电气工程及其自动化专业英语课件.ppt

1、 电气工程及其自动化专业英语电气工程及其自动化专业英语 Specialized English for Electrical Engineering Its Automation Contents Part 1 Electrics and Electronics Part 2 Electric Machinery Part 3 Electrical Engineering Part 4 Modern Computer Control Techniques Unit 1 Specialized English Wordscircuit components 电路元件 circuit parame

2、ters 电路参数the dielectric 电介质 storage battery 蓄电池electric circuit 电路 wire导线electrical device 电气设备 electric energy 电能energy source 电源 primary cell 原生电池secondary cell 再生电池 energy converter 电能转换器e.m.f.electromotive force 电动势 unidirectional current 单方向电流circuit diagram 电路图 load characteristic 负载特性terminal

3、 voltage 端电压 external characteristic 外特性Conductor 导体 load resistance 负载电阻generator 发电机 heating appliance 电热器direct-current(D.C.)circuit 直流电路 magnetic and electric field 电磁场time-invariant 时不变的 self-(or mutual-)induction 自（互）感displacement current 位移电流 voltage drop 电压降 conductance 电导 volt-ampere charac

4、teristics 伏安特性metal-filament lamp 金属丝灯泡 carbon-filament lamp 碳丝灯泡non-linear characteristics 非线性特性Unit 1 Circuit Elements and Parameters An electric circuit(or network)is an interconnection of physical electrical devices.The purpose of electric circuits is to distribute and convert energy into some o

5、ther forms.Accordingly,the basic circuit components are an energy source(or sources),an energy converter(or converters)and conductors connecting them（连接（连接它们的）它们的）.An energy source(a primary or secondary cell,a generator and the like)converts chemical,mechanical,thermal or some other forms of energy

6、 into（将（将-转换成转换成-）electric energy.An energy converter,also called load(such as a lamp,heating appliance or electric motor),converts electric energy into light,heat,mechanical work and so on.Events in a circuit can be defined in terms of（用（用-，根据，根据-）e.m.f.(or voltage)and current.When electric energy

7、is generated,transmitted and converted under conditions such that the currents and voltages involved remain constant with time,one usually speaks of direct-current(D.C.)circuits.With time-invariant currents and voltages,the magnetic and electric fields of the associated electric plant are also time-

8、invariant.This is the reason why no e.m.f.s of self-(or mutual-)induction（自感或互（自感或互感）感）appear in D.C.circuits,nor are there（倒装结构）（倒装结构）any displacement currents（位移电流）（位移电流）in the dielectric surrounding the conductors(导体周围的电介质导体周围的电介质）.Fig.1.1 shows in simplified form a hypothetical circuit with a st

9、orage battery as the source and a lamp as the load.The terminals of the source and load are interconnected by conductors(generally but not always wires).As is seen,the source,load and conductors form a closed conducting path.The e.m.f.of the source causes a continuous and unidirectional current to c

10、irculate round this closed path.This simple circuit made up of a source,a load and two wires isseldom,if ever,met with in practice.Practical circuits may contain a large number of sources and loads interconnected in a variety of ways Fig.1.1（按不同方式连接的）（按不同方式连接的）.To simplify analysis of actual circuit

11、s,it is usual to show them symbolically in a diagram called a circuit diagram,which is in fact a fictitious or,rather,idealized model of an actual circuit of network.Such a diagram consists of interconnected symbols called circuit elements or circuit parameters.Two elements are necessary to represen

12、t processes in a D.C.circuit.These are a source of e.m.f.E and of internal(or source)resistance RS,and the load resistance(which includes the resistance of the conductors)R(Fig.1.2)Fig.1.2 Whatever its origin(thermal,contact,etc.),the source e.m.f.E(Fig.1.2(a)is numerically equal to the potential di

13、fference between terminals 1 and 2 with the external circuit open,that is,when there is no current flowing through the source E=?1?2=V12 (1.1)The source e.m.f.is directed from the terminal at a lower potential to that（代替代替terminal）at a higher one（代替代替potential）.On diagram,this is shown by arrows（箭箭头

14、）.When a load is connected to the source terminals(the circuit is then said to be loaded)and the circuit is closed,a current begins to flow round it.Now the voltage between source terminals 1 and 2(called the terminal voltage)is not equal to its e.m.f.because of the voltage drop VS inside the source

15、,that is,across the source resistance RS VS=RSI Fig.1.3 shows a typical so-called external characteristic V=?1?2=V(I)of a loaded source(hence another name is the load characteristic of a source).As is seen,increase of current from zero to II1 causes the terminal voltage of the source to decrease lin

16、early V12=V=EVS=ERSIFig.1.3 In other words,the voltage drop VS across the source resistance rises in proportion to the current.This goes on until a certain limit is reached.Then as the current keeps rising,theproportionality between its value and the voltage drop across the source is upset,and the e

17、xternal characteristic ceases to be（不再是）（不再是）linear.This decrease in voltage may be caused by a reduction in the source voltage,by an increase in the internal resistance,or both.The power delivered by a source is given by the equality（等式）（等式）PS=EI (1.2)where PS is the power of the source.It seems re

18、levant at this point to dispel a common misconception about power.Thus one may hear that power is generated,delivered,consumed,transmitted,lost,etc.In point of fact,however,it is energy that can be generated,delivered,consumed,transmitted or lost.Power is just the rate of energy input or conversion,

19、that is,the quantity of energy generated,delivered,transmitted etc per unit time.So,it would be more correct to use the term energy instead of power in the above context.Yet,we would rather fall in with the tradition.The load resistance R as a generalized circuit element,gives an idea about the cons

20、umption of energy,that is,the conversion of electric energy into heat,and is defined as P=RI2 (1.3)In the general case,the load resistance depends solely on the current through the load,which in fact is symbolized by（用符号）（用符号）the function R(I).By Ohms law,the voltage across a resistance is V=RI (1.4

21、)In circuit analysis,use is often made of the reciprocal of the resistance,termed the conductance,which is defined as g=1/R In practical problems,one often specifies the voltage across a resistance as a function of current V(I),or the inverse relation I(V)have come to be known as volt-ampere charact

22、eristics.Fig.1.4 shows volt-ampere curves for a metal-filament lamp V1(I),and for a carbon-filament lamp V2(I).As is seen,the relation between the voltage and the current in each lamp is other than linear.The resistance of the metal-filament lamp increases,and that of the carbon-filament lamp decrea

23、ses with increase of current.Fig.1.4Electric circuits containing components with non-linear characteristic（含有非线性特性元件的）（含有非线性特性元件的）are called non-linear.If the e.m.f.and internal resistances of sources and associated load resistances are assumed to be independent of the current and voltage,respective

24、ly,the external characteristic V(I)of the sources and the volt-ampere characteristic V1(I)of the loads will be linear.Electric circuits containing only elements with linear characteristic are called linear.Fig.1.5 Most practical circuits may be classed as linear.Therefore,a study into the properties

25、 and analysis of linear circuits is of both theoretical and applied interest.of interest=interestingUnit 2 Specialized English Words ideal source 理想电源 series and parallel equivalent circuit 串并联等值电路 internal resistance 内阻 double subscript 双下标 ideal voltage source 理想电压源 active circuit elements有源电路元件 p

26、assive circuit elements 无源电路元件 power transmission line 输电线 sending end 发送端 receiving end 接收端 leakage current 漏电流 ideal current source 理想电流源 Unit 2 Ideal Sources Series and Parallel Equivalent Circuits Consider an elementary circuit containing a single source of e.m.f.E and of internal resistance RS,

27、and a single load R(Fig.2.1).The resistance of the conductors of this type of circuit may be neglected.In the external portion of the circuit,that is,in the load R,the current is assumed to flow from the junction a(which is at a higher potential such that?a=?1)to the junction b(which is at a lower p

28、otential such that?b=?2).The direction of current flow may be shown either by a hollow arrowhead or by supplying the current symbol with a double subscript whose first digit identifies the junction at a higher potential and the second(省略了省略了identifies)the junction at a lower potential.Thus for the c

29、ircuit of Fig.2.1,the current I=Iab.We shall show that the circuit of Fig.2.1 containing a source of known e.m.f.E and source resistance R may be represented by two types of equivalent circuits.As already started,the terminal voltage of a loaded source is lower than the source e.m.f.by an amount equ

30、al to the voltage drop across the source resistance V=?1?2=E VS=E RSI (2.1)On the other hand,the voltage across the load resistance R is Fig.2.1 Since?1=?a and?2=?b,from Eqs.(2.1)and(2.2)it follows that E-RsI=RI,or E=RSI+RI (2.3)And I=E/(RS+R)V=?a?b=RI (2.2)From the last equation we conclude that th

31、e current through the source is controlled by both the load resistance and the source resistance.Therefore,in an equivalent circuit diagram the source resistance R may be shown connected in series with（与（与-串联）串联）the load resistance R.This configuration may be called the series equivalent circuit(usu

32、ally known as the Thevenin equivalent source-戴维宁等效电源戴维宁等效电源).Depending on the relative magnitude of the voltages across Rs and R,we can develop two modifications of the series equivalent circuit（串联等效电路）（串联等效电路）.Fig.2.2 In the equivalent circuit of Fig.2.2(a),V is controlled by the load current and i

33、s decided by the difference between the source e.m.f.E and the voltage drop V.If RSR and,for the same current,VSV(that is,if the source is operating under conditions very close to（接近）（接近）no-load or an open-circuit),we may neglect the internal voltage drop,put VS=RI=0(very nearly)and obtain the equiv

34、alent circuit of Fig.2.2(b).What we have got is a source whose internal resistance is zero(R=0).It is called an ideal voltage source.In diagrams it is symbolized by（用（用-符号表示）符号表示）a circle with(with结构）结构）an arrow inside and the letter E beside it.When applied to a network,it is called a driving force

35、 or an impressed voltage source.The terminal voltage（端电压）（端电压）of an ideal voltage source is independent of the load resistance and is always equal to the e.m.f.E of the practical source it represents.Its external characteristic is a straight line parallel to the x-axis（与（与X轴平行轴平行的）的）(the dotted line

36、 ab in Fig.1.3).The other equivalent circuit in Fig.2.3 may be called the parallel equivalent circuit(usually known as the Norton equivalent-诺顿等效电路诺顿等效电路).It may also have two modifications.To prove this,we divide the right-and left-hand sides of Eq.(2.3)by RS E/RS=I+V/RS=I+VgS or J=I+IS (2.4)where

37、J=E/RS,current with the source short-circuited(with R=0);IS=V/RS=VgS-current equal to the ratio of the terminal source voltage to the source resistance（-与与-的比率）的比率）;I=V/R=Vg-load current.Eq.(2.4)is satisfied by the equivalent circuit of Fig.2.3(a)in which the source resistance RS is placed in parall

38、el with（与（与-并联）并联）the load resistance R.If gSR and,for the same voltages across RS and R,the current IS0 and i0.The segments of the curve between points a and b or O and c cover a complete cycle of current alternations over one period.Fig.4.1The number of cycles or periods per second is the frequenc

39、y of a periodic current.It is reciprocal of its period f=1/TIt is usually to specify the frequency of any periodic quantity in cycles per second（每秒周数每秒周数）.Thus the frequency of a periodic current will be 1 cycle per second,if its period is 1 second,or 1 cycle/sec.A direct current may be regarded as

40、a special case of a periodic current whose period is infinitely long（无穷大）（无穷大）and the frequency is thus zero.The term（术语）（术语）alternating current is often used in the narrow sense of a periodic current whose constant(direct-current)component is zero,or The frequencies of alternating current encounter

41、ed in practice（在（在实际中遇到的）实际中遇到的）range over（涉及）（涉及）very wide values.The mains frequency is 50Hz in the Soviet Union and Europe,and 60Hz in the United States.Some industrial processes use frequencies from 10 Hz to 2.5109 Hz.in radio practice（在无线电应用中）（在无线电应用中）,frequencies up to 31010 Hz are employed.01

42、0TidtT The definitions for currents just introduced(and,indeed,those that will be introduced shortly)fully apply to periodic voltages,e.m.f.s,magnetic fluxes and any other electrical and magnetic quantities.Some additional remarks are only needed with regard to the sign of alternating voltages and e

43、.m.f.s.An alternating voltage between two points A and B,determined along a specified path l,periodically changes sign,so that if it is assumed to be positive in the direction from A to B（沿沿A到到B的的方向方向）,it will be negative in the direction from B to A at the same instant of time【1】.This is why it is

44、important to label which of the two directions is assumed positive.In diagrams,such a direction is labeled either by arrows or subscripts in the symbols for voltages and is regarded to be the positive reference direction of a voltage(or of an e.m.f.).Electrical engineering uses the simplest and comm

45、onest type of alternating current,the one which varies sinusoidally with time;(按正弦规律变化）按正弦规律变化）hence the term（is called/termed）a harmonic or a sinusoidal current.The preference for sinusoidal currents is explained by the fact that non-sinusoidal currents entail increased energy losses,induced over-v

46、oltages,and excessive interference with（对（对-的干扰）的干扰）communications circuits.The transmission of information over a distance(wire or radio communications circuits,remote control,etc.)also uses sinusoidal currents modulated by the signal in amplitude,frequency or phase.Periodic non-sinusoidal currents

47、 may likewise be treated as composed of sinusoidal currents at a variety of frequencies occurring simultaneously.This is why thorough of sinusoidal-current circuits is of primary importance.The A.C.Generator An A.C.generator consists of a stationary part,the stator,and a revolving part,the rotor.As

48、a rule the rotor carries magnetic poles with coils around them.These are the field coils of the generator,because they establish a magnetic field in the machine.They are energized with direct current through slip rings and brushes.The slots of the stator stacked up from electrical-sheet steel punchi

49、ngs receive the coils of the stator winding【2】.The stator coils are connected in series,as shown by the full and dotted lines in the drawing.The e.m.f.induced in a stator conductor is given by E=Blv where Bmagnetic induction of the field moving relative to the conductor;lactive length of the conduct

50、or;vspeed with which the magnetic field moves relative to the conductor.Since l and v are constant,the induced e.m.f.will vary exactly as B varies.If the induced e.m.f.is to be sinusoidal(which is usually sought),the distribution of B around the circumference of the stator should be as close to sinu