An embodiment of the invention provides an AC motor that is driven by an AC voltage. The AC motor includes a motor coil, a switch circuit, a position detector and a controller. The motor coil receives the AC voltage to drive an axis of the motor. The switch circuit is coupled to the motor coil and controls a current passing through the motor coil. The position detector detects the position of a motor rotor to output a polarity signal. The controller controls the switch circuit according to the polarity signal and the AC voltage to make the current to be a first current with a first direction or a second current with a second direction.