Show Me the Swirl
High pole pass frequency (Fp) peaks as a sideband around electrical line frequency or as an independent frequency in a current demodulated spectrum are both indications of a rotor bar defect. However, it is important to remember that mechanically induced load changes can also create frequency peaks at or near Fp that mirror the effects of a broken rotor bar. If a trend is not your friend, meaning you have not collected enough historical data to identify a trend, then you should always rely on a third indication before calling the rotor bad. One very good indication is to monitor the Swirl Effect. Best described in detail by the late Gerald Kliman, the Swirl Effect is the 180° reversal in phase of the air gap flux across a broken bar. This disturbance of flux is easily transferred to the stator and can be detected with current signature technology like the EMAX focusing on the 5th harmonic as a primary location for slip related sideband activity. So, if you’re trying to make a decision on a possible rotor defect, don’t forget the swirl.
To learn more about the Swirl Effect from a broken rotor bar Click Here to view a short video.
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