Modeling of motor bearing currents in PWM inverter drives

Pulse-width modulated (PWM) inverters have recently been found to be a major cause of motor bearing failures in inverter-fed motor drive systems. Specifically, all inverters generate common-mode voltages relative to the earth ground. These voltages provide coupling or bearing currents through motor parasitic capacitances to the rotor iron which flow via the bearings to the grounded stator case. In this paper, a model of bearing currents caused by PWM inverters is proposed. The model is based on transmission line theory which uses an equivalent lumped parameter π-network to describe the parasitic coupling phenomenon. The model parameters are then identified by matching the calculated model outputs with those of experimental measurement. The validation of the method is demonstrated by the fact that the model can reproduce a variety of experimental results obtained on a test motor. An application of this method also gives a motor grounding current model. As the conducted electromagnetic interference (EMI) in motor drive systems is related to the grounding currents, the grounding current model can be used for the analysis of conducted EMI in motor-drive systems