Английский язык для моряков / Англ Труханова

Ь) different from those of series motors; с) similar to those of series motors.

а) а slightly in !ined curve;

Ь) а consid raЬ\y inclined curve; с) а straight line.

12.The properties of shunt-compound motors are: а) similar to those of shunt motors;

Ь) absolutely identical to those of shunt motors; с) different from those of shunt motors.

IIL Mindyour Grammar:

Put the verbs in brackets in the correctform - Pre ent Simple Ac­ tive or Present Simple Passive.

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fans, pumps, compressors etc.

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UNIT 5.

TRANSFORMERS

1.PURPOSE, CONSTRUCTION, ТУРЕS

Аtransfonner is an e\ectromagnetic device converting an alter­ nating current of one voltage into an altemating current of another volt­ age but of the same frequency. Although there are many types of trans­ formers and а great variety of different applications, the operating principle is the same in each case.

Аtransfonner (fig.6) is composed ofа \aminated iron core and two separate windings. The core is made ofe\ectrical sheet steel. The wind­

ing, to which the voltage U1 is led, is ca\led the primary winding WI ' the one from which the voltage U2 is collected, is called the secondary winding.

The operating principle ofа transfonner is based upon the phenom­

enon ofmutual induction. Ifthe a\temating current with the voltage U1 is led to the primary winding, the current 11 passing through it will pro­ duce the altemating magnetic tlux Ф in the core which cutting the tums

ofthe primary windings will induce the emfofself-induction Е1 in it, but in the tums of the secondary winding the emf of mutual induction

Е2 will Ье induced. Since the magnetic flux is the same, in each tum of both the primary winding and the secondary winding there will Ье in­ duced one and the same in its magnitude emf - е. Therefore, the emfof the primary winding Е1 = ewI' the one ofthe secondary winding Е2 =

ew2• If we disregard the voltage drop in the windings, emfmay Ье equated with the voltages: И1 ::::: Е1 = ew1 and И2 ::::: Е2 = ew2 • Dividing one equation Ьу another, we oЬtain:

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where k is а transformer ratio, i.e. the ratio of the connected volt­

age to the obtained one or the ratio of the number oftums in the prima­ ry winding to that of the secondary winding.

The currents in the transformer windings are inversely proportion­ al to their voltages.

In construction, transformers are divided into two types - а bar­ type transformer and an ironclad transformer. Their cooling may Ье ei­ ther airy or oily.

On board ships, in accordancewith the work safety rules, only air­ cooled transformers are used.

In the number of phases, transformers are one-phased, two-phased, or multiphased; in the number ofwindings they are either two-circuit or multicircuit. There are also transformers which have windings with spe­ cial tappings.

In application, transformers are classified as power transformers, special purpose transformers and autotransformers.

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1.What kind of electromagnetic device is а transformer?

2.What elements is а transforrner composed ot'?

3.What is the transforrner core made of?

4.What sort of windings does а transformer have?

5.What is the operating principle of а transformer based upon?

6.How are the emfs of self-induction and of mutual induction in-

9. What types of transforrners do you know according to their l)con­ struction, 2)number of phases, З)number of windings, 4)application?

1О. What kind of cooling is used in shipboard transformers?

Fig. 6. Operational diagram of а transformer

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2.THREE-PHASE TRANSFORMERS

Аthree-phase transfonner is usually formed as а group of three one-phase transfonners either staror delta-connected. TherefQre, all processes taking place iл а one-phase transfonner are identica\ to those of а three-phase transformer. The latter is composed of а three-leg core, each leg having both а primary field winding and а secondary field winding of the same phase. The beginning of the field windings of upper voltage is marked Ьу the capitals А,В,С, the ends - Ьу the capitals X,Y,Z. Respectively, the beginning of the field windings oflower voltage is marked Ьу small letters а,Ь,с, the ends - Ьу small letters x,y,z. Standardized field winding connections of three-phase transfonners are the fol\owing (fig.7):

a)douЫe star connection (the second star is the one with а de­

picted central point); Ь)star-delta connection;

c)star-delta connection (the star is the one with а depicted cen­ tral point).

The last column of Fig.7 represents а number of figures (0(12),11,11). Each figure is referred to а group of connections which revea\s at what angle line-to-line lower voltage is shifted c\ockwise with respect to line-to-line upper voltage, e.g. figure 0(12) shows that the shift is equal to 360°, or zero, but figure 11 - the shift equal to 330°, as 30° is supposed to Ье unity.

These shifts make по difference to ал energy consumer. They are only needed for putting transfonners in paral\el operation.

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VOCABULARY

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QUESTIONS

l. What is а three-phase transformer formed as?

2.How are one-phase transformers connected in а three-phase trans­ former assemЫy?

3.Is the construction of а three-phase transformer identical to that of an ordinary transformer? What is it?

4.How is the beginning of the field windings of upper voltage marked? How is the one of the lower voltage marked?

5.What types of standardized connections does а three-phase trans­ former have?

6.What are figures 12(0) and 11 referred to?

7.What are the shifts of voltage mentioned in the text needed for?

3. SPECIAL TRANSFORMERS

Special transformers are availaЫe in three types: l) current mea­ suring transformers, 2) voltage measuring transformers, 3) welding transformers.

Transformers measuring current and voltage are designed to widen measurement limits of ammetres, voltmetres, wattmetres etc. in а.с. cir­ cuits. Above all, they provide operational safety of measuring instru­ ments in а high voltage network.

The rated secondary voltage of voltage transformers is equal to 1OOV, the one of shipboard voltage transformers is 127V, therefore mea­ suring instruments should Ье rated in accordance with this voltage.

The rated secondary current value of current transformers is equal to SA, the one of shipboard transformers should Ье IA. Fuses are not mounted in current transformers' secondary circuits since in case they burn off, the transformer magnetic flux and the emf of the secondary open winding will go up to dangerous values: winding insulation break­ down, overheating of the core and electric shock hazard of an operating personnel become possiЫe.

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lf measuring transformers are connected to а high voltage network, their secondary windings and their bodies are grounded. The value of Фе primary (high) voltage U1 in а measuring circuit is obtained Ьу mul­

tiplying U2 of the secondary winding Ьу tl1e transformer ratio к

и, = И2 К-

Тhе current value 11 in а measuring circuit is equal to the product of 12 of the transformer secondary winding and the current transformer

Welding transformers are applied for arc welding Ьу using an al­ temating one-phase current. Their external characteristic is steeply drooping. The voltage in the secondary winding ranges between 30 - 70V. Тhеу supply the arc across the reactive resistance, i.e. the reactor. The current value may Ье adjusted Ьу varying the air-gap of the reactor magnetic conductor the current value of which may rise to 2,500А high.

Оп board ships only air-cooled transformers are in use. They differ in the following types:

-one-phase dry open transformer,

-one-phase dry waterprooftransformer,

-one-phase dry splashproof transformer,

-three-phase dry waterprooftransformer, -three-phase dry splashproof transformer,

-one-phase dry ironclad waterproof type.

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QUESTIONS

1.What types are special transformers availaЫe in?

2.What are current measuring transformers and voltage measuring transformers designed for?

3.In accordance with what should measuring transformers Ье rated?

4.Why are fuses not mounted in current transformers' secondary cir­ cuits?

5.In what case are the measuring transformers' secondary windings and bodies grounded?

6.How is the value of the primary voltage U1 obtained?

7.What is the current value 11 equal to?

8.What are welding transformers applied for?

9.What range does the voltage in the welding transformer's second­

ary winding vary in?

10. How can the welding transformer's current value Ье adjusted? ll. What types of air-cooled transformers are in use on board ships?

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