In our discussion of the three-phase asynchronous motor, the rotating magnetic field is composed of three-phase sinusoidal AC stator windings pass into the power of each phase windings have the corresponding magnetic field, due to the size and direction of the current is continuously changing with time, so each phase winding direction of the magnetic field is constant change, through the most simple in star connection poles of the stator winding can be seen as an example, the synthesis of magnetic field produced by the three-phase winding power is a pair of magnetic poles, the two pole motor we said, and with the change of three-phase alternating current for a week, the synthesis of the magnetic field in a clockwise direction for a week and it can be proved that the synthesis of magnetic field of three-phase stator winding is generated by a rotating magnetic field.