PURPOSE: To perform speed control over a wide range by obtaining a two-phase AC current having 90° phase shift from a trapezoidal wave having variable frequency and exciting two primary windings of a motor individually.

CONSTITUTION: A square wave A is converted through a two-phase timing circuit 16 into square waves B1, C1 having 90° phase difference and inverse waves B2, C2 thereof, and then they are provided to a two-phase exciting circuit 13. A triangular wave S is converted through trapezoidal wave circuits 17, 18 into trapezoidal waves D1, D2 having 90° phase difference. The two-phase exciting circuit 13 feeds two-phase AC currents having 90° phase difference to two field windings L1, L2 of a motor 11, with such phase and frequency as corresponding to those of the trapezoidal waves D1, D2, thus exciting the field windings L1, L2 individually. Consequently, the motor 11 is driven by the rotary field produced through the two-phase AC currents and when the frequency of the two-phase AC currents is varied through variation of DC input V1, rotary speed of the motor 11 can be controlled over a wide range.