Types of Current

6. Read about current transformers. Answer the questions that follow. Current Transformers

Current is a flow of electricity through a circuit. Let us consider two main types of current: direct and alternating. A direct current (d.c.) flows through a conducting circuit in one direction only. It flows pro­vided a direct voltage source is applied to the circuit.

An alternating current (a.c.) is a current that changes its direction of

flow through a circuit. It flows provided an alternating voltage source is

Current transformers are used for operating ammeters, wattmeters, aJapplied to the circuit. Alternating current flows in cycles. The number

other measuring devices. They produce in the meters a current lower thai of cycles per second is called the frequency of the current. In a 60-cycle

the measured current but proportional to it. Current transformers also insulate the instrument from the circuit whiq is being measured. This is necessary for high voltage circuits. 1. What is a current transformer used for? 2. What type of current does it produce?

fhft mMSlirpH rilTTPnt Kilt nmnnrtinnol «л i4 I.... _,. ,• • sn •

alternating current circuit the current flows m one direction 60 times and in the other direction 60 times per second.

It is easy to transform a.c. power from one voltage to another by a transformer. Transformers are also used to step down the voltage at the receiving point of the line to the low values that are necessary for use.

When necessary a.c. can be changed into d.c. but this is seldom nec­essary.

4. Complete the sentences using the correct variant:

3. In an alternating cur- a) current flows in one direction 60 times per rent circuit second. b) current flows in one direction 60 times and in the other direction 60 times per second. 4. A.c. a) can be changed into d.c. b) cannot be changed into d.c.

1. a) Cover the right column and read the English words. Translatij them into Russian and check your translation.

b) Cover the left column and translate the Russian words back int<| English.

alternating diVect diVection flow necessary to con'sider use

[.oilta'neitin] [flou] ['nesisgn] [ju:s]

переменный

прямой

направление

течение

необходимый

рассматривать

использование

1. D.c. is a current that

2. A.c. flows provided

a) changes its direction of flow.

b) flows in one direction.

a) a direct voltage source is applied.

b) an alternating voltage source is applied.

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3*

3S

to provide to touch to bring that is definite

5. Complete these sentences using while. Follow the model on page 13.

1. An alternating current changes its direction of flow

2. A direct current flows provided a direct voltage source is applied

6. Answer the following questions:

1. What is current?

2. What types of current do you know?

3. When does a direct current flow?

4. What type of current is called an alternating current?

5. What type of current is called a direct current?

6. What is called the frequency of current?

7. What device is used to transform a.c. power from one voltage to another?)

8. Is it often necessary to change a.c. into d.c?

7. Read about frequency, answer the question that follows.

Frequency

The number of cycles per second is the frequency of an alternating cul rent. There are two frequencies: the standard for Europe is 50 cycles pa second while the standard for the USA is 60 cycles per second. A standard frequency has a great advantage since different systems can be interco nected.

What is the advantage of a standard frequency?

1. a) Cover the right column and read the English words, them into Russian and check your translation.

Translal b) Cover the left column and translate the Russian words back in

English.
in'ductance		индуктивность
coil	[koil]	катушка
size	[saiz]	размер
unit	['ju:mt]	единица
fast	[fast]	быстрый
mutual	['mjuttjual]	взаимный
to induce	[m'dju:s]	индуктировать

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[praVaid]	обеспечить
[tAtj]	касаться
	приносить, подносить
	то есть
I'defmit]	определенный

Translate into Russian and put down the Russian equivalents. Then translate them back into English (orally).

a. definite value

primary coil

wire coil

mutual inductance _______________________________

varying current _______________________________

one ampere per second ________________________________

Ь. 1. Coils of wire are called inductors.

2. Two coils are brought close together. ______

3. A source of current is applied to one of the coils. _______

4. Mutual inductance is measured in henries.

3. Which of the words are nouns and which are verbs?

resistor, resist, resistance; induce, induction, inductor, inductance; conductor, conduct, conductance; compute, computer

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