High performance switch mode three phase AC to single phase AC cycloconverters

Abstract

In this thesis, a three phase AC to single phase AC Ćuk topology based switch mode cycloconverter is proposed. Switched mode converters are of various configurations, such as Buck, Boost, Buck-Boost, Ćuk and Sepic etc. The Ćuk topology is chosen in this research work for its unique characteristics. In the proposed circuit the three phase AC supply is rectified by a three phase rectifier. There are two converters used in the circuit-one is P-converter and other is Nconverter. These two converters supply power to the load according to the switching of IGBTs. When p-converter is active the current flows through the load in one direction and activation of n-converter invert the direction of power flow through load. The duration of switching of two converters determines the frequency of the output of the cycloconverter. The topology reduces the number of switches in the circuit. Conventional three-phase AC to single-phase AC SCR cycloconverter requires at least 12 switches, whereas, only four switches are needed in the proposed topology. The reduction of switches has four advantages compared to conventional cycloconverter topologies. It reduces the switching losses and thereby increases the efficiency. The proposed circuits have to deal with less number of isolation schemes. The number of gate/base drives is reduced and the proposed topology has higher reliability as the number of switches is reduced. It also provides mechanism to have high input power factor, low input current THD and use of small input and output filters. The proposed circuit may operate in either buck or boost voltage mode. The performance of the proposed topology is evaluated in terms of THD, input power factor, output voltage, output frequency and efficiency for wide range of the duty cycle of the control signal. The control scheme employed here is the open-loop control. This scheme directly controls the gate pulses of the solid state switches without any current sensing and feedback. The outcome of this work is a new topology of three-phase AC to single-phase AC cycloconverter based on Ćuk topology. The new topology provides adjustable voltage and frequency with improved input current. The input current has low total harmonic distortion (THD). Input power factor and the efficiency are high in the circuit. The circuit can provide non integer multiple of supply cycle to be at the output without significant change in supply input current. The performance of the proposed topology is investigated and compared with configuration of conventional phase control type cycloconverter. The performance of the proposed three-phase to single-phase AC cycloconverter is found to be better than the conventional SCR three-phase to single-phase cycloconverters.