The difference between synchronous and asynchronous

The difference betiween synchronous and asynchronous is rotor and stator rotating speed with the magnetic field, if the rotor  and stator speed is the same, it is called synchronous motor, and if not, is called asynchronous motor.

when the number of pole is fixed, the motor speed and frequency have fixed too that is synchronous motor. Asynchronous motors, also called induction motor, the rotor speed work is always less than the synchronous motor.

So-called "synchronous" is the winding current flowing in the armature (stator), Form a rotating magnetic field in the air gap, while  the direction of rotation and the rotation speed of the magnetic field is same, the same speed, so for synchronization. Asynchronous motor, the rotating magnetic field and the rotor there is a relative speed, which produce torque