Squirrel-cage rotor
Squirrel-cage rotor or squirrel cage rotors are rotors of induction motors, called the wound instead of a wire, a coil supplied through slip rings in the laminated core permanently short-circuited cage (solid turns) possess. The squirrel-cage was developed in 1889 by Mikhail Ossipowitsch Doliwo - Dobrowolski, following preparatory work of Galileo Ferraris. Already at the beginning of the 1890s were built and used in induction motors at the General Electric Company ( AEG) squirrel-cage rotor.
Construction
The squirrel cage is much simpler than the rotor of the slip-ring motor. The rotor is composed of a laminated iron core, the metal rods are embedded in non-ferrous metal. The laminated iron core is made of 0.5 mm thick, mutually insulated sheets into the slots are punched to accommodate the rotor bars. The sheets for runners of smaller machines are manufactured in a complete cut, the sheets of larger runners are in the hack section method. The rotor bars are provided with double-sided metallic short circuit rings. For engines with less power up to 100 kilowatts, the " squirrel cage " poured into corresponding recesses of the laminated iron core ( grooves or holes ) in the aluminum die-casting. With great achievements the cage winding in the laminated iron core of the rotor from copper, brass or bronze rods, one which will be soldered in on both sides of external short circuit rings of the same material. The rotor bars are run when short-circuit rotor usually as round bars. One speaks in this design from round bar runners. The bars are not as deep in the rotor core, as with other types of construction. The grooves for the cage rotor run somewhat obliquely in the rule. The rotor bars are constructed as simple geschränkte or twice geschränkte cages. Cages with double geschränkten rotor bars are also called relay runners. Through the cabinets of the rotor bars are achieved more favorable starting conditions, so in this way the Nutenpfeifen, an inhomogeneous torque, magnetic eddying, jarring and stunts are reduced.
In short runners the number of slots in the lamination stack is different than the number of slots of the stator, it can be larger or smaller either. In general, the squirrel-cage rotor has a smaller number of slots as the stand. This design has several reasons. Firstly, the different number of slots is used as a measure to overcome the saddle torque. In addition, as designed rotor can be used for engines with different numbers of pole pairs. If, for the special rotor cage resistance alloys, these runners have an increased slip, they are therefore called rotor resistance or slip runner. A special design of the cage rotor is the rotor current displacement. In order to achieve better efficiency, special rotors have been developed with rotor bars made of copper and used since 2003 in engines. Such as built rotors are called copper rotors. The damper cage ( damper winding ) of a synchronous machine is similar in structure to the squirrel cage.
Operation
Due to the rotating magnetic field of the stator coils, a rotor voltage is induced in the metal cage. Because of the mutually shorted metal rods to flow in the rotor bars corresponding rotor currents that generate their own magnetic field. The currents of the rotor change sinusoidally, they form a polygon in the vector diagram. The coupling of the stator rotating field of the squirrel-cage - field leads to the rotation of the rotor. With increasing speed decrease both the induced rotor voltage, and the inductor current. In addition, reduces the rotor reactance, which has the consequence that the phase shift between the rotor voltage and rotor current becomes smaller.
Performance
Motors with squirrel-cage rotor behave in operation as shorted slip ring motors. Due to the rods they have a large inrush current and a smaller torque. The unfavorable torque is due to the low ohmic resistance of the rotor bars. From about 1 /7 of the synchronous speed, there are often a recess of the characteristic. This saddle is caused by harmonics. Then the motor reaches not have the required starting torque, it may happen that the rotor is held at this speed and not running up to its rated speed. Once the rotor is at rated speed, the speed falls under load only slightly. The motor is a shunt behavior. Since three-phase motors with squirrel-cage rotor for certain applications still have a too large starting torque, is used where a smoother start-up is required, the KUSA - circuit. The starting current for motors with round bar runner is the eight to ten times the rated current. The rotor current can be produced by the design of the rotor during operation do not change. Asynchronous motors with squirrel-cage rotor can, under certain conditions work as asynchronous.