A Simplified square wave PWM technique: for speed control of low power 3-phase induction motors

Abstract

AC induction motors are used today for a wide range of applications demanding variable speed. This has created a high demand for AC induction type variable speed motor drives. These drives mainly use PWM and Space vector controlling techniques to achieve variable speed. PWM appears in Square wave, Sinusoidal and Hysteresis (band-band) forms. When alternating current is applied to the stator windings of an AC induction motor, a rotating magnetic field is developed. The rotating magnetic field cuts the bars of the rotor and induces a current in them due to generator action. The direction of this current flow can be found using the left-hand rule for generators. This induced current will produce a magnetic field, opposite in polarity of the stator field, around the conductors of the rotor, which will try to line up with the magnetic field of the stator. Since the stator field is rotating continuously, the rotor cannot line up with, or lock onto, the stator field and, therefore, must follow behind it [1,2,4].