A Fast Transient Model for Bearing Fault Analysis in Induction Machine Drives

In this paper, a common-mode and medium-range frequency model for an electrical drive, which includes an inverter, an electrical machine, and bearings, is proposed. The bearing model is a piecewise linear model that can model simultaneously most types of bearing current mechanisms. This common-mode model can be used in the stage of designing an electrical machine to evaluate the machine against the maximum permissible level of bearing voltage, which is stated in the standards, such as NEMA-MG1. Instead of using spectrum analysers, an impulse generator circuit, with voltage which is high enough, is presented for finding the impulse response of the drive in common mode. Then, an effective and low-cost method is suggested to identify bearing failures. In this method, the conditions of the bearing of an inverter-driven electrical machine can be monitored, and failures can be detected by analyzing the measured time waveforms of the voltages of bearing ends or the magnitude of the fast Fourier transform of ground currents near the frame, while neither the installation of complex mechanical measurement devices nor sophisticated spectral analysis techniques for finding correct frequencies is required. Finally, to validate the failure detection method, the numerical results of the proposed model with a bearing failure are compared with the experimental measurements.