A New Three-Phase Static Variable Frequency Changer

A new three-phase ac to ac static frequency changer is described. It accepts power from a conventional three-phase constant frequency (e.g., 60 Hz) source, and control signals from an auxiliary circuit, also three phase. The converter output may be represented by positive, negative, and zero sequence voltage systems, each system characterized by a particular frequency. By appropriate control, the lowest (fundamental) output frequency becomes fc -fs Hz, where fc is continuously controlled and fs is the constant supply frequency. Fundamental output frequency is associated with positive and zero sequence voltages. The process of conversion involves both free and forced commutation. It may be accomplished by four thyristors per phase, triggered by the square-wave control signals.