Some novel thyristor circuits for controlled DC and 3-phase AC output from 1-phase supply for drive applications

The paper describes the development of new topologies of thyristorised power converters for controlled DC as well as 3-phase AC output using single phase supply with possible application to electric traction. The input side power factor is kept high and the thyristors are load commutated with the help of two AC capacitors. The circuits can work at considerably high frequencies using not too fast thyristors. The turn-on and turn-off losses are kept low due to soft switching. In all, three stages of power conversion are proposed. The first stage is a 1-phase AC to DC boost converter which converts the AC input into fixed DC voltage with a magnitude higher than the peak of the input voltage. This is followed by a second stage which is a fast response DC to DC chopper with reduced switching losses. The output of this stage can be directly fed to a DC motor (for DC traction). The third stage of power conversion consists of a new inverter circuit and is meant for AC motor drives. The inverter is directly connected to the output terminals of the second stage. The proposed inverter circuit does not need any commutation elements and the thyristors used can be of converter grade. Only one snubber circuit is needed. However the inverter needs 12 thyristors in place of the usual 6 thyristors and 8 diodes. The operation of the above circuits is explained briefly and the experimental oscillograms of currents and voltages at different stages as obtained from laboratory scale set-ups (along with necessary controllers) are given