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Natural circulating current control of a cycloconverter

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Natural circulating current control of a cycloconverter - Industry Applications Society Annual Meeting, 1993., Conference Record of the 1993 IEEE Natural Circulating Current Control of a Cycloconverter S.G. Bosga, J.L. Duarte, L.J. Offringa, A.J. Vandenput Eindhoven University of Technology PO Box 513, 5600 MB Eindhoven, The Netherlands Abstract - In order to perform experimental verifieations of theoretical analyses and simulations related to field oriented controlled ac drives, especially at low frequencies, a 20 kVA power convertor was needed. This converter should act as a voltage amplifier, allowing the load current to change direction without discontinuities. For this purpose a &pulse cycloconverter with circulating current was chosen. A control method is needed to avoid that the circulating current becomes zero. Two methods from the Literature were theoretically compared: a first one, in which the circulating current is controlled to a constant value, and a second one, whieh makes the input reactive power constant. In this paper a new control strategy is introduced, which is based on the shape of the natural (uncontrolled) circulating current. With simulations it is shown that using this method, with a relatively simple control circuitry, the circulating current can be controlled in such a way that it never becomes zero. As a result the load current is free to change direction without becoming discontinuous and without a dead zone, and the output voltage can be controlled independently of the load current. Practical experiments confirmed the theory and the simulations. I. INTRODUC~ION Cycloconverters are frequently used in high power AC drive systems to provide a voltage with variable amplitude and frequency. While studying the characteristics of different field oriented control system, especially at low frequencies, a converter was needed which acts as a voltage amplifier, providing the requested output voltage independent of the

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