Главная Случайная страница


Категории:

ДомЗдоровьеЗоологияИнформатикаИскусствоИскусствоКомпьютерыКулинарияМаркетингМатематикаМедицинаМенеджментОбразованиеПедагогикаПитомцыПрограммированиеПроизводствоПромышленностьПсихологияРазноеРелигияСоциологияСпортСтатистикаТранспортФизикаФилософияФинансыХимияХоббиЭкологияЭкономикаЭлектроника






Look at Fig. 12 and speak on power generation in Russia.

Retell the text.

LESSON NINETEEN

THE STRUCTURE OF TRANSFORMERS

The transformer is a device for changing the electric current from one voltage to another. As a matter of fact, it is used for increasing or decreasing voltage. A simple trans­former is a kind of induction coil. It is well known that in its usual form it has no moving parts. On the whole, it re­quires very little maintenance, provided it is not misused and is not damaged by lightning.

We may say that the principal parts of a transformer are two windings, that is coils, and an iron core. They call the coil which is supplied with current "the primary winding," or just "primary" for short. The winding from which they take the current is spoken about as the "secondary winding" or "secondary", for short. It is not new to you that the former is connected to the source of supply, the latter being connected to the load.

The primary alternating current produces an alternating magnetic flux in the iron core, and this alternating magnetic flux passes through the turns of the secondary winding. Accord­ing to well-known electro-magnetic laws, this flux produces an alternating e. m. f., or voltage, in the secondary winding. In spite of the fact that there is no electrical connection be­tween the two circuits - the primary and the secondary - the application of a voltage to one is known to produce a voltage at the terminal of the other.

When the number of turns of wire on the secondary is the same as the number on the primary, the secondary voltage is the same as the primary, and we get what is called a "one-to-one" transformer. In case, however, the number of turns on the secondary winding is greater than those on the primary,

the output voltage is larger than the input voltage and the transformer is called a step-up transformer. On the other hand, the secondary turns being fewer in number than the primary, the transformer is known as a step-down transformer. One should add in this connection that to "step up" means to in­crease the voltage. On the contrary, to "step down" means "to reduce".

A transformer operates equally well to increase the volt­age and to reduce it. By the way, the above process needs a negligible quantity of power. It is important to point out that the device-under consideration will not work on d. c, but it is rather often employed in direct-current circuits.

Inefficiency in a transformer is caused mainly by heat losses due not only to current flowing in the coils, but also to unwanted current induced in the core of the transformer.


 

Currents induced in the core are generally called "eddy cur­rents." The flow of eddy currents is stopped in its progress and the efficiency of the transformer is increased by constructing the transformer core of flat sheets of soft iron,

In Fig. 13 it is seen how transformers are used in stepping up the voltages for distribution or transmission over long distances and then in stepping these voltages down, where «4hey are to be used. In this figure, one may see three large step-up transformers which are used to increase the potential to 275,000 volts for transmission over long-distance transmis­sion lines. At the consumer's end of the line, in some distant locality, three step-down transformers are made use of to reduce that value (i. е., 275,000 volts) to 2,300 volts. Local transformers, in their turn, are expected to decrease the 2,300 volts to lower voltages,


 

suitable for use with small mo­tors arid lamps. One could have some other transformers in the system, such as those to be employed for operating door bells or powering radio sets and television sets, for example. They lower the voltage even further. On the whole all radio sets and all television sets are known to use two or more kinds of transformers. These familiar examples of electronic equip­ment could not work without transformers. The facts you have been given above illustrate the wide use of transformers and their great importance.

Another alternating-current system of transmission and distribution is shown in Fig. 14. You are asked to follow the whole process,, that is, to describe it from beginning to end.

 

Exercises

1. Learn the following active words:

Damage повреждение

eddy current вихревой ток

flux течение, поток

induce вызывать, стимулировать

input подводимый ток

maintenance тех. обслуживание

output выход, выработка

primary первичный

secondary вторичный

2. Translate the following sentences:

(a) 1. The new discovery was much spoken about. 2. This house is lived in. 3. This apparatus is often made use of.

(b)1. The students were asked to carry on the experiment. 2. You will be given two new magazines. 3. I was told to translate the instructions.

(c) 1. This substance was supposed to have some important features. 2. This device is assumed to be the best for converting heat into work. 3. The new power plant is known to have been put into operation. 4. This invention was considered to be of great practical importance. 5. A magnetic flux is assumed to consist of magnetic lines of force taken as a whole.

3. Translate the following sentences:

1. Говорят, что этот прибор описан в предыдущей статье. 2. Считали, что ток течет от положительного потен­циала к отрицательному. 3. Говорят, что мой друг хороший математик. 4. Известно, что Ломоносов основал Москов­ский университет. 5. Кажется, что это вещество имеет некоторые другие свойства. 6. Известно, что переменный ток меняет свое направление.

4. Form as many words as possible using suffixes and prefixes. De­fine what parts of speech the new words are and translate them:

engine, apply, differ, electrify, value, opposite, trans­former, magnet, conductance

5. Form nouns from the following verbs using either suffix –ment or -tion:

construct, develop, consider, distribute, deflect, equip, connect, require, produce, state

6. What is the English equivalent for the following:

на основе (чего-либо), теплотехника, по этой причине само собой разумеется, повышать напряжение, повышать ток,понижать ток, оказывать сопротивление

7. Arrange the following words and expressions in pairs of syno­nyms:

(a) amount, among, matter, application, at present, engine, tube

(b) now, pipe, quantity, substance, between, use, machine

8. Arrange the following words and expressions in pairs of antonyms:

(a) left, increase, beginning, d.c., above, step-up, at rest, high, short, more

(b) end, low, long, step-down, in motion, less, decrease, below, a.c., reght

9. Form statements that are not true to the fact. Ask other students
to correct them.

10. Translate the following sentences paying special attention to
the words in bold type:

1. The Fahrenheit scale is mainly used in English-speaking countries but it is not used in the Our country. 2. His scien­tific activity lasted but twenty years but in these twenty years he did very much.3. Motors are widely employed not only in industry but also in our daily life. 4. There is but one measuring scale in the instrument. 5. Everyone took an exam­ination in physics but that student. 6. A simple trans­former is but a kind of an induction coil.

11. Compare:

1. A solenoid and an electromagnet.

2. A direct current and an alternating current.

3. A step-up transformer and a step-down transformer.

4. A stator and a rotor.

LESSON TWENTY

Последнее изменение этой страницы: 2016-07-23

lectmania.ru. Все права принадлежат авторам данных материалов. В случае нарушения авторского права напишите нам сюда...