MICROPROCESSOR BASED VSI FED INDUCTION MOTOR DRIVE

Abstract

This work concerns the open-loop and the closed-loop control of a low rating squirrel cage induction motor. The drive has a speed error loop for its variable speed operation. The power converter is a three phase voltage-source inverter (V.5.I.) employing thyristors. At low operating frequencies the inverter is operated in the Pulse Width Modulated mode while at higher operating frequencies the inverter is operated in the quasi square wave mode. The thyristors are forced commutated and the commutation circuit used is a.modified version of the McMurray commutation circuit. Here the pre-charging.of the commutating capacitors is not required. The circuit is ready for commutation at any instant of time which results in a very flexible and programmable commutation control. Higher circuit efficiency as compared to the McMurray commutation circuit is an additional advantage. The power circuit has a built in protection against shoot through. An attempt has been made to further enhance this feature in the software. The three-phase P.W.M. control signals have been generated on line by the micro-computer. The complete control scheme including the generation of PWM, non PWM waveforms for inverter, PI processing of the speed error signal and measurement of feed back signals, has been realized on a micro-computer system using a Personal Computer. Experimental performance of the drive under open-loop and closed-loop conditions has been investigated.