外文翻譯--- 直流電機(jī)的介紹

1、<p><b>　　直流電機(jī)的介紹</b></p><p>　　直流電機(jī)的特點(diǎn)是他們的多功用性。依靠不同的并勵(lì)、串勵(lì)和他勵(lì)勵(lì)磁繞組的組合，他們可以被設(shè)計(jì)為動(dòng)態(tài)的和靜態(tài)的運(yùn)轉(zhuǎn)方式從而呈現(xiàn)出寬廣范圍變化的伏安特性或速度轉(zhuǎn)矩特性。因?yàn)樗?jiǎn)單的可操縱性，直流系統(tǒng)經(jīng)常被用于需要大范圍發(fā)動(dòng)機(jī)轉(zhuǎn)速或精確控制發(fā)動(dòng)機(jī)的輸出量的場(chǎng)合。</p><p>　　直流電機(jī)的總貌如圖所示

2、。定子上有凸極，而且由一個(gè)或幾個(gè)勵(lì)磁線圈勵(lì)磁。氣隙磁通量以磁極中心線為軸線對(duì)稱分布。這條軸線叫做磁場(chǎng)軸線或直軸。</p><p>　　我們都知道，在每個(gè)旋轉(zhuǎn)電樞線圈中產(chǎn)生的交流電壓，經(jīng)由一與電樞聯(lián)接的旋轉(zhuǎn)的換向器和靜止的電刷，在電樞線圈出線端轉(zhuǎn)換成直流電壓。換向器－電刷組合構(gòu)成了一個(gè)機(jī)械整流器，它形成了一個(gè)直流電樞電壓和一個(gè)被固定在空間中的電樞磁勢(shì)波形。電刷的位置應(yīng)使換向線圈也處于磁極中性區(qū)，即兩磁極之間。這樣，

3、電樞磁勢(shì)波的軸線與磁極軸線相差90度,也就是在交軸上。在示意圖中，電刷位于交軸上，因?yàn)檫@是線圈和電刷相連的位置。這樣，電樞磁勢(shì)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度，這是因?yàn)樵哪┒诵螤顦?gòu)成圖示結(jié)果與換向器相連）。電刷上的電磁轉(zhuǎn)矩和旋轉(zhuǎn)電勢(shì)與磁通分布的空間波形無(wú)關(guān)；為了方便我們可以假設(shè)在氣隙中有一個(gè)正弦的磁通密度波形。轉(zhuǎn)矩可以從磁場(chǎng)的觀點(diǎn)分析得到。</p><p>

4、　　轉(zhuǎn)矩可以用每個(gè)磁極的直軸氣隙磁通和電樞磁勢(shì)波的空間基波分量相互作用的結(jié)果來(lái)表示。在交軸上的電刷和這個(gè)磁場(chǎng)的夾角為90度，其正弦值等于1，對(duì)于一臺(tái)極電機(jī)</p><p><b>　?。?-1）</b></p><p>　　式中帶負(fù)號(hào)被去掉因?yàn)檗D(zhuǎn)矩的正方向可以由物理的推論測(cè)定出來(lái)。鋸齒電樞磁勢(shì)波的空間基波是它最大值的。代替上面的等式可以給出：</p>&

5、lt;p><b>　?。?-2）</b></p><p>　　其中：=電樞外部點(diǎn)路中的電流；</p><p>　　=電樞繞組中總導(dǎo)體數(shù)；</p><p>　　=通過(guò)繞組的并聯(lián)支路數(shù)；</p><p>　　及 （1-3）</p>

6、<p>　　其為一個(gè)由繞組設(shè)計(jì)而確定的常數(shù)。</p><p>　　簡(jiǎn)單的單個(gè)線圈的電樞中的整流電壓前在面已被討論過(guò)。將繞組分散在幾個(gè)槽中的效果可用圖形表示，在圖示中每一個(gè)整流的正弦波是在線圈中產(chǎn)生的電壓,換向線圈邊處于磁中性區(qū)。從電刷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，在圖中標(biāo)以的文波表示。每個(gè)磁極用12個(gè)或更多換向片,可以使波動(dòng)變得很小。從電刷中觀測(cè)到平均產(chǎn)生的電壓等于整流線圈電壓的

7、平均值的總和。電刷之間整流電壓，即旋轉(zhuǎn)電勢(shì)為</p><p><b>　?。?-4）</b></p><p>　　為常數(shù)。分布繞組的整流電壓與集中繞組有相同的平均值，不同的是波動(dòng)大大減低了。</p><p>　　在上面的等式中，所有的變量都是標(biāo)準(zhǔn)國(guó)際單位制。</p><p><b>　?。?-5）</b&

8、gt;</p><p>　　這個(gè)等式清楚地說(shuō)明，與旋轉(zhuǎn)電勢(shì)相關(guān)的瞬間功率等于與磁場(chǎng)轉(zhuǎn)矩有關(guān)的瞬時(shí)機(jī)械功率，能量的流向是由設(shè)備確定是發(fā)動(dòng)機(jī)還是發(fā)電機(jī)。</p><p>　　直軸氣隙磁量由勵(lì)磁繞組的合成磁勢(shì)產(chǎn)生，其磁通—磁勢(shì)曲線就是電機(jī)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, 假設(shè)電樞磁勢(shì)波的軸線與磁場(chǎng)軸垂直，因此假定電樞磁勢(shì)對(duì)直軸磁通不產(chǎn)生作用。在本文的后面有必要重新檢驗(yàn)這一假

9、設(shè)，飽和效應(yīng)會(huì)深入研究。因?yàn)殡姌须妱?shì)是與磁通、時(shí)間、速度成比例，所以通常用恒定轉(zhuǎn)速下的電樞電勢(shì)來(lái)表示磁化曲線更為方便。任意轉(zhuǎn)速電壓時(shí)，任一給定磁通下的電壓與轉(zhuǎn)速成正比，也就是說(shuō)</p><p><b>　?。?-6）</b></p><p>　　圖中磁化曲線只有一個(gè)勵(lì)磁繞組勵(lì)磁的，這種曲線可以通過(guò)測(cè)試的方法輕松獲得，不需要任何設(shè)計(jì)步驟的知識(shí)。</p>&

10、lt;p>　　大范圍勵(lì)磁下的鐵磁阻與空氣氣隙相比可以忽略不計(jì)，在這種情況下磁通與勵(lì)磁繞組的總磁勢(shì)成線性比例關(guān)系，比例常數(shù)就是直軸的氣隙導(dǎo)磁性。</p><p>　　直流電機(jī)的顯著優(yōu)勢(shì)源自于通過(guò)選擇勵(lì)磁繞組的勵(lì)磁方式而獲得不同的運(yùn)轉(zhuǎn)方式。勵(lì)磁繞組可以從外部直流電源以他勵(lì)的方式勵(lì)磁，也可以以自勵(lì)的方式勵(lì)磁。換句話，直流電機(jī)可以提供自身勵(lì)磁。勵(lì)磁方式不僅極大地影響它的靜態(tài)特性，而且極大地影響在控制系統(tǒng)中電機(jī)的動(dòng)態(tài)

11、性能。</p><p>　　他勵(lì)發(fā)電機(jī)的聯(lián)接圖解已經(jīng)給出的。所需的勵(lì)磁電流只是電樞電流中的一小部分。在勵(lì)磁電路中少量的功率可以控制相對(duì)一大部分電樞電路的功率。換句話說(shuō)，發(fā)電機(jī)是一個(gè)功率放大器，當(dāng)需要在大范圍控制電樞電壓時(shí)，他勵(lì)發(fā)電機(jī)通常在反饋控制系統(tǒng)中使用。自勵(lì)發(fā)電機(jī)的勵(lì)磁繞組可以有三種不同的供電方式。勵(lì)磁線圈可以與電樞串聯(lián)起來(lái)，這便是串勵(lì)發(fā)電機(jī)；勵(lì)磁繞組可以與電樞并聯(lián)在一起，這便是并勵(lì)發(fā)電機(jī)。也可以同時(shí)以兩種方

12、式相連接組成一個(gè)復(fù)勵(lì)發(fā)電機(jī)。為了引起自勵(lì)過(guò)程，在自勵(lì)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(dòng)機(jī)速度恒定，穩(wěn)態(tài)電動(dòng)勢(shì)與端電壓之間的關(guān)系為</p><p><b>　?。?-7）</b></p><p>　　其中是電樞輸出電流，是電樞回路電阻。在發(fā)動(dòng)機(jī)中，大于。電磁轉(zhuǎn)矩是一個(gè)反轉(zhuǎn)矩。</p>&l

13、t;p>　　他勵(lì)發(fā)電機(jī)的端電壓隨著負(fù)載電流的增大而輕微的減小，主要是因?yàn)殡妷涸陔姌须娮枭系膲航怠４畡?lì)發(fā)電機(jī)中的勵(lì)磁電流與負(fù)載電流相同，所以氣隙磁通和電壓隨負(fù)載變化很大，因此很少采用串勵(lì)發(fā)電機(jī)。并勵(lì)發(fā)電機(jī)電壓隨負(fù)載增加會(huì)有所下降，但在許多應(yīng)用場(chǎng)合，這并不妨礙使用。復(fù)勵(lì)發(fā)電機(jī)的連接通常使串勵(lì)繞組的磁勢(shì)與并勵(lì)繞組磁勢(shì)相加，其優(yōu)點(diǎn)是通過(guò)串勵(lì)繞組作用，每極磁通隨著負(fù)載增加，從而產(chǎn)生一個(gè)隨負(fù)載增加近似為常數(shù)的輸出電壓。通常，并勵(lì)繞組匝數(shù)多，導(dǎo)

14、線細(xì)；而繞在外部的串勵(lì)繞組由于它必須承載電機(jī)的整個(gè)電樞電流，所以其構(gòu)成的導(dǎo)線相對(duì)較粗。不論是并勵(lì)還是復(fù)勵(lì)發(fā)電機(jī)的電壓都可借助并勵(lì)磁場(chǎng)中的變阻器在適度的范圍內(nèi)得到調(diào)節(jié)。</p><p>　　所有勵(lì)磁的方法在電動(dòng)機(jī)上同樣適用。在電動(dòng)機(jī)典型的靜態(tài)轉(zhuǎn)速—轉(zhuǎn)矩特性中，電機(jī)端電壓假設(shè)由恒壓源供電，在電動(dòng)機(jī)中感應(yīng)的電勢(shì)與路端電壓間關(guān)系是</p><p><b>　?。?-8）</b>

15、;</p><p>　　是電樞輸入電流。電勢(shì)小于端電壓。電樞電流與發(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>　　對(duì)于并勵(lì)與他勵(lì)電動(dòng)機(jī)來(lái)說(shuō)，磁場(chǎng)磁通基本近似為常數(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(shí)為允許增大的電流通過(guò)小的電樞電阻，要求反電勢(shì)稍有減少。由于反電勢(shì)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(dòng)機(jī)類似，并勵(lì)電動(dòng)機(jī)實(shí)際是一種從空

16、載到滿負(fù)荷的速度基本上只有5%的下降的恒速電動(dòng)機(jī)。從起動(dòng)轉(zhuǎn)矩到達(dá)到最大轉(zhuǎn)矩之間一直是被電樞電流所控制可以正常交替進(jìn)行。</p><p>　　并勵(lì)電動(dòng)機(jī)的一個(gè)顯著優(yōu)點(diǎn)是速度控制，通過(guò)在并勵(lì)繞組回路裝上內(nèi)部變阻器，勵(lì)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(shì)大致等于外施加端電壓。用這種方法我們可以獲得最大調(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(guò)改變外施加電樞電壓，可以獲

17、得很寬的調(diào)速范圍。</p><p>　　對(duì)于串勵(lì)電動(dòng)機(jī)來(lái)說(shuō)，電樞電流、電樞磁勢(shì)波以及定子磁場(chǎng)磁通隨負(fù)載增長(zhǎng)而增長(zhǎng)。因?yàn)橛捎谪?fù)載增大而造成的磁通增大，速度必須降低，這樣才可以維持反電勢(shì)與外加電壓之間的平衡。此外，由于磁通增加，所以轉(zhuǎn)矩增大所引起電樞電流的增大比并勵(lì)電動(dòng)機(jī)中的要小。因此串勵(lì)電動(dòng)機(jī)是一種具有明顯下降的轉(zhuǎn)速-負(fù)載特性的變速發(fā)電機(jī)。對(duì)于要求轉(zhuǎn)矩過(guò)載很多的應(yīng)用場(chǎng)合，由于對(duì)應(yīng)的過(guò)載功率隨相應(yīng)的轉(zhuǎn)速下降而維持在一

18、個(gè)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(shí)還帶來(lái)非常有用的起動(dòng)特性。</p><p>　　在復(fù)勵(lì)電動(dòng)機(jī)中，串勵(lì)磁場(chǎng)可以連接成積復(fù)勵(lì)式，使其磁勢(shì)與并勵(lì)磁場(chǎng)相加；也可以連接成差復(fù)勵(lì)式，兩磁場(chǎng)方向相反,差復(fù)勵(lì)很少使用。積復(fù)勵(lì)電動(dòng)機(jī)具有界于串勵(lì)和并勵(lì)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(lì)磁場(chǎng)和串勵(lì)磁場(chǎng)的相對(duì)安匝數(shù)。這種電動(dòng)機(jī)沒(méi)有像串勵(lì)電動(dòng)機(jī)那樣輕載高轉(zhuǎn)速的缺點(diǎn)，但它在相當(dāng)

19、的程度上保持著串勵(lì)方式的優(yōu)點(diǎn)。</p><p>　　直流電機(jī)的應(yīng)用優(yōu)勢(shì)是可以連接成串勵(lì)、并勵(lì)及復(fù)勵(lì)式等各種勵(lì)磁方式。其中的一些特性我們已在本文中的提及到了。如果增加電刷可以通過(guò)換向器獲得更多的電壓，那么還會(huì)存在更多的應(yīng)用場(chǎng)合，不論對(duì)人工的還是自動(dòng)控制的適應(yīng)性是它們的顯著特性。</p><p>　　一個(gè)直流電機(jī)是由兩個(gè)基本元素組成：</p><p>　　定子是電機(jī)固定

20、的部分。它由以下基礎(chǔ)組成；在結(jié)構(gòu)中有一個(gè)磁軛；勵(lì)磁磁極和繞組；換向極和繞組；有滑動(dòng)軸承的端罩；電刷和電刷固定器；出線盒。</p><p>　　轉(zhuǎn)子是電機(jī)旋轉(zhuǎn)的部分。它構(gòu)成了一個(gè)中心，這個(gè)中心是安放在設(shè)備軸上并且已經(jīng)平均地隔開(kāi)，把電樞繞組放入槽中。還有一個(gè)換向器和一個(gè)風(fēng)扇組成，被放在設(shè)備的軸上。</p><p>　　它用螺栓和底座固定在地板上。低壓電機(jī)的磁軛和本身的結(jié)構(gòu)是一體的，穿過(guò)勵(lì)磁磁極

21、閉合而產(chǎn)生的磁通量。它的結(jié)構(gòu)和磁軛是用生鐵和鑄鋼制造成的，有時(shí)候也用焊接的鋼板。在低壓和可控補(bǔ)償整流器電機(jī)中，磁軛是由0.5～1毫米的薄鐵板制成的。磁軛經(jīng)常被安放在一個(gè)非鐵磁性的結(jié)構(gòu)內(nèi)（通常是由鋁合金制成，為了縮減重量）。在內(nèi)部有兩個(gè)端蓋并且都包含球體和滑動(dòng)軸承。</p><p>　　勵(lì)磁磁極由用0.5～1mm的鐵片通過(guò)用螺栓釘牢互相支撐。磁極被放入結(jié)構(gòu)內(nèi)的依靠螺栓固定。它們支撐繞組，讓它運(yùn)送勵(lì)磁流動(dòng)。在電樞上，

22、磁極鐵心的末端是極靴，它通過(guò)氣隙有助于磁通量的分布。繞組被放置在一個(gè)絕緣結(jié)構(gòu)內(nèi)的中心處，被極靴保護(hù)。</p><p>　　勵(lì)磁繞組是由絕熱的圓形物或矩形的導(dǎo)體制成，并且和另一個(gè)連續(xù)或平行的相連接。繞組是以一個(gè)磁極的磁通量穿過(guò)氣隙，然后被指引由極靴向電樞（北極），下一個(gè)磁極的磁通量由電樞到極靴（南極）。換向極的磁極就像主磁極，它組成了一個(gè)中心，末端在極靴中并且一個(gè)繞組繞在中心周?chē)Ｋ鼈儽环旁趦蓚€(gè)主磁極中間的對(duì)稱軸，

23、拴在磁軛上。換向極的磁極是由生鐵或鑄鐵制成。換向極的繞組是由周?chē)^緣的或垂直的導(dǎo)線制成。它們相互平行或首尾連接，帶動(dòng)設(shè)備的主電流。</p><p>　　轉(zhuǎn)子的中心是由0.5～1毫米硅合金薄板制成。薄板是通過(guò)清漆薄膜或氧化物涂層和其他物質(zhì)絕緣。絕緣物質(zhì)厚度為0.03～0.05毫米。目的是當(dāng)它在磁場(chǎng)中旋轉(zhuǎn)時(shí)渦流升高時(shí)，減少渦流。它變得很熱將導(dǎo)致能量損失。在實(shí)心物的中心，它損失得很高，減少電機(jī)的效率和產(chǎn)生劇烈的熱量。轉(zhuǎn)

24、子的中心包含了一些金屬薄片。軸向的冷卻管（8～10毫米）被嵌在金屬薄片中給它更好的冷卻。壓力施加在中心的兩端。轉(zhuǎn)子的長(zhǎng)度超過(guò)磁極2～5毫米，作用是減少磁力滲透性導(dǎo)致的軸向位移。轉(zhuǎn)子的外圍提供了槽放入電樞繞組。每個(gè)轉(zhuǎn)子繞組包含了一個(gè)線圈直接繞在轉(zhuǎn)子槽中依靠特殊設(shè)計(jì)機(jī)械或成行的線圈。繞組是絕緣的，依靠木制的或絕緣物質(zhì)制成的槽楔保護(hù)它。繞組過(guò)載是其彎曲，用鋼絲相互連接為了抵抗由地心引力產(chǎn)生的變形。轉(zhuǎn)子繞組的線圈交叉點(diǎn)連接到放在電樞軸的換向器上

25、。換向器是圓柱體含有少量的銅。換向片是絕緣的。轉(zhuǎn)子線圈被焊接在換向片上。低壓電機(jī)的換向器片被分割成一個(gè)獨(dú)立的單元，依靠合成樹(shù)脂互相絕緣，例如人造樹(shù)脂。為了連接電樞繞組固定接線端，一組電刷在換向器的表面上依靠支架滑動(dòng)。電刷通過(guò)彈簧給予不變的壓力連接換向片?？ㄡ敯卜旁诙松w上并且支撐碳刷支架。奇數(shù)的炭</p><p>　　Introduction to D.C. Machines</p><p>

26、;　　D.C. machines are characterized by their versatility. By means of various combinations of shunt-, series-, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-t

27、orque characteristics for both dynamic and steady state operation. Because of the ease with which they can be controlled, systems of D.C. machines are often used in applications requiring a wide range of motor speeds or

28、precise control of motor output.</p><p>　　The essential features of a D.C. machine are shown schematically. The stator has salient poles and is excited by one or more field coils. The air-gap flux distributi

29、on created by the field winding is symmetrical about the centerline of the field poles. This is called the field axis or direct axis.</p><p>　　As we know, the A.C. voltage generated in each rotating armature

30、 coil is converted to D.C. in the external armature terminals by means of a rotating commutator and stationary brushes to which the armature leads are connected. The commutator-brush combination forms a mechanical rectif

31、ier, resulting in a D.C. armature voltage as well as an armature m.m.f. Wave then is 90 electrical degrees from the axis of the field poles, i.e. in the quadrature axis. In the schematic representation the brushes ar<

32、/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 continue to assume a sinusoidal flux-densit

33、y 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 air-gap flux per pole and space-fu

34、ndamental 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 a pole machine</p><p><b>

35、;　?。?-1）</b></p><p>　　In which the minus sign gas been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental of the sawtooth armature m.

36、m.f.wave is times its peak. Substitution in above equation then gives </p><p><b>　?。?-2）</b></p><p>　　Where, =current in external armature circuit;</p><p>　　=total num

37、ber of conductors in armature winding;</p><p>　　=number of parallel paths through winding.</p><p><b>　　And</b></p><p><b>　　（1-3）</b></p><p>　　is

38、 a constant fixed by the design of the winding.</p><p>　　The rectified voltage generated in the armature has already been discussed before for an elementary single-coil armature. The effect of distributing t

39、he winding in several slots is shown in figure. In which each of the rectified sine wave is the voltage generated in one of the coils, commutation taking place at the moment when the coil sides are in the neutral zone. T

40、he generated voltage as observed from the brushes and is the sum of the rectified voltages of all the coils in series between brus</p><p><b>　?。?-4）</b></p><p>　　where is the design

41、 constant. The rectified voltage of a distributed winding has the same average value as that of a concentrated coil. The difference is that the ripple is greatly reduced.</p><p>　　From the above equations, w

42、ith all variable expressed in SI units,</p><p><b>　　（1-5）</b></p><p>　　This equation simply says that the instantaneous power associated with the speed voltage equals the instantaneo

43、us mechanical power with the magnetic torque. The direction of power flow being determined by whether the machine is acting as a motor or generator.</p><p>　　The direct-axis air-gap flux is produced by the c

44、ombined 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 magnetization curve, it is assumed that the armature –m.m.f. Wave

45、 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 thoroughly. Because the armature e.m.f. is proportional

46、to flux tim</p><p><b>　?。?-6）</b></p><p>　　There is the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods, no knowledge of an

47、y 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 proportional t

48、o 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 D.C. machines arise from the wide variety of operatin

49、g characteristics that can be obtained by selection of the method of excitation of the field windings. The field windings may be separately excited from an external D.C. source, or they may be self-excited; i.e. the mach

50、ine 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>　　The con

51、nection 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 large a

52、mount 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 required

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

54、ted e.m.f. and the terminal voltage is </p><p><b>　?。?-7）</b></p><p>　　where is the armature current output and is the armature circuit resistance. In a generator, is larger than

55、 and the electromagnetic torque is a counter torque opposing rotation.</p><p>　　The terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally beca

56、use of the voltage drop in the armature 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

57、generators are normally connected so that the m.m.f. of the series winding aids that of the shunt winding. The advantage is that through the action of the </p><p>　　Any of the methods of excitation used for

58、generators can also be used for motors. In the typical steady-state speed-torque characteristics, it is assumed that motor terminals are supplied from a constant-voltage source. In a motor the relation between the e.m.f.

59、 generated in the armature and terminal voltage is </p><p><b>　?。?-8）</b></p><p>　　where is now the armature current input. The generated e.m.f. is now smaller than the terminal

60、voltage , the armature current is in the opposite direction to that in a generator, and the electron magnetic torque is in the direction to sustain rotation of the armature.</p><p>　　In shunt and separately

61、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. to allow this

62、 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 substantially a con

63、stant-speed motor having about 5% dr</p><p>　　An outstanding advantage of the shunt motor is case of speed control. With a rheostat in the shunt-field circuit, the field current and flux per pole can be vari

64、ed 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 I can be obtained by this

65、 method. The limitation again being commutating conditions. By variation of the impressed armature voltage, very speed </p><p>　　In the series motor, increase in load is accompanied by increase in the armatu

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

67、.f. Moreover, the increased in armature current caused by increased torque is varying-speed motor with a markedly drooping speed-load characteristic. For applications requiring heavy torque overloads, t</p><p&

68、gt;　　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 rarely used

69、. 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 shunt and seri

70、es fields. It does not have disadvantage of very high light-load </p><p>　　The application advantages of D.C. machines lie in the variety of performance characteristics offered by the possibilities of shunt,

71、 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 can be obtai

72、ned from the commutator. Thus the versatility of D.C. machine system and their adaptability to control, both manual and automatic, are their outstan</p><p>　　A D.C machines is made up of two basic components

73、:</p><p>　　－The stator which is the stationary part of the machine. It consists of the following elements: a yoke inside a frame; excitation poles and winding; commutating poles (composes) and winding; end s

74、hield with ball or sliding bearings; brushes and brush holders; the terminal box.</p><p>　　－The rotor which is the moving part of the machine. It is made up of a core mounted on the machine shaft. This core

75、has uniformly spaced slots into which the armature winding is fitted. A commutator, and often a fan, is also located on the machine shaft.</p><p>　　The frame is fixed to the floor by means of a bedplate and

76、bolts. On low power machines the frame and yoke are one and the same components, through which the magnetic flux produced by the excitation poles closes. The frame and yoke are built of cast iron or cast steel or sometim

77、es from welded steel plates.</p><p>　　In low-power and controlled rectifier-supplied machines the yoke is built up of thin (0.5～1mm) laminated iron sheets. The yoke is usually mounted inside a non-ferromagne

78、tic frame (usually made of aluminum alloys, to keep down the weight). To either side of the frame there are bolted two end shields, which contain the ball or sliding bearings.</p><p>　　The (main)excitation p

79、oles are built from 0.5～1mm iron sheets held together by riveted bolts. The poles are fixed into the frame by means of bolts. They support the windings carrying the excitation current.</p><p>　　On the rotor

80、side, at the end of the pole core is the so-called pole-shoe that is meant to facilitate a given distribution of the magnetic flux through the air gap. The winding is placed inside an insulated frame mounted on the core,

81、 and secured by the pole-shoe.</p><p>　　The excitation windings are made of insulated round or rectangular conductors, and are connected either in series or in parallel. The windings are liked in such a way

82、that the magnetic flux of one pole crossing the air gap is directed from the pole-shoe towards the armature (North Pole), which the flux of the next pole is directed from the armature to the pole-shoe (South Pole).</p

83、><p>　　The commutating poles, like the main poles, consist of a core ending in the pole-shoe and a winding wound round the core. They are located on the symmetry (neutral) axis between two main poles, and bolte

84、d on the yoke. Commutating poles are built either of cast-iron or iron sheets.</p><p>　　The windings of the commutating poles are also made from insulated round or rectangular conductors. They are connected

85、either in series or in parallel and carry the machine's main current.</p><p>　　The rotor core is built of 0.5～1mm silicon-alloy sheets. The sheets are insulated from one another by a thin film of varnish

86、 or by an oxide coating. Both some 0.03～0.05mm thick. The purpose is to ensure a reduction of the eddy currents that arise in the core when it rotates inside the magnetic field. These currents cause energy losses that tu

87、rn into heat. In solid cores, these losses could become very high, reducing machine efficiency and producing intense heating.</p><p>　　The rotor core consists of a few packets of metal sheet. Redial or axial

88、 cooling ducts (8～10mm inside) are inserted between the packets to give better cooling. Pressure is exerted to both side of the core by pressing devices foxed on to the shaft. The length of the rotor usually exceeds that

89、 of the poles by 2～5mm on either side－the effect being to minimize the variations in magnetic permeability caused by axial armature displacement. The periphery of the rotor is provided with teeth and slots in</p>

90、<p>　　The rotor winding consists either of coils wound directly in the rotor slots by means of specially designed machines or coils already formed. The winding is carefully insulated, and it secured within the slots

91、 by means of wedges made of wood or other insulating material.</p><p>　　The winding overcharge are bent over and tied to one another with steel wire in order to resist the deformation that could be caused by

92、 the centrifugal force.</p><p>　　The coil-junctions of the rotor winding are connected to the commutator mounted on the armature shaft. The commutator is cylinder made of small copper. Segments insulated fro

93、m one another, and also from the clamping elements by a layer of minacity. The ends of the rotor coil are soldered to each segment.</p><p>　　On low-power machines, the commutator segments form a single unit,

94、 insulated from one another by means of a synthetic resin such as Bakelite.</p><p>　　To link the armature winding to fixed machine terminals, a set of carbon brushes slide on the commutator surface by means

95、of brush holders. The brushes contact the commutator segments with a constant pressure ensured by a spring and lever. Clamps mounted on the end shields support the brush holders.</p><p>　　The brushes are con

96、nected electrically－with the odd-numbered brushes connected to one terminal of the machine and the even-numbered brushes to the other. The brushes are equally spaced round the periphery of the commutator－the number of ro