3 phase AC motors as cnc servo motors ?

This might be a stupid question, but I'm going to ask it anyway: why
aren't small (1/4, 1/3,1/2Hp+) 3 phase AC motors suitable for use as
servo motors ?

We run several small and medium sized 3 phase AC motors on machine
equipment with variable frequency drives. Their low speed torque seems
good. Speed control is generally very good.

Why couldn't a quasi step-direction/variable frequency drive device be
made to utilize small 3 phase motors as servo motors ? The device would
accept (step, direction) input from the computer and encoder feedback
from the motor. It would output power to the 3 phases of the motor.

At first I thought that the number of poles in the motor might limit the
"step" resolution, but I suspect that the DC motors used as servo motors
have the same limitation. Or do they use a very large number of poles ?

I was wondering if the slip characteristics of an induction motor might
have something to do with it, or if the back emf at low speeds is low
and rotor stability might be a problem.

Thanks.

--
Kim Lux <lux@...>