电气工程及其自动化专业英语.ppt
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1、电气工程及其自动化专业英语 Specialized English for Electrical Engineering Its Automation 戴文进 主 编 杨植新 副主编,Part 2 Electric Machinery,Unit 11 The Transformer on No Load Unit 13 Construction and Application of D.C. Machines Unit 16 Three-Phase Induction (Asynchronous) Machine with the Rotor at Standstill Unit 19 Arm
2、ature Reaction of Salient-Pole Synchronous Machine a Two-Reaction Theory,Unit 11 The Transformer on No Load- Specialized English Words,in close proximity (to) 紧密耦合 permeability 磁导率 magnetizing current 激磁电流 power component of current 电流的有功分量 sinusoidal variations 正弦变量 a mutually induced e.m.f. 互感电动势
3、the applied voltage 外施电压 zero-power-factor 零功率因数 the no-load power factor 空载功率因数 formulate 用公式表示,系统地阐述 saturation 饱和,Unit 11 The Transformer on No Load,The Simple Two-Coil Transformer The transformer is a straightforward application of Faradays Law of Electromagnetic Induction. The simple transforme
4、r consists of two coils in close proximity. One coil of N1 turns is excited with alternating current and therefore establishes a flux 11 which alternates with the current (随时间交变). The other coil is linked by (与-交链)most of this flux and thus has a mutually induced e.m.f. of value e2=N2d21/dt. This e.
5、m.f. would drive a load current through any circuit connected to the terminals of the second coil. Energy would then be transferred through the medium of the magnetic field(磁场媒介) from coil 1 to coil 2.,The transformation could be from any convenient input voltage to any convenient output voltage. Th
6、is apparently simple function of the transformer makes it as vital to modern industry as the gear train which, as a “transformer” of speed and torque represents an interesting analogy(句子的主语是“function”,谓语为“make”,“as vitalas”意为“与一样重要”,“which”引导一个定语从句“which represents an interesting analogy as atransfo
7、rmerof speed and torque”,修饰“gear train”,意为“齿轮箱与变压器表现出一种有趣的相似,齿轮箱被称为速度和转矩的变换器。” The coils would be on an iron core (铁芯)and so at the cost of introducing iron losses, the value of flux per ampere is increased several hundred times because of the change of permeability from 0 to 0 r .,The exciting or m
8、agnetizing current (励磁电流)can thus be very small. Further, the proportion of the total flux which is linked mutually by the two coils is greatly increased. Looked at another way, the leakage flux 1 which links N1 turns alone and “leaks” between the two windings without linking the N2 turns, is a much
9、 smaller fraction of the total, making for(有助于,倾向于) more efficient energy transfer. The mutual flux would be a much larger percentage of the total and the flux pattern (磁力线图)would be rather more involved(复杂的), but simplified diagrams of this kind are quite adequate for the understanding of the princ
10、iples to be discussed in this chapter. The exciting coil which initiates (激励)the flux changes is called the primary winding, and the coupled coil which receives energy as a result (因此), is called the secondary winding.,It should not be difficult to realize that the two functions are interchangeable:
11、 if coil 2 were excited instead, a mutual e.m.f. would be induced in coil 1 which would then become the secondary winding(二次绕组). It should be obvious that since the primary winding also experiences flux changes, there is a primary e.m.f. self-induced this time. With winding 1 excited, the primary e.
12、m.f. would be equal to N1d 11/dt. This is practically in phase opposition to (与-反相)the applied terminal voltage V1 and limits the current to a very much smaller value than V1/R1, where R1 is the primary resistance(原边电阻). As explained later, the resistance drop is normally very small so that the back
13、 e.m.f. is virtually equal in magnitude to V1.,If the applied voltage is alternating sinusoidally at frequency f=/2 Hz, a vector diagram can be drawn, and for an air-core transformer, neglecting resistance. As shown I1 is the magnetizing current producing 11 which rises and falls with it in time pha
14、se. It is therefore in quadrature with (与-正交)V1 and the circuit is a zero-power-factor lagging load. With the secondary circuit open, the primary coil is an inductor whose voltage drop is +N1d 11/dt or in vector form=j L11I1. The two components of flux linking N1 (与N1交链的磁通)are shown and though I1 is
15、 in phase with both of them,it will be found that when the secondary is carrying current, the mutual flux is no longer in phase with the primary current due to the reaction of the secondary ampere-turns(磁势).,The phase of component 1 is unchanged, however, since it is produced by primary current alon
16、e and is therefore in phase with it. Further, even with an iron core the reluctance offered to 1 is still predominantly due to its path in air so that the leakage flux remains virtually proportional to the primary current providing(=provided) the iron path is not greatly saturated(饱和). The e.m.f. N1
17、d 1/dt may thus be treated as if it were due to an unsaturated inductance of magnitude N121. The corresponding leakage reactance is a dominant factor in transformer behavior(characteristic). For the iron-cored transformer which will form the subject of this chapter, the total back e.m.f. of the prim
18、ary, N1d 11/dt, will be considered in two components, the one due to leakage and the other due to the mutual flux(互感磁通).,The first one will normally be treated in terms of the voltage drop required to sustain the current. This is equal to +N1d 1/dt, or vectorially for frequency f it is given by j L1
19、I1 =jX1I1which is a voltage leading the current by 90(领先电流90度). The second component is due to the mutual flux and is represented by a vector E1, lagging 90behind the flux (滞后磁通90度)time vector. This too could be treated as a voltage drop jMI1N1/N2=jXmI1=E1, but the mutual flux is in iron, subject to
20、 permeability changes and so the mutual inductance M varies with the flux level. Here is the vector diagram for an iron-cored transformer with its secondary winding open circuited, the no-load condition. Allowance has been made for the iron losses (对铁耗损进行补偿)which require a power component of current
21、 Ip.,This is drawn in phase with E1, the component of applied voltage necessary to sustain +E1. E1Ip is the iron loss in watts. The ampere-turns required to produce the flux are given by ImN1 so that on no load the primary takes a current I0 which is made up of the two components Im and Ip. In pract
22、ice Ip is very much smaller than Im and the no-load power factor is very low. The mutual flux is shown asm and is equal to21 on no load. When the secondary carries current, I1 increases from I0;m is then due to the combined effect of primary and secondary ampere-turns, but is little changed from21.
23、Consequently the magnetizing and power requirements of the flux are nearly constant so that I0 can be regarded as a substantially constant component of I1 at any load.,It will be noticed from the diagram that since the e.m.f.s E1 and E2 are due to the same flux m, the voltage ratio E1/E2 is the same
24、 as the turns ratio N1/N2. In fact, on no load (空载时), the impedance drops I0R1 and I0X1 are so small that V1 is virtually the same magnitude as E1 and the turns ratio is equal to the no-load voltage ratio V 1/E2. Thus any voltage V 1 can be transformed to (变换成)any other voltage E2 by suitable choice
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