Английский язык для моряков / Англ Труханова

winding are connected to the bars. The commutator is designed for the conversion of an altemating emf induced in the armature winding into

а direct one.

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

The commutator and the brushgear are the most essential parts of а

d.c. machine, its reliaЫe operation being dependent оп their condition. The operative principle of а d.c. machine is built upon the laws

of electromagnetic induction and e\ectromagnetic force. As the arma­ ture is rotated in the magnetic flux ofthe main poles Ьу а drive motor, the e.m.f. is induced in the armature winding. In the \oaded 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 Ьу а prime mover. As this takes place, the machine operates as а generator. The mechanica\ power consumed from the drive motor is converted into the electrica\ one and is given up to the mains.

When а d.c. machine is connected to the electrical source of sup­ ply, 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 opposi­ tion to the current direction is induced in the armature winding. In this case the machine works as а motor. The electrical power consumed from the mains is converted into the mechanical one.

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QUESTIONS

l.What type of d.c. machine is referred to as а d.c_. generator?

2.What type of d.c. machine is spoken of as а d.c. motor?

3.What are the two main parts of а d.c. machines?

4.What is the frame composed of?

5.Is the construction of the commutating poles identical to the one of the main poles?

6.Are the commutating poles intended for the same purposes as the main poles?

7.What does the armature make up?

8.What is the commutator designed for?

9.What are the elements the brushgear is constructed of? What is it designated for?

10.What laws is the operative principle of а d.c. machine built upon?

11.How can the operative principle of а d.c. machine Ье described?

12.In what case does а d.c. machine operate as а d.c. generator?

13.Under what condition does а d:c. machine work as а d.c. motor?

14.What happens to the electrical power consumed from the mains when а d.c. machine works as а motor?

Fig. 1.., Four-pole d.c. generator

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2. D.C. GENERATORS AND THEIR CHARACTERISTICS

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

In а separately excited generator, the field winding obtains its sup­ ply from а separate current source. lt is connected in series with the armature winding in а series-wound generator and in parallel - in а shunt-wound generator. А 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 arrnafure winding. The parallel field winding concentrates the current ranging from 1 to 6% of the rated armature current. lt is made from copper conductors provided with а large number of tums of relatively small section. The series field wind­ ing carries the entire arrnature 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 а sep­ arate cuпent source for their excitation. The current supplying the wind­ ings is derived from the generator armafure.

The generator properties depend upon the method of the genera­ tor excitation. They may Ье expressed Ьу definite characteristics, that is, the. relationships between the e.m.f., the voltage, the arrnature cur­ rent and the excitation, all of them being responsiЫe for the operation ofad.c. machine on different loads. The most important of the indi­ cated magnitudes is the voltage which is dependent on the above men­ tioned excitation current, the arrnature current and the rotational speed.

The main characteristics which reveal the properties of d.c. gener­ ators may Ье described as follows.

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No-load characteristic is defined as the relationship between the generator e.m.f. Е and the excitation current 1•• with the armature cur­ rent 1 = О (curve l, fig.2,a): Е = f(I..). This characteristic lets us judge the saturation ofthe magnetic circuit and may Ье applied for plotting the other characteristics.

Short-circuit characteristic (fig. 2,Ь) shows the relationship between the armature current lsh and the excitation current with the short­ circuited armature and the voltage equal to zero: I,h = F (1..). This char­ acteristic is plotted as а straight line since under short-circuit conditions the generator magnetic circuit is practically not saturated.

External characteristic (fig. 2,с) is the relationship between the voltage U and the armature current I with the resistance in the ex­ citation circuit R = const. The equation is U = f (l). Тhе extemal char­ acteristics plotted in fig. 2,с are those ofа separately excited generator (1), ofа shunt-wound generator (2), of а series-wound generator (3) and ofа compound-wound generator when its field winding is either con­ nected accordantly (4) or in opposition (5) to the armature winding. With the excess m.m.f. ofа series winding the compound-wound generator voltage shows an increase when the armature current rises (6).

Regulation characteristic (fig. 2,d) makes up the relationship between the excitation current and the armature cuпent with the gener­ ator voltage U = const: 1.. = f (1). Fig. 2,d shows Ьу graphical display the regulation characteristics ofa shunt-wound generator (1), ofa sep­ arately-excited generator (2) and of а compound-wound generator (3) with its windings connected accordantly.

Load characteristic indicates the relationship between the volt­ age U and the excitation cuпent 1..: U = f (1•.) with 1 = const. The load characteristics of а separately-excited generator (2) and of а compound­ wound generator (3) with its windings connected accordantly appear in fig. 2, а. As illustrated in the figure, curve 3 passes above the no­ load curve (1). This may Ье explained Ьу the action ofthe m.m.f. ofа series winding.

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VOCABULARY

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QUESTIONS

l.How many armature windings and field windings are d.c. genera­ tors provided with?

2. What types of d.c. generators depending оп the way of the arma­ ture winding and field winding interconnection do you know?

3. What fact does а separately excited generator take its name from? 4. In what way is the field winding connected with the armature wind­

ing in а shunt-wound generator?

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

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

7. Why are d.c. generators of shunt excitation, of series excitation and of compound excitation considered to Ье self-excited?

8. What do the generator properties depend on?

9. What are the magnitudes responsiЫe for а d.c. machine operation оп different loads?

10. Which of the magnitudes is the most important?

11. What relationship is the no-load characteristic defined as? 12. What relationship does the short-circuit characteristic show?

13. What magnitudes are presented in the relationship which the external characteristic makes up?

14. What are the magnitudes displayed in the relationship Ьу which the regulation characteristic is expressed?

Iex

.х

Fig.2. Characteristics of D.C. Generators:

а - no-load and load characteristics; Ь - short-circuit characteristic; с - external characteristics; d - regulation characteristics

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3. APPLICATION OF D.C. GENERATORS AND THEIR PARALLEL

OPERATION

Practically all d.c. generators installed on vessels for supplying aux­ iliary power are of the self-excited type.

Shunt-wound generators are usually used as the exciters of sep­ arately excited generators and have the added feature of charging accu­ mulative batteries. The latter is associated with the fact that with а re­ verse current they aren't remagnetized in view of the unchangeaЫe 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 fтequent over­ loads and short-circuits, 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 а wide­ range voltage regulation is required, that is in the electric drives of steer­ ing 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 neces­ sary that two conditions Ье met: 1) their polarity should Ье the same as the one in the mains to which they are connected; 2) their e.m.f. is required to Ье equal to the mains voltage. The device responsiЫe for fulfilling the indicated conditions is а voltmeter of а magnetic electric system. 1n order to transmit the power fтom one generator operating in parallel to another, the excitation currents of the generator to which the power is transferred, should Ье increased but those of the genera­ tor from which it is collected, should Ье reduced. The generators are required to Ье loaded as uniformly and proportionally to their rated capacities as possiЫe. If this condition is not fulfilled, the efficiency will Ье lower.

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QUESTIONS

1.What type of d.c. geпerators installed on vessels is used for supply­ ing auxiliary power?

2.What purposes are shuпt-wound generators applied for?

3.Why are shunt generators сараЫе of charging accumulative batter­ ies?

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4.Is а series-wound type of d.c. generators employed on board ships? If it isn't, why?

5.Why are compound-wound generators not subject to frequent overloads and short-circuits?

6.Where are compound-wound generators employed?

7.Why are separately-excited generators used in driving steering gears, windlasses, winches etc.?

8.As what are separately-excited generators applied in the electric propulsion plants?

9.What conditions should Ье met to put d. . generators in parallel operation?

1О. What do you call the device which is responsiЫe for fulfilling the conditions necessary for putting d.c. generators in parallel?

11.What is done to the excitation currents to transmit the power from one generator to another?

12.How should generators working in parallel Ье loaded?

EXERCISES

/. Check yourselfin your vocabulary.

What do уои са//:

1.the rotating part of а d.c. machine?

2.the poles which are designed to ensure non-sparking operation of an electric machine?

3.the element of а d.c. machine which is intended to convert altemating e.m.f. induced in the armature into direct one?

4.the element which brings into rotation the annature?

5.the type of d.c. generator the field winding of which obtains its sup­ ply from а separate current source?

6.the stationary part of а d.c. machine?

7.the element of а d.c. machine that is designed for collecting the current from the armature winding and Jeading it to the latter?

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