Transformers

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8. Carbon decreases its resistance

a) when the temperature in­creases.

b) when the temperature de­creases.

9. Metals have

c) a positive temperature coefl ficient of resistance.

d) a negative temperature coed ficient of resistance.

8. Complete the sentences using while. Follow the model on page 13.

1. Conductors have a low resistance.............

2. Current passes through insulators with great difficulty................

3. Metals are common conductors.............

4. To make insulators conduct, currents of great value must be applia

5. Carbon decreases its resistance when the temperature increases....

6. Metals have a positive temperature coefficient of resistance....

9. Pair work. Put these questions to your groupmate, and ask him/her t< answer them:

1. What is the difference between conductors and insulators?

2. How does current pass through insulators?

3. What materials are commonly used to produce insulators?

4. What materials are commonly used to produce conductors?

5. In what case do insulators conduct current?

6. How does resistance change when the temperature decreases?

понижать частота благодаря, из-за

['friikwansi] ['dju: ta]

to step down frequency due to

\x. Put down the Russian for:

primary winding secondary winding step-up transformer step-down transformer

iron core ____________

closed core ____________

input voltage ____________

output voltage ____________

U_t<V,

и_гщ

A transformer is used to transfer energy. Due to the transformer electric power may be transferred at a high voltage and reduced at the point where it must be used to any value. Besides, a transformer is used to change the voltage and current value in a circuit.

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

b) Cover the left column and translate the Russian words back inti English.

core winding turn to step up

['wamdin]

сердечник обмотка виток повышать

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Fig. 10 Fig. 11

A two-winding transformer consists of a closed core and two coils (windings). The primary winding is connected to the voltage source. It receives energy. The secondary winding is connected to the load resis­tance and supplies energy to the load.

The value of voltage across the secondary terminal depends on the number of turns in it. In case it is equal to the number of turns in the Primary winding the voltage in the secondary winding is the same as in ^the primary.

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In case the secondary has more turns than the primary the outpii voltage is greater than the input voltage. The voltage in the secondary i; greater than the voltage in the primary by as many times as the numbei of turns in the secondary is greater than the number of turns in the pri тагу. A transformer of this type increases or steps up the voltage and ii called a step-up transformer. In case the secondary has fewer turns thai] the primary the output voltage is lower than the input. Such a trans-former decreases or steps down the voltage, it is called a step-dowi transformer. Compare T\ and T₂ in the diagram. 7\ has an iron core. For this real son it is used for low-frequency currents. T₂ has an air core and is usee for high frequencies. Common troubles in transformers are an open in the winding, j short between the primary and the secondary, and a short betweei turns. In case a transformer has a trouble it stops operating or operates badly. A transformer with a trouble should be substituted. 3. Complete the sentences using the correct variant:

6. A step-up transformer is a) to step down or decrease the secondary used voltage. b) to step up or increase the primary volt­age. 7. A step-down transformer a) to step down the secondary voltage, is used b) to step down the primary voltage.

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

The secondary winding of a transformer is connected to the load re­sistance.... The primary winding receives energy.... A step-down transformer decreases the primary voltage.... An air core transformer is used for high-frequency currents........... In a step-up transformer the number of turns of the secondary winding is greater than the number of turns of the primary winding............

1. A transformer is used

2. Electric power is trans­ferred at a high voltage and reduced to any value

3. A transformer consists of

4. The function of the pri­mary is

a) to store charge.

b) to prevent the change of energy.

c) to transfer energy.

d) to change the voltage and current valuj in a circuit.

a) due to resistors.

b) due to capacitors.

c) due to transformers.

a) cores only.

b) the primary and the secondary winding^

c) a core and the primary and the secori dary windings.

a) to prevent the change of voltage.

b) to supply energy.

c) to receive energy.

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5. The function of the sec­ondary is

8. A transformer with an iron core

9. A transformer with an air core is used

10. In a step-up transformer

11. A transformer should be substituted

a) to receive energy.

b) to supply energy.

c) to transfer energy.

d) to decrease the value of charge.

a) is used for high-frequency currents.

b) is used for low-frequency currents.

a) for high-frequency currents and for low-frequency currents.

b) for high-frequency currents only.

a) the number of turns of the secondary winding is greater than the number of turns of the primary.

b) the number of turns of the primary winding is greater than the number of turns of the secondary.

c) in case it has an open in the winding.

d) in case it has a short between the pri­mary and the secondary.

e) in case it has a short between turns.

5. Pair work. Put these questions to your groupmate and ask him/her * Read the words and write down their Russian equivalents: answer them.

['saikl] cycle [taip] type

What is a transformer used for? What does a transformer consist of? What is the function of the primary winding? What is the function of the secondary winding? What type of transformer is called a step-up transformer? What type of transformer is used for high-frequency currents? What type of transformer is called a step-down transformer? What type of transformer is used for low-frequency currents? What is the relation between the number of turns in the windings an] the value of current? 10. What are common troubles in a transformer? 11. What should be done in case a transformer has a trouble?

[рэ 'seksnd] per second

к Put down the Russian for:

direct voltage source alternating voltage source direction of flow

Lne time ______________

five times ______________

kixty times _____________

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