Chapter XLII

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 principle 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 referred to 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. 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 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 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.

O. Translate the given sentences, pick out all electrical terms and learn them.

1. In Russia, the use of transformers for supplying current to arc lamps was suggested for the

first time in 1876 by Yablochkov, the inventor of the arc lamp.

2. A good deal of work on transformer design was done by another Russian scientist,

I.F.Usaguin, who suggested the transformer as a source of power for loads other arc

lamps.

3. A transformer is a device which depends for its action on mutual induction and which

serves to convert electric power in an A-C system at one voltage or current into electric

power at some other voltage or current without change in the frequency.

4. An elementary transformer consists of a steel core which carries two windings insulated

from both the core and each other.

5. The winding connected to the supply circuit is called the primary and the winding

connected to the receiver circuit is the secondary.

6. The alternating current traversing the primary sets up an alternating magnetic field which

interlinks the secondary and induces an EMF in it.

7. Since the magnetic field is an alternating field, the induced EMF in the secondary is

likewise alternating at a frequency equals to that of the current in the primary.

8. The magnitudes of the two EMFs depend on the frequency of the magnetizing current, the

number of turns in the windings, and the magnetic flux in the core.

9. The difference between the EMFs and the terminal voltages is so small that the terminal

voltages of the primary and secondary are practically proportional to the number of turns.

10. The difference between the EMF and the terminal voltage of the primary is especially small,

when the secondary is open and the current in it is zero.

11. The current flowing in the primary when the secondary is open-circuited is very small

( 3.5 to 10 per cent of the load current); and is known as the exciting ( or magnetizing,

or no-load) current of a transformer. Also, the secondary terminal voltage is equal to the

EMF induced in it.

12. In Russian terminology the ratio of two voltages is referred to as the transformation ratio, it is

designated by the letter k.

13 .When the transformers reduce the applied voltage, they are called step-up transformers .Their

transformation ratio is greater than unity.

14. When the transformers raise the applied voltage, they are called step-down transformers.

Their ratio is transformation ratio is less than unity.

15. When the primary of a transformer is connected to an A-C source and the secondary is open,

the transformer is said to be at no-load ( there is no load on the secondary).

Р. Translate the following sentences from Russian into English, using new words and

words combinations.

Q. Answer the following questions to the exercise N.

1. What is a transformer?

2. What is a transformer used for?

3. Are there any moving parts in a transformer?

4. What are the principal parts of a transformer?

5. How many windings are there in a transformer?

6. What winding is connected to a load?

7. What is the purpose of a step-down transformer?

8. What is known as a step-up transformer?

Chapter XLII

"Monsieur ne mange rien[1]," said Sister St. Joseph.

"Monsieur's palate is ruined by Manchu cooking," replied the Mother Superior.

The smile left Sister St. Joseph's face and she assumed an expression of some primness. Waddington, a roguish glance in his eyes, took another cake. Kitty did not understand the incident.

"To prove to you how unjust you are, ma mere, I will ruin the excellent dinner that awaits me."

"If Mrs. Fane would like to see over the convent I shall be glad to show her." The Mother Superior turned to Kitty with a deprecating smile. "I am sorry you should see it just now when everything is in disorder. We have so much work and not enough Sisters to do it. Colonel Yu has insisted on our putting our infirmary at the disposal of sick soldiers and we have had to make the refectoire into an infirmary for our orphans."

She stood at the door to allow Kitty to pass and together, followed by Sister St. Joseph and Waddington, they walked along cool white corridors. They went first into a large, bare room where a number of Chinese girls were working at elaborate embroideries. They stood up when the visitors entered and the Mother Superior showed Kitty specimens of the work.

"We go on with it notwithstanding the epidemic because it takes their minds off the danger."

They went to a second room in which younger girls were doing plain sewing, hemming and stitching, and then into a third where there were only tiny children under the charge of a Chinese convert. They were playing noisily and when the Mother Superior came in they crowded round her, mites of two and three, with their black Chinese eyes and their black hair; and they seized her hands and hid themselves in her great skirts. An enchanting smile lit up her grave face, and she fondled them; she spoke little chaffing words which Kitty, ignorant though she was of Chinese, could tell were like caresses. She shuddered a little, for in their uniform dress, sallow-skinned, stunted, with their flat noses, they looked to her hardly human. They were repulsive. But the Mother Superior stood among them like Charity itself. When she wished to leave the room they would not let her go, but clung to her, so that, with smiling expostulations, she had to use a gentle force to free herself. They at all events found nothing terrifying in this great lady.

"You know of course," she said, as they walked along another corridor, "that they are only orphans in the sense that their parents have wished to be rid of them. We give them a few cash for every child that is brought in, otherwise they will not take the trouble, but do away with them." She turned to the Sister. "Have any come to-day?" she asked.

"Four."

"Now, with the cholera, they are more than ever anxious not to be burdened with useless girls."

She showed Kitty the dormitories and then they passed a door on which was painted the word infirmerie. Kitty heard groans and loud cries and sounds as though beings not human were in pain.

"I will not show you the infirmary," said the Mother Superior in her placid tones. "It is not a sight that one would wish to see." A thought struck her. "I wonder if Dr. Fane is there?"

She looked interrogatively at the Sister and she, with her merry smile, opened the door and slipped in. Kitty shrank back as the open door allowed her to hear more horribly the tumult within. Sister St. Joseph came back.

"No, he has been and will not be back again till later."

"What about number six?"

"Pauvre garçon[2], he's dead."

The Mother Superior crossed herself and her lips moved in a short and silent prayer.

They passed by a courtyard and Kitty's eyes fell upon two long shapes that lay side by side on the ground covered with a piece of blue cotton. The Superior turned to Waddington.

"We are so short of beds that we have to put two patients in one and the moment a sick man dies he must be bundled out in order to make room for another." But she gave Kitty a smile. "Now we will show you our chapel. We are very proud of it. One of our friends in France sent us a little while ago a life-size statue of the Blessed Virgin."

Date: 2014-12-28; view: 984