Analysis of Rotor Currents under the Starting for the Detection of Stator Asymmetries in Wound Rotor Induction Motors

Wound rotor induction motors (WRIM) are commonly used in applications requiring high starting torques. This is due to the possibility of adding external rheostats in series with the rotor winding that enhance their speed-torque characteristics. However, they are more prone to failures due to their greater constructive complexity. Previous works demonstrated that the combined application of steady-state (Motor Current Signature Analysis) and transient stator current-based methodologies enables a robust diagnosis of rotor asymmetries. However, few works have considered thedetection of stator winding asymmetries (a type of failure that is rather frequent in field WRIM). On the other hand, even fewer works have explored the suitability of rotor current analysis for diagnostic purposes. This work proposes the advanced analysis of rotor currents for the detection of stator asymmetries in WRIM. The proposed novel methodology is based on the combined application of steady-state and transient-based methods to rotor current signals. Unlike what happens in Cage Induction Motors (CIM), rotor currents can be easily measured in WRIM in a simple, reliable and economic way; these are interesting advantages for a reliable WRIM condition monitoring compared to other solutions. The laboratory results 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. Also, 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 an industrial environment in two different large field motors operating in cement plants. This proves the powerfulness of the approach to diagnose the presence of the considered failure.