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THREE – PHASE INDUCTION MOTOR




The three-phase induction motor is the most commonly used type. It has been widely used in recent years. Normally an induction motor consists of a cylindrical core (the stator) which carries the primary coils in slots on its inner periphery. The primary coils are arranged for a three-phase supply and serve to produce a revolving magnetic field. The stator encircles a cylindrical rotor carrying the secondary winding in slots on its outer periphery.

The rotor winding may be one of two types: squirrel-cage and slip-ring for wound-rotor. In a squirrel-cage machine the rotor winding forms a complete closed circuit in itself. The rotor winding of a slip-ring machine is completed when the slip rings are connected either directly together or through some resistance external to the machine. The rotor shaft is coupled to the shaft of the driven mechanism.

The rotor is stationary at some instant of time. The revolving magnetic field of the stator winding cuts across the stationary rotor winding at synchronous speed and induces an e.m.f. in it. The e.m.f. will give rise to a current which sets up a magnetic field. The rotor starts rotating.

It is the interaction between the rotor current and the revolving magnetic field that has created torque and has caused the rotor to rotate in the same direction as the revolving magnetic field. Tine speed of the rotor is 98–95 per cent of the synchronous speed of the revolving magnetic field of the stator. Hence another name for this type of motor is the asynchronous motor. As a matter of fact, the speed of the rotor cannot be equal to synchronous speed. If it were equal to the latter, the revolving magnetic field would not be able to cut the secondary conductors and there would not be any current induced in the secondary winding and no interaction between the revolving field and the rotor current, and the motor would not run.

II. Translate the sentences, paying attention to the translation of the wordone.

1. One should distinguish between single-phase and three-phase induction motors.

2. The new device is better the old one.

3. The three-phase induction motor type is the most commonly used one.

4. The rotor winding may be one of two types.

5. As a matter of fact the speed of the rotor cannot be equal to synchronous one.

III. Translate the sentences from the text paying attention to the Nominative Absolute Participle Constructions:

1. In the split-phase method an auxiliary stator winding is provided for starting purposes only, this winding being displaced from the main stator winding by 90 electrical degrees.

2. The starting torque of such motions being limited, they are frequently constructed with the rotor arranged to revolve freely on the shaft at starting until nearly normal speed is reached.

3. An auxiliary winding may be connected to the single-phase line through an external inductance and a switch, the introduction of the inductance in the auxiliary winding splitting the phase as before.

IV. Answer the following questions:

1. What way does the single-phase motor differ from the three-phase one?

2. What is the starting of single-phase induction motors accomplished by?

3. How can an auxiliary winding be connected to the single-phase line?

4. What parts does an induction motor consist of?

5. What are the two types of the rotor winding?

V. Work out the plan of the text.

VI. State 5 questions to the text.

UNIT 15

I. Read the text.

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. (General Question).

2. Transformers change voltage through electromagnetic induction. (How …).

3. Transformer is used for increasing or decreasing voltage.

4. The primary winding is connected to the source of power. (…or…).

5. Transformers are placed inside a steel tank. (Question-tag).

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?

VII. Topics for discussion:

1) Transformer as an electric device;

2) Main parts and principles of a transformer action.

UNIT 16

I. Read the text.

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, oilcooled and water-cooled transformers.

By the number of phases transformers are divided into singlephase 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 radiosets 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 smith; 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?

VII. Topics for discussion:

1) Types of transformers;

2) Use of transformers in everyday life.

UNIT 17

I. Read the text.





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