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with the rotor arranged to revolve freely on the shaft at starting until nearly
normal speed is reached, at which time the load is pitched up by the automatic
action of a centrifugal clutch.
Second: an auxiliary winding may be connected to the single-phase line
through an external inductance and a switch (for disconnecting the auxiliary
winding from the circuit after the motor has reached normal speed), the intro-
duction of the inductance in the auxiliary winding splitting the phase as before.
TEXT 2
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 car-
rying 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 com-
plete closed circuit in itself. The rotor winding of a slip-ring machine is com-
pleted 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 synchro-
nous 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 di-
rection 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 word
-one-
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 synchro-
nous one.
III. Translate the sentences from the text paying attention to the Nomina-
tive 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 un-
til 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 transform-
ers 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 func-
tion 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 ele-
mentary 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 secon-
dary 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 in-
duce 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) in-
sulated 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) об-
мотка; 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?
VI. 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 classi-
fied into step-up transformers and step-down transformers. In a step-up trans-
former 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 transfor-
mation of a current at a high voltage into proportionate current at a low volt-
age. Current transformers are used in conjunction with a.-c. meters or instru-
ments 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 tele-
vision sets are known to use two or more kinds of transformers. These are fa-
miliar examples showing that electronic equipment cannot do without trans-
formers.
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) выходя-
щее / входящее напряжение; 4) число витков; 5) механическое устройст-
во; 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?
VI. Topics for discussion:
1. Types of transformers;
2. Use of transformers in everyday life.
UNIT 17
I. Read the text
Measurements of Electric Values
The measurement of any physical quantity applies a determination of its
magnitude in terms of some appropriate unit. In the case of simple fundamen-
tal quantities such as length, mass or time, the units themselves are simple.
Electrical and magnetic quantities are, however, much less simple than
length, mass or time and cannot be measured directly by comparison with a ma-
terial stand. The units in which these quantities are expressed have to be defined
in terms of their observable affects obtained in experimental work, e.g. the
weight of silver deposited in one second by a current when it is passed through a
solution of silver nitrate is a measure of the magnitude of this current.
Electrical measurements can be classified broadly as neither absolute meas-
urements, nor secondary measurements, but the first class of such measure-
ments is rarely undertaken.
II. Guess the meaning of the following international words:
1) physical; 2) system; 3) fundamental; 4) material; 5) experimental;
6) absolute; 7) class.
III. Give the English equivalents to the words below:
1) измерение; 2) определение; 3) соответствующая единица; 4) быть
соответствующим; 5) сравнение; 6) достигать; 7) серебро; 8) широко;
9) заботиться; 10) длина.
IV. Translate into Russian the words and expressions from the text:
1) magnitude; 2) electrical and magnetic quantities; 3) to define; 4) ob-
servable affects; 5) to deposit; 6) secondary measurements; 7) to undertake.
V. Insert the words:
1. Magnitude of any … (физическая величина) must be determined in
terms of some appropriate … (единица).
2. … (единицы) are simple for simple … (основных) quantities.
3. … (электрические) and (магнитные) quantities cannot be measured
simply.
4. These units must be … (определены) in terms of their … (наблю-
даемые) effects obtained in… (экспериментальная работа).
5. Absolute … (измерения) are … (редко) undertaken.
VI. Answer the questions:
1. What do we need to measure any physical quantity?
2. What simple units for measuring of simple fundamental quantities do
you know?
3. Can electrical and magnetic quantities be measured directly by com-
parison with a material stand?
4. How can we get units for defining electrical and magnetic quantities?
5. What types of measurement do you know?
VII. State questions to the underlined words:
1. Before we can measure, we must decide upon a system of units.
2. Electric and magnetic quantities are much less simple than fundamen-
tal quantities.
3. These quantities cannot be measured directly by comparison with a
material stand.
4. Electrical measurements can be classified as neither absolute, nor
secondary measurements. (Question-tag)
VIII. Topics for discussion:
1. Measurement of any physical quantity;
2. Measurement of electric and magnetic quantities.
UNIT 18
I. Read the text
MAIN TYPES OF AMMETERS AND VOLTMETERS
Ammeters and voltmeters are made to operate on the same principle. The
two principle kinds are the moving coil and moving iron types.
The electro-magnetic effect of the current is the one chiefly made use of
for measuring purposes. Moving iron instruments employ this effect. The mov-
ing-iron instrument consists of a fixed coil of wire carrying the current which
magnetizes a small piece of soft iron mounted on the instrument spindle. In con-
struction there are two varieties: the repulsion type having two pieces of iron;
and the attraction type having only one.
In the attraction type of the instrument the bobbin carrying the wire is ob-
long instead of circular, and has only a narrow slot-shaped opening in the cen-
ter. A thin flat piece of iron, which is mounted on the instrument spindle, is
sucked into this opening by magnetic attraction when the current flows. Either
gravity or spring control can be used on moving-iron instruments and damping
is usually by means of an air-dash-pot.
A moving-coil instrument may be compared to a miniature direct-current mo-
tor in which the armature never moves more than about a quarter of a revolution.
When a current flows through the coil of a moving-coil type ammeter, it
becomes a magnet, one face being of north, and the other of south polarity.
These poles are attracted by the poles of opposite polarity of the permanent
magnet, and the coil tends to turn until its axis is parallel with the line joining the
pole pieces of the permanent magnet. This movement is proportional to the cur-
rent flowing and is opposed by the control springs. A pointer fixed to the coils
moves over a graduated scale and indicates the current flowing in amperes. The
scale of this type of instrument is evenly divided, but the positive terminal must
be connected to the positive terminal of the supply or the instrument tends to
read backward. Such an instrument is only suitable for d. c. circuits.
Moving-coil instruments are more accurate and sensitive, but more ex-
pensive than those of moving-iron types.
II. Give the English equivalents for the following words and word-
combinations:
1) электромагнитный тип; 2) магнитно-электрический тип; 3) ось;
4) репульсионный тип; 5) притягивающий тип; 6) продолговатый; 7) ус-
танавливать; 8) втягивать; 9) воздушный успокоитель.
III. Translate into Russian:
1) purpose; 2) employ; 3) slot-shaped; 4) magnetic attraction; 5) damp;
6) revolution; 7) pole; 8) axis; 9) pointer; 10) graduated scale.
IV. Answer the questions:
1. What are the two principle kinds of ammeters and voltmeters?
2. What is the construction of a moving iron instrument?
3. What are the two types of moving iron instrument?
4. How does a moving coil instrument work?
5. What instrument is suitable only for d.c.?
6. What instruments are more expensive and sensitive: moving coil or
moving iron instruments?
V. State questions to the underlined words:
1. Ammeters and voltmeters are made to operate on the same principle.
2. Moving-iron instruments employ this effect. (General Question)
3. Moving iron instrument consists of a coil, small piece of iron and a
spindle. (what … of)
4. The repulsion type instrument has two pieces of iron. (…or…)
5. A pointer moves over a graduated scale. (Question-tag)
VI. Insert the words:
1. In the attraction type of the … (механизмы) the bobbing is … (про-
долговатый) instead of … (круглый).
2. A small piece of … (железо) is mounted on the instrument … (ось).
3. … (Амортизация) is usually by means of an … (воздушный успо-
коитель)
4. The … (якорь) never moves more than about a quarter of a … (пол-
ный оборот) in a miniature d. c. motor.
5. … (катушечный) movement is proportional to … (движение тока)
and is opposed by the … (пружинный механизм).
6. … (Стрелка) indicates the … (ток) flowing in … (ампер).
7. The … (положительный) terminal must be connected to the … (по-
ложительный) terminal of the … (питание) or the … (механизм) tends to
read … (наоборот).
VII. Topics for discussion:
1. Moving iron instruments;
2. Moving coil instruments.