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POWER TRANSMISSION

A transformer is an electrical device by which the electromotive force of a source of alternating current may be increased or decreased. They are widespread in long­distance power transmission as well as in telephones, radio transmitters and receivers, television etc. Nearly all transformers come under one of the two follow­ing classes: step-up, and step-down transformers. In the transmission of electrical energy over wires for long distance, transformers are practically indispensable. At the power house in the distant mountains, for example, electric current is generated by huge alternating current generators at the relatively low voltage of several thousand volts. If an attempt were made to transmit this electrical energy, at a voltage of say 2,200 volts, over many miles of wire cable to a distant city, the current would be so large that nearly all of the energy would be consumed in heating the power line. The heat generated (it should be remembered) is proportional to the square of the current (heat = 0.24I^2 Rt).

To avoid large heat losses, transformers at the power house step the voltage up to some 220,000 volts before switching the current onto the power line. Since the voltage in the case cited is increased one-hundred fold, the current drops by the same proportion to one-hundredth. Since the square of 1/100 is 1/10.000, the heat loss along the transmission line is only one ten thousandth of what it would have been had the transformer not been used.

At the city end of the power line, a transformer substation steps the voltage down to something like its original value of 2,200 volts. From there branch lines distribute the power to various sections of the city where smaller transformers, one near each group of several houses, step it down again to the relatively safe voltage of 110 to 220 volts.

УПРАЖНЕНИЯ

I. Переведите следующие словосочетания:

long-distance power transmission, radio transmitters, alternating current generators, wire cable, heat losses, power line, transmission line, at the city end

II. Сгруппируйте синонимы:

to step up, part, to step down, to increase, to decrease, not far, too, as, since, as well as, section, different, near, various

III. Образуйте с помощью суффиксов слова однокоренные с данными:

generate, relate, transmit, transform, receive, distant, consume

IV. Выполните задание:

Describe the scheme of the use of transformers in the transmission of electrical energy.

Задание 13

Заменители модальных глаголов

Для выражения долженствования употребляются глаголы to have, to be+инфинитив смыслового глагола. Глагол to have выражает необходимость ввиду опре­деленных обстоятельств, глагол to be — необходимость в связи с предварительной договоренностью, планом.

Например:

Не had to leavehis scientific work.— Ему пришлосьоставить научную деятельность.

Не is to makethe experiments. —Ему нужно (пред­стоит)провести эксперименты.

При этом глаголы to be, to have стоят в соот­ветствующем времени.

Глагол саn имеет эквивалент to be able to. Глагол may имеет эквивалент to be allowed to.

Примечание: см. таблицу № 31, с. 186. УПРАЖНЕНИЕ

I. Переведите на русский язык. Обратите внимание на выражение модальности:

1. Can you tell me the right time? 2. Can you speak English? 3. You can take this picture if you like it. 4. He can hardly have meant that. 5. He was not able to get there in time. 6. I shall be able to come on Saturday. 7. May I ask a question? 8. May I trouble you to explain me this problem? 9. Maybe he will help you. 10. I put on my raincoat because I thought it might rain. 11. Smoking is not allowed here. 12. I am afraid the teacher will not allow me to rewrite the composition. 13. I must get up early every day. 14. He must be ill. 15. He must be somewhere here. 16. To make the air hot enough, the Diesel engine has to use higher pressure than the petrol engine.

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A DIESEL ENGINE

A Diesel engine is a machine which produces power by burning oil in air which has been quickly compressed to a high pressure by a piston moving in a closed cylinder. In Fig. 14a the piston is shown at its lowest position, about to start upwards on its compression stroke. As it moves upwards it compresses the air above it into a smaller and smaller volume. If the final volume of the air is one twenty-fifth of its original volume, the engine is said to have a compression ratio of 25.

In Fig. 14 a the piston is shown at its highest position. Here the air in the cylinder is highly compressed and is also very hot. (Have you not noticed how your bicycle-pump gets hot when you pump up your tyres?) At this stage of the cycle of operation oil is injected into the hot air. The oil ignites instantaneously. The heat of its combustion raises the temperature of the air still higher and so the pressure of the enclosed air is very much increased also.

The piston is therefore forced down again violently. This second stroke of the piston is called the working-stroke, as shown in Fig. I4b.

After reaching the bottom of its working-stroke the piston begins to rise again. Its purpose now is to sweep the products of combustion from the cylinder. So a valve is opened (Fig. I4c) to let the spent gases escape to the exhaust pipe as the piston rises to the top of its third stroke.

The fourth stroke of the piston is downwards again. The exhaust valve is closed and an inlet valve is opened to allow fresh air to refill the cylinder (Fig. 14d). After four strokes, two up and two down, the cylinder is again in the condition we began with. We say that it has completed its four-stroke cycle.

A Diesel engine usually has four cylinders which fire in turn. The four pistons are connected to a rotating crankshaft so that the working-stroke of one piston is always available to drive the three simultaneously non-working-strokes of the other pistons. The remaining energy of the working-stroke is available for the external work the Diesel engine has to do.

УПРАЖНЕНИЯ

I. Переведите на русский язык следующие термины и словосочетания:

a Diesel engine, a compression stroke, a compression ratio, bicycle-pump, working-storoke, exhaust-pipe, exhaust-valve, inlet-valve, the four-stroke cycle, rotating crankshaft, non-working-stroke, a petrol engine, induction coil, piston, cylinder

II.Образуйте производные от слов:

press, inject, work, produce, burn, operate

III.Составьте предложения, употребив следующие группы слов:

to produce power ...; the piston moves up (down) ...; to compress the air; if we say that the engine has a compression ratio of ..., we mean...; oil is injected into ...; the oil ignites ...; during the working-stroke...; the exhaust valve is opened during ...; in order ...; the exhaust valve is closed and the ... is opened ...; the four pistons of the Diesel engine are connected to ...

IV.Проанализируйте в тексте все ing-формы и опреде­лите функции слов с окончанием -ed.

V. Дайте полные ответы на вопросы:

1. How does a Diesel engine produce power? 2. How does the piston in Fig. 14a compress the air? 3. When is the engine said to have a compression ratio of 25? 4. What happens to the air in the cylinder when the piston is at its highest position? 5. How does the oil ignite? 6. When is the piston forced down again? 7. What is the second stroke of the piston called? 8. When does the piston begin to rise again?

9. How does fresh air refill the cylinder? 10. How are the four pistons connected to the rotating crankshaft?

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