Squirrel cage rotor . In its installed form, it is a cylinder mounted on a shaft . Internally it contains longitudinal aluminum or copper conductive bars with grooves and connected together at both ends by short-circuiting the rings that form the cage .
The name is derived from the similarity between this ring and bar cage and a hamster’s wheel (probably similar wheels exist for domestic squirrels). The rotor base is constructed of a stacked rolling iron .
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- 1 Operation
- 2 Types of rotors
- 1 Single Squirrel Cage Rotor
- 2 Double squirrel cage rotor
- 3 Rotor with deep groove
- 4 Slip Ring Rotor
- 3 Common applications
- 4 Sources
The windings inductors in the stator of an engine of induction urge the magnetic field to rotate around the rotor. The relative movement between this field and the rotation of the rotor induces electric current , a flow in the conductive bars. Alternately these longitudinally flowing currents in the conductors react with the motor’s magnetic field producing a force that acts tangent to the rotor, resulting in a torque to turn the axis. In effect the rotor is carried around the magnetic field but at a slightly slower rate of rotation. The difference in speed is called slip and increases with load.
Often conductors lean slightly along the length of the rotor to reduce noise and to reduce torque fluctuations that might result, at some speeds, and due to interactions with the stator bars. The number of bars in the squirrel cage is determined according to the currents induced in the stator coils and therefore according to the current through them. Constructions that offer fewer regeneration problems use prime numbers of bars.
The iron core serves to carry the magnetic field through the motor. In structure and material it is designed to minimize losses. The thin sheets, separated by the varnish insulation , reduce eddy currents flowing from eddy currents. The material, a low carbon but high silicon steel , with several times the resistance of pure iron, in the additional reducer. The low carbon content makes it a smooth magnetic material with low hysteresis loss .
The same basic design is used for single and three phase motors over a wide range of sizes. Three phase rotors have variations in depth and bar shape to meet design requirements. This motor is very useful in variable speed drives.
There are several types of these elements, but those that are most used in the industry; that is, the rotors for asynchronous AC motors are 4.
Single squirrel cage rotor
The black circles shown in the figure represent the grooves of the rotor where the winding is inserted. There are several types of grooves, hence there are several types of rotors.
This type of rotor is used for small motors, at which start the nominal current exceeds 6 or 8 times the nominal current of the motor. Withstands peak loads poorly. It is being replaced by double squirrel cage rotors on medium power engines. Its starting torque does not exceed 140% of normal.
Double Squirrel Cage Rotor
The slot is double, for this reason it has the name of double squirrel cage. The two slots are physically separated, although it is not observed in the drawing.
This type of rotor has a starting current of 3 or 5 times the nominal current, and its starting torque can be 230% normal. These characteristics make this type of rotor very interesting compared to the single squirrel cage rotor. It is the most widely used today, withstands overloads well without the need to slow down, which gives it better stability.
Rotor with deep groove
It is a variant of the single squirrel cage rotor, but is called a deep groove rotor. Its characteristics are equal to that of the single cage rotor. It is used for low power motors that need continuous starts and stops.
Slip Ring Rotor
Slip ring rotors are called because each end of the winding is connected to a ring located on the axis of the rotor. The winding phases go outside by means of brushes that rub on the rings. By connecting external resistances to the brushes, it is possible to increase the rotor resistance, thus, it is possible to vary the starting torque, which can be, depending on said external resistances, 150% and 250% of the normal torque. The nominal current does not exceed 2 times the nominal motor current.