Asynchronous motor is the induction motor, the same thing with a different name just.
Both of them produce a rotating magnetic field that rotates continuously and can be imagined as a large magnet that spins constantly.
Synchronous motor Rotor is the DC excitation or permanent magnet, it can be imagined as a large magnet, by the stator magnet attracted to non-stop rotation. They are attracted to the rotation must ensure the same speed (if the load is too large, the stator magnet can not absorb the rotor, the rotor is slow, a period of time between the magnet becomes repulsive force, this situation of the motor can not work properly, called the step Out).
The rotor of an asynchronous motor is usually an iron bar connected together to form a large conductor. Put it in the rotating magnetic field of the stator, the stator magnetic field constantly "cutting" iron bars, the equivalent of iron bars do not stop the movement of magnetic lines, resulting in induced current, then the iron bar this has electric currents in the rotor magnetic field by the role of electromagnetic force, non-stop rotation. Such a rotor is called a squirrel-cage rotor (like a rat cage that keeps circling around a pet mouse), Such an asynchronous motor is called a squirrel-cage asynchronous motor. Also useful for coils around the rotor, the principle is similar to the squirrel cage rotor. Because this kind of motor works by the current, it is also called AC inductive motor, which is AC induction motors.
Because the asynchronous motor must rely on the rotor and stator magnetic field relative movement cutting magnetic lines, so the speed of the rotor must be slower than the stator speed, not synchronous, so domestic usually called it asynchronous motor. The rotational speed of the stator rotating magnetic field is called synchronous speed, and the ratio of the two speeds is called slip.