電氣專業(yè)外文翻譯---直流電機(jī)導(dǎo)論負(fù)載運(yùn)行的變壓器

1、<p><b>　　附錄A 譯文</b></p><p>　　直流電機(jī)導(dǎo)論負(fù)載運(yùn)行的變壓器</p><p>　　直流電機(jī)以其多功用性而形成了鮮明的特征。通過并勵、串勵和特勵繞組的各種不同組合，直流電機(jī)可設(shè)計(jì)成在動態(tài)和穩(wěn)態(tài)運(yùn)行時呈現(xiàn)出寬廣范圍變化的伏-安或速度-轉(zhuǎn)矩特性。由于直流電機(jī)易于控制，因此該系統(tǒng)用于要求電動機(jī)轉(zhuǎn)速變化范圍寬或能精確控制電機(jī)輸出的場合

2、。</p><p>　　定子上有凸極，由一個或一個以上勵磁線圈勵磁。勵磁繞組產(chǎn)生的氣隙通以磁極中心線為軸線對稱分布，這條軸線稱為磁場軸線或直軸。</p><p>　　我們知道，每個旋轉(zhuǎn)的電樞繞組中產(chǎn)生的交流電壓，經(jīng)由一與電樞連接的旋轉(zhuǎn)的換向器和靜止的電刷，在電樞繞組出線端轉(zhuǎn)換成直流電壓。換向器一電刷的組合構(gòu)成機(jī)械整流器，它產(chǎn)生一直流電樞電壓和一在空間固定的電樞磁勢波形。電刷的放置應(yīng)使換向線

3、圈也處于磁極中性區(qū)，即兩磁極之間。這樣，電樞磁勢波形的軸線與磁極軸線相差90°電角度，即位于交軸上。在示意圖中，電刷位于交軸上，因?yàn)榇颂幷桥c其相連的線圈的位置。這樣，如圖所示電樞磁勢波的軸線也是沿著電刷軸線的。（在實(shí)際電機(jī)中，電刷的幾何位置大約偏移圖例中所示位置90°電角度，這是因?yàn)樵哪┒诵螤顦?gòu)成圖示結(jié)果與換向器相連。）</p><p>　　電刷上的電磁轉(zhuǎn)矩和速度電壓與磁通分布的空間波形

4、無關(guān)；為了方便起見，我們假設(shè)氣隙中仍然是正弦磁密波，這樣便可以從磁場分析著手求得轉(zhuǎn)矩。</p><p>　　轉(zhuǎn)矩可以用直軸每極氣隙磁通和電樞磁勢波的空間基波分量相互作用的結(jié)果來表示。電刷處于交軸時，磁場間的角度為90°電角度，其正弦值等于1，則對于一臺P極電機(jī)</p><p>　　式中由于轉(zhuǎn)矩的正方向可以根據(jù)物理概念的推斷確定，因此負(fù)號已經(jīng)去掉。電樞磁勢鋸齒波的空間基波是峰值的8

5、/。上式變換后有</p><p>　　式中 =電樞外部電路中的電流；</p><p>　　=電樞繞組中的總導(dǎo)體數(shù)；</p><p>　　=通過繞組的并聯(lián)支路數(shù)；</p><p><b>　　且</b></p><p>　　其為一個由繞組設(shè)計(jì)而確定的常數(shù)。</p><p>

6、　　簡單的單個線圈的電樞中的整流電壓前面已經(jīng)討論過了。將繞組分散在幾個槽中的效果可用圖形表示，圖中每一條整流的正弦波形是一個線圈產(chǎn)生的電壓，換向線圈邊處于磁中性區(qū)。從電刷端觀察到的電壓是電刷間所有串聯(lián)線圈中整流電壓的總和，在圖中由標(biāo)以的波線表示。當(dāng)每極有十幾個換向器片，波線的波動變得非常小，從電刷端觀察到的平均電壓等于線圈整流電壓平均值之和。電刷間的整流電壓即速度電壓，為</p><p>　　式中為設(shè)計(jì)常數(shù)。分布

7、繞組的整流電壓與集中線圈有著相同的平均值，其差別只是分布繞組的波形脈動大大減小。</p><p>　　將上述幾式中的所有變量用SI單位制表達(dá)，有</p><p>　　這個等式簡單地說明與速度電壓有關(guān)的瞬時功率等于與磁場轉(zhuǎn)矩有關(guān)的瞬時機(jī)械功率，能量的流向取決于這臺電機(jī)是電動機(jī)還是發(fā)電機(jī)。</p><p>　　直軸氣隙通由勵磁繞組的合成磁勢產(chǎn)生，其磁通-磁勢曲線就是電機(jī)

8、的具體鐵磁材料的幾何尺寸決定的磁化曲線。在磁化曲線中，因?yàn)殡姌写艅莶ǖ妮S線與磁場軸線垂直，因此假定電樞磁勢對直軸磁通不產(chǎn)生作用。這種假設(shè)有必要在后述部分加以驗(yàn)證，屆時飽和效應(yīng)會深入研究。因?yàn)殡姌须妱菖c磁通成正比，所以通常用恒定轉(zhuǎn)速下的電樞電勢來表示磁化曲線更為方便。任意轉(zhuǎn)速時，任一給定磁通下的電壓與轉(zhuǎn)速成正比，即</p><p>　　圖中表示只有一個勵磁繞組的磁化曲線，這條曲線可以很容易通過實(shí)驗(yàn)方法得到，不需要任

9、何設(shè)計(jì)步驟的知識。</p><p>　　在一個相當(dāng)寬的勵磁范圍內(nèi)，鐵磁材料部分的磁阻與氣隙磁阻相比可以忽略不計(jì)，在此范圍內(nèi)磁通與勵磁繞組總磁勢呈線性比例，比例常數(shù)便是直軸氣隙磁導(dǎo)率。</p><p>　　直流電機(jī)的突出優(yōu)點(diǎn)是通過選擇磁場繞組不同的勵磁方法，可以獲得變化范圍很大的運(yùn)行特性。勵磁繞組可以由外部直流電源單獨(dú)激磁，或者也可自勵，即電機(jī)提供自身的勵磁。勵磁防哪個法不僅極大地影響控制系

10、統(tǒng)中電機(jī)的靜態(tài)特性，而且影響其動態(tài)運(yùn)行。</p><p>　　他勵發(fā)電機(jī)的連接圖已經(jīng)給出，所需勵磁電流是額定電樞電流的很小一部分。勵磁電路中很小數(shù)量的功率可以控制電樞電路中相對很大數(shù)量的功率，也就是說發(fā)電機(jī)是一種功率放大器。當(dāng)需要在很大范圍內(nèi)控制電樞電壓時，他勵發(fā)電機(jī)常常用于反饋控制系統(tǒng)中。自勵發(fā)電機(jī)的勵磁繞組可以有三種不同的供電方式。勵磁繞組可以與電樞串聯(lián)起來，這便形成了串勵發(fā)電機(jī)；勵磁繞組可以與電樞并聯(lián)在一起

11、，這便形成了并勵發(fā)電機(jī)；或者勵磁繞組分成兩部分，其中一部分與電樞串聯(lián)，另一部分與電樞并聯(lián)，這便形成復(fù)勵發(fā)電機(jī)。為了引起自勵過程，在自勵發(fā)電機(jī)中必須存在剩磁。</p><p>　　在典型的靜態(tài)伏-安特性中，假定原動機(jī)恒速運(yùn)行，穩(wěn)態(tài)電勢和端電壓關(guān)系為：</p><p>　　式中為電樞輸出電流，為電樞回路電阻。在發(fā)電機(jī)中，比大，電磁轉(zhuǎn)矩T是一種阻轉(zhuǎn)矩。</p><p>　

12、　他勵發(fā)電機(jī)的端電壓隨著負(fù)載電流的增加稍有降低，這主要是由于電樞電阻上的壓降。串勵發(fā)電機(jī)中的勵磁電流與負(fù)載電流相同，這樣，氣隙磁通和電壓隨負(fù)載變化很大，因此很少采用串勵發(fā)電機(jī)。并勵發(fā)電機(jī)電壓隨負(fù)載增加會有所下降，但在許多應(yīng)用場合，這并不防礙使用。復(fù)勵發(fā)電機(jī)的連接通常使串勵繞組的磁勢與并勵繞組磁勢相加，其優(yōu)點(diǎn)是通過串勵繞組的作用，每極磁通隨著負(fù)載增加，從而產(chǎn)生一個隨負(fù)載增加近似為常數(shù)的輸出電壓。通常，并勵繞組匝數(shù)多，導(dǎo)線細(xì)；而繞在外部的串

13、勵繞組由于它必須承載電機(jī)的整個電樞電流，所以其構(gòu)成的導(dǎo)線相對較粗。不論是并勵還是復(fù)勵發(fā)電機(jī)的電壓都可借助并勵磁場中的變阻器在適度的范圍內(nèi)得到調(diào)節(jié)。</p><p>　　任何用于發(fā)電機(jī)的勵磁方法都可用于電動機(jī)。在電動機(jī)典型的靜態(tài)轉(zhuǎn)速-轉(zhuǎn)矩特性中，假設(shè)電動機(jī)兩端由一個恒壓源供電。在電動機(jī)電樞中感應(yīng)的電勢與端電壓間的關(guān)系為</p><p>　　式中此時為輸入的電樞電流。電勢此時比端電壓小，電樞電

14、流與發(fā)電機(jī)中的方向相反，且電磁轉(zhuǎn)矩與電樞旋轉(zhuǎn)方向相同。</p><p>　　在并勵和他勵電動機(jī)中磁場磁通近似為常數(shù)，因此轉(zhuǎn)矩的增加必須要求電樞電流近似成比例增大，同時為允許增大的電流通過小的電樞電阻，要求反電勢稍有減少。由于反電勢決定于磁通和轉(zhuǎn)速，因此，轉(zhuǎn)速必須稍稍降低。與鼠籠式感應(yīng)電動機(jī)相類似，并勵電動機(jī)實(shí)際上是一種從空載到滿載速降僅約為5%的恒速電動機(jī)。起動轉(zhuǎn)矩和最大轉(zhuǎn)矩受到能成功換向的電樞電流的限制。<

15、;/p><p>　　并勵電動機(jī)的突出優(yōu)點(diǎn)是易于調(diào)速。在并勵繞組回路裝上變阻器，勵磁電流和每極磁通都可任意改變，而磁通的變化導(dǎo)致轉(zhuǎn)速相反的變化以維持反電勢大致等于外施端電壓。通過這種方法得到最大調(diào)速范圍為4或5比1，最高轉(zhuǎn)速同樣受到換向條件的限制。通過改變外施電樞電壓，可以獲得很寬的調(diào)速范圍。</p><p>　　在串勵電動機(jī)中，電樞電流、電樞電勢和定子磁場磁通隨負(fù)載增加而增加（假設(shè)鐵芯不完全飽

16、和）。因?yàn)榇磐S負(fù)載增大，所以為了維持外施電壓與反電勢之間的平衡，速度必須下降，此外，由于磁通增加，所以轉(zhuǎn)矩增大所引起的電樞電流的增大比并勵電動機(jī)中的要小。因此串勵電動機(jī)是一種具有明顯下降的轉(zhuǎn)速-負(fù)載特性的變速電動機(jī)。對于要求轉(zhuǎn)矩過載很多的應(yīng)用場合，由于對應(yīng)的過載功率隨相應(yīng)的轉(zhuǎn)速下降而維持在一個合理的范圍內(nèi)，因此，這種特性具有特別的優(yōu)越性。磁通隨著電樞電流的增大而增大，同時還帶來非常有用的起動特性。</p><p&g

17、t;　　在復(fù)勵電動機(jī)中，串勵磁場可以連接成積復(fù)勵式，使其磁勢與并勵磁場相加；也可以連接成差復(fù)勵式，兩磁場方向相反。差復(fù)勵連接很少使用。積復(fù)勵電動機(jī)具有界于并勵和串勵電動機(jī)之間的速度-負(fù)載特性，轉(zhuǎn)速隨負(fù)載的降低取決于并勵磁場和串勵磁場的相對安匝數(shù)。這種電動機(jī)沒有像串勵電動機(jī)那樣輕載高轉(zhuǎn)速的缺點(diǎn)，但它在相當(dāng)?shù)某潭壬媳３种畡罘绞降膬?yōu)點(diǎn)。</p><p>　　直流電機(jī)的應(yīng)用優(yōu)勢在于可接成并勵、串勵和復(fù)勵等各種勵磁方式，

18、因而可提供多種性能各異的運(yùn)行特性。其中有一些特性在本文中已大致提及。如果增加附加的電刷組以至于從換向器上另外可得到一些電壓，那么還會存在更多的運(yùn)用場合，因此直流電機(jī)系統(tǒng)的多用性，及其不論對人工還是自動控制的適應(yīng)性，是它們的顯著特性。</p><p><b>　　附錄Ｂ 外文原文</b></p><p>　　Introduction to DC MachinesThe

19、 Transformer on load</p><p>　　DC machines are characterized by their versatility. By means of various combination of shunt, series, and separately excited field windings they can be designed to display a wid

20、e variety of volt-ampere or speed-torque characteristics for both dynamic and steadystate operation. Because of the ease with which they can be controlled , systems of DC machines are often used in applications requiring

21、 a wide range of motor speeds or precise control of motor output.</p><p>　　The essential features of a DC machine are shown schematically. The stator has salient poles and is excited by one or more field coi

22、ls. The air-gap flux distribution created by the field winding is symmetrical about the centerline of the field poles. This axis is called the field axis or direct axis.</p><p>　　As we know , the AC voltage

23、generated in each rotating armature coil is converted to DC in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush comb

24、ination forms a mechanical rectifier, resulting in a DC armature voltage as well as an armature m.m.f. wave which is fixed in space. The brushes are located so that commutation occurs when the coil sides are in the neutr

25、al zone , midway between the fie</p><p>　　The magnetic torque and the speed voltage appearing at the brushes are independent of the spatial waveform of the flux distribution; for convenience we shall continu

26、e to assume a sinusoidal flux-density wave in the air gap. The torque can then be found from the magnetic field viewpoint. </p><p>　　The torque can be expressed in terms of the interaction of the direct-axis

27、 air-gap flux per pole and the space-fundamental component of the armature m.m.f. wave . With the brushes in the quadrature axis, the angle between these fields is 90 electrical degrees, and its sine equals unity. For

28、a P pole machine</p><p>　　In which the minus sign has been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental of the sawtooth armature m.m.

29、f. wave is 8/ times its peak. Substitution in above equation then gives </p><p>　　Where =current in external armature circuit;</p><p>　　=total number of conductors in armature winding;</p>

30、<p>　　=number of parallel paths through winding;</p><p><b>　　And </b></p><p>　　Is a constant fixed by the design of the winding.</p><p>　　The rectified voltage ge

31、nerated in the armature has already been discussed before for an elementary single-coil armature. The effect of distributing the winding in several slots is shown in figure ,in which each of the rectified sine waves is t

32、he voltage generated in one of the coils, commutation taking place at the moment when the coil sides are in the neutral zone. The generated voltage as observed from the brushes is the sum of the rectified voltages of all

33、 the coils in series between brushes</p><p>　　Where is the design constant. The rectified voltage of a distributed winding has the same average value as that of a concentrated coil. The difference is that t

34、he ripple is greatly reduced. </p><p>　　From the above equations, with all variable expressed in SI units:</p><p>　　This equation simply says that the instantaneous electric power associated wit

35、h the speed voltage equals the instantaneous mechanical power associated with the magnetic torque , the direction of power flow being determined by whether the machine is acting as a motor or generator.</p><p&

36、gt;　　The direct-axis air-gap flux is produced by the combined m.m.f. of the field windings, the flux-m.m.f. characteristic being the magnetization curve for the particular iron geometry of the machine. In the magnetizat

37、ion curve, it is assumed that the armature m.m.f. wave is perpendicular to the field axis. It will be necessary to reexamine this assumption later in this chapter, where the effects of saturation are investigated more th

38、oroughly. Because the armature e.m.f. is proportional to flux tim</p><p>　　Figure shows the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods, no know

39、ledge of any design details being required.</p><p>　　Over a fairly wide range of excitation the reluctance of the iron is negligible compared with that of the air gap. In this region the flux is linearly pro

40、portional to the total m.m.f. of the field windings, the constant of proportionality being the direct-axis air-gap permeance.</p><p>　　The outstanding advantages of DC machines arise from the wide variety of

41、 operating characteristics which can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external DC source, or they may be self-excited; i.e.,

42、 the machine may supply its own excitation. The method of excitation profoundly influences not only the steady-state characteristics, but also the dynamic behavior of the machine in control systems.</p><p>　

43、　The connection diagram of a separately excited generator is given. The required field current is a very small fraction of the rated armature current. A small amount of power in the field circuit may control a relatively

44、 large amount of power in the armature circuit; i.e., the generator is a power amplifier. Separately excited generators are often used in feedback control systems when control of the armature voltage over a wide range is

45、 required. The field windings of self-excited generators may </p><p>　　In the typical steady-state volt-ampere characteristics, constant-speed prime movers being assumed. The relation between the steady-stat

46、e generated e.m.f. and the terminal voltage is </p><p>　　Where is the armature current output and is the armature circuit resistance. In a generator, is large than ; and the electromagnetic torque T is a

47、 countertorque opposing rotation.</p><p>　　The terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally because of the voltage drop in the armature

48、 resistance. The field current of a series generator is the same as the load current, so that the air-gap flux and hence the voltage vary widely with load. As a consequence, series generators are not often used. The volt

49、age of shunt generators drops off somewhat with load. Compound generators are normally connected so that the m.m.f. </p><p>　　Where is now the armature current input. The generated e.m.f. is now smaller t

50、han the terminal voltage , the armature current is in the opposite direction to that in a motor, and the electromagnetic torque is in the direction to sustain rotation of the armature.</p><p>　　In shunt and

51、separately excited motors the field flux is nearly constant. Consequently, increased torque must be accompanied by a very nearly proportional increase in armature current and hence by a small decrease in counter e.m.f. t

52、o allow this increased current through the small armature resistance. Since counter e.m.f. is determined by flux and speed, the speed must drop slightly. Like the squirrel-cage induction motor ,the shunt motor is substan

53、tially a constant-speed motor having about 5 pe</p><p>　　An outstanding advantage of the shunt motor is ease of speed control. With a rheostat in the shunt-field circuit, the field current and flux per pole

54、can be varied at will, and variation of flux causes the inverse variation of speed to maintain counter e.m.f. approximately equal to the impressed terminal voltage. A maximum speed range of about 4 or 5 to 1 can be obtai

55、ned by this method, the limitation again being commutating conditions. By variation of the impressed armature voltage, very wide s</p><p>　　In the series motor, increase in load is accompanied by increase in

56、 the armature current and m.m.f. and the stator field flux (provided the iron is not completely saturated). Because flux increases with load, speed must drop in order to maintain the balance between impressed voltage and

57、 counter e.m.f.; moreover, the increase in armature current caused by increased torque is smaller than in the shunt motor because of the increased flux. The series motor is therefore a varying-speed motor with a m</p&

58、gt;<p>　　In the compound motor the series field may be connected either cumulatively, so that its.m.m.f.adds to that of the shunt field, or differentially, so that it opposes. The differential connection is very r

59、arely used. A cumulatively compounded motor has speed-load characteristic intermediate between those of a shunt and a series motor, the drop of speed with load depending on the relative number of ampere-turns in the shun

60、t and series fields. It does not have the disadvantage of very high light-lo</p><p>　　The application advantages of DC machines lie in the variety of performance characteristics offered by the possibilities

61、 of shunt, series, and compound excitation. Some of these characteristics have been touched upon briefly in this article. Still greater possibilities exist if additional sets of brushes are added so that other voltages c