Abstract: A control apparatus for an induction motor comprises a variable voltage, variable frequency inverter feeding the motor, which is controlled by the PWM control. A secondary current component and an exciting current component of the motor are obtained by the calculation on the basis of a motor current and an actual slip frequency of the motor. From the thus obtained secondary current component, an actual torque produced by the motor is calculated. A frequency reference for the PWM control is obtained by the comparison result of the actual torque and a torque reference. A modulation factor reference for the PWM control is obtained by the comparison of the exciting current component and an exciting current reference. According to this control apparatus, the torque of the induction motor can be controlled to follow the torque reference with a high accuracy.

Abstract: A power converter such as an inverter or chopper using self-turn-off type semiconductor devices supplies power to an inductive load and free wheel diodes are provided so that the current flowing through this load circulates not through the power supply but through the free wheel diodes. When it is detected that the load current exceeds a predetermined value, the self-turn-off-type semiconductors which are conductive are turned off. At this time, the self-turn-off-type semiconductors which are turned on and immediately thereafter cause the recovery current to flow in the free wheel diodes are turned off not immediately but after the recovery current disappears.