Bearing raceway damage with high frequency bearing currents in inverter-fed 90 W-cage induction motors and effect of current rectification

In automotive applications the rotors of the inverter-driven electric AC motors might be grounded e. g. by a coupled gear, causing a common-mode rotor-to-ground current flow via the bearings. In case of a not grounded rotor at full lubrication of bearings, the electrical potential difference between rotor and housing may lead to discharge bearing currents. Their occurrence rate is smaller than for rotor-to-ground currents, where the bearing stays in a conductive state. So, for equal high bearing current amplitudes the dissipated power of the high frequency bearing currents in the bearing is much bigger for the rotor-to-ground bearing currents. This is demonstrated by 1000 hours-tests for three to five small inverter-fed 90 W-cage induction motors, where roughly at the same maximum bearing current amplitude the discharge bearing currents caused only a grey "frosting" of the bearing raceway, whereas the rotor-to-ground bearing currents caused strong fluting. In addition, the effect of an added DC component to the bearing current was investigated by rectifying the discharge bearing currents with diodes, which also led within 1000 hours to pronounced fluting, confirming that bearing currents with a constant polarity are more dangerous to generate fluting than corresponding AC currents of same amplitude.