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What Are the Basics of Electric Motor Inspection Electrical Testing?



2020-12-electric motor test instrumentsElectric motor inspection testing is an important part of establishing the condition of the motor and the beginning the troubleshooting process. There are several different tests involved, and a basic knowledge of what the tests are can help you understand the repair data you receive back from your electric motor repair shop.

Electric Motor Inspection Tests

The most critical motor inspection tests include the following:

  • Winding phase-to-phase resistance
  • Insulation resistance (IR) to ground
  • DC hi-pot
  • Surge comparison
  • Polarization index
  • AC and DC voltage drop (DC motors)

These tests should be performed by experienced technicians using the methods and voltages prescribed by  EASA (Electrical Apparatus Service Association) and IEEE (Institute of Electrical and Electronics Engineers) standards. In addition, all phase-to-phase resistance tests and IR tests must be passed prior to performing the high voltage DC hi-pot and surge comparison tests. 

Note that electric motor inspection test values are compared between the initial inspection test data and the final test data to ensure that improvements were made as a result of the repair or remanufacturing process.

Winding Phase-to-Phase Resistance Test

Winding phase-to-phase resistance tests are used to detect any large differences in resistance that exist between the windings and is a common AC motor repair test. The resistance of each winding should essentially be the same. A standard low resistance ohmmeter is usually used for this type of test.

IR to Ground Test

An IR to ground test is extremely useful because anomalies here are a sure sign that a motor is about to fail or already has. In this type of test for a three phase motor, the insulation resistance is measured between each of the motor windings and the motor frame (which serves as the ground). This type of test requires an insulation resistance tester, or simply IR tester), to be able to read the high resistance levels that are expected.

When IR to ground tests are being performed, it’s important to keep in mind that there is a relationship between temperature and resistance: as the temperature changes, the resistance will change. For IR values to be meaningful, they need to be taken with a reference temperature for the winding, and then the IR values corrected to align with a 40°C standard reference temperature. This allows the IR results to be compared across multiple tests.

DC Hi-Pot Test

Th DC hi-pot test, which can easily go up to 75,000V, serves as a stress test for the insulation and requires the use of a DC hi-pot tester. This is considered an over-voltage test because the voltage applied is higher than what the electric motor would normally be exposed to. 

Surge Comparison Test

Electric motors go through both incoming and final surge comparison tests. The purpose of a surge comparison test is to detect insulation weaknesses and shorts (usually turn-to-turn, phase-to-phase, or coil-to-coil) as well as incorrect turn counts or improper internal connections. This type of test is performed with a multi-function surge tester.

Polarization Index Test

The polarization index test is essentially an insulation resistance to ground test that is performed over a 10-minute period. It provides information regarding the condition of the insulation with regard to moisture and cleanliness. This particular test is performed according to the IEEE 43-2000 standard. Once the data has been gathered, the value at the ten minute mark is divided by the value at the one minute mark to provide a ratio that is meaningful as it is tracked over time for a motor.

DC Voltage Drop Test

The DC voltage drop test is specific to DC motor repair and is the primary testing method used to identify shorted turns in the field windings of a DC motor, as well as the shunts, series, and interpoles. This test is performed by applying voltage to the shunt fields and interpoles and then measuring the votlage drop between the like feeds. And, even though this type of test is performed on DC motors, both AC and DC voltages can be applied. Using AC voltages at 60 Hz allows more problems to be detected.

A Note on Cleaning

There are times when the electric motor windings are dirty enough that they need to be cleaned before the condition of the windings can be accurately evaluated. If the IR values are too low, for example, it may be necessary to clean and bake the windings to obtain more meaningful readings from the inspection tests. In fact, some windings may have test results that look like the winding needs to be replaced when all it actually needs is a thorough cleaning. 

And this is especially common when it comes to DC motor armatures, where carbon and dirt can be hidden beneath the armature banding and commutator. As a result, the IR will be much lower than it should, but the primary problem is one of cleanliness and not a motor defect that requires a rewind.


At HECO, we take electric motor inspection testing very seriously. We carefully document the results of each test performed and share those results with you. If we get poor results for a test that could be influenced positively by cleaning, then we will perform additional cleaning before we tell you that you need a coil or armature rewind. And, as an EASA accredited motor repair shop, you can count on us for the high quality repairs and reliable motors.

Author: Justin Hatfield at 

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