Variable Frequency Three-Phase Square Wave Generator with Fixed Phase Sequence

The design of a three-phase square wave generator with continuously variable output frequency in the 40-400 Hz range is described. The basic generator circuit consists of an astable multivibrator, and a divide-by-three logic network followed by a divide-by-two logic network with a synchronizing gate. Frequency control is arranged in the multivibrator. The synchronizing gate assures fixed output signal phase sequence. Noninverted and inverted output is available. In order to provide adequate output power, push-pull load driving stages preceded by emitter followers are incorporated. Transformer coupled output is used in the prototype to drive SCRs in a high-power three-phase static frequency changer. Custom wound toroidal core transformers have been used. The rise time of the square wave at the generator output is below 3μs.