单回路网络-电力牵引交流传动系统.ppt

Unit3 Analysis of a Single-loop Network—单回路网络分析Ohm’s Law for a Circuit Branch with an e.m.f.As will be recalled, for the potential of any point to be determined uniquely, it should be related to some reference, or datum, point.分析：as…插入语，for…介词短语做目的状语主句it指代any point ， reference point， datum point，这几个词组意思相同且为同位语同位语省译)翻译：含一个电动势支路的欧姆定律如前所述，要唯一地确定任一点的电位应该取定一个参考点(也就是基准点)New Words & Expressions:Ohm’s Law 欧姆定律 circuit branch 支路 Single-Loop Network 单回路网络/电路vIf in the network shown in Fig.2.2(a) the reference or datum potential is φ2=constant=C, then, by definition, the potential at point 1’ will be higher than φ2 by an amount equal to the e.m.f. E (equation).如果图2.2(a)所示电路的参考电位是φ2=常量C，那么根据定义，点1’的电位将比φ2高一个电动势E(公式)。

vAs already stated, the current in the external circuit, or generally in any passive element, is directed from a point at a higher potential a to a point at a lower potential b. Therefore the potential φ1= φa at point 1 is higher than the potential φ2 at point 2 翻译：如前所述, 外电路（通常是无源元件）的电流是从高电势点a流向低电势点b因此，点1 的电势φ1= φa高于点2的电势φ2valso φ1’= φ1+RsIvFrom Eqs.(1) and (2) it follows that φ1’= φ2+E=φ1+RsIHence the current (eq.3)同时由式(1)和式(2)可推出于是电流vIn the same vein, we may write an equation for the current in any branch of an arbitrarily complex circuit with any number of e.m.f.s and resistances and a specified P.D. across that branch (Fig.3.1).v同样，我们也可以对含有多个电动势和电阻、端电压已知的任意复杂的任一支路列出电流方程。

New Words & Expressions:any number of adj. 许多 P.D.=potential drop 电压降vThe current I in a branch containing an e.m.f. (an active branch) may be directed either from point a to point b, or the other way round. If the direction of current flow is not specified in advance, it should be assumed so that equation of the form Eq.(3) may be written.v对于包含有一个电动势的支路（即有源支路），电流I既可能从a流向b，也可能从b流向a如果没有提前指定电流方向，就应该假设一个电流方向，以便写成式（3）的方程式New Words & Expressions:the other way round 方向相反, 倒过来vSuch an assumed direction is termed the positive reference direction and is denoted exactly as the actual direction by a hollow arrowhead or a current symbol with suitable subscripts.v这个假定的方向称为正参考方向，认为它准确地指示电流的实际方向，并用一个空心箭头或者带有合适下标的电流符号表示。

New Words & Expressions:positive reference direction 正参考方向vIf the positive reference direction of I in the branch of Fig.3.1 is from a to b, the potential φb will be expressed in terms of φa thus (eq.)v如果图3.1中支路I的正参考方向是从a到b，那么电势φb可用φa来表示（公式）vA minus sign in an answer obtained by Eq.(4) indicates an incorrect guess of the direction of current flow (that in the actual direction of current flow is the other way round).v式(4)得出的答案如果是负值，则表明对电流正方向的猜想是错误的，也就是说，电流实际的流向跟之前假定的电流方向是相反的vFrequently it is necessary to guess also the direction of the voltage between any points in a circuit. The assumed positive direction for a voltage is indicated by the subscripts of the voltage symbol V or by an arrow in the diagram.v通常，假定电路中任意两点之间的电压方向也是非常必要的，假设的电 压正方向由电压符号V的下标或者电路图中的箭头表示。

vIn our further discussion we shall place an arrow pointing from a to b for a voltage Vab= φa –φb deeming point a to be at a higher potential than point b. Thus in current (calculations), voltage should be regarded as an algebraic quantity in calculations. v在接下来的讨论中，我们将用一个从a指向b的箭头来表示Vab= φa –φb ，认为a点电势高于b点这样，在电流的计算中，电压被认为是一个代数量New Words & Expressions:algebraic quantity 代数元(素), 代数量vThis “guess” technique also applies to e.m.f.s of voltage sources and currents of current sources, if their directions are not specified in advance. The convention is the same: use either double subscripts or an arrow.v若电压源的电动势和电流源的电流方向事先没有确定，以上假定正方向的做法同样适用。

规则是相同的，即可用双下标也可用箭头表示vFor branches containing only passive elements, current and voltage will always be chosen in the same direction, which makes redundant a separate arrow for voltage.v分析：注意which引导的非限定性定语从句sth makes sth adj.此处由于宾语较长，故将adj放在宾语后面v翻译：对于仅含有无源元件的支路来说，我们总是将电压和电流取相同的方向，这样便可以省去表示电压方向的箭头Potential Distribution in a Single-Loop Network--单回路网络的电势分布vThe potential distribution in a single-loop network can be conveniently represented by a suitable plot. Fig.3.2 shows an elementary single-loop network with two sources of e.m.f.s E1 and E2 and of source resistances Rs1 and Rs2, and two load resistances.New Words & Expressions:potential distribution 电势分布单回路网络的电势分布可以方便地用恰当的曲线来表示。

图3.2给出一个基本的单回路电路，其中包括两个电动势分别为E1和E2、内阻分别为Rs1 和 Rs2的电源和两个负载电阻R1 和 R2vLet E1 be greater than E2. then the actual current is (eq.) and flows in the same direction as E1.设E1>E2，那么实际的电流是：（公式），且电流方向与E1相同vSo that the potential of any point in the network can be determined unambiguously, we take the point a to be at zero potential (or earthed, as is seen in Fig.3.2) (and take)φa to be the datum potential. Now the potentials of any other points can be readily stated by reference to it. 为使电路中任一点电位值是唯一确定的 ，我们将a点定为零电位（或接地，如图3.2所示），φa为基准电位。

这样电路中任意其他各点的电位参考它便可很容易确定vThe potential at b is lower than that at a (eq.).vOn moving through the first source, the potential rises by an amount equal to E and decreases by an amount equal to。