Selection Criteria for Servo Motor Drives

Superior power density and acceleration characteristics of ac servo drives are being recognized over its dc counterpart. The ac servo family consists of permanent magnet synchronous, dc brushless, induction and switched reluctance motor drives. This ac servo family comprises of radial (known also as the sausage type) and axial (known also 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 in this paper 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, peâk 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 help to appreciate the various facets of ac servos and hence contrast them for comparison.