IEC 60034-18-41:2014

IEC 60034-18-41 ®
Edition 1.0 2014-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Rotating electrical machines –
Part 18-41: Partial discharge free electrical insulation systems (Type I) used in
rotating electrical machines fed from voltage converters – Qualification and
quality control tests
Machines électriques tournantes –
Partie 18-41: Systèmes d’isolation électrique sans décharge partielle (Type I)
utilisés dans des machines électriques tournantes alimentées par des
convertisseurs de tension – Essais de qualification et de contrôle qualité

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IEC 60034-18-41 ®
Edition 1.0 2014-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Rotating electrical machines –

Part 18-41: Partial discharge free electrical insulation systems (Type I) used in

rotating electrical machines fed from voltage converters – Qualification and

quality control tests
Machines électriques tournantes –

Partie 18-41: Systèmes d’isolation électrique sans décharge partielle (Type I)

utilisés dans des machines électriques tournantes alimentées par des

convertisseurs de tension – Essais de qualification et de contrôle qualité

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 29.160 ISBN 978-2-8322-1416-9

– 2 – IEC 60034-18-41:2014 © IEC 2014
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Machine terminal voltages arising from converter operation . 13
5 Electrical stresses in the insulation system of machine windings . 17
5.1 General . 17
5.2 Voltages stressing the phase/phase insulation . 18
5.3 Voltages stressing the phase/ground insulation . 18
5.4 Voltages stressing the turn and strand insulation . 18
5.5 Mechanisms of insulation degradation . 19
6 Types of machine insulation . 20
7 Stress categories for Type I insulation systems used in converter fed machines . 20
8 Design qualification and type tests for Type I insulation systems . 22
8.1 General . 22
8.2 Design qualification test . 22
8.3 Type test . 22
9 Test equipment . 22
9.1 PD measurement at power frequency . 22
9.2 PD measurement during voltage impulses. 22
9.3 Voltage impulse generators . 23
9.4 Sensitivity . 23
9.5 PD tests . 23
9.5.1 Power frequency voltage . 23
9.5.2 Impulse excitation . 23
10 Qualification of the design of Type I insulation systems . 23
10.1 General . 23
10.2 Approach . 24
10.2.1 General . 24
10.2.2 Twisted pair or equivalent arrangement . 24
10.2.3 Motorette (random wound) or formette (form-wound) . 24
10.2.4 Complete windings . 24
10.3 Preparation of test objects . 25
10.3.1 General . 25
10.3.2 Turn/turn insulation samples . 25
10.3.3 Motorette/formette test samples or complete windings . 25
10.4 Design qualification tests . 26
10.4.1 General . 26
10.4.2 Pre-diagnostic tests . 26
10.4.3 Diagnostic tests . 26
10.4.4 Ageing cycle . 26
10.4.5 PD tests . 26
10.5 Pass criterion for the design qualification test . 27
11 Type test procedure for Type I insulation systems . 27
11.1 General . 27

11.2 Power frequency PD tests . 27
11.3 Impulse PD tests . 28
12 Routine tests . 28
13 Analysis, reporting and classification . 28
Annex A (informative) Derivation of possible terminal voltages in service for a
converter-fed machine . 29
A.1 Calculation of d.c. bus voltage . 29
A.2 Calculation of maximum peak voltages for a 2-level converter . 30
Annex B (normative) Derivation of test voltages for Type I insulation systems . 32
B.1 Stress categories . 32
B.2 Requirements for the applied impulse voltage . 32
B.3 Enhancement factors for PD tests . 33
B.4 Voltage for design qualification and type tests . 34
B.5 Examples of maximum peak/peak operating voltages . 37
B.6 Calculation of test voltages . 37
Annex C (normative) Derivation of allowable voltages in service . 39
C.1 Impulse voltage insulation class (IVIC) of the machine . 39
C.2 Impulse voltage insulation class assigned in special designs . 39
Bibliography . 41

Figure 1 – Voltage impulse waveshape parameters . 13
Figure 2 – Five step phase to phase voltage at the terminals of a machine fed by a 3-
level converter . 15
Figure 3 – Jump voltage (U ) at the machine terminals associated with a converter
j
drive . 15
Figure 4 – Voltage enhancement at the terminals of a motor due to reflection as a
function of cable length for various impulse rise times . 17
Figure 5 – Example of a random wound design . 18
Figure 6 – Example of a form-wound design . 18
Figure 7 – Worst case voltage stressing the turn/turn insulation in a variety of random
wound stators as a function of the rise time of the impulse . 19
Figure A.1 – Circuit diagram for a converter/machine system . 29
Figure B.1 – Forbidden zone (shaded) for impulse tests . 33
Figure B.2 – Examples of test waveforms . 33
Figure B.3 – Comparison of phase/phase, phase/ground, and turn/turn voltages for a
2-level converter . 35
Figure B.4 – Impulse test voltage waveforms and the levels for applying the same
peak/peak voltage of 2aU on the turn/turn insulation (schematic representation) . 36
j
Figure B.5 – Test voltages for phase/ground and turn/turn impulse tests using a
unipolar impulse . 38

Table 1 – Common ranges of characteristics of the terminal voltages of converter fed
machines . 14
Table 2 – Definition of symbols . 14
Table 3 – Influence of features of the machine terminal voltage on components of Type
I insulation systems . 21
Table 4 – Stress categories for Type I insulation systems based on a 2-level converter . 21
Table 5 – Allowable voltage waveforms for testing system components . 25

– 4 – IEC 60034-18-41:2014 © IEC 2014
Table A.1 – Examples of maximum peak voltages . 31
Table B.1 – Summary of stress categories . 32
Table B.2 – Summary of enhancement factors to be applied to the operating voltages . 34
Table B.3 – Maximum peak/peak operating voltages related to U for a 2-level
dc
converter according to the stress categories of Table 4 . 36
Table B.4 – Examples of maximum peak/peak operating voltage for a 500 V r.m.s. rated
winding fed from a 2-level converter, according to the stress categories of Table 4. 37
Table B.5 – Examples of maximum peak/peak test voltage for a 500 V rated winding fed
from a 2-level converter, according to the stress categories of Table 4 and with EF 1,25 . 37
Table B.6 – Turn/turn PD test levels for special windings and twisted pairs . 38
Table C.1 – Maximum allowable operating voltage at the machine terminals in units of U . 39
N
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ROTATING ELECTRICAL MACHINES –

Part 18-41: Partial discharge free electrical insulation systems (Type I)
used in rotating electrical machines fed from voltage converters –
Qualification and quality control tests

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60034-18-41 has been prepared by IEC technical committee 2:
Rotating machinery.
IEC 60034-18-41 cancels and replaces IEC/TS 60034-18-41 (2006).
The text of this standard is based on the following documents:
FDIS Report on voting
2/1728/FDIS 2/1738/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 6 – IEC 60034-18-41:2014 © IEC 2014
NOTE A table of cross-references of all IEC TC 2 publications can be found in the IEC TC 2 dashboard on the
IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
The approval of electrical insulation systems for use in rotating electrical machines driven
from voltage converters is set out in two IEC documents. They divide the systems into those
which are not expected to experience partial discharge activity within specified conditions in
their service lives (Type I) and those which are expected to withstand partial discharge
activity in any part of the insulation system throughout their service lives (Type II). For both
Type I and Type II insulation systems, the drive system integrator (the person responsible for
co-ordinating the electrical performance of the entire drive system) shall inform the machine
manufacturer what voltage will appear at the machine terminals in service. The machine
manufacturer will then decide upon the severity of the tests appropriate for qualifying the
insulation system. The severity is based on the impulse rise time, the peak to peak voltage
and, in the case of Type II systems, the impulse repetition rate. After installation of the
converter/machine system, it is recommended that the drive system integrator measures the
phase/phase and phase/ground voltages between the machine terminals and ground to check
for compliance.
IEC 60034-18-41
The Type I systems are dealt with in this standard. They are generally used in rotating
machines rated at 700 V r.m.s. or less and tend to have random wound windings. The
procedures described here are directed at:
– Qualification of the insulation system.
– Type and routine testing of the complete windings of service machines.
Before undertaking any testing, the machine manufacturer shall decide upon the level of
severity that the system will be required to withstand. The severity is based on how large the
voltage overshoot and how short the impulse rise time will be at the machine terminals. The
machine designer then makes a choice from a table in which the range of expected overshoot
voltage is divided into bands. Testing is performed at the extreme value of each band. A
default value of 0,3 µs is attributed to the impulse rise time. Other values of impulse rise time
or voltage overshoot are dealt with as special cases.
In qualification testing, the insulation system is used to construct various representative test
objects. These are subjected to the range of tests described in IEC 60034-18-21 or
IEC 60034-18-31 with the addition of a high frequency voltage test and a partial discharge
test. For the latter, it may be necessary to use impulse test equipment, as described in
IEC/TS 61934. If the test object is partial discharge free under the specified test conditions at
the end of the sequence of testing, the insulation system is qualified for the severity band that
has been selected.
Type and optional routine tests are performed on complete windings to demonstrate that they
are partial discharge free under sinewave or impulse voltage conditions (as appropriate) for
the band of severity that the manufacturer has chosen. An impulse voltage insulation class is
then assigned to the machine. A mechanism is described for dealing with special cases.
IEC/TS 60034-18-42
The tests for qualification and acceptance of electrical insulation systems chosen for Type II
rotating electrical machines are described in this technical specification. These insulation
systems are generally used in rotating machines and tend to have form-wound coils, mostly
rated above 700 V r.m.s. The qualification procedure is completely different from that used for
Type I insulation systems and involves destructive ageing of insulated test objects under
accelerated conditions. The rotating machine manufacturer requires a life curve for the
insulation system that can be interpreted to provide an estimate of life under the service
conditions with converter drive. Great importance is attached to the qualification of any stress
grading system that is used and testing here should be performed under repetitive impulse
conditions. If the insulation system can be shown to provide an acceptable life under the

– 8 – IEC 60034-18-41:2014 © IEC 2014
appropriate ageing conditions, it is qualified for use. Acceptance testing is performed on coils
made using this insulation system when subjected to a voltage endurance test.

ROTATING ELECTRICAL MACHINES –

Part 18-41: Partial discharge free electrical insulation systems (Type I)
used in rotating electrical machines fed from voltage converters –
Qualification and quality control tests

1 Scope
This part of IEC 60034 defines criteria for assessing the insulation system of stator/rotor
windings which are subjected to voltage-source pulse-width-modulation (PWM) drives. It
applies to stator/rotor windings of single or polyphase AC machines with insulation systems
for converter operation.
It describes qualification tests and quality control (type and routine) tests on representative
samples or on completed machines which verify fitness for operation with voltage source
converters.
This standard does not apply to:
– rotating machines which are only started by converters;
– rotating electrical machines with rated voltage ≤ 300 V r.m.s.;
– rotor windings of rotating electrical machines operating at ≤ 200 V (peak).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60034-18-1:2010, Rotating electrical machines – Part 18-1: Functional evaluation of
insulation systems – General guidelines
IEC 60034-18-21, Rotating electrical machines – Part 18-21: Functional evaluation of
insulation systems – Test procedures for wire-wound windings – Thermal evaluation and
classification
IEC 60034-18-31, Rotating electrical machines – Part 18-31: Functional evaluation of
insulation systems – Test procedures for form-wound windings – Thermal evaluation and
classification of insulation systems used in rotating machines
IEC/TS 60034-18-42, Rotating electrical machines – Part 18-42: Qualification and acceptance
tests for partial discharge resistant electrical insulation systems (Type II) used in rotating
electrical machines fed from voltage converters
IEC/TS 60034-25:2007, Rotating electrical machines – Part 25: Guidance for the design and
performance of a.c. motors specifically designed for converter supply
_______________
This TS is in the process of being transformed into an IS.

– 10 – IEC 60034-18-41:2014 © IEC 2014
IEC/TS 60034-27, Rotating electrical machines – Part 27: Off-line partial discharge
measurements on the stator winding insulation of rotating electrical machines
IEC 60172, Test procedure for the determination of the temperature index of enamelled
winding wires
IEC 60664-1, Insulation co-ordination for equipment within low voltage systems – Part 1:
Principles, requirements and tests
IEC/TS 61800-8, Adjustable speed electrical power drive systems – Part 8: Specification of
voltage on the power interface
IEC/TS 61934, Electrical insulating materials and systems – Electrical measurement of partial
discharges (PD) under short rise time and repetitive voltage impulses
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
partial discharge
PD
electric discharge that only partially bridges the insulation between electrical conductors
Note 1 to entry: It may occur inside the insulation or adjacent to an electrical conductor.
3.2
partial discharge inception voltage
PDIV
lowest voltage at which partial discharges are initiated in the test arrangement when the
voltage applied to the test object is gradually increased from a lower value at which no such
discharges are observed
Note 1 to entry: With sinusoidal applied voltage, the PDIV is defined as the r.m.s. value of the voltage. With
impulse voltages, the PDIV is defined as the peak to peak voltage.
3.3
partial discharge extinction voltage
PDEV
voltage at which partial discharges are extinguished in the test arrangement when the voltage
applied to the test object is gradually decreased from a higher value at which such discharges
are observed
Note 1 to entry: With sinusoidal applied voltage, the PDEV is defined as the r.m.s. value of the voltage. With
impulse voltages, the PDEV is defined as the peak to peak voltage.
3.4
peak (impulse) voltage
U
p
maximum numerical value of voltage reached during a unipolar voltage impulse (e.g. U in
p
Figure 1)
Note 1 to entry: For bi-polar voltage impulses, it is half the peak to peak voltage (see Figure 2).
Note 2 to entry: The definition of peak to peak voltage is clarified in Clause 4.
3.5
steady state impulse voltage magnitude
U
a
final magnitude of the voltage impulse (see Figure 1)

3.6
voltage overshoot
U
b
magnitude of the peak voltage in excess of the steady state impulse voltage (see Figure 1)
3.7
peak to peak impulse voltage
U’
pk/pk
peak to peak voltage at the impulse repetition rate (see Figure 2)
3.8
peak to peak voltage
U
pk/pk
peak to peak voltage at the fundamental frequency (see Figure 2)
3.9
repetitive partial discharge inception voltage
RPDIV
minimum peak-to-peak impulse voltage at which more than five PD pulses occur on ten
voltage impulses of the same polarity
Note 1 to entry: This is a mean value for the specified test time and a test arrangement where the voltage applied
to the test object is gradually increased from a value at which no partial discharges can be detected.
3.10
unipolar impulse
voltage impulse, the polarity of which is either positive or negative
Note 1 to entry: The term impulse is used to describe the transient stressing voltage applied to the test object and
the term pulse is used to describe the partial discharge signal.
3.11
bipolar impulse
voltage impulse, the polarity of which changes alternately from positive to negative or vice
versa
3.12
impulse voltage repetition rate
f
inverse of the average time between two successive impulses of the same polarity, whether
unipolar or bipolar
3.13
impulse rise time
t
r
time for the voltage to rise from 10 % to 90 % of its final value (see Figure 1)
3.14
electrical insulation system
insulating structure containing one or more electrical insulating materials together with
associated conducting parts employed in an electrotechnical device
3.15
formette
special test model used for the evaluation of the electrical insulation systems for form-wound
windings
– 12 – IEC 60034-18-41:2014 © IEC 2014
3.16
motorette
special test model used for the evaluation of the electrical insulation systems of random-
wound windings
3.17
(electric) stress
electric field in volts/mm
3.18
rated voltage
U
N
voltage assigned by the manufacturer for a specified power frequency operating condition of a
machine and indicated on its rating plate
3.19
impulse voltage insulation class
IVIC
safe peak to peak voltage assigned by the manufacturer in relation to the rated voltage for a
specified converter-driven machine and indicated in its documentation and on its rating plate
3.20
fundamental frequency
first frequency, in the spectrum obtained from a Fourier transform of a periodic time function,
to which all the frequencies of the spectrum are referred.
Note 1 to entry: For the purposes of this standard, the fundamental frequency of the machine terminal voltage is
the one defining the speed of the converter fed machine.
3.21
impulse duration
impulse width
interval of time between the first and last instants at which the instantaneous value of an
impulse reaches a specified fraction of its impulse magnitude or a specified threshold.
3.22
jump voltage
U
j
change in voltage at the terminals of the machine occurring at the start of each impulse when
fed from a converter (see Figure 3)
3.23
DC bus voltage
U
dc
voltage of the intermediate circuit of the voltage converter (dc-link-circuit)
Note 1 to entry: For a two level converter U is equal to U in Figure 1.
dc a
Note 2 to entry: For a multilevel converter, U is equal to ½ Upk/pk minus the overshoot in Figure 2.
dc
3.24
overshoot factor
ratio of the voltage appearing at the machine terminals and the voltage at the converter for
each converter level
3.25
power drive system
complete drive module and rotating machine together with the connecting cable if necessary

t
r
IEC  0561/14
Key
U voltage
t time
Figure 1 – Voltage impulse waveshape parameters
4 Machine terminal voltages arising from converter operation
Modern converter output voltage rise times may be in the 0,05 µs – 2,0 µs range due to power
semiconductor switching characteristics. The voltage appearing at the terminals of a converter
driven machine may be calculated using IEC/TS 61800-8 and depends upon several
characteristics of the power drive system, such as,
a) operating line voltage of the converter;
b) architecture and control regime of the converter;
c) filters between the converter and machine;
d) length and type of cable between them;
e) design of the machine winding;
f) design and configuration of the installation.
In order to apply this Standard to the qualification and testing of the insulation system of a
winding, it is necessary to specify the required parameters of the voltage appearing at the
machine terminals (Clause 7).
The amplitude and rise time of the voltage at the machine terminals depend upon the
grounding system, various design aspects of the cable, the machine surge impedance and the
presence of any filters that increase the impulse rise time. Common ranges of characteristics
of converter impulses at the machine terminals are given in Table 1.

– 14 – IEC 60034-18-41:2014 © IEC 2014
Table 1 – Common ranges of characteristics of the terminal
voltages of converter fed machines
Characteristics Range of values
(depending on ratings, characteristics and
service conditions of the drive system)
Peak/peak voltage 0,5 kV – 7 kV
Impulse rise time 0,05 µs – 2,0 µs
Impulse voltage repetition rate 100 Hz – 20 000 Hz
Impulse duration
10 µs – 10 000 µs
Shape Rectangular
Polarity Unipolar or bipolar
Fundamental frequency 5 Hz – 1 000 Hz
Mean time between impulses ≥ 0,6 µs
For the purpose of this standard, the symbols in Table 2 are used.
Table 2 – Definition of symbols
Symbol Parameter Units Type of feed
U Phase to phase (rated) voltage V r.m.s. Line
line
U Phase to neutral voltage V r.m.s. Line

phase
Maximum phase/neutral voltage V Line
U = U
max phase
U Peak to peak voltage V Converter
pk/pk
U DC bus voltage V Converter
dc
In the case of 2-level or other U converters, depending on the rise time of the voltage impulse
at the converter output and on the cable length and machine impedance, the impulses
generate voltage overshoots at the machine terminals (typically U up to 2U between
p dc
phases). The voltage overshoot is created by reflected waves at the interface between cable
and machine or converter terminals due to surge impedance mismatch. It is fully explained by
transmission line and travelling wave theory.
Figure 2 shows the voltage that appears (during one period at the fundamental frequency) at
the machine terminals when fed from a 3-level converter.

U
U pk/pk
t
U'
pk/pk
IEC  0562/14
Figure 2 – Five step phase to phase voltage at the terminals
of a machine fed by a 3-level converter

U
t
U j
IEC  0563/14
Figure 3 – Jump voltage (U ) at the machine terminals associated
j
with a converter drive
The maximum change in voltage, U , at the impulse frequency is shown in Figure 3. This
j
parameter is important in defining the voltage enhancement that can occur across the first or
last coil in the winding. A double jump transition is possible but it is the duty of the drive
system integrator to ensure that the software controlling the converter drive prevents this from
happening.
– 16 – IEC 60034-18-41:2014 © IEC 2014
For an “n” level converter, the phase/phase voltage can be estimated as follows:
Peak/peak fundamental frequency voltage = 2(U + U ) (1)
dc b
Peak/peak impulse frequency voltage = U /(n–1) + 2U
dc b
The phase/ground values are estimated as follows:
Peak/peak fundamental frequency voltage = 0,7 × 2(U + U ) (2)
dc b
Peak/peak impulse frequency voltage = 0,7(U /(n–1) + 2U )
dc b
The jump voltage is given by 0,7(U /(n–1) + U ) (3)
dc b
The proportion of jump voltage appearing across the first turn is obtained from Figure 7.
The value of U in these formulae is the value shown in Figure 1 for the phase/phase voltage
b
on the machine terminals. The values of the phase/ground voltages estimated from these
formulae may be higher or lower in practice, depending upon the grounding system, converter
control regime and other factors. It is known that a sudden rise can occur in the machine
ground voltage level with respect to the d.c. zero point in the converter. The theoretical rise is
determined capacitively to be 1/3 which gives a residual effect of about 0,7. This would apply
to simple systems where only travelling wave theory determines the factor, i.e. stress
categories A, B and C (see Clause 7).
Examples of the enhancement that is produced for various rise times and cable lengths in the
case of a motor driven from a converter are given in Figure 4. In this case, the enhancement
to the voltage for an impulse rise time of 1,0 µs is insignificant below about 15 m and only
exceeds 1,2 when the cable length is greater than about 50 m.
Voltages above 2U can be produced at the terminals of the machine by drive double
dc
transition and by a converter fed drive algorithm that does not allow a minimum time between
successive pulses. Double transition occurs, for example, when one phase switches from
minus to plus d.c. bus voltage at the same instant that another phase switches from plus to
minus. This generates a 2U voltage wave which travels to the machine and can then
dc
increase in magnitude when reflected at the machine terminals. If there is no minimum
impulse time control in the drive and if the time between two impulses is matched with the
time constant of the cable between the converter and the machine, an over voltage >2U can
dc
be generated at the machine terminals. The reflection can be reduced or prevented by using a
filter in the converter, at the machine terminals or both.
In the event of an earth fault on one of the phases of a system where the neutral star point is
not grounded, the machine may be permitted by the manufacturer to run for a period of
several hours until a suitable outage can be arranged for repairs. In this case, the voltage
stress on the turn to ground insulation in the other phases will increase.

U /U
p a
IEC  0564/14
Key
● t = 0,05 µs   ○ t = 0,1 µs  ▼ t = 0,2 µs   ∇ t = 1,0 µs
r r r r
l (m) cable length
U /U ratio of peak voltages at the machine and at the converter terminals
p a
Figure 4 – Voltage enhancement at the terminals of a motor due to reflection
as a function of cable length for various impulse rise times
5 Electrical stresses in the insulation system of machine windings
5.1 General
If a winding experiences short rise time voltage impulses of significant magnitude, high
voltage stresses will be created, for example, in the following locations (Figures 5 and 6):
• between conductors in different phases,
• between a conductor and ground,
• between adjacent turns in the line-end coil.
Due to space and surface charge creation within the insulation components, the electric
stress is not only defined by the instantaneous voltage itself but also by the peak voltages
that have been stressing the insulation previously. Generally, it has been shown by
experience that, within certain limits valid for drive systems, the stressing parameter is the
peak/peak voltage. This is also the reason why a unipolar voltage produces the same stress
as a bi-polar voltage having a peak/peak voltage of the same value [1] .
_______________
Numbers in square brackets refer t
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