II. Read the statements and define if they are true or false (according to the text content). If the statement is false, correct it.

1. All substances show similar resistances to the flow of electricity through them.

2. The external force applied to a circuit to overcome the opposition to the flow of current is called voltage.

3. With the increase in the temperature the resistance of carbon, insulating materials increases.

4. The ampere is an electrical unit for measuring the strength of electric current in a circuit.

5. The device used for measuring delivery of electric energy is called ohmmeter.

III. Find all definitions in the text, learn them by heart. Be ready to define the electrical units and measuring devices.

UNIT 2: Electric Machines

Text 2A

Ø Read and translate the text, using the given vocabulary. Learn the words and word combinations by heart.

to convert	преобразовывать
reversible	обратимый
identical	одинаковый, идентичный
stationary	неподвижный
rotating	вращающийся
frame	станина, корпус
main poles	главные полюса
commutating poles	добавочные полюса
to ensure	обеспечивать
armature	якорь
the former	первый из двух названных
commutator	коллектор
in the form of	в виде
closed magnetic conductor	замкнутый магнитопровод
for the purpose of	в целях
losses	потери
sheet steel laminations	листы электротехнической стали
to mount	монтировать, располагать
field winding	обмотка возбуждения
non-sparking operation	безыскровая работа
cylindrical core	цилиндрический сердечник
slot	паз
sections' leads	выводы секций
brush gear	щёточный аппарат
brush-holder	щёткодержатель
brush-stud	щёточный палец
brush-rocker	щёточная траверса
current-collecting bar	токособирающая шина
essential parts	ответственные узлы
drive motor	приводной двигатель
loaded machine	машина, включенная на нагрузку
to bring into existence	приводить к появлению
to coincide in direction	совпадать по направлению
electromagnetic torque	электромагнитный момент
directed in opposition to	направленный навстречу
prime mover	первичный двигатель
consumed from	потребляемый от
is given up to	передаётся в
to bring into rotation	приводить к вращению
the mains	сеть

Ø Check pronunciation of the following words:

identical, armature, commutator, winding, isolated, cylindrical, coincide, torque.

Direct Current Electric Machines

D.C. machines converting mechanical energy into electrical one are referred to as d.c. generators. Those, on the other hand, which convert electrical energy into mechanical one, are spoken of as d.c. motors. D.c. machines are reversible and, therefore, identical in construction. They are composed of two main parts – the stationary part which is called the frame provided with the main poles and the commutating poles, and the rotating part which is said to be the armature, its windings and com­mutator being positioned on it.

The frame is made of steel in the form of a closed magnetic con­ductor. The main polesfor the purposes of decreasing the losses are assembled of sheet steel laminations. Mounted on them, the field wind­ings are built of copper isolated conductors. The main poles are in­tended to produce the main magnetic flux.

The commutating polesare also made of steel and arranged mid­way between the main poles. Their windings as well as those of the main poles are built of copper conductors. The commutating poles with the windings are designed to ensure non-sparking operation of an elec­tric machine.

The armature makes up a cylindrical core made of sheet steel lam­inations, a two-layer winding being fixed in their slots. The former is built of sections made of isolated copper conductors.

The commutatoris constructed of separate copper bars isolated from each other and from the frame. The section leads of the armature winding are connected to the bars. The commutator is designed for the conversion of an alternating e.m.f. induced in the armature winding into a direct one.

The brush gearis an element designed for collecting the current from the armature winding and leading it to the latter. It is composed of brushes, brush-holders, brush-studs, brush-rockers and current col­lecting bars.

The commutator and the brush gear are the most essential parts of a d.c. machine, its reliable operation being dependent on their condition.

The operative principleof a d.c. machine is built upon the laws of electromagnetic induction and electromagnetic force. As the arma­ture is rotated in the magnetic flux of the main poles by a drive motor, the e.m.f. is induced in the armature winding. In the loaded machine, the e.m.f. brings into existence the current with which it coincides in direction. This current interacting with the magnetic flux produces the electromagnetic torque directed in opposition to that produced by a prime mover. As this takes place, the machine operates as a generator. The mechanical power consumed from the drive motor is converted into the electrical one and is given up to the mains.

When a d.c. machine is connected to the electrical source of sup­ply, the current is generated in the armature winding. Interacting with the magnetic flux of the poles, it produces the electromagnetic torque which brings into rotation the armature. The e.m.f. directed in opposi­tion to the current direction is induced in the armature winding. In this case the machine works as a motor. The electrical power consumed from the mains is converted into the mechanical one.

Exercises

I. a) Find the Russian equivalents of the following English word combinations:

1. stationary part; 2. field winding; 3. to ensure non-sparking operation; 4. made of sheet steel laminations; 5. induced in the armature winding; 6. current-collecting bar; 7. coincide in direction; 8. interacting with the magnetic flux; 9. bring into rotation; 10. consumed from the mains;

1. обеспечивать безыскровую работу; 2. совпадать по направлению; 3. сделанный из листов электротехнической стали; 4. обмотка возбуждения; 5. индуктируемая в якорной обмотке; 6. неподвижная часть; 7. потребляемый от сети; 8. взаимодействуя с магнитным потоком; 9. токособирающая шина; 10. приводить к вращению.

b) Translate these Russian words and phrases from the text into English:

одинаковый, вращающийся, станина, якорь, обмотка возбуждения, добавочные полюса, устанавливать, сердечник, щёточный аппарат, приводной двигатель, совпадать, электромагнитный момент, сеть, обеспечивать.

II. Make sentences out of two given parts:

III. Answer the questions according to the text context:

1) What type of a d.c. machine is referred to as a d.c. motor?

2) What are the two main parts of a d.c. machine?

3) What is the frame composed of?

4) Is the construction of the commutating poles identical to the one of the main poles?

5) What does the armature make up?

6) What is the commutator designed for?

7) What are the most essentials parts of a d.c. machine?

8) What elements is the brush gear constructed of?

9) What laws is the operative principle of a d.c. machine based upon?

10) In what case does a d.c. machine operate as a d.c. generator?

11) Under what condition does a d.c. machine work as a d.c. motor?

IV. Put the verbs in brackets into the correct form – Present Simple Active or Present Simple Passive:

1. The frame (to make) of steel in the form of a closed magnetic conductor. 2. The commutating poles (to design) to ensure non-sparking operation of an electric machine. 3. The main poles (to assemble) of sheet steel laminations and (to decrease) the losses. 4. D.c. motor (to convert) electrical energy into mechanical one. 5. The operative principle of a d.c. machine (to build) upon the laws of electromagnetic induction and electromagnetic force. 6. When a d.c. machine (to connect) to the electrical source of supply, the current (to generate) in the armature winding. 7. Interacting with the magnetic flux of the poles, this current (to produce) the electromagnetic torque which (to bring) into rotation the armature.

V. Speak on:

§ the construction of d.c. machines: the frame, toe poles, the armature, the commutator, the brush gear;

§ the principle of operation: d.c. generators, d.c. motors.

Text 2B

Ø Read and translate the text, using the given vocabulary. Learn the words and word combinations by heart.

Types of D.C. Generators

The d.c. current in the field coils is called the 'excitation current', and may be supplied either from a separate d.c. voltage source, or by using the d.c. output of the generator itself. If the field is supplied with current from an external source, the generator is said to be 'separately-excited'; but if some of the generator output is used to supply the field current, it is said to be 'self-excited'. In a self-excited generator the field coils may be connected either in series with the armature coils ('series'), in parallel with the armature coils ('shunt'), or partly in series and partly in parallel with them ('compound').

Separately-excited d.c. generators have two circuits: the field current, consisting of the field coils connected across a separate d.c. source, and the armature circuit, consisting of the armature coil and the load resistance. In the generators of this type the field is independent of the armature, since it is supplied with current from either another generator (exciter), an amplifier or a battery. The separately-excited generator is frequently used in automatic motor control systems. In such systems the field current is controlled by an amplifier, and the output of the generator supplies the current which drives a motor.

In a series generator the field coils are connected in series with the armature so that the whole armature current flows through both the field and the load. If the generator is not connected across a load, the circuit is incomplete and no current will flow to excite the field. The series field winding contains relatively few turns of wire.

A shunt d.c. generator has its field winding connected in shunt (in parallel) with the armature. Therefore the current through the field coils is determined by the terminal voltage and the resistance of the field. The shunt field windings have a large number of turns and require a relatively small current to produce the necessary field flux.

A compound generator is a combined series and shunt generator. There are two sets of field coils – one in series with the armature, one in parallel. One shunt coil and one series coil are always mounted on a common pole piece. If the series field is connected so that its field aids the shunt field, the generator is called cumulatively compound. If the series field opposes the shunt field, the generator is called differentially compound. Compound generators were designed to overcome the drop in terminal voltage which occurs in a shunt generator, when the load is increased. This voltage drop is undesirable where constant voltage loads such as lighting systems are used.

Exercises

I. Translate the following words and phrases:

a) from English into Russian: separate, amplifier, supply, output, shunt, relatively, determine, aid, series, occur, cumulatively compounded;

b) from Russian into English: катушка возбуждения, генератор независимого возбуждения, внешний источник, требовать, генератор смешанного возбуждения, противодействовать, нагрузка, незамкнутый.