Non-uniform voltage distribution in stator winding of PWM fed induction motor

Abstract

The stator turn-to-ground and turn-to-turn insulation failures are caused by thermal, mechanical and dielectric stresses, the major contributory factor being the dielectric stresses arising from switching surges due to adjustable speed drives (ASDs) fed by PWM inverters. The numerical evaluation of the electrical stress in the line end coil of the stator winding of an induction motor fed by a pulse width modulated (PWM) inverter seems to be indispensable for a rational design of the machine. In order to fulfill such a task, the system composed of a PWM inverter, feeder cable and a stator winding is modelled and simulated by using EMTP software. The present work on induction motor over voltages is therefore aimed to resolve these issues. This paper aims to make contributions towards a comprehensive understanding of the internal voltage distribution in the stator winding taking into account simultaneously both the effects of self, mutual inductance and capacitance parameters as well as their difference between the slot embedded and the end winding portion, for a fast switching impulse voltage applied at the motor terminals. Hence this leads to non-uniform distribution of wave front over coil. Because the wave front duration of most surges is very short, as compared to the propagation time of a coil, the line end coil, under the majority of cases, receives the most stress on both its turn to turn and turn to ground insulations. This has been shown for surges with wave front durationpsilas of 0.5 mus or less. Under surge conditions 80% of incident transient voltage may appear across the line end coil. Simulation and experimental studies are carried out and presented in detail in this paper. Solution to minimize the non-uniformity is also explained in this paper.