AC motors and the squirrel-cage induction motor

An AC motor is an electric machine driven by alternating current. The most important type in industry is the induction, or asynchronous, motor, and within that family the squirrel-cage induction motor is the workhorse. Its name comes from the rotor, which is built from conducting bars short-circuited at each end by rings, forming a cage. There are no electrical connections to the rotor at all, which makes the machine simple, rugged and inexpensive.

The stator carries windings, usually three-phase, that produce a magnetic field rotating at a speed set by the supply frequency and the number of poles. This rotating field induces currents in the cage bars, and those induced currents create their own field that the stator field drags around, producing torque. Crucially, the rotor must turn slightly slower than the rotating field; this difference is called slip. Without slip there would be no induced current and no torque, so an induction motor never quite reaches synchronous speed under load.

The relationship between torque, slip and speed gives the motor its characteristic curve: torque rises from standstill to a peak and then falls as the rotor approaches synchronous speed. Synchronous motors, by contrast, run exactly at the field speed because their rotors are magnetised independently and lock to the rotating field. Induction motors dominate fans, pumps and conveyors, while synchronous machines are chosen where constant speed or power-factor control is needed.

Frequently asked questions

What is slip in an induction motor?

Slip is the difference between the speed of the stator's rotating magnetic field and the actual rotor speed. Some slip is essential, because it is what induces rotor current and produces torque.

Why is it called a squirrel-cage motor?

The rotor consists of conducting bars joined by end rings, forming a shape resembling a cage. This cage carries the induced currents that make the motor turn.

How do synchronous and induction motors differ?

A synchronous motor runs exactly at the speed of the rotating field, while an induction motor always runs slightly slower so that current can be induced in its rotor.