电气电子毕业设计161河南理工大学发电厂电气部分设计
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电气电子毕业设计161河南理工大学发电厂电气部分设计,毕业设计论文
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河南理工大学毕业设计（论文） 1 附录 1. Energy conversion and conservation The conversion of mechanical energy to heat is by no means new to us. We are also familiar with other transformations of energy. Chemical energy is converted into heat when fuel burns. Electrical energy is transformed into heat and light in electrical lamps and electrical stoves. Radiant energy turns into heat when sunlight strikes an object which absorbs it. “All contradictory things are interconnected; not only do they coexist in a single entity in given conditions, but in other given conditions, they also transform themselves into each other.” In a word, all energies maybe converted from one form to another and what is more, they all can transform into heat by themselves. Heat is an energy of irregular motion of particles in a substance, at ordinary temperature it is less unable than any of the other energies. However, at high temperatures heat energy may be converted into energy of more usable forms. Some people have made different kinds of machines to convert heat into mechanical energy. Diesel and gasoline engines are designed to convert heat that is developed by the burning of fuel into mechanical energy for running tractors, trucks, and cars. The mechanical energy transformed from heat in a steam turbine is made to operate generators. And the generators, in turn, convert the mechanical energy. All these transformations are taking place every minute and everywhere in our daily life and production. In any energy transformation, there is some loss, but no energy is destroyed. The part that is lost is simply wasted. If all of the energies nts河南理工大学毕业设计（论文） 2 that are wasted were added to that used, the total would be found to be equal to the total supplied. The form may be changed, but the amount remains unchanged. The fact that energy can be changed from one form to another, but can neither be created nor destroyed, constitutes one of the most important laws in science, the law of conservation of energy. No one form of energy can be long conserved, but the total is conserved at any time. A machine may be designed to lift a much larger weight than the force that is applied, but it can never produce more work than was supplied to it. In other words, a machine cannot have an efficiency greater than one. Since man cannot create or destroyed energy, he must use the energy that is available to him. Some devices were designed for the purpose of doing work without the need of supplying energy. These are the so-called perpetual-motion machines. We say that such machines are impossible because they violate the law of conservation of energy. The attempt has never been successful. And it will never be successful. 2. Generator and electricity (1) Faraday and his Generator The electric current in our homes is produced in power stations which usually contain several generators. These are machines which generate electric current when they are turned. So there has to be some kind of engine to turn them. What kind of engine can we use? Steam engines are suitable, and so are oil engines. Sometimes the water of a great river can turn the generators, and so power stations are often built near dams. The water which is stored behind a dam flows out with great force nts河南理工大学毕业设计（论文） 3 when it is allowed to do so. We can use this force to turn machines which are called turbines. The water is led through big pipes to the turbines, and then they turn the generators. These supply the country with useful current. Michael Faraday (1791-1867) made the first generator. He was a great scientist. He studied gases and changed some of them into liquids. He made many discoveries in chemistry and electricity. Before his time scientists got their electric current from electric cells. Several cells together form a battery. An Italian, Volta, made the first battery and it produced a small current. Modern cells are boxes which contain acids and other materials such as metals or carbon rods. Faraday knew about Voltas work, but he wanted to produce an electric current by using magnets. An electric current which flows through a coil of wire round an iron rod produces magnetism in it. Faraday wanted to do the opposite: he wanted to produce a current in a wire by using magnetism. He tried to do this for a long time, but he failed completely until he moved a wire near the magnets. Then his instruments showed that a small current was flowing in the wire. Either the magnet or the wire had to move. He made a small machine to turn a coil of wire near the magnets, and this generated a current. It was the first generator in the world. All modern generators depend on Faradays work. The magnets in them are usually electromagnets; even in an electromagnet a little magnetism remains in the iron after the current is switched off. As soon as the generator turns, a small current appears. This increases the magnetism, and so the current increases. This again increases the nts河南理工大学毕业设计（论文） 4 magnetism, and so on. In a few seconds there is quite a big current flowing in the wires. If a river turns the turbines, it does all the necessary work, and no fuel is needed. Those countries which have big and powerful rivers are lucky because they can get a lot of electric power from them. (2) Direct and Alternating Currents A direct current is, of course, useful. The electric system in a car uses the direct current. Besides, direct current is also used to meet some of he industrial requirements. However, at present, most cities make use of another kind of electric current going first in one direction and then in another, we give it the name of an alternating current. In spite of its being very useful a direct current system has one great disadvantage; namely, there is no easy, economical way in which one can increase or decrease its voltage. The alternating current does not have this disadvantage, its voltage may be increased or decreased with little energy loss by the use of a transformer. Using a transformer it is possible to transform power at low voltage into power at high voltage, and vice versa. In that manner, current can be generated at a voltage which is suitable for any given machine. In large power stations, the best suited voltage is often 6,300 or 10,500 V. Power being transmitted over long distances with less loss at high voltage than at low voltage, it is more economical to increase the voltage to 35,000 or 110,000 or even 220,000 V for transmission. Wherever the power is to be used, it is lowered to the voltage which satisfies that particular purpose, such as nts河南理工大学毕业设计（论文） 5 220 V in homes, or 380 V in factories, etc. (3) Voltage and Current All metals are good conductors because there is a great number of free electrons in them. These free electrons usually do not move in a regular way so that there is no current. However, when an electric field is set up, all the free electrons will be made to move in one direction. And an electric current is formed. Or to say, in order that an electric current can be produced in a conductor, an electric field must be built in it. An electric field is usually set up by applying a voltage between the two terminals of the conductor. Thus, the free electrons form an electric current in the conductor. There are two kinds of electric currents: direct current (D.C.) and alternating current (A.C.). Direct current is an electric current the charges of which move in one direction only. It is constant in value, unless the circuit conditions, such as the applied voltage or circuit resistance are changed. The changes of an alternating current change their direction regularly. First they flow one way, then the other. The difference between A.C. and D.C. depends upon the voltage applied. If the electric field applied is unchanged, the current produced is D.C. If the electric field applied is alternating, the current produced is A.C. Both A.C. and D.C. have their advantages and disadvantages and they are respectively used in different applications. Electric power is made at power stations, but it is usually needed far away. How is the current taken to far-off places? Thick wires usually carry it across the country, and steel pylons hold the wires above the ground. The pylons are so high that nobody can nts河南理工大学毕业设计（论文） 6 touch the wires at the top. The wires are not usually copper wires; they are made of aluminum, and thirty wires together form one thick cable. Aluminum is so light that the pylons can easily hold the cables up. It would not he cheap to drive very large currents through these cables. Large currents need very thick wires. If thin wires are used, they get hot or melt, and so the currents ought to be as small as possible. Can we send a lot of power if we use a small current? We can do so if the voltage is high. We need a small current and a high voltage; or a large current with a low voltage. The small current is cheaper because the wires need not be thick. The result is that the voltage has to be very high. The pressure in the aluminum cables may be 132,000 volts, and this is terribly high. The voltage of a small battery is usually between 1 and 9 volts. The is the kind of battery which we carry about in our pockets. A car battery has a voltage of 6 or 12 volts. In a house the pressure in the wires may be 230 volts, or something like that. Even 230 volts is high enough to kill a person, so what would happen if we touched one of the aluminum cables? The high voltage would drive a heavy current through our bodies to the earth. The wires are placed high up so that nobody can touch them. When they lead down to a house or a railway, the voltage is made lower. It can be changed easily; but if the voltage is lower, the current must be higher. If it is not, we will lose power. So the wires have to be thicker. The wires must never tough the steel pylons. If they did that, the current would escape to the earth through the steel. Steel is a good conductor of electricity, and so are most metals. We have to separate nts河南理工大学毕业设计（论文） 7 the wires from the pylons, and we do this with insulators. An insulator does not allow an electric current to flow through it; but a conductor lets it flow easily. Paper, air and glass are examples of good insulators. Another is porcelain. Porcelain is such a good insulator that it is widely used, and the aluminum cables hang down from the pylons on several separated porcelain insulators. Parts of these have to keep dry even when it rains, because water is a good conductor. So the insulators have a special shape and the rain cannot reach all parts of them. (4) Resistance Resistance is the opposition to the flow of electrons. The greater the resistance of a wire is, the less electric current will pass through it under the same voltage. The resistance of a wire depends mainly on the length, the cross-section, the material and the temperature of the wire. Copper is one of the best conductors that are used in electrical engineering. A long copper wire has a larger resistance than a short copper wire with the same cross-section. If two copper wires are equal in length, the wire with a larger cross-section will show smaller resistance. Now lets study the effect of temperature on resistance. Measure the resistance of a conductor when a small current is passing though it, and then measure its resistance when a large current causes it red-hot. You will find the electrons meet more resistance when the conductor is hot than when it is cold. Accordingly a conductor which has a resistance of 100 ohms at 0 will have a resistance of about 150 ohms at 100 . The higher its temperature is, the more resistance it shows. nts河南理工大学毕业设计（论文） 8 3. Electric equipments (1) Electric wires Electric wire is usually made of copper. Copper lets the electric current flow easily through it. We say that it has a low resistance. Some other metals also have a low resistance, but copper is the most useful. There are copper wires in millions of houses in the world. These wires carry the current to our lamps. There is a thin wire inside an electric lamp; you can see it if you look carefully. A thin wire has a higher resistance than a thick one. It tries to stop the flow of current. Then it gets very hot. The thin wire is not made of copper; it is made of tungsten. All metals melt when they get hot. (Mercury melts at a lower temperature than our usual ones.) Tungsten does not melt easily. It has to be very hot indeed before it melts. When the tungsten gets hot, it also gets bright. It shines and gives a good light. It also lasts a long time without breaking. An American, Edison, invented the first small electric lamp. He wanted a thin wire for his lamp, and tried to make one; but he had a lot of trouble. Thin wires easily melt if they are made of copper. He decided to use carbon because it does not melt. He tried cotton and hundreds of other materials to make his thin piece of carbon. But at first all of them broke. They were too thin and weak. They had to be thin because they had to shine brightly. Thick pieces do not have a high resistance. So they did not get hot enough, and they gave no light. Edison did not stop trying; and after a lot of trouble he made his first lamp. nts河南理工大学毕业设计（论文） 9 Our tungsten lamps are better than the old carbon lamps. They are brighter and they last longer. The tungsten does not easily melt or break. There is not much air inside an electric lamp; we have to take it out. Air contains oxygen, and the hot tungsten could burn in it. Usually we put some gas in the place of the air. Electric fires also have wires which get hot. These wires are thick, but they are not made of copper. They have a high resistance. A large current flows though them and makes them hot. So we can use electric fires in winter to keep us warm. In some houses an electric current also makes the water hot. This is useful when we want a bath. The wires get hot like the wires of electric fires; but we must keep them away from the water. We have to separate the wires from the water with some special material. It is not safe to let an electric wire tough water. Water has a low resistance to an electric current. Sometimes a person touches an electric wire with a wet hand; he ought not to do this. He might kill himself. The water lets the current flow easily to his body. Then it can escape to the ground through his legs. The current can easily flow through his body; and it can go through his heart. Then his heart will stop beating. (2) Switches and fuses An electric switch is often on a wall near the door of a room. Two wires lead to the lamp in the room. The switch is fixed in one of them. The switch can cause a break in this wire, and then the light goes out. The switch can also join the two parts of the wire again; then we get a light. Switches can control many different things. Small switches control nts河南理工大学毕业设计（论文） 10 lamps and radio sets because these do not take a large current. Larger switches control electric fires. Other switches can control electric motors. Good switches move quickly. They have to stop the current suddenly. If they move slowly, an electric spark appears. It jumps across the space between the two ends of the wire. This is unsafe and it heats the switches are sometimes placed in oil. Sparks do not easily jump though oil, and so the oil makes the switch safer. A large current makes a wire hot. If the wire is very thin, even a small current makes it hot. This happens in an electric lamp. The electric wires in a house are covered with some kind of insulation. No current can flow through the insulation; so the current can never flow straight from one wire to the other. But the insulation on old wires can tough. A large current may flow; and if this happens, the wires will get very hot. Then the house may catch fire. Fuses can stop this trouble. A fuse is only a thin wire which easily melts. It is fixed in a fuse-holder. The fuse-holder is made of some material which cannot burn. A large current makes the fuse hot and then it melts away. We say that the fuse “blows”. The wire is broken, and no current can flow. So the house does not catch fires go out because there is no current. When a fuse blows, something is wrong. We must find the fault first. Perhaps two wires are touching. We must cover them with new insulation of some kind. Then we must find the blown fuse and repair it. We put a new piece of fuse-wire in the holder. (Sometimes we can find the others are cold.) If we do not repair the fault first, the new fuse will nts河南理工大学毕业设计（论文） 11 blow immediately. Some people get angry when a fuse blows. So they put a thick copper wire in the fuse-holder! Of course this does not easily melt; if the current rises suddenly, nothing stops it. The thick wire easily carries it. Then the wires of the house may get very hot, and the house may catch fire. Some of the people in it may not be able to escape. They may lose their lives. So it is always best to use proper fuse-wire. This will keep everyone and everything in the house safe. (3) Autotransformers A transformer in which the primary and secondary windings are connected electrically as well as magnetically is called an autotransformer. Figure shows a connection diagram of an autotransformer. If this transformer is to be used as a step-down transformer, the entire winding ac forms the primary winding and the section ab forms the secondary winding. In other words, the section ab is common to both primary and secondary. As in the standard two-winding transformer, the ratio of voltage transformation is equal to the ratio of primary to secondary turns if the losses and exciting currents are neglected and Figure 11 represents an autotransformer winding with a total of 220 turns, with the sections ab and bc having 150 and 70 turns respectively. If a voltage of 440 V is applied to the winding ac, the voltage across each tur
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