电力英语论文

DC GENENRATORS

ABSTRACT：This paper introduces the DC generator winding connection , working voltage adjustment , and according with excitation into the two major categories . One is separately excited , another is self-motivation, which is divided into shunt , series excited , compound , then followed by a brief introduction of their circuit diagrams and the voltage characteristics.

KEYWORDS : DC genenrator separately excited self-excited shunt series compound

1. INTRODUCTION

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. 2. FIELD WINDING CONNECTIONS

The general arrangement of brushes and field winding for a four-pole machine 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.

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.

Figure 1 Sketch of four-pole dc matchine

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):

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.

3. VOLTAGE REGULATION

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. 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 determined. This merely requires an adjustment of all points on the curve according to where the quantities values at the various speeds.

E'Gn'EGxnLet 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.

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,

VtEGIARAwhere 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 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. 4. SHUNT OR SELF-EXCIITED GENRATORS

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.

Figure 5 Shunt generator:(a)circuit;(b)load characteristic

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.

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. 5． SERIES GENERATORS

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.6 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 where RS is the series field winding resistance.

VtEGIA(RARS)

Figure 6 Series generator: (a)circuit diagram;(b)load characteristics

6．COMPOUND GENERATORS

The compound generator has both a shunt and a series field winding, the latter winding wound on top of the shunt winding. Fig.7 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.

Figure 7 Terminal generators (a)circuit diagram;(b)load characteristics

The shunt connection illustrated in Fig.7 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 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.

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.

reference :

1. Zheng Zhaoke, Tang Shunhua. Electrical science. Shanghai: Tongji University press.

1990

2. Zhao Jun, Xu Yimin, Zhang Ling, Li Jianxing. Electrical science. Beijing: Chinese Power Press，2005

3. Long net www.zhulong.com

直 流 发 电 机

摘 要：本文简要介绍了直流发电机的绕组连接方式、工作电压调整，以及根据励磁方式分成

的两大类，一是它励，二是自励，其中自励又分为并励、串励、复励，之后又简要介绍了它们的电路图和伏安特性。

关键词： 直流发电机 它励 自励 并励 串励 复励 1. 介绍

对于所有实用目的而言，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用。

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。

2. 励磁绕组连接

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

们这样连接是为了交替产生N,S极。

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

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）：

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。

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）：

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。

3. 电压调整

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。

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

4. 并励或自并励

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。

图5 并励发电机：（a）电路；(b)负载特性

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。

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直流电源上几秒钟，这个过程正是快速建立励磁。

5. 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6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6 串励发电机：（a）电路图；（b）负载特性

6. 复励发电机

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7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7复励发电机：（a）电路图；（b）负载特性

图7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。

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。电压控制的其他方法是可变电阻的使用，。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。

参考文献：

1．郑朝科，唐顺华. 电机学. 上海：同济大学出版社，1990

2．赵君有，徐益敏，张玲，李建兴 . 电机学. 北京：中国电力出版社，2005 3．筑龙网 www.zhulong.com