Evaluation of the bearing voltage and the overshoot phase voltage in PWM inverter-fed by means of a Multiconductor Transmission Line Model

In Adjustable Speed Drives (ASDs) for industrial and traction applications the pulse width modulation (PWM) inverters may produce a shaft voltage due to the capacitive couplings between the windings and the rotor and between the rotor and the stator. Such shaft voltage may be so intense to generate breakdown of the lubricating film in the motor bearings and the associated impulsive currents damage the bearing elements thus affecting the overall ASD reliability. In order to analyze these phenomena and carry out appropriate system design, suitable numerical models are needed which accurately take into account the effects of electromagnetic wave propagation and reflection and the frequency-dependent distributed losses in the system. For this purpose, by means of a model based on a lossy multiconductor transmission lines (MTLs) approach previously developed, the prediction of the voltages and currents at critical sections of a full stator winding (of the wound type) of a three phase high power traction motor is carried out. The MTL model is studied in the time-domain by using the telegraphers' equation system with suitable boundary conditions. The phase voltage at the input terminals of the motor and the voltage across the two bearings are calculated. Moreover, the simulated input voltages are shown to be in good agreement with those obtained by measurements on a real traction drive.