A synchronous motor having multi-phase armature windings and a field winding is drived by a frequency converter. Actual torque is determined from a sinusoidal position signal representative of rotational position of the synchronous motor and the armature current. The actual torque value is compared with a command torque signal, whereby the armature current is controlled in dependence on difference or deviation resulting from the comparison thereby to cancel the torque ripple. Torque ripple can thus be suppressed to a satisfactory degree even at a low rotation speed of the synchronous motor.