Two Minute Tips  

Performing a Demodulation of the Current Signal – Rotor Evaluation

Noah Bethel | Vice President-Product Development, PdMA Corporation

Six independent methods of rotor evaluation can be used from the MCEMAX technology to analyze the condition of a squirrel cage AC induction motor: Pole-pass frequency (Fp) sidebands around line frequency, 5th harmonic, demodulated Fp frequency, Rotor Influence Check (RIC), In/Rush-Start/Up, and average inductance. Over the next six tips we will discuss each of these methods in detail and provide examples.

Performing a demodulation of the current signal and displaying it in the frequency domain provides a look into the electro-mechanical world of the electric motor. Previously hidden in the noise platform of the current spectrum, now peaks related to belts, gears, fans and other machine train components can be easily identified and band alarmed. A further advantage of the demodulated current spectrum is that the pole-pass frequency (Fp = #poles x slip) has proven to be far more sensitive and accurate as an indicator of rotor bar defects than just trending Fp sidebands around line frequency. Especially for large two pole motors.

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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.