Introduction of synchronous machine-Alternator

It is known that the electric supply used, nowadays for commercial as well as domestic purposes, is of alternating type.
Similar to d.c. machines, the a.c. machines associated with alternating voltages are also classified as generators and motors.
The machines generating a.c. e.m.f. is called alternators or synchronous generators. While the machines accepting input from a.c. supply to produce mechanical output are called Synchronous motors. Both these machines work at a specific constant speed called synchronous speed and hence in general called synchronous machines.
All the modern power stations consist of large capacity three-phase alternators. In this topic, the construction, working principle and the emf equation of three-phase alternator are discussed.

Difference between D.C. generator and Alternator
It is seen that in case of a dc generator, basically, the nature of the induced emf in the armature conductors is of alternating type.  By using commutator and brush assembly it is converted to dc and made available to the external circuit. if commutator is dropped from a dc generator and induced emf is tapped from an armature directly outside, the nature of such emf will be alternating. Such a machine without a commutator, providing an ac emf to the external circuit is called an alternator. The obvious question is how is it possible to collect and emf from the rotating armature without a commutator.

So The arrangement which is used to collect an induced emf from the rotating armature and make it available to the stationary circuit is called slip ring and brushes assembly.

Concept of Slip Rings and Brush Assembly-

Whenever there is a need for developing contact between the rotating element and the stationary circuit without conversion of an emf from ac to dc, the slip rings and brush assembly can be used.
       In the case of three-phase alternators, the armature consists of three-phase winding and an ac emf 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.

The brushes are stationary. Hence as brushes make contact with the slip rings, the three-phase supply is now available across the stationary brushes.

Hence any stationary load can then be connected across these stationary terminals available from the brushes.

The schematic arrangement is shown in the figure-

Not only the induced emf can be taken out from the rotating winding outside but an induced emf 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 emf 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 dc generator. as the armature is connected to commutator in a dc generator, armature must be a rotating member while fielding 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

I hope that this article will be informative for you all.

Thanks with Regards

Er. Abhishek Srivastava