Detection of Stator Asymmetries in Wound Rotor Induction Motors through the Advanced Analysis of Rotor Currents

The constructive characteristics of wound rotor induction motors (WRIM) make them more prone to failures than cage induction motors (CIM). The participation of certain elements such slip ring/brushes systems or starting rheostats imply a higher probability for the occurrence of certain modes of failure. Previous works proved that the analysis of the stator current demanded by the motor was a powerful tool to diagnose some of these failures; in this regard, it was demonstrated that the combined application of steady-state approaches (MCSA) and transient current-based methodologies enables a robust diagnosis of rotor asymmetries, which were, indeed, quite frequent in these machines. Nonetheless, most of these approaches relied on the analysis of stator currents, whereas few works have investigated the suitability of rotor currents analysis to diagnose certain faults in WRIM. This work proposes the advanced analysis of rotor currents for the detection of stator asymmetries in WRIM (a type of fault for which there is a lack of diagnosis options in the field). The methodology is based on the combined application of steady-state and transient-based methods to rotor current signals. Unlike what happens in CIM, these currents can be easily measured in WRIM using the same equipment that is required for the measurement of stator currents. The laboratory results of this work prove that it is possible to discern the harmonics caused by different levels of stator asymmetries in the FFT spectrum of steady-state rotor currents and, moreover, the fault also yields characteristic patterns in the time-frequency maps resulting from the analyses of rotor currents under the starting. These results are validated with a field 1.65 MW WRIM operating in a cement plant, which proves the powerfulness of the approach to diagnose the presence of the considered failure.