The benefits of testing a 3-phase electric motor

Testing a three-phase motor with a polarization index test (PI), megger test, or surge test on a regular basis helps extend the life of the motor.


A three-phase electric motor is often able to run for many years before any serious problems occur. Testing the motor on a regular basis as part of a preventative maintenance program helps extend the life of the motor. If problems happen to occur, it is possible to test the motor on-site to determine what happened and how to correct the issue.

Testing the 3-phase motor

Image courtesy: Bob Vavra, CFE MediaThe first step to take when testing an electric motor is to disconnect it from the power supply. Mark all power leads and motor cables clearly to ensure that there are not problems with the rotation after the testing process is complete. Check the motor with a volt-ohmmeter set to ohms. Attach the leads from the motor to each of the power connections. The motor reads either over-limit (OL) or infinite ohms. If not, the motor is bad, and it may be necessary to replace it.

There are also many other ways to test a three-phase motor, including the following:

  • Polarization index test (PI): Sometimes called a 10 min to 1 min insulation resistance test, the PI checks the insulation resistance. It also monitors the health of the motor and determines if there is excessive moisture present.
  • Megger testing: This test determines if there is a problem with the insulation. It is a good option for testing a three-phase motor because it does not cause more damage during the testing process. It uses a high-voltage insulation tester to check the insulation within the circuit.
  • Surge test: The surge test is an old test, but it is still an efficient way to determine if the winding insulation is experiencing problems. It also determines if there are defects in coil-to-coil, face-to-face, or turn to turn sometimes missed by other types of testing processes.

Performing in-house tests often saves considerable time and frustration. These tests also save a significant amount of money. It is also important to use qualified technicians that understand these testing processes to get the best results.

David Manney is a marketing administrator at L&S Electric. This article originally appeared on L&S Electric Watts New Blog. L&S Electric is a CFE Media content partner.

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