AC cooling fan is provided for the use of alternating power: Only a single-phase AC motor windings. When single-phase sinusoidal current through the stator windings, the motor will produce an alternating magnetic field, the strength and direction of the magnetic field at any time intercropping sine law changes, but in the spatial orientation is fixed, so called the alternating magnetic field is pulsating magnetic field. This alternating pulsating magnetic field can be decomposed into two at the same rotational speed, the rotational direction of the rotating magnetic field opposite to each other, when the rotor is stationary, both two equal-sized rotary magnetic field generation in the rotor, the opposite torque direction, such that the synthesis of torque is zero, so the motor can not turn. When we use the force of the motor to rotate in one direction (e.g., clockwise), then cutting magnetic lines clockwise movement of the rotor and the rotating magnetic field rotation direction becomes smaller; between the rotor and the rotating magnetic field direction of counterclockwise rotation cutting magnetic field lines become larger. Such a balance is broken, the total electromagnetic torque generated by the rotor will no longer be zero, the rotor will push along the direction of spin.