Steep-fronted overvoltages in inverter-fed induction motors: numerical identification of critical parameters

Due to the trends in power-systems, steep-fronted switching surges are often encountered in many applications as, for example, in induction motors fed by pulse-width modulated (PWM) inverters. In such a case, the effect of these voltage transients can result in undesirable electrical and thermal stresses in the stator windings, which can lead to the premature failure of the insulating material between the coils and, definitely, to the failure of the whole component. Up to now, the dimensioning of the interturn insulation of such motors is based empirically on previous service experience; a more rational design can be performed if a more exact knowledge of the voltage distribution within the stator winding is reached. In the paper the first results of a simulation carried out in order to predict the voltage distribution between the coils of a stator winding fed by a PWM inverter are presented. Such distributions depend on a very large number of parameters (insulation dimension, coils size, length of connecting cable, shape of the applied voltage, etc.). In the following the influence played on such a distribution by the cable length and applied voltage rise-time is discussed.