Method for controlling the torque of an ac motor

Abstract

The present invention relates to a method for controlling an ac motor supplied by an inverter having several switches. In order that the torque can be adjusted to be as desired as rapidly as possible both with maximum voltage and in the vicinity thereof, one proceeds in accordance with the invention in such a way that a value (.DELTA..gamma.) representing the difference between the reference value and actual value of the angle between the stator flux and rotor flux of the motor is determined on the basis of a value representing the requirement for torque change of the motor, e.g. by means of the difference (.DELTA.T) between the reference valve (T*) and actual valve (T) of the torque, and a quantity representing the magnetic state of the motor, the next switching moment of the switches of the inverter, i.e. the so called limit between flux sectors (.DELTA..phi.), is shifted with a first quantity (k.sub.1 .DELTA..gamma.) proportional to the determined differential value to change the angle (.gamma.) between the stator flux and the rotor flux, i.e. the torque of the motor, in a stepwise manner to be as desired, and that switching moment of the switches of the inverter which is next in turn is shifted with second quantity (k.sub.2 .DELTA..gamma.) proportional to the determined differential value and reverse to the first quantity (k.sub.1 .DELTA..gamma.) to change the rotating speed of the stator flux.