WRM – Pole Pass Frequency With No Rotor Bars?

A Wound Rotor Motor (WRM) is an induction motor typically found in cranes, hoists, compressors, conveyors, mixers, fans, blowers, mills, and pumps. However, unlike the very common squirrel cage induction motor, the WRM rotor has windings in the rotor slots rather than rotor bars. A common rotor analysis technique on squirrel cage induction motors is the use of the pole pass frequency (Fp) amplitude trend. The Fp is the rate at which the magnetic field on the stator passes the magnetic field on the rotor. This event creates a load modulation that is easily seen on the current spectrum in the form of a sideband peak around line frequency. Without rotor bars in a WRM do we still have the Fp to analyze the health of a rotor? The answer is Yes!  Both a squirrel cage and wound rotor will create a rotor magnetic field that will interact with the stator magnetic field to create a Fp peak. The big difference is that a WRM rotor fault zone is much more complicated and extends to slip rings, brushes, and a variable resistor bank with contacts. When a defect occurs in one of the many components of a WRM rotor circuit it often has a dramatic effect on the Fp and should be acted on quickly to reduce any secondary damage. Also, the traditional alarm levels associated with Fp sideband amplitudes come from squirrel cage induction motors and should not be directly applied for severity levels without historical trend.

To watch a case study showing a rotor circuit anomaly on a WRM visit the PdMA YouTube channel at https://www.youtube.com/watch?v=YKSQBVL0bos