Study on Bearing Impedance Properties at Several Hundred Kilohertz for Different Electric Machine Operating Parameters

The possibility of bearing damage caused by inverter-induced bearing currents in modern variable-speed drive systems has been well recognized. However, the breakdown and current conduction mechanisms have still not been well understood today. We present results on the HF impedance properties of rolling element bearings at 300-kHz and 1.5-MHz signals-frequencies taken from the typical spectra of HF circulating bearing currents. Different motor speeds, bearing temperatures, voltages across the bearing, and two different motor sizes are investigated. Notably, we analyze the frequency of transitions between ohmic and capacitive behavior (or vice versa)-the “mode transitions”-as a function of these operating parameters: For some operating conditions, the bearing impedance alternates between capacitive and ohmic behavior without any modification of the macroscopic conditions it is subjected to.