Numerical simulation and measurement of common-mode and circulating bearing currents

Steep wavefront voltages of pulse width modulated (PWM) inverters cause common-mode (CM) bearing currents due to the capacitive coupling between stator windings and rotor core. The CM bearing currents represent a current unbalance of the three-phase winding system producing a high-frequency (HF) magnetic flux around the motor shaft that induces the so-called circulating bearing currents. The circulating currents flow through the electrically conductive loop consisting of the motor shaft, bearings and stator frame. Both the bearing current components cause bearing material corrosion and early mechanical failure. This paper presents a method for computing the bearing currents based on advanced 2-D and 3-D electromagnetic field simulations for obtaining the parameters of a detailed motor equivalent circuit. The suggested methodology was tested on a chosen synchronous motor and the obtained results were verified by comparison with measurements.