1、附錄1 外文資料翻譯A1.1 譯文直流發電機1.介紹 對于所有實際目的來說，直流發電機僅用于特殊場合和地方性發電廠。這個局限性是由于換向器要把發電機內部的電壓整流為直流電壓，因此使大規模直流發電不能實行。 結果，所有大規模生產的電能都以三相交流電的形式生產和分配。今天固態轉換器的應用使交流變直流成為可能。而且，直流發電機的操作特性一直重要，因為大部分的理論能被應用到所有其它機器上。 2.勵磁繞組連接對于一個有四個電極的機器其電刷和勵磁繞組的一般布置如圖1所示。四個電刷安在換向器上，正極電刷和A1端子相連，負極電刷和A2端子相連。正如在草圖中所示，電刷被放置在電極下接近中間的位置，它們與線圈相接

2、觸，這些線圈產生很少或不產生電動勢，因為它們邊被安在電極之間。 圖1 四極發電機模型四個勵磁磁極通常串聯在一起，并且它們的末端與標注F1和F2的端子相連。它們這樣連接是為了交替產生N,S極。直流發電機的類型以勵磁繞組提供的方式來劃分。一般來說，用來連接勵磁繞組和電樞繞組的方式可歸結為以下幾組（看圖2）：圖2 直流發電機勵磁連接：(a)它勵發電機；(b)自勵，自并勵；(c)串勵發電機；(d)復勵發電機，短并勵連接；(e)復勵發電機，長并勵連接。1 它勵發電機，勵磁繞組被連接到一個獨立的直流供電源上。2 自勵發電機，它們可以進一步劃分為：（a） 并勵發電機，勵磁繞組和轉子端部相連。（b） 串勵發電

3、機，勵磁繞組以串聯方式和轉子繞組相連。（c） 復勵發電機，勵磁由一個并聯和串聯的復合繞組提供。并聯繞組包括很多匝相對較細的細線，它們只能承載一個較小的電流，僅為額定電流的很小一個百分比。另一方面，串聯繞組有很少匝粗線，因為它和轉子串聯，因而承載較重的電流。在討論直流發電機端部特性之前，讓我們測試一下發電機在空載時的電壓和勵磁電流之間的關系。發電機電動勢和每個電極的磁通及發電機給定的轉速成正比，即，EG=kn,通過控制讓轉速為定值，可以顯示出電勢EG直接依賴于磁通，在實際的發電機上測試這種依賴關系并不是非常實際的，因為它要牽涉到磁通的測量。磁通由勵磁線圈的安培匝數產生；磁通必需依賴于勵磁電流的大

4、小，因為勵磁線圈的匝數是恒定的。這種關系并不是線性的，因為在勵磁電流達到某一個值后將出現磁飽和，EG對勵磁電流If的變化關系可以磁化曲線或開路特性曲線來表示，對于這臺給定以恒速運轉的發電機，沒有帶負載電流，并且它的勵磁是它勵方式。If從0逐漸增大到一個適宜的值，使發電機機端電壓達到額定電壓以上，并測量相對應If的每個機端電壓EG的值，產生的曲線入圖3所示，當If=0時，即勵磁回路為開路，由于剩磁，測量到一個很小的電壓Er，隨著勵磁電流的增大，產生的電動勢線性地增大到磁化曲線的拐點處，過了這個點以后，增大勵磁電流逐漸引起磁路飽和。圖3 它勵支直流發電機的磁化曲線或開路特性曲線 這意味著使電壓達到

5、一定值時需要一個更大的勵磁電流。 因為產生的電壓EG也直接與轉速成比例，因此一旦這條曲線確定，對于任何其它速度，這條磁化曲線能被描出來，這僅僅要求依照EG=EG*n/n 在這條曲線上所有點進行調整。3.電壓調整 讓我們進一步考慮在發電機上增加一個負載的情況。因為電樞繞組上有電阻，所以機端電壓將要下降，除非采取一些措施保持它恒定，顯示機端電壓隨負載電流變化關系的曲線被叫做負載特性曲線或外特性曲線。圖4 （a）直流它勵發電機負載特性；(b)電路圖圖4顯示了它勵發電機的外特性，機端電壓下降主要是因為電樞電阻RA,即Vt=EG-IARA此處Vt是機端電壓，IA是發電機帶負載時的電樞電流（或負載電流）。

6、 另一個導致機端電壓下降的因素是由于電樞反應而導致磁通的減少。電樞電流建立一個磁動勢，這個磁動勢使主磁通發生畸變，導致弱磁效應，這種情況尤其在無附加磁極機器上表現更為突出，這種效應叫做電樞反應。正如圖4所示，因為鐵心的非線形，機端電壓對于負載電流并沒有成線形下降。由于電樞反應依賴于電樞電流，使得曲線呈下傾特性。四.并勵或自并勵并勵發電機的并勵勵磁繞組電樞繞組平行連接，以便機器本身提供它的自己的勵磁，正如圖5所示。電壓的建立正如所說的，在勵磁磁極中要有剩磁。通常，假如發電機以前已經用過，將會有剩磁存在。我們已經在第三部分中看到假如勵磁沒連上的話如果發電機已經以某速度運轉，因為有剩磁將要有小的電壓

7、Er產生，這個小的電壓將提供給并勵繞組并驅動一個小的電流從勵磁回路中流過，假如在并勵繞組中的這個小的電流的方向正好使剩磁減弱，則這個電壓將接近于零，機端電壓不能建立。這種情況下這個弱化主磁極的磁通與剩磁抵消。圖5 并勵發電機：（a）電路；(b)負載特性假如關系是這樣：弱化主磁極的磁通助增了剩磁通，導致電壓變的更大，這反過來意味著更大的電壓提供給了主勵磁，機端電壓快速增大一個常值，這個建立的過程易看成是漸增的，然后更大的增大了勵磁電流，它反過來又增大了電壓，等。這個過程終止于一個有限的電壓值的原因是磁路的非線性。這個電路僅有直流電流，以致勵磁電流僅依賴于勵磁回路的電阻Rf,這可能由勵磁繞組電阻加

8、上與它相串聯的可變電阻Rin組成。對于一給定值的勵磁回路電阻Rf ，按照歐姆定律，勵磁電流依賴于所產生的電壓。應該是明顯的，在一臺新機器上或一臺閑置了很常時間已經失去剩磁的機器上，必須要建立磁場，通常做法是通過連接勵磁繞組到一獨立直流電源上幾秒鐘，這個過程正是快速建立勵磁。 總之，阻止電壓建立有四種條件，發電機電壓極性取決于轉動的方向，假如一臺發電機在其它條件都滿足的情況下不能建立電壓，那肯定是電刷的極性反了，可以通過顛倒轉動方向來解決 ，顛倒方向后關于剩磁通的主磁極性也將顛倒，假如現在電壓還不能建立，它意味著主勵磁和剩磁是對立的。 串勵發電機正如前面提到的，串勵發電機的勵磁繞組和電樞繞組串聯

9、因為它承載負荷電流，因此勵磁線圈僅由幾匝細導線。空載時，僅有剩磁，機端電壓小，當加上負載時，磁通增加，機端電壓也增加，圖7顯示了串勵發電機在某轉速運轉時的負載特性，虛線指示同臺機器電樞開路且它勵情況下所產生的電動勢，這兩條曲線的差值簡直就是在串勵繞組和電樞繞組上的IR的壓降，例如，Vt=EG-IA(RA+RS)此處，RS是串勵繞組電阻圖7 串勵發電機：（a）電路圖；（b）負載特性復勵發電機 復勵發電機有一個并勵和一個串勵勵磁繞組，后者在并勵繞組的頂部，圖8顯示了這個電路圖，這兩個繞組通常這樣連接是為了使它們的安培匝數在相同方向，正因為如此，這種發電機被稱作積復勵。 圖8的并聯連接被稱作長復勵。

10、假如并勵繞組直接和電樞端部連接在一塊，這種連接被稱作短復勵，實際中這種連接很少應用，因為和滿負荷電流相比，并勵繞組承載的電流小，此外串勵繞組匝數少，這意味著它的電阻也小，在滿負荷時在它上面所對應的電壓降是最小的。 圖9曲線僅僅反映了并勵繞組外特性，正如所示隨著一個小串勵繞組的增加，機端壓降隨負荷增加而減小，這樣的發電機被稱作欠復勵，通過增加串勵匝數，空載和滿載時機端電壓能夠相等，這種發電機被稱作平復勵。假如串勵匝數比需要的多些以補償電壓降，這種發電機被稱作過復勵，在這種情況下，滿載電壓比空載時還高。 圖8復勵發電機 圖9復勵發電機外特性與并勵發電機外特性比較過復勵可能被用于負荷與發電機存在一定

11、距離的場合，在饋電線上的電壓降隨著負載增加而得到補償。顛倒和并勵相對應的串勵繞組的極性時，勵磁將彼此抵消，且隨著負荷電流的增加而尤為突出，這樣的發電機被稱作差復勵，它被用于負荷可能發生或接近短路的場合，例如，饋電線可能斷線或短接發電機，不過短路電流仍被限制在一個安全的值，這種類型的發電機的外特性也顯示在圖9中。因為復勵發電機的外特性能被設計的有很廣的變化范圍，故這種發電機比其他類型的有更廣的用途。 正如插圖中所示，在復勵合適的角度下，滿載時機端電壓能被保持在空載時的值上。電壓控制的其他方法是可變電阻的使用，。例如，裝在勵磁回路上。不過，隨著負荷的變化，要求恒定調節可變電阻來保持電壓。 一個更有

12、用的現在普遍使用的東西是用一臺發電機電壓自動調節裝置，在本質上，電壓調節器是一個反饋控制系統，發電機輸出的電壓能夠被感知并于一個固定的參考電壓相比較，任何輸出電壓只要偏離參考電壓，就將發出一誤差信號，并送入功率放大器，而這個功率放大器提供勵磁電流，假如誤差信號為正，例如，輸出電壓大于設定電壓，則功率放大器蔣減小它的電流驅動，如此，直到偏差信號減小為零。譯自<< College English Reading For Students Of Electric Reading>>A1.2 原文DC GENENRATORS1. INTRODUCTION For all pra

13、ctical 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

14、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

15、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.

16、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 j

17、oined 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 fie

18、ld and armature windings falls into the following groups (see Fig.2): Figure 2 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 fi

19、eld 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 t

20、he 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 i

21、t 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

22、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 characteristi

23、c. 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. Th

24、e 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

25、 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

26、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.3. VOLTAGE REGULATION Let us next c

27、onsider 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

28、.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, where Vt is the terminal voltage and IA is the armature current (or load current IL) supplied by the generator to the load

29、. 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

30、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

31、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

32、 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 sp

33、eed. 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 th

34、e terminal voltage does not build up. In this situation the weak main pole flux opposes the residual flux. 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, const

35、ant 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

36、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

37、 field circuit resistance Rf, the field current depends on the generated voltage in accordance with Ohms 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

38、winding only to a separate dc source for a few seconds. This procedure is generally known as flashing the field.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 on

39、ly 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 ge

40、nerated 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 thatwhere RS is the series field winding resistance. Figure 7 Series generator: (a)circuit dia

41、gram;(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

42、 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 is o

43、f 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.

44、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-loa

45、d 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. Figu

46、re 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