Originally Posted by
probinson
The difference between AC and DC servos is just electronic commutation. Some power is lost in brushes, they wear out, and they take up space in the motor which means that an all electronic motor can be a more powerful than the same size DC motor and be virtually maintenance free.
AC servos are cheaper to build but they are newer technology, higher power density, and maintenance free so they can fetch a higher price. Your best bang for the buck, however, will probably be a DC servo system, especially if you are shopping the surplus market.
If you were building a CNC that had to run 24/7, needed maximum power in a small space, or needed really massive motors then AC servos would be the only way to go. For hobby or small business applications, the bushes will never need replacing and you won't see any difference in performance between the two. You are shopping for power per dollar in your servo system, not power per cubic inch or bragging rights.
I think I went over my 2 cents worth. Oh well, inflation..
Pete