Forced commutation for variable speed motor

Abstract

A variable speed motor control system includes a power control circuit and a forced commutation control circuit for controlling the torque and rotational speed respectively of an induction motor. The power control circuit and the forced commutation control circuit are independently controlled by signals from a speed control circuit. An overcurrent sensor monitors the motor current and, if excessive current is detected, a speed and power monitor limits the response of the power control circuit and forced commutation control circuit to the speed command. The forced commutation control circuit and power drive circuit contain silicon-controlled rectifiers both for controlling the amount of power fed to the induction motor as well as for controlling the switching of power between combinations of motor windings. A re-pulse generator produces a high-frequency pulse signal with which the gate signals for the silicon-controlled rectifiers are gated to ensure that, if a silicon-controlled rectifier fails to fire when the gate signal is first applied, it will be triggered into conduction with minimum delay. In a current-source inverter embodiment of the invention, a self-regenerating switch momentarily shorts the DC power line feeding silicon-controlled rectifiers controlling the application of power to the motor windings. In preferred embodiments of the invention, two windings of a high-frequency inductor are connected in series with the DC power to the motor-control silicon-controlled rectifiers. The two windings of the high-frequency inductor are tapped for connection to the switch.