MODELING OF MOTOR BEARING CURRENTS IN PWM INVERTER DRIVES

Pulsewidth modulated (PWM) inverters have recently been found to be a major cause of motor bearing failures in inverter-motor drive systems, Specifically, all inverters generate common mode voltages relative to the earth ground, The voltages provide coupling or bearing currents t hrough motor parasitic capacitances to the rotor iron which flow via t he bearings to the grounded stator case,In this paper, a model of bear ing currents caused by PWM inverter is proposed, The model is based on transmission line theory which uses an equivalent lumped parameter pi -network to describe the parasitic coupling phenomenon. The model para meters are then identified by matching the calculated model outputs wi th those of experimental measurement, The validation of the method is demonstrated by the fact that the model can reproduce a variety of exp erimental results obtained on a test motor, An application of this met hod also gives a motor grounding current model, As the conducted elect romagnetic interference (EMI) in drive systems is related to the groun ding currents, the grounding current model can be used for the analysi s of conducted EMI in motor-drive systems.