AC servo motors
These servo motors are basically two-phase, reversible, induction motors modified for servo operation. Ac servo motors are used in applications requiring rapid and accurate response characteristics. To achieve these characteristics, these ac servo motors have small diameter, high resistance rotors. The ac servo motor's small diameter provides low inertia for fast starts, stops, and reversals. High resistance provides nearly linear speed-torque characteristics for accurate servo motor control.
An induction motor designed for servo use is wound with two phases physically at right angles or in space quadrature. A fixed or reference winding is excited by a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servoamplifier. The servo motor windings are often designed with the same voltage/turns ratio, so that power inputs at maximum fixed phase excitation, and at maximum control phase signal, are in balance.
The inherent damping of servo motors decreases as ratings increase, and the servo motors are designed to have a reasonable efficiency at the sacrifice of speed-torque linearity.
Induction type servo motors are available in fractional and integral horsepower sizes.