Experimental ball bearing impedance analysis under different speed and electrical conditions

Ball bearings are commonly used in inverter driven electrical machines, where they bluecan be subject to high frequency common mode currents which accelerate their degradation. This paper investigates the impact of shaft speed, voltage amplitude and frequency on the bearing impedance, including during electrical discharges, with the aim to provide an experimentally validated approach to model their high frequency behaviour as function of operating conditions. This analysis can help the development of improved models to predict bearing currents in electric drives. A dedicated experimental setup, completely isolated from ground, is presented. It proposes dielectric plate and bolts to insulate the bearing from the support (which is connected to the ground), a dielectric coupling to insulate the dyno from bearing, dielectric sheets to insulate the brush holder from the ground. Different voltage and frequency amplitudes are applied between the inner and outer bearing raceways for several shaft speeds. The measured impedance amplitude and phase, currents and voltages are presented. Due to the large amount of data, a statistical approach is proposed for studying the bearings behaviour. A simple analytical bearing model is presented in order to provide an interpretation of the experimental measurements. The analysis shows that the voltage amplitude and frequency have a significant impact on electrical discharge in ball bearings, especially for certain shaft speeds strongly affecting the impedance amplitude and phase during the breakdown state.