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A squirrel cage rotor is the rotating part often used in an AC induction motor. An electric motor with a squirrel cage rotor is sometimes called a squirrel cage motor. In overall shape it is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars of aluminium 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 (presumably similar wheels exist for pet squirrels).
The core of the rotor is built of a stack of iron laminations. The drawing shows only three laminations of the stack but many more are used.
The 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 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 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.
The iron core serves to carry the magnetic field across the motor. In structure and material it is designed to minimize losses. The thin laminations, separated by varnish insulation, reduce stray circulating currents that would result in eddy current loss. The material is a low carbon but high silicon iron with several times the resistivity of pure iron, further reducing eddy-current loss. The low carbon content makes it a magnetically soft material with low hysteresis loss.
The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. Rotors for three-phase will have variations in the depth and shape of bars to suit the design classification.
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.
Induction generators
Three phase squirrel cage induction motors can also be used as generators. For this to work the motor must either be connected to a grid supply or have and arrangement of capacitors to the inputs/outputs. If the motor is run as a self exciting induction generator (SEIG) the capacitors can either be connected in a delta or c2c arrangement. The c2c method is for producing a single phase output and the delta method is for a three phase output. For the motor to work as a generator instead of a motor the rotor must be spun just faster than its nameplate speed, this will cause the motor to generate power after building up its residual magnetism.
See also
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- This page was last modified on 5 October 2008, at 15:21.
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