Dc servomotors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servomotors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens motor response to command signals. Servomotors include permanent-magnetic, printed-circuit, and moving-coil (or shell) motors. The rotor of a shell 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 field is provided by cast AlNiCo magnets whose magnetic axis is radial. The motor may have two, four, or six poles.

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

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

The final servosystem design is usually the least sophisticated that meets the performance specifications reliably. 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 performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servosystem and choose a motor type.