IEC 60034-18-31:2012

IEC 60034-18-31 ®
Edition 2.0 2012-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
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

Machines électriques tournantes –
Partie 18-31: Evaluation fonctionnelle des systèmes d'isolation – Procédures
d'essai pour enroulements préformés – Evaluation thermique et classification
des systèmes d'isolation utilisés dans les machines tournantes

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IEC 60034-18-31 ®
Edition 2.0 2012-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
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

Machines électriques tournantes –

Partie 18-31: Evaluation fonctionnelle des systèmes d'isolation – Procédures

d'essai pour enroulements préformés – Evaluation thermique et classification

des systèmes d'isolation utilisés dans les machines tournantes

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX U
ICS 29.160 ISBN 978-2-83220-159-6

– 2 – 60034-18-31 © IEC:2012
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 General considerations . 8
3.1 Reference insulation system . 8
3.2 Test procedures . 8
4 Test objects and test specimens . 9
4.1 Construction of test objects . 9
4.2 Verification of effects of minor changes in insulation systems . 9
4.3 Number of test specimens . 9
4.4 Quality control . 9
4.5 Initial diagnostic tests . 10
5 Test procedures . 10
5.1 Thermal ageing sub-cycle . 10
5.2 Ageing temperatures and sub-cycle lengths . 10
5.3 Method of heating. 11
5.4 Thermal ageing of test objects . 12
6 Diagnostic sub-cycle. 12
6.1 Conditioning procedure . 12
6.1.1 General . 12
6.1.2 Mechanical conditioning . 12
6.1.3 Moisture conditioning . 13
6.2 Diagnostic tests . 13
6.2.1 Voltage withstand test . 13
6.2.2 Method . 14
6.2.3 Mainwall insulation test. 14
6.2.4 Turn insulation test . 14
6.3 Informative tests . 15
7 Reporting and functional evaluation of data from candidate and reference systems . 15
7.1 General . 15
7.2 Determining qualification . 15
7.2.1 Overview . 15
7.2.2 Case A: Qualification for the same class temperature and same
expected service life . 16
7.2.3 Case B: Qualification for the same class temperature and a different
expected service life . 17
7.2.4 Case C: Qualification for a different class temperature and same
expected service life . 18
7.2.5 Case D: Qualification for a different class temperature and different
expected service life . 19
7.2.6 Non-linearity of regression lines . 20
7.2.7 Reduced evaluation . 20
Annex A (informative) Example of test object (formette) construction . 22
Bibliography . 27

60034-18-31 © IEC:2012 – 3 –
Figure 1 – Candidate system qualified for the same thermal class and the same
expected service life . 17
Figure 2 – Candidate system qualified for the same thermal class and different
expected service life . 18
Figure 3 – Candidate system qualified for a different class temperature and the same
expected service life . 19
Figure 4 – Candidate system qualified for a different service life and different thermal
class from the reference . 20
Figure A.1 – Typical slot assembly . 23
Figure A.2 – Typical slot assembly . 24
Figure A.3 – Formette for testing DC armature coils . 25
Figure A.4 – Test fixture for rotor slot section . 26

Table 1 – Thermal classes . 10
Table 2 – Recommended temperatures and ageing sub-cycle exposure periods . 11
Table 3 – Conditions for qualification of candidate system . 16

– 4 – 60034-18-31 © IEC:2012
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
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

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 60034-18-31 has been prepared by IEC technical committee 2:
Rotating machinery.
This second edition cancels and replaces the first edition published in 1992, and its
amendment 1 (1996), of which it constitutes a technical revision.
The main technical changes with regard to the previous edition include:
– Definition of the test method and sub-cycles required to establish a consistent
standardized platform for thermal ageing of insulation systems for form-wound windings.
– Recommendations for establishing a thermal life curve based on confidence intervals.
– Comparison of candidate and reference system performance for specific requirements of
thermal class, within feasible limits.

60034-18-31 © IEC:2012 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
2/1662/FDIS 2/1671/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.
NOTE A table of cross-references of all IEC TC 2 publications can be found on 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.
– 6 – 60034-18-31 © IEC:2012
INTRODUCTION
IEC 60034-18 comprises several parts, dealing with different types of functional evaluation
and special kinds of test procedures for insulation systems of rotating electrial machines.
IEC 60034-18-1 provides general guidelines for such procedures and qualification principles.
The subsequent parts IEC 60034-18-21, IEC 60034-18-31, IEC 60034-18-32,
IEC 60034-18-33, IEC 60034-18-34, IEC 60034-18-41 and IEC 60034-18-42 give detailed
procedures for the various types of windings.
IEC 60034-18-31 describes thermal evaluation and classification of insulation systems for
form-wound windings. It provides standard thermal ageing techniques and diagnostic test
procedures.
Parts relevant to this document are:
– IEC 60034-18-1: General guidelines
– IEC 60034-18-21: Test procedures for wire-wound windings
– IEC 60034-18-41: Qualification and type tests for Type I electrical insulation systems used
in rotating electrical machines fed from voltage converters
– IEC 60034-18-42: Qualification and acceptance tests for partial discharge resistant
electrical insulation systems (Type II) electrical insulation systems used in rotating
electrical machines fed from voltage converters

60034-18-31 © IEC:2012 – 7 –
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

1 Scope
This part of IEC 60034 describes thermal endurance test procedures for classification of
insulation systems used in a.c. or d.c. rotating electrical machines with indirect cooling and
form-wound windings.
The test performance of a candidate insulation system is compared to the test performance of
a reference insulation system with proven service experience.
The test procedures described in IEC 60034-18-31 are intended to compare the thermal
endurance performance of the mainwall insulation between conductor(s) and ground and,
where required by the design of the coil or bar, the insulation between the turns.
The test is not intended to simulate the in-service mechanical stresses experienced by the
endwinding bracing or support materials. It does not include the evaluation of thermo-
mechanical deterioration by expansion and contraction of insulation during temperature
cycling.
IEC 60034-18-1 describes general testing principles applicable to thermal endurance testing
of insulation systems used in rotating electrical machines. The principles of IEC 60034-18-1
are followed unless otherwise stated in IEC60034-18-31.
The thermal class for the insulation system refers to its maximum allowed (“hot spot”)
temperature. The average temperature measured in service should not exceed the allowed
temperature rise according to IEC 60034-1.
NOTE 1 Large machines, especially synchronous generators using bars, may require special thermal evaluation
test procedures which are not included in this part.
NOTE 2 Recommended parameters for the diagnostic test may be applied according to IEC 60034-18-42 to form-
wound coils designed with Type II insulation systems for use in converter applications.
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 60034-15:2009, Rotating electrical machines – Part 15: Impulse voltage withstand levels
of form-wound stator coils for rotating a.c. machines
IEC 60034-18-1:2010, Rotating electrical machines – Part 18-1: Functional evaluation of
insulation systems – General guidelines

– 8 – 60034-18-31 © IEC:2012
IEC 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 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60085, Electrical Insulation – Thermal evaluation and designation
IEC 60216-1, Electrical insulating materials – Properties of thermal endurance – Part 1:
Ageing procedures and evaluation of test results
IEC 60216-4-1, Electrical insulating materials – Thermal endurance properties – Part 4-1:
Ageing ovens – Single-chamber ovens
IEC 60216-5, Electrical insulating materials – Thermal endurance properties – Part 5:
Determination of relative thermal endurance index (RTE) of an insulating material
IEC 60505, Evaluation and qualification of electrical insulation systems
3 General considerations
3.1 Reference insulation system
A reference insulation system, as described in 4.3 of IEC 60034-18-1, shall be tested using
the same procedure as that used for the candidate system.
Acceptable service life of a low voltage system does not automatically qualify a high voltage
(HV) system, especially since HV systems usually feature additional materials: for example
corona suppression systems and enhanced turn insulation. These materials may affect the
design and thermal endurance of the system.
A manufacturer may wish to qualify a new system with reduced mainwall thickness. It should
be appropriate to compare the candidate to the reference by modifying the thickness or the
rated voltage (U ) of the diagnostic test, provided the test sample designs include all the
N
specified materials and processing.
3.2 Test procedures
Each thermal endurance test consists of a series of cycles, where each cycle comprises a
thermal ageing sub-cycle followed by a diagnostic sub-cycle, consisting of a conditioning
procedure and a diagnostic test. The conditioning procedure shall include application of
mechanical stress and moisture, in that order. The diagnostic tests are described in Clause 6.
The mechanical conditioning requires shaking the test object on a table equipped to vibrate
with specified amplitude.
A condensation chamber is required for moisture conditioning of an unsealed insulation
system. The condensation chamber and complete immersion are required for moisture
conditioning of a sealed insulation system.
For sealed systems, the diagnostic voltage withstand test is performed while the specimens
are immersed. For unsealed systems, the diagnostic voltage test may be performed outside
the condensation chamber, provided that the samples are still thoroughly wetted. The voltage
withstand test in either case shall be applied across the mainwall insulation and between
turns, where appropriate to the design of the coil or bar. Additional voltage withstand tests
may be applied as appropriate to the design.

60034-18-31 © IEC:2012 – 9 –
In addition to the required tests, non-destructive informative diagnostic tests may be used to
characterize the insulation system or obtain periodic measurements of its response to the
thermal and diagnostic sub-cycles.
4 Test objects and test specimens
4.1 Construction of test objects
The various insulating materials and components of an insulation system to be evaluated by
these test procedures should be screened beforehand. The temperature indices (TI) of
insulating materials may be used, but they provide only an indication of potential performance
in the thermal functional tests and do not constitute qualification of insulation systems.
Wherever feasible, test objects should closely represent the actual construction of the
insulation system to be used in a rotating machine. Usually this requires coils of full cross-
section with actual clearances and creepage distances, mounted in a fixture that simulates
the arrangement of coils in the machine. The coils or bars are the test specimens, and the
complete fixture with specimens in place is the test object.
Where the test specimens are coils or bars, they should represent the full insulation design,
including insulation thickness, coil-to-coil (or bar-to-bar) clearance, and any required corona
suppression or grading materials. The specimens tested shall represent the design for the
intended maximum rated voltage and equipment standards.
For large and high-voltage machines, test specimens representing a partial coil or bar may
be used, provided that:
a) representative factors of influence can be applied to the test specimens to investigate
ageing processes specific to that part of the system, and
b) each material employed in the higher voltage design is represented in the test specimens,
and arranged and processed on the specimens as it would be in service.
Test objects known as formettes simulate the geometry, clearance and placing of a series of
form-wound coils in a winding. They have been used successfully for thermal evaluation
testing. Examples of formettes are illustrated in Annex A.
4.2 Verification of effects of minor changes in insulation systems
A minor change is described in IEC 60034-18-1. An example of a minor change in a form-
wound insulation system may include purchasing a key component material from a new
supplier without changing the material specification. If thermal ageing evaluation is
appropriate to evaluate a minor change to a service-proven insulation system, it is acceptable
to use one temperature to age one test object consisting of no fewer than the recommended
number of test specimens.
This reduced evaluation should be performed using an ageing temperature cycle within the
range of known thermal endurance data for the service-proven system.
4.3 Number of test specimens
Tests should be conducted using no fewer than five test specimens per ageing temperature,
per insulation system. This is the minimum recommended number for statistical confidence.
4.4 Quality control
Each insulating material intended for the preparation of test objects should be tested
separately before assembly to determine that it meets specifications. The quality tests chosen
should ensure that each material is suitable for the processes required to produce the
individual specimens and test object assemblies.

– 10 – 60034-18-31 © IEC:2012
Defective test objects should be identified by visual examination followed by over-voltage
tests consistent with the machine or coil tests used in the manufacturing facility, or as
described in the appropriate subclauses for diagnostic tests, whichever is more stringent. Any
widely deviating test object should be discarded or inspected to determine the reason for the
deviation.
NOTE When appropriate, additional screening (or qualifying) tests may be used, including the following:
– insulation resistance measurement;
– loss tangent and capacitance measurement;
– partial discharge inception voltage measurement;
– balance of phase currents while running;
– repetitive surge;
– leakage current;
– high-voltage test.
4.5 Initial diagnostic tests
Each prepared test object shall be subjected to the complete sequence of tests selected for
the diagnostic sub-cycle and any chosen informative tests before starting the first thermal
ageing sub-cycle.
5 Test procedures
5.1 Thermal ageing sub-cycle
Experience has shown that a thermally degraded and thus usually brittle insulation system
can be detected by exposure to mechanical stress, followed by exposure to moisture and
voltage withstand test. Cracks produced in the aged specimens by the mechanical stress will
be identified by the combination of moisture and applied voltage tests.
5.2 Ageing temperatures and sub-cycle lengths
The intended thermal class (or class temperature) of the candidate insulation system and the
known class of the reference system shall be selected from Table 1, which is a subset of the
thermal classes defined in IEC 60085 and IEC 60505.
Table 1 – Thermal classes
Thermal class rating Thermal class
°C
105 (A) 105
120 (E) 120
130 (B) 130
155 (F) 155
180 (H) 180
200 (N) 200
NOTE 1 Thermal classes 105, 120 and 200 are rarely used in modern insulation systems and are not listed in
IEC 60034-1.
Table 2 lists the recommended ageing temperatures and corresponding periods of exposure
in each thermal ageing sub-cycle for insulation systems of the various thermal classes. The
values in the table are derived approximately on the doubling of life for each 10 K decrease in
ageing temperature. Because the thermal class of the reference system and the intended

60034-18-31 © IEC:2012 – 11 –
thermal class of the candidate system may be different, the ageing temperatures and sub-
cycle exposure periods should be selected appropriately.
At least three different ageing temperatures should be chosen. Although the basis for
qualification is comparison of the candidate performance to that of a reference, certain
minimum criteria for life should ensure that the candidate system can withstand periodic
thermal excursions above the thermal class while in service. The lowest test temperature
should not exceed the class temperature of the reference system by more than 25 K and
should produce a mean test life of at least 5 000 h, and the highest temperature should
produce a mean test life of at least 100 h. The ageing temperatures should be separated by
intervals of at least 20 K. If more than three temperatures are used, the interval may be
reduced to 10 K.
The lengths of ageing sub-cycles should be selected to give a mean life of about 10 cycles for
each ageing temperature.
Table 2 is designed to permit laboratories to choose ageing times and temperatures to
optimize labour and facilities. To permit comparison of the systems, any variations to the
cycles shall be applied equally to all specimens.
To perform a reduced evaluation, the middle ageing temperature cycle may be used to obtain
data more quickly.
Table 2 – Recommended temperatures and ageing sub-cycle exposure periods
Anticipated
105 120 130 155 180 200
thermal class
T < T ≤ T T T T T T T T T T T T T Days per
1 A 2 1 2 1 2 1 2 1 2 1 2 1 2
ageing
sub-cycle
170 180 185 195 195 205 220 230 245 255 265 275 1 – 2
Suggested
range
160 170 175 185 185 195 210 220 235 245 255 265 2 – 3
for ageing
150 160 165 175 175 185 200 210 225 235 245 255 4 – 6
temperature
(T ) °C
A 140 150 155 165 165 175 190 200 215 225 235 245 7 – 10
130 140 145 155 155 165 180 190 205 215 225 235 14 – 21
120 130 135 145 145 155 170 180 195 205 215 225 28 – 35
110 120 125 135 135 145 160 170 185 195 205 215 45 – 60

NOTE 2 Ageing sequences may be designed to accomodate a 5-day working week. For example, an ageing sub-
cycle would always start on a Friday and the diagnostic tests on a Monday (e.g. 3, 10, 17, 31 and 59 days of
ageing).
5.3 Method of heating
Experience has shown that ovens provide a convenient and economical method of thermal
ageing (see IEC 60216-4-1). The oven subjects all the parts of the insulation system to the
full ageing temperature, while in actual service a large proportion of the insulation can operate
at considerably lower temperatures than the hot-spot temperature. Ageing temperatures shall
be controlled and held constant within ± 2 K up to 180 °C and ± 3 K above 180 °C up to
300 °C.
Where ovens are not feasible or do not adequately represent the conditions of service,
alternative heating methods are permitted. Examples of more direct means of heating to
closely simulate certain service conditions include:
– direct heating by electric current applied to the specimen leads;
– application of flexible heaters to the test specimens.

– 12 – 60034-18-31 © IEC:2012
Some materials may deteriorate more rapidly when the products of decomposition remain in
contact with the insulation surface, whereas other materials deteriorate more rapidly when the
decomposition products are continually removed. Ideally, the concentration of the
decomposition products should not change with the ageing temperature but in practical testing
this can be unrealistic. The products of decomposition are likely to remain near the insulation
during oven ageing whereas they can be carried away by ventilation in actual operation.
The same conditions of oven ventilation and rate of replacement of air during thermal ageing
shall be maintained for both the candidate and the reference systems.
5.4 Thermal ageing of test objects
When ovens are used, the test objects should be loaded directly from a room temperature air
environment at the beginning of the ageing sub-cycle into a pre-heated oven. If the oven is
not pre-heated, the rate of temperature increase should be controlled and consistent between
each cycle at each temperature. The purpose is to prevent thermal shock in each cycle.
The thermal ageing sub-cycle of the test objects begins as soon as a calibrated temperature
detector placed on the surface of the insulation on the most shielded area of the test object
reaches the ageing temperature.
Test objects should be removed from the oven directly to room-temperature air at the end of
the sub-cycle, or cooled by other appropriate means, to subject them to uniform thermal shock
on cooling. The ageing sub-cycle ends as soon as the source of heat is removed, or when the
test objects are removed from the oven to ambient conditions. After the thermal ageing sub-
cycle, the test objects are allowed to cool in air to room temperature before starting the
diagnostic sub-cycle.
6 Diagnostic sub-cycle
6.1 Conditioning procedure
6.1.1 General
Following each sub-cycle of thermal ageing, each test specimen shall be subjected to a series
of conditioning sub-cycles, including mechanical stress and moisture exposure.
The tests will be applied using a consistent order and procedure with every cycle. The
sequence and detailed parameters of the diagnostic sub-cycles shall be recorded.
6.1.2 Mechanical conditioning
The procedure for applying mechanical stress may be designed to accommodate specific
types of test objects and the intended service. The mechanical stress is applied to test
objects at room temperature and without applied voltage.
A widely used method for applying mechanical stress is by vibration of the test specimen
endwindings. Each test object is mounted on a horizontal shaking table and subjected to
oscillation at 50 Hz or 60 Hz for 1 h. The motion of the table should be normal to the plane of
the test specimens so that the coil ends will vibrate radially, as expected under the end
winding forces typical of an actual rotating machine.
Where complete machines are used as test objects, a start-stop or reversing cycle may be
used to apply mechanical stress to the windings instead of the shaking table. The procedure
should be designed to accommodate the increase in severity of the stress with increasing
machine size.
The preferred peak-to-peak amplitude of vibration is 0,2 mm at 60 Hz or 0,3 mm at 50 Hz.
This amplitude corresponds to an acceleration of approximately 1,5 times the acceleration of
gravity (i.e.,15 m/s ).
60034-18-31 © IEC:2012 – 13 –
6.1.3 Moisture conditioning
6.1.3.1 Overview
The combination of moisture and electrical stress often strongly affects the properties of
electrical insulation. The absorption of moisture by solid insulation gradually increases
dielectric loss, reduces insulation resistance and contributes to a decrease in electric
strength. Cracks and porosity can be more easily detected by elevated voltage when moisture
is deposited on the insulation surfaces.
Moisture is deposited on the surface of each test specimen so that all exposed surfaces are
completely wetted. For unsealed insulation systems, the moisture is applied in a controlled,
high-humidity environmental chamber. For sealed insulation systems, moisture is applied in
a controlled, high-humidity environmental chamber followed by complete immersion of the
test objects in water.
Experience has shown that at least 48 h of exposure is required for moisture to penetrate the
winding and stabilize the insulation resistance level. This degree of moisture exposure
demonstrates a more severe condition than is met in normal service.
6.1.3.2 Humidity method
A visible and continuous moisture deposit may be achieved by enclosing the test object in a
humidity or condensation chamber. Each test object shall be exposed to humidity for at least
48 h, which is sufficient to produce a visible moisture deposit on all exposed surfaces. The
temperature of the test objects should be 25 °C ± 10 °C and the actual temperature recorded.
No voltage is applied during the exposure period. If necessary for safety requirements, the
test object may be removed from the condensation chamber before applying the withstand
voltage test.
6.1.3.3 Water immersion method
Each of the test specimens in a test object should be subjected to humidification exposure
and subsequent voltage exposure according to 6.4.1.
Immediately following the voltage exposure after humidification, each of the specimens in a
test object shall be immersed, including the lead connections of each specimen, for a period
of 30 min in tap water at room temperature containing a non-ionic wetting agent (surfactant) in
sufficient concentration to reduce the surface tension to no greater than 3,1 µN/m at 25 °C.
After 30 min, while the test objects are still submerged, a voltage shall be applied to the test
specimens as described in 6.4.1. Following the voltage test, the test objects shall be rinsed in
tap water without any added wetting agent. The units shall be allowed to dry in room
temperature air, preferably overnight, before starting the next thermal ageing sub-cycle.
Before applying the voltage test it may be useful and informative to measure the insulation
resistance (IR) of the test object using a voltage chosen for the insulation system voltage
rating.
A sealed system test requires special construction of the sample leads to permit voltage
testing of the samples while they are immersed. Long leads or those specially extended to
clear the water surface may require additional bracing to prevent cracking at their base when
subjected to the mechanical test.
6.2 Diagnostic tests
6.2.1 Voltage withstand test
An elevated voltage withstand test is used to check the condition of the specimens and
determine the end of test life. The test is performed at room temperature, and the actual

– 14 – 60034-18-31 © IEC:2012
ambient temperature shall be recorded. The test voltage is specified according to the
insulation voltage rating. When power frequency voltage is specified, the frequency shall be
45 Hz – 65 Hz (as defined in IEC 60060-1).
The test equipment shall be capable of applying a sufficiently high voltage stress to reveal a
failure. The test voltage is applied to determine the condition of the test specimens and the
end of test life.
Failure is indicated by insulation breakdown. Failure in any component of the insulation
system constitutes failure of the entire test specimen. The failure location may be determined
by re-applying the voltage gradually from zero; failed specimens will not be able to hold the
voltage. Excess current, localized heating or smoke may also be observed. Minor surface
sparking and electrical discharge occurring during the voltage test should be recorded but do
not constitute a failure.
6.2.2 Method
It is important to test each part of the insulation system separately to identify areas that may
have cracked and/or delaminated during the thermal and mechanical stress cycles. The
voltage shall be applied in the following manner and sequence:
– between turns of the test specimen, where multi-turn specimens are used (as a test of the
insulation between turns), and
– between the test specimens and the frame of the test object (as a test of the mainwall
insulation).
6.2.3 Mainwall insulation test
For the test of the mainwall insulation, the frame of the test object shall be earthed. To aid in
identifying coil failure sites resulting from the mainwall insulation test, each sample within the
frame may be tested individually, provided that the frame and other samples within it are
earthed.
For unsealed systems and for sealed systems following the humidity chamber exposure and
before immersion, a power frequency test voltage at approximately room temperature shall be
applied for 1 min while test specimens are still wet from exposure, i.e., there is visible
moisture on all surfaces of the coil. The power-frequency test voltage level should be 2 U or
N
1 000 V, whichever is higher. U is the intended maximum rated voltage of the insulation
N
system under test.
For immersed sealed system test specimens, a power frequency test voltage of 1,15 U is
N
applied for 1 min between coil and frame. The potential of the aqueous solution and frame
shall be equal during the test.
Optionally, a coil-to-coil test may be performed in which the frame and the coil(s) adjacent to
the test coil are earthed and the power frequency test is applied. The dimensions shown in
the fixture diagrams are intended for tests of the mainwall insulation to ground. It will be
necessary to design a special arrangement for the coil-to-coil test.
6.2.4 Turn insulation test
Three test object configurations are proposed to allow a test of the insulation between turns:
– Samples with two parallel turns with full turn insulation around each strand, placed back to
back and with ends splayed, where a.c. voltage can be applied to one turn with the other
earthed. An example is shown in IEC 60034-18-42. For sealed and unsealed systems, the
voltage magnitude of the a.c. voltage test shall be 0,2 multiplied by the peak value of U ,
N
plus 1 kV, for 60 s.
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