Mitigation of bearing electro-erosion of inverter-fed motors through passive common mode voltage suppression

Investigations of bearing current caused by voltage source inverters (VSIs) have emphasized the effect of steep wavefronts and high carrier frequencies. Such investigations have led to suggested bearing current mitigation techniques incorporating frequency dependent devices such as low-pass VSI output filters and low-inductance ground paths. Alternatively, others have addressed the problem from an electrostatic standpoint within the motor and have offered solutions such as shaft grounding brushes, insulated bearings and electrostatically shielded stator windings. This paper examines VSI output common mode voltage as the driving source of bearing currents and develops, from a system level, the electrostatic circuit approach as a device to give insight into the technique of reducing common mode voltage at the motor terminals and thus at the rotor. The technique is passive, low cost, and retrofittable where allowed by electrical codes and is capable of reducing common mode voltage by levels of magnitude. Such reductions can profoundly extend bearing service life by reducing the electro-erosion of the bearing surfaces.