difference between servo motor and ac squirell cage induction motor

im a lil confused about the difference between servo motor and ac squirell cage induction motor. they both have encoders.and i i saw applications in positioning that both uses servo and ac motor in a same application. whats the difference in the theoretical sense and application wise.

One difference commonly cited:

A servo motor uses permanent magnets to generate the field.
A squirrel cage motor uses a magnetisation current to generate the field.

Technically, a servo motor is any motor that has feedback and closed loop control. If the motor has an encoder and closed loop control, its a servo motor. So technically, attaching an encoder (or resolver or other feed back device) to a general purpose three phase squirrel cage motor and closing the control loop makes it a servo motor. http://en.wikipedia.org/wiki/Servomechanism

Now in practice the technical definition doesn't allways wash. Its a matter of semantics, but usually when people think of a servo motor they are meaning a motor that is made to much more exacting tolerances and is designed to have a lower rotor inertia yet also be able to withstand larger currents than a similar power general purpose motor. As has been mentioned, it can be a squirrel cage induction motor or a DC permanent magenet motor. And technically, one of these precision motors, used without any feedback or without a closed loop control, is not a servo, no matter how special the motor is.

Zero speed with 100% Torque.

Originally posted by jlcann0n:

Zero speed with 100% Torque.

Click to expand...

You can do that today with an AC induction motor and a vector control drive.

As Alaric said, the term 'servomotor' is pretty misleading. But generally speaking servomotors are separated from others by high peak torque capacity and very low rotor inertia. This translates into very high accelerations and high bandwidth.

Keith

If I put a photoeye above my living room ceiling fan to count the fan blades, converted it to motor speed feedback, and tied that back to the fan speed controller, would my fan motor count as a servo motor? Technically, yes.

Its not what we generally think of when talking about servo motors, but its at least as good as some servo setups in a budget conscious college laboratory.

thats not quite accurate sir, as far as i know, servomotor usually has permanent magnet (i have never encounter a servomotor without permanent magnet)and ac motor doesnt have any.dc motor has magnet(if i remember it right) but usually servomotor works in ac provided by an amplifier.thats why usually to check wheter a servomotor is good or not is to short two power leads(this is not powered) and rotate the motor(by hand) if it has resistance(i.e. stiff to rotate) then the servomotor is said to be ok.you cant do that in induction(ac squirell cage) motor. the ac squirell cage motor we talking about here is the one you usually start with softstarters or wye-delta starters or inverters.

What makes things tougher to tell apart is when someone like these people http://www.ssddrives.com/usa/Products/AC890/AC890.php
make a controller that controls both servo motors as well as plain induction motors in V/F control.

interruptedz said:

... servomotor usually has permanent magnet (i have never encounter a servomotor without permanent magnet)and ac motor doesnt have any...

Click to expand...

Conventional use of the word "servo motor" often refers to motors with permanent magnets, but as I said, SERVO technically means feedback and closed loop control (see link in previous post). What most people understand when we talk about it is something else, but there is absolutely nothing at all that mandates that a servo have a permanent magnet - indeed, there are hundreds of models on the market that do not have a permanent magnent. For that matter, a servo motor doesn't even have to be electric, it can be a pneumatic, vacuum, or hydraulic motor.

thanks pal, i also thought about it that way at first because im a "theory" guy, i know the term servo control means something about feedback and master slave thing.but the industry blurred it so much to make me confuse sometimes