Английский язык для моряков / Англ Труханова

6. Towards the end of starting of а d.c. motor the starting rheostat should Ье:

а) completely withdrawn;

Ь) placed in series with the armature circuit; с) placed in parallel with the armature circuit.

1/L Mindyour Grammar.

1. Notice the use ofthe Future Simple ("will") in the main c/ause. Ореп the brackets using the correct verbform. Trans/ate the sentences.

1)_If the external force is removed, the electromagnetic force . . .

(preserve) the same direction.

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

3)lf the armature rotates in the direction opposing that of the genera­ tor rotation, the d.c. machine . . . (change over) to а motor mode.

in the field winding, the reversal . . . (not take place).

6)As soon as we place а 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).

2.Complete the sentences using опе ofthese verbs in Present Sim­ p/e Passive.

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1)Тhе electromagnetic force F 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 extemal 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)А step-by-step speed regu\ation . . . Ьу putting an adjusting rheo­

stat in series with the armature circuit.

8) lf the magnetic flux . . . , а stepless wide-range regulation . . . .

9) Towards the end of starting the starting rheostat . . . completely

. . . from the armature circuit.

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UNIT 4.

ТУРЕS OF D.C. MOTORS AND THEIR

DISTINCTIVE FEATURES

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1. SHUNT-WOUND MOTORS

This term is referred to а particular type of d.c. motor having the field winding connected in parallel with the armature winding. Fig. 3, а illustrates the diagram of а shunt-wound motor, on which А stands for the armature, ShW - for the shunt winding, SR - for the starting rheo­ stat and Rrт - for the regulating rheostat. This type of motor exhiЬits the following properties:

1. With the load incre se from zero to its rated value the speed of rotation is altered moderately - Ьу 1 - 5% of its rated value; predomi­ nantly at the expense of the voltage drop in the armature. Consequent­ ly, the speed characteristic curve (fig.3, Ь) is "rigid", i.e., slightly in­ clined:

With the change in the Ioad the speed of rotation of а shunt-wound motor is kept constant. This can Ье explained Ьу the fact that the field current and hence the flux remain unchangeaЫe.

2.Тhе motor may Ье started idle. Stepless speed regulation within wide limits is accomplished Ьу putting both а regulating rheostat in the field current and an additional resistor in the armature circuit.

3.The starting torque of the motor is rather small, directly propor­ tional to the armature current; this being so, the current consumed from

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the mains is proportional to the load on the shaft. The motor torque char­

Ву virtue of the fact that а shunt-wound motor features а small starting torque, а "rigid" speed characteristic curve and а stepless speed regulation, it fmds application in electric drives of fans, pumps, com­ pressors and other mechanisms.

Flg. 3. The diagram of а shunt-wound motor:

а - the dlagram; Ь, с - performance characteristlcs

2.SERIES-WOUND MOTORS

Аseries-wound motor possesses а field winding connected in series with the armature circuit (fig.4, а).

Аseries-wound motor manifests the following properties:

1. While running idle or at load which is 20 - 25% less than the rated one, the motor tends to overspeed. The reason is that at light loads the current value both in the armature circuit and in the field winding is small and, therefore, the flux and the voltage drop in the field wind­ ing are both moderate and, as а consequence, the speed of rotation is significant:

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И -Ja(Ra + Rsw) n= сФ

With the load increase the current value in the field winding and thus the flux rise. Moreover, the intemal voltage drop grows and in con­ sequence the speed ofrotation abruptly drops off. The motor speed char­ acteristic makes up а "mild" curve, i.e., consideraЫy inclined. The mo­ tor is only started under Ioad.

2. The starting torque ofа series motor is large, proportional to the square value of the annature current (fig. 4, с):

М=к.1 102

3. The regulation ofthe speed ofrotation is accomplished Ьу sev­ eral methods, but the one which is economically attтactive is performed Ьу shunting the field winding, i.e., placing an adjusting rheostat in its parallel. As а result, the speed of rotation shows an increase when com­ pared to the rated one.

А series-wound motor is used where а large starting torque is re­ quired. They driver haulage mechanisms (starters, turning gears etc.)

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Fig.4. The diagram of а series-wound motor: а - diagram; Ь, с - performance characterlstlcs

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З. COMPOUND-WOUND MOTORS

This tenn relates to а specific type of d.c. motor which has two field windings: the shunt one and the series one (fig.?). According to the method of the field winding connection the above mentioned mo­ tors may Ье either differentially compounded or cumulatively compound­ ed. А differential compound motor has the fluxes directed in opposi­ tion: Ф = Фslnv - Фsiv . lt features а small starting torque and, therefore, hasn't found application оп shipboard. А cumulative compound mo­

tor has the fluxes that add up: Ф = Фshw + Фsiv. This type of d.c. mo­ tor is subclassified into series-compounded and shunt-compounded.

Series-compound motors are those provided with а predominant series field winding and an auxiliary shunt field winding. ln properties, they are quite similar to series-wound motors which manifest а consid­ eraЫe starting torque. Nevertheless, series-compound motors while run­ ning idle don't tend to overspeed because of their shunt field winding. They· are used for driving windlasses, capstans, cranes etc.

Shunt-compound motors are those which have the shunt field winding as predominant and the series field winding as auxiliary. ln prop­ erties, they are similar to shun-wound motors, but exhiblt а consider­ aЫe starting torque. Shunt-compound motors are in use at electric drives

Fig. 5. The diagram of а compound-wound motor:

а- the diagram; Ь, с - performance characteristics, where ShC - those of

аshunt-compound motor, SC - those of а series-compound motor

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QUESTIONS

1.What is the term of "shunt-wound motor" referred to?

2.How is the speed of rotation of а shunt-wound motor altered with

the load increase?

3. What can the. speed characteristic curve of а shunt-wound motor Ье described as?

4.Is it possiЫe to start а shunt-wound motor idle?

5.How is stepless speed regulation of а shunt-wound motor accom­ plished?

6.What can you say about the starting torque of а shunt-wound motor?

7.What does the shunt motor torque characteristic make?

8.Where exactly on board ship do shunt motors find application?

9.What do you call "series-wound motor"?

1О. Why does а series-wound motor tend to overspeed?

11.What can the speed characteristic curve of а series motor Ье described as?

12.What can you say about the starting torque of а series-wound motor?

13.What is shunting? ls shunting economically attractive?

14.What mechanism do, series-motors drive?

15.What do you call "compound-wound motor"?

16.What is the difference between а cumulative compound motor and а differential compound motor?

17.What type of compound motor is called "series-compound motor"? Where is it used?

18.What type of compound motor is called "shunt-compound motor"? Where is it used?

EXERCISES

L Check yourselfin your vocabulary.

What do you call:

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1.the type of d.c. motor which possesses а field winding connected in series with the armature circuit?

2.the shunt-wound motor characteristic which makes а "rigid", slightly inclined curve?

3.the method of speed regulation which is performed Ьу placing an adjusting rheostat in parallel with the field winding?

4.the type of d.c. motor which has the field winding connected in par­ allel with the armature winding?

5.the shunt-wound motor characteristic which makes а straight line?

6.the property which а series motor manifests while running idle or at load that is 20 - 25% less than the rated one?

7.the type of motor which is only started under load?

8.the series motor characteristic which makes а "mild", consideraЫy inclined curve?

9.the type of d.c. motor which has two field windings?

1О. the type of compound motor which hasn't found application on board ship?

11.the type of compound motor which features а small starting torque?

12.the type of compound motor which is provided with а predominant series field winding and an auxiliary shunt-field winding?

13.the type of d.c. motor which while running idle doesn't tend to overspeed?

14.the type of compound motor having а predominant shunt field winding and an auxiliary series field winding?

15.the type of compound motor which has the fluxes that add up?

II.Make sure уои 've properly realized the contents ofthe texts given above.

1.А shunt-wound motor is the one which has:

а) two field windings;

Ь) one field winding connected in parallel with the armature winding;

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с) one field winding connected in series with the arrnature winding.

2.Stepless speed regulation within wide limits is accomplished Ьу put­ ting:

а) а regulating rheostat in the armature circuit; Ь) an additional resistor in the armature circuit;

с) both а regulating rheostat and an additional resistor in the armature circuit.

3.The starting torque of а shunt-wound motor is:

а) small, directly proportional to the armature current;

Ь) large, proportional to the square value of the armature current; с) consideraЫe, inversely proportional to the armature current.

4. А series-wound motor is the one which:

а) doesn't tend to overspeed while running idle;

Ь) doesn't tend to overspeed while running at load which is 20 - 25% \ess than the rated one;

с) tends to overspeed while running idle or at load which is 20 - 25% less than the rated one.

5.The speed characteristic of а series-wound motor makes: а) а slightly inclined curve;

Ь) а consideraЬly inclined curve; с) а straight line.

6.А series-wound motor is used to drive:

а) starters, tuming gears etc.;

Ь) windlasses, capstans, cranes etc.; с) pumps, compressors, fans etc.

7.А differential compound motor has the fluxes which: а) add up;

Ь) are directed in opposition; с) remain constant.

8.The shunt motor torque characteristic makes:

а) а slightly inclined curve;

Ь) а consideraЫy inclined curve;

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