First, we will discuss how the AC Induction Motor rotates, which involves the interaction of magnetic fields of the rotor and the stator. For this type of motor, the stator windings are usually connected to a supply in one or three phase form. By applying a voltage across the winding, a radial rotating magnetic field is formed. The rotor has layers of conductive strands along its periphery. The rotating magnetic fields produced by the stator induce a current into the conductive loops of the rotor. Once that occurs, the magnetic field causes forces to act on the current carrying conductors, which results in a torque on the rotor.

An AC Synchronous Motor has a stator very similar to that of the AC induction motor. Windings are placed in slots throughout the periphery. The quantity of windings and slots is determined in part by the number of phases (usually 3 or 1) and the number of poles (usually 2 or 4). The stator produces a rotating magnetic field that is proportional to the frequency supplied.

The main difference between the synchronous motor and the induction motor is that the rotor of this motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor either has permanent magnets or dc excited currents, which are forced to lock into a certain position when confronted with another magnetic field.

Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. There are three main types of DC motors. These are:

Shunt Wound Motor (with its field current independent of the load torque);

Series Wound Motor (whose field current varies directly with the power input);

Compound Wound Motor with both shunt and series windings in varying proportions.

All DC motors other than the relatively small brushless types use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to a squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.

By definition, a servo system is one in which some output variable is measured, fed back, and compared to some desired input function, at which point corrections are made to correct the error between the two. Thus any closed loop system is theoretically a servo system. This however does not call for every motor with a feedback device to be labeled a servo motor.

A servo system needs to rapidly respond to changes in speed and position, which require high acceleration and deceleration. This requires extremely high intermittent torque.

These motors are excellent for applications that require very precise speed control or any type of positioning control. For precise speed control, the motor can be driven with either a trap drive or a sine wave drive. In order to attain positioning control, you will also need a motion controller.