发电机和电气设备毕业论文外文翻译.doc

Generator and Electrical EquipmentsGeneratorIntroduction Electric generators convert mechanical energy to electrical energy, which is more easily transmitted to remotely located points of application. The first large electric generating systems used direct-current (dc) generators, mainly because direct current was better understood than alternating current (ac). However, dc generators are limited to generating power at relatively low voltages, largely due to problems at their commutates. As power networks developed, higher and higher voltages were required to transmit large blocks of power over longer and longer distances. Electric transformers can easily change the normally low voltage generated to the high voltages needed for efficient power transmission, and of course, transformers only work on alternating current. Ac generators, or alternators as they are commonly called, are so much simpler mechanically, so much more efficient, and require so much less maintenance than dc machines that all large generating plants output alternating current today. Although de transmission lines can transport extremely large blocks of power very efficiently over long distances, the power is always generated as alternating current, transformed to the voltage required, rectified and transmitted as direct current, and then inverted back to alternating current at the point of application.Mechanical Energy The mechanical energy for driving the generator must be derived from a source with enough reliability and capacity to make it economically feasible to develop and transmit the energy electrically to the point of use. A small water supply running only during exceptionally wet years or located at a great distance from electrical consumers would probably not be suitable. Mechanical energy sources which cannot be moved, such as hydraulic turbines or even wind machines, must have the cost of transporting the energy produced (among other factors) taken into account when overall costs are calculated. Steam-turbine power plants, however, can be located near a coal seam, lumber mill, or a reliable source of cooling water to save on transportation costs. Some mechanical power may be obtained from sources more easily located near the point of utilization. Gas turbines and reciprocating gas or diesel engines fall into this category. Except for standby emergency power generators, even here it might be more economical to install large units and transmit the power to the point of use. Large power plants will generally have better operating efficiencies than small ones, and it may be desirable to locate a large plant near the center of use and then distribute the power generated outward, assuming the fuel supply is transportable.Each type of mechanical driver has its own peculiarities, and some have a sizable impact on the generator configuration. There are marked differences as to the engine output Speeds available, the speed pulsations possible, the chances of overspeed , etc.Normally, the generator shaft is horizontal and direct-connected to the driver. Sometimes speed-changing gear boxes are installed between a high-speed turbine and a lower speed generator. These allow the turbine to run at its most efficient speed, a speed that may be too high for the generator. Small hydraulic turbines usually have their shafts mounted horizontally; large hydraulic machines have their shafts direct-connected and vertically mounted. The generator may include special bearings to carry the thrust imposed by the water flowing through the turbine. Criteria like these for providing mechanical energy impose special designs on the generating machines.Basic Principle and Construction There are two quite distinct forms of modem alternator. While the principle of operation of each is the same, i.e. , the movement of magnetic poles past stationary coils, their constructions are very different. The reason for this is that each has been designed to 'match' its prime mover, i.e., to suit the mechanical device that is to tap the two principal natural power resources-failing water, on the one hand, and steam, generated by heat from fossil fuels or nuclear fuels, on the other. To match the output of the turbo alternators, the water wheel generators must therefore be multi-polar and hence of large diameter and small axialength. There is a limit to the length of a turbo alternator, based largely on the mechanical considerations involved in supporting a large rotor mass between a bearing at each end. At 3 000 or 3 600 r/min (50 or 60 Hz) the rotor must be extremely well balanced and its surface smooth. With the lower speed water powered machine, such precautions can be relaxed with a view to making the larger rotor cheaper to make. The fundamental difference in shape between the rotors of the two types of machine is consequent upon the above considerations, but now a secondary difference is introduced by what could be termed the experience and skill of the designer. It is necessary to produce a sine wave of induced e.m.f. The factors that affect the instantaneous value e of this are the flux density b , the length I of the conductor, and the velocity v (the use of small letters indicating the instantanous values). Thus: = b l v (4.1) A reasonably clear definition of what constitutes a ' pole' can be given by defining a pole pitch rather than a pole per se. A pole pitch is the distance ( p ) between points where the current flow is a maximum. The number of poles in the machine is then the periphery ( 2r ) divided by p . This definition fits easily into linear motor technology, where the number of poles need be neither even nor an integer. The speed of rotation expressed in Table 4.1 as 2 f/n r/s, where n was the number of poles, can always be converted to a linear speed, for the periphery 2 r contains n pole pitches each of length p so that 2r = n p . Hence the rotational speed 2 f/n /r/s 'translates ' into a linear speed, v s , such that v s=(2 f / n) (2r)= (2 f / n) (n p)=2pf (4.2) which is simply the 'common sense' statement that a travelling wave moves two pole pitches ( = one wavelength; A ) each cycle of events. (This corresponds to the well known formula v = f for all wave motions. ) Three-phase machine stator shown in Figure 4. l (a). It does not, as it appears at first sight, have six poles, even though it has six obvious 'polar projections'. These are to be seen as six ' teeth' in a slotted stator with a three-phase 'distributed' winding, except that the distribution has virtually disappeared except insofar as there are three phases. Unless such a diagram makes clear how the two coils in each phase are connected, no one can say whether it has two poles or four. It is worth studying Figure 4.1 carefully, first to appreciate the differences between(b)and (c), hence to ' see' the kind of difficulty that can arise in the mind of the student being confronted with the problem for the first time, and finally to demolish the problem so that it never arises in the future. For the connections shown at (b), both red-phase coils assist each other in driving flux diametrically across the machine. So do both yellow-phase coils and both blue-phase coils. So, whatever instantaneous currents flow in the system as a whole, the resultant flux will be the vector sum of three diametral fluxes which therefore is itself diametral and the machine corresponds to the two-pole system. But if opposite pairs send opposing fluxes into the rotor then the only possible resultant flux pattern corresponds to that of Figure 4.1 (c) and the machine has four poles. In this crude example the lack of winding 'distribution' is now obvious, since a two-pole, three-phase machine with only six slots has one slot per pole per phase, or, as is now more 'fashionably' written, one slot per pole and phase, The four-pole version has only half a slot per pole and phase, which gives a very ' lumpy' kind of travelling field to be avoided in practice if at all possible by having a larger number of slots. The reader w-ill appreciate , however, that if a more realistic example of two-and four-pole machines with, say , twenty-four slots each had been chosen, the diagrams might have become too obscure to make the point about 'pole counting'. ( 1 ) Stators The rapidly varying magnetic flux in the stator iron causes hysteresis losses as the iron resists changes in the flux density. The varying magnetic flux also causes electric currents, called eddy currents, to flow in the iron laminations; losses also result from this current flow. The stator is built from thin laminations to minimize the electrical losses and of specially rolled silicon steel to minimize the hysteresis losses. For small machines, the laminations are circular, in the shape of the finished stator. For large machines, the laminations are punched as semicircles and then assembled into the finished circular stator. Slots are punched for future installation of the windings. The winding slots are suitably insulated to provide both electrical insulation between the windings and the grounded stator and protection from abrasion damage to the windings by the stator iron. Windings are specified with the proper span, wire size, and amount of insulation required by the machine rating.For smaller machines, the windings are wound with loose coils of round wire', which are inserted into the slots provided in the stator, mm by mm, and fastened with slot wedges to prevent movement of the windings. To get as much conductor and stator iron as possible into the machine, large units are wound with square or rectangular wire, which is formed into rigid coils with insulation both between the individual wires and around the coils themselves. The coils are inserted into the stator slots, which have parallel walls to provide a snug fit between the coils and the stator iron; slot wedges hold the coils in place. Coil ends are connected into the proper groupings to provide the configuration of poles, voltage, and other parameters for which the machine is rated. (2) Rotors Two basic types of rotors see service in synchronous alternators. High-speed machines (two-and four-pole) are built with round rotors; slots are cut into the rotor for the field windings. These alternators are referred to as uniform-air-gap machines. Slower-speed machines have field poles that stick out from the rotor shaft, with the field winding wound around the projecting poles. The air gap obviously is not uniform. These alternators are called salient-pole machines. Each pole on the alternator rotor has a winding through which direct current, usually at 63 125 250, or 375 V, is circulated to "excite" the field and create a magnetic field. The power required for field excitation is normally only a small percentage of the output, about 1 to 2 percent of the alternator rating. The dc excitation is obtained either from direct-connected machines driven by a prime mover or from separately mounted exciters that &five their power from other sources. The exciter output voltage level must be adjustable and have enough capacity to enable the alternator to produce rated voltage at rated output. (3) Exciters Over the years, field excitation has been provided by three main exciter designs rotating brush, rotating brush less , and static types. Rotating Brush Type. Rotating, compound-wound de designs were the only exciters used for many years. Exciters driven via a speed-increasing belt-and-pulley arrangement were sometimes specified so that less expensive, higher-speed exciters could be paired with slower-speed alternators. Direct current is delivered to the alternator rotor slip rings, which consist of two circular brass-alloy rings mounted on and insulated from the alternators shaft. Connections are made from the slip rings to the alternator field. Brushes riding on the slip rings are connected to the exciter.The rotating brush exciter still sees service, but continual maintenance problems are These problems, together with the development of reliable, inexpensive semiconductors, make the brush less exciter the dominant choice today. Brush less Type. The brush less exciter is simply a special type of alternator mounted on the same shaft as the main exciter. It is special because its field, which must be excited with direct current, is stationary , and its ac output comes from the rotating parts. The output is rectified and connected to the main alternator' s field by means of cables run along and fastened to the alternator shaft. Brushes, commutators, slip rings, and their maintenance are eliminated. Static Type. As prices go down and the reliability and ratings of semiconductors go up, special cubicle-mounted controlled rectifiers, called static exciters, are becoming an increasingly popular choice. Their lower cost, reduced losses, reduced maintenance, and more flexible outputs also make them good choices for replacements of damaged rotating exciters. A static exciter consists of an input transformer, silicon controlled rectifiers (SCRs), rectifier controls, and voltage regulator controls. The complete assembly functions to rectify the incoming ac voltage into a properly controlled dc exciter voltage required by the alternator. Static exciter input may be connected to any convenient ac powersource, such as station power (assuming it is available when the alternator is not running), but it is normally connected to the alternator output leads. Fuses and disconnect switches are installed between the alternator and exciter to protect against faults in the system. Once an alternator field winding has had direct current passed through it, a small amount of residual magnetism remains. When the alternator is run again at rated speed, without excitation, an ac voltage of 2 to 10 percent of rated can be measured at the alternator' s output terminals. This voltage is generated by the residual magnetic flux in the rotor acting on the stator windings. When it is connected to the alternator' s output, the static exciter rectifies this residual ac voltage into direct current, which is applied to the alternator field windings. This action further increases the excitation, which builds up until, in a very short time, the rated output voltage is obtained. Obviously, the correct connections must be made; if the output of the static exciter is in opposition to that of the residual voltage, no buildup will occur.The exciter output is connected to the alternator field via the slip rings, which will require some brush and ring maintenance, but not as much as is required by the brash and commutator arrangement in a rotating exciter. Sometimes the residual magnetism is lost or it