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Testing Motors and Generators

It is of great importance, therefore, to make regular tests of insulation resistance of all machinery so as to detect possible faults. Different conditions may influence the value of the insulation resistance.

It is advisable, therefore, to make the test of the machine as soon after it has been shut down as possible, when the insulation resistance is likely to be lowest. If, after the motor has just been shut down the insulation resistance is found to be satisfactory, it may be assumed that it will be better at any other time provided that the machine does not stand idle for long in a humid atmosphere.

Faults on electrical machinery must be due to one pf two causes. One is the absence of continuity in the conductor which is supposed to be carrying the current. The other is the absence, or partial absence, of insulation. The latter is by far the more common and the more dangerous of the two. A burnt out armature, for example, is usually due to insulation failure.

A drop in insulation resistance may often be accounted for, for example, by damp weather.

As regards the effect of temperature it should be noted that the insulation resistance of motors and generators is generally lower when they are hot than when they are cold as the insulating varnishes used in the building of the machines have a lower resistance when hot than when cold.

Unit V

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Nuclear Power Plants.

The energy for operating a nuclear power plant comes from the heat released during the fissioning of uranium or plutonium atoms in a nuclear reactor. This fission heat is used to generate steam, which drives a turbine generator. Thus, there are two main differences between a nuclear power plant and a steam-electric power plant: the nuclear plant uses a nuclear fuel instead of a fossil fuel, and it uses a nuclear reactor instead of a boiler.

The fissioning of uranium-235 or plutonium-239 atoms – the primary nuclear fuels – is caused by the impacts of neutrons on these atoms. The fission process not only produces heat but also several additional neutrons that can cause fissioning of other uranium-235 or plutonium-239 atoms. Thus, by proper arrangement of the atoms of fuel, a sustained chain reaction can be maintained to provide a steady source of heat for operating a power plant. This chain reaction is controlled by regulating the number and energy of the neutrons as they proceed from one fission reaction to another.

There are various types of nuclear reactors. The major differences between them are the form of fuel, the methods for controlling the number and energy of the neutrons, and the type of liquid or gas used to remove the heat from the reactor core.

Windscale Accident.

Throughout the years, accidents causing a release of radioactive material to the environment have occurred. Since World War II all over the world large scale nuclear facilities have been built and operated both for civil and defense purposes. Some of the sites on which these facilities were located are heavily contaminated with radioactive substances.

One of first nuclear reactor accidents of environmental concern was the Windscale accident in October 1957. During the accident the military air-cooled graphite-moderated natural-uranium reactor used for plutonium production caught fire during the liberation of energy in the graphite. Emission from the Windscale lasted for 18 hours. Radioactive material was detectable in many parts of Western Europe but the majority of it deposited in the United Kingdom.

Chernobyl Accident.

The accident, which was of global concern was the accident in Ukraine in the Chernobyl power plant located in Polesye on the River Pripyat.

On 26 April, 1986, Unit 4 of the Chernobyl nuclear plant suffered a major accident. The Chernobyl 4 reactor was a graphite-moderated, light-water-cooled system. The installed electrical generating capacity was 1 GW. The accident follower some engineering tests of a generator.

During the tests, basic operating safely rules were being violated. Most control rods were withdrawn from the core and the safely systems were switched off. Two explosions and a fire that followed them damaged the reactor and the containment building. The graphite started to burn. Explosive energy was released, which resulted in the 1000 ton cover plate of the reactor being lifted up.

A prolonged release of large quantities of radioactive products transported by the cloud from Chernobyl was detected not only in northern and southern Europe but also in Canada, Japan, and the US.

The major part of release took place over the period of about ten days. There were two peaks in release rate (26th April and 5th May). Later on, the release continued for many weeks at a lower rate before the destroyed reactor was finally sealed, which took place some five months later.

Initially the cloud of radioactive materials was carried over the Baltic Sea into Scandinavia. After a few days the wind direction rotated clockwise and the cloud travelled eastwards across the USSR and southwards to Turkey.

The total mass of the radioactive particles released in the accident was about 6000-8000 Kg. More than half of it was deposited near the plant but the rest travelled thousands of kilometres (see Fig. 26).

There is no doubt that the nuclear plant accidents offer a number of lessons to be learnt.

At present, over 200 nuclear power reactors for commercial electricity production operate in Europe.

The accident at the Chernobyl nuclear plant has shown that largescale accidents in nuclear power plants can lead to contamination of the entire continent.

 

Vocabulary

to release- высвобождать

instead of- вместо

boiler- котел

to cause- вызывать, быть причиной

additional-дополнительный

proper- соответствующий

to provide- обеспечивать

steady- постоянный

to maintain- поддерживать

chain- цепь

liquid- жидкость

to remove- удалять

core- ядро, центр

environment- окружающая среда

facility-средство

civil- гражданский

to locate- располагать

to contaminate- удалять, устранять

emission-излучение

 

2. Выполните упражнения:

1) Ответьте на вопросы:

а) How long did the emission from the Windscale reactor last?

б) What did the emission result in?

в) What was the cause of the Chernobyl accident?

г) Were does the energy for operating a nuclear plant come from?

д) What are the differences between various of nuclear reactors?

е) How is a chain reaction maintained?

ж) Why do accidents occur?

з) How many atoms are there in uranium?

2) найдите все формы причастия, укажите их и переведите предложения:

1. The energy lost in the capacitor appears in the from of heat being generated in the dielectric.

2. The problem being discussed is of no great importance for practice.

3. The generators constructed at the plant have no commutators.

4. The code widely used is called Morse code.

5. While passing through the conductor, resistance results in the production of heat.

6. Having been insulated with polythene, the line was tested under unfavorable conditions.

7. Having made a number of tests, the researched got some useful results.

8. Having been tested under different conditions, the motors were put to use.

9. When being rubbed, some substances produce electric charges.

10. Decelerating trains and descending elevators use negative, or braking, torque.

11. In what way is the transmitter controlled in an amplitude-modulated system?