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Circulating type motor bearing current in inverter drives

Motor bearing currents in inverter drives have become an important issue as very fast turn-off switching devices have become available for medium horsepower applications (10-100 HP). In previous work a noncirculating type of bearing current caused by parasitic capacitive coupling from stator windings to the rotor was identified. In this paper a new circulating inductive type of bearing current is reported. A theory for the existence of this new type of bearing currents in inverter-motor drives is proposed. It identifies the fact that unbalanced currents flow in the two coil sides of a motor winding due to distribution of capacitive coupling currents. The current unbalance produces a high frequency magnetic flux surrounding the motor shaft and thus causes a bearing current to circulate in the conductive loop formed by the stator case, rotor shaft and end bearings. The amplitude of the circulating bearing current depends on the common mode component of capacitive coupling currents. The analysis is verified by experimental measurement or bearing currents. The findings also provide insight to determining solutions for attenuating these destructive bearing currents.