Squirrel-cage rotor

A squirrel-cage rotor is the rotating part (rotor) used in the most common form of AC induction motor. It consists of a cylinder of steel with aluminum or copper conductors embedded in its surface. An electric motor with a squirrel-cage rotor is termed a squirrel-cage motor. It consist of a stationary and a rotating parts. The rotating rotor is known as the squirrel cage rotor as it resembles a squirrel cage, whereas the stator is completely wound by copper wires with a proper insulation.

Galileo Ferraris described an induction machine with a two-phase stator winding and a solid copper cylindrical armature in 1885. In 1888, Nikola Tesla received a patent on a two-phase induction motor with a short-circuited copper rotor winding and a two-phase stator winding. Developments of this design became commercially important. In 1889, Mikhail Dolivo-Dobrovolsky developed a wound-rotor induction motor, and shortly afterward the cage-type rotor winding. By the end of the 19th century induction motors were widely applied on the growing alternating-current electrical distributions systems.

The motor rotor shape is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually made of aluminium or copper) set into grooves and connected at both ends by shorting rings forming a cage-like shape. The name is derived from the similarity between this rings-and-bars winding and a squirrel cage.

The solid core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows one of many laminations used. The rotor has a smaller number of slots than the stator and must be a non-integer multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth at the starting instant.

The rotor bars may be made either of copper or aluminium. A very common structure uses die cast aluminium poured into the rotor after the laminations are stacked. Some larger motors have aluminium or copper bars which are welded or brazed to end-rings. Since the voltage developed in the squirrel cage winding is very low, no intentional insulation layer is present between the bars and the rotor steel.

The field windings in the stator of an induction motor set up a rotating magnetic field through the rotor. The relative motion between this field and the rotor induces electric current in the conductive bars. In turn these currents lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent orthogonal to the rotor, resulting in torque to turn the shaft. In effect the rotor is carried around with the magnetic field but at a slightly slower rate of rotation. The difference in speed is called slip and increases with load.

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