Squirrel Cage Rotor

An electric motor with a squirrel cage rotor is sometimes called a squirrel cage motor. A squirrel cage rotor is the rotating part commonly used in an AC induction motor. In overall shape it is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars of aluminum or copper set into grooves and connected together 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 hamster wheel.

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The squirrel-cage rotor field windings in the stator of an induction motor set up a rotating magnetic field around the rotor. The relative motion between this field and the rotation of the rotor induces electrical current flow in the conductive bars. In turn these currents flowing lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent 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.

The conductors are often skewed slightly along the length of the rotor to reduce noise and smooth out torque fluctuations that might result at some speeds due to interactions with the pole pieces of the stator. The number of bars on the squirrel cage determines to what extent the induced currents are fed back to the stator coils and hence the current through them. The constructions that offer the least feedback employ prime numbers of bars.

Use in synchronous motors
Synchronous motors must use other types of rotors although they may employ a squirrel cage winding to allow them to reach near-synchronous speed while starting. Once operating at synchronous speed, the magnetic field is rotating at the same speed as the rotor, so no current will be induced into the squirrel cage windings and they will have no further effect on the operation of the synchronous motor.

Electric Motors