Smarten Up About Rotor Analysis

Rotors are among the many variables that can affect the reliability of squirrel-cage AC induction motors. According to Noah Bethel of PdMA Corp. (Tampa, FL, pdma.com), a study sponsored by the Electric Power Research Institute (Palo Alto, CA, epri.com), and performed by General Electric in the 1980s estimated that rotor defects were responsible for approximately 10% of failures in such motors. Things certainly have changed since then—including motor-testing equipment and methods. Yet, even now, Bethel notes, one of the biggest problems in electrically analyzing squirrel-cage induction rotors continues to be access to the rotor itself. The solution? “We don’t want to disassemble every motor just to look at its rotor,” he explained. “We have to be a little smarter and a little more equipped with the right kind of tools and techniques.” As an example, he points to the following six types of rotor analysis. Keep them in mind.

Fp SIDEBAND

Fp (pole-pass frequency) sideband amplitude is one of the more established methods of rotor evaluation using the current-signature analysis test. The slip between the rotating rotor and stator magnetic fields creates a modulation of the stator current at Fp presented as a peak on a spectrum plot in the frequency domain. Differential amplitudes between the Fp and line frequency can be trended to identify rotor-bar defects.

DEMODULATED Fp

Demodulating the current and displaying it in the frequency domain provides a look into rotor health, as well as the electro-mechanical machine-train components of the motor. Research has found that the Fp identified in the demodulated spectrum is the most sensitive indication of developing rotor-bar anomalies for large two-pole motors.

5th HARMONIC

Broken rotor bars result in a 180-deg. phase shift in rotor magnetic flux. This can be seen in a rotor-evaluation current spectrum as three peaks separated by Fp to the left of the 5th harmonic peak.

ROTOR INFLUENCE CHECK

Inductance measurements of the de-energized stator windings at different rotor positions can be plotted to create a graphical representation of the rotor magnetic flux. High resistance joints and broken rotor bars will change the impedance reflected back onto the stator windings creating a rotor-defect flux pattern.

IN-RUSH/START-UP

High resistance connections or broken rotor bars change the reflected impedance on the stator windings causing a drop in the start-up current and start-up torque. This drop in startup torque will result in a longer acceleration time for the motor.

AVERAGE INDUCTANCE

Broken rotor bars result in an increased inductance value measured on the stator windings of a de-energized motor. Trending this value will give ample warning of developing rotor defects. EP

Noah Bethel is vice president of Product Development for PdMA Corp., Tampa, FL. For more information on motor-testing and analysis topics and solutions, visit pdma.com.