Don't lose your bearings

The major cause of parasitic bearing currents is the common-mode voltage source developed by the feeding inverter. Different investigated mitigation techniques are efficient for parasitic bearing currents during inverter-operation depending on the type of bearing current. These are shielded cable, passive filters and insulated/hybrid bearings. The use of shielded rather than unshielded motor cables is not an effective mitigation technique for bearing currents of the electrostatic discharge machining (EDM) and circulating types. EDM bearing currents are not related to the stator ground current. Passive filters suppress the common-mode current but not the common-mode voltage. Insulated bearings are steel bearings with an insulating aluminum oxide layer deposited on the outer surface of the outer bearing ring. These bearings have been used to suppress classical bearing currents caused by magnetic asymmetries in line-fed motors. Hybrid bearings consist of steel bearing rings and ceramic bearing rolls or balls. It completely suppress EDM, circulating and rotor ground currents.