Control apparatus for induction motor

Claim 1

A control apparatus for controlling a polyphase induction motor, comprising in combination, a source of polyphase alternating current, one main trigger controllable electric valve connected to each of a plurality of phases of said source between said source and said polyphase induction motor, a single-phase transformer including a primary winding connected across said source and a secondary winding having a center tap connected to one of the phases of said induction motor, a pair of controlling trigger controllable electric valves connected to both ends of said secondary transformer winding with the same polarity, respectively, and having respective output terminals connected together to another phase of said induction motor, motor ignition circuits connected to said main electric valves and to said controlling electric valves to control firing angles of the associated electric valves respectively, a tachometer generator operatively coupled for sensing the speed of said motor, pattern generator means operatively connected for generating a pattern of speed instruction for said motor, distribution amplifier means coupled to said tachometer generator and said pattern generator means for detecting a difference between an output from said tachometer generator and an output from said pattern generator means and producing an output dependent upon the detected difference and operatively coupled to said ignition circuits for applying its output to said ignition circuits for said main electric valves when said pattern generator means is higher in output than said tachometer generator and applying said output to said ignition circuits for said controlling electric valves when said pattern generator means is less in output than said tachometer generator, whereby said induction motor is controlled in accordance with the output from said pattern generator means and by continuously changing a voltage applied to said motor.