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Inductive Imbalance in a Wound Rotor Motor

Noah Bethel | Vice President-Product Development, PdMA Corporation

Inductive Imbalance in a Wound Rotor Motor

Wound rotor motors are used for a variety of applications where high torque is needed at low speeds, such as conveyors, cranes, shredders, and more. Standing next to a 7000 hp wound rotor motor driving a shredder at a steel-recycling plant while an old car with a 454 big block engine gets shredded is something we should all experience.

Testing wound rotor motors can be tricky depending on what and where you are testing. Different from a squirrel cage motor, a wound rotor motor has windings on the stator and rotor. This makes it similar to a transformer in that changes in the secondary (rotor) circuit can cause large influences to measurements taken on the primary (stator) windings.

For baseline or troubleshooting a wound rotor motor, we recommend testing the rotor circuit, resistor bank, and stator windings separately. If a large inductive imbalance is seen on the stator, remember that it could be caused by something changing in the rotor or resistor bank circuits.

Contamination between slip rings connected to the rotor circuit may create a false symptom on the stator winding. When the brushes are lifted, they are more likely to touch each other, possibly shorting out a phase of the resistor bank, which will also have a large influence on the stator winding, leading an analyst to suspect stator winding defects. Normally a wound rotor motor in the warehouse will have the brushes lifted, so look before you test.

Go to to view a case study on our YouTube channel about a wound rotor motor fault.

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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.