Motor according to current points, can be divided into DC motor and AC motor, where the AC motor can be divided into synchronous motor and asynchronous motor.
Compared with AC motor, DC motor has several advantages such as excellent speed controling performance, heavy starting torque and so on. And most of DC motors are used for the machinery which has higher requirments for speed control performance and starting performance. However, the manufacturing process of DC motor is complex, production cost is high, maintenance is difficult and reliablity is poor.
The difference between DC motor and AC motor is : a converter. The rotor for the DC motor is energized by direct current, under the action of a magnetic field, the direction of the magnetic force that subject to rotation is constantly changing. And the role of a converter is to keep the direction of its rotation constant. The role of the converter in a DC motor is the opposite, and it turns AC electromotive force into DC electromotive force and then output it.
The reason why it's called asynchronous motor is because in the rotating magnetic field formed by the stator, the rotor generates the induced electromotive force and current, thereby generating the electromagnetic force. When it is loaded, the rotor speed decreases, the relative motion between the speed and the synchronous speed increases, and the corresponding output torque increases. The transfer rate - "s" is an important parameter. The asynchronous motors generally have three working states: Braking, S>1, the direction of the speed is opposite to the synchronous speed. Electricing, S<1, the direction of the speed is the same as that of the synchronous speed.
Generating, S<1, the direction of the speed is opposite to the synchronous speed.