When the speed needs to be adjusted, the excitation current is increased to improve the magnetic flux potential of the N. S polarity phase-modulated rotor. Because the iron core of the circular induction rotor has longitudinal magnetic slots, the two independent magnetic flux potentials of the stator and the phase-modulated rotor cannot self-circulate. As a result, the torque current and lag angle of the rotor decrease. Therefore, by controlling the excitation current of the phase-modulated rotor, the magnetic flux vector of the circular induction rotor and the input power of the stator can be controlled, so that the electromagnetic torque T of the circular induction rotor can be effectively controlled. The motor stepless speed regulation adopts multi-tap three-phase autotransformer with equal spacing, and each phase of the three-phase autotransformer is connected into a triangle from end to end; Or a three-phase transformer with a plurality of equally spaced taps in the secondary, and the end and the end of the secondary of the three-phase transformer are connected into a triangle. From a multi-tap three-phase autotransformer or three taps corresponding to the secondary of the three-phase transformer to an asynchronous motor, the corresponding taps are sequentially and circularly switched step by step in the way of motor driving or triggering the gate to turn off the thyristor, and the phase shift is gradually accumulated. The speed of asynchronous motor is changed by changing the tap speed.