外文翻譯--- 直流電機的介紹

1、<p><b>　　直流電機的介紹</b></p><p>　　直流電機的特點是他們的多功用性。依靠不同的并勵、串勵和他勵勵磁繞組的組合，他們可以被設(shè)計為動態(tài)的和靜態(tài)的運轉(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。因為它簡單的可操縱性，直流系統(tǒng)經(jīng)常被用于需要大范圍發(fā)動機轉(zhuǎn)速或精確控制發(fā)動機的輸出量的場合。</p><p>　　直流電機的總貌如圖所示

2、。定子上有凸極，而且由一個或幾個勵磁線圈勵磁。氣隙磁通量以磁極中心線為軸線對稱分布。這條軸線叫做磁場軸線或直軸。</p><p>　　我們都知道，在每個旋轉(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)成了一個機械整流器，它形成了一個直流電樞電壓和一個被固定在空間中的電樞磁勢波形。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，因為這是線圈和電刷相連的位置。這樣，電樞磁勢波的軸線也是沿著電刷軸線的（在實際電機中，電刷的幾何位置大約偏移圖例中所示位置90度，這是因為元件的末端形狀構(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)電勢與磁通分布的空間波形無關(guān)；為了方便我們可以假設(shè)在氣隙中有一個正弦的磁通密度波形。轉(zhuǎn)矩可以從磁場的觀點分析得到。</p><p>

4、　　轉(zhuǎn)矩可以用每個磁極的直軸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。在交軸上的電刷和這個磁場的夾角為90度，其正弦值等于1，對于一臺極電機</p><p><b>　?。?-1）</b></p><p>　　式中帶負號被去掉因為轉(zhuǎn)矩的正方向可以由物理的推論測定出來。鋸齒電樞磁勢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>　　（1-2）</b></p><p>　　其中：=電樞外部點路中的電流；</p><p>　　=電樞繞組中總導(dǎo)體數(shù)；</p><p>　　=通過繞組的并聯(lián)支路數(shù)；</p><p>　　及 （1-3）</p>

6、<p>　　其為一個由繞組設(shè)計而確定的常數(shù)。</p><p>　　簡單的單個線圈的電樞中的整流電壓前在面已被討論過。將繞組分散在幾個槽中的效果可用圖形表示，在圖示中每一個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，在圖中標以的文波表示。每個磁極用12個或更多換向片,可以使波動變得很小。從電刷中觀測到平均產(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，不同的是波動大大減低了。</p><p>　　在上面的等式中，所有的變量都是標準國際單位制。</p><p><b>　　（1-5）</b&

8、gt;</p><p>　　這個等式清楚地說明，與旋轉(zhuǎn)電勢相關(guān)的瞬間功率等于與磁場轉(zhuǎn)矩有關(guān)的瞬時機械功率，能量的流向是由設(shè)備確定是發(fā)動機還是發(fā)電機。</p><p>　　直軸氣隙磁量由勵磁繞組的合成磁勢產(chǎn)生，其磁通—磁勢曲線就是電機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)電樞磁勢波的軸線與磁場軸垂直，因此假定電樞磁勢對直軸磁通不產(chǎn)生作用。在本文的后面有必要重新檢驗這一假

9、設(shè)，飽和效應(yīng)會深入研究。因為電樞電勢是與磁通、時間、速度成比例，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)速電壓時，任一給定磁通下的電壓與轉(zhuǎn)速成正比，也就是說</p><p><b>　　（1-6）</b></p><p>　　圖中磁化曲線只有一個勵磁繞組勵磁的，這種曲線可以通過測試的方法輕松獲得，不需要任何設(shè)計步驟的知識。</p>&

10、lt;p>　　大范圍勵磁下的鐵磁阻與空氣氣隙相比可以忽略不計，在這種情況下磁通與勵磁繞組的總磁勢成線性比例關(guān)系，比例常數(shù)就是直軸的氣隙導(dǎo)磁性。</p><p>　　直流電機的顯著優(yōu)勢源自于通過選擇勵磁繞組的勵磁方式而獲得不同的運轉(zhuǎn)方式。勵磁繞組可以從外部直流電源以他勵的方式勵磁，也可以以自勵的方式勵磁。換句話，直流電機可以提供自身勵磁。勵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)中電機的動態(tài)

11、性能。</p><p>　　他勵發(fā)電機的聯(lián)接圖解已經(jīng)給出的。所需的勵磁電流只是電樞電流中的一小部分。在勵磁電路中少量的功率可以控制相對一大部分電樞電路的功率。換句話說，發(fā)電機是一個功率放大器，當需要在大范圍控制電樞電壓時，他勵發(fā)電機通常在反饋控制系統(tǒng)中使用。自勵發(fā)電機的勵磁繞組可以有三種不同的供電方式。勵磁線圈可以與電樞串聯(lián)起來，這便是串勵發(fā)電機；勵磁繞組可以與電樞并聯(lián)在一起，這便是并勵發(fā)電機。也可以同時以兩種方

12、式相連接組成一個復(fù)勵發(fā)電機。為了引起自勵過程，在自勵發(fā)電機中必須存在剩磁。</p><p>　　在典型的靜態(tài)伏-安特性中，假定原動機速度恒定，穩(wěn)態(tài)電動勢與端電壓之間的關(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)動機中，大于。電磁轉(zhuǎn)矩是一個反轉(zhuǎn)矩。</p>&l

13、t;p>　　他勵發(fā)電機的端電壓隨著負載電流的增大而輕微的減小，主要是因為電壓在電樞電阻上的壓降。串勵發(fā)電機中的勵磁電流與負載電流相同，所以氣隙磁通和電壓隨負載變化很大，因此很少采用串勵發(fā)電機。并勵發(fā)電機電壓隨負載增加會有所下降，但在許多應(yīng)用場合，這并不妨礙使用。復(fù)勵發(fā)電機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)點是通過串勵繞組作用，每極磁通隨著負載增加，從而產(chǎn)生一個隨負載增加近似為常數(shù)的輸出電壓。通常，并勵繞組匝數(shù)多，導(dǎo)

14、線細；而繞在外部的串勵繞組由于它必須承載電機的整個電樞電流，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>　　所有勵磁的方法在電動機上同樣適用。在電動機典型的靜態(tài)轉(zhuǎn)速—轉(zhuǎn)矩特性中，電機端電壓假設(shè)由恒壓源供電，在電動機中感應(yīng)的電勢與路端電壓間關(guān)系是</p><p><b>　?。?-8）</b>

15、;</p><p>　　是電樞輸入電流。電勢小于端電壓。電樞電流與發(fā)電機中的方向相反，且電磁轉(zhuǎn)矩與電樞旋轉(zhuǎn)方向相同。</p><p>　　對于并勵與他勵電動機來說，磁場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)電動機類似，并勵電動機實際是一種從空

16、載到滿負荷的速度基本上只有5%的下降的恒速電動機。從起動轉(zhuǎn)矩到達到最大轉(zhuǎn)矩之間一直是被電樞電流所控制可以正常交替進行。</p><p>　　并勵電動機的一個顯著優(yōu)點是速度控制，通過在并勵繞組回路裝上內(nèi)部變阻器，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>　　對于串勵電動機來說，電樞電流、電樞磁勢波以及定子磁場磁通隨負載增長而增長。因為由于負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電動機中的要小。因此串勵電動機是一種具有明顯下降的轉(zhuǎn)速-負載特性的變速發(fā)電機。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

18、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>　　在復(fù)勵電動機中，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)勵很少使用。積復(fù)勵電動機具有界于串勵和并勵之間的速度—負載特性，轉(zhuǎn)速隨負載的降低取決于并勵磁場和串勵磁場的相對安匝數(shù)。這種電動機沒有像串勵電動機那樣輕載高轉(zhuǎn)速的缺點，但它在相當

19、的程度上保持著串勵方式的優(yōu)點。</p><p>　　直流電機的應(yīng)用優(yōu)勢是可以連接成串勵、并勵及復(fù)勵式等各種勵磁方式。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換向器獲得更多的電壓，那么還會存在更多的應(yīng)用場合，不論對人工的還是自動控制的適應(yīng)性是它們的顯著特性。</p><p>　　一個直流電機是由兩個基本元素組成：</p><p>　　定子是電機固定

20、的部分。它由以下基礎(chǔ)組成；在結(jié)構(gòu)中有一個磁軛；勵磁磁極和繞組；換向極和繞組；有滑動軸承的端罩；電刷和電刷固定器；出線盒。</p><p>　　轉(zhuǎn)子是電機旋轉(zhuǎn)的部分。它構(gòu)成了一個中心，這個中心是安放在設(shè)備軸上并且已經(jīng)平均地隔開，把電樞繞組放入槽中。還有一個換向器和一個風(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。低壓電機的磁軛和本身的結(jié)構(gòu)是一體的，穿過勵磁磁極

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，有時候也用焊接的鋼板。在低壓和可控補償整流器電機中，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)部有兩個端蓋并且都包含球體和滑動軸承。</p><p>　　勵磁磁極由用0.5～1mm的鐵片通過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，讓它運送勵磁流動。在電樞上，

22、磁極鐵心的末端是極靴，它通過氣隙有助于磁通量的分布。繞組被放置在一個絕緣結(jié)構(gòu)內(nèi)的中心處，被極靴保護。</p><p>　　勵磁繞組是由絕熱的圓形物或矩形的導(dǎo)體制成，并且和另一個連續(xù)或平行的相連接。繞組是以一個磁極的磁通量穿過氣隙，然后被指引由極靴向電樞（北極），下一個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，它組成了一個中心，末端在極靴中并且一個繞組繞在中心周圍。它們被放在兩個主磁極中間的對稱軸，

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圍絕緣的或垂直的導(dǎo)線制成。它們相互平行或首尾連接，帶動設(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。目的是當它在磁場中旋轉(zhuǎn)時渦流升高時，減少渦流。它變得很熱將導(dǎo)致能量損失。在實心物的中心，它損失得很高，減少電機的效率和產(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)子的長度超過磁極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。每個轉(zhuǎn)子繞組包含了一個線圈直接繞在轉(zhuǎn)子槽中依靠特殊設(shè)計機械或成行的線圈。繞組是絕緣的，依靠木制的或絕緣物質(zhì)制成的槽楔保護它。繞組過載是其彎曲，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。低壓電機的換向器片被分割成一個獨立的單元，依靠合成樹脂互相絕緣，例如人造樹脂。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。電刷通過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>　　（1-2）</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>　?。?-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>　?。?-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>　　（1-6）</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>　　（1-8）</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>　?。璗he 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