電氣專(zhuān)業(yè)外文翻譯---負(fù)載運(yùn)行的變壓器及直流電機(jī)導(dǎo)論

1、<p>　　The Transformer on load﹠Introduction to DC Machines</p><p>　　The Transformer on load</p><p>　　It has been shown that a primary input voltage can be transformed to any desired open-ci

2、rcuit secondary voltage by a suitable choice of turn’s ratio. is available for circulating a load current impedance. For the moment, a lagging power factor will be considered. The secondary current and the resulting amp

3、ere-turns will change the flux, tending to demagnetize the core, reduce and with it . Because the primary leakage impedance drop is so low, a small alteration to will cause an appreciable </p><p>　　The ph

4、ysical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary, is shown unchanged because the total back e.m.f., （）is still equal and opposite to . How

5、ever, there has been a redistribution of flux and the mutual component has fallen due to the increase of with . Although the change is small, the secondary demand could not be met without a mutual flux and e.m.f. altera

6、tion to permit primary current to change. The net flux</p><p>　　If a low enough leading power factor is considered, the total secondary flux and the mutual flux are increased causing the secondary terminal v

7、oltage to rise with load. is unchanged in magnitude from the no load condition since, neglecting resistance, it still has to provide a total back e.m.f. equal to . It is virtually the same as , though now produced by th

8、e combined effect of primary and secondary ampere-turns. The mutual flux must still change with load to give a change of and permit more</p><p>　　Two more points should be made about the figures. Firstly, a

9、 unity turns ratio has been assumed for convenience so that . Secondly, the physical picture is drawn for a different instant of time from the vector diagrams which show , if the horizontal axis is taken as usual, to be

10、the zero time reference. There are instants in the cycle when primary leakage flux is zero, when the secondary leakage flux is zero, and when primary and secondary leakage flux is zero, and when primary and secondary lea

11、</p><p>　　The equivalent circuit already derived for the transformer with the secondary terminals open, can easily be extended to cover the loaded secondary by the addition of the secondary resistance and le

12、akage reactance.</p><p>　　Practically all transformers have a turn’s ratio different from unity although such an arrangement is sometimes employed for the purposes of electrically isolating one circuit from

13、another operating at the same voltage. To explain the case where the reaction of the secondary will be viewed from the primary winding. The reaction is experienced only in terms of the magnetizing force due to the secon

14、dary ampere-turns. There is no way of detecting from the primary side whether is large and small </p><p>　　With changes to , since the e.m.f.s are proportional to turns, which is the same as .</p>

15、<p>　　For current, since the reaction ampere turns must be unchanged must be equal to .i.e. .</p><p>　　For impedance, since any secondary voltage becomes , and secondary current becomes , then any s

16、econdary impedance, including load impedance, must become . Consequently, and .</p><p>　　If the primary turns are taken as reference turns, the process is called referring to the primary side.</p>&l

17、t;p>　　There are a few checks which can be made to see if the procedure outlined is valid.</p><p>　　For example, the copper loss in the referred secondary winding must be the same as in the original second

18、ary otherwise the primary would have to supply a different loss power. Must be equal to . does in fact reduce to .</p><p>　　Similarly the stored magnetic energy in the leakage field which is proportional

19、to will be found to check as . The referred secondary .</p><p>　　The argument is sound, though at first it may have seemed suspect. In fact, if the actual secondary winding was removed physically from the c

20、ore and replaced by the equivalent winding and load circuit designed to give the parameters ,,and , measurements from the primary terminals would be unable to detect any difference in secondary ampere-turns, demand or c

21、opper loss, under normal power frequency operation.</p><p>　　There is no point in choosing any basis other than equal turns on primary and referred secondary, but it is sometimes convenient to refer the prim

22、ary to the secondary winding. In this case, if all the subscript 1’s are interchanged for the subscript 2’s, the necessary referring constants are easily found; e.g. ,; similarly and .</p><p>　　The equivalen

23、t circuit for the general case where except that has been added to allow for iron loss and an ideal lossless transformation has been included before the secondary terminals to return to .All calculations of internal v

24、oltage and power losses are made before this ideal transformation is applied. The behavior of a transformer as detected at both sets of terminals is the same as the behavior detected at the corresponding terminals of thi

25、s circuit when the appropriate parameters are i</p><p>　　Very little error is introduced if the magnetizing branch is transferred to the primary terminals, but a few anomalies will arise. For example, the cu

26、rrent shown flowing through the primary impedance is no longer the whole of the primary current. The error is quite small since is usually such a small fraction of. Slightly different answers may be obtained to a partic

27、ular problem depending on whether or not allowance is made for this error. With this simplified circuit, the primary and referred </p><p><b>　　And </b></p><p>　　It should be pointed

28、out that the equivalent circuit as derived here is only valid for normal operation at power frequencies; capacitance effects must be taken into account whenever the rate of change of voltage would give rise to appreciabl

29、e capacitance currents,. They are important at high voltages and at frequencies much beyond 100 cycles/sec. A further point is not the only possible equivalent circuit even for power frequencies .An alternative , treatin

30、g the transformer as a three-or four-te</p><p>　　There are two ways of looking at the equivalent circuit:</p><p>　　viewed from the primary as a sink but the referred load impedance connected acr

31、oss ,or</p><p>　　Viewed from the secondary as a source of constant voltage with internal drops due to and. The magnetizing branch is sometimes omitted in this representation and so the circuit reduces to a

32、 generator producing a constant voltage (actually equal to ) and having an internal impedance (actually equal to ).</p><p>　　In either case, the parameters could be referred to the secondary winding and thi

33、s may save calculation time.</p><p>　　The resistances and reactances can be obtained from two simple light load tests.</p><p>　　Introduction to DC Machines</p><p>　　DC machines are

34、characterized by their versatility. By means of various combination of shunt, series, and separately excited field windings they can be designed to display a wide variety of volt-ampere or speed-torque characteristics fo

35、r both dynamic and steady state operation. Because of the ease with which they can be controlled, systems of DC machines are often used in applications requiring a wide range of motor speeds or precise control of motor o

36、utput.</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 coils. The air-gap flux distribution created by the field wind

37、ing 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 generated in each rotating armature coil is converted to DC

38、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 rectifier, resulting in a DC arma

39、ture 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 neutral zone, midway between the field</p><p>　　The

40、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-density wave in the air gap.

41、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 the space-fundamental component

42、 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 P pole machine</p><p>　　In which the minus s

43、ign has been dropped because the positive direction of the torque can be determined from physical reasoning. The space fundamental of the saw tooth armature m.m.f. wave is 8/ times its peak. Substitution in above equati

44、on then gives </p><p>　　Where =current in external armature circuit;</p><p>　　=total number of conductors in armature winding;</p><p>　　=number of parallel paths through winding;<

45、;/p><p><b>　　And </b></p><p>　　Is a constant fixed by the design of the winding.</p><p>　　The rectified voltage generated in the armature has already been discussed before

46、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 the voltage generated in one of the coils, commutation taki

47、ng 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 the coils in series between brushes</p><p>　

48、　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 the ripple is greatly reduced. </p><p>　　From

49、the above equations, with all variable expressed in SI units:</p><p>　　This equation simply says that the instantaneous electric power associated with the speed voltage equals the instantaneous mechanical po

50、wer 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>　　The direct-axis air-gap flux is produced by the combin

51、ed 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 is p

52、erpendicular 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 to fl

53、ux tim</p><p>　　Figure shows the magnetization curve with only one field winding excited. This curve can easily be obtained by test methods, no knowledge of any design details being required.</p><

54、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 to the total m.m.f. of the field windings, the co

55、nstant of proportionality being the direct-axis air-gap permeance.</p><p>　　The outstanding advantages of DC machines arise from the wide variety of operating characteristics which can be obtained by selecti

56、on 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., the machine may supply its own excitation. The method of e

57、xcitation profoundly influences not only the steady-state characteristics, but also the dynamic behavior of the machine in control systems.</p><p>　　The connection diagram of a separately excited generator i

58、s 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 amount of power in the armature circuit; i.e., the g

59、enerator 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. The field windings of self-excited generators ma

60、y </p><p>　　In the typical steady-state volt-ampere characteristics, constant-speed prime movers being assumed. The relation between the steady-state generated e.m.f. and the terminal voltage is </p>

61、<p>　　Where the armature is current output and is the armature circuit resistance. In a generator, is large than ; and the electromagnetic torque T is a counter torque opposing rotation.</p><p>　　The

62、 terminal voltage of a separately excited generator decreases slightly with increase in the load current, principally because of the voltage drop in the armature resistance. The field current of a series generator is the

63、 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 voltage of shunt generators drops off somewhat with load. Compo

64、und 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 than the terminal voltage , the armature current is in the op

65、posite 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 separately excited motors the field flux is nearly constant.

66、 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 increased current through the small armature re

67、sistance. 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 constant-speed motor having about 5 pe</p>&

68、lt;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 can be varied at will, and variation of flux causes the inve

69、rse 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 obtained by this method, the limitation again being commutating c

70、onditions. By variation of the impressed armature voltage, very wide s</p><p>　　In the series motor, increase in load is accompanied by increase in the armature current and m.m.f. and the stator field flux (

71、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 counter e.m.f.; moreover, the increase in armature current

72、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><p>　　In the compound motor the series field may b

73、e 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. A cumulatively compounded motor has speed-load c

74、haracteristic 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 series fields. It does not have the disadvantage of ve

75、ry high light-lo</p><p>　　The application advantages of DC machines lie in the variety of performance characteristics offered by the possibilities of shunt, series, and compound excitation. Some of these cha

76、racteristics 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 obtained from the commutator. Thus the versatility of D

77、C machine systems and their adaptability to control, both manual and automatic, are their outstandi</p><p>　　負(fù)載運(yùn)行的變壓器及直流電機(jī)導(dǎo)論</p><p><b>　　負(fù)載運(yùn)行的變壓器</b></p><p>　　通過(guò)選擇合適的匝數(shù)比

78、，一次側(cè)輸入電壓可任意轉(zhuǎn)換成所希望的二次側(cè)開(kāi)路電壓?？捎糜诋a(chǎn)生負(fù)載電流，該電流的幅值和功率因數(shù)將由而次側(cè)電路的阻抗決定。現(xiàn)在，我們要討論一種滯后功率因數(shù)。二次側(cè)電流及其總安匝將影響磁通，有一種對(duì)鐵芯產(chǎn)生去磁、減小和的趨向。因?yàn)橐淮蝹?cè)漏阻抗壓降如此之小，所以的微小變化都將導(dǎo)致一次側(cè)電流增加很大，從增大至一個(gè)新值。增加的一次側(cè)電流和磁勢(shì)近似平衡了全部二次側(cè)磁勢(shì)。這樣的話，互感磁通只經(jīng)歷很小的變化，并且實(shí)際上只需要與空載時(shí)相同的凈磁勢(shì)。一次側(cè)

79、總磁勢(shì)增加了，它是平衡同量的二次側(cè)磁勢(shì)所必需的。在向量方程中，，上式也可變換成。滿(mǎn)載時(shí)，電流只約占滿(mǎn)載電流的5%，因而近似等于。記住，近似等于的輸入容量也就近似等于輸出容量。</p><p>　　一次側(cè)電流已增大，隨之與之成正比的一次側(cè)漏磁通也增大。交鏈一次繞組的總磁通沒(méi)有變化，這是因?yàn)榭偡措妱?dòng)勢(shì)仍然與相等且反向。然而此時(shí)卻存在磁通的重新分配，由于隨的增加而增加，互感磁通分量已經(jīng)減小。盡管變化很小，但是如果沒(méi)有互

80、感磁通和電動(dòng)勢(shì)的變化來(lái)允許一次側(cè)電流變化，那么二次側(cè)的需求就無(wú)法滿(mǎn)足。交鏈二次繞組的凈磁通由于產(chǎn)生的二次側(cè)漏磁通（其與反相）的建立而被進(jìn)一步削弱。盡管圖中和是分開(kāi)表示的，但它們?cè)阼F芯中是一個(gè)合成量，該合成量在圖示中的瞬時(shí)是向下的。這樣，二次側(cè)端電壓降至，它可被看成兩個(gè)分量，即，或者向量形式。與一次側(cè)漏磁通一樣，的作用也用一個(gè)大體為常數(shù)的漏電感來(lái)表征。要注意的是，由于它對(duì)互感磁通的作用，一次側(cè)漏磁通對(duì)于二次側(cè)端電壓的變化產(chǎn)生部分影響。這兩

81、種漏磁通，緊密相關(guān)；例如，對(duì)的去磁作用引起了一次側(cè)的變化，從而導(dǎo)致了一次側(cè)漏磁通的產(chǎn)生。</p><p>　　如果我們討論一個(gè)足夠低的超前功率因數(shù)，二次側(cè)總磁通和互感磁通都會(huì)增加，從而使得二次側(cè)端電壓隨負(fù)載增加而升高。在空載情形下，如果忽略電阻，幅值大小不變，因?yàn)樗蕴峁┮粋€(gè)等于的反總電動(dòng)勢(shì)。盡管現(xiàn)在是一次側(cè)和二次側(cè)磁勢(shì)的共同作用產(chǎn)生的，但它實(shí)際上與相同?；ジ写磐ū仨毴噪S負(fù)載變化而變化以改變，從而產(chǎn)生更大的一次側(cè)

82、電流。此時(shí)的幅值已經(jīng)增大，但由于與是向量合成，因此一次側(cè)電流仍然是增大的。</p><p>　　從上述圖中，還應(yīng)得出兩點(diǎn)：首先，為方便起見(jiàn)已假設(shè)匝數(shù)比為1，這樣可使。其次，如果橫軸像通常取的話，那么向量圖是以為零時(shí)間參數(shù)的，圖中各物理量時(shí)間方向并不是該瞬時(shí)的。在周期性交變中，有一次側(cè)漏磁通為零的瞬時(shí)，也有二次側(cè)漏磁通為零的瞬時(shí)，還有它們處于同一方向的瞬時(shí)。</p><p>　　已經(jīng)推出的變

83、壓器二次側(cè)繞組端開(kāi)路的等效電路，通過(guò)加上二次側(cè)電阻和漏抗便可很容易擴(kuò)展成二次側(cè)負(fù)載時(shí)的等效電路。</p><p>　　實(shí)際中所有的變壓器的匝數(shù)比都不等于1，盡管有時(shí)使其為1也是為了使一個(gè)電路與另一個(gè)在相同電壓下運(yùn)行的電路實(shí)現(xiàn)電氣隔離。為了分析時(shí)的情況，二次側(cè)的反應(yīng)得從一次側(cè)來(lái)看，這種反應(yīng)只有通過(guò)由二次側(cè)的磁勢(shì)產(chǎn)生磁場(chǎng)力來(lái)反應(yīng)。我們從一次側(cè)無(wú)法判斷是大，小，還是小，大，正是電流和匝數(shù)的乘積在產(chǎn)生作用。因此，二次側(cè)繞

84、組可用任意個(gè)在一次側(cè)產(chǎn)生相同匝數(shù)的等效繞組是方便的。</p><p>　　當(dāng)變換成，由于電動(dòng)勢(shì)與匝數(shù)成正比，所以，與相等。</p><p>　　對(duì)于電流，由于對(duì)一次側(cè)作用的安匝數(shù)必須保持不變，因此，即。</p><p>　　對(duì)于阻抗，由于二次側(cè)電壓變成，電流變?yōu)椋虼俗杩怪?，包括?fù)載阻抗必然變?yōu)椤Ｒ虼?，，?lt;/p><p>　　如果將一次側(cè)匝

85、數(shù)作為參考匝數(shù)，那么這種過(guò)程稱(chēng)為往一次側(cè)的折算。</p><p>　　我們可以用一些方法來(lái)驗(yàn)證上述折算過(guò)程是否正確。</p><p>　　例如，折算后的二次繞組的銅耗必須與原二次繞組銅耗相等，否則一次側(cè)提供給其損耗的功率就變了。必須等于，而事實(shí)上確實(shí)簡(jiǎn)化成了。</p><p>　　類(lèi)似地，與成比例的漏磁場(chǎng)的磁場(chǎng)儲(chǔ)能，求出后驗(yàn)證與成正比。折算后的二次側(cè)。</p&

86、gt;<p>　　盡管看起來(lái)似乎不可理解，事實(shí)上這種論點(diǎn)是可靠的。實(shí)際上，如果我們將實(shí)際的二次繞組當(dāng)真從鐵芯上移開(kāi)，并用一個(gè)參數(shù)設(shè)計(jì)成，，，的等效繞組和負(fù)載電路替換，在正常電網(wǎng)頻率運(yùn)行時(shí)，從一次側(cè)兩端無(wú)法判斷二次側(cè)的磁勢(shì)、所需容量及銅耗與前有何差別。</p><p>　　在選擇折算基準(zhǔn)時(shí)，無(wú)非是將一次側(cè)與折算后的二次側(cè)匝數(shù)設(shè)為相等，除此之外再?zèng)]有什么更要緊的了。但有時(shí)將一次側(cè)折算到二次側(cè)倒是方便的，

87、在這種情況下，如果所有下標(biāo)“1”的量都變換成了下標(biāo)“2”的量，那么很容易得到必需的折算系數(shù)，例如。值得注意的是，對(duì)于一臺(tái)實(shí)際的變壓器，，；同樣地，。</p><p>　　的通常情形時(shí)的等效電路，它除了為了考慮鐵耗而引入了，且為了將折算回而在二次側(cè)兩端引入了一理想的無(wú)損耗轉(zhuǎn)換外，其他方面是一樣的。在運(yùn)用這種理想轉(zhuǎn)換之前，內(nèi)部電壓和功率損耗已進(jìn)行了計(jì)算。當(dāng)在電路中選擇了適當(dāng)?shù)膮?shù)時(shí)，在一、二次側(cè)兩端測(cè)得的變壓器運(yùn)行情

88、況與在該電路相應(yīng)端所測(cè)得的請(qǐng)況是完全一致的。將線圈和線圈并排放置在一個(gè)鐵芯的兩邊，這一點(diǎn)與實(shí)際情況之間的差別僅僅是為了方便。當(dāng)然，就變壓器本身來(lái)說(shuō)，兩線圈是繞在同一鐵芯柱上的。</p><p>　　如果將激磁支路移至一次繞組端口，引起的誤差很小，但一些不合理的現(xiàn)象又會(huì)發(fā)生。例如，流過(guò)一次側(cè)阻抗的電流不再是整個(gè)一次側(cè)電流。由于通常只是的很小一部分，所有誤差相當(dāng)小。對(duì)一個(gè)具體問(wèn)題可否允許有細(xì)微差別的回答取決于是否允許

89、這種誤差的存在。對(duì)于這種簡(jiǎn)化電路，一次側(cè)和折算后二次側(cè)阻抗可相加，得和</p><p>　　需要指出的是，在此得到的等效電路僅僅適用于電網(wǎng)頻率下的正常運(yùn)行；一旦電壓變化率產(chǎn)生相當(dāng)大的電容電流時(shí)必須考慮電容效應(yīng)。這對(duì)于高電壓和頻率超過(guò)100Hz的情形是很重要的。其次，即使是對(duì)于電網(wǎng)頻率也并非唯一可行的等效電路。另一種形式是將變壓器看成一個(gè)三端或四端網(wǎng)絡(luò)，這樣便產(chǎn)生一個(gè)準(zhǔn)確的表達(dá)，它對(duì)于那些把所有裝置看成是具有某種傳

90、遞性能的電路元件的工程師來(lái)說(shuō)是方便的。以此為分析基礎(chǔ)的電路會(huì)擁有一個(gè)既產(chǎn)生電壓大小的變化，也產(chǎn)生相位移的匝比，其阻抗也會(huì)與繞組的阻抗不同。這種電路無(wú)法解釋變壓器內(nèi)類(lèi)似飽和效應(yīng)等現(xiàn)象。</p><p>　　等效電路有兩個(gè)入端口形式：</p><p>　　從一次側(cè)看為一個(gè)U形電路，其折合后的負(fù)載阻抗的端電壓為；</p><p>　　從二次側(cè)看為一其值為，且伴有由和引起內(nèi)

91、壓降的恒壓源。在這種電路中有時(shí)可省略激磁支路，這樣電路簡(jiǎn)化為一臺(tái)產(chǎn)生恒值電壓（實(shí)際上等于）并帶有阻抗（實(shí)際上等于）的發(fā)電機(jī)。</p><p>　　在上述兩種情況下，參數(shù)都可折算到二次繞組，這樣可減小計(jì)算時(shí)間。</p><p>　　其電阻和電抗值可通過(guò)兩種簡(jiǎn)單的輕載試驗(yàn)獲得。</p><p><b>　　直流電機(jī)導(dǎo)論</b></p>

92、<p>　　直流電機(jī)以其多功用性而形成了鮮明的特征。通過(guò)并勵(lì)、串勵(lì)和特勵(lì)繞組的各種不同組合，直流電機(jī)可設(shè)計(jì)成在動(dòng)態(tài)和穩(wěn)態(tài)運(yùn)行時(shí)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(dòng)機(jī)轉(zhuǎn)速變化范圍寬或能精確控制電機(jī)輸出的場(chǎng)合。</p><p>　　定子上有凸極，由一個(gè)或一個(gè)以上勵(lì)磁線圈勵(lì)磁。勵(lì)磁繞組產(chǎn)生的氣隙通以磁極中心線為軸線對(duì)稱(chēng)分布，這條軸線稱(chēng)為磁場(chǎng)軸線或直軸。&l

93、t;/p><p>　　我們知道，每個(gè)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(shì)波形。電刷的放置應(yīng)使換向線圈也處于磁極中性區(qū)，即兩磁極之間。這樣，電樞磁勢(shì)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。

94、這樣，如圖所示電樞磁勢(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°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無(wú)關(guān)；為了方便起見(jiàn)，我們假設(shè)氣隙中仍然是正弦磁密波，這樣便可以從磁場(chǎng)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(shì)波的

95、空間基波分量相互作用的結(jié)果來(lái)表示。電刷處于交軸時(shí)，磁場(chǎng)間的角度為90°電角度，其正弦值等于1，則對(duì)于一臺(tái)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(hào)已經(jīng)去掉。電樞磁勢(shì)鋸齒波的空間基波是峰值的8/。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)；<

96、/p><p>　　=通過(guò)繞組的并聯(lián)支路數(shù)；</p><p><b>　　且</b></p><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面已經(jīng)討論過(guò)了。將繞組分散在幾個(gè)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(gè)線圈產(chǎn)生的電壓，換向線圈邊處于磁中性區(qū)。從電刷端

97、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(gè)換向器片，波線的波動(dòng)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(dòng)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&l

98、t;/p><p>　　這個(gè)等式簡(jiǎn)單地說(shuō)明與速度電壓有關(guān)的瞬時(shí)功率等于與磁場(chǎng)轉(zhuǎn)矩有關(guān)的瞬時(shí)機(jī)械功率，能量的流向取決于這臺(tái)電機(jī)是電動(dòng)機(jī)還是發(fā)電機(jī)。</p><p>　　直軸氣隙通由勵(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，因?yàn)殡姌写艅?shì)波的軸線與磁場(chǎng)軸線垂直，因此假定電樞磁勢(shì)對(duì)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(shí)飽和效

99、應(yīng)會(huì)深入研究。因?yàn)殡姌须妱?shì)與磁通成正比，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)速時(shí)，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(gè)勵(lì)磁繞組的磁化曲線，這條曲線可以很容易通過(guò)實(shí)驗(yàn)方法得到，不需要任何設(shè)計(jì)步驟的知識(shí)。</p><p>　　在一個(gè)相當(dāng)寬的勵(lì)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(lì)磁繞組總磁勢(shì)呈線性