Английский язык для моряков / Англ Труханова

4.AUTOTRANSFORMERS

The primary and the secondary windings of ordinary transforrn­ ers interact e\ectromagnetically. Autotransforrners only have one wind­ ing which is primary and secondary at the same time. This winding is arra ged оп а c\osed magnetic conductor; therefore , the conventional primary and secondary windings of ai1 autotransforrner are not on\y con­ nected e\ectromagnetically, but also e\ectrically. The common part of an autotransforrner winding is wound Ьу the wire of smaller section, which makes it possiЬ\e to save the mass and the volume of the copper expended and, therefore, to reduce the stee\ mass. When the steel mass is decreased, the magnetic losses are decreased too; this being so, autotransforrners are cheaper, but their efficiency \evel is higher in comparison with that of ordinary transforrners of the sarne power.

The ratio between voltages and currents in an autotransforrner is identica\ to that of an ordinary transforrner:

Autotransforrners are used for starting а.с. e\ectric motors, regu­ \ating light, etc. Their use is prohiЬited, however, in low voltage porta­ Ыe lightingbecause of the electrical type of connection between the wind­ ings of lower and upper voltage, which сап \ead to the penetration of high voltage in а low voltage network and cause injury of an attending personne\.

Autotransformers may Ье of а step-down and step-up type, one­ phased and multiphased. If the autotransforrner winding possesses а \arge number of tappings, the secondary voltage U1 may Ье adjusted in the range between zero and the mains voltage U2 •

The disadvantage of autotransforrners is the e\ectrical type of con­ nection between the secondary and primary circuits, which makes their field of application narrower.

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QUESTIONS

1. How many windings does an autotransformer have?

2. Is the autotransformer winding primary and secondary at the same time?

3. Where is the autotransformer winding arranged?

4. In what way are the conventional autotransformer windings con­ nected to each other?

5. What makes it possiЫe to save the mass and the volume of the cop­ per expended in autotransformers?

6. Why are autotransformers cheaper but their efficiency level is higher than that of ordinary transformers of the same power?

7. Where are autotransformers used?

8. What сап lead to the penetration of high voltage in а low voltage network and injure the attending personnel?

9. What types of autotransformers do you know? 1О. What is the disadvantage of autotransformers?

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EXERCISES

I. Check yourselfin your vocabulary.

What do you call:

1.the transformer element which is usually made of electrical sheet steel?

2.the phenomenon upon which the transformer operating principle is based?

3.the ratio of the number of tums in the transformer primary winding to that of the transfonner secondary winding?

4.the transfonners possessing airy cooling?

5.the transfonners having а large number of windings?

6.the core which has three legs?

7.the type of the winding connection which is realized Ьу two stars?

8.the type of the winding connection which is realized Ьу both а star and а triangle?

9.the transformers having an electrical type of connection between the windings of lower and upper voltage?

1О. the type of lighting in which the use of autotransformers is prohibited?

11. the losses which are decreased when the steel mass is decreased?

12.the transformers that provide operational safety of measuring instruments?

13.the phenomenon of breaking the winding insulation?

14.the things related to а measuring transformer which are usually grounded when the latter is connected to а high voltage network?

15.the type of resistance across which welding transformers supply the arc?

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II. Test yourselfhow wellуои have understood "the senses" of the texts dealing with transformers.

1. А transformer is а device which converts:

а) mechanical energy into electrical energy; Ь) altemating current into direct current;

с) the altemating current of one voltage into the altemating current of another voltage but of the same frequency.

2.The operating principle of transformers is based upon the phe­ nomenon of:

а) self-induction; Ь) mutual induction; с) electric induction.

3.The ends of the three-phase transformer windings of upper voltage are marked Ьу:

а) the capita\s А,В,С; Ь) the capita\s X,Y,Z; с) the small \etters а,Ь,с.

4.An autotransformer possesses:

а) one winding which is primary and secondary at the same time; Ь) two windings;

с) many windings.

5.The efficiency of an autotransformer in comparison with that of an ordinary transformer of the same power is:

а) higher; Ь) \ower;

с) absolutely the same.

6.The rated secondary voltage of shipboard voltage measuring instru­ ments is:

а) 120 V; Ь) 147 V; с) 127 V.

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7.The rated secondary current of shipboard current measuring trans­ formers is:

а) lA; Ь) 5А;

с) ЗА.

8.Welding transformers are used for: а) contact welding;

Ь) arc-welding;

с) electron-beam welding.

IIL Mindyour Grammar:

1. Complete the sentences with the correct form of Present Sim­ ple Active. Choosefrom the verbs in the Ьох. Use each verb опсе оп/у.

2)Current and voltage measuring transformers . . . operational safety of ammetres, voltmetres, wattmetres etc.

3)The winding insulation breakdown and the overheating of the core

... possiЬ\e.

4)The voltage in the secondary windings . . . between 30 and

70 V.

5)Welding transformers . . . the arc across the reactor.

6)Autotransformers only . . . one winding.

7)The transformer primary and secondary windings . . . electro­ magnetically.

8)The penetration of high voltage in а low voltage network . . .

injury of an attending personnel.

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2. Make опе sentence from two using ап -ing clause.

1)А transformer is an e\ectromagnetic device. It converts an altemat­ ing current of one voltage into an a\temating current of another volt­ age.

2)The current 11 produces the a\ternating magnetic flux Ф in the core. It passes through the primary winding.

3)We obtain the transformer ratio. We divide one equation Ьу anoth­ er.

4)The altemating magnetic flux Ф in the core induces the emf of self-induction. It cuts the tums of the primary winding.

5)We obtain the value of the primary voltage U1 in а measuring cir­ cuit. We multiply И2 of the secondary winding Ьу the transformer ratio к.

6)An autotransformer is а device. It has one winding - the primary and the secondary at the same time.

7)The transformer magnetic flux and the ernf of the secondary open winding go up to dangerous values. They cause the insulation break­ down of the winding and the overheating of the core.

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UNIT 6.

МARINE А.С. МACHINES AND SPECIAL

PURPOSE МACHINES

1. SYNCHRONOUS ALTERNATORS

This term refers to а: particular type of generator the number of revolutions of which with standard frequency is coordinated with the number of pairs of poles. The synchronous number of revolutions

п1 = 60-. Тhе standard frequency f= 50 Нz.

р

Synchronous altemators of small power are sometimes built like d.c. genetators equipped with stationary poles and а rotating armature. The difference is that not the commutator but the armature winding is connected to the slip-rings causing its altemating current to flow into the extemal circuit.

Taking into account that it is difficult to collect large power Ьу means of sliding contacts, synchronous alternators are usually provid­ ed with rotating poles (the rotor) and а stationary armature (the stator). The power ofsuch altemators is practically unlimited (1,400,000 kVA).

The stator is found in а steel frame with the core mounted inside and assemЫed of 0,35 - 0,5 mm electrical steel laminations. The stator winding is laid in the core slots, its leads being brought out to the ter­ minal Ьох. Synchronous altemators may Ье either one-phased or three­ phased. А three-phase altemator has the stator windings either staror delta-connected in the terminal Ьох.

Тhе greater part of shipboard altemators operates at the speed of

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rotation ranging from 500 to 1500 rev/min and has the rotor equipped with salient poles.

The rotor is а steel shaft оп which the cores of the poles with mounted оп them excitation windings are fixed, their 'supply being ob­ tained through slip-rings and brushes. The source of direct current is usually а shunt-wound generator (the exciter) which is mounted with а synchronous altemator оп the same shaft.

The principal characteristics for synchronous altemators are the following:

No-load characteristic, which is the relationship between the al­ temator emf and the excitation current Е = f(I •.) with the speed ofrota­ tion n = О. This characteristic makes it possiЫe to judge the degree of steel saturation.

External characteristic U = f(I) which detects the relationship be­ tween the altemator voltage and the load current with the speed of rota­ tion n = const, the excitation current Iex = const and the power factor cos {О =const. The extemal characteristics show the change of the volt­ age when the load current value and character are varied.

Regulation characteristic 1•• = f(I) which reveals the relationship between the excitation current and the load current with the altemator voltage U = const, the speed of rotation n = const, and the power factor cos? = const. The regulation characteristics indicate in what way the excitation should Ье varied with the alteration of the load current value and character in order to keep the voltage at the altemator terrni­ nals unchangeaЫe.

Synchronous altemators in marine application are those with sil­ icon rectifiers in the excitation system, those with static excitation sys­ tem, brushless alternators etc. Self-excited brushless alternators are the most widespread because they do not have either а commutator, slip-rings or brushes, which makes them more reliaЫe and easier to operate.

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VOCAВULARY

QUESTIONS

1.What does the teпn of "synchronous alternator" refer to?

2.What is the equation Ьу means of which the synchronous number of revolutions is expressed?

3.What is the standard frequency for synchronous alternators equal to?

4.What are synchronous alternators equipped with when they are built like d.c. generators?

5.Is there any difference between а synchronous alternator of small power and а d.c. generator?

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6.What are the main constructive elements of synchronous alterna­ tors of large power?

7.How is the stator of а synchronous alternator of large power con­ structed ?

8.What types of synchronous altemators сап Ье distinguished accord-

ing to their number of phases?

9. How is the rotor of shipboard altemators constructed?

1О. What are the principal characteristics for synchronous alternators?

11.What magnitudes are in relation to each other when the no-load characteristic is considered?

12.What sort of relationship does the extemal characteristic detect?

13.What sort of relationship does the regulation characteristic reveal?

14.What types of synchronous altemators in marine application do you know?

15.Which of synchronous altemators in marine application are the most widespread on board ships and why?

2.INDUCTION (ASYNCHRONOUS) MOTORS.

АTHREE-PHASE CAGE MOTOR

Induction motors are simple in construction, economical and re­ liaЫe in operation, owing to which they are widely used in all indus­ tries. On board ships they are afforded to drive various machines, mech­ anisms and devices; they make 80-90% ofthe total number ofthe electric motors installed.

Induction motors are the machines the rotor ofwhich rotates asyn­ chronously with the magnetic field. The rotor speed ofrotation varies with the change in the load on the shaft ofthe asynchronous motor. The ratio between the speed ofrotation ofthe magnetic field n1 and the rotor speed ofrotation n2 is defmed Ьу the slip S = (n1 -n2)/n1 •

The construction of induction motors (fig.8 ) provides for two main parts: the stator which is the stationary part and the rotor which is the moving part. The stator is а steel пате 1 with the core 2 mounted

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