Whenever there is a need of connecting the rotating member of the machine to the stationary external circuit, then slip rings and brush assembly is used.
Consider a three phase rotating star connected winding as shown in the Fig. 1. It is required to connect external three stationary star connected resistances to this windings. The winding must keep on rotating and external resistance must remain stationary and still there should be contact between the two. This is possible by slip rings and brushes.
|Fig. 1 Concept of slip rings and brush|
The three rings made up of conducting material called slip rings are mounted on the same shaft with which winding is rotating. Each terminal of winding is connected to an individual slip ring, permanently. Thus three ends R-Y-B of winding are available at the three rotating slip rings. The three brushes are then used. Each brush is resting on the corresponding slip ring, making contact with the slip ring but the brushes are stationary. So rotating three ends R-Y-B are now available at the brushes which are stationary as shown in the Fig. 1. Now stationary external circuit can be connected to the brushes which are nothing but the three ends of the winding.
Thus the external stationary circuit can be connected to the rotating internal part of the machine with the help of slip rings and brush assembly. Not only the external circuit can be connected but the voltage also can be injected to the rotating winding, by connecting stationary supply to the brushes externally.
Key point : Such slip rings and brush assembly plays an important role in the working of slip ring induction motor.