Optically-triggered half-frequency ac converter

Abstract

An optically-triggered converter directly changes the frequency of an incoming voltage waveform to half frequency thereby causing an induction motor, driving a ventilation fan or the like, to operate at half speed. At this lower speed, the induction motor consumes significantly less power. A center-tapped power transformer is used to lower the magnitude of the incoming voltage waveform and a number of optically-triggered thyristor switches are connected between the transformer secondary and the induction motor. The novel trigger circuit includes a synchronous motor connected to receive the input voltage waveforms so that a cylinder attached thereto is rotated in synchronism with the input voltage waveforms. The optically-triggered thyristors are circumferentially positioned about the rotating cylinder and are triggered by a light pulse emerging through an aperture in the cylinder sidewall. The arrangement of the thyristors around the cylinder is selected so that the thyristors are enabled in a predetermined sequence forming fundamental three-phase half-frequency waveforms which are then presented to the induction motor.