Effects of the Speed Loop on the Diagnosis of Rotor Faults in Induction Machines

The development of high-performance static converters and advanced control techniques has led to a widespread diffusion of asynchronous drive systems in industrial plants. One of the strong points of the induction machines is their reliability; however, an unexpected failure may still result in a considerable economic loss. This fact has stimulated a prolific research activity concerning the continuous monitoring of the wear status of induction machines. The knowledge of their condition allows implementing condition-based maintenance techniques, which have proven to be very effective in critical processes. In previous papers, the authors have focused their attention on the detection of broken rotor bars in inverter-fed induction machines. Two indexes have been defined: Both of them are simple methods to estimate the same fault indicator, namely, the ratio between the amplitude of the power oscillation and the average power draw of the drive system. However, this fault indicator exhibits a noticeable dependence on the mechanical load as well as on the speed loop parameters. In this paper, a deep investigation about these dependences is provided. The study suggests introducing a new fault indicator which is much less sensitive with respect to the torque developed by the motor. Furthermore, the analysis allows predicting how the speed loop affects the fault detection techniques. The results of the study and the effectiveness of the new fault indicator have been first verified by means of numerical simulations and then validated through an experimental activity.