Numerical characterization of electrical stresses on dielectric grease of rolling bearings in induction motors fed by PWM inverters

Pulsewidth modulated (PWM) inverters are one of the major causes of motor bearing failure in inverter-motor drive systems. The new switching devices, such as the IGBT´s, characterized by high switching frequencies and faster switching times, on one side have increased the efficiency, performance and controllability of adjustable speed drives but, on the other side, have brought some relevant drawbacks. In fact, the output voltage from the inverter is not purely sinusoidal and, in particular, steep front pulses can be generated in correspondence to the commutations of switching devices. The presence of high frequency harmonics causes an uneven voltage distribution along motor windings, producing critical stresses on the motor electrical insulation; furthermore the steep pulses can excite the capacitive coupling between the stator and the rotor, resulting in shaft voltages. The latter may cause the breakdown of the lubricating film in the bearings, and the associated electric discharge machining (EDM) currents can damage the bearing and reduce its lifetime. For design purposes it seems of major importance to predict the electrical stresses on the lubricating film of rolling bearings. The present paper is dedicated to the presentation of results of numerical simulations carried out on a traction motor obtained by implementing a Multiconductor Transmission Line (MTL) model of the overall system composed of the power voltage, the feeding cable, the motor stator winding, the rotor shaft and the bearings. The influence on the shaft voltages of parameters like the amplitude and slew rate (dV/dt) of the applied voltage, the cable length, is discussed.