EDM Damage Assessment and Lifetime Prediction of Motor Bearings Driven by PWM Inverters

Common-mode voltages produced by pulse width modulation inverters produce electric discharge machining (EDM) currents in a motor's bearings, which can cause premature failure of the bearings. This paper investigates the impact of these discharges, for both Si- and SiC-based motor drive applications, on the mechanical damage produced on the bearing ball and race surfaces with a finite element model, the results of which are compared with actual EDM damage spots on 5hp motor bearings (6306) tested experimentally. Then, experimental results are presented that demonstrate a correlation between EDM discharge energy and the number of discharges until bearing failure (i.e., lifetime), which allows for lifetime predictions of motor bearings. In this paper a bearing lifetime prediction model is developed given typical discharge conditions from both Si and SiC inverters using a bearing state of health (SOH) metric.