PURPOSE: To suppress the variation of a torque caused by the variation of a magnetic flux density, by simulating the variation of the magnetic flux density for the section of 120 degrees in the air gap part of a brushless motor with a cosine wave signal, and by adding the cosine waveshapes to currents flowing through stator windings according to the rotating position of a rotor.

CONSTITUTION: One period of pulses SL corresponds to the section of 120 degrees in an air gap part, and the pulses SL clear a counter IC1. Pulses SP are the output of a speed sensor 7, and a fixed number of the pulses SP are generated while a rotor rotates from 0 to 120 degrees. Cosine wave signal values every time interval obtained by dividing the one period of SL by the fixed number of SP for the section of 120 degrees are stored in a ROM circuit IC2. Therefore, an output Vm of a D/A converter IC3 becomes the cosine wave whose period is identical with the one of SL. the signal pulse-width-modulated by a cosine wave S2 is sent to an inverter circuit 3. Consequently, the respective current waveshapes in stator windings become irregular so as to compensate the variation of the magnetic flux density in the section of 120 degrees, i.e., according to the irregularity of the magnetic flux density. Thereby, the variation of a torque caused by the variation of the magnetic flux density can be suppressed.