This thesis describes a novel pulse-width-modulated, voltage fed, unipolar inverter drive scheme for a squirrel-cage induction motor, which enables effective shoot through protection. Standard PWM techniques were employed. The proposed scheme was tested using standard two pole, totally enclosed, fan cooled, three phase induction motors, rated at 4 kW and 415 Vac. Simple alterations are required to the motor windings, and these were made by the manufacturer.

A review of inverter technology covers the principal categories of inverters, switching devices and detail circuitry. The modes of operation of unipolar schemes are discussed and their advantages and disadvantages identified.

A description of the experimental drive and its operating details follows, reference being made to measured current waveforms. Measured torque-speed and efficiency-speed graphs are presented and discussed.

For a more detailed examination of the performance a theoretical model of the entire inverter-induction motor scheme is developed. The model for the motor is based on a mutually coupled coils approach. The predictions confirm the explanation of the operation given previously. Theoretical comparisons are made between the proposed unipolar drive and equivalent conventional inverter drives.

There is then an extensive study of the inverter under fault conditions, where it is shown that the behaviour of the inverter under fault conditions is predictable and controllable. Suitable methods of fault detection are described briefly.

The power rating of the motor under unipolar inverter feed at 50 Hz is investigated using experimental tests and theoretical calculations. From this a simple rule for derating a motor is suggested, and a table of standard frame sizes and their ratings, under both mains feed and unipolar inverter operation, is drawn up.

Finally, it is concluded that the unipolar inverter, cage induction motor drive is a viable alternative to existing types of drive, and offers high reliability and low maintenance.