ASTM D3145-20

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D3145 − 18 D3145 − 20
Standard Test Method for
Thermal Endurance of Electrical Insulating Varnishes by the
Helical Coil Method
This standard is issued under the fixed designation D3145; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This test method covers the determination of the thermal endurance of electrical insulating varnishes alone or in combinations
with magnet wire insulation. Changes in the helical coil bond strength are used as the test criteria. The coils are made from bare
aluminum or copper wire, or from film- or fiber-insulated magnet wire.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
NOTE 1—There is no similar or equivalent IEC standard.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use. For a specific precautionary statement, see Section 7.
1.4 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:
D1711 Terminology Relating to Electrical Insulation
D1932 Test Method for Thermal Endurance of Flexible Electrical Insulating Varnishes
D2307 Test Method for Thermal Endurance of Film-Insulated Round Magnet Wire
D2519 Test Method for Bond Strength of Electrical Insulating Varnishes by the Helical Coil Test
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 International Electrotechnical Commission Publications:Publication:
IEC 60216-1 Guide for the Determination of Thermal Endurance Properties of Electrical Insulation Materials (Part 1)
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.01 on Electrical Insulating Products.
Current edition approved Nov. 1, 2018Aug. 1, 2020. Published November 2018September 2020. Originally approved in 1973. Last previous edition approved in 20132018
as D3145 – 13.D3145 – 18. DOI: 10.1520/D3145-18.10.1520/D3145-20.
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.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
*A Summary of Changes section appears at the end of this standard
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D3145 − 20
3. Terminology
3.1 Definitions
3.1.1 For definitions of terms used in the test method, refer to Terminology D1711.
3.1 Definitions:
3.1.1 For definitions of terms used in the test method, refer to Terminology D1711.
3.1.2 varnish, electrical insulating, n— a liquid resin system that is applied to and cured on electrical components providing
electrical, mechanical, and environmental protection.
3.1.2.1 Discussion—
There are two types of electrical insulating varnish: solvent-containing and solventless. The solvent-containing varnish is a
solution, dispersion, or emulsion of a polymer or mixture of polymers in a volatile, nonreactable liquid. The solventless type is
a liquid resin system free of volatile, nonreactable solvents.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 bond strength, n—a measure of the force required to separate surfaces which have been bonded together.
3.2.2 magnet wire, n—a metal electrical conductor, covered with electrical insulation, for use in the assembly of electrical
inductive apparatus such as coils for motors, transformers, generators, relays, magnets, and so forth.
3.2.2.1 Discussion—
The electrical insulation is usually composed of a film covering formed from a magnet wire enamel applied over a bare conductor.
In some specific applications, fibrous coverings, either taped or linear filament served, are also used as electrical insulation.
3.2.3 varnish, electrical insulating, n— a liquid resin system that is applied to and cured on electrical components providing
electrical, mechanical and environmental protection.
3.2.3.1 Discussion—
There are two types of electrical insulating varnishes—solvent-containing and solventless. Solvent-containing types are solutions,
dispersions or emulsions of a polymer or a mixture of polymers in a volatile, nonreactable liquid. Solventless types are liquid resin
systems free of volatile, nonreactable solvents.
4. Summary of Test Method
4.1 Flexural strength of the helical coils is measured periodically after exposure to several exposure temperatures. The time to
reach an arbitrarily selected value of bond strength at each exposure temperature is determined. The logarithms of these times in
hours are plotted as a function of the reciprocal temperature (1/K) to give an Arrhenius plot.
5. Significance and Use
5.1 This test method is used to determine the effect of exposure to elevated temperatures on the bond strength of combinations
of magnet wire insulations and electrical insulating varnishes. The results are used as a guide for the comparison and selection of
varnishes and combinations of varnishes and magnet wire insulation for specific applications. Test Methods D1932 and D3251
describe additional tests for determining the thermal endurance of insulating varnishes. A comprehensive evaluation of thermal
characteristics includes a comparison of the thermal endurance determined in these different ways.
5.2 This test method is useful for research and product qualifications purposes.
6. Apparatus
6.1 Testing Machine, see Test Method D2519.
6.2 Test Fixture, see Test Method D2519.
6.3 Ovens, see Specification D5423, Type II.
D3145 − 20
7. Hazards
7.1 It is unsafe to use varnish at temperatu
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