Scaling effects of inverter-induced bearing currents in ac machines

In this survey paper, we show by straightforward mathematical scaling considerations that small inverter-fed ac motors of up to typical 20 kW at 1500/min are likely to suffer from discharge (electric discharge machining) bearing currents, whereas larger motors are likely to be subjected to high-frequency (HF) circulating bearing currents. When comparing motors that operate at the same voltage level, the resulting bearing-current density is high for very small and very large motors. Smaller values occur in between these two extremes with medium size motors in the range of 10-100 kW. Therefore, electric bearing insulation is useful for larger motors to interrupt the HF circulating bearing-current path, whereas small motors need rotor shielding, common-mode voltage filters, or hybrid bearings.