Circulating Type Motor Bearing Current in Inverter Drives

Pulse width modulated (PWM) inverters have been associated with induction motor bearing currents and thus bearing damage in recent studies [1-4]. The high dv/dt common mode voltages and parasitic capacitances in a drive system were found to account for the generation of motor shaft voltages and bearing currents. Based on a theory proposed in references [3, 4], the common mode voltages excite the parasitic capacitances of a drive system and produce the so-called common mode currents. A small portion of the common mode currents flows from the windings through parasitic capacitors to the rotor and then finds its path via the conductive bearings back to the grounded motor case. Therefore, a type of bearing current is recognized as being related to chose common mode currents that flow from the windings to the rotor. Since this type of current passes through the bearings unidirectionally from the rotor iron to the stator iron, rather than circulating in the conductive loop formed by the stator case, rotor shaft, and bearings, it is termed the noncirculating type of bearing current.