串并联等值电路-电力牵引交流传动系统解读.ppt

Unit2 Ideal Sources Series and Parallel Equivalent Circuits ——理想电源的串并联等效电路 Consider an elementary circuit containing a single source of e.m.f. E and of internal resistance Rs, and a single load R (Fig.1). The resistance of the conductors of this type of circuit may be neglected. New Words Expressions: internal resistance? ? 内阻 如图1所示，考虑一个由电动势为E、内阻为Rs的电源和负载为R构成的基本电路（此句将英语的祈使句翻译成汉语的无主句）。在这种类型的电路中，导线的电阻一般可以忽略不计。 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 φa=φ1) to the junction b (which is at a lower potential such that φb=φ2). 在电路的外部，即在负载 R 处，假定电流从端点 a（位于φ a = φ 1的高电势）流向端点 b（位于φ b = φ 2的低电势）。 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 (identifies) the junction at a lower potential. Thus for the circuit of Fig.2.1, the current I=Iab. 分析：整句是一个具有定语从句的主从复合句。从句里有并列句，第二句省略谓语动词。 翻译：电流的方向既可以用一个空心箭头来表示，又可用带双下标的电流符号来表示，且第一个下标表示高电位点，第二个下标表示低电位点。因此，对于图2.1中的电路，电流表示为I=Iab。 We shall show that the circuit of Fig.2.1 containing a source of known e.m.f. E and source resistance Rs may be represented by two types of equivalent circuits. 分析：that引导宾语从句。从句的主语又有定语修饰。 下面将证明，对于图2.1所示的含一个已知电动势E和内阻Rs的电源的电路，可以用两种类型的等效电路来表示。 New Words Expressions: source resistance? ? 内阻 As (it is) 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 如前所述，电源加负载后的端电压要比电源电动势低一个值，这个值等于电源内阻两端的电压降： On the other hand, the voltage across the load resistance R is Since and , from Eqs. (2.1) and (2.2) it follows that , or and . 另外，负载电阻R两端的电压是 由于 、 ，从公式（2.1）和（2.2）可以得出 或者 和 From the last equation