电气工程及其自动化专业英语课件.ppt

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 ElectronicsPart 2 Electric Machinery Part 3 Electrical Engineering Part 4 Modern Computer Control Techniques,ContentsPart 1 Electrics an,Unit 1 Specialized English Words,circuit components 电路元件 circuit parameters 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 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 characteristics 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 Specialized English Wor,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 other forms. Accordingly, the basic circuit components are an energy source (or sources), an energy converter (or converters) and conductors connecting them（连接它们的）.An energy source (a primary or secondary cell, a generator and the like) converts chemical, mechanical, thermal or some other forms of energy into （将-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.,Unit 1 Circuit Elements and Pa,Events in a circuit can be defined in terms of （用-，根据-）e.m.f. (or voltage) and current. When electric energy 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-invariant. This is the reason why no e.m.f.s of self- (or mutual-)induction（自感或互感）appear in D.C. circuits, nor are there （倒装结构）any displacement currents （位移电流）in the dielectric surrounding the conductors(导体周围的电介质）.,Events in a circuit can be def,Fig.1.1 shows in simplified form a hypothetical circuit with a storage 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 circulate 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）.,Fig.1.1 shows in simplified fo,To simplify analysis of actual circuits, 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 represent 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,To simplify analysis of actual,Whatever its origin (thermal, contact, etc.), the source e.m.f. E (Fig.1.2 (a) is numerically equal to the potential difference 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terminal） at a higher one（代替potential）. On diagram, this is shown by arrows（箭头）.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, that is, across the source resistance RS VS=RSI,Whatever its origin (thermal,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 linearly V12=V=EVS=ERSI,Fig.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 external characteristic ceases to be （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.,Fig.1.3 shows a typical so-cal,The power delivered by a source is given by the equality（等式） PS=EI (1.2) where PS is the power of the source.It seems relevant 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, 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 power delivered by a sourc,The load resistance R as a generalized circuit element, gives an idea about the consumption 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） the function R(I).By Ohms law, the voltage across a resistance is V=RI (1.4) 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 characteristics.,The load resistance R as a gen,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 decreases with increase of current.,Fig.1.4,Electric circuits containing components with non-linear characteristic （含有非线性特性元件的）are called non-linear.,Fig.1.4 shows volt-ampere curv,If the e.m.f. and internal resistances of sources and associated load resistances are assumed to be independent of the current and voltage, respectively, 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 and analysis of linear circuits is of both theoretical and applied interest.,of interest=interesting,If the e.m.f. and internal res,Unit 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 passive 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 Specialized English Wo,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, 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 potential 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identifies) the junction at a lower potential. Thus for the circuit of Fig.2.1, the current I=Iab .,Unit 2 Ideal Sources Series an,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 equal 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),We shall show that the cir,From the last equation we conclude that the 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-串联） the load resistance R. This configuration may be called the series equivalent circuit (usually known as the Thevenin equivalent source-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）.,Fig.2.2,From the last equation we conc,In the equivalent circuit of Fig.2.2(a), V is controlled by the load current and is 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） no-load or an open-circuit), we may neglect the internal voltage drop, put VS=RI=0 (very nearly) and obtain the equivalent 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-符号表示）a circle with (with结构）an arrow inside and the letter E beside it. When applied to a network, it is called a driving force or an impressed voltage source.,In the equivalent circuit of F,The terminal voltage （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X轴平行的） (the dotted line 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). 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 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-的比率）;I=V/R=Vg - load current.,The terminal voltage （端电压,Eq.(2.4) is satisfied by the equivalent circuit of Fig.2.3(a) in which the source resistance RS is placed in parallel with （与-并联）the load resistance R.If gSR and, for the same voltages across RS and R, the current ISI (that is, if the source is operating under conditions approaching a short-circuit), we may put IS=VgS=0 (very nearly) and get the equivalent circuit of Fig.2.3(b).,Fig.2.3,What we have got is a source of zero internal conductance, gS=0(RS=). It is called an ideal current source. When applied to a circuit, it is called a driving force of an impressed current source.The current of an ideal current source is independent of the load resistance R and is equal to E/RS.,Eq.(2.4) is satisfied by the F,The external characteristic of an ideal current source is a straight line parallel to the y-axis （平行于Y轴的）(the dotted line cd in Fig.1.3). Thus whether a real energy source may be represented by an ideal voltage source or an ideal current source depends on the relative magnitude of RS and R. A real source, though, may be represented by an ideal voltage or current source also when RS is comparable with R. In such a case, either RS, or gS=1/RS (Fig.2.2 (a) and Fig.2.3(a), respectively) should be removed from the source and lumped with R or g=1/R. Ideal voltage and current sources are active circuit elements（有源电路元件）, while resistances and conductance are passive elements(无源元件）.,The external characteristic of,In developing an equivalent circuit, it is important to take into account, as much as practicable（实际上）,the known properties （已知的特性）of each device and of the circuit as a whole（整体上）.Let us develop an equivalent circuit for a two-wire power transmission line （输电线）of length I, diagrammatically shown（用图形表示） in Fig.2.4 (a). There is a generator of e.m.f.E and of source resistance RS at the sending end and a load of resistance R2 at the receiving end of the line.,Fig.2.4,In developing an equivalent ci,It is obvious that the receiving end voltage will be less than the sending end one by an amount equal to the voltage drop across the resistance of the line conductors. The current at the receiving end will be smaller than that at the sending end by an amount equal to the leakage current （漏电流）(due to imperfect insulation-绝缘不完善).A less than B by an amount (which is) equal to -; A比B少-Let each line conductor have a resistance R0/2 and a conductance g0 per unit length of the line. We divide the line into length elements dx (Fig.2.4(a). Then each length element will have associated with it the combined resistance of the “go” and “return” wires, R0dx=(R0/2)dx+(R0/2)dx and a conductance g0dx（这样，一长度元将含有与其相关的、含有去与来两段组合的电阻即 R0dx(R0/2)dx+(R0/2)dx）和电导 g0dx ）.,It is obvious that the receivi,Accordingly, the entire line may be represented by a network of elements each of resistance R0dx and conductance g0dx (Fig.2.4 (b). The sending end generator in this network is represented by a voltage source (of e.m.f. E and of source resistance RS). From this equivalent circuit we can find （计算出） the voltage and current at any point on the line in terms of specified voltage and current （根据给定的电压和电流）at the sending or receiving end. If the leakage current of the line is only a small fraction of （只占小部分）the load current, we may neglect it and remove all conductance g0dx from the network（把所有的电导从网络中移开）.,Accordingly, the entire lin,Thi