Two Minute Tips  

Troubleshooting a De-energized Motor

Noah Bethel | Vice President-Product Development, PdMA Corporation

This week we look at troubleshooting techniques and considerations when a motor is not running (de-energized), requiring controlled test signals to be sent into the motor. The fault zones related to a de-energized motor are power circuit, insulation, stator, rotor, and air gap. In less than five minutes the MCE® test can give a basic evaluation of the insulation health using resistance and capacitance-to-ground measurements, and power circuit and stator winding health using phase resistance and inductance readings. For insulation health, the PdMA default setpoints for resistance-to-ground, based on accepted industry standards, is a good place to start in determining if the insulation is acceptable for continued operation. If the resistance-to-ground is close to the limit, additional insulation tests such as a Polarization Index or Step Voltage Test may be required. For power circuit and stator health of a three- phase motor, resistance and inductance values of each phase should be nearly the same depending on the rotor influence. A comparison to baseline or last test can be critical in a troubleshooting situation, so work very hard to get a baseline test as soon as possible when the motor is running normally. Elevated phase resistance on one of the phases indicates a power circuit or high resistance connection anomaly. Reduced inductance in one phase can indicate lost or shorted turns in the stator windings. Remember that copper winding has a positive temperature coefficient, so higher temperatures means higher phase resistance. However, changing temperatures has little to no effect on inductance values.

To listen to a discussion on troubleshooting for electric motor reliability visit our PdMA YouTube Channel at


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About the Author

Noah Bethel Vice President-Product Development, PdMA Corporation

Noah has over twenty-five years of broad operations and electrical systems maintenance experience in industrial, commercial, and military settings ranging from nuclear submarines to world class amusement parks. His experience includes high and low voltage, AC and DC, power generation, power distribution, motors, and motor controllers. Noah is currently in charge of product development for new and existing PdM technology at PdMA Corporation.

Noah is a graduate of the University of the State of New York and the Naval Nuclear Power School and Training Unit. He is a Certified Maintenance Reliability Professional, with field experience in motor circuit analysis, current signature, power analysis, thermography, vibration analysis, oil analysis and ultrasonic testing.