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UBC Theses and Dissertations

Three-phase frequency conversion Berg, Gunnar Johannes


This thesis discusses a principle of continuous speed regulation of AC motors, which is based on frequency conversion, and describes an experimental three-phase converter built for speed test purposes. The converter accepts power from a three-phase 60 cps. source and control signals from an auxiliary three-phase square-wave generator whose fundamental output frequency can be varied between 70 and 570 cps. When the two three-phase inputs are of opposite sequence, fundamental output frequency can be varied between 10 and 510 cps. The converter is of the static switching type. It consists of three identical units, one in each phase. Each unit has four power transistors which are operated in the on-off mode. Control signals are fed to the base-emitter circuits through isolating transformers. A free-running multivibrator in the control unit determines the period of the square-wave signals. The signals are taken from three bistable circuits, triggered in proper sequence through a system of gates. Output voltages from the converter have a relatively high harmonic content. Higher harmonic currents forming zero-sequence systems can be suppressed entirely. Current harmonics of positive and negative sequences must in general be tolerated. A discussion is given on the performance of AC motors when powered by this type of converter. No-load speed tests on small induction motors confirm the, principle on which the experimental work has been based.

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