A New Control Technique for AC-AC Converters

Abstract

This work presents the fundamentals of a new control strategy for an AC-AC converter which uses power transistors in the switching mode as the main power elements and which connects directly the three-phase source to the load. This converter works with a fixed switching frequency and the modulation is performed by using a high-frequency dither (carrier) signal superimposed to a low-frequency control (modulating) voltage. The control strategy proposed here is based on the concept of Ficticious Bipolar Source. This concept is introduced in this work and it makes unnecessary the use of a direct current link for controlling this converter.The study is performed by considering that the converter is composed by controlled bi-directional switches. Preliminary results have been obtained via a digital simulation by using the Continuous System Modelling Program (CSMP).The control technique developed in this work allows to change the load frequency between 0 and more than 150 Hz; this output frequency is not limited by the frequency of the three-phase source. The low-frequency component of the load voltage can be varied continuously between 0 % and 100 %. The harmonics in the load and in the source currents can be easily removed by using simple filters. This converter presents no restrictions to the direction of power flow and it is well suited to be used in the four-quadrant speed control of DC and AC machines.