Abstract: The present invention is an induction motor control system which prevents an excessive induced voltage across the thyristors providing DC braking control. The amount of DC braking torque is controlled by the time of application of the DC braking voltage to the induction motor through a phase-control capacitor which sets the time of firing the braking thyristors within each half cycle of the AC voltage. In a first embodiment the firing of all the braking thyristors is inhibited upon application of the DC braking control signal until the half cycle of the AC voltage associated with a predetermined braking thyristor. In a second embodiment the firing of a predetermined braking thyristor is inhibited during the half cycle of the AC voltage of the application of the DC braking control signal and the next succeeding half cycle. In both embodiments the braking thyristors are fired during their associated half cycle of the AC voltage thereafter to apply a continuing DC braking voltage to the induction motor.

Abstract: In a three-phase AC voltage control circuit containing reverse parallel connections of a thyristor and a diode or reverse parallel connections of thyristors in at least two phases, each asymmetrical AC voltage obtained by applying a line voltage of the power source having 30 degrees of phase lead from that of the phase voltage of the power source in the phase containing the thyristor to each polarized delay circuit is used as the power source for a firing angle control circuit of the thyristor in the phase.

Abstract: The disclosed elevator speed control system accelerates an induction motor for an elevator car in accordance with a voltage dependent upon a differential signal between a command speed signal and the actual speed signal and passed through a saturation generator. The saturation generator issues a command for applying across the motor its rated voltage upon the closure of a normally open contact set disposed in it. The system includes a sensor circuit is responsive to the difference signal less than a predetermined magnitude to decrease an output from the saturation generator. Alternatively the sensor circuit may responds to a negative load on the motor exceeding a predetermined magnitude to decrease the output from the saturation generator.