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1、Chapter 4Electric Machinery Section 3 Synchronous machinesn Textn New Words and Expressionsn Exercisesn Endn Addition or Omission of Chinese wordsSection 3 Synchronous Machines A synchronous machine is an AC machine whose speed under steady-state conditions is proportional to the frequency of the cu
2、rrent in its armature.Polyphase DC excited synchronous machines comprise a polyphase armature winding and a DC field winding.At synchronous speed,the rotating magnetic field created by the armature currents travels at the same speed as the field created by the field current,and a steady torque resul
3、ts.An elementary physical picture of how a synchronous machine works has already been given elsewhere,Section 3 Synchronous Machineswith emphasis on torque production in terms of the interactions among its magnetic fields.The purpose of this section is to develop a quantitative theory from which bot
4、h the electrical and mechanical characteristics of synchronous motors and generators can be determined.The synchronous machine is idealized to the extent that the effects of salient poles and magnetic saturation are not included.Section 3 Synchronous Machines3600-rpm machines.Because of the economie
5、s of high-speed high-temperature high-pressure steam turbines,much study and some real pioneering work have been devoted to improvements in materials and design of both generators and turbines,and the maximum ratings for which 3600-rpm machines have been built have approximately doubled before the d
6、ecade.The advantages of large units are:somewhat increased efficiency,somewhat lower capital cost per kilowatt,and lower plant operating cost becauseSection 3 Synchronous Machinesof the greater ease of operating a generating station consisting of a relatively few large units as compared with one con
7、sisting of many smaller units.The field windings are embedded in axial slots cut in the rotor and are held in place by metal wedges.They usually consist of concentric coils of insulated copper strap laid flat in the slots.The rotor coil ends and ends connections are firmly held in place by retaining
8、 rings shrunk on over the ends of the coils.The leads are connected to slip rings which make contact with carbon brushes through which the field current is introduced.Section 3 Synchronous MachinesThe mmf wave created by such a winding is shown by the step wave.The height of each step in the wave is
9、 proportional to the total current in the slot below it.Because of the high rotational stresses,the rotors of turbine generators must be designed for as small a diameter as is consistent with other requirements.At the same time,limitations are imposed the axial length of the rotor by vibration consi
10、derations.Hence the design of the rotor is indeed a difficult problem,and the design of the whole machine isSection 3 Synchronous Machines The air gaps of turbine generators usually are much longer than in other types of machines.A long air gap reduces the reactance of the armature winding and impro
11、ves voltage regulation and stability.It is also necessary for ventilation.An obvious disadvantage of a long air gap is that it necessitates a greater field mmf to produce a specified air-gap flux.The cooling problem in electrical apparatus in general increases in difficulty with increasing size,beca
12、use the surface area from which the heat mustSection 3 Synchronous Machinesbe carried away increases roughly as the square of the dimensions,while the heat developed by the losses is roughly proportional to the volume and therefore increases approximately as the cube of the dimensions.Because of the
13、ir compactness,this problem is a serious one with large turbine generators.The use of a totally enclosed ventilating system brings up the possibility of using hydrogen as the cooling medium.Hydrogen has many properties which make it well suited to the purpose.ReturnNew Words and Expressionscylindric
14、al-rotor隐极式转子，鼓极转子steam-turbine-driven generator汽轮发电机Y-connected windings 星形连接绕组turbine generator 涡轮发电机，汽轮发电机，水轮发电机cps=cycles per second周/秒，赫rpm=revolutions per minute 转/分steam turbine汽轮机ratingn.额定值，额定参数；标称值，定额 New Words and Expressionsdiameter n.直径vibration n.振荡field current励磁电流excitern.励磁机shaft n.
15、轴regulation n.调节stabilityn.稳定性，稳定 New Words and Expressionsventilation n.通风，换气，通风装置flux n.通量，磁力线 Return(2)When you turn a switch,you can easily operate lighting,heating or power-driven electrical devices.旋转开关，就能轻而易举地控制照明装装置置、供热装装置置或电动装置。(3)The letter I represents the current in amperes,E the electro
16、motive force in volts,and R the resistance in ohms.字母I代表电流的安培数，E代代表表电动势的伏特数，R代表代表电阻的欧姆数。Addition or Omission of Words6.增加表示句子主语的词增加表示句子主语的词 当被动句中的谓语是表示“知道”、“了解”、“看见”、“认为”、“发现”、“考虑”等意思的动词时，通常可在其前增加“人们”、“我们”、“有人”等词语，译成汉语的主动句。(1)With the development of modern electrical engineering,power can be transmi
17、tted to wherever it is needed.随着现代化电气工程的发展，人人们们可以把电力输送到任何所需要的地方。Addition or Omission of Words(2)Weak magnetic fields are known to come from the human body.我们我们知道，人体能产生微弱的磁场。(3)It is said that numerical control is the operation of machine tools by numbers.人们人们说，数控就是机床用数字加以操纵。Addition or Omission of W
18、ords7.增加表示被动意义的词增加表示被动意义的词 英语中的被动句，当要强调动作或突出被动者时，译文常须增加“由”、“为”、“受”、“让”、“给”、“遭”、“加以”、“予以”、“为所”等词。不定式作定语，如果是被动形式，都含有将来的意思，在译文时需增加“将要”、“应该”、“有待”、“设法”等词语。例如：(1)The method to be used is 要要采用的方法是。(2)The lighthouse to be installed is eight meters high and 2.2 meters in diameter.即将即将安装的灯塔高8米，直径2.2米。Addition
19、 or Omission of Words8.增加具体化、明确化的词语增加具体化、明确化的词语 有些英语句子如果直译成汉语，意思表达不够具体和明确，需要增译相关词语。1.(1)Floppy disks have always been cheap to make and relatively easy to copy.2.软盘制作成本比较低成本比较低，而且也比较容易拷贝。(2)At the moment,developed areas in Europe,the United States and Asia have already started studying the possibili
20、ty of an electronic currency.目前，欧洲、美国以及亚洲一些发达地区已经开始研究推行电力货币的可能性可能性。Addition or Omission of WordsExercises I.Choose the best answer into the blank 1.The speed of a synchronous machine is directly proportional to in its armature under steady-state conditions.A.the amplitude of the current B.the freque
21、ncy of the current C.the amplitude of the voltage D.the flux Exercises I.Choose the best answer into the blank 2.The field winding in a synchronous generator is supplied by power source.A.a DC B.an AC C.no D.any type 3.The cylindrical rotor machine is used in .A.low-speed motors B.high-speed motors
22、C.high-speed steam-turbine-driven generators D.low-speed hydro-turbine-driven generators Exercises I.Choose the best answer into the blank 4.The turbine generators with two poles for 60-cps are machines.A.3600-rpm B.1800-rpm C.5400-rpm D.2400-rpm5.The field current of a turbine generator is usually
23、obtained from .A.a motor B.an AC generator C.an exciter D.a battery Exercises III.Translate the following into Chinese A squirrel-cage rotor consists of a number of identical bars of copper or aluminum sunk into slots in a laminated steel core.It is cheap to produce,but has the disadvantage of a low
24、 starting torque and a lack of control of speed.In the case of large motors,it is desirable that there should be an adequate starting torque and some control over speed.For large motors therefore,a phase-wound rotor is used,with the windings connected at one end to each other and the other end to slip-rings,thus enablingExercises III.Translate the following into Chinese the resistance of the rotor to be varied at will,providing a greater starting torque,and some speed control.Return