Diagnosis of rotor faults in three-phase induction motors under time-varying loads

Condition monitoring schemes for IM rotor faults are usually developed under the assumption of a constant load torque. The most accepted technique is the so-called motor current signature analysis (MCSA), which consists in the analysis of specific spectrum components of the stator currents, related to certain motor faults. Broken rotor bars are detected by monitoring current spectral components, which appear as sidebands around the supply frequency, at twice the slip frequency and its multiples. On the other hand, an oscillating load torque with an ac component at the frequency equal to twice the slip frequency gives rise to the appearance of the same current spectral components, hence not being possible to effectively discriminate these two working conditions using this diagnostic method. In this paper, a new diagnostic method to diagnose rotor faults in operating three-phase induction motors under the presence of time-varying loads is presented. The proposed method is based on the analysis of the zero crossing time (ZCT) signal of the (emf) obtained from an external search coil, which allows perfect simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque. The 2D finite element analysis results show the interest and the efficiency of this proposed diagnostic technique.