance coils. The dielectric stress of the winding insulation is
determined by the peak voltage, short rise time and high repetition rate of the impulses
produced by the converter, the characteristics and the length of the connection leads between
converter and motor, the design of the winding and other systems parameters.

Motors with random wound windings with enamelled round wires will typically endure the pulse
voltages of Figure 7 at the terminals without significant reduction of lifetime.

The combination of fast switching inverters with cables will cause peak voltages due to
transmission line effects. For motors rated at voltages less than or equal to 500 V a.c. the
insulation system should typically give satisfactory life when subjected to peak voltages shown
in Figure 7. Care must be taken to avoid variable speed applications that involve rapid speed
changes as these can cause regenerative voltages at the converter output up to twice the rated
motor voltage.

--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---

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 DD IEC/TS 60034-17:2006
60034-17 – 14 –

ÛLL V
2 000

1 500

1 000
--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---

500

0
0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
tr µs
IEC 790/06

Figure 7 – Limiting curve of admissible impulse voltage Û LL (including voltage reflection

and damping) at the motor terminals as a function of the rise time t r

For motors rated over 500 V a.c., supplied from a fast switching inverter, an enhanced
insulation system and/or filters at the converter output (designed to increase the rise time
and/or to limit the peak voltages) may be required.

The term peak rise time is based on the following definition which takes into account the
transient phenomena within the winding (see Figure 8).

u (t1) 100 %

90 %

tr

∆u = u (t1) − u (t0)

10 %

u (t0) 0%
t0 t1 t
IEC 791/06

Figure 8 – Definition of the peak rise time t r of the voltage

at the motor terminals

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 – 15 – DD IEC/TS 60034-17:2006

The voltage range ∆u is the difference between the instantaneous values of the voltage directly
before and after the voltage impulse. This impulse is finished at the instant when the voltage
has reached its first maximum. The peak rise time t r is defined as the interval during which the
voltage changes from 10 % to 90 % of the whole voltage range ∆u.

In view of the complex interrelations, a careful design of the complete drive is suggested.

10 Bearing currents

When operating a motor at sinusoidal voltage, a shaft voltage is induced in the conducting loop
comprising the shaft, the bearings, the end-shields and the housing (see Figure 9) by a ring
flux in the stator yoke. The ring flux is caused by irregularities within the yoke (e.g. dovetailed
punchings to clamp the core, ventilation ducts, magnetic anisotropies of the laminations).
Normally the shaft voltage is predominantly of power frequency, superimposed by a component
of three-times the power frequency caused by saturation effects. If the shaft voltage does not
exceed approximately 500 mV (peak), no protective devices are necessary according to
experience of long standing. Shaft voltages above approximately 500 mV (peak) may produce
circulating currents in the conducting loop indicated above, which may destroy the bearings
within a relatively short period of time. Insulation of one bearing, preferably at the non-drive
end, is sufficient to avoid circulating currents through both bearings and eventually through
bearings of the driven equipment in case of a conducting coupling. The insulation of bearings is
neither necessary nor customary in case of motors within the scope of this specification, when
operated at sinusoidal supply voltage and fabricated according to the current state of the art.

icirc
φring (t)

IEC 792/06
--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---

Figure 9 – Ring flux including shaft voltage and

resulting circulating current i circ

During current source converter operation the shaft voltage is increased slightly caused by the
harmonics of the supplying currents. The same approximate upper limit 500 mV (peak) as for
sinusoidal supply is recommended.

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 DD IEC/TS 60034-17:2006
60034-17 – 16 –

A completely new source of bearing currents exists for voltage source converter operation, that
is the so-called common mode voltage of the converter. The common mode voltage is inherent
to the topology and to the control algorithm of the converter, it consists in particular of
frequency components related to the harmonic content of the output voltage. Its peak value is
in the range of 50 % of the d.c. voltage in the intermediate circuit of the converter or in the
range of 72 % of the line-to-line voltage at the converter input in case of an uncontrolled 6-
pulse rectifier. The common mode voltages of the three phases are in phase and can be
considered as a zero-sequence component of the voltages. The equivalent circuit of the
common mode model is shown in Figure 10. Contrary to the performance at power frequency,
--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---

the capacitances play an important role at high frequencies. The amplitude of the common
mode current depends on the impedance, especially the reactance, of the common mode
circuit; the common mode current flows finally back to the neutral point of the converter.
Generally, the common mode current can take three different paths through the motor bearings
as detailed below:

• A circulating current passing the same loop as described above may be caused by a ring
flux of high frequency which is not associated with irregularities in the stator yoke, but with
currents flowing through the capacitances between the winding and the core. The latter
effect results in different currents in both coil sides of one turn. Therefore, the net ampere
turns of the complete winding are not zero, and consequently a ring flux is generated
according to Ampere’s law.

Depending on the switching frequency, on the rise time of the pulse and on the motor
rating, the shaft voltage contains peaks of high frequency, possibly in excess of 10 V which
may cause the puncture of the lubrication film of the bearings. Tests showed that the
current flow may be sustained by the components of operating frequency and its third
harmonic, even when the amplitude of the low frequency shaft voltage is less than 500 mV
(peak). Therefore, a motor with low shaft voltage at sinusoidal supply is advantageous to
avoid circulating currents at converter supply.

If it is intended to measure the shaft voltage during converter operation, appropriate

precautions should be taken and specific instrumentation and shielded measuring cables
have to be used. Otherwise, the results would be falsified by the components of higher
frequencies.

• Pulse-shaped capacitive currents through the motor bearings, especially the bearing at the
drive end, arise, if the potential of the shaft is closer to earth potential of the converter than
the potential of the motor frame. This configuration is fostered, when motor and driven
machine are connected by a conductive coupling and the motor frame is not grounded
adequately. Amplitudes of more than 10 A were measured in motors of shaft height above
315 mm, by which the motor bearings were destroyed within several hundred operating
hours.

• A capacitively coupled voltage, the so-called bearing voltage, at the radial clearance of the
bearings can be measured, if the stator core and the frame are well grounded (see Figure
10). The bearing voltage is a mirror image of the common mode voltage. Its percentage of
the common voltage is called BVR (Bearing Voltage Ratio) and depends on the
capacitances between the stator winding and the rotor, between the rotor and the housing,
and the capacitance of the bearing itself. According to measurements, typically, the bearing
voltage is in the range of 10 V to 30 V (peak). Short-time discharging currents (so-called
EDM 1 currents) arise if the bearing voltage exceeds its breakdown value. The repetition
rate of the EDM currents increases with increasing values of bearing voltage and switching
frequency. Peak values of the EDM breakdowns in the range of several amperes with a
repetition rate of 50 to 100 per 20 ms were measured. The EDM currents cannot be

___________
1 EDM = Electrostatic Discharge Machining.

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 – 17 – DD IEC/TS 60034-17:2006

suppressed by the insulation of one bearing only. The EDM breakdowns cause pitting of the
bearing races and can be disclosed in the beginning by an increase of the bearing noise.

If a proper grounding system is installed, experiences shows that:

• motors within the scope of this specification and with shaft heights up to and including
280 mm seldom experience bearing failure caused by converter operation. Nevertheless,
the dielectric stress on the bearings varies widely with the type of control algorithm and
especially with the switching frequency of the converter. When using converters having a
switching frequency greater than 10 kHz and an output voltage greater than 400 V rms,
consideration should be given to the insulation of one bearing;

• for motors within the scope of this specification the insulation of an antifriction bearing can
be achieved by replacement with an insulated bearing of the same dimensions. For motors
with shaft height 315 mm and above the use of bearing insulation with the insulation
impedance at least of 100 Ω at 1 MHz is advisable.

The need to insulate both motor bearings is seldom necessary. In such a case, the
examination of the whole drive system by an expert is highly recommended and should
include the driven machine (insulation of the coupling) and the grounding system (possibly
use of an earthing brush);

• for machines within the scope of this specification and with shaft heights above 315 mm,
for which the insulation of the motor bearing is not possible or not desirable, it is
recommended either:

– to reduce the du/dt of the converter output voltage,

– or to use a converter with a filter designed to reduce the zero-sequence component of

--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---
the phase voltages (so-called common mode voltage).
C B

R0 L0 Csr

RB/2

ucm usng ubrg

Csf Crf 2CB
iEDM

iB

IEC 793/06

Key

B represents the bearing i EDM is the electrostatic discharge machining (EDM) current
C represents the cable L0 is the leakage inductance
C sr is the stator rotor capacitance RB is the bearing resistance
C B is the bearing capacitance R0 is the winding resistance
C sf is the stator frame capacitance u sng is the stator neutral ground voltage
C rf is the rotor frame capacitance u brg is the bearing voltage
iB is the bearing current u cm is the common mode voltage

Figure 10 – Common mode circuit model and bearing voltage u brg

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 DD IEC/TS 60034-17:2006
60034-17 – 18 –

11 Installation (cabling, grounding, bonding)

In order to reduce the endangering of the motor bearings by the effects of the common mode
voltage, all efforts must be undertaken to minimize the penetration of common mode currents
into the interior of the motor. Instead of, the common mode currents should be led back to the
converter through the cable shield and/or the earth conductors of the motor.

To meet this requirement (and the EMC-requirements, too) shielded symmetrical multi-core
cables should be used. The shield should be made by copper or aluminium in order to achieve
a low impedance also for high frequencies. The shield must be connected to PE at both ends.
Cables, in which the earth conductor(s) is placed symmetrical to all phase conductors, are
recommendable. If the shield is used as a protective conductor, its conductivity must be
selected carefully.

--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---
When installing the cable it should be ensured that the shield is high frequency connected to
both the converter and the motor enclosure. It is, therefore, necessary that the terminal box of
the motor is made by an electrical conductive material and that it is high frequency electrically
connected to the enclosure. The impedance between the motor housing and the cable shield or
the earth conductor(s) at the motor end should be less than 1 Ω at 1 MHz.

All exposed metallic parts of an installation must be connected to the earthing system to meet
safety requirements. In the case of converter fed motors, the bonding connections should also
meet requirements for low inductance at high frequencies. Especially broad braided straps
made by copper are suitable for this purpose.

Such bonding straps can also be used to equalize the potential of the motor housing and the
terminal box.

Depending on the grounding conditions of the driven machine, a potential equalization

connection between the motor and the machinery is sometimes needed for motors with shaft
heights above 315 mm. Typical applications are well grounded machines like water pumps,
which are connected to the driving motor by a metallic coupling.

Auxiliary devices, for example tachometers, should be electrically insulated from the motor in
order to prevent parasitic currents. The shield of the tachometer cable should be grounded at
the converter, but insulated from the tachometer frame. The use of a double shielded cable is
preferred for a pulse encoder. The cable routing of auxiliary devices should be separated from
that of the power cabling.

12 Maximum safe operating speed

If a motor is intended to be operated at speeds above its rated speed, the maximum safe
operating speed is obtained from 9.6 of IEC 60034-1. Depending on the motor design, the
operation at higher speeds may be permitted, but this possibility should be verified by the
manufacturer.

When operating at speeds above rated speed, noise and vibration levels will increase. It may
also be required to refine the balance for acceptable operation above rated speed.

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 – 19 – DD IEC/TS 60034-17:2006

Operation at speeds close to the maximum safe operating speed for extended periods of time
may cause considerable shortening of the service life of the bearings. Moreover, the shaft
seals and/or the regreasing intervals (or the grease service life in the case of greased-for-life
bearings) may be affected.

13 Power factor correction

Power factor correction at the input of the converter should never be undertaken without
harmonic analysis.

The use of power capacitors for power factor correction on the load side of an electronic
control connected to an induction motor is not recommended; damage to the control may occur
and power factor capacitors are not generally rated for the high frequencies to which they are
subjected.

Power factor correction at the input of a voltage source converter can be achieved by the use
of a converter with an active front end.

____________

--`,``,,`,```,,,````,`,``,```,,-`-`,,`,,`,`,,`---

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 DD IEC/TS
60034-17:2006
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