Abstract: A thyristor cycloconverter for frequency changing a 3-phase input supply to, commonly, a single-phase or 3-phase output supply. The output frequency can be near to, and even higher than, the input frequency without intolerable harmonics. A local oscillator generates a reference wave of output frequency, single-phase or 3-phase. Each reference wave is regularly sampled, twice each input supply cycle, and the sampled value stored to provide a step-form modified reference wave. Thyristor firing pulse trains are generated, during the period of the appropriate input supply commutation intervals, following the instants of amplitude equality of each step-form modified reference wave and each supply phase. A comparator provides a pulse output during alternate successive wave crossings. Output supply positive half-cycle thyristor firing pulse trains commence from the pulse trailing edges and negative half-cycle thyristor firing pulse trains commence from the pulse leading edges.

Abstract: A method of energizing a single-, three-, or poly-phase A.C. motor for continuously-variable speed control from a fixed-frequency supply of corresponding phase. Each phase is modulated by a continuously-variable modulation wave of sub-supply frequency to provide two continuously-variable sideband frequencies, either of which can be used to supply the motor. The modulation wave is sampled in each cycle of the A.C. supply and a full-wave rectified resultant modulation wave compared with a cosine wave of supply frequency. Each phase is gated by solid-state switch means for current flow from amplitude equality to next-following cosine zero. Optionally, secondary modulation, by a square wave of supply frequency, is used.