Natural circulating current control of a cycloconverter

In order to perform experimental verifications of theoretical analyses and simulations related to field-oriented controlled AC drives, especially at low frequencies, a 20 kVA power converter was needed. This converter should act as a voltage amplifier, allowing the load current to change direction without discontinuities. For this purpose, a six-pulse cycloconverter with natural circulating current was chosen. A control strategy is introduced for this device which is based on the shape of the natural (uncontrolled) circulating current. Simulations show 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