Squirrel cage

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Principle drawing of a squirrel cage rotor (example with only three sheet iron lamellas)
Individual lamella of a sheet iron package of a rotor (inside) and stator (outside)
Removed squirrel cage

Squirrel cage or squirrel-cage rotor , the rotors of induction motors known that instead of a wire wound, supplied via slip rings coil a permanently short-circuited during lamination stack cage (massive windings) have. The squirrel cage was developed in 1889 by Mikhail Ossipowitsch Doliwo-Dobrowolski after preliminary work by Galileo Ferraris . At the beginning of the 1890s, the Allgemeine Elektricitäts-Gesellschaft (AEG) built squirrel cage rotors and used them in asynchronous motors.


The squirrel cage rotor has a much simpler structure than the rotor of the slip ring motor . The rotor consists of a sheet iron package in which metal rods made of non-ferrous metals are embedded. The sheet iron package consists of 0.5 millimeter thick, mutually insulated sheets into which grooves are punched to accommodate the rotor bars . The sheets for runners of smaller machines are made using the complete cut process, the sheets for larger runners are made using the chopping process.

The rotor bars are provided with metal short-circuit rings on both sides. For motors with a lower output of up to 100 kilowatts and lower efficiency, the "cage winding" is cast into corresponding recesses in the sheet iron package (slots or holes) using the aluminum die-casting process. In newer, more efficient motors with a lower output (see also efficiency of electric motors ), copper is used instead of aluminum, as this reduces losses due to the lower specific resistance . In the case of high outputs, the cage winding in the sheet iron package of the rotor is built up from copper, brass or bronze rods, which are soldered into external short-circuit rings made of the same material on both sides . In squirrel cage rotors, the rotor bars are usually designed as round bars. This construction is called a round bar runner. The bars are not as deep in the arm as in other designs. The grooves for the squirrel cage are usually a little oblique. The rotor bars are constructed as single or double set cages. Cages with double-set runners are also called relay runners. By setting the rotor bars, more favorable start-up conditions are achieved, so that the grooves whistling, an inhomogeneous torque, magnetic eddies, shaking forces and braking are reduced.

With squirrel cage rotors, the number of slots in the laminated core is different from the number of slots in the stator; it can either be larger or smaller. As a rule, the squirrel cage has a smaller number of slots than the stator. There are different reasons for this construction. On the one hand, the different number of grooves serves as a measure to overcome the saddle torque. In addition, rotors designed in this way can be used for motors with different numbers of pole pairs . If special resistance alloys are used for the rotor cage, these rotors have an increased slip, which is why they are called resistance rotors or slip rotors . The current displacement rotor is a special type of squirrel cage rotor . In order to achieve better efficiency , special rotors with rotor bars made of copper have been developed and have also been used in motors since 2003. Such rotors constructed in this way are called copper rotors. The structure of the damper cage (damper winding) of a synchronous machine is similar to the squirrel cage rotor.

Mode of action

A rotor voltage is induced in the metal cage by the rotating magnetic field of the stator coils . Due to the short-circuited metal rods, corresponding rotor currents flow in the rotor rods, which generate their own magnetic field. The rotor currents change sinusoidally, they form a polygon in the vector diagram . The coupling of the rotating stator field with the squirrel cage field leads to the rotation of the rotor. As the speed increases , both the induced rotor voltage and the rotor current decrease. In addition, the rotor reactance is reduced, with the result that the phase shift between the rotor voltage and the rotor current is smaller.

Operating behavior

Motors with squirrel cage rotors behave like short-circuited slip-ring motors during operation. Due to the round bars, they have a large starting current and a smaller tightening torque . The unfavorable tightening torque is due to the low ohmic resistance of the rotor bars. At about 1/7 of the synchronous speed, there is often an indentation of the characteristic. This saddle is caused by harmonics. If the motor then does not reach the required ramp-up torque, it can happen that the rotor is held at this speed and does not continue to run up to its nominal speed. As soon as the rotor is at the nominal speed, the speed drops only slightly under load. The motor shows a shunt behavior . As three-phase motors with squirrel-cage rotors draw too high a starting current for certain applications or have too high a starting torque, the KUSA circuit is used where a smoother start-up is required . The starting current for motors with round rod rotors is eight to ten times the rated current. Due to the construction of the rotor, the rotor current cannot be changed during operation. Three-phase asynchronous motors with squirrel cage rotors can work as asynchronous generators under certain conditions .


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Web links