Gone Outdoors

How to Test a 3-Phase Motor

by G.K. Bayne

Testing a 3-phase motor is as simple as holding the leads of a volt ohmmeter. The trick is knowing which leads to hold on to and which wires make no difference at all. Once you have tested a few 3-phase motors, the process will come easily.

Unscrew the motor's wire cover box from the motor after turning off all power that feeds that electric motor. Mark all the pairs of wires from the motor to the power feed so rotation will remain the same after checking the motor.

Remove any wire nuts or tape that are insulating the connections. You must have bare copper wires for testing the motor leads.

Switch the volt ohmmeter to ohms and place the leads from the motor case to each motor power connection lead one at time. Each reading should be infinite ohms, or OL, on the meter. If the meter does not read OL, you may have a direct short in the motor windings. Replace the motor.

Items you will need
  • Volt ohmmeter
  • Wire pliers
  • Phillips and slotted screwdrivers

Tips

  • Reading the ohmmeter in the range of 2.5 Meg ohms to infinite may be fine for some motors operation. If the motor has a history of blowing fuses, chances are the motor windings are getting weak under the load. You should replace the motor.
  • If the motor is reading fine for ohms, there is no need to check the windings, as the complete circuit has been checked.
  • By the same conclusion, if the motor is reading poorly, there is no need to check the individual motor windings to find the bad circuit. Most of the windings are internal, and it will take a complete motor rewind to correct the problem.
  • Follow all manufacturer's instructions and all local and state electrical codes when replacing motors.