Text22. Generators

Principles of Operation

Generation of E.M.F. — The fundamental principle on which all a.c. generators depend is that an e.m.f. is induced when a conduc­tor is made to cut across a magnetic flux. In order to obtain an e.m.f. of suitable magnitude, many conductors in series and a strong magnetic flux are employed, while the rate of cutting is made as high as practicable.

During the first 90° of rotation the rate of cutting flux is gradu­ally increasing, while during the second 90° it is gradually decreas­ing, owing to the alteration in the angle at which the conductors cut the flux. During the second half of the revolution the conductors cut the flux in the reverse direction, and during this period the reverse half of the e.m.f. wave is induced. One complete cycle of e.m.f. is induced in one complete revolution.

If a second turn were rigidly attached to the first, but at right angles to it, an e.m.f. of the same r.m.s. value would be induced in this second turn, but it would be 90° out of phase with the e.m.f. induced in the first turn. The arrangement would constitute a two-phase generator.

In practice three-phase generators are the most common, and a three-phase supply would be obtained by mounting three turns on the game shaft, these three turns being rigidly fixed at 120°to one Another.

Rotating Field. — In d.c. generators the armature winding is mounted on the rotating member which rotates between the poles of n fixed inatfiiut system, but with a.c. generators, or alternators, the standard practice is to place the armature winding upon the stationary clement, now called the stator, while the field system is mounted on the rotating* element, now called the rotor. This meth­od of construction has many advantages from the designer's point of view, and is always adopted except in the case of a few special purpose machines.

The d. с exciting current has to be led into the rotor, this being done by means of two insulated slip rings on which press a number of carbon brushes. This d.c. exciting current is usually provided by a special d.c. generator called an exciter, this being directly driven from the shaft of the main a.c. generator. In certain cases where a d.c. supply is available, this exciter is omitted, but in power station work each a.c. generator is provided with its own d.c. exciter.

Notes

flux - потік; постійний рух

horizontal - горизонтальний

inactive - пасивний» бездіяльний

merely - тільки, лише; просто

angle - кут

rigid - негнучкий

r.m.s. = root means square

to omit - випускати; нехтувати; пропускати

Comprehension questions:

1. What is the fundamental principle on which all a.c. generators depend?

2. What did you learn from the text about the behavior of e.m.f. during the generation?

3. What can you say about d.c. exciting current? Is it important in rotating field or not?