A new concept for generating three-phase sine wave voltages with semiconductor power switches

A rational method for using switching circuits to produce precision three-phase sine wave power from dc or ac power sources has been developed. A three-phase sine wave format is considered to be composed of voltage segments consisting of stepped waves and square waves generated by separate circuits and combined on a threephase line to produce voltage waveforms with a total harmonic content of 4% prior to filtering. The stepped portions of the wave, called X and Y functions, are generated by a tapped autotransformer and appropriate switches. Only about 20% of the power is generated in the form of steps. A simple circuit that generates a square wave voltage called the "center" function, handles more than 80% of the inverter power. The inverter has the following characteristics: high input power factor (typically 0.9 to 0.95); low output impedance; can energize nonlinear loads without significant change in total harmonic content; and easy to parallel because of low energy storage in the output filter. The inverter has an efficiency of 88-93% and weighs about 120 lb (breadboard). Power rating is 10 k W with 200% overload capacity and output frequency is 60 or 400 Hz.