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The power density and acceleration characteristics of the ac servo drive are being recognized as being superior to those of the dc counterpart. The ac servo family consists of permanent-magnet synchronous, dc brushless, induction, and switched reluctance motor drives. This ac servo family is comprised of radial (also known as the sausage type) and axial (also known as the pancake type) field motors, thus enlarging the spectrum of ac servo motors to the users. Realizing that the ac servos are of recent development, it is obvious that their potentials and pitfalls have not been published, thus making it difficult for the user and application engineer to choose one servo from the family of ac servos for a particular application. In this regard, an attempt is made here to develop a set of selection criteria to differentiate the various servo drives of the ac family. The factors proposed to do that are cost, power density, acceleration rates, peak torque capability, speed range, torque per unit ampere, thermal capability, parameter sensitivity, cogging torque, ripple torque, braking, rotor position feedback device, etc. Application of these criteria would promote appreciation for the various facets of ac servos through their comparison with other drive types.