Usually the rotor of the motor is a permanent magnet, and when the current flows through the stator windings, the stator windings produce a vector magnetic field. The magnetic field will drive the rotor rotation angle, so that a pair of magnetic field direction of the rotor and the stator's magnetic field direction. When the stator's vector magnetic field rotates an angle. The rotor also has an angle with the magnetic field. Each input an electric pulse, the motor rotates an angle forward one step. The angular displacement of the output is proportional to the input pulse number, and the rotational speed is proportional to the pulse frequency. Changes in the order of the winding power, the motor will be reversed.