Modulations Diagnosis of Asynchronous Motors Stator Current in Case of Load Torque Oscillations

This paper proposes a novel method for the prediction of stator current modulations applied to the detection of load torque oscillations in asynchronous drives. In faulty cases, the stator current presents amplitude and phase modulation (AM - PM) leading to sideband frequency components in the current spectrum. The characteristics of these modulations depends on the electrical and mechanical parameters of the drive, on the fault frequency and on the operating point. The knowledge of these properties mean helpful information for the definition of a detection scheme. A full analytic approach is then proposed to predict the faulty stator current behavior. This theoretical analysis is based on the state space representation of the induction machine and allows expressing the frequency variation law of the sideband components for a given drive. Amplitude and phase modulation indexes are determined using proposed model that is validated with experimental results.