Rotor impedance of the high frequency circulating bearing current path in inverter-fed AC machines

The circulating bearing current depends on the common mode stator ground current and the stator laminated core impedance, but also on the bearing impedances and on the rotor impedance of the circulating bearing current path. In the literature, the rotor impedance was neglected, or it was considered as a small air gap and end-winding cavity inductance. It is shown here by calculation and measurement that the rotor impedance is not negligible, and it is much larger than the air gap and end-winding cavity inductance. Moreover, it is shown that the circulating bearing currents can be mitigated, if the rotor impedance is increased by at least ten times the stator impedance. For increasing the rotor impedance, nanocrystalline magnetic materials are added on the machine shaft. The circulating bearing currents are measured for a standard 110 kW machine and for the same machine with the increased rotor impedance.