PURPOSE: To enable speed change over a wide range, by shortcircuiting a plurality of conductors arranged on a plurality of rotor cores through resistor materials between rotor cores, thereby obtaining rated torque even when the speed is near to zero.

CONSTITUTION: A pulse motor 35 is operated to rotate first and second stators 24, 25 so as to produce phase difference between them thus varying the rotary speed of a rotor 8. A plurality of conductors 5 mounted on a plurality of rotor cores 2, 3 are shortcircuited through resistor material (r) between the rotor cores. Consequently, current flows through the resistor material (r) corresponding to the phase shift thus producing a large torque.