New improved modulation method for a cycloconvertor driving an induction motor

The authors describe new modulation techniques that remove all the major limitations of the noncirculating current cycloconvertor, including low maximum frequency, subharmonics, poor bank crossover timing, and distortion due to discontinuous current, when used to drive an induction motor. The new modulation techniques work by keeping the flux in the induction motor (sensed by voltage feedback) as close to sinusoidal as possible and give superior performance to the present current feedback techniques. An experimental 18-thyristor 3-pulse cycloconvertor showed that the new modulation techniques allow smooth speed control of an induction motor from 0 to 25 Hz (limited by software) with a peak output voltage of 95% of the input voltage. It is expected that the same techniques applied to a 36-thyristor 6-pulse cycloconvertor will allow smooth speed control up to 50 Hz. To obtain fast speed response, vector control can be easily added to the new modulation techniques. The new modulation techniques can also be used to advantage in other cyclo-convertor and convertor applications