ASTM A937 Standard Test Method for Determining Interlaminar Resistance of Insulating Coatings Using Two Adjacent Test Surfaces

Get Certified ASTM A937 Testing for Reliable Electrical Steel Insulation 

Manufacturers use ASTM A937 testing because a coating’s interlaminar resistance is directly related to its ability to prevent eddy-current losses between electrical machinery laminations. Even coating enables electrical steel to perform consistently in transformers and rotating machines and minimizes core losses. Certified testing confirms that a batch of coated electrical steel meets its intended interlaminar resistance requirement before it is used in production.

The scope of ASTM A937 includes: 

• Material type: Flat-rolled electrical steel laminations, having an electrically insulating coating, especially inorganic coating. 
• Purpose: To assess interlaminar resistance and test the effectiveness of the coating for reducing interlaminar losses. 
• Properties measured: Interlaminar resistance between two adjacent test surfaces coated, in kΩ. 
• Method: Two-surface tester, where the voltage, pressure, and temperature are controlled between two coated laminates. 
• Result: The values of resistance are measured directly; the maximum measurable resistance is determined by the current setting used for measurement. 
• Test Limitations: Results differ from those obtained by Test Method A717/A717M and are not directly comparable. 
• Environmental conditions: The test is conducted under the predetermined voltage, pressure, and temperature conditions as specified by the method. 
• Applications: Quality control of insulating coating application and evaluation of electrical steel used for transformers and motors. 

What are the Uses of ASTM A937 Testing?

Manufacturers use ASTM A937 to assess interlaminar resistance of insulating coatings in a consistent manner, and engineers use the test to ensure the function of a coating for use on electrical machinery. This standard-

• Measures the interlaminar resistance between two adjacent coated electrical steel laminations.
• Assists in quality control for use of insulating coatings.
• Helps indicate the effectiveness of surface coatings for limiting interlaminar losses.
• Helps test the electrical steels coated with inorganic insulating coatings.
• Performs with insulation resistivities greater than those in Test Method A717.
• Helps compare coating performance between several laminations of the same lot. 

What Materials Can Be Tested Under ASTM A937? 

ASTM A937 applies mainly to flat-rolled electrical steel coated with electrically insulating coatings, and it is particularly useful for, but not limited to, steels coated with inorganic insulating coatings. The common application is in electrical sheet steels, which are used in electrical machines such as transformers, motors, etc., where reducing interlaminar losses is significant. Laboratories sample laminations from the same batch of electrical steel and test the uniformity of the coating throughout the steel. 

Why is ASTM A937 Important? 

The ASTM A937 standard helps manufacturers ensure that the quality of the coating is consistent, which ensures that the electrical steel used in the manufacture of transformers and motors works efficiently and does not lead to excessive core losses during operation. The interlaminar resistance is directly proportional to the interlaminar eddy current resistance of the insulating coating of an electrical machine. The thickness of the coating varies throughout the surface of the electrical steel, so further readings may be taken by using different contact positions, or may be necessary on several laminations from the same lot to get a representative thickness reading. 

ASTM A937 Testing Process and Data Collection

The process begins with an analyst taking the representative laminations from the same batch of coated electrical steel. The technician then places two laminations in contact in the two-surface tester and sets voltage, pressure, and temperature conditions. The operator takes the interlaminar resistance during measurement at the current set point. Analysts then re-read at various contact locations and compare the readings to quality control expectations. 

Common Challenges and Troubleshooting

To get accurate ASTM A937 results, the contact pressure needs to be consistent, the temperature steady, and the current set appropriately for the range of resistance anticipated. If the thickness of the coating is not uniform over the surface of the lamination, then the thickness measurement of a single contact position may yield inconsistent results. Technicians minimize these problems by retesting at various contact positions and testing multiple laminations from each lot. Laboratories also get more consistent results when the two surface testers are in good calibration and use the same test conditions. 

• The laboratory reports interlaminar resistance directly in kω, with reference to the setting used during measurement.
• The technician takes a number of readings from different contact positions to cover up the variation in coating thickness across the surface. The analyst notes that the results from this method are not of the same quantity as those from test method A717.
• The engineer considers applied voltage, pressure, and temperature when he interprets the results of the resistance.
• The manufacturer uses the measured interlaminar resistance as a quality control of the insulating coating application. 

Link to ASTM A937