What is the difference between Synchronous servo motor and Asynchronous servo motor?

If I'm not mistaken, I believe an asynchronous servo motor is an induction motor. Its torque is generated by the difference in rotational velocity of the rotor's magnetic field and the stator's magnetic field. AC voltage is applied to the stator and this induces a current in the rotor. These motors are usually used where high power is required.

A synchronous servo motor uses a permanent magnet. The stator is similar to the induction motor but the rotor has permanent magnets mounted on it. The magenetic fields of the rotor and stator move independently but synchronously at the same rotational velocity. A torque is generated when a load is applied as the positional alignment of the rotor and stator is changed. The greater the positional misalignment, the greater the torque.

I suggest looking up :

"electric motors" on HowStuffWorks.com.

Or for more information on both types of servo motors follow this link.

The fundamental difference between the two is that if you command a synchronous motor to move 1/16th of a turn, it moves exactly 1/16th of a turn. With an AC induction motor, you only get a rough idea of what the rotor is doing. The amount of movement you get depends on lots of factors, like load. So synchronous means that what the rotor does is "synchronized" with what it is commanded to do. Asynchronous means that the rotor does approximately what it is commanded to do.

Synchronous motors are thus considerably more complicated to build and control. That's why you see a correspondingly higher price. But they can do nifty things, like precision positioning and high torque at zero speed.

Copied from an original post by James Ingraham