GENERATORS

The dynamo invented by Faraday in 1831 is certainly a primitive apparatus compared with the powerful, highly efficient generators and alternators that are in use now. Nevertheless, these machines operate on the same principle as the one invented by the great English scientist. When asked what use his new invention had, Faraday asked in turn: “What is the use of a new –born child?” As a matter of fact, “the new born child” soon became an irreplaceable device we cannot do without.

Although used to operate certain devices requiring small currents for their operation, batteries and cells are unlikely to supply light, heat and power on the large scale. We need electricity to light up millions of lamps, to run trains, to lift things and to drive the machines. Batteries could not supply electricity enough to do all this work.

That dynamo-electric machines are used by means of which mechanical energy is turned directly into electrical energy with a loss of only a few per cent. It is calculated that the produce more than 99.99 per cent of the entire world’s electric power.

There are two types of dynamo, namely, the generators and the alternators. The former supplies d-c which is similar to the current from a battery and the latter, as its name implies provides a-c. To generate electricity both of them must be continuously provided with energy from some outside source of mechanical energy such as steam engines, steam turbines or water turbines.

Both generators and alternators consist of the following principal parts: an armature and an electromagnet. The electromagnet of a d-c generator is usually called a stator for it is in a static condition while the armature the rotor is rotating. Alternators may be divided into two types: 1. alternators that have a stationary armature and a rotating electromagnet; 2.alternators whose armature serves as a rotor but this is seldom done. In order to get a strong e m f,

the rotors in large machines rotate at a speed of thousands of revolutions per minute (r.p.m.).

The faster they rotate, the greater the output voltage the machine produces.

In order to produce electricity under the most economical conditions, the generators must be as large as possible. In addition to it, they should be kept as fully loaded as possible all the time. It is interesting to note here that the biggest generators ever installed at any hydro-electric station in the world are those installed in Russia.

F. Define the following terms.

electromotive force, electric circuit, heating effect of an electric current, magnetic effect of an

electric current, electromagnet, generator, alternator, armature, revolution, scale, amplitude.

H. Translate the following sentences, using the new word stock.

1. The synchronous generator is the only type of a-c generator now in general use.

2. It consists of a unit which supplies a magnetic field (the field structure) and a unit in which an EMF is induced (the armature).

3. Synchronous generators may be constructed with either the armature or the field structure as the revolving unit.

4. Small generators are made with a revolving armature.

5. The required magnetic field is produced by d-c electromagnets placed on the stationary unit (the stator) and the current generated is collected by means of brushes and slip-rings on the revolving unit.

6. All large synchronous generators are made with a revolving field.

7. There must be three slip-rings for a revolving armature while only two are necessary for the low-voltage revolving field.

8. The prime mover of a synchronous generator may be a steam turbine, a water wheel,

an internal- combustion engine ( most often a diesel engine), an electric motor.

9. The armature has slots which receive the armature winding.

10. There are two types of field construction: the salient-pole type and the cylindrical or

non-salient-pole type.

11. The salient-pole type consists of a cylindrical steel forging whose rim carries the pole cores.

12. The slip-rings carry brushes which are connected to a d-c source.

13. The d-c generator is called the exciter.

14. The rated voltage of the exciter can be between 60 and 350 V.

15. Synchronous generators may be cooled either by air or by hydrogen.

16. Hydrogen cooling is a good method of generator ventilation.

I. Read and translate the text, paying attention to the Grammar and new lexical units.

Date: 2014-12-28; view: 1728