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Slip Ring Rotor or Wound Rotor in three phase induction motor

In this type of construction, rotor winding is exactly similar to the stator. The rotor carries a three phase star or delta connected, distributed winding, wound for same number of poles as that of stator. The rotor construction is laminated and slotted. The slots contain the rotor winding. The three ends of three phase winding, available after connecting the winding in star or delta, are permanently connected to the slip rings. The slip rings are mounted on the same shaft. We have seen that slip slip rings are used to connect external stationary circuit to the internal rotating circuit. So in this type of rotor, the external resistances can be added with the help of brushes and slip ring arrangement, in series with each phase of the rotor winding. This arrangement is shown in the Fig. 1. 

Fig. 1  Slip rings or wound rotor

Key point : This way the value of rotor resistance per phase ca be controlled. This helps us to control some of the important characteristics of the motor like starting torque, speed etc.
       In the running condition, the slip rings are shorted. This is possible by connecting a metal collar which gets pushed and connects all the slip rings together, shorting them. At the same time brushes are also lifted from the slip rings. This avoids wear and tear of the brushes due to friction. The possibility of addition of an external resistance in series with the rotor, with the help of slip sings is the main feature of this type of rotor.
 
Comparison of Squirrel Cage and Wound Rotor

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