ASTM D2307-21

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Designation: D2307 − 07a (Reapproved 2013) D2307 − 21
Standard Test Method for
Thermal Endurance of Film-Insulated Round Magnet Wire
This standard is issued under the fixed designation D2307; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers determination of the thermal endurance of film-insulated round magnet wire in air at atmospheric
pressure. It is not applicable to magnet wire with fibrous insulation, such as cotton or glass.
1.2 This test method covers the evaluation of thermal endurance by observing changes in response to ac proof voltage tests. The
evaluation of thermal endurance by observing changes in other properties of magnet wire insulation requires the use of different
test methods.
1.3 It is possible that exposure of some types of film insulated wire to heat in gaseous or liquid environments in the absence of
air will give thermal endurance values different from those obtained in air. Consider this possibility when interpreting the results
obtained by heating in air with respect to applications where the wire will not be exposed to air in service.
1.4 It is possible that electric stress applied for extended periods at a level exceeding or even approaching the discharge inception
voltage will change significantly the thermal endurance of film insulated wires. Under such electric stress conditions, it is possible
that comparisons between materials will also differ from those developed using this method.
1.5 This test method is similar to IEC 60172. Differences exist regarding specimen preparation.
1.6 The values stated in inch-pound units are to be regarded as the standard. The SI units in parentheses are provided for
information only.only and are not considered standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1676 Test Methods for Film-Insulated Magnet Wire
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.17 on Fire and Thermal Properties.
Current edition approved April 1, 2013July 1, 2021. Published April 2013August 2021. Originally approved in 1964. Last previous edition approved in 20072013 as
D2307 – 07a.D2307 – 07a (2013). DOI: 10.1520/D2307-07AR13. 10.1520/D2307-21
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2307 − 21
D1711 Terminology Relating to Electrical Insulation
D3251 Test Method for Thermal Endurance Characteristics of Electrical Insulating Varnishes Applied Over Film-Insulated
Magnet Wire
D5423 Specification for Forced-Convection Laboratory Ovens for Evaluation of Electrical Insulation
2.2 Other Standards:
IEC 60172 Statistical Analysis of Thermal Life Test DataTest Procedure For The Determination Of The Temperature Index Of
Enamelled Winding Wires
IEEE 101 Guide for the Statistical Analysis of Thermal Life Test Data
3. Terminology
3.1 Definitions:
3.1.1 temperature index, n—a number which permits comparison of the temperature/time characteristics of an electrical insulating
material, or a simple combination of materials, based on the temperature in degrees Celsius which is obtained by extrapolating the
Arrhenius plot of life versus temperature to a specified time, usually 20 000 h.
3.1.2 thermal endurance, n—an expression for the stability of an electrical insulating material, or a simple combination of
materials, when maintained at elevated temperatures for extended periods of time.
FIG. 1 Device for Preparing Twisted Pair Specimens,
Motorized Unit
3.2 Definitions of Terms Specific to This Standard:
3.2.1 specimen failure time, n—the hours at the exposure temperature that have resulted in a specimen failing the proof test (see
9.1).
3.2.2 thermal endurance cycle, n—one oven exposure period followed by a proof voltage test.
3.2.3 time to failure, n—the log average hours calculated for a set of specimens, calculated from the individual specimen failure
times at an exposure temperature (see 9.2).
3.3 For definitions of terms related to electrical insulation, see Terminology D1711.
4. Summary of Test Method
4.1 This test method specifies the preparation of specimens, the exposure of these specimens at elevated temperatures, and the
periodic testing of the specimens by applying a preselected proof voltage.
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D2307 − 21
4.2 The cyclic exposure to temperature is repeated until a sufficient number of specimens have failed to meet the proof test, and
the time to failure is calculated in accordance with Section 9. The test is carried out at three or more temperatures. A regression
line is calculated in accordance with Section 10, and the time to failure values plotted on thermal endurance graph paper (see Fig.
6) as a function of the exposure temperature.
5. Significance and Use
5.1 This test method is useful in determining the thermal endurance characteristics and thermal indices of film-insulated round
magnet wire in air (see 1.3) (see Test Method D3251). This test method is used as a screening test before making tests of more
complex systems or functional evaluation. It is also used where complete functional systems testing is not feasible.
5.2 Experience has shown that film-insulated wire and electrical insulating varnishes or resins can affect one another during the
thermal exposure process. Test Method D3251 provides indications on the thermal endurance for a combination of insulating
varnish or resin and film insulated wire. It is possible that interaction between varnish or resin and film insulation will increase
or decrease the relative thermal life of the varnish and film insulated wire combination compared with the life of the film insulated
wire tested without varnish.
5.3 The conductor type or the surface condition of the conductor will affect the thermal endurance of film-insulated magnet wire.
This test method is used to determine the thermal endurance characteristics of film insulation on various kinds of conductors. The
use of sizes other than those specified in 7.1.1 is permissible but is not recommended for determining thermal endurance
characteristics.
5.4 The temperature index determined by this test method is a nominal or relative value expressed in degrees Celsius at 20 000
h. It is to be used for comparison purposes only and is not intended to represent the temperature at which the film insulated wire
could be operated.
5.5 There are many factors that influence the results obtained with this test method. Among the more obvious are the following:
5.5.1 Wire size and film thickness.
5.5.2 Moisture conditions during proof voltage tests.
5.5.3 Oven construction:
FIG. 2 Device for Preparing Twisted Pair Specimens,
Hand-Operated Hand-operated Unit
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Metric Equivalents
in. mm in. mm in. mm
0.03 0.8 0.38 9.7 1.25 31.8
0.046 1.2 0.44 11.2 2.00 50.8
0.064 1.6 0.50 12.7 3.00 76.2
0.12 3.0 0.62 15.7 4.75 120.7
0.140 3.6 0.88 22.4 4.88 124.0
0.250 6.4 1.00 25.4 6.81 173.0
0.30 7.6 1.12 28.4
FIG. 3 A Specimen Holder
5.5.3.1 Velocity of air.
5.5.3.2 Amount of replacement air.
5.5.3.3 Elimination of products of decomposition during thermal exposure.
5.5.3.4 Oven loading.
5.5.3.5 Accuracy with which the oven maintains temperature.
5.5.4 In most laboratories, the number of thermal endurance ovens is limited and, therefore, many different sets of specimens are
thermally exposed in the same oven. All specimens are not necessarily removed each time the oven is opened. This extra
temperature cycling will possibly have a degrading influence.
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FIG. 4 A Specimen Holder
FIG. 5 A Specimen Holder and Electrical Connection Device
5.5.5 Care with which specimens are handled, especially during latter cycles when the insulation becomes brittle.
5.5.6 Vibration of specimens will have a degrading effect during the later thermal endurance cycles.
5.5.7 Electrical characteristics of dielectric test instrument. Refer to 8.4 and 8.5.
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NOTE 1—This graph should contain all appropriate information regarding the insulating materials.
FIG. 6 Example of a Regression Line Plot (Table 5)
5.5.8 Environmental factors such as moisture, chemical contamination, and mechanical stresses, or vibration are factors that will
possibly 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
6.1 Voltage Source Source—(seeSee 8.3, 8.4, and 8.5).
6.2 Oven Oven—(seeSee Specification D5423 Type 2).2.
6.3 Device for Preparing Twisted Pair Specimens Specimens—(seeSee Figs. 1 and 2).
6.4 Specimen Holders Holders—(seeSee Figs. 3-5).
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7. Test Specimens
7.1 Preparation:
7.1.1 Film-insulated round magnet wire having bare wire diameters ranging from 0.0113 to 0.1019 in. (0.287 to 2.588 mm)
2.588 mm) 10 to 29 AWG inclusive are evaluated as described in this test method. If the dimensions of the magnet wire are not
known, determine them using Test Methods D1676.
7.1.2 Form a length of wire approximately 16 in. (400 mm) long into a U shape and twist together for a distance of 4.75 6 0.25
in. (120 6 6 mm) with a device similar to those shown in Figs. 1 and 2. The winding weight applied to the wire specimen while
being twisted and the number of twists (full 360° rotations of the head of the twist maker) are given in Table 1.
7.1.3 If specimens are to evaluated with a varnish, see Test Method D3251.
7.2 Number of Test Specimens—The accuracy of the test results depends largely upon the number of test specimens exposed at
each temperature. A greater number of test specimens is required to achieve an acceptable degree of accuracy if there is a wide
spread in results among the specimens exposed at each temperature. Use a minimum of 10ten specimens for each temperature. It
is permissible to evaluate a greater number of specimens if desired.
7.3 Specimen Holder—It has been found that individual handling of the twisted specimens will introduce premature failures. It
is, therefore, mandatory that the specimens be placed in a suitable holder. Design the holder in a manner that will protect the twisted
specimens from external mechanical damage and warpage. An example of a suitable holder is shown in Figs. 3 and 4. Construct
the holder so as to allow for the electrical connection of the twists for the proof testing (see Fig. 5 for an example).
7.4 Electrical Connection—Provide a suitable electrical connection to the test specimens in the holder that will not induce
mechanical stress to the specimens. Non-mechanical connections are preferred. A typical device is shown in Fig. 5. The specimens
are connected to a voltage source as described in 8.3 and 8.4.
8. Procedure
8.1 Prior to the first exposure cycle, make sure all specimens pass the proof-voltage test (see Table 2). Expose the specimens at
elevated temperatures in accordance with Table 3. Remove the specimens from the oven and cool to room temperature before
testing. Test by applying the voltage specified in Table 2. Take care to prevent damage to the specimens.
8.2 Exposure Times—The exposure times given in Table 3 are selected to subject the test specimen to approximately ten cycles
before all specimens fail. It is permissible to extend Table 3 at the high end of the exposure temperature range to accommodate
special high-temperature film insulations. The thermal endpoint time of the specimens will possibly be affected by the number of
cycles. Log average or median hour values, obtained from test specimens subjected to less than eight cycles or more than twenty
cycles at the exposure temperature are possibly unreliable. Therefore, to ensure the number of cycles to failure will be within the
parameters, adjust the exposure time. For example, if a set of test specimens has been exposed for eight cycles and less than half
TABLE 1 Tension and Number of Twists for Twisted Pair Construction
Nominal Bare Wire Diameter Winding Weight on Specimens (± 2%)
Wire Size Total
A
AWG Twists
in. mm kg lb
0.102 to 0.091 2.59 to 2.30 10 to 11 3 10.8 24
0.081 to 0.064 2.05 to 1.63 12 to 14 4 5.4 12
0.057 to 0.045 1.45 to 1.15 15 to 17 6 2.7 6
0.040 to 0.032 1.02 to 0.81 18 to 20 8 1.35 3
B
0.029 to 0.023 0.72 to 0.57 21 to 23 12 0.70 1.5
0.020 to 0.016 0.51 to 0.40 24 to 26 16 0.34 .
0.014 to 0.011 0.36 to 0.29 27 to 29 20 0.17 .
A
Prepare test specimens, of intermediate diameters, in accordance with the requirements for the next smaller AWG size.
B
For weights less than 1.5 lb, use kilogram weights.
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TABLE 2 Proof-Voltage Test
Difference Between the Bare Wire
AC Test
A
and Insulated Wire Diameters
Voltage, V
± 5%±5%
in. mm
0.0015 to 0.0020 0.036 to 0.050 500
0.0021 to 0.0027 0.051 to 0.070 700
0.0028 to 0.0035 0.071 to 0.090 1000
0.0036 to 0.0051 0.091 to 0.130 1200
A
For self-bonding magnet wires, the self-bonding layer is included in the film build
thickness determination.
TABLE 3 Recommended Exposure Times in Days Per Cycle
Estimated Temperature Index
Exposure
Temperature (°C)
105 130 155 180 200 220 240
320 1
310 2
300 1 4
290 2 7
280 1 4 14
270 2 7 28
260 1 4 14 56
250 2 7 28
240 4 14 56
230 1 7 28
220 2 14 56
210 1 4 28
200 2 7 56
190 1 4 14
180 2 7 28
170 4 14 56
160 7 28
150 14 56
140 28
130 56
have failed, it is recommended that the exposure time should be approximately doubled, and if the test shows a 30 % or greater
failure rate by the fourth cycle, it is recommended that the exposure time should be reduced by one-half. Expose test specimens
to at least three temperatures. It is recommended that exposure temperatures be at least 10°C 10 °C apart. Select the lowest test
temperature to be no more than 20°C 20 °C above the estimated temperature index of the magnet wire.
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