It is seen that in case of a d.c. generator, basically the nature of the induced e.m.f. in the armature conductors is of alternating type. By using commutator and brush assembly it is converted to d.c. and made available to the external circuit. If commutator is dropped from a d.c. generator and induced e.m.f. is tapped from an armature directly outside, the nature of such e.m.f. will be alternating. Such a machine without commutator, providing an a.c. e.m.f. to the external circuit is called an alternator. The obvious question is how is it possible to collect an e.m.f. from the rotating armature without commutator ?
Note : So the arrangement which is used to collect an induced e.m.f. from the rotating armature and make it available to the stationary circuit is called slip ring and brush assembly.
1.1 Concept of Slip Rings and Brush Assembly
Whenever there is a need of developing a contact between rotating element and the stationary circuit without conversion of an e.m.f. from a.c. to d.c., the slip rings and brush assembly can be used.
In case of three phase alternators, the armature consist of three phase winding and an a.c. e.m.f. gets induced in these windings. After connecting windings in star or delta, the three ends of the windings are brought out. Across these terminals three phase supply is available. But the armature i.e. these terminals are rotating and hence stationary load can not be connected directly to them. Hence slip rings, made up of conducting material are mounted on the shaft. Each terminal of winding is connected to an individual slip ring, permanently. Hence three phase supply is now available across the rotating slip rings. The brushes are resting on the slip rings, just making contact.
Note : The brushes are stationary. Hence as brushes make contact with the slip rings, the three phase supply is now available across the brushes which are stationary.
Hence any stationary load can then by connected across these stationary terminals available from the brushes. The schematic arrangement is shown in the Fig. 1.
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| Fig. 1 Arrangement of slip rings |
Not only the induced e.m.f. can be taken out from the rotating winding check outside but an induced e.m.f. can be injected to the rotating winding from outside with the help of slip ring and brush assembly. The external voltage can be applied across the brushes, which gets applied across the rotating due to the springs.
Now the induced e.m.f. is basically the effect of the relative motion present between an armature and the field. Such a relative motion is achieved by rotating armature with the help of prime mover, in case of a d.c. generator. As armature is connected to commutator in a d.c. generator, armature must be rotating member while field as a stationary. But in case of alternators it is possible to have,
1) The rotating armature and stationary field.
2) The rotating field and stationary armature.
Note : But practically most of the alternators prefer rotating field type construction with stationary armature due to certain advantages.