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 up to typical 20 kW at 1500/min are likely to suffer from discharge (EDM) bearing currents, while larger motors are likely to be subject to high-frequency circulating bearing currents. When comparing motors operated at the same voltage level, the resulting bearing current density is high for very small and for very large motors. Smaller values occur in-between these two extremes with medium size motors in the range (10... 100) M Therefore, electric bearing insulation is useful for larger motors to interrupt the HF circulating bearing currents path, whereas small motors need either rotor shielding, common mode voltage filters, or hybrid bearings.