Polygon-connected autotransformer-based 24-pulse ac-dc converter for vector-controlled induction-motor drives

This paper presents a novel autotransformer-based 24-pulse ac-dc converter feeding vector-controlled induction motor drives (VCIMDs) for improving power quality at the point of common coupling. The dc-ripple-reinjection technique is used in achieving the pulse multiplication in a 12-pulse ac-dc converter. The design of the proposed autotransformer is given, and necessary modifications are made in it to provide the same output de voltage as a 6-pulse diode-bridge ac-dc converter to make it suitable for retrofit applications. The proposed 24-pulse ac-dc converter is found capable of suppressing up to the 21st harmonics in the supply current along with the power-factor improvement close to unity in the wide operating range of the drive. A set of power-quality indexes at input ac mains and on dc bus for a VCIMD fed from various ac-dc converters are also given to compare their performance. A prototype of the proposed autotransformer-based 24-pulse ac-dc converter is developed, and test results are presented to validate the developed design procedure and the simulation models of this ac-dc converter under varying loads.