Capacitance of bearings for electric motors at variable mechanical loads

Bearing currents in converter-fed three-phase motors are significantly dependent on the lubricant film condition. As a consequence, the lubricant film thickness and the resulting capacitance are important input quantities for a simulation model aiming at predicting bearing currents and harmful electric discharges, which was presented in corresponding work. In addition to speed, load and temperature, there is another influence on motor bearing film thickness: Vibrations caused by external excitation. A new test rig for ball bearings was designed to investigate the influence of vibrations on the lubricant film. After a brief description of the test rig, test results are presented. Based on these results, the probability of capacitive versus ohmic behaviour of the bearing is estimated.