Drive circuit for electrical load

1. A drive circuit for an electrical load, the drive circuit comprising a multi-tap non-phase shifting transformer, and a set of switchable power cells, the multi-tap non-phase shifting transformer having a primary winding and a secondary winding, the secondary winding having multiple output taps each corresponding to a different voltage ratio, the set of switchable power cells connected to an output of each phase of the transformer for each output tap, each power cell being switchable to provide electrical power to an output of the circuit, each power cell comprising a rectifier circuit having an AC input and being connected across serially connected DC link capacitors, an inverter connected across the DC link capacitors and a bi-directional switch connected from the DC link capacitors to between each phase of the AC input.

2. A circuit as claimed in claim 1 , including a bypass circuit connected across the inverter output and being configurable to connect the output to the electrical load.

3. A circuit as claimed in claim 2 in which the power cells are connected in series with each phase of the load through the respective bypass circuits.

4. A circuit as claimed in claim 1 in which each bi-directional switch comprises a pair of transistors connected with common emitters and a bypass diode connected across each transistor.

5. A circuit as claimed in claim 1 in which each bidirectional switch comprises gate turn-off thyristors connected in parallel and to conduct in opposite directions.

6. A circuit as claimed in claim 1 in which the bidirectional switch comprises pairs of diodes in series connected in parallel across a transistor.

7. A circuit as claimed in claim 1 in which the bidirectional switch comprises a pair of source-coupled MOSFET devices.

8. A circuit as claimed in claim 1 in which the inverter comprises an H-bridge circuit.

9. A circuit as claimed in claim 1 in which the rectifier circuit is a diode rectifier circuit.