Cepstral analysis of the stator current for monitoring mechanical unbalance in squirrel cage motors

Elderly loads cannot impose to the driving rotation process a strict unvarying torque. This truth can occur even with new loads because of inherent manufacturing imperfections. In the majority of circumstances, the load torque varies according to the machine rotor position and these variations, which are not related to the motor health condition, involve an amplitude modulation of the machine stator current at rotational frequency. Thus, harmonics caused by an eventual mechanical unbalance can be overlapped and the user can not distinguish a normal operating condition from a failure mode. Since an induction machine becomes sufficiently loaded, classical diagnosis technique based on the spectral analysis of the stator phase current becomes insufficient to monitor such fault incidence; in this paper we suggest an alternative diagnosis practice based on the analysis of the complex cepstrum. Simulation and experimental results show the effectiveness of the proposed method.