An accurate evaluation of electric discharge machining bearings currents in inverter-driven induction motors

In inverter motor drives an increasing fraction of faults is associated to the bearings damages induced by circulating currents. Current flow through the bearings is a consequence of two phenomena which, in turn, may depend on the motor speed. The first one gives rise to a conduction current through the motor bearing if a metallic contact takes place between the shaft and frame (for example, by bearing ball contact at low speed). The second one determines a discharge current when the voltage across the bearing lubricating film exceeds the film breakdown voltage (typically at high rotor angular velocity). These currents are responsible of the so called electric discharge machining (EDM) of the bearings. In order to account for such damaging mechanisms an accurate model for the evaluation of bearings currents in inverter-driven induction motors has been developed and is presented in this paper. The statistical occurrence of the discharges is also taken into account in a simple but accurate way. The model allows to estimate the characteristics of the discharge pulses in typical PWM inverter driven traction motors and hence the energy levels producing the onset of the bearings' damage.