Three basic types of servo motors are used in modern servo systems: ac servo motors, based on induction motor designs; dc servo motors, based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs.

AC servo motors are used in ac servo mechanisms and computers which require rapid and accurate response characteristics. To obtain these characteristics, servo motors have small-diameter high-resistance rotors. The small diameter provides low inertia for fast starts, stops, and reversals, while the high resistance provides a nearly linear speed-torque relationship for accurate control.

In an ideal servo motor, torque at any speed is directly proportional to control-winding voltage. In practice, however, this relationship exists only at zero speed because of the inherent inability of an induction motor to respond to voltage input changes under conditions of light load.

Servomotors are wound with two phases physically at right angles or in space quadrature. A fixed or reference winding is excited from a fixed voltage source, while the control winding is excited by an adjustable or variable control voltage, usually from a servo amplifier. The windings are usually 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 motors have a reasonable efficiency at the sacrifice of speed-torque linearity. Most larger motors have integral auxiliary blowers to maintain temperatures within safe operating ranges. Servomotors are available in power ratings from less than 1 to 750 W, in sizes ranging from 0.5 to 7-in. OD. Most designs are available with modular or built-in gear heads.

DC servo motors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these servo motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens servo motor response to command signals. Servo motors include permanent-magnetic, printed-circuit, and moving-coil (or shell) dc servo motors. The rotor of a shell dc servo motor consists of a cylindrical shell of copper or aluminum wire coils which rotate in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core. The servo motor features a field, which is provided by cast AlNiCo magnets whose magnetic axis is radial. Servo motors usually have two, four, or six poles.

Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.

DC Servo Motor Selection: The first selection approach is to choose a servo motor large enough for a machine that has already been designed; the second is to select the best available servo motor with a specific feature and then build the system around it; and the third is to study servo motor performance and system requirements and mate the two.

The final servo motor system design is usually the least sophisticated that meets the performance specifications reliably. Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances. This says that the servo designer must define the system carefully, establish the servo motor's performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servo system and choose a servo motor type.