Bibliography: pages 135-136.This thesis covers the design, manufacture and testing of a frequency converter, that transforms three phase AC 380V, into one phase AC 50 to 120Hz, 100 to 650V. The inverter output is intended to power a Soudronic VAA2 O welding machine. The input to the converter was stepped down and rectified to generate an unregulated DC bus of 250V. A full bridge transistorised inverter was controlled by a 6809 microprocessor that generated pulse width modulated waveforms to derive a desired inverter output current and frequency. A base drive was developed to control the power transistor in the inverter. It facilitates the rapid switching of the transistors and provides them with overcurrent protection. The inverter was originally constructed in push-pull configuration. At 20KVA this type of inverter was found to be undesirable, so a full bridge configuration was used in the final design. The converter has been installed and is operating successfully. Many recommendations are made for the improvement of future converters. The changes will improve the operation of the converter and can also reduce the size, cost and weight of it
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