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.
