ASTM D2307-95

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: D 2307 – 95 An American National Standard
Standard Test Method for
Thermal Endurance of Film-Insulated Round Magnet Wire
This standard is issued under the fixed designation D 2307; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope Ovens for Evaluation of Electrical Insulation
2.2 IEEE Standards:
1.1 This test method covers determination of the thermal
IEEE 101 Statistical Analysis of Thermal Life Test Data
endurance of film-insulated round magnet wire in air at
atmospheric pressure. It is not applicable to magnet wire with
3. Terminology
fibrous insulation, such as cotton or glass.
3.1 Definitions:
NOTE 1—Other solid conductors such as coated resistance wire may be
3.1.1 temperature index, n—a number which permits com-
evaluated by this test method.
parison of the temperature/time characteristics of an electrical
1.2 The values stated in inch-pound units are to be regarded
insulating material, or a simple combination of materials, based
as the standard. The SI units in parentheses are provided for
on the temperature in degrees celsius which is obtained by
information only.
extrapolating the Arrhenius plot of life versus temperature to a
1.3 This test method covers the evaluation of thermal
specified time, usually 20 000 h.
endurance by observing changes in response to ac proof
3.1.2 thermal endurance, n—an expression for the stability
voltage tests. The evaluation of thermal endurance by observ-
of an electrical insulating material, or a simple combination of
ing changes in other properties of magnet wire insulation
materials, when maintained at elevated temperatures for ex-
requires the use of different test methods.
tended periods of time.
1.4 Exposure of some types of film insulated wire to heat in
3.2 Definitions of Terms Specific to This Standard:
gaseous or liquid environments in the absence of air may give
3.2.1 specimen failure time, n—the hours at the exposure
thermal endurance values different from those obtained in air.
temperature that have resulted in a specimen failing the proof
This fact should be considered when interpreting the results
test (see 9.1).
obtained by heating in air with respect to applications where
3.2.2 time to failure, n—the hours calculated for a set of
the wire will not be exposed to air in service.
specimens, calculated from the individual specimen failure
1.5 Electric stress applied for extended periods at a level
times at an exposure temperature (see 9.2).
exceeding or even approaching the discharge inception voltage
4. Summary of Test Method
may change significantly the thermal endurance of film insu-
lated wires, varnished or unvarnished. Under such electric 4.1 This test method specifies the preparation of specimens,
stress conditions, comparisons between materials may also
the aging of these specimens at elevated temperatures, and the
differ from those developed using this method. periodic testing of the specimens by applying a preselected
1.6 This standard does not purport to address all of the
proof voltage.
safety concerns, if any, associated with its use. It is the
4.2 The cyclic exposure to temperature is repeated until a
responsibility of the user of this standard to consult and sufficient number of specimens have failed to meet the proof
establish appropriate safety and health practices and deter-
test, and the time to failure is calculated in accordance with
mine the applicability of regulatory limitations prior to use. Section 9. The test is carried out at three or more temperatures.
A regression line is calculated in accordance with Section 10,
2. Referenced Documents
and the time to failure values plotted on thermal endurance
2.1 ASTM Standards:
graph paper (see Fig. A1.1) as a function of the exposure
D 115 Test Methods for Varnishes Used for Electrical Insu-
temperature.
lation
5. Significance and Use
D 5423 Specification for Forced-Convection Laboratory
5.1 This test method is useful in determining the thermal
endurance characteristics and a temperature index of the
This test method is under the jurisdiction of ASTM Committee D-9 on
film-insulated round magnet wire in air (see 1.4), alone or in
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.10 on Magnet Wire Insulation. Originally published as
D 2307 – 64T. Last previous edition D 2307 – 78.
Current edition approved Sept. 10, 1995. Published November 1995. Originally Annual Book of ASTM Standards, Vol 10.02.
published as D 2307 – 68. Last previous edition D 2307 – 94. Available from the Institute of Electrical and Electronics Engineers, Inc., 345
Annual Book of ASTM Standards, Vol 10.01. E. 47th St., New York, NY 10017.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 2307
combination with insulating varnish (see Test Methods D 115). 6.2 Oven (Specification D 5423).
This test method may be used as a screening test before making 6.3 Device for Preparing Twisted Pair Specimens (See Fig.
tests of more complex systems or functional evaluation. It may 1 and Fig. 2).
also be used where complete functional systems testing is not 6.4 Specimen Holders (see Fig. 3, Fig. 4, and Fig. 5).
feasible.
7. Test Specimens
5.2 Experience has shown that film-insulated wire and
7.1 Preparation:
electrical insulating varnishes or resins can affect one another
7.1.1 Film-insulated magnet wire having noninsulated wire
during the thermal aging process. Interaction between varnish
diameters ranging from 0.0113 to 0.1019 in. (0.287 to 2.588
or resin and film insulation may increase or decrease the
mm) 10 to 29 AWG inclusive can be evaluated as described in
relative thermal life of the varnish and film insulated wire
this test method.
combination compared with the life of the film insulated wire
7.1.2 Form a length of wire approximately 16 in. (400 mm)
tested without varnish. This test method may give indications
long into a U shape and twist together for a distance of 4.75 in.
on the thermal endurance for a combination of insulating
(120 mm) with a device as shown in Fig. 1 and Fig. 2. The
varnish or resin and film insulated wire.
winding weight applied to the wire specimen while being
5.3 The conductor type or the surface condition of the
twisted and the number of twists are given in Table 1.
conductor may also affect the thermal endurance of film-
7.1.3 When a solvent varnish is used, dilute it with a
insulated magnet wire. This test method, may be used to
suitable solvent to obtain the required coating thickness.
determine the thermal endurance characteristics of film insu-
7.1.4 Use solventless varnishes as received.
lation on various kinds of conductors. Sizes other than those
7.1.5 Dip the twisted specimens that are to be varnish coated
specified in 7.1.1 may be used but are not recommended for
to the depth to cover the wire specimens 0.75 in. (19 mm)
determining thermal endurance characteristics.
beyond the twist area for not less than 30 s, then slowly
5.4 The temperature index determined by this test method is
withdraw at a uniform rate of approximately 4 in./min (100
a nominal or relative value expressed in degrees Celsius at
mm/min). Cure the specimens for the time and at the tempera-
20 000 h. It is to be used for comparison purposes only and is
ture recommended by the varnish manufacturer. If the appli-
not intended to represent the temperature at which the film
cation requires it, reverse dip and cure in the opposite direction.
insulated wire could be operated.
7.2 Number of Test Specimens—The accuracy of the test
5.5 There are many factors that may influence the results
results depends largely upon the number of test specimens aged
obtained with this test method. Among the more obvious are
at each temperature. A greater number of test specimens is
the following:
required to achieve an acceptable degree of accuracy if there is
5.5.1 Wire size and film thickness.
a wide spread in results among the specimens exposed at each
5.5.2 Moisture conditions during aging and during voltage
temperature. Use a minimum of 10 specimens for each
tests.
temperature. More specimens may be aged if desired.
5.5.3 Oven construction:
7.3 Specimen Holder—It has been found that individual
5.5.3.1 Velocity of air.
handling of the twisted specimens may introduce premature
5.5.3.2 Amount of replacement air.
failures. It is, therefore, mandatory that the specimens be
5.5.3.3 Elimination of products of decomposition during
placed in a suitable holder. The recommended holder is shown
aging.
in Fig. 3 and Fig. 4. Design the holder in a manner that will
5.5.3.4 Oven loading.
protect the twisted specimens from external mechanical dam-
5.5.3.5 Accuracy with which the oven maintains tempera-
age and warpage. Construct the holder so as to allow the ends
ture.
of the twist to protrude from the holder to make electrical
5.5.4 In most laboratories, aging ovens are limited and,
connection for the proof testing as shown in Fig. 5.
therefore, many different sets of specimens are aged in the
7.4 Electrical Connection Device—Provide a suitable elec-
same oven. All specimens are not necessarily removed each
trical connection device to make nonmechanical electrical
time the oven is opened. This extra temperature cycling may
have a degrading influence.
5.5.5 Care with which specimens are handled, especially
during latter cycles when the insulation becomes brittle.
5.5.6 Vibration of specimens. This may have a degrading
effect during the later aging cycles.
5.5.7 Electrical characteristics of dielectric test instrument.
Refer to 8.4 and 8.5.
5.5.8 Environmental factors such as moisture, chemical
contamination, and mechanical stresses, or vibration are factors
that may result in failure after the film insulated wire has been
weakened by thermal deterioration and are more appropriately
evaluated in insulation system tests.
6. Apparatus
FIG. 1 Device for Preparing Twisted Pair Specimens, Motorized
6.1 Voltage Source (see 8.3 and 8.4). Unit
D 2307
median hours are less than 100, do not use the data. Use aging
ovens of the forced-draft design preferably conforming to
Specification D 5423.
8.3 Test Voltages—The voltages given in Table 2 are se-
lected in order to subject the insulation to a stress of approxi-
mately 300 V/mil (12 kV/mm). This value is above the air
breakdown value for the space afforded by the insulation films
separating the wires. These relatively high values are chosen so
that crazing, or other deterioration of the coating is readily
detected.
8.4 The voltage to be applied shall be an ac voltage and
have a nominal frequency of 50 or 60 Hz of an approximately
sine-wave form, the peak factor being within the limits of 2
=
6 5 % (1.34 to 1.48). The test transformer shall have a rated
power of at least 500 V-A and shall provide a current of
essentially undistorted waveform under test conditions.
8.5 To detect failure, the fault detection device shall operate
when a current of 1.5 to 15 mA flows in the high voltage
circuit. The test voltage source shall have a capacity to supply
the detection current (1.5 to 15 mA) with a maximum voltage
FIG. 2 Device for Preparing Twisted Pair Specimens, Hand-
drop of 10 %.
Operated Unit
8.6 Apply the proof voltage to the test specimens for
approximately 1 s. A relatively short time of application of the
connection to the test specimens in the holder. The device is
test voltage is desirable to minimize the effects of corona and
connected to a voltage source described in 8.3 and 8.4. A
dielectric fatigue.
typical device is shown in Fig. 5.
9. Calculation
8. Procedure
9.1 Specimen Failure Time—The specimen failure time is
8.1 Prior to the first aging cycle, make sure all specimens the sum of the total hours at the time of failure minus one-half
pass the proof-voltage test (see Table 2). Age the specimens at
the hours of the last cycle. As an example, suppose a given
elevated temperatures in accordance with Table 3. Remove the specimen failed to withstand the proof voltage following the
specimens from the aging oven and cool to room temperature
ninth 100-h exposure. Thus the total hours would be 900 h
before testing. Test by applying the voltage specified in Table minus one-half the hours of the last cycle, 100 h/2 5 50 h, for
2. Take care to prevent damage to the specimens. a failure time of 850 h.
8.2 Exposure Times—The exposure times given in Table 3 9.2 Time to Failure—Calculate the time to failure of a set of
are selected to subject the test specimen to approximately ten specimens at one exposure temperature using either the median
cycles before all specimens fail. Table 3 may be extended at the or the logarithmic mean. For many materials, the median
high end of the exposure temperature range to accommodate endurance is statistically valid when specimen failure times are
special high-temperature film insulations. The life of the normally distributed. In most cases, the use of the median will
specimens may be affected by the number of cycles. Log significantly reduce testing time, since the test ceases when the
average or median hour values, obtained from test specimens median value has been obtained. Exposure times (8.2) are
subjected to less than eight cycles or more than twenty cycles consistent regardless of the method used to obtain the end
at the exposure temperature, may not be reliable. Therefore, the point.
exposure times should be adjusted during the aging to ensure 9.2.1 Median Calculation Method:
that the number of cycles to failure are within these parameters. 9.2.1.1 Calculate the time to failure as follows: If the
For example, if a set of test specimens has been exposed for number of specimens at each temperature is n, and if n is even,
eight cycles and less than half have failed, the exposure time the median endurance of the group of specimens is the average
should be approximately doubled, and if the test shows a 30 % of the failure times of specimens n/2 and (n + 2)/2. If n is odd,
or greater failure rate by the fourth cycle, the exposure time
use the specimen failure time of specimen number (n + 1)/2.
should be reduced by one-half. Expose test specimens to at 9.2.1.2 For instance, if n is 10, the logarithm of the failure
least three temperatures. Test temperatures should be at least
times of the fifth and sixth specimens must be added and
10°C apart. Select the lowest test temperature to be no more averaged. The time to failure (median log average hours) of the
than 20°C above th
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