VIII. Say some sentences about the types of electric current and its properties.
I. Read the text.
The Nature of Electricity
Practical electricity is produced by small atomic particles known as electrons. It is the movement of these particles which produces the effects of heat and light.
The pressure that forces these atomic particles to move, the effects they encounter opposition and how these forces are controlled are some of the principles of electricity.
Accepted atomic theory states that all matter is electrical in structure. Any object is largely composed of a combination of positive and negative particles of electricity. Electric current will pass through a wire, a body, or along a stream of water. It can be established in some substances more readily than in others, that all matter is composed of electric particles despite some basic differences in materials. The science of electricity then must begin with a study of the structure of matter.
Matter is defined as any substance which has mass (or weight) and occupies space. This definition should be broad enough to cover all physical objects in the universe. Wood, water, iron, and paper are some examples of matter. Energy is closely related to, but not to be confused with, matter. Energy does not have mass, and it does not occupy space. Heat and light are examples of energy.
The smallest particle of matter which can be recognized as an original substance was thought to be a unit called the atom. Recently scientists have found particles even smaller than atoms, but our theories are still based on the atom. The atom consists of a nucleus and a cloud of electrons. It is generally agreed that the electrons are small particles of electricity, which are negative in nature. These particles orbit the nucleus in much the same fashion that planets orbit a sun.
II. Guess the meaning of the following international words:
Electricity, electron, effect, structure, combination, material, mass, energy, atom, orbit.
III. Give the English equivalents for the words below:
1) производить; 2) частица; 3) тепло и свет; 4) напряжение; 5) сила; 6) вещество; 7) положительный; 8) отрицательный; 9) электрический ток; 10) вес; 11) ядро.
IV. Translate into Russian the words and expressions from the text:
1) atomic particle; 2) effects of heat and light; 3) encounter opposition; 4) principles of electricity; 5) composed (of); 6) pass through a wire; 7) structure of matter; 8) occupy space; 9) physical objects; 10) a cloud of electrons; 11) in the same fashion.
V. Complete the sentences using the text:
1. Electricity is produced by …
2. The effects of heat and light are produced by …
3. According to the accepted atomic theory all matter is …
4. Any object is composed of …
5. Matter is defined as …
6. Energy must not be confused with …
7. The atom consists of …
8. The smallest particle of matter is …
9. Most theories are based on …
10. Electrons are …
VI. Answer the questions:
1) What are the principles of electricity? 2) What must the science of electricity begin with? 3) Are there any differences between energy and matter? What are they? 4) What is recognized as an original substance now?
VII. Topics for discussion:
1) The nature of electricity;
2) The nature of matter;
3) Contents of atomic theory.
UNIT 2
I. Read the text.
Electric current
The electric current is a quantity of electrons flowing in a circuit per second of time. The unit of measure for current is ampere. If one coulomb passes a point in a circuit per second then the current strength is 1 ampere. The symbol for current isI.
The current which flows along wires consists of moving electrons. The electrons move along the circuit because the electromotive force drives them. The current is directly proportional to the e. m. f.
In addition to traveling through solids, however, the electric current can flow through liquids as well and even through gases. In both cases it produces some most important effects to meet industrial requirements.
Some liquids, such as melted metals for example, conduct current without any change to themselves. Others, called electrolytes, are found to change greatly when the current passes through them.
When the electrons flow in one direction only, the current is known to be d.c., that is, direct current. The simplest source of power for the direct current is a battery, for a battery pushes the electrons in the same direction all the time (i.e., from the negatively charged terminal to the positively charged terminal).
The letters a.c. stand for alternating current. The current under consideration flows first in one direction and then in the opposite one. The a.c. used for power and lighting purposes is assumed to go through 50 cycles in one second. One of the great advantages of a.c. is the ease with which power at low voltage can be changed into an almost similar amount of power at high voltage and vice versa. Hence, on the one hand alternating voltage is increased when it is necessary for long-distance transmission and, on the other hand, one can decrease it to meet industrial requirements as well as to operate various devices at home.
Although there are numerous cases when d.c. is required, at least 90 per cent of electrical energy to be generated at present is a.c. In fact, it finds wide application for lighting, heating, industrial, and some other purposes.
II. Guess the meaning of the following international words:
electric, ampere, symbol, proportional, industrial, metal, electrolyte, battery, generate.
III. Give the English equivalents for the words and word combinations below:
1) течь, протекать; 2) цепь, схема; 3) единица измерения; 4) провод; 5) электродвижущая сила; 6) твердое тело; 7) жидкость; 8) проводить (ток); 9) источник энергии; 10) постоянный ток; 11) переменный ток; 12) напряжение.
IV. Give Russian equivalents for the following:
1) to meet industrial requirements; 2) melted metals; 3) to push in the same direction; 4) negatively (positively) charged terminal; 5) power and lightning purposes; 6) long-distance transmission; 7) to operate devices; 8) to find wide application.
V. Say whether these sentences are true or false:
1. The symbol for current isI.
2. The electric current can flow only through liquids.
3. The current can be of two types: direct current and alternating current.
4. The alternating current flows in one direction.
5. A battery is the simplest source of power for the direct current.
6. Direct current finds wider application than alternating current.
7. Electrolytes don’t change greatly when current passes through them.
8. One of the great advantages of alternating current is the ease with which voltage can be changed.
VI. Fill in the blanks, using the words from the box:
|
A quantity of moving electrons flowing in a circuit is the a) _______. The current can flow through b) ________ and c) ________. Some liquids d) _______ current without any change to themselves. When the electrons flow in one direction only, the current is known to be e) _______. The current flowing first in one direction and then in the opposite one is f) _______. Such advantage of alternating current as alternating g) _______ finds wide industrial and household application.
VII. State the questions to the underlined words:
1. Melted metals conduct current without any change to themselves.
2. Alternating voltage can be changed to operate various devices at home.
3. A battery pushes the electrons in the same direction.
4. The alternating current is used for power and lightning purposes.
5. Alternating current accounts for 90 per cent of electrical energy generated now.
Unit 3
I. Read the text.
UNIT 4
I. Read the text.
Electric Circuits
The concepts of electric charge and potential are very important in the study of electric currents. When an extended conductor has different potentials at its ends, the free electrons of the conductor itself are caused to drift from one end to the other. The potential difference must be maintained by some electric source such as electrostatic generator or a battery or a direct current generator. The wire and the electric source together form an electric circuit, the electrons are drifting around it as long as the conducting path is maintained.
There are various kinds of electric circuits such as: open circuits, closed circuits, series circuits, parallel circuits and short circuits.
To understand the difference between the following circuit connections is not difficult at all. If the circuit is broken or “opened” anywhere, the current is known to stop everywhere. The circuit is broken when an electric device is switched off. The path along which the electrons travel must be complete otherwise no electric power can be supplied from the source to the load. Thus the circuit is “closed” when an electric device is switched on.
When electrical devices are connected so that the current flows from one device to another, they are said “to be connected in series”. Under such conditions the current flow is the same in all parts of the circuit as there is only a single path along which it may flow. The electrical bell circuit is considered to be a typical example of a series circuit. The “parallel” circuit provides two or more paths for the passage of current. The circuit is divided in such a way that part of the current flows through one path and part through another. The lamps in the houses are generally connected in parallel.
The “short” circuit is produced when the current can return to the source of supply without control. The short circuits often result from cable fault or wire fault. Under certain conditions the short circuit may cause fire because the current flows where it was not supposed to flow. If the current flow is too great a fuse is used as a safety device to stop the current flow.
II. Guess the meaning of the following international words:
concept, potential, electrostatic generator, aluminum, parallel, typical, control.
III. Give the English equivalents for the following words and word combinations:
1) электрические цепи, 2) электрический заряд, 3) проводник, 4) сопротивление, 5) движение электронов, 6) изолятор, 7) короткое замыкание, 8) энергия.
IV. Say whether these sentences are true or false:
1. When an extended conductor has the same potential at its ends, free electrons are drifting from one end to another.
2. The wire and the electric source together form an electric circuit.
3. A path of any material will allow current to exist.
4. Silver, copper and gold oppose very strongly.
5. The slighter the opposition is, the better the insulator is.
6. There is only one type of electric circuit.
7. We close the circuit when we switch on our electric device.
V. Complete the sentences using the text:
1. The potential difference must be maintained by …
2. Materials that offer slight opposition are called …
3. The best insulators are …
4. There are various kinds of electric circuits such as …
5. We “open” the circuit when …
6. We “close” the circuit when …
7. The “short” circuit is produced when …
8. A fuse is …
VI. Answer the questions:
1. What concepts are very important in study of electric current?
2. What forms an electric circuit?
3. What materials are the best conductors and insulators?
4. What kinds of electric circuits do you know?
5. How can we open and close the circuit?
6. When are electrical devices connected in series?
7. What is an example of a series circuit?
8. What can you say about “parallel” circuits?
9. What does the short circuit often result from?
UNIT 5
I. Read the text.
Alternating Current
Current is defined as increment of electrons. The unit for measuring current was named in honor of A.M. Ampere, the French physicist. Thus it is called ampere. The symbol for current is I. The electric current is a quantity of electrons flowing in a circuit per second of time. The electrons move along the circuit because the e.m.f. drives them. The current is directly proportional to the e.m.f.
A steam of electrons in a circuit will develop a magnetic field around the conductor along which the electrons are moving. The strength of the magnetic field depends upon the current strength along the conductor. The direction of the field is dependant upon the direction of the current.
If the force causing the electron flow is indirect, the current is called direct (d.c.). If the force changes its direction periodically the current is called alternative (a.c.).
Alternating current is the current that changes direction periodically. The electrons leave one terminal of the power supply, flow out along the conductor, stop, and then flow back toward the same terminal. A voltage that caused current reverses its polarity periodically. This is properly called an alternating voltage. The power supply that provides the alternating voltage actually reverses the polarity of its terminals according to a fixed periodic pattern. A given terminal will be negative for a specific period of time and drive electrons out through the circuit. Then, the same terminal becomes positive and attracts electrons back from the circuit. This voltage source cannot be a battery. It must consist of some types of rotating machinery.
II. Guess the meaning of the following international words:
1) physicist, 2) ampere, 3) symbol, 4) second, 5) polarity, 6) period, 7) battery.
III. Translate into Russian the words and expression from the text:
1) increment of electrons; 2) measuring; 3) to drive; 4) directly proportional; 5) conductor; 6) strength; 7) causing force; 8) terminal; 9) to flow; 10) to reverse.
IV. Give the English equivalents for the words below:
1) переменный ток, 2) за секунду, 3) количество электронов, 4) поток электронов, 5) магнитное поле, 6) направление, 7) зависеть, 8) усиление, 9) источник напряжения, 10) ротационный механизм.
V. Complete the sentences using the text:
1. The electric current is …
2. The unit for measuring current is …
3. A steam of electrons in a circuit will develop …
4. The current is called direct if …
5. The current is called alternating if…
6. Alternating voltage is …
7. Alternating voltage source cannot be …
VI. Answer the questions:
1. Why do electrons move along the circuit?
2. What does the strength of the magnetic field depend upon?
3. What does the direction of the field depend upon?
4. What is the way of alternating current electrons?
5. How does the alternating voltage power supply reverse the polarity of terminals?
UNIT 6
I. Read the text.
CONDUCTORS AND INSULATORS
All substances have some ability of conducting the electric current, however, they differ greatly in the ease with which the current can pass through them. Solid metals conduct electricity with ease while non-metals do not allow it to flow freely. Thus, there are conductors and insulators.
What do the terms "conductors" and "insulators" mean?
This difference is expressed by what is called electrical conductivity of the body. It depends upon the atomic constitution of the body. Substances through which electricity is easily transmitted are called conductors. Any material that strongly resists the electric current flow is known as an insulator.
Conductance, that is the conductor's ability of passing electric charges, depends on the four factors: thesize of the wire used, its length and temperature as well as the kind of material to be employed.
A large conductor will carry the current more readily than a thinner one. To flow through a short conductor is certainly easier for the current than through a long one in spite of their being made of similar material. Hence, the longer the wire, the greater is its opposition, that is resistance, to the passage of current.
There is a great difference in the conducting ability of various substances. Almost all metals are good electric current conductors. The best conductors are silver, copper, gold and aluminum. Nevertheless, copper carries the current more freely than iron; and silver, in its turn, is a better conductor than copper. Copper is the most widely used conductor. The electrically operated devices are connected to the wall socket by copper wires.
A material which resists the flow of the electric current is called an insulator.
The higher the opposition is, the better the insulator is. There are many kinds of insulation used to cover the wires. The kind used depends upon the purposes the wire or cord is meant for. The insulating materials generally used to cover the wires are rubber, asbestos, glass, plastics and others. The best insulators are oil, rubber and glass.
Rubber covered with cotton, or rubber alone is the insulating material usually used to cover desk lamp cords and radio cords.
Glass is the insulator to be often seen on the poles that carry the telephone wires in city streets. Glass insulator strings are usually suspended from the towers of high voltage transmission lines. One of the most important insulators of all, however, is air. That is why power transmission line wires are bare wires depending on air to keep the current from leaking off.
Conducting materials are by no means the only materials to play an important part in electrical engineering. There must certainly be a conductor, that is a path, along which electricity is to travel and there must be insulators keeping it from leaking off the conductor.
II. Give the Russian equivalents for the words and word combinations below:
1) conductors; 2) insulators; 3) transmit; 4) resistance; 5) passage of current; 6) socket; 7) to connect to; 8) cord; 9) high voltage transmission line; 10) leak off.
III. Find in the text the sentences with the following related words and translate them:
conducting – conductor – conductivity – conductance.
UNIT 7
I. Read the text.
SEMICONDUCTORS
There are materials that really occupy a place between the conductors of the electric current and the non-conductors. They are called semiconductors. These materials conduct electricity less readily than conductors but much better than insulators.
Semiconductors include almost all minerals, many chemical elements, a great variety of chemical compounds, alloys of metals, and a number of organic compounds. Like metals, they conduct electricity but they do it less effectively. In metals all electrons are free and in insulators they are fixed. In semiconductors electrons are fixed, too, but the connection is so weak that the heat motion of the atoms of a body easily pulls them away and sets them free.
Minerals and crystals appear to possess some unexpected properties. It is well known that their conductivity increases with heating and falls with cooling. As a semiconductor is heated, free electrons in it increase in number, hence, its conductivity increases as well.
Heat is by no means the only phenomenon influencing semiconductors. They are sensitive to light, too. Take germanium as an example. Its electrical properties may greatly change when it is exposed to light. With the help of a ray of light directed at a semiconductor, we can start or stop various machines, effect remote control, and perform lots of other useful things. Just as they are influenced by falling light, semiconductors are also influenced by all radiation. Generally speaking, they are so sensitive that a heated object can be detected by its radiation.
Such dependence of conductivity on heat and light has opened up great possibilities for various uses of semiconductors. The semiconductor devices are applied for transmission of signals, for automatic control of a variety of processes, for switching on engines, for the reproduction of sound, protection of high-voltage transmission lines, speeding up of some chemical reactions, and so on. On the one hand they may be used to transform light and heat energy directly into electric energy without any complex mechanism with moving parts, and on the other hand, they are capable of generating heat or cold from electricity.
Russian engineers and scientists turned their attention to semiconductors many years ago. They saw in them a means of solving an old engineering problem, namely, that of direct conversion of heat into electricity without boilers or machines. Semiconductor thermocouples created in Russia convert heat directly into electricity just as a complex system consisting of a steam boiler, a steam engine and a generator does it.
II. Give the English equivalents for the words and word combinations below:
1) полупроводник; 2) химическое соединение; 3) сплав; 4) освобождать; 5) свойство; 6) увеличивать(ся); 7) охлаждение; 8) чувствительный к; 9) выставлять; 10) луч; 11) направлять на; 12) дистанционное управление; 13) находить, обнаруживать; 14) защита; 15) ускорение; 16) решить инженерную проблему; 17) термоэлемент.
III. Guess the meaning of the following international words:
element, organic, mineral, crystal, phenomenon, automatic, control, process, reproduction, conversion, boiler.
IV. Join the beginnings and ends:
| Semiconductors are sensitive to… | … conductors of the electric current and non-conductors. |
| Semiconductors convert heat into … | … dependence of conductivity on heat and light. |
| Semiconductors occupy a place between … | … heat and light. |
| Semiconductors conduct electricity … | …into electricity without machines. |
| Great possibilities for various uses of semiconductors are connected with … | … less effectively than metals. |
| As a semiconductor is heated … | … its conductivity increases as well. |
V. Insert words and expressions:
1) Semiconductors include a great variety of (химические соединения), (сплавы металлов).
2) Minerals and crystals appear to possess some unexpected (свойства). Their conductivity increases with (нагревание) and falls with (охлаждение).
3) With the help of a ray of light directed at a semiconductor, we can effect (дистанционное управление).
4) The semiconductor devices are applied for (автоматический контроль) of a variety of processes, for the (воспроизведение) of sound, (ускорение) of some chemical reactions.
5) (Термоэлементы) created in Russia convert heat directly into electricity.
VI. Answer the questions:
1) What do semiconductors include? 2) How does the atomic structure of semiconductors influence their properties? 3) What phenomena influence semiconductors? 4) What are the semiconductor devices applied for? 5) How do semoconductors help in solving engineering problems?
UNIT 8
I. Read the text.
ELECTRICITY AND MAGNETISM
TEXT 1
Electromotive Force
When free electrons are dislodged from atoms, electrical energy is released. Chemical reaction, friction heat and electromagnetic induction will cause electrons to move from one atom to another. Whenever energy in any form is released, a force called electromotive (e. m. f.) is developed.
If the force exerts its effort always in one direction, it is called direct; and if the force changes its direction of exertion periodically, it is called alternating. The chemical reaction in a dry cell, heat and friction are sources of a unidirectional force. Electromagnetic induction produces an alternating force. The direction of force depends on the direction in which the field is cut. Whenever an e. m. f. is developed, there is also a field of energy called an electrostatic field, which can be detected by an electroscope and measured by an electrometer.
TEXT 2
Electromagnetic Induction
An electromotive force is induced in the conductor when there is a change in the magnetic field surrounding a conductor. This induced electromotive force may be produced in several ways as follows:
a) A conductor may move in a stationary magnetic field of constant strength.
b) A stationary conductor may be exposed to a moving magnetic field of constant strength.
c) The strength of the field surrounding the conductor may change without any motion of conductor or magnetic circuit.
The electromotive force induced by motion of a conductor or a magnetic flux is the same when the conductor rotates and the flux is stationary or the flux rotates and the conductor is stationary. If both, conductor and flux, rotate in the same direction at the same speed, no electromotive force will be produced, if they rotate at the same speed but in opposite directions, the electromotive force induced would be twice as that which would be induced, if one of them was stationary. An electromotive force is not induced when a conductor is moved parallel to the lines of force, but only when it moves at an angle with these lines.
Any motion across the direction of the lines, however, will produce an electromotive force in the conductor. For this reason, the conductor is said to „cut" the lines of force. The actual electromotive force induced in the conductor depends upon the nature at which the flux is cut.
TEXT 3
UNIT 9
I. Read the text.
Dynamos
The term „dynamo" is applied to machines which convert either mechanical energy into electrical energy or electrical energy into mechanical energy by utilizing the principle of electromagnetic induction. A dynamo is called a generator when mechanical energy supplied in the form of rotation is converted into electrical energy. When the energy conversion takes place in the reverse order the dynamo is called a motor. Thus a dynamo is a reversible machine capable of operation as a generator or motor as desired.
A generator does not create electricity, but generates or produces an induced electromotive force, which causes a current to flow through a properly insulated system of electrical conductors external to it. The amount of electricity obtainable from such a generator is dependent upon the mechanical energy supplied. In the circuit external to a generator the e.m.f. causes the electricity to flow from a higher or positive potential to a lower or negative potential. In the internal circuit of a generator the e.m.f. causes the current to flow from a lower potential to a higher potential. The action of a generator is based upon the principles of electromagnetic induction.
The dynamo consists essentially of two parts: a magnetic field, produced by electromagnets, and a number of loops or coils of wire wound upon an iron core, forming the armature. These parts are arranged so that the number of the magnetic lines of force of the field threading through the armature, coils will be constantly varied, thereby producing a steady e.m.f. in the generator or a constant torque in the motor.
II. Fill in the gaps with the words given below:
IV. Answer the questions.
1. What term can be applied to machines converting mechanical energy into electrical and vice versa?
2. What kind of machine is a dynamo?
3. What is the function of a generator?
4. What is the action of a generator based upon?
5. What parts does the dynamo consist of?
UNIT 10
I. Read the text.
GENERATORS
The powerful, highly efficient generators and alternators that are in use today operate on the same principle as the dynamo invented by the great English scientist Faraday in 1831.
Dynamo-electric machines are used to supply light, heat and power on a large scale. These are the machines that produce more than 99.99 per cent of all the world's electric power.
There are two types of dynamos – the generator and the alternator. The former supplies d.c. which is similar to the current from a battery and the latter provides a.c. To generate electricity both of them must be continuously provided with energy from some outside source of mechanical energy such as steam engines, steam turbines or water turbines.
A generator is an electric machine, which converts mechanical energy into electric energy. There are direct-current (d.c.) generators and alternating-current (a.c.) generators. Their construction is much alike. A d.c. generator consists of stationary and rotating elements. The stationary elements are: the yoke or the frame and the field structure. The yoke forms the closed circuit for the magnetic flux. The function of the magnetic structure is to produce the magnetic field.
The rotating elements are: true armature and the commutator. They are on the same shaft. The armature consists of the core and the winding. The winding is connected to the commutator. With the help of the brushes on the commutator that conduct the electric current to the line the winding is connected to the external circuit. The stationary element of an a-c. generator is called a stator. The rotating element is called a rotor.
The essential difference between a d.c. generator and a.c. generator is that the former has a commutator by means of which the generated e. m. f. is made continuous, i.e. the commutator mechanically rectifies the alternating e.m.f. so that it is always of the same polarity.
D.c. generators are used for electrolytic processes such as electroplating. Large d.c. generators are employed in such manufacturing processes as steel making. The d.c. generator of small capacities is used for various special purposes such as arc welding, automobile generators, train lighting systems, etc. It also finds rather extensive use in connection with communication systems.
II. Give the Russian equivalents for the following English words and word combinations:
1) generator; 2) alternator; 3) steam turbine; 4) water turbine; 5) armature; 6) rotor; 7) stationary; 8) commutator; 9) stator; 10) yoke; 11. brushes; 12) core; 13) frame; 14) winding.
III. Fill in the blanks
1. A generator is an electric machine, which a) … mechanical energy into electrical energy.
2. A direct-current generator consists of в) … .
3. The dynamo was invented by c) … in 1831.
4. The d.c. generator is used for various purposes such as d) … .
IV. Work out the plan of the text.
V. Speak on the following points:
1. The construction of a generator.
2. The direct – current generators and their industrial application.
3. Industrial application of D.C. Generators.
UNIT 11
I. Read the text.
UNIT 12
I. Read the text.
The Alternator
The alternator is an electric machine for generating an alternating current by a relative motion of conductors and a magnetic field. The machine usually has a rotating field and a stationary armature. In a synchronous alternator the magnetic field is excited with a direct current. The direction of an induced e.m.f. is reversed each time when a conductor passes from a pole of one polarity to a pole at another polarity. Most machines of this type are used for lighting and power, but there are alternators with a revoking armature and a stationary field. They are used in small sizes mostly for special purposes.
Any electrical machine is reversible. When a machine is driven by a source of mechanical power, it works as a generator and delivers electrical power. If it is connected to a source of electrical power, it produces mechanical energy, and operates as a motor. The alternator may also be operated as a motor.
The a.c. generator, or alternator, does not differ in principle from the d.c. generator. The alternator consists of a field structure and an armature. The field structure is magnetized by a field winding carrying a direct current. An electromotive force is generated in tine winding of the armature. In alternators the field is usually the rotating element and the armature is stationary. This construction has a number of advantages. Only two rings are needed with a rotating field. These rings carry only a relatively light field current, at a voltage generally of 125, and seldom exceeding 250. The insulation of such rings is not difficult. A stationary armature requires no slip rings. The leads from the armature can be continuously insulated from the armature winding to the switchboard, leaving no bare conductor. The alternator with a rotating field may be further divided into the vertical and the horizontal types.
The vertical type is usually applied for large water-wheel generators where it is desirable to mount the water turbine below the generator. The more common horizontal type is used with diesel and steam engine drive. A low-speed alternator of this type is suitable for a diesel engine drive, a high speed alternator is suitable for a steam turbine drive.
UNIT 13
I. Read the text.
The Induction Motor
An induction motor like any other motor consists of a stationary part, the stator, and a rotating part, the rotor. The rotor of an induction motor is not connected electrically to the source of power supply. The currents which circulate in the rotor conductors are the result of voltage induced in the rotor in the magnetic field set up by the stator. The rotor is fitted with a set of conductors in which currents flow. As these conductors lie in the magnetic field produced by the stator, a force is exerted on the conductors and the rotor begins to revolve.The operation of the motor depends upon the production of a rotating magnetic field. The speed at which the field of an induction motor turns is called the synchronous speed of the field or of the motor.
The induction motor is the simplest of the various types of electric motors and it has found more extensive application in industry than any other type. It is made in two forms – the squirrel cage and the wound rotor, the difference being in the construction of the rotor.
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. The rotor construction, however, is entirely different.
The squirrel-cage rotor is a simpler form and has been used in many machines.
Instead of coils the winding consists of heavy copper bars.
The wound-rotor type has a winding made up of well-insulated coils, mounted in groups whose end connections are brought out to fill in rings. The purpose of this winding is to provide for variation in the amount of resistance included in the rotor circuit.
Provision for ventilation is made by leaving passageways through the core and frame, through which air is forced by fan vanes mounted on the rotor. In main cases the motors now built in as an integral part of the machine it is to drive.
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. With but a single-phase winding on the stator, however, the torques produced in the two halves of the rotor would be in apposition, and the motor would not start. With more than one set of windings two for a two-phase motor, three for a three-phase motor a resultant field is produced which has the effect of cutting across the rotor conductors and induces voltages in them. This field is considered to be revolving at uniform speed.
The term „revolving field" should not be taken to mean actual revolution of flux lines. The magnetic field from the coils of each phase varies in strength with changes in current value but does not move around the stator. The revolutions are those of the resultant of the three, or two, phases, as the case may be. A motor with a single-phase winding is not self-starting but must be provided with an auxiliary device of some kind to enable the motor to develop a starting torque. The effect of the revolving field is the same as would result from actual revolution of a stator having direct-current poles. As voltages have been induced in the bars or windings of the rotor, currents start flowing as a result of these voltages, and a torque is produced which brings the motor up to speed.
II. Find in the text the English equivalents for the word combinations given below:
1) асинхронный двигатель; 2) неподвижная часть; 3) вращающаяся часть; 4) проводник; 5) одновременная скорость; 6) широкое применение; 7) паз; 8) механизм обмотки; 9) трансформатор; 10) вращающий момент.
UNIT 14
I. Read the text.
Types of Induction Motors
TEXT 1
SINGLE-PHASE MOTOR
The single-phase induction motor differs from poly-phase type principally in the character of its magnetic field, as an ordinary single-phase winding will not produce a rotating field, but a field that is oscillating, and the induced currents and poles produced in the rotor by this field will tend to produce equal torque in opposite directions, therefore, the rotor cannot start to revolve. However, if the rotor can in some manner be made to rotate at a speed corresponding to the frequency of the current in the stator windings then the reaction of the stator and rotor flux is such as to produce a torque that will keep the rotor revolving.
In practice the starting of single-phase induction motors is accomplished by two general methods applicable to small-sized motors only.
First: the split-phase method, in which an auxiliary stator winding is provided for starting purposes only, this winding being displaced from the main stator winding by 90 electrical degrees. It has a higher inductance than the main stator winding, thus causing the currant in it to lag far enough behind the current in the main winding to produce a shifting or rotating field during the starting period, which exerts a starting torque on the rotor sufficient to cause rotation.
When nearly normal speed has been reached the auxiliary winding is out of circuit by a switch and clutch in the motor, which operates automatically by centrifugal force, and the rotor continues to run as a single-phase motor. 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, 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 introduction of the inductance in the auxiliary winding splitting the phase as before.
TEXT 2
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.
UNIT 18
I. Read the text.
UNIT 19
I. Read the text.
SUPPLEMENTARY TEXTS
Part I
HISTORY OF ELECTRICITY:
TEXT 1
GEORGE SYMON OHM
GEORGE SYMON OHM (1784–1854) is a famous German physicist. In 1805 he entered the Erlangen University. Though he did not graduate from this University, he managed to write and defend a thesis in 1811. Later, he was a teacher at the gymnasiums of Gottstadt and Wamburg. Beginning from 1833 he became professor at the Polytechnical School in Nuremberg, and since 1849 – at the München University.
He is the most famous for establishment of the general law of the electric circuit, stating the relation between resistance, electromotive force, and strength of the current in the electric circuit. The law was discovered experimentally and first formulated in 1826. Further investigations made use of this law. The unit of resistance was named after Ohm at the International Congress of Electricians in1881.
TEXT 2
Ohm's Law
One of Ohm's major contributions was the establishment of a definite relationship between voltage, resistance, and current in a closed circuit. A circuit consists of a voltage source and a complete path for current. Ohm stated this relationship as follows:
Current is directly proportional to voltage and inversely proportional to resistance.
As a formula, it appeals like this:
This formula is commonly known as Ohm's Law.
About 1817 Ohm discovered that a simple correlation exists between resistance, current and voltage. That is: the current that flows in the circuit is directly proportional to the voltage and inversely proportional to the resistance. A current is measured in amperes, a voltage, or potential difference is measured in volts. A resistance is measured in ohms.
TEXT 3
Faraday's Law
MICHEL FARADAY was a great British physicist, the founder of the theory of electron field, a member of the London Royal Society. He was born in London in the family of a smith. Spending a few years in the primary school, he continued his studies all by himself, reading books and listening public lectures. Greatly impressed by lectures of a well-known English chemist H. Davy, he sent him a letter asking for a job at the Royal Institute. In 1813 Davy gave him a job of a laboratory assistant. Thanks to the brilliant talent of an experimenter, Faraday soon made himself known. All his future scientific work was carried out in the Royal Institute laboratories.
Faraday's law is formulated as follows: (a) the induced E.M.F. in a conductor is proportional to the rate at which the conductor cuts the magnetic lines of force. (b) The induced E.M.F. in a circuit is proportional to the rate of change of the number of lines of force threading the circuit.
TEXT 4
EMIL LENZ. Lenz's Law
EMIL LENZ was born on the 12 of February 1804 and died on the 29 of January 1865 in Derpt. He became a prominent Russian physicist, an Academician.
At the age of 16 he entered the Derpt University. In 1823, when being a student, he joined a 3 year round-the-world trip on board of the ship “Enterprise” as a physicist. The chief of the expedition was Kotzebu, a famous Russian seaman and explorer. In 1828 Lenz was elected adjunct-professor of the St.Petersburg Academy of Sciences for his outstanding investigations in geophysics.
In the 30 s of the 19th century, Lenz reorganized a physical laboratory of the Academy of Sciences where he began his famous studies on electricity and magnetism. He discovered the law of the electric current emitting heat in conductors. This law laid the foundation for the discovery of the Law of conservation and conversion of energy.
The direction of the induced current is such that its effect opposes the change producing it. The right-hand rule enables one to predict the direction of the induced current, and may be shown to conform with Lenz's law.
The induction coil, the dynamo, the transformer, and the telephone are practical application of electromagnetic induction.
TEXT 5
Kirchhoff's Laws
GUSTAV ROBERT KIRCHHOFF (1824–1887) is a famous German scientist. He graduated from the Königsberg University in 1846. Since 1850 he had been an extraordinary professor of physics at the University of Breslau, and since 1854 – an ordinary professor of experimental and theoretical physics in Heidelberg University, in 1875 he became the chief of the Chair of mathematical physics in Berlin University.
His first works (1845–49) were dedicated to studies of the electric current in various kinds of conductors, series and parallel circuits, and to distribution of electricity in the conductors. Together with Bunsen, he was the author of spectral analysis.
G. R. Kirchhoff expanded and clarified Ohm's law with two statements which may be paraphrased as follows:
1. The current entering a point is equivalent to the current leaving the point.
2. The sum of the voltage drops around a closed loop is equal to the applied voltage.
Kirchhoff intended his statements to apply to all circuits.
The two main principles of circuit analysis are:
(1) Kirchhoff's Current Law. The sum of the currents directed away from the junction is equal to the sum of the currents directed toward the junction.
(2) Kirchhoff's E. M. F. Law. The sum of the voltage drops around any closed loop of a network equals the sum of the voltage rises around this loop.
TEXT 6
TEXT 7
CHARLES COULOMB
CHARLES COULOMB (1736–1806), a member of the Paris Academy of Sciences, an outstanding French physicist in the period from 1785 to 1789 stated the law of electrostatic and magnetic interaction. His work in this field laid foundation for the future theoretic investigations in the electrostatics and magnetstatics.
Coulomb’s law is one of the principal laws of electrostatics. It established a relationship between the force of interaction of two static electric charges, their quantities, and the distance between them. According to Coulomb’s law the absolute value of the force of repulsion of two like charges or the force of attraction between two unlike charges el and e2, which size is much less than the distance between them, is inversely proportional to the square of the distance between them. He also stated the laws of rotation, dry friction, laws of interaction between magnetic poles. All these laws were named in honor of Ch. Coulomb.
TEXT 8
ANDRE MARIE AMPERE
ANDRE MARIE AMPERE (1775–1836) was an outstanding physicist and mathematician of French origin. He is one of the founders of modern electrodynamics. He was born in aristocratic family in Lyon. By the age of 14 he has read all the 20 volumes of “The Encyclopedia” by Diderot and D’Alambert. His scientific interests were very diverse.
In 1801 Ampere headed the Chair of Physics in Burge, in 1805 he became a teacher of physics at the Polytechnical School in Paris. Since 1814 he was elected Member of The Institute, which later transformed into the French Academy of Sciences. After 1824 he occupied the post of professor at the Ecole Normale in Paris.
Ampere’s studies on the effects of the electric current flow on the magnetic needle were his greatest contribution to physics. In 1820 in the report to the Paris Academy, he made the announcement of the so-called “Ampere Rule”, which is since used to define the deflection of the needle affected by the electric current. This led him to the discovery of interactions between electric currents. The fundamental laws of this interaction got his name.
TEXT 9
JAMES CLERC MAXWELL
JAMES CLERC MAXWELL, a British physicist, was born in 1831. In 1847–50, he studied at the Edinborough University and later in Cambridge. On graduating from the Cambridge University, he was offered a post of a teacher there. In 1860 he headed the Chair of Physics in the King’s College in London. In 1871 he went back to Cambridge where he headed a newly-organized laboratory named in honor of H. Cavendish.
His scientific interests lay in the field of electro-magnetism, molecular physics, optics, mechanics, and other. Maxwell published his first scientific paper when he was only 15. He founded the theory of electro-magnetic field, the electromagnetic theory of light. He is credited with the studies of the Saturnus rings. He described all known facts of electrodynamics by means of system of equations, known as Maxwell’s equations of electrodynamics.
TEXT 10
TEXT 11
TEXT 12
Text 13
Nature of Electricity
The first recorded observation on electricity was made by the ancient Greek philosopher Phales. He stated that a piece of amber rubbed with fur attracted light objects. But more than 22 centuries passed before the study of magnetism and of electrical phenomena began by Galileo and other scientists.
It was well known that not only amber, but many other substances having been rubbed behave like amber i.e. can be electrified. It was discovered that any 2 dissimilar substances forced into contact and then separated became electrified, or acquired electrical charges.
During the 19th century the idea of the nature of electricity was completely revolutionized. The atom was regarded as the ultimate subdivision of matter. Today the atom is regarded as an electrical system. In this electrical system there is a nucleus containing positively charged particles called protons. The nucleus is surrounded by lighter negatively charged units – electrons. So the most essential constituent of matter is made up of electrically charged particles. Matter is neutral and produces no electrical effects when it has equal amounts of both charges.
But when the number of negative charge is unlike the number of positive ones, matter will produce electrical effects. Having lost some of its electrons, the atom has a positive charge: having an excess of electrons – it has a negative charge.
TEXT 14
ATMOSPHERIC ELECTRICITY
Electricity plays such an important part in modern life that in order to get it, men have been burning millions of tons of coal. Coal is burned instead of its being mainly used as a source of valuable chemical substances which it contains. Therefore, finding new sources of electric energy is a most important problem that scientists and engineers try to solve.
Hundreds of millions of volts are required for a lightning spark about one and a half kilometre long. However, this does not represent very much energy because of the intervals between single thunderstorms. As for the power spent in producing lightning flashes all over the world, it is only about 1/10,000 of the power got by mankind from the sun, both in the form of light and that of heat. Thus, the source in question may interest only the scientists of the future.
Atmospheric electricity is the earliest manifestation of electricity known to man. However, nobody understood that phenomenon and its properties until Benjamin Franklin made his kite experiment. On studying the Leyden jar (for long years the only known condenser), Franklin began thinking that lightning was a strong spark of electricity. He began experimenting in order to draw electricity from the clouds to the earth. The story about his famous kite is known all over the world.
On a stormy day Franklin and his son went into the country taking with them some necessary things such as: a kite with a long string, a, key and so on. The key was connected to the lower end of the string. "If lightning is the same as electricity," Franklin thought, "then some of its sparks must come down the kite string to the key." Soon the kite was flying high among the clouds where lightning flashed. However, the kite having