Basics of AC Electric Motors

An AC motor is one of two types of electric motors, the name is derived from the alternate current used to drive it, with the other type being a direct current (DC) motor. AC motors further fall into two categories: synchronous and asynchronous or induction. There are two main parts of an AC motor: the primary winding stator, which is stationary and is connected to the power source, and the rotor, which carries the current.

Synchronous Motors

In a synchronous motor, the current passes through a magnetic field in the rotors at the same rate as the alternating current resulting in no slip. The main function of the stator in a motor is to set up a rotating or oscillating field. In a synchronous motor, the rotor's magnetic poles are fixed and the motor's speed is determined by the number of poles in the motor and the frequency of the current supplied to it.
Single-phase synchronous motors are continuous duty motors and are most popular in everyday use appliances such as clocks and tape players. For applications above 10 horsepower, larger synchronous motors are used, having a higher efficiency rate than induction motors.

Induction Synchronous Motors

A synchronous motor requires a starter to achieve starting torque. In an induction synchronous motor, there is no starter. The rotor windings are instead short-circuited allowing the motor to operate as an induction motor. Brushes---electric contacts on the commuter---are required to achieve DC magnetic excitation of the rotor windings which allows for synchronous operation.

Induction Motors

In an induction motor, the stator's oscillating magnetic field causes or induces current in the rotor. These motors rely on the rotor turning slower than the rotation of the stator fields because as the speed of the rotor approaches the speed of the stator field, the torque diminishes. This difference is called slip and is what makes the induction motor operate. There are two types of rotors common in induction motors, the squirrel cage and the wound rotor. The squirrel cage type looks like a hamster wheel and the rotor is practically self contained. These types of rotors are used for motors smaller than one horsepower. The wound rotor consists of insulated coils of wire that are not permanently short-circuited but are connected in regular succession to form a defined polar area having the same number of poles as the stator.

Single Phase Induction Motors

The two most common types of AC motors are single-phase and three-phase.
Single-phase motors can be subdivided into split-phase and commutator. Split-phase motors can be further subdivided into resistance-start, reactor-start, split-capacitor and capacitor-start motors. Commutator motors are subdivided into two groups: series and repulsion and these are subdivided even further and all are modified to produce the necessary starting torque.

Three-Phase Induction Motors

Three-phase induction motors do not need centrifugal switches or capacitors as they generate their own rotating magnetic torque. These types of motors are used for machine tools; fans, air compressors and air conditioning equipment and industrial pumps.