毕业设计翻译——电气工程师手册-张.doc

译 文 原文题目：Newnes Electrical Engineers Handbook译文题目： 纽恩斯电气工程师手册节选 学 院： 电子信息学院 专业班级：电气工程及其自动化2008级04班学生姓名： 张 学 号： 408030404 1西安工程大学本科毕业设计（译文）Newnes Electrical Engineers HandbookD.F. WarneChapter 1 Introduction 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 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 electrical equipment has the specific aim of making it operable or user-friendly without special training or knowledge. 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 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 each chapter. More comprehensive coverage of all the subjects covered in this pocket book is available in the Newnes Electrical Engineers Reference Book. More so now than ever before, the specification and performance of electrical equipment is governed by national 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 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 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 each chapter. More comprehensive coverage of all the subjects covered in this pocket book is available in the Newnes Electrical Engineers Reference Book.More so now than ever before, the specification and performance of electrical equipment is governed by national 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 of the book is based around three groups of chapters, which address:l fundamentals and general material l the design and operation of the main classes of electrical equipment l special technologies which apply to a range of equipmentThe first group comprises three chapters which set out fundamentals and principles running through all aspects of electrical technology.The opening chapter deals with fundamentals of electric and magnetic fields and circuits, with energy and power conversion principles.This is followed by a review of the materials 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 increases 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 homes and motorcars. Developments in insulating mataids mean that increased reliability and operation at higher temperatures can now readily be achieved. Under the heading of conductors there are continuing advances in superconductors, which are now able to operate in liquid nitrogen, and of course semiconductor development has transformed the way in which equipment can be controlled and the processing power in computers.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 moving 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.The following group of nine chapters make up the main core of the book and cover the essential groups of electrical equipment found today in commerce and industry.The opening five chapters here cover generators, transformers, switchgear, 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. Exciting 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 compaction of miniature and moulded-case circuit breakers. A new section in the wire and cables chapter addresses the growing technology of optical fibre cables; although the main use for this technology is in telecommunications, which is outside the scope of the book, a chapter on wires and cables would not be complete without it and optical fibres have a growing number of applications in electrical engineering.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 electrical 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 this chapter to explain the fundamentals of power electronic design and technology. Rotating converters were important for many of the duties now handled by power electronics, but these are now in decline and are not covered here. The range of batteries being developed and appearingin a variety of applications is now very large and this is the subject of a special chapter, which also covers the techniques of battery charging and the emerging related technology of fuel cells. If fuel cells fulfil their promise and start to play a greater part in the generation of electricity in the future then we can expect to see coverage of this area grow and perhaps move to the generation section in future editions.Another major electricity consumer is the range of technologies generally known as electroheat. This covers a spectrum of technologies from arc furnaces through microwave heating to ultraviolet drying techniques which are described in a special chapter. The final group of three chapters cover subjects that embrace a range of technologies and equipment. There is a chapter on 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 supply 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 chapter in this group concerns electromagnetic compatibility (emc); with the growing amount of electronic and high-frequency equipment in use today it is imperative that precautions are taken to prevent interference and legislation has been introduced to enforce this prevention. The techniques for tackling this are complex and influence a wide range of equipment. Finally there is a chapter describing the design and use of equipment for operation in hazardous and explosive environments; this again covers a wide range of equipments and there are a number of different classifications of protection.Chapter 2 Principles of electrical engineering2.1 Nomenclature and units This book uses notation in accordance with the current British and International Standards. Units for engineering quantities are printed in upright 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 appropriate, 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 instantaneous value is printed in lower case italic, peak value in lower case italic with caret (*), and rms value in upper case, e.g. i, i, I. Symbols for the important electrical quantities with their units are given in Table 2.1, and decimal prefix symbols are shown in Table 2.2. Graphical symbols for basic electrical engineering components are shown on Fig. 2.1. 2.2 Electromagnetic fields2.2.1 Electrlc fields 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 electrons. 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 is given by （2.1）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 electric field strength becomes (2.2)Any charged object or particle experiences a force when inside an electric field. The force F (N) experienced by a charge Q (C) in an electric field strength E(Vim) is given byF=QE (2.3)Electric field strength is a vector quantity. The direction of the force 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. Work must be done to move charges 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 normally assumed that the earth is at zero potential.) Positively charged objects have a positive potential relative to the earth. If a positively charged object is held some distance above the ground, 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 pointing away from the object, towards the ground. The electric field strength is equal to the potential gradient, and opposite in direction. (2.4)2.2.2 Electric currents 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 if one coulomb passes along the conductor every second . （2.5）A given current flowing through a thin wire represents a greater density of 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 （2.6）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.Ohms law gives this relationship between the potential difference V(V) and the current I (A) as or (2.7)where R () is the resistance, and G (S) = 1/R is the conductance (Fig.2.2). For wire of length I and cross-section area A, these quantities depend on the resistivity(m) and conductivity (Wm) of the materialFig. 2.2 Ohms law or （2.8）For materials normally described as conducitorsis small, while for insulators is large. Semicondz6ctors have resistivity in between these extremes, and are usually very dependent on purity and temperature. (2.9)for a conductor with resistance RT, at reference temperature To. This is explained in more detail 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 metallinsulat or object, which is a function of its geometry. The charge is related to the potential by (2.10)A simple parallel-plate capacitor, with plate area A, insulator thickness d and relative permittivity E, has capacitance (2.11)2.2.3 Magnetic fields A flow of current through a wire produces 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 m-1) is a vector quantity whose magnitude at a distance d from a current Z is given by （2.12）For a more complicated geometry, Amptrek law relates the number 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. （2.13）where Fm (ampere-turns) is the magnetomotive force (mmf). This only works for situations where H is uniform along the lines of magnetic field. The magnetic field produced by a current does not depend on the 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 a magnetic flux density B (T) in air or vacuum （2.14）where the permeability of fire space 0 = 4 x l0-7 Wm. In a ferromagnetic material with relative permeability r （2.15）A second conductor of length I carrying an electric current Z will experience a force F in a magnetic flux density BF=BIL （2.16）The force is at right angles to both the wire and the magnetic field. Its direction is given by Flemings left-hand rule (Fig. 2.4). If the magnetic 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.A flow of magnetic flux (Wb) is caused by the flux density in a given cross-section area A as=BA （2.17）The mmf F, required to cause a magnetic flux toflow through a region of length I and cross-section area A is given by the reluctance R, ( A N b ) or the permeance A (Wb/A) of the region or （2.18）Where （2.19）In ideal materials, the flux density B is directly proportional to the magnetic fieldstrength H.In ferromagnetic materials the relation between 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 (Fig. 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 3.2.2.2.4 ElectromagnetismAny 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 field. 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 （2.20） The direction of the induced voltage is given by Flemingsright-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 be found using Faradayslaw.If a magnetic flux 0 passes through a coil of N turns, the magneticflux linkage Y(Wb-t) is （2.21）Faradays law says that the induced emf is given by （2.22）The direction of the induced emf is given by Lenzs law, which says that the induced voltage is in the direction such that, if the 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.A current flowing 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 the induced voltage to the rate of change of current （2.23）Two coils placed close together will interact. The magnetic field of one coil will link with the wire 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) （2.24）Placing 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 each proportional to theirnumber of turns, so