(369f) Numerical Simulation to Predict Motor Bearing Damage By Electrical Pulses

Currently, silicon-based power semiconductors are widely used in variable frequency drive (VFD) systems. It is highly desirable to have high dv/dt pulse-width modulated (PWM) inverter output pulses to minimize the turn-on/turn-off times/switching loss from a device efficiency point of view. However, the high dv/dt PWM inverter output pulses have adverse effects on the machines they are driving due to motor terminal over-voltages stressing the insulation and damaging bearing currents [1], which can lead to bearing destruction within a few months of VFD operation and is thus expensive in motor repair and downtime. Therefore, a simulation model that can predict the damages would be necessary.

The simulation model is developed based on the gap between the bearing ball and the race. Since the grease used between the gap has a partial insulating effect, charge accumulates on the rotor assembly until the voltage exceeds the dielectric capability of the bearing grease, which leads to a frequently repeated flash-over current and in time can cause electroplating and pitting of balls and races. Numerical simulations are used to explore characteristic parameters (such as frequency, dv/dt, di/dt, motor running speed, etc.) of the bearing, and optimized by matching the experimental data, and eventually, predict the failure time of the bearing.

Reference:

[1] A. von Jouanne, P. Enjeti, W. Gray, “Application Issues for PWM Adjustable Speed AC Motor Drives,” IEEE Industry Applications Magazine, Sept/Oct. 1996, pp. 10-18. Prize Paper Award.