III. Complete the following sentences according to the contents of the text

1. The Induction Motor is …….. of electric motors and is more extensively applied in industry than any other type.

2. The purpose of this winding is …….. for variation in the amount of resistance included in the rotor circuit.

3. The effect of …. is the same as would result from actual revolution of a stator having direct-current poles.

IV. Answer the following questions:

1. What parts does the induction motor consist of?

2. What are the names of its rotating and stationary parts?

3. What does the motor operation depend on?

4. How can the difference between stator and rotor construction be explained?

5. What does the term “revolving field” mean?

V. Translate the sentences from the text paying attention to the Participle Constructions:

1. The induction motor is made in two forms – the squirrel cage and the wound rotor, the difference being in the construction of the rotor.

2. The stator of the induction motor has practically the same slot and winding arrangement as the alternator and has the coils arranged to form a definite number of poles, the number of poles being a determining factor in connection with the speed at which the motor will operate.

3. There being no electrical connection between the rotor circuits of the induction motor and the stator circuits, or supply line, the currents which flow in the rotor bars or windings correspond to the induced voltages, the action being similar to that of a transformer with a movable secondary.

9

TRANSFORMERS

One of the great advantages in the use of the alternating current is the ease with which the voltage may be changed by means of a relatively simple device known as a transformer. Although there are many different types of transformers and a great variety of different applications, the principles of action are the same in each case.

The transformer is a device for changing the electric current from one voltage to another. It is used for increasing or decreasing voltage. So the function of a transformer is to change voltage and current of an alternating system to meet requirements of the equipment used. It is known to be simple in elementary principle, and in construction that is it involves no moving parts.

Transformers change voltage through electromagnetic induction.

The principle parts of a transformer are: an iron core and, usually, two coils of insulated windings. One of them is called primary, another is called the secondary. The primary coil is connected to the source of power. The secondary coil is connected to the load. Thus, the primary is the coil to which power is supplied. The secondary is the coil from which power is taken. In scientific terms to produce an alternating magnetic flux in the iron core an alternating current must be passed through the primary coil. This flux is considered to induce electromotive force in both primary and secondary coils. The secondary coil is open – circuited. Current flows in the secondary coil when the latter is connected to the external circuit or load. The flow of current in the secondary coil tends to reduce the flux in the core. Transformers are placed inside a steel tank usually with oil to improve the insulation and also to cool the device.

II. Guess the meaning of the following international words:

1) transformer; 2) type; 3) principle; 4) electric; 5) function; 6) elementary; 7) construction; 8) induction.

III. Translate into Russian the words and expressions from the text:

1) advantage; 2) voltage; 3) relatively simple; 4) application; 5) increase; 6) to decrease; 7) to meet requirements; 8) moving parts; 9) iron core; 10) insulated windings; 11) load; 12) electromotive force; 13) to induce.

IV. Give the English equivalents to the words below:

1) переменный ток; 2) прибор; 3) принцип работы (действия); 4) электромагнитная индукция; 5) катушка; 6) первичная (вторичная) обмотка; 7) источник питания; 8) магнитный поток; 9) стальной контейнер; 10) остужать.

V. State questions to the underlined words:

1. Voltage may be changed by a transformer.

2. Transformers change voltage through electromagnetic induction.

3. Transformer is used for increasing or decreasing voltage.

4. The primary winding is connected to the source of power.

5. Transformers are placed inside a steel tank.

VI. Answer the questions:

1. What kind of device is a transformer?

2. What are the functions of a transformer?

3. What are the principle parts of a transformer?

4. What is the primary coil connected to?

5. What is the secondary coil connected to?

6. What are the principles of action of a transformer?

7. Where are transformers usually placed?

10

TYPES OF TRANSFORMERS

There are different types of transformers. By the purpose they are classified into step-up transformers and step-down transformers. In a step-up transformer the output voltage is larger than the input voltage, because the number of turns on the secondary winding is greater than that of the primary. In a step down transformer the output voltage is less than input voltage as the number of turns on the secondary is fewer than that on the primary.

By the construction transformers are classified into core-type and shell type transformers. In the core-type transformers the primary and the secondary coils surround the core. In the shell type transformers the iron core surrounds the coils. Electrically they are equivalent. The difference is in the mechanical construction.

By the methods of cooling transformers are classified into air – cooled, oil – cooled and water – cooled transformers.

By the number of phases transformers are divided into single – phase and polyphase transformers.

Instrument transformers are of two types, current and potential.

A current transformer is an instrument transformer used for the transformation of a current at a high voltage into proportionate current at a low voltage.

Current transformers are used in conjunction with a.-c. meters or instruments where the current to be measured must be of low value. They are also used where high – voltage current has to be metered. A voltage transformer, which is also called a potential transformer, may be defined as an instrument transformer for the transformation of voltage from one value to another. This transformer is usually of a step – down type because it is used when a meter is installed for use on a high – voltage system.

Transformers operate equally well to increase the voltage and to reduce it.

The above process needs a negligible quantity of power.

Transformers are widely used in our everyday life. All radio – sets and all television sets are known to use two or more kinds of transformers. These are familiar examples showing that electronic equipment cannot do without transformers.

II. Guess the meaning of the following international words:

1) to classify; 2) method; 3) phase; 4) instrument; 5) system; 6) process; 7) radio; 8) television.

III. Give the English equivalents for the words below:

1) цель; 2) повышающий / понижающий трансформатор; 3) выходящее / входящее напряжение; 4) число витков; 5) механическое устройство; 6) монофазные / полифазные трансформаторы; 7) высокое / низкое напряжение; 8) определять; 9) работать; 10) незначительное количество.

IV. Translate into Russian the words and expression from the text:

1) core-type / shell-type transformers; 2) air-cooled / oil – cooled / water – cooled transformers; 3) current / potential transformers; 4) in conjunction with smth.; 5) to reduce; 6) electronic equipment.

V. Complete the sentences using the text:

1. By the purpose transformers are …

2. By the construction transformers are …

3. By the methods of cooling transformers are …

4. By the number of phases transformers are …

5. Transformers operate equally well…

6. Process of voltage changing needs…

7. Familiar examples of transformer applications are …

VI. Answer the questions:

1. What voltage is larger in a step-up transformer and why?

2. What voltage is less in a step – down transformer and why?

3. What is the construction of a core – type transformer?

4. What is the construction of a shell – type transformer?

5. What are the two types of instrument transformers?

6. What are current transformers used for?

7. What are potential transformers used for?

11

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