变压器及双绕组变压器的工作原理

变压器及双绕组变压器的工作原理 外文文献翻译 要从远端发电厂送出电能，必须应用高压输电。因为最终的负荷，在一些点高电压 必须降低。变压器能使电力系统各个部分运行在电压不同的等级。本文我们讨论的原则 和电力变压器的应用。 变压器的最简单形式包括两个磁通相互耦合的固定线圈。两个线圈之所以相互耦 合，是因为它们连接着共同的磁通。 在电力应用中，使用层式铁芯变压器(本文中提到的)。变压器是高效率的，因为它 没有旋转损失，因此在电压等级转换的过程中，能量损失比较少。典型的效率范围在92到99%，上限值适用于大功率变压器。 从交流电源流入电流的一侧被称为变压器的一次侧绕组或者是原边。它在铁圈中建 立了磁通φ，它的幅值和方向都会发生周期性的变化。磁通连接的第二个绕组被称为变 压器的二次侧绕组或者是副边。磁通是变化的；因此依据楞次定律，电磁感应在二次侧 产生了电压。变压器在原边接收电能的同时也在向副边所带的负荷输送电能。这就是变 压器的作用。 当二次侧电路开路是，即使原边被施以正弦电压V，也是没有能量转移的。外加电p 压在一次侧绕组中产生一个小电流I。这个空载电流有两项功能：（1）在铁芯中产生电θ 磁通，该磁通在零和φ之间做正弦变化，φ是铁芯磁通的最大值；（2）它的一个分,mm 量说明了铁芯中的涡流和磁滞损耗。这两种相关的损耗被称为铁芯损耗。 变压器空载电流I一般大约只有满载电流的2%—5%。因为在空载时，原边绕组中θ 的铁芯相当于一个很大的电抗，空载电流的相位大约将滞后于原边电压相位90º。显然可见电流分量I= Isinθ，被称做励磁电流，它在相位上滞后于原边电压V 90º。就m00P是这个分量在铁芯中建立了磁通；因此磁通φ与I同相。 m 1 外文文献翻译 第二个分量I=Isinθ，与原边电压同相。这个电流分量向铁芯提供用于损耗的电e00 流。两个相量的分量和代表空载电流，即 I = I+ I0me 应注意的是空载电流是畸变和非正弦形的。这种情况是非线性铁芯造成的。 如果假定变压器中没有其他的电能损耗一次侧的感应电动势E和二次侧的感应电p压E可以表示出来。因为一次侧绕组中的磁通会通过二次绕组，依据法拉第电磁感应定s 律，二次侧绕组中将产生一个电动势E，即E=NΔφ/Δt。相同的磁通会通过原边自身，产生一个电动势E。正如前文中讨论到的，所产生的电压必定滞后于磁通90º，因此，p 它于施加的电压有180º的相位差。因为没有电流流过二次侧绕组，E=V。一次侧空载ss电流很小，仅为满载电流的百分之几。因此原边电压很小，并且V的值近乎等于E。原pp边的电压和它产生的磁通波形是正弦形的；因此产生电动势E和E的值是做正弦变化ps的。产生电压的平均值如下 给定时间内磁通变化量E = turns× avg给定时间 即是法拉第定律在瞬时时间里的应用。它遵循 2,mE = N = 4fNφ avgm1/(2)f 其中N是指线圈的匝数。从交流电原理可知，有效值是一个正弦波，其值为平均电压的 1.11倍；因此 E = 4.44fNφ m 因为一次侧绕组和二次侧绕组的磁通相等，所以绕组中每匝的电压也相同。因此 E = 4.44fNφ ppm 并且 E = 4.44fNφ ssm 其中N和E是一次侧绕组和二次侧绕组的匝数。一次侧和二次侧电压增长的比率称做变ps 比。用字母a来表示这个比率，如下式 EpNpa = = ssEN 假设变压器输出电能等于其输入电能——这个假设适用于高效率的变压器。实际上 我们是考虑一台理想状态下的变压器；这意味着它没有任何损耗。因此 2 外文文献翻译 P = P mout 或者 VI × primary PF = VI × secondary PF ppss 这里PF代表功率因素。在上面公式中一次侧和二次侧的功率因素是相等的；因此 VI = VI ppss 从上式我们可以得知 VpIpEp = ? ? a sssVIE 它表明端电压比等于匝数比，换句话说，一次侧和二次侧电流比与匝数比成反比。 匝数比可以衡量二次侧电压相对于一次恻电压是升高或者是降低。为了计算电压，我们 需要更多数据。 终端电压的比率变化有些根据负载和它的功率因素。实际上, 变比从标识牌数据获得, 列出在满载情况下原边和副边电压。 当副边电压V相对于原边电压减小时，这个变压器就叫做降压变压器。如果这个电s 压是升高的，它就是一个升压变压器。在一个降压变压器中传输变比a远大于1(a>1.0)，同样的，一个升压变压器的变比小于1(a<1.0)。当a=1时，变压器的二次侧电压就等于 起一次侧电压。这是一种特殊类型的变压器，可被应用于当一次侧和二次侧需要相互绝 缘以维持相同的电压等级的状况下。因此，我们把这种类型的变压器称为绝缘型变压器。 显然，铁芯中的电磁通形成了连接原边和副边的回路。在第四部分我们会了解到当 变压器带负荷运行时一次侧绕组电流是如何随着二次侧负荷电流变化而变化的。 VpIpEp从电源侧来看变压器，其阻抗可认为等于V / I。从等式 = ? ? appsssVIE中我们可知V = aV并且I = I/a。根据V和I，可得V和I的比例是 pspssspp 2saVVpaVs = = spIIIa/s 但是V / I负荷阻抗Z，因此我们可以这样表示 ss L 2Z (primary) = aZ mL 2这个等式表明二次侧连接的阻抗折算到电源侧，其值为原来的a倍。我们把这种折算方式称为负载阻抗向一次侧的折算。这个公式应用于变压器的阻抗匹配。 3 外文文献翻译 一次侧电压和二次侧电压有着相同的极性，一般习惯上用点记号表示。如果点号同 在线圈的上端，就意味着它们的极性相同。因此当二次侧连接着一个负载时，在瞬间就 有一个负荷电流沿着这个方向产生。换句话说，极性的标注可以表明当电流流过两侧的 线圈时，线圈中的磁动势会增加。 因为二次侧电压的大小取决于铁芯磁通大小φ，所以很显然当正常情况下负载电0 势E没有变化时，二次侧电压也不会有明显的变化。当变压器带负荷运行时，将有电流s I流过二次侧，因为E产生的感应电动势相当于一个电压源。二次侧电流产生的磁动势ss NI会产生一个励磁。这个磁通的方向在任何一个时刻都和主磁通反向。当然，这是楞ss 次定律的体现。因此，NI所产生的磁动势会使主磁通φ减小。这意味着一次侧线圈中ss0 的磁通减少，因而它的电压E将会增大。感应电压的减小将使外施电压和感应电动势之p 间的差值更大，它将使初级线圈中流过更大的电流。初级线圈中的电流I的增大，意味p着前面所说明的两个条件都满足：（1）输出功率将随着输出功率的增加而增加（2）初级线圈中的磁动势将增加，以此来抵消二次侧中的磁动势减小磁通的趋势。 总的来说，变压器为了保持磁通是常数，对磁通变化的响应是瞬时的。更重要的是， 在空载和满载时，主磁通φ的降落是很少的（一般在）1至3%。其需要的条件是E降0 落很多来使电流I增加。 p ’’在一次侧，电流I在一次侧流过以平衡I产生的影响。它的磁动势NI只停留在pspp一次侧。因为铁芯的磁通φ保持不变，变压器空载时空载电流I必定会为其提供能量。00 ’’故一次侧电流I是电流I与I的和。 pp0 因为空载电流相对较小，那么一次侧的安匝数与二次侧的安匝数相等的假设是成立 的。因为在这种状况下铁芯的磁通是恒定的。因此我们仍旧可以认定空载电流I相对于0满载电流是极其小的。 当一个电流流过二次侧绕组，它的磁动势（NI）将产生一个磁通，于空载电流Iss0产生的磁通φ不同，它只停留在二次侧绕组中。因为这个磁通不流过一次侧绕组，所0 以它不是一个公共磁通。 另外，流过一次侧绕组的负载电流只在一次侧绕组中产生磁通，这个磁通被称为一 次侧的漏磁。二次侧漏磁将使电压增大以保持两侧电压的平衡。一次侧漏磁也一样。因 此，这两个增大的电压具有电压降的性质，总称为漏电抗电压降。另外，两侧绕组同样 具有阻抗，这也将产生一个电阻压降。把这些附加的电压降也考虑在内，这样一个实际 4 外文文献翻译 的变压器的等值电路图就完成了。由于分支励磁体现在电流里，为了分析我们可以将它 忽略。这就符我们前面计算中可以忽略空载电流的假设。这证明了它对我们分析变压器 时所产生的影响微乎其微。因为电压降与负载电流成比例关系，这就意味着空载情况下 一次侧和二次侧绕组的电压降都为零。 5 外文文献翻译 对于所有实际目的来说，直流发电机仅用于特殊场合和地方性发电厂。这个局限性 是由于换向器要把发电机内部的电压整流为直流电压，因此使大规模直流发电不能实 行。 结果，所有大规模生产的电能都以三相交流电的形式生产和分配。今天固态转换器 的应用使交流变直流成为可能。而且，直流发电机的操作特性一直重要，因为大部分的 理论能被应用到所有其它机器上。 对于一个有四个电极的机器其电刷和励磁绕组的一般布置如图1所示。四个电刷安在换向器上，正极电刷和A1端子相连，负极电刷和A2端子相连。正如在草图中所示，电刷被放置在电极下接近中间的位置，它们与线圈相接触，这些线圈产生很少或不产生 电动势，因为它们边被安在电极之间。 图1 四极发电机模型 四个励磁磁极通常串联在一起，并且它们的末端与标注F1和F2的端子相连。它们这样连接是为了交替产生N,S极。 直流发电机的类型以励磁绕组提供的方式来划分。一般来说，用来连接励磁绕组和 电枢绕组的方式可归结为以下几组（看图2）： 6 外文文献翻译 图2 直流发电机励磁连接：(a)它励发电机；(b)自励，自并励；(c)串励发电机；(d)复励发电机，短并励连接；(e)复励发电机，长并励连接。 1．它励发电机，励磁绕组被连接到一个独立的直流供电源上。 2．自励发电机，它们可以进一步划分为： （a） 并励发电机，励磁绕组和转子端部相连。 （b） 串励发电机，励磁绕组以串联方式和转子绕组相连。 （c） 复励发电机，励磁由一个并联和串联的复合绕组提供。 并联绕组包括很多匝相对较细的细线，它们只能承载一个较小的电流，仅为额定电 流的很小一个百分比。另一方面，串联绕组有很少匝粗线，因为它和转子串联，因而承 载较重的电流。 在讨论直流发电机端部特性之前，让我们测试一下发电机在空载时的电压和励磁电 流之间的关系。发电机电动势和每个电极的磁通及发电机给定的转速成正比，即，EG=knφ,通过控制让转速为定值，可以显示出电势EG直接依赖于磁通，在实际的发电机上测 试这种依赖关系并不是非常实际的，因为它要牵涉到磁通的测量。磁通由励磁线圈的安 培匝数产生；磁通必需依赖于励磁电流的大小，因为励磁线圈的匝数是恒定的。这种关 系并不是线性的，因为在励磁电流达到某一个值后将出现磁饱和，EG对励磁电流If的变化关系可以磁化曲线或开路特性曲线来表示，对于这台给定以恒速运转的发电机，没 7 外文文献翻译 有带负载电流，并且它的励磁是它励方式。 If从0逐渐增大到一个适宜的值，使发电机机端电压达到额定电压以上，并测量相 对应If的每个机端电压EG的值，产生的曲线入图3所示，当If=0时，即励磁回路为开路，由于剩磁，测量到一个很小的电压Er，随着励磁电流的增大，产生的电动势线性 地增大到磁化曲线的拐点处，过了这个点以后，增大励磁电流逐渐引起磁路饱和。 图3 它励支直流发电机的磁化曲线或开路特性曲线 这意味着使电压达到一定值时需要一个更大的励磁电流。 因为产生的电压EG也直接与转速成比例，因此一旦这条曲线确定，对于任何其它速度， 这条磁化曲线能被描出来，这仅仅要求依照 EG‘=EG*n’/n 在这条曲线上所有点进行调整。 让我们进一步考虑在发电机上增加一个负载的情况。因为电枢绕组上有电阻，所以机 端电压将要下降，除非采取一些措施保持它恒定，显示机端电压随负载电流变化关系的 曲线被叫做负载特性曲线或外特性曲线。 8 外文文献翻译 图4 （a）直流它励发电机负载特性；(b)电路图 图4显示了它励发电机的外特性，机端电压下降主要是因为电枢电阻RA,即Vt=EG-IARA 此处Vt是机端电压，IA是发电机带负载时的电枢电流（或负载电流）。 另一个导致机端电压下降的因素是由于电枢反应而导致磁通的减少。电枢电流建立一 个磁动势，这个磁动势使主磁通发生畸变，导致弱磁效应，这种情况尤其在无附加磁极 机器上表现更为突出，这种效应叫做电枢反应。正如图4所示，因为铁心的非线形，机端电压对于负载电流并没有成线形下降。由于电枢反应依赖于电枢电流，使得曲线呈下 倾特性。 并励发电机的并励励磁绕组电枢绕组平行连接，以便机器本身提供它的自己的励 磁，正如图5所示。 电压的建立正如所说的，在励磁磁极中要有剩磁。通常，假如发电机以前已经用过， 将会有剩磁存在。我们已经在第三部分中看到假如励磁没连上的话如果发电机已经以某 速度运转，因为有剩磁将要有小的电压Er产生，这个小的电压将提供给并励绕组并驱 动一个小的电流从励磁回路中流过，假如在并励绕组中的这个小的电流的方向正好使剩 磁减弱，则这个电压将接近于零，机端电压不能建立。这种情况下这个弱化主磁极的磁 通与剩磁抵消。 9 外文文献翻译 图5 并励发电机：（a）电路；(b)负载特性 假如关系是这样：弱化主磁极的磁通助增了剩磁通，导致电压变的更大，这反过来 意味着更大的电压提供给了主励磁，机端电压快速增大一个常值，这个建立的过程易看 成是渐增的，然后更大的增大了励磁电流，它反过来又增大了电压，等。这个过程终止 于一个有限的电压值的原因是磁路的非线性。 这个电路仅有直流电流，以致励磁电流仅依赖于励磁回路的电阻Rf,这可能由励磁绕组电阻加上与它相串联的可变电阻Rin组成。对于一给定值的励磁回路电阻Rf ，按 照欧姆定律，励磁电流依赖于所产生的电压。 应该是明显的，在一台新机器上或一台闲置了很常时间已经失去剩磁的机器上，必 须要建立磁场，通常做法是通过连接励磁绕组到一独立直流电源上几秒钟，这个过程正 是快速建立励磁。 总之，阻止电压建立有四种条件，发电机电压极性取决于转动的方向，假如一台发 电机在其它条件都满足的情况下不能建立电压，那肯定是电刷的极性反了，可以通过颠 倒转动方向来解决 ，颠倒方向后关于剩磁通的主磁极性也将颠倒，假如现在电压还不 能建立，它意味着主励磁和剩磁是对立的。 串励发电机 正如前面提到的，串励发电机的励磁绕组和电枢绕组串联因为它承载负荷电流，因 此励磁线圈仅由几匝细导线。空载时，仅有剩磁，机端电压小，当加上负载时，磁通增 加，机端电压也增加，图7显示了串励发电机在某转速运转时的负载特性，虚线指示同 10 外文文献翻译 台机器电枢开路且它励情况下所产生的电动势，这两条曲线的差值简直就是在串励绕组 和电枢绕组上的IR的压降，例如， Vt=EG-IA(RA+RS) 此处，RS是串励绕组电阻 图7 串励发电机：（a）电路图；（b）负载特性 复励发电机有一个并励和一个串励励磁绕组，后者在并励绕组的顶部，图8显示了这个电路图，这两个绕组通常这样连接是为了使它们的安培匝数在相同方向，正因为 如此，这种发电机被称作积复励。 图8的并联连接被称作长复励。假如并励绕组直接和电枢端部连接在一块，这种连 接被称作短复励，实际中这种连接很少应用，因为和满负荷电流相比，并励绕组承载的 电流小，此外串励绕组匝数少，这意味着它的电阻也小，在满负荷时在它上面所对应的 电压降是最小的。 图9曲线仅仅反映了并励绕组外特性，正如所示随着一个小串励绕组的增加，机端 压降随负荷增加而减小，这样的发电机被称作欠复励，通过增加串励匝数，空载和满载 时机端电压能够相等，这种发电机被称作平复励。假如串励匝数比需要的多些以补偿电 压降，这种发电机被称作过复励，在这种情况下，满载电压比空载时还高。 11 外文文献翻译 图8复励发电机 图9复励发电机外特性与并励发电机外特 性比较 过复励可能被用于负荷与发电机存在一定距离的场合，在馈电线上的电压降随着负 载增加而得到补偿。颠倒和并励相对应的串励绕组的极性时，励磁将彼此抵消，且随着 负荷电流的增加而尤为突出，这样的发电机被称作差复励，它被用于负荷可能发生或接 近短路的场合，例如，馈电线可能断线或短接发电机，不过短路电流仍被限制在一个安 全的值，这种类型的发电机的外特性也显示在图9中。因为复励发电机的外特性能被的有很广的变化范围，故这种发电机比其他类型的有更广的用途。 正如插图中所示，在复励合适的角度下，满载时机端电压能被保持在空载时的值上。 电压控制的其他是可变电阻的使用，。例如，装在励磁回路上。不过，随着负荷的 变化，要求恒定调节可变电阻来保持电压。 一个更有用的现在普遍使用的东西是用一台发电机电压自动调节装置，在本质上， 电压调节器是一个反馈控制系统，发电机输出的电压能够被感知并于一个固定的参考电 压相比较，任何输出电压只要偏离参考电压，就将发出一误差信号，并送入功率放大器， 而这个功率放大器提供励磁电流，假如误差信号为正，例如，输出电压大于设定电压， 则功率放大器蒋减小它的电流驱动，如此，直到偏差信号减小为零。 12 外文文献翻译 The high-voltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications. A transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. The coils are said to be mutually coupled because they link a common flux. In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers. The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing; therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenz’s law. Thus the primary receives its power from the source while the secondary supplies this power to the load. This action is known as transformer action. 13 外文文献翻译 When a sinusoidal voltage V is applied to the primary with the secondary p open-circuited, there will be no energy transfer. The impressed voltage causes a small current I to flow in the primary winding. This no-load current has θ two functions: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and φ, where φ is the maximum value of the core ,mmflux; and (2) it provides a component to account for the hysteresis and eddy current losses in the core. There combined losses are normally referred to as the core losses. The no-load current I is usually few percent of the rated full-load current θ of the transformer (about 2 to 5%). Since at no-load the primary winding acts as a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90º. It is readily seen that the current component I= Isinθ, called the magnetizing current, is 90º in phase behind the primary m00 voltage V. It is this component that sets up the flux in the core; φ is P therefore in phase with I. m The second component, I=Isinθ, is in phase with the primary voltage. It is e00 the current component that supplies the core losses. The phasor sum of these two components represents the no-load current, or I = I+ I0me It should be noted that the no-load current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material. If it is assumed that there are no other losses in the transformer, the induced voltage In the primary, E and that in the secondary, E can be shown. Since ps the magnetic flux set up by the primary winding，there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, E=NΔφ/Δt. This same flux also links the primary itself, inducing in it an 14 外文文献翻译 EMF, E. As discussed earlier, the induced voltage must lag the flux by 90º, p therefore, they are 180º out of phase with the applied voltage. Since no current flows in the secondary winding, E=V. The no-load primary current I is small, ss0 a few percent of full-load current. Thus the voltage in the primary is small and V is nearly equal to E. The primary voltage and the resulting flux are pp sinusoidal; thus the induced quantities E and E vary as a sine function. The psaverage value of the induced voltage given by changeinfluxinagiventime E = turns× avggiventime which is Faraday’s law applied to a finite time interval. It follows that 2,mE = N = 4fNφ avgm1/(2)fwhich N is the number of turns on the winding. Form ac circuit theory, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage; thus E = 4.44fNφ mSince the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence E = 4.44fNφ ppmand E = 4.44fNφ ssmwhere E and Es are the number of turn on the primary and secondary windings, p respectively. The ratio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a, it is seen that EpNpa = = ssENAssume that the output power of a transformer equals its input power, not a bad sumption in practice considering the high efficiencies. What we really are saying is that we are dealing with an ideal transformer; that is, it has no losses. Thus 15 外文文献翻译 P = P moutor VI × primary PF = VI × secondary PF ppsswhere PF is the power factor. For the above-stated assumption it means that the power factor on primary and secondary sides are equal; therefore VI = VI ppssfrom which is obtained VpIpEp = ? ? a sssVIEIt shows that as an approximation the terminal voltage ratio equals the turns ratio. The primary and secondary current, on the other hand, are inversely related to the turns ratio. The turns ratio gives a measure of how much the secondary voltage is raised or lowered in relation to the primary voltage. To calculate the voltage regulation, we need more information. The ratio of the terminal voltage varies somewhat depending on the load and its power factor. In practice, the transformation ratio is obtained from the nameplate data, which list the primary and secondary voltage under full-load condition. When the secondary voltage V is reduced compared to the primary voltage, the s transformation is said to be a step-down transformer: conversely, if this voltage is raised, it is called a step-up transformer. In a step-down transformer the transformation ratio a is greater than unity (a>1.0), while for a step-up transformer it is smaller than unity (a<1.0). In the event that a=1, the transformer secondary voltage equals the primary voltage. This is a special type of transformer used in instances where electrical isolation is required between the primary and secondary circuit while maintaining the same voltage level. Therefore, this transformer is generally knows as an isolation transformer. As is apparent, it is the magnetic flux in the core that forms the connecting link between primary and secondary circuit. In section 4 it is shown how the 16 外文文献翻译 primary winding current adjusts itself to the secondary load current when the transformer supplies a load. Looking into the transformer terminals from the source, an impedance is seen VpIpEpwhich by definition equals V / I. From = ? ? a , we have V = pppsssVIEaV and I = I/a.In terms of V and I the ratio of V to I is spssspp 2saVVpaVs = = spIIIa/s But V / I is the load impedance Z thus we can say that ssL 2Z (primary) = aZ mLThis equation tells us that when an impedance is connected to the secondary side, 2it appears from the source as an impedance having a magnitude that is a times its actual value. We say that the load impedance is reflected or referred to the primary. It is this property of transformers that is used in impedance-matching applications. The primary and secondary voltages shown have similar polarities, as indicated by the “dot-making” convention. The dots near the upper ends of the windings have the same meaning as in circuit theory; the marked terminals have the same polarity. Thus when a load is connected to the secondary, the instantaneous load current is in the direction shown. In other words, the polarity markings signify that when positive current enters both windings at the marked terminals, the MMFs of the two windings add. Since the secondary voltage depends on the core flux φ, it must be clear that 0the flux should not change appreciably if E is to remain essentially constant s under normal loading conditions. With the load connected, a current I will s flow in the secondary circuit, because the induced EMF E will act as a voltage ssource. The secondary current produces an MMF NI that creates a flux. This ssflux has such a direction that at any instant in time it opposes the main flux 17 外文文献翻译 that created it in the first place. Of course, this is Lenz’s law in action. Thus the MMF represented by NI tends to reduce the core flux φ. This means ss0that the flux linking the primary winding reduces and consequently the primary induced voltage E, This reduction in induced voltage causes a greater p difference between the impressed voltage and the counter induced EMF, thereby allowing more current to flow in the primary. The fact that primary current I increases means that the two conditions stated earlier are fulfilled: (1) p the power input increases to match the power output, and (2) the primary MMF increases to offset the tendency of the secondary MMF to reduce the flux. In general, it will be found that the transformer reacts almost instantaneously to keep the resultant core flux essentially constant. Moreover, the core flux φ drops very slightly between n o load and full load (about 1 to 3%), a 0 necessary condition if E is to fall sufficiently to allow an increase in I. pp ’On the primary side, I is the current that flows in the primary to balance the p ’demagnetizing effect of I. Its MMF NI sets up a flux linking the primary spp only. Since the core flux φ remains constant. I must be the same current 00that energizes the transformer at no load. The primary current I is therefore p ’the sum of the current I and I. p0 Because the no-load current is relatively small, it is correct to assume that the primary ampere-turns equal the secondary ampere-turns, since it is under this condition that the core flux is essentially constant. Thus we will assume that I is negligible, as it is only a small component of the full-load current. 0 When a current flows in the secondary winding, the resulting MMF (NI) creates ssa separate flux, apart from the flux φ produced by I, which links the 00secondary winding only. This flux does no link with the primary winding and is therefore not a mutual flux. In addition, the load current that flows through the primary winding creates a flux that links with the primary winding only; it is called the primary leakage flux. The secondary- leakage flux gives rise to an induced voltage 18 外文文献翻译 that is not counter balanced by an equivalent induced voltage in the primary. Similarly, the voltage induced in the primary is not counterbalanced in the secondary winding. Consequently, these two induced voltages behave like voltage drops, generally called leakage reactance voltage drops. Furthermore, each winding has some resistance, which produces a resistive voltage drop. When taken into account, these additional voltage drops would complete the equivalent circuit diagram of a practical transformer. Note that the magnetizing branch is shown in this circuit, which for our purposes will be disregarded. This follows our earlier assumption that the no-load current is assumed negligible in our calculations. This is further justified in that it is rarely necessary to predict transformer performance to such accuracies. Since the voltage drops are all directly proportional to the load current, it means that at no-load conditions there will be no voltage drops in either winding. For all practical purposes, the direct-current generator is only used for special applications and local dc power generation. This limitation is due to the commutator required to rectify the internal generated ac voltage, thereby making largescale dc power generators not feasible. Consequently, all electrical energy produced commercially is generated and distributed in the form of three-phase ac power. The use of solid state converters nowadays makes conversion to dc economical. However, the operating characteristics of dc generators are still important, because most concepts can be applied to all other machines. The general arrangement of brushes and field winding for a four-pole machine 19 外文文献翻译 is as shown in Fig.1. The four brushes ride on the commutator. The positive brusher are connected to terminal A1 while the negative brushes are connected to terminal A2 of the machine. As indicated in the sketch, the brushes are positioned approximately midway under the poles. They make contact with coils that have little or no EMF induced in them, since their sides are situated between poles. Figure 1 Sketch of four-pole dc matchine The four excitation or field poles are usually joined in series and their ends brought out to terminals marked F1 and F2. They are connected such that they produce north and south poles alternately. The type of dc generator is characterized by the manner in which the field excitation is provided. In general, the method employed to connect the field and armature windings falls into the following groups (see Fig.2): 20 外文文献翻译 Figure2 Field connections for dc generators:(a)separately excited generator;(b)self-excited,shunt generator;(c)series generator;(d)compound generator;short-shunt connection;(e)compound generator,long-shunt connection. The shunt field contains many turns of relatively fine wire and carries a comparatively small current, only a few percent of rated current. The series field winding, on the other hand, has few turns of heavy wire since it is in series with the armature and therefore carries the load current. Before discussing the dc generator terminal characteristics, let us examine the relationship between the generated voltage and excitation current of a generator on no load. The generated EMF is proportional to both the flux per pole and the speed at which the generator is driven, EG=kn. By holding the speed constant it can be shown the EG depends directly on the flux. To test this dependency on actual generators is not very practical, as it involves a magnetic flux measurement. The flux is produced by the ampere-turns of the field coils: in turn, the flux must depend on the amount of field current flowing since the number of turns on the field winding is constant. This relationship is not linear because of magnetic saturation after the field current reaches a certain value. 21 外文文献翻译 The variation of EG versus the field current If may be shown by a curve known as the magnetization curve or open-circuit characteristic. For this a given generator is driven at a constant speed, is not delivering load current, and has its field winding separately excited. The value of EG appearing at the machine terminals is measured as If is progressively increased from zero to a value well above rated voltage of that machine. The resulting curve is shown is Fig.3. When Ij=0, that is, with the field circuit open circuited, a small voltage Et is measured, due to residual magnetism. As the field current increases, the generated EMF increases linearly up to the knee of the magnetization curve. Beyond this point, increasing the field current still further causes saturation of the magnetic structure to set in. Figure 3 Magnetization curve or open-circuit characteristic of a separately excited dc machine The means that a larger increase in field current is required to produce a given increase in voltage. Since the generated voltage EG is also directly proportional to the speed, a magnetization curve can be drawn for any other speed once the curve is 22 外文文献翻译 determined. This merely requires an adjustment of all points on the curve according to 'n',EExGGn where the quantities values at the various speeds. Let us next consider adding a load on generator. The terminal voltage will then decrease (because the armature winding ha resistance) unless some provision is made to keep it constant. A curve that shows the value of terminal voltage for various load currents is called the load or characteristic of the generator. Figure 4 (a) directs current it to urge the generator load characteristics; (b) circuit diagram Fig.4 shows the external characteristic of a separately excited generator. The decrease in the terminal voltage is due mainly to the armature circuit resistance RA. In general, V,E,IRtGAA where Vt is the terminal voltage and IA is the armature current (or load current IL) supplied by the generator to the load. Another factor that contributes to the decrease in terminal voltage is the 23 外文文献翻译 decrease in flux due to armature reaction. The armature current established an MMF that distorts the main flux, resulting in a weakened flux, especially in noninterpole machines. This effect is called armature reaction. As Fig.4 shows, the terminal voltage versus load current curve does not drop off linearly since the iron behaves nonlinear. Because armature reaction depends on the armature current it gives the curve its drooping characteristic. A shunt generator has its shunt field winding connected in parallel with the armature so that the machine provides its own excitation, as indicated in Fig.5. The question arises whether the machine will generate a voltage and what determines the voltage. For voltage to “build up” as it is called, there must be some remanent magnetism in the field poles. Ordinarily, if the generator has been used previously, there will be some remanent magnetism. We have seen in Section 3 that if the field would be disconnected, there will be small voltage Ef generated due to this remanent magnetism, provided that the generator is driven at some speed. Connecting the field for self-excitation, this small voltage will be applied to the shunts field and drive a small current through the field circuit. If this resulting small current in the shunt field is of such a direction that it weakens the residual flux, the voltage remains near zero and the terminal voltage does not build up. In this situation the weak main pole flux opposes the residual flux. 24 外文文献翻译 Figure 5 Shunt generator:(a)circuit;(b)load characteristic If the connection is such that the weak main pole flux aids the residual flux, the induced voltage increases rapidly to a large, constant value. The build-up process is readily seen to be cumulanve. That is, more voltage increases the field current, which in turn increases the voltage, and so on. The fact that this process terminates at a finite voltage is due to the nonlinear behavior of the magnctic circuit. In steady state the generated voltage is causes a field current to flow that is just sufficient to develop a flux required for the generated EMF that causes the field current to flow. The circuit carries only dc current, so that the field current depends only on the field circuit resistance, Rf. This may consist of the field circuit resistance Rf, the field current depends on the generated voltage in accordance with Ohm’s law. It should be evident that on a new machine or one that has lost its residual flux because of a long idle period, some magnetism must be created. This is usually done by connecting the field winding only to a separate dc source for a few seconds. This procedure is generally known as flashing the field. Series Generators 25 外文文献翻译 As mentioned previously, the field winding of a series generator is in series with the armature. Since it carries the load current the series field winding consists of only a few turns of thick wire. At no load, the generated voltage is small due to residual field flux only. When a load is added, the flux increases, and so does the generated voltage. Fig.7 shows the load characteristic of a series generator driven at a certain speed. The dashed line indicates the generated EMF of the same machine with the armature open-circuited and the field separately excited. The difference between the two curves is simply the IR drop in the series field and armature winding, such that V,E,I(R，R)tGAAS where RS is the series field winding resistance. Figure 7 Series generator: (a)circuit diagram;(b)load characteristicsCompound Generators The compound generator has both a shunt and a series field winding, the latter winding wound on top of the shunt winding. Fig.8 shows the circuit diagram. The two windings are usually connected such that their ampere-turns act in the same direction. As such the generator is said to be cumulatively compounded. The shunt connection illustrated in Fig.8 is called a long shunt connection. If the shunt field winding is directly connected across the armature terminals, the connection is referred to as a short shunt. In practice the connection used 26 外文文献翻译 is of little consequence, since the shunt field winding carries a small current compared to the full-load current. Furthermore, the number of turns on the series field winding. This implies it has a low resistance value and the corresponding voltage drop across it at full load is minimal. Curves in Fig.9 represents the terminal characteristic of the shunt field winding alone. By the addition of a small series field winding the drop in terminal voltage with increased loading is reduced as indicated. Such a generator is said to be undercompounded. By increasing the number of series turns, the no-load and full-load terminal voltage can be made equal; the generator is then said to be flatcompounded. If the number of series turns is more than necessary to compensate for the voltage drop, the generator is overcome pounded. In that case the full-load voltage is higher than the no-load voltage. Figure 9 Terminal characteristics of compound generators compared with that of the shunt generator The overcompounded generator may be used in instances where the load is at some distance from the generator. The voltage drops in the feeder lines are the compensated for with increased loading. Reversing the polarity of the series field in relation to the shunt field, the fields will oppose each other more and more as the load current increase. Such a generator is said to be differentially compounded. It is used in applications where feeder lines could 27 外文文献翻译 occur approaching those of a short circuit. An example would be where feeder lines could break and short circuit the generator. The short-circuit current, however, is then limited to a “safe” value. The terminal characteristic for this type of generator is also shown in Fig.9. Compound generators are used more extensively than the other types because they may be designed to have a wide varity of terminal characteristics. As illustrated, the full-load terminal voltage can be maintained at the no-load value by the proper degree of compounding. Other methods of voltage control are the use of rheostats, for instance, in the field circuit. However, with changing loads it requires a constant adjustment of the field rheostat to maintain the voltage. A more useful arrangement, which is now common practice, is to use an automatic voltage regulator with the generator. In essence, the voltage regulator is a feedback control system. The generator output voltage is sensed and compared to a fixed reference voltage deviation from the reference voltage gives an error signal that is fed to a power amplifier. The power amplifier supplies the field excitation current. If the error signal is positive, for example, the output voltage is larger than desired and the amplifier will reduce its current drive. In doing so the error signal will be reduced to zero. 28