Microprocessor control of a 3-phase to 3-phase 6-pulse noncirculating cycloconverter with alternative firing strategies

The authors describe a controller which can produce the firing signals for a 3-phase to 3-phase 6-pulse noncirculating current cycloconverter using a dual-processor approach. A digital signal processor is used to perform the time critical task of generating the firing signals, using a sample based algorithm, and a conventional processor controls the bridge change-over process. The unit is compact and has been constructed on a single 6U Eurocard. Firing signals have been produced, using the normal control firing strategy usually used with cycloconverters, and also with controlled flywheeling which led to a reduction in output voltage harmonics, giving more continuous load current and hence an improvement in output waveforms. Controlled flywheeling also gives a significant reduction in the VAr required by the converter when operating at low modulation depths