Английский язык для моряков / Англ Труханова

8.the poles which are intended to produce the main magnetic flux?

9.the type of d.c. generator the field winding of which is connected in series with the armature winding?

1О. the characteristic ot· а d.c. generator which is defined as the relationship between the generator e.m.f. Е and the excitation current lex with the armature current 1 = О?

11.the type of d.c. generator the field winding of which is connected in parallel with the armature winding?

12.the characteristic of а d.c. generator that shows the relationship between the armature current 15h and the excitation current with the short-circuited armature and the voltage equal to zero?

13.the type of d.c. generator which has two field windings?

14.the characteristic of а d.c. generator which is the relationship between the voltage U and the armature current I with the resistance in the excitation circuit Rex = const?

1S. the power which is for the most part supplied Ьу self-excited generators?

16.the characteristic of а d.c. generator that makes up the relationship between the excitation current and the armature current with the generator voltage U = const?

17.the type of d.c: generator which doesn't require а separate current source for its excitation?

18.the phenomena to which compound-wound generators are not subject, for their series field winding demagnetizes them?

19.the characteristic of а d.c. generator which indicates the relationship between the voltage U and the excitation current lex with 1 = const?

20.the set in which separately-excited generators are used as main generators and exciters?

IJ. Make sureуои've understood the texts given above in а prop­ er way:

1. А d.c. generator is а machine which converts:

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a)electrical energy into mechanical one; Ь) alternating e.m.f. into direct one;

с) mechanical energy into electrical one.

commutator.

7. The most important of the magnitudes responsiЫe for operation of а d.c. machine is:

а) the armature current; Ь) the e.m.f.; с) the voltage.

8.External characteristic of а d.c. generator shows the relationship between:

а) the generator e.m.f. and the excitation current; Ь) the excitation current and the armature current; с) the voltage U and the armature current.

9.Load characteristic of а d.c. generator makes up the relationship

between:

а) the voltage U and the excitation current lex; Ь) the voltage U and the armature current;

Ь) the excitation current and the armature current. 1 О. Series-wound generators are those which:

а) are not used оп board ships; Ь) are used in welders; Ь) are used as exciters of separately-excited generators.

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11. The device responsiЫ e for meeting the conditions of paral\el operation of d.c. generators is:

а) voltmeter; Ь) wattmeter; с) ammeter.

· 12 . Shunt-wound generators are сара Ые of charging accumulative batteries as with а reverse current:

а) they aren't remagnetized; Ь) they are demagnetized; с) they are remagnetized.

I/L Mindyour Grammar.

1. Test yourselfin some ofthe auxi/iaries. Put опе ofthefollow­

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

8). . . separately-excited generators require а separate current

11). . . the generator magnetic circuit saturated under shortcircuit conditions?

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

13)How . the generators operating in parallel required to Ье loaded?

14). . . the commutator constructed of separate copper bars?

15)What relationship . . . short-circuit characteristic show?

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2. Make sure уои are good at building the English interrogative sentence. Риt thefollowing words iп the right order to ask а question.

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UNIT 3.

D.C. MOTORS

1. CONVERTIBILIТY

When а d.c. machine is operating in а generator mode, the con­ ductors of the arrnaturewinding acrosswhich the current is flowing in­ teractwith the magnetic flux of the po\es. As а resu\t, the e\ectromag­ netic force F opposing the arrnature rotation is produced. In order to overcome this force, the extemal force should Ье exerted upon the gen­ erator armature.

lf the extemal force is removed, the polarity of the poles is main­ tained and the current of the same direction is passed through the arma­ turewinding, the e\ectromagnetic forcewill preserve the sarne direction. When acted upon Ьу the e\ectromagnetic force, the arrnaturewill rotate in the direction opposing that of the generator rotation, and hence the d.c. machine will change over to а motor mode. The property Ьу virtue ofwhich any d.c. machine maywork either in а generator mode or in а motor mode is called convertibllity.

2. E.M.F., TORQUE, POWER

When the d.c. motor arrnature is rotating, the e.m.f. the direction ofwhich opposes the current direction is induced in the armaturewind­ ing. This e.m.f.which is called the counter e.m.f. is defined as much the sameway as the generator e.m.f.: Е = с0 n Ф.

The equation for the e.m.f. and voltages of а d.c. motor is the fol­ lowing:

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(1)

where 1, is the aпnature current, R,- the aпnature resistance.

From Equation (1) it follows that the aпnature current and the speed of the armature rotation can Ье obtained from the equations:

where р is the number of pairs of poles; а - the number of pairs of para\lel paths; cm - the design constant of а d.c. machine.

The power of а d.c. motor consumed from the mains is described Ьу the following equation:

where U is the mains voltage; 1 - the current consumed Ьу the motor.

The power of the motor shaft, or the output power, can Ье deter­ mined in the following way:

where М is the motor torque.

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3. STARTING, SPEED REGUATION, REVERSAL

While starting the d.c. motor, its armature is stationary (n = О) and hence the counter e.m.f. Е = О. The starting annature current is many times larger than the rated current, which may Ье explained Ьу а small quantity of the armature resistance R1• Such а current is dangerous for the armature winding and the machine as а 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 ofrotation increases, the counter e.m.f. rises, but the armature cur­ rent I, rapidly decreases. As the armature current decreases, the arma­ ture resistance is reduced. Towards the end ofstarting, the starting rheo­ stat should Ье completely withdrawn.

There exist three methods of d.c. motor speed regulation:

1)Ьу varying the connected voltage U, owing to which stepless reg­ ulation сап Ье accomplished (the method is used in adjustaЫe-potential systems);

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

3)Ьу varying the magnetic flux Ф; the regulation is stepless, wide­ ranged; it is attained Ьу changing the field current Ьу means of an ad­ justing rheostat placed in the field circuit.

In order to alter the direction of the motor rotation, it is neces­ sary 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.

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QUESTIONS.

1.How is the electromagnetic force produced?

2.What should Ье done to overcome the e\ectromagnetic force?

3.What are the conditions under which the e\ectromagnetic force preserves the same direction?

4.How will а d.c. machine change over to а motor mode?

5.What kind of property is ca\led convertibllity?

6.HO\V 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 а whole?

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

1О. What should Ье 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 regu\ation?

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

14.What is to Ье done to a\ter the direction of the motor rotation?

15.Does the change in the current direction of the armature or of the field winding арр\у in practice?

EXERCISES

1.Checkyourselfin your vocabulary. What do уои са//:

1.the e.m.f. having the direction opposing the current direction and induced when the armature is rotating?

2.the event which happens to а d.c. motor when the current direction either in the armature or in the field winding a\ters?

3.the property of а d.c. machine Ьу virtue of which it may work in either а generator mode or in а motor mode?

4.the power on the motor shaft which is designated Ьу Р2?

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5.the speed regulation which is accomplished Ьу varying the magnet­ ic flux Ф?

6.the device which is placed in series with the armature circuit in order to limit the starting current?

7.the method of d.c. motor speed regulation which is used in adjust­ aЬ\e-potential systems?

8.the device which is put in series with the armature circuit in order to obtain step-by-step speed regu\ation?

IL Makesureуои have properly understood the textsgiven above:

1.То overcome the electromagnetic force, it is necessary that the externa\ force:

а) Ье removed;

Ь) Ье exerted upon the generator armature;

с) preserve the same direction.

2.As the speed of rotation increases, the counter e.m.f. а) remains the same;

Ь) rises;

с) decreases.

3.The d.c. motor reversal will never take place if:

а) the current direction in the armature changes;

Ь) the current direction in both the armature and the field winding changes;

с) the current direction in the field winding changes. 4. The starting armature current is:

а) many times \ess then the rated current; Ь) as large as the rated current;

с) many times larger than the rated current.

5. А d.c. machine will change over to а motor mode if:

а) the armature is rotating in the direction opposing that of the generator rotation;

Ь) the armature is stationary;

с) the conductors of the armature winding across which the current is flowing interact with the magnetic flux of the poles.

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