An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current.
A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as d.c. generators. They also consist of an armature winding and a magnetic field. But there is one important difference between the two. Whereas in d.c. generators, the armature rotates and the field system is stationary, the arrangement in alternators is just the reverse of it.
Q. 1. What are the two types of turbo-alternators ?
Ans. Vertical and horizontal.
Q. 2. How do you compare the two ?
Ans. Vertical type requires less floor space and while step bearing is necessary to carry the weight of
the moving element, there is very little friction in the main bearings. The horizontal type requires
no step bearing, but occupies more space.
Q. 3. What is step bearing ?
Ans. It consists of two cylindrical cast iron plates which bear upon each other and have a central
recess between them. Suitable oil is pumped into this recess under considerable pressure.
Q. 4. What is direct-connected alternator ?
Ans. One in which the alternator and engine are directly connected. In other words, there is no
intermediate gearing such as belt, chain etc. between the driving engine and alternator.
Q. 5. What is the difference between direct-connected and direct-coupled units ?
Ans. In the former, alternator and driving engine are directly and permanently connected. In the latter
case, engine and alternator are each complete in itself and are connected by some device such as
friction clutch, jaw clutch or shaft coupling.
Q. 6. Can a d.c. generator be converted into an alternator ?
Ans. Yes.
Q. 7. How ?
Ans. By providing two collector rings on one end of the armature and connecting these two rings to
two points in the armature winding 180° apart.
Q. 8. Would this arrangement result in a desirable alternator ?
Ans. No.
Q. 9. How is a direct-connected exciter arranged in an alternator ?
Ans. The armature of the exciter is mounted on the shaft of the alternator close to the spider hub. In
some cases, it is mounted at a distance sufficient to permit a pedestal and bearing to be placed
between the exciter and the hub.
Q. 10. Any advantage of a direct-connected exciter ?
Ans. Yes, economy of space.
Q. 11. Any disadvantage ?
Ans. The exciter has to run at the same speed as the alternator which is slower than desirable. Hence,
it must be larger for a given output than the gear-driven type, because it can be run at high speed
and so made proportionately smaller.