Influence of circuit parameters on the electric discharge machining of the bearings of a PWM inverter driven motor

In inverter motor drives an increasing fraction of faults is associated to the bearings damages induced by circulating currents. In particular, the capacitive couplings between the stator and the rotor give rise to a shaft voltage which, in turn, may produce the breakdown of the lubricating film in the bearings and the associated electric discharge machining (EDM) currents. By using an accurate circuital model for an inverter-driven induction motor for traction applications such damaging mechanisms, the statistical occurrence of the discharges and the associated energy level are evaluated. In order to investigate the relations among the currents amplitude and frequency and the influencing parameters of the drive, such as the cable length, the geometrical and physical characteristics affecting the bearings impedance, the Response Surface Method is applied. The most influencing parameter, the sensitivity level and the mutual effects induced by the parameters variations are obtained. The resulting information can be used in the design phase. The same approach can be applied also for correlating different performances of the motor with further influencing parameters.