The circuit uses a current converter supplying the AC for the motor, the frequency of which controls the rotation rate of the motor. A comparator (1) provides a control signal to a voltage/frequency converter (6), dependent on the difference between the signal from a tachogenerator (16) monitoring the actual motor rotation and a reference signal representing the required speed. A commutation frequency generator (8) responds to the output of the voltage/frequency converter (6) synchronised with the network supply, to provide six commutation signals with relative phase offsets of 60 degrees. These signals are used to control load switches (11), via which each stator winding (15) of the motor (M) is coupled to all three phases of the network.
