Two Minute Tips  

Troubleshooting the Rotor and Air Gap on a De-energized Motor

Noah Bethel
Noah Bethel | Vice President-Product Development, PdMA Corporation

Today we look at troubleshooting techniques and considerations on a de-energized motor for the rotor and air gap fault zones. One of the measurements taken during the five-minute MCE® standard test is inductance, and with the three phases of inductance, inductive imbalance is calculated. Different rotor positions will change the reflected impedance onto the stator windings changing the inductance measurement uniquely for each phase of the stator winding.  However, because of the nature of a three-phase balanced system, inductive imbalance should not change and a steady inductive imbalance is a healthy sign. Although stator phase inductance readings rise and fall for each residual field passing the stator phase, rising trends in average inductance are indicative of developing rotor anomalies such as broken, cracked, or high resistance rotor bar and end rings.  Performing a detailed Rotor Influence Check (RIC) shows phase resistance measurements for each predefined degree of rotation of the rotor giving a graphical representation of the residual rotor magnetic field. The RIC will allow confirmation of the source of inductance changes as coming from the rotor or stator making the troubleshooting effort much easier. Additionally, the RIC test will allow the analyst to identify air gap offsets due to a larger air gap causing a reduction in the inductance values.

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

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About the Author

Noah Bethel
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.