Английский язык для моряков / Труханова Англ для моряков

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

autotransformers.

VOCABULARY

1.What kind of electromagnetic device is a transformer?

2.What elements is a transformer composed of?

3.What is the transformer core made of?

4.What sort of windings does a transformer have?

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

6.How are the emfs of self-induction and of mutual induction induced in the transformer windings?

7.How is the transformer ratio obtained?

8.In what proportion are the transformer currents to the transformer voltages?

9.What types of transformers do you know according to their 1)construction, 2)number of phases, 3)number of windings, 4)application?

10.What kind of cooling is used in shipboard transformers?

2. THREE-PHASE TRANSFORMERS

A three-phase transformer is usually formed as a group of three one-phase transformers either staror

delta-connected. Therefore, all processes taking place in a one-phase transformer are identical to those of a three-phase transformer. The latter is composed of a three-leg core, each leg having both a primary field winding and a secondary Held winding of the same phase. The beginning of the field windings of upper voltage is marked by the capitals A,B,C, the ends - by the capitals X,Y,Z. Respectively, the beginning of the field windings of lower voltage is marked by small letters a,b,c, the ends - by small letters x,y,z. Standardized field winding connections of three-phase transformers are the following (fig.7):

The last column of Fig.7 represents a number of figures (0(12),11,11). Each figure is referred to a group of connections which reveals at what angle line-to-line lower voltage is shifted clockwise 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 be unity.

These shifts make no difference to an energy consumer. They are only needed for putting transformers in parallel operation.

Fig.7. Diagram of field winding connections of a three-phase transformer.

QUESTIONS

1) What is a three-phase transformer formed as?

2)How are one-phase transformers connected in a three-phase transformer assembly?

3)Is the construction of a 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 a three-phase transformer 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 available in three types: 1) current measuring 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 A.C. circuits. Above all, they provide operational safety of measuring instruments in a high voltage network.

The rated secondary voltage of voltage transformers is equal to 100V, the one of shipboard voltage transformers is 127V, therefore measuring instruments should be rated in accordance with this voltage.

The rated secondary current value of current transformers is equal to 5 A, the one of shipboard transformers should be 1A. 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 breakdown, overheating of the core and electric shock hazard of an operating personnel become possible.

If measuring transformers are connected to a high voltage network their secondary windings and their bodies are grounded. The value the primary (high) voltage U] in a measuring circuit is obtained by multiplying U2 of the secondary winding by the transformer ratio

The current value I1 in a measuring circuit is equal to the product of I2 of the transformer secondary winding and the current transform ratio:

Welding transformers are applied for arc welding by using an alternating one-phase current. Their external characteristic is steeply drooping. The voltage in the secondary winding ranges between 30 70V.

They supply the arc across the reactive resistance, i.e. the reactor The current value may be adjusted by varying the air-gap of the reactor magnetic conductor the current value of which may rise to 2,500A high|

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

-one-phase dry open transformer,

-one-phase dry waterproof transformer,

-one-phase dry splashproof transformer,

-three-phase dry waterproof transformer,

-three-phase dry splashproof transformer,

-one-phase dry ironclad waterproof type.

VOCABULARY

QUESTIONS

1.What types are special transformers available in?

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

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

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

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

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

7.What is the current value Ij equal to?

8.What are welding transformers applied for?

9.What range does the voltage in the welding transformer's secondary winding vary in?

10.How can the welding transformer's current value be adjusted?

11.What types of air-cooled transformers are in use on board ships?

4. AUTOTRANSFORMERS

The primary and the secondary windings of ordinary transformers interact electromagnetically. Autotransformers only have one winding which is primary and secondary at the same time. This winding is arranged on a closed magnetic conductor; therefore , the conventional primary and secondary windings of an autotransformer are not only connected electromagnetically, but also electrically. The common part of an autotransformer winding is wound by the wire of smaller section, which makes it possible to save the mass and the volume of the copper expended and, therefore, to reduce the steel mass. When the steel mass is decreased,' the magnetic losses are decreased too; this being so, autotransformers are cheaper, but their efficiency level is higher in comparison with that of ordinary transformers of the same power.

The ratio between voltages and currents in an autotransformer is identical to that of an ordinary transformer:

Autotransformers are used for starting a.c. electric motors, regulating light, etc. Their use is prohibited, however, in low voltage portable lighting because of the electrical type of connection between the windings of lower and upper voltage, which can lead to the penetration of high voltage in a low voltage network and cause injury of an attending personnel.

Autotransformers may be of a step-down and step-up type, onephased and multiphased. If the autotransformer winding possesses a large number of tappings, the secondary voltage Ux may be adjusted in the range between zero and the mains voltage U2 .

The disadvantage of autotransformers is the electrical type of connection between the secondary and

primary circuits, which makes their field of application narrower.

VOCABULARY

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 connected to each other?

5.What makes it possible to save the mass and the volume of the copper 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 can lead to the penetration of high voltage in a low voltage network and injure the attending personnel?

9.What types of autotransformers do you know?

10.What is the disadvantage of autotransformers?

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

8)The penetration of high voltage in a low voltage network . injury of an attending personnel.

9)The welding transformers' external characteristic ... steeply drooping.

2. Make one sentence from two using an -ing clause.

1)A transformer is an electromagnetic device. It converts an alternating current of one voltage into an alternating current of another voltage.

2)The current produces the alternating magnetic flux Ф in the core. It passes through the primary winding.

3)We obtain the transformer ratio. We divide one equation by another.

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

5)We obtain the value of the primary voltage Ul in a measuring circuit. We multiply U2 of the secondary

winding by the transformer ratio к.

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

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

UNIT 6.

MARINE A.C. MACHINES AND SPECIAL PURPOSE

MACHINES

1. SYNCHRONOUS ALTERNATORS

This term refers to a 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

The standard frequency

Synchronous alternators of small power are sometimes built like D.C. generators equipped with stationary poles and a rotating armature. The difference is that not the commutator but the armature winding is connected to the slip-rings causing its alternating current to flow into the external circuit.

Taking into account that it is difficult to collect large power by means of sliding contacts, synchronous alternators are usually provided with rotating poles (the rotor) and a stationary armature (the stator).1 The power of such alternators is practically unlimited (1,400,000 kVA).

The stator is found in a steel frame with the core mounted inside and assembled 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-