電氣專(zhuān)業(yè)畢業(yè)設(shè)計(jì)外文翻譯 10

1、<p><b>　　譯 文</b></p><p>　　原文題目：Newnes Electrical Engineers Handbook</p><p>　　譯文題目： 《紐恩斯電氣工程師手冊(cè)》節(jié)選 </p><p>　　學(xué) 院： 電子信息學(xué)院 </p><p>　　專(zhuān)

2、業(yè)班級(jí)：電氣工程及其自動(dòng)化2008級(jí)04班</p><p>　　學(xué)生姓名： 張 </p><p>　　學(xué) 號(hào)： 408030404 </p><p>　　Newnes Electrical Engineers Handbook</p><p>　　

3、D.F. Warne</p><p>　　Chapter 1 Introduction </p><p>　　There seems to be a trend in the public perception of engineering and technology that to be able to operate a piece of equipment or a system

4、is to understand how it works. Nothing could be further from the truth. The gap between the ability to operate and a genuine understanding is, if anything, widening because much of the complexity added to modem electrica

5、l equipment has the specific aim of making it operable or ‘user-friendly’ without special training or knowledge. The need for a basic expla</p><p>　　how various important and common classes of electrical equ

6、ipment works , has never been stronger. Perhaps more so than in its predecessor, Newnes Electrical Pucker Book, an attempt is made to address fundamentals in this book, and the reader is encouraged to follow through

7、any areas of interest using the references at the end of each chapter. More comprehensive coverage of all the subjects covered in this pocket book is available in the Newnes Electrical Engineer’s Reference Book. More so

8、no</p><p>　　The need for a basic explanation of principles, leading to a simple description of how various important and common classes of electrical equipment works, has never been stronger. Perhaps more so

9、 than in its predecessor, Newnes Electrical Pucker Book, an attempt is made to address fundamentals in this book, and the reader is encouraged to follow through any areas of interest using the references at the end of

10、 each chapter. More comprehensive coverage of all the subjects covered in this pocket</p><p>　　More so now than ever before, the specification and performance of electrical equipment is governed by national

11、 and international standards. While it would be inappropriate in a pocket book to cover standards in any detail, a summary of key standards is included for reference purposes at the end of each chapter. The structure

12、of the book is based around three groups of chapters, which address:</p><p>　　fundamentals and general material </p><p>　　the design and operation of the main classes of electrical equipment &

13、lt;/p><p>　　special technologies which apply to a range of equipment</p><p>　　The first group comprises three chapters which set out fundamentals and principles running through all aspects of elect

14、rical technology.The opening chapter deals with fundamentals of electric and magnetic fields and circuits, with energy and power conversion principles.</p><p>　　This is followed by a review of the materials

15、 that are so crucial to the design of electrical equipment, and these are grouped into sections on magnetic, insulating and conducting materials. In each of these areas technology is moving ahead rapidly. The great incr

16、eases in the strength of permanent magnets in the past ten years has done much to make possible the miniaturization of equipment such as the Sony Walkman and the introduction of so many small motors and actuators in our

17、 homes and mot</p><p>　　Finally in this opening group there is a chapter on measurement and instrumentation. A classical textbook on electrical measurement would in the past have included sections on mov

18、ing iron and moving coil instruments, but these have been omitted here in favour of the oscilloscope and sensors which now dominate measurements in most areas.</p><p>　　The following group of nine chapters m

19、ake up the main core of the book and cover the essential groups of electrical equipment found today in commerce and industry.</p><p>　　The opening five chapters here cover generators, transformers, switchge

20、ar, fuses and wire and cables. These are the main technologies for the production and handling of electrical energy, from high power and high voltage levels down to the powers and voltages found in the household. Excit

21、ing developments in this area include the advances made in high voltage switchgear using SF6 as an insulating medium, the extension of polymer insulation into high voltage cables and the continuing compacti</p>

22、<p>　　The following four chapters describe different groups of equipment that use or srore electrical energy. Probably the most important here is electric motors, since these use almost two-thirds of all electr

23、ical energy generated. Static power supplies are also of growing importance in applications such as emergency standby for computers; this technology is now based on power electronics and the opportunity is taken in t

24、his chapter to explain the fundamentals of power electronic design and t</p><p>　　The final group of three chapters cover subjects that embrace a range of technologies and equipment. There is a chapter on

25、power systems which describes the way in which generators, switchgear, transformers, lines and cables are connected and controlled to transmit and distribute our electrical energy. The privatization of electricity su

26、pply in countries across the world has brought great changes in the way in which power systems are operated and these are touched upon here. The second c</p><p>　　Chapter 2 Principles of electrical engine

27、ering</p><p>　　2.1 Nomenclature and units </p><p>　　This book uses notation in accordance with the current British and International Standards. Units for engineering quantities are printed in u

28、pright roman characters, with a space between the numerical value and the unit, but no space between the decimal prefix and the unit, e.g. 275 kV. Compound units have a space, dot or / between the unit elements as appro

29、priate, e.g. 1.5 N m, 300 ds, or 9.81 m-C2. Variable symbols are printed in italic typeface, e.g. V. For ac quantities, the instantaneo</p><p>　　2.2 Electromagnetic fields</p><p>　　2.2.1 Ele

30、ctrlc fields </p><p>　　Any object can take an electric or electrostatic charge. When the object is charged positively, it has a deficit of electrons, and when charged negatively it has an excess of electr

31、ons. The electron has the smallest known charge, -1.602 x lO-” C. Charged objects produce an electric field. The electricfield strength E (V/m) at a distance d(m) from an isolated point charge Q(C) in air or a vacuum i

32、s given by</p><p><b>　?。?.1）</b></p><p>　　where the permittivity offree space E, = 8.854 x 10-12 F/m. If the charge is inside aninsulating material with relative permittivity q the

33、electric field strength becomes</p><p><b>　　(2.2)</b></p><p>　　Any charged object or particle experiences a force when inside an electric field. The force F (N) experienced by a cha

34、rge Q (C) in an electric field strength E(Vim) is given by</p><p>　　F=QE (2.3)</p><p>　　Electric field strength is a vector quantity. The direction of the for

35、ce on one charge due to the electric field of another is repulsive or attractive. Charges with the same polarity repel; charges with opposite polarities attract. </p><p>　　Work must be done to move charges

36、of the same polarity together. The effort required is described by a voltage or electrostatic potential. The voltage at a point is defined as the work required to move a unit charge from infinity or from earth. (It is

37、normally assumed that the earth is at zero potential.) Positively charged objects have a positive potential relative to the earth. </p><p>　　If a positively charged object is held some distance above the gro

38、und, then the voltage at points between the earth and the object rises with distance from the ground, so that there is apotenrinl gradient between the earth and the charged object. There is also an electric field p

39、ointing away from the object, towards the ground. The electric field strength is equal to the potential gradient, and opposite in direction.</p><p><b>　　(2.4)</b></p><p>　　2.2.2 Ele

40、ctric currents </p><p>　　Electric charges are static if they are separated by an insulator. If charges are separated by a conductor, they can move giving an electric current. A current of one ampere flows

41、if one coulomb passes along the conductor every second </p><p>　　. （2.5）</p><p>　　A given current flowing through a thin wire represents a greater density o

42、f current than if it flowed through a thicker wire. The current densiq J (Nm2) in a wire with cross-section area A (m2) carrying a current I (A) is given by</p><p><b>　?。?.6）</b></p>

43、<p>　　For wires made from most conducting materials, the current flowing through the wire is directly related to the difference in potential between the ends of the wire.</p><p>　　Ohm's law gives th

44、is relationship between the potential difference V(V) and the current I (A) as</p><p>　　or (2.7)</p><p>　　where R (Ω) is the resistance, and G (S) = 1/R is th

45、e conductance (Fig.2.2). For wire of length I and cross-section area A, these quantities depend on the resistivity(Ωm) and conductivity σ (Wm) of the material</p><p>　　Fig. 2.2 Ohm's law</p><

46、;p>　　or （2.8）</p><p>　　For materials normally described as conducitorsρis small, while for insulators ρis large. Semicondz6ctors have resistivity in between these extremes, and

47、 are usually very dependent on purity and temperature.</p><p><b>　　(2.9)</b></p><p>　　for a conductor with resistance RT, at reference temperature To. This is explained in more det

48、ail in section 3.4.1. Charges can be stored on conducting objects if the charge is prevented from moving by an insulator. The potential of the charged conductor depends on the capacitance C (F) of the metallinsul

49、at or object, which is a function of its geometry. The charge is related to the potential by</p><p><b>　　(2.10)</b></p><p>　　A simple parallel-plate capacitor, with plate area A, ins

50、ulator thickness d and relative permittivity E, has capacitance</p><p><b>　　(2.11)</b></p><p>　　2.2.3 Magnetic fields </p><p>　　A flow of current through a wire produce

51、s a magnetic field in a circular path around the wire. For a current flowing forwards, the magnetic field follows a clockwise path, as given by the right-hand corkscrew rule (Fig. 2.3). The magnetic field strength H (A

52、 m-1) is a vector quantity whose magnitude at a distance d from a current Z is given by</p><p><b>　?。?.12）</b></p><p>　　For a more complicated geometry, Amptrek law relates the numbe

53、r of turns N in a coil, each carrying a current I, to the magnetic field strength H and the distance around the lines of magnetic field l.</p><p><b>　?。?.13）</b></p><p>　　where Fm (a

54、mpere-turns) is the magnetomotive force (mmf). This only works for situations where H is uniform along the lines of magnetic field. </p><p>　　The magnetic field produced by a current does not depend on t

55、he material surrounding the wire. However, the magnetic force on other conductors is greatly affected by the presence of ferromagnetic materials, such as iron or steel. The magnetic field produces </p><p>

56、　　a magnetic flux density B (T) in air or vacuum</p><p><b>　　（2.14）</b></p><p>　　where the permeability of fire space μ0 = 4π x l0-7 Wm. In a ferromagnetic material with relative

57、 permeability μr</p><p><b>　?。?.15）</b></p><p>　　A second conductor of length I carrying an electric current Z will experience a force F in a magnetic flux density B</p><p

58、>　　F=BIL （2.16）</p><p>　　The force is at right angles to both the wire and the magnetic field. Its direction is given by Fleming’s left-hand rule (Fig. 2.4). If the mag

59、netic field is not itself perpendicular to the wire, then the force is reduced; only the component of B at right angles to the wire should be used.</p><p>　　A flow of magnetic flux @ (Wb) is caused by the fl

60、ux density in a given cross-section area A as</p><p>　　Φ=BA （2.17）</p><p>　　The mmf F, required to cause a magnetic flux @toflow through a region of length I a

61、nd cross-section area A is given by the reluctance R, ( A N b ) or the permeance A (Wb/A) of the region</p><p>　　or （2.18）</p><p><b>　　Where</b></p>&

62、lt;p><b>　　（2.19）</b></p><p>　　In ideal materials, the flux density B is directly proportional to the magnetic field</p><p>　　strength H.In ferromagnetic materials the relation bet

63、ween B and H is non-linear (Fig. 2.5(a)), and also depends on the previous magnetic history of the sample. The magnetization or hysteresis or BH loop of the material is followed as the appliedmagnetic field is changed (F

64、ig. 2.5(b)). Energy is dissipated as heat in the material as the operating point is forced around the loop, giving hysteresis loss in the material.These concepts are developed further in section 3.2.</p><p>

65、　　2.2.4 Electromagnetism</p><p>　　Any change in the magnetic field near a wire generates a voltage in the wire byelectromagneticinduction.The changing field can be caused by moving the wire inthe magnetic fi

66、eld. For a length 1 of wire moving sideways at speed v ( d s ) across a magnetic flux density B, the induced voltage or electromotiveforce (emf)is given by</p><p><b>　?。?.20）</b></p><p

67、>　　The direction of the induced voltage is given by Fleming’sright-handnile (Fig. 2.6).An emf can also be produced by keeping the wire stationary and changing the magnetic field. In either case the induced voltage can

68、 be found using Faraday’slaw.If a magnetic flux 0 passes through a coil of N turns, the magneticflux linkage Y(Wb-t) is</p><p><b>　?。?.21）</b></p><p>　　Faraday’s law says that the in

69、duced emf is given by</p><p><b>　　（2.22）</b></p><p>　　The direction of the induced emf is given by Lenz’s law, which says that the induced voltage is in the direction such that, if t

70、he voltage caused a current to flow in the wire, the magnetic field produced by this current would oppose the change in Y. The negative sign indicates the opposing nature of the emf.</p><p>　　A current flowi

71、ng in a simple coil produces a magnetic field. Any change in thecurrent will change the magnetic field, which will in turn induce a back-emfin the coil. The self-inductance or just inductance L (H) of the coil relates t

72、he induced voltage to the rate of change of current</p><p><b>　?。?.23）</b></p><p>　　Two coils placed close together will interact. The magnetic field of one coil will link with the w

73、ire of the second. Changing the current in the primary coil will induce a voltage in the secondary coil, given by the nzurual inductance M (H)</p><p><b>　?。?.24）</b></p><p>　　Placing

74、 the coils very close together, on the same former, gives close coupling of the coils. The magnetic flux linking the primary coil nearly all links the secondary coil. The voltages induced in the primary and secondary are

75、 each proportional to their</p><p>　　number of turns, so that</p><p><b>　　(2.25)</b></p><p>　　and by conservation of energy, approximately</p><p><b>　

76、?。?.26）</b></p><p>　　A two-winding transformer consists of two coils wound on the same ferromagnetic core. An autotransformer has only one coil with tapping points. The voltage across each section is p

77、roportional to the number of turns in the section.Transformer action is described more fully in section 6.1.</p><p>　　2.3 Circuits</p><p>　　2.3.1 DC circuits</p><p>　　DC power is su

78、pplied by a battery, dc generator or rectifying power supply from the mains. The power flowing in a dc circuit is the product of the voltage and current</p><p><b>　　(2.27)</b></p><p>

79、;　　Power in a resistor is converted directly into heat.but their voltages must be added together (Fig. 2.7)</p><p>　　When two or more resistors are connected in series, they carry the same current</p>

80、<p><b>　　(2.28)</b></p><p>　　Fig. 2.7 Seriesresistors</p><p><b>　　(2.29)</b></p><p>　　When two or more resistors are connectedinparallel, they have the

81、 same voltage but their currents must be added together (Fig. 2.8)</p><p><b>　　(2.30)</b></p><p>　　Fig. 2.8 Parallel resistors</p><p>　　The total resistance is given by&

82、lt;/p><p><b>　　(2.31)</b></p><p>　　A complicatedcircuit is made of severalcomponentsof brunches connectedtogether at nodes forming one or more complete circuits, loops or meshes. At eac

83、h node, Kii-chhofS’scirrrent law (Fig. 2.9(a)) says that the total current flowing into the node must be balanced by the total current flowing out of the node. In each loop, the sum of all the voltages taken in order aro

84、und the loop must add to zero, by KirchhofS’s volrage law (Fig. 2.9(b)). Neither voltage nor current can be lost in a circuit.</p><p>　　DC circuits are made of resistors and voltage or current sources. A cir

85、cuit with only two connectionsto the outside world may be internally complicated. However, to the outside world it will behave as if it contains some resistance and possibly a source of voltage or current. The Tlzivvenin

86、 equivalent circuit consists of a voltage source and a resistor (Fig. 2.10(a)), while the Norton equivalentcircuit consists of a current sourceand a resistor (Fig. 2.10(b)). Theresistor equals the internalresi</p>

87、<p>　　2.3.2 AC circuits</p><p>　　AC power is supplied by the mains from .large ac generators or alternators, by a local</p><p>　　alternator, or by an electronic synthesis. AC supplies are

88、normally sinusoidal, so that</p><p>　　at any instant the voltage is given by</p><p>　　= （2.32）</p><p>　　V is the peak voltage or amplitude, ois the angul

89、arfrequency (rad s8) and the</p><p>　　phase angle (rad). The angular frequency is related to the ordinaryfreqzrency f(Hz)</p><p><b>　?。?.33）</b></p><p>　　and the period

90、is llf. The peak-to eak or pk-pk voltage is 2Vm, and the mot meansquare or rms voltage is V,, I$ It is conventional for the symbols V and Zin accircuits to refer to the rms values, unless indicated otherwise. AC voltages

91、 andcurrents are shown diagrammatically on a phasor diagram (Fig. 2.12).It is convenient to represent ac voltages using complex numbers. A sinusoidalvoltage can be written</p><p><b>　?。?.34）</b>&

92、lt;/p><p>　　A resistor in an ac circuit behaves the same as in a dc circuit, with the current and voltage in phase and related by the resistance or conductance (Fig. 2.13). The current in an inductor lags the v

93、oltage across it by 90"( d 2 rad) (Fig. 2.14). The ac resistance or reactance X of an inductor increases with frequency</p><p>　　XL=ωL （2.35）</p><p>　　The

94、 phase shift and reactance are combined in the complex impedance Z</p><p><b>　?。?.36）</b></p><p>　　Inductors in series behave as resistors in series</p><p><b>　?。?

95、.37）</b></p><p>　　and inductors in parallel behave as resistors in parallel</p><p><b>　?。?.38）</b></p><p>　　For a capacitor, the current leads the voltage across i

96、t by 90" (1d2 rad) (Fig. 2.15).The reactance decreases with increasing frequency</p><p><b>　?。?.39）</b></p><p>　　In a capacitor, the current leads the voltage, while in an induc

97、tor, the voltage leads the current. The impedance is given by</p><p><b>　?。?.40）</b></p><p>　　Capacitors in series behave as resistors in parallel (eqn 2.41) and capacitors in parall

98、el behave as resistors in series (eqn 2.42)</p><p><b>　　（2.41）</b></p><p><b>　?。?.42）</b></p><p>　　The direction of the phase shift in inductors and capacito

99、rs is easily remembered by the mnemonic CIVIL (i.e. C-IV, VI-L). Imperfect inductors and capacitors have some inherent resistance, and the phase lead or lag is less than 90".The difference between the ideal phase an

100、gle and the actual angle is called the loss angle 6. For a component of reactance X having a series resistance R</p><p><b>　　(2.43)</b></p><p>　　The reciprocal of impedance is admitt

101、ance</p><p><b>　?。?.44）</b></p><p>　　Combinationsof resistors,capacitorsand inductorswill have a variation of impedance or admittancewith frequency whichcan be used to selectsignalsa

102、t certainfrequencies in preference to others. The circuit acts as aBltel; which can be low-pass, high-pass, band-pass, or band-stop.</p><p>　　An important filter is the resonant circuir. A series combination

103、 of inductor and capacitor has zero impedance (infinite admittance) at its resonant frequency</p><p><b>　?。?.45）</b></p><p>　　A parallel combination of inductor and capacitor has inf