A Flexible Hybrid-Firing Controller for Cycloconverter Drives

A firing controller for cycloconverter drives has been developed using a unique combination of digital and analog methods. The cosine wave crossing method of control is used, incorporating digital sinewave generation techniques to produce accurate timing waves that are easily synchronized with a noisy ac line. Conversion to analog signals using multiplying D/A converters provides fast digital control of the output amplitude. Frequency and phase are also precisely controlled with 8-bit digital inputs. A unique circuit detects the conduction state of each SCR and is used to prevent line-to-line shorts, thus simplifying the requirements for an accurate bank selection signal and eliminating the need for large interphase reactors.