Slip speed control in an induction motor drive with a phase locked loop

This paper presents a digital method, using a phase locked loop as frequency multiplier, to control the slip frequency in inverter driven squirrel cage induction motor drives. The digital tachometer used is a disc with a low number of teeth and a magnetic pick-up. The paper also provides a detailed description of the operation and design of the digital control scheme including the phase locked loop, the adder-subtracter and the slip speed generator circuits.