AC induction motors operate on the principle of EM induction. They have no rotating coil; the coils act as electromagnets in the stator and current is not fed into the rotor but instead is induced. They can either have single phase or three phase AC power supply fed into the stator. A single-phase AC power supply only consists of AC fed through one circuit, meaning that the mag field changes in strength and direction over time. Three phase power supply consists of power fed through three circuits which are out of phase with each other by 120 degrees. Each circuit is connected to 1 pair of opposing stator coils, which together produce a rotating magnetic field with a constant magnitude.
Assembly of parallel conductive bars, embedded into two circular end rings, producing an open cylinder structure, known as a squirrel cage. The bars and rings are made from copper or aluminium which are capable of carrying large eddy currents, and are symmetrically arranged. The rotor rotates about the axis of the motor shaft. The squirrel cage structure is encased with laminated soft iron core to: minimise heating due to EC and intensify mag field lines produced by stator coils. The cage sits inside the stator.
Stationary, hollow cylinder which forms outer frame of motor. Contains 3 opposing stator coils wound around iron cores, which are connected to the frame of the motor. The sequential excitation in opposing coils with three phase AC, produces a rotating mag field. Current flows in such a manner that one coil will always be a N pole and another will be a S pole.
For each stator every half revolution, the current is reversed to ensure constant rotation, hence an AC power source is required. As a result of the rotating field, current is induced in rotor (Faradays Law), which opposes the change that caused it (Lenzs Law). Therefore, the rotor attempts to minimise the relative motion between it and the rotating field, by generating torque on the squirrel cage which then rotates in the same direction as the field. The rotor continually chases the mag field, but never catches up. If the rotor rotated at the same speed as the field, then no current would be induced. Hence the rotor would slow down and a current would be induced again, which would causes it to speed up, and this cycle would continue indefinitely, resulting in a varying speed of the motor.