Файл: Методическое пособие для студентов iv курса очного и заочного обучения специальности 26. 05. 07 "Эксплуатация судового электрооборудования и средств автоматики".doc

2. Read and translate the text.

D.C. GENERATORS

D.C. generators are provided with the armature winding and one of the two field windings. Depending on the type of the armature winding and field winding interconnection D.C. generators may be recognized as those of separate excitation, of shunt excitation, of series excitation, and of compound excitation.

In a separately excited generator, the field winding obtains its supply from a separate current source. It is connected in series with the armature winding in a series-wound generator and in parallel - in a shunt-wound generator. A compound-wound generator has two field windings positioned on the main poles, one of them being connected in shunt, the other - in series with the armature winding. The parallel field winding concentrates the current ranging from 1 to 6% of the rated armature current. It is made from copper conductors provided with a large number of turns of relatively small section. The series field winding carries the entire armature current and hence its conductors are of large section.

Shunt-wound generators, series-wound generators and compound- wound generators are self-excited; that is to say they don't require a separate current source for their excitation. The current supplying the windings is derived from the generator armature.

Practically all D.C. generators installed on vessels for supplying aux­iliary power are of the self-excited type.

Shunt-wound generators are usually used as the exciters of sep­arately excited generators and have the added feature of charging accu­mulative batteries. The latter is associated with the fact that with a re­verse current they aren't remagnetized in view of the unchangeable current direction in the field winding.

Series-wound generators have the voltage which varies abruptly with the load and, therefore, this type of D.C. generators is not used on board ships.

Compound-wound generators are not subject to frequent overloads and short-circuit, for the series field winding demagnetizes them. They are employed in welders as well as in some electric drives on the voltage-control system.

Separately excited generators find application where a wide- range voltage regulation is required, that is in the electric drives of steer­ing gears, windlasses, winches etc. as well as in the electric propulsion plants as main generators and exciters.

When putting D.C. generators in parallel operation, it is neces­sary that two conditions be met: 1) their polarity should be the same as the one in the mains to which they are connected; 2) their e.m.f. is required to be equal to the mains voltage. The device responsible for fulfilling the indicated conditions is a voltmeter of a magnetic electric system. In order to transmit the power from one generator operating in parallel to another, the excitation currents of the generator to which the power is transferred, should be increased but those of the generator from which it is collected, should be reduced. The generators are required to be loaded as uniformly and proportionally to their rated capacities as possible. If this condition is not fulfilled, the efficiency will be lower.
3. Match the following English and Russian word combinations.

a) to vary abruptly,tocharge accumulative batteries,to be subject to,

frequent overloads,short-circuit, find application, to fulfill the conditions

b) частые перегрузки, находить применение, короткое замыкание, выполнять условия, резко меняться, быть подверженным, заряжать аккумуляторные батареи.

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4. Put auxiliaries into each gap.

is are do does

1. . . . D.C. machines identical in construction?

2. What parts ...a D.C. machine composed of?

3.What ...the field windings built of?

4. . . . the armature make up a cylindrical body?

5.Where ...the e.m.f. induced?

6.What ...the electromagnetic torque bring into rotation?

7. . . . those generators of the self-excited type?

8. . . . separately-excited generators require a separate current source?

9. What amount of current ...the parallel field winding of a compound-wound generator concentrate?

10. Where . . . separately-excited generators find application on board ships?

11. How ...the generators operating in parallel required to be loaded?

12. . . . the commutator constructed of separate copper bars?
5. Put the words in the right order to ask a question.

1. D.C. machines reversible are ?

2. the frame made is of what ?

3. assembled what the main poles? of are

4. where the commutating poles are arranged

5. isolated the commutating bars are from each other

6. what kind of the brushgear element is

7. is from consumed a drive motor mechanical energy

8. does the field winding obtain where from its supply

9. how many a compound-wound generator does have field windings

10.where the current is from derived

11. find where separately-excited generators do application on board ships

12. the e.m.f. required is to be equal to what

13. compound-wound generators used are in welders

14. they be can as exciters used
6. Answer the questions.

1. How many armature windings and field windings are D.C. generators provided with?

2. What types of D.C. generators depending on the way of the arma­ture winding and field winding interconnection do you know?

3. What fact does a separately excited generator take its name from?

4. In what way is the field winding connected with the armature wind­ing in a shunt-wound generator?

5. In what way is the field winding connected with the armature wind­ing in a series-wound generator?

6. How many field windings does a compound-wound generator have? How are they connected to the armature winding?

7. What purposes are shunt-wound generators applied for?

8. Is a series-wound type of D.C. generators employed on board ships?

9. Where are compound-wound generators employed?

10. What conditions should be met to put D.C. generators in parallel operation?
7. Speak about d.c. generators.

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UNIT III. D.C. MOTOR

1. Read and remember the following words and word combinations.

convertibility	обратимость
generator mode	генераторный режим
oppose	препятствовать
overcome	преодолеть
exert	приложить (силу, усилие)
remove	убирать
maintain	сохранять
pass through	пропустить через
preserve	сохранять
when acted upon by the electromagnetic force	под действием электромагнитной силы
change over	переключаться, переходить
motor mode	двигательный режим
counter e.m.f.	противо э.д.с.
starting	пуск
small quantity	малая величина
in order to limit	для ограничения
place (put) in series with the armature circuit	последовательно включать в цепь якоря
speed regulation	регулирование частоты вращения
regulating (adjusting) rheostat	регулировочный реостат
towards the end of starting	к концу пуска
withdraw	выводить
connected voltage	подводимое напряжение
stepless regulation	плавное регулирование
step-by-step regulation	ступенчатое регулирование
wide-ranged regulation	регулирование в широком диапазоне

2. Read and translate the text.

D.C. MOTOR

When a D.C. machine is operating in a generator mode, the conductors of the armature winding across which the current is flowing interact with the magnetic flux of the poles. As a result, the electromagnetic force opposing the armature rotation is produced. In order to overcome this force, the external force should be exerted upon the generator armature.

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If the external force is removed, the polarity of the poles is maintained and the current of the same direction is passed through the armature winding, the electromagnetic force will preserve the same direction. When acted upon by the electromagnetic force, the armature will rotate in the direction opposing that of the generator rotation, and hence the D.C. machine will change over to a motor mode. The property by virtue of which any D.C. machine may work either in a generator mode or in a motor mode is called convertibility.

While starting the D.C. motor, its armature is stationary (n = 0) and hence the counter e.m.f. E = 0. The starting armature current is many times larger than the rated current, which may be explained by a small quantity of the armature resistance. Such a current is dangerous for the armature winding and the machine as a whole.

In order to limit the starting current, an additional resistor (the starting rheostat) is placed in series with the armature circuit. As the speed of rotation increases, the counter e.m.f. rises, but the armature current rapidly decreases. As the armature current decreases, the armature resistance is reduced. Towards the end of starting, the starting rheostat should be completely withdrawn.

There exist three methods of D.C. motor speed regulation:

1) by varying the connected voltage, owing to which stepless regulation can be accomplished (the method is used in adjustable-potential systems);

2) by varying the resistance in the armature circuit; the regulation is step-by-step, not economical; it is obtained by putting an adjusting rheostat in series with the armature circuit;

3) by varying the magnetic flux; the regulation is stepless, wide- ranged; it is attained by changing the field current by means of an adjusting rheostat placed in the field circuit.

In order to alter the direction of the motor rotation, it is necessary to change the current direction either in the armature or in the field winding. If the current direction in both of them is varied, the reversal will never take place. In practice the change in the current direction of the armature usually applies.
3. Match the following English and Russian word combinations.

a) to interact with the magnetic flux, to overcome the force, to maintain, to preserve the same direction, speed regulation, motor mode, to accomplish the regulation, to limit the starting current, to alter the direction

b) преодолеть силу, выполнять регулировку, сохранять полярность, взаимодействовать с магнитным потоком, двигательный режим, сохранять одно и то же направление, ограничить пусковой ток, менять направление, регулирование частоты вращения
4. Answer the questions.

1. How is the electromagnetic force produced?

2. What should be done to overcome the electromagnetic force?

3. What are the conditions under which the electromagnetic force preserves the same direction?

4. How will a D.C. machine change over to a motor mode?

5. What kind of property is called convertibility?

6. How is the counter e.m.f. induced in the armature winding?

7. In what way is the counter e.m.f. defined?

8. Why is the starting armature current dangerous for the armature winding and the machine as a whole?

9. What should be done in order to limit the starting current?

10. What should be done to the starting rheostat towards the end of starting?

11. What are three methods of the D.C. motor speed regulation?

12. Which methods are used to attain stepless regulation?

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13. Which method is used to accomplish step-by-step regulation?

14. What is to be done to alter the direction of the motor rotation?

15. Does the change in the current direction of the armature or of the field winding apply in practice?
5. Open the brackets using the correct verb form. Translate the sentences.

1. If the external force is removed, the electromagnetic force . . (preserve) the same direction.

2. If we don't change the field current, we . . . (not accomplish) stepless wide-ranged speed regulation.

3. If the armature rotates in the direction opposing that of the generator rotation, the D.C. machine . . . (change over) to a motor mode.

4. When the armature is acted upon by the electromagnetic force, it . . . (rotate) in the direction opposing that of the generator rotation

5. If we don't change the current direction either in the armature in the field winding, the reversal . . . (not take place).

6. As soon as we place a starting rheostat in series with the armature circuit, we . . . (limit) the starting current.

7. If we don't maintain the polarity of the poles, the electromagnetic force . . . (not preserve) the same direction.

8. When the speed of rotation increases, the counter e.m.f. . . . (rise) but the armature current . . . rapidly (decrease).
6. Complete the sentences using one of these verbs in Present Simple Passive.

attain	pass through	place	Induce	vary
withdraw	produce	remove	Maintain	define
accomplish

1. The electromagnetic force opposing the armature rotation ....

2. When the D.C. motor armature is rotating, the counter e.m.f . . . in the armature winding.

3. If the external force . . . , the polarity of the poles . . . and the current of the same direction ...the armature winding, the electromagnetic force will preserve the same direction.

4. The counter e.m.f. ...as much the same way as the generator e.m.f.

5. The armature current and the speed of the armature rotation . . . from the following equations.

6. In order to limit the starting current, an additional resistor . . . in series with the armature circuit.

7. A step-by-step speed regulation ...by putting an adjusting rheostat in series with the armature circuit.

8. If the magnetic flux . . . , a stepless wide-range regulation ....

9. Towards the end of starting the starting rheostat . . . completely . . . from the armature circuit.
7. Speak about D.C. motor.

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