Power quality studies on a Soft-start for an induction motor

Three-phase induction motor of ratings beyond 50 kW take very large low power factor currents while being started directly from a 3-phase supply. Normally soft-starts are used for avoiding this problem and to achieve smooth starting of large capacity induction motors. A 3-phase AC voltage controller is employed as a soft-start. But, this takes harmonic-rich current especially while operating at large firing angles. In this paper, the effect of inserting a shunt active filter to provide harmonic and reactive power compensation in a soft-start has been studied. The shunt active filter has been inserted between the AC voltage controller and power supply to take care of the reactive power requirement of the motor and AC voltage controller and also to provide harmonic compensation. Both simulation hardware studies have been done on a 22 kW induction motor with and without soft-start arrangement. The shunt-active filter uses the IcosPhi control algorithm for calculating the compensation currents. The results with the insertion of shunt active filter show a substantial reduction in the current drawn from the source and its harmonic content. This will be a very useful scheme especially in industries where multiple induction motors of large capacity are frequently started from a 3-phase supply.