AC motors are made up of two assemblies, namely the stator and the rotor.  The stator structure is composed of steel laminations shaped to form poles.  Copper wire coils are wound around these poles.  These primary windings are connected to a voltage source to produce a rotating magnetic field. Standard electric motors for industrial, commercial and residential use are three-phase motors with windings spaced 120 electrical degrees apart.

The rotor is the second assembly composed of laminations over a steel shaft core.  Radial slots around the laminations’ periphery house rotor bars, cast-aluminum or copper conductors shorted at the ends and positioned parallel to the shaft.  The way in which the rotor bars are arranged resembles a squirrel cage; hence, the term squirrel-cage induction motor.  The name “induction motor” comes from the AC “induced” into the rotor via the rotating magnetic flux produced in the stator.

Torque is generated through the interaction of currents flowing in the rotor bars and the stators’ rotating magnetic field.  When fully operational, rotor speed always lags the magnetic field’s speed, thus allowing the rotor bars to cut magnetic lines of force and produce useful torque.  This speed difference is commonly known as slip speed.  Slip also increases with load and is necessary for torque production.