ASTM A937/A937M-01
(Test Method)Standard Test Method for Determining Interlaminar Resistance of Insulating Coatings Using Two Adjacent Test Surfaces
Standard Test Method for Determining Interlaminar Resistance of Insulating Coatings Using Two Adjacent Test Surfaces
SCOPE
1.1 This test method covers a means of testing the interlaminar resistance of electrically insulating coatings as applied to adjacent laminations of flat-rolled electrical steel, under predetermined conditions of voltage, pressure and temperature. It indicates the effectiveness of surface coatings on electrical sheet steels for limiting interlaminar losses in electrical machinery. The interlaminar resistance is measured directly in units of resistance (kΩ).
1.2 This test method is particularly useful for, but not limited to, electrical steels coated with inorganic insulating coatings.
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 and health practices and determine the applicability of regulatory limitations prior to use.
1.4 The values and equations stated in customary (cgs-emu and inch-pound) or SI units are to be regarded separately as standard. Within this standard, SI units are shown in brackets. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with this standard.
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Designation: A 937/A937M – 01
Standard Test Method for
Determining Interlaminar Resistance of Insulating Coatings
Using Two Adjacent Test Surfaces
This standard is issued under the fixed designationA937/A937M; 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 3.1.1 interlaminar resistance—the average resistance of
two adjacent insulating surfaces in contact with each other,
1.1 This test method covers a means of testing the inter-
under conditions specified in this standard.
laminar resistance of electrically insulating coatings as applied
3.1.2 surface insulation resistivity—a) the effective resistiv-
to adjacent laminations of flat-rolled electrical steel, under
ity of a single insulating layer tested between applied bare
predetermined conditions of voltage, pressure and temperature.
metal contacts and the base metal of the insulated test
It indicates the effectiveness of surface coatings on electrical
specimen, as per Test MethodA 717; b) the resistance of a unit
sheet steels for limiting interlaminar losses in electrical ma-
area per test strip calculated from a measurement of the
chinery. The interlaminar resistance is measured directly in
electrical resistance of a stack of strips as per Test Method
units of resistance (kV).
A 718.
1.2 This test method is particularly useful for, but not
3.1.3 two-surface tester—the apparatus used in this test
limited to, electrical steels coated with inorganic insulating
method.
coatings.
3.1.4 four-terminal measuring technique, often referred to
1.3 This standard does not purport to address all of the
as four-probe measuring technique—a common method to
safety concerns, if any, associated with its use. It is the
measure resistance when a high degree of accuracy is re-
responsibility of the user of this standard to establish appro-
quired. In this standard, the circuit configuration for this
priate safety and health practices and determine the applica-
technique is referred to as a four-probe configuration.Inthe
bility of regulatory limitations prior to use.
two-surface tester, this configuration features two probes con-
1.4 The values and equations stated in customary (cgs-emu
nected to the top lamination test surface and two probes
and inch-pound) or SI units are to be regarded separately as
connected to the bottom lamination test surface. One of the
standard. Within this standard, SI units are shown in brackets.
probes in each pair carries the measuring current, and the other
The values stated in each system may not be exact equivalents;
provides a contact for the voltage measurement. Because of the
therefore, each system shall be used independently of the other.
extremely high impedance of the measuring circuit, very little
Combining values from the two systems may result in noncon-
current flows through the voltage contacts, and thus very little
formance with this standard.
voltage is produced across the contacts to influence the true
2. Referenced Documents
reading, that is, any effect from contact resistance is avoided or
reduced to a negligible amount. The two-surface tester has
2.1 ASTM Standards:
provision to check the integrity of the contacts made between
A 34 Practice for Sampling and Procurement Testing of
the probes and the test surfaces.
Magnetic Materials
3.1.5 bad contact—a contact which results in a voltage
A 717 Test Method for Surface Insulation Resistivity of
drop in excess of 0.6 V as described in 6.1.2.
Single-Strip Specimens
3.1.6 test specimen—two electrical steel laminations, each
A 718 Test Method for Surface Insulation Resistivity of
having a minimum size of 25 3 25 cm [250 3 250 mm] and
Multi-Strip Specimens
each having an electrically insulating coating on both sides.
3. Terminology
The two electrical steel laminations are placed one on top of
the other for the interlaminar resistance measurement, Fig. 1.
3.1 Definitions of Terms Specific to This Standard:
3.1.7 exposed test surface—the insulating top surface of the
top lamination or the insulating bottom surface of the bottom
This test method is under the jurisdiction of ASTM Committee A06 on
lamination of the test specimen.
Magnetic Properties and is the direct responsibility of SubcommitteeA06.01 onTest
Methods.
Current edition approved May 10, 2001. Published August 2001. Originally
published as A 937–95. Last previous edition A 937–95. Harris,F.K., Electrical Measurements,RobertE.KriegerPublishingCompany,
Annual Book of ASTM Standards, Vol 03.04. Huntington, New York, 1975, pp. 220–224.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
A 937/A937M
FIG. 1 Schematic Illustration of Four-Probe Configuration
4. Summary of Test Method 5. Significance and Use
4.1 The test method measures the average resistance of two 5.1 This test method is particularly suitable for quality
adjacent insulating surfaces. Intimate physical contact of these control in the application of insulating coatings. This test
surfaces is achieved via test heads which force a defined method measures the interlaminar resistance of insulating
surface area into contact under a specified pressure. For the coatings, as defined in 3.1.1. Interlaminar resistance is the
interlaminar resistance measurement, electrical contact is es- measure of the insulating quality of the coating. Interlaminar
tablished between the test specimen and a constant direct resistance is reported in units of kV.
current source using metallic contacts (drill bits). The tester 5.2 The interlaminar resistance determined in accordance
utilizes two sets of metallic contacts which penetrate the with this test method is not the same quantity determined by
exposed test surfaces into the base metal, to form a four-probe Test Method A 717 or Test Method A 718.
configuration, Fig. 1. A continuous electrical path is formed 5.3 This test method is particularly useful for electrical
between the contacts and constant current source when the steels coated with inorganic insulating coatings having surface
2 2
metallic contacts penetrate through the coating on the exposed insulation resistivities in excess of 0.3 kV·cm [30 kV·mm ]
test surfaces to the underlying base metal, and the insulating when tested using Test Method A 717 (a Franklin current less
surfaces are held in intimate contact by the test head. When than 0.02 A). This test method can readily be extended to any
current flows in the circuit, the dc voltage developed in the range of insulation resistivity that the equipment comprising
circuit may be easily measured by means of a digital voltmeter. the two-surface tester allows. For the equipment specified
(Note: The current range settings for the two-surface tester are herein, the maximum measurable resistance is 1200 kV for the
1-µA dc and 10-µA dc, thus enabling the resistance to be read 10-µA current setting and 12 000 kV for the 1-µA current
directly from the voltmeter. The current select switch is setting; the maximum voltage for the test system is 12 V.
designed to shift the decimal point appropriately so as to 5.4 Repeat readings on the same set of two electrical steel
provide a resistance reading in units of kV.) A block diagram laminations using different contact positions, as well as the
of the two-surface test system is illustrated in Fig. 2. testing of multiple laminations from the same lot of electrical
A 937/A937M
FIG. 2 Block Diagram of Two-Surface Test System
steel, are recommended. Several readings are suggested be- used to house and rotate the drill bit through the coating on the
cause the coating thickness may vary across the surface of a exposed surfaces of the test specimen, Fig. 4.
given electrical steel lamination. Additionally, the coating
6.1.1 The associated measuring equipment, which may be
thickness may vary across several laminations taken from the remotely located, includes an electronic voltmeter with a
same lot of electrical steel. Such variations in coating thickness
digital readout, a source of constant current, a contact check
are likely to yield variations in the measured interlaminar unit, a calibration module and a display unit. A block diagram
resistance. The required number of readings depends on the
of the electronic system for the two-surface tester is shown in
nature of the coating and the accuracy required. Fig. 2. A computer compatible interface, although not manda-
tory, is recommended for data collection and analysis. This
6. Apparatus
function is a standard feature on many commercial electronic
voltmeters.
6.1 The two-surface tester measures the resistance between
twolaminationsthatareinintimatecontactwithoneanotheras 6.1.1.1 The two-surface tester measures the interlaminar
resistance as follows: The constant current source of the tester
shown in Fig. 1. The two-surface tester shall consist of two
pressure pads, each of which is mounted to one of the heads of forces a constant current through the insulation. The voltage
which results is measured by a suitable voltmeter. The resis-
a hydraulic or pneumatic press. The diameter of each pressure
pad is 3.57 cm [35.7 mm], giving a nominal area of 10 cm tance of the insulation is then determined by Ohm’s law. The
results are communicated to the operator via a display.
[1000 mm ]. The diameter of the pressure head is 11.50 cm
[115 mm]. Each pressure head circumscribes two metallic 6.1.2 Constant Direct Current Source—The use of a source
contacts(drillbits)whichareusedtomakecontactwiththetest which supplies constant measuring current, independent of
specimen.Aschematic diagram of the pressure head, pads and surface resistance, is recommended. The use of a properly
metallic contacts is shown in Fig. 3. Commercially available adjusted dc constant current source enables resistance to be
rollnut actuators, which convert rotary to linear motion, can be read directly in units of resistance (kV).
A 937/A937M
FIG. 3 Diagram of Pressure Head Assembly
6.1.2.1 In the two-surface tester, constant direct current is 6.1.3 Contact Check Unit—The contact check unit serves to
supplied using a feedback circuit incorporating a high gain verify the integrity of the contacts and to ensure that bad
operational amplifier. Current is set by means of an adjustable contacts do not interfere with the coating resistance measure-
resistor. Once set, the current is held constant by means of the ment. The contact resistance is measured using the four-probe
feedback control circuit. The test system is configured such configuration shown in Fig. 1. Note that there are two current
that an indicator light will turn on if the current goes out of probes and two voltage probes. The contact resistance is
regulation. measured at the start of a test by temporarily connecting the
6.1.2.2 Furthermore, the test system described herein is voltage contacts together with a switch. This causes current to
equipped with two constant current range settings, 10 and 1 be diverted as shown in Fig. 5. The contacts are considered
µA. For interlaminar resistance values less than 1200 kV, the acceptableifthevoltageacrosseitherpairofprobesislessthan
10-µA range is suitable. For interlaminar resistance values 5 % of the product of the maximum measurable resistance for
greater than 1200 kV, the 1-µA range is recommended to the selected current and the selected current, that is, an
optimize the accuracy of the measurement. This system is associated voltage drop of 0.6 V for either current range.
equipped with an indicator light that warns the operator if the (Specifically, V = i 3 R. For a current setting of 1 µA and a
current range is not appropriate for a given specimen (because maximum measurable resistance of 12 000 kV; 0.05 3 (1
the current goes out of regulation). µA 3 12 000 kV) = 0.6 V. Similarly, 0.05 3 (10 µA 3 1200
6.1.2.3 Because the current source setting is an exact kV) = 0.6 V.) If either contact shows a voltage drop in excess
multiple of ten, the resistance of the test specimen is numeri- of 0.6 V, a bad contact indicator light is lit and the measure-
cally equal to the voltage reading to within a multiple of ten. ment of interlaminar resistance is inhibited. The penetrations
Therefore, the resistance, in kV, may be directly read on the may be reestablished until satisfactory results are obtained. If
voltmeter. The decimal point is properly positioned by the the measurable voltage drop is less than 0.6 V, subsequent
current range select switch. interlaminar resistance measurement is enabled.
A 937/A937M
FIG. 4 Schematic Diagram of Metallic Contact/Rollnut Actuator Assembly
NOTE 1—When the switch of the Contact Check Unit (Item C–Fig. 2)
is in the “open” position, the voltage drop across the “Insulation Coating
NOTE 1—When the switch of the Contact Check Unit (Item C–Fig. 2)
Layers Under Test–(Item C–Fig. 1)” is measured. (The contacts 1 and 3
is in the “closed” position, the voltage contacts are temporarily connected.
are at the same potential. The contacts 2 and 4 are at the same potential.)
The voltage drop across contacts between 1 and 3, and between 2 and 4
FIG. 6 Current Path During the Measurement of the Coating
is evaluated. A “bad contact” indicator light is triggered if this voltage
Interlaminar Resistance Value
drop is in excess of 0.6 volts. The measurement of the coating’s
interlaminar resistance is inhibited under “bad contact” conditions.
FIG. 5 Current Path During the Contact Check Process
resistors which may be connected to the two-surface tester by
means of cables. The calibration module substitutes precisely
6.1.3.1 Once suitable contact resistance values are obtained, known values of resistance for the insulation under test. It also
the switch connecting the voltage contacts is opened, and substitutes a variable contact resistance for each of the upper
current is redirected to flow through the insulation instead of and lower contacts, allowing the operation of the contact
the switch, Fig. 6. The interlaminar resistance can then be resistancesystemtobeindependentlyverified.Byselectingthe
measured. appropriate jumper, a precision resistor is substituted for the
6.1.4 Calibration Module—The calibration module is pro- insulation. The meter reading should correctly read the value
vided to allow precise calibration of the two-surface tester.The selected by the jumper (provided that it is w
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