Rotary electric motors involve the interaction of two magnetic fields, that of the stator and that of the rotor. In a brushed-type dc motor, the dc voltage is applied to the stator, creating a stationary magnetic field. DC is also fed to the commutator, which is attached to the armature, part of the rotor. Because the commutator is segmented, the polarity of the current fed to the rotor winding periodically reverses. As a result, the polarity of the magnetic field associated with the rotor also reverses.

It is the continuous alternation of one magnetic field with respect to the other that causes rotation. The fluctuating field can be associated with either the stator or rotor, while the other field is static. The static field can be created by permanent magnets or by dc voltage. You could have batteries mounted on the rotor, but nobody would do that. Rather than being created by a commutator that is part of the rotor, commutation can be external, from an electronic or mechanical controller.

Another possibility is to let the utility do the work. The electrical power supply throughout most of the world is 50- or 60-Hz alternating current. Viewed through an oscilloscope, it is a reasonably pure sine wave, a consequence of the rotary nature of the generating equipment.

If this ac power is wired into the stator coils, a rotating magnetic field is established. The rotor can have permanent magnets or windings energized by rectified dc, fed via a brush and slip ring arrangement.

The speed of the rotating stator field is called the synchronous speed. The frequency of the power supply and the number of poles of the machine determine the synchronous speed. A synchronous motor is one in which the rotor turns at the same speed as the rotating magnetic field in the stator. Step motors, dc brushless, variable reluctance motors, switched reluctance and hysteresis motors, and dc brush motors all typically operate as synchronous motors.

In contrast, an asynchronous motor is one in which the rotor turns at a speed slower than the synchronous speed. Induction motors are asynchronous motors. In an induction motor, a changing magnetic field induces currents in the rotor, so the rotor will always rotate more slowly than the synchronous speed of the magnetic field in the stator. The difference between these two speeds is called “slip” and is usually given as a percentage of the synchronous speed.

There are many varieties of synchronous motors but in all cases, their rotors turn at the synchronous speed of the stator. Perhaps the most widely used synchronous motor is the dc brushless motor. Its stator resembles that of an induction motor but the rotor is made of permanent magnets rather than conductive bars.

In small sizes, ac synchronous motors serve in clocks and timers. Because the armature speed is synchronized to the frequency of the power supplied by the utility, timekeeping accuracy is assured. Utilities watch their output closely, and if the frequency (which depends upon the speed of their rotor) deviates at all, they adjust the speed to get back on track.

Synchronous motors also have an important additional advantage: they can be used for power factor correction. A synchronous motor’s excitation can be varied, enabling it to operate at lagging, leading or unity power factor. At minimum excitation current, the power factor is unity. In an industrial setting, large synchronous motors, besides performing useful work, can serve to modify a harmful power factor. Often non-working synchronous motors are kept online only because this is less expensive than deploying power factor correction capacitors.

Due to the inertia of rest, a large synchronous motor with a heavy armature is not self-starting. Such motors can be equipped with internal induction windings or an external pony motor to get them up to speed.