Capacitively coupled discharging currents in bearings of induction motor fed from PWM (pulsewidth modulation) inverters

This paper examines ac motor bearing currents (Electric Discharge Machining – EDM) resulting from a shaft voltage when operated under pulse-width modulation voltage source inverters. The mechanism for electrostatic shaft voltage buildup and bearing current occurrence is briefly explained. The basic cause of bearing currents is an instantaneous electrical asymmetry on the output of the inverter and distributed stray capacitance couplings in the drive system, especially inside of the motor. It is pointed out that bearing current event is an electrostatic discharge phenomenon. Using the experimental data and a simulation model, it is shown that EDM currents do not flow to a grounding arrangement as has been suggested in previous works, but circulate inside of the motor.