To improve reliability of a motor to realize speed control in a wider range by connecting in series a second bothway thyristor and an extra winding, applying an AC power supply to a motor and realizing speed control through phase control by means of the first and second bothway thyristors.

A second bothway thyristor 5b and an extra winding 3 are connected in series and it is then connected to an AC power supply 6 to form a motor 1 of the series connection system. A first trigger circuit 7a and a second trigger circuit 7b are connected to the gates of the first bothway thyrisor 5a and the second bothway thyristor 5b and a speed control means through the phase control is also provided. Thereby, when the motor is rotating at a high speed, the first bothway thyristor 5a is continuously turned on to realize the control only with the phase control by means of only the second bothway thyristor 5b and when the motor is rotating at a low speed, simultaneous control is performed through the phase control by means of both first and second bothway thyristors 5a, 5b. As a result, reliability of motor can be improved and speed control in wider range can be made in a low noise level.