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What Test Instruments are Used to Electrically Test an Electric Motor?


2020-12-Low resistance ohm meter for testing electric motors-1Performing tests on an electric motor is no easy task. You have to be careful about what voltage to use, what parameters are correct for the type of motor, and what specifications the test needs to be performed according to. And there are certain electric motor test instruments that are used to gather key data to evaluate the condition of a motor. Even if you're not the one performing the tests, it's still a good idea to have an idea of what those tests are and what test instruments are involved.

These tests are referred to as "standard" however, there are many opinions out there on voltage levels, what tests can be used when, etc. Watch the video to learn more.

Motor Inspection Tests

The most critical motor inspection tests include the winding phase-to-phase resistance, insulation resistance (IR) to ground, DC hi-pot (High-Potential), and surge comparison. These tests are performed by an experienced technician according to both EASA (Electrical Apparatus Service Association) and IEEE (Institute of Electrical and Electronics Engineers) standards to ensure you have reliable, high-quality outcomes. 

These motor tests cannot be performed in a “vacuum”: pre-repair values must be compared to post-repair values to ensure that progress has been made in improving motor performance. In addition, the tests must be performed in the order just discussed. All phase-to-phase resistance tests and IR tests must be passed prior to performing high voltage tests. If those tests are not passed, high voltage tests cannot be performed.

Electrical Test Instruments for Motors

For inspection testing of electric motors, there are four key test instruments that are needed: a low resistance ohmmeter, a multi-voltage IR tester, a DC hi-pot tester, and a multi-function surge tester. Each of these serves its own purpose and provides key data for evaluating the condition and performance of electric motors. And these instruments provide the data that repair vendors should provide you once the repairs are complete.

Low Resistance Ohmmeter

One of the often-used test instruments is the low resistance ohmmeter. This meter accurately measures very, very low resistance down to 𝜇Ω (micro ohms). In general, if a resistance measurement falls below 1Ω then it is considered a low-resistance measurement. Such resistance measurements require different equipment to accurately measure, which is why a standard ohmmeter isn’t suitable. 

Standard ohmmeters and their probes are usually not able to obtain accurate low resistance measurements because of errors introduced by the test lead and/or the contact resistance between the probe and the material where the test is being conducted.

Multi-Voltage IR Tester

The concept behind insulation resistance testing is simple: we know that if something is a good insulator then it will have a high level of resistance; if it is a poor insulator, the level of resistance will be very low. This means that electrical insulation can be measured by looking at the electrical insulation. In order to test IR to ground, a multi-voltage IR tester is used. 

The multi-voltage option is important because different voltage levels are required for different motors. To cover most motors, an IR tester that offers 500V, 1000V, 2500V, and 5000V is usually sufficient. The resistance values associated with a multi-voltage IR tester are far greater than what we see with the low resistance ohmmeter: they range into the megaohms (1x106 Ω).

DC Hi-Pot Tester

The objective of performing a DC Hi-Pot (High-Potential) test is to ensure that there is no current flow between two test points called "leakage". It basically determines the adequacy of the electric insulation for certain high voltage levels for a limited amount of time. For inspection testing of electric motors, a DC Hi-pot tester that goes up to 75,000V is typically used. This level of voltage may seem unnecessarily high, but it really isn’t. Sometimes customers ask that sample coils for windings be tested to failure, and that can easily involve voltages in excess of 48kV. AC Hi-Pot testing is sometime performed on larger horsepower, high voltage jobs with new windings. 

Multi-Function Surge Tester

Another key tool for effective motor inspection testing is the multi-function surge tester. This type of test instrument can serve a variety of purposes and automate much of the surge testing process. The technician connects it to the motor, enters the necessary information about the motor being tested (particularly, the voltage), and then selects a parameter for testing. From that point, the tester will perform an automated sequence of tests and report the results for review.


Here at HECO, we take electrical motor inspection testing very seriously and follow the appropriate IEEE and EASA standards as they relate to the sequence of tests performed and the parameters to be used for those tests. Included in our suite of motor test instruments are low resistance ohmmeters, multi-voltage IR testers, DC hi-pot testers, and multi-function surge testers. 

However, we know that the best equipment doesn’t take the place of knowledgeable technicians who know how to use the equipment correctly. When you send your motors to HECO for troubleshooting, repair, or rewinds, you can rest assured that our team has the experience and know-how to provide high-quality service.

Author:  Justin Hatfield at 

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