直流調速系統(tǒng)概述外文翻譯

1、<p>　　DC governor system outlined</p><p>　　DC governor is artificially or automatically change the direct current generator rotational speed to meet the For mechanical requirements. From the mechanica

2、l properties point of view, is that by changing the parameters or electric motors plus The voltage electric motors and other methods to change the mechanical properties, thus changing electric motors and mechanical pro

3、perties Mechanical properties of the claimed identity work for the stability of operation of electric motors changed. T</p><p>　　DC governor advantage </p><p>　　Direct current generator is a

4、 good, braking performance, appropriate in a broader context, smooth mechanics, rolling mill, mine hoist, excavator, marine rigs, metal cutting machine tools, paper planes, such as the need for high-performance high-lift

5、 electricity fees Controllable areas received widespread application. In recent years, exchanges governor system developed rapidly, but direct current fees system in theory and practice are relatively mature, and from th

6、e feedback loop control perspec</p><p>　　Double-loop system integral DC governor </p><p>　　Rotational speed, the current two-loop systems theory governor figure 2-1. By the plan shows that the

7、 system has two feedback circuit, a 2 loop circuit (so called double-loop). One is the current regulator Lt and current testing - the current part of a feedback link, and another by the speed regulator St rotational spee

8、d detection-a feedback link speed link. As part of the Central buildings currents, the currents that threatened to Central (boys Deputy Central), said speed link to cry (that the </p><p>　　DC mechanics of th

9、e digital system </p><p>　　With microelectronic technology, microprocessors and computer software development for the control of the governor through a variety of functions in almost all microprocessors, th

10、e use of software to achieve. From the past to control the simulation modelling - controlled digital mixed development, the final realization of the entire digital. In digital systems, with the exception of the governor

11、of conventional functions, but also a failure warning, diagnosis and shows, and other functions, and </p><p>　　DC governor system and the current development </p><p>　　Nearly 30 years ago, for

12、 the following reasons, and promoted the rapid development of power control systems exposed, one is to control the development of theory, a best control, the smart control self control, intelligence control. Correspondin

13、g control system is also exposed the practice gradually taking shape. The second is the development of electronic devices, control systems composition fees brought significant changes. Changes in the past used rotary flo

14、w units (electric motors to exchange</p><p>　　Outlook </p><p>　　From the traditional system, although in recent years the rapid exchange of electrical mechanics technology development in many

15、areas is direct current machine mechanics technology expansion in DC mechanics research and manufacturing process, the design and assembly of hardware accounted for a large proportion. Manufacture and the manufacture of

16、electrical devices require a large number of mechanics and skilled workers, a certain quality of personnel requirements. Foreign related industries rela</p><p>　　Introduction to DC Machines</p><p&

17、gt;　　DC machines are characterized by their vcrsatility．By means of various combinanons of shunt，serie5,and separately excited field windings they can be designed to display a wide variety of volt—ampcre or speedtorque c

18、haraclersics for both dynamic and stcady—state operation．Because of the ease with which they can be contro11ed，systems of DC machines are often used in applications requiring a wide range of motor speeds or precise contr

19、o 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 of more field coils．The air-gap flux distribution created by the

20、 field winding is symmetrical about the centerline ofthe field Poles．This axis called the field axis of direct axis．</p><p>　　As we know,the AC voltage generated in each rotating armtue coiI is converled to

21、DC in the external armature terminals by means of a rotating commutator and stationonary brushes to which the armature leads are connected．The commutingor—brush combination forms a mechanical rectifier ，resulting in a D

22、C armature voltage as wellas 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 po</p><p>

23、　　The magnetic torque and the speed voltage appearing at the brushes are independent of the spatial waveform of the flux distributions; for convcenience we shall continue to assume a sinusoidal flux—density wave in the

24、air gap．The torque can then be found from the magnetic field viewpoint.</p><p>　　The torque can be expressed in terms 0f the interaction of the direct—axis air—gap flux per pole and the space—fundamental co

25、mponent of the armature m．m．f．wave．With the brushes in the quadrature axis the angle between these fields is 90 elcctrical de8rees，</p><p>　　and its sine equals unity．For a　 P　pole machine：</p>&l

26、t;p>　　in which the minus sign gas been dropped because the positve difection of the torque can irection of the torque can bedetermined from Physical reasoning．The space fundamental of thc sawtooth arma—ture m．m．fwav

27、e　times its peak．Substitution in above equation then gives</p><p>　　where =currcnt in external armature circuit；</p><p>　　Ca＝total number of conductors in armature winding;</p><p>

28、;　　m＝number of parallel paths through winding；</p><p>　　and is a constant fixed by the design of the winding.</p><p>　　The rectified voltage generated in the armatufe has already been disscus

29、sed 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 recufied sine waves is the voltage generrated in one of the coils，commut

30、aion taking place at the moment when the coil sides are in the neutral</p><p>　　zone．The generated voltage as observed from the brushes shown by the rippling line labeled in figure．With a dozen or so commuta

31、tor segments per pole，The ripple becomes very small and the average generated voltage observed from the brushes equals the sum of the average values of the rectified coil voltages．The rectified voltage ea between b

32、rushes，known also as the speed voltage,is</p><p>　　where Ka is the design constant．The rechfied voltage of a distributed winding has the same average value as that or a concentrated coil．The differcnce is th

33、at the ripple is greatly reduced.</p><p>　　From the abovc equations，with a11 variable expressed in SI units：</p><p>　　This equation simply says that the instantaneous electrlc power associated

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

35、gt;　　The direct—axis air—gap flux is produced by the combined m．m．f． of thc field windings，thc flux—m．m．f． charactcristic being the magnetization curve for the panlcular iron geonletry of the machine．In the magnetizat

36、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 effccts of saturation are investigated more thorou

37、ghly. Because the armature 　e．m．f． is proportional to flux times</p><p>　　Figure shows the magnetZation curve with only one field winding exeited．This curve can easily be obtained by test methods，no knowled

38、ge of any design details being required．</p><p>　　Over a fairly wide rage of exeitation the reluctance of the iron is negligible compared with that of the air gap．In this region the flux is 1inearly propodio

39、nal to the tota lm．m．f of the field windings，the constant of proportionality being the derect-axis air-gap permeance.</p><p>　　The outstanding advantages of DC machines arise from the wide variety of operati

40、ng characteriulcs 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．，the machine

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

42、on diagram of a separately excited genefator is given．The required field currcent is a very small fraction of the rated armature current. A small amount of power in the field circuit may contol a relatively large amoun

43、t of Power in the armature circuit; i．e.，the generator is a power amplifier. Separately excited generators are often used in feed back control systems when control of the armature voltage over a wide range is required．&l

44、t;/p><p>　　The field windings of self—excited generators may be supplied in three different ways．The field may be connected in series with the armature,resulting in a series gencrator．The field may be connected

45、 in shunt with the armature,resulting in a shunt gencrator，or the field may be in two sections，one of which is connected in series and the othcr in shunt with the</p><p>　　armature,resulting in a compound ge

46、nerator. With self-excited generators residual magnetism must be present in the machine iron to get the self—excitation process started．</p><p>　　In the typical steady-state volt-ampere characteristics，c

47、onstant—speed prime movers being as sumed.The relation between the steady-state generated e. m．f．Ea and the terminal voltage Vt is　　</p><p>　　where Ia is the armature cufrent output and Ra is the armature

48、cifcuit resistance．In a gen—erator，Ea is larger than Vt; and the electromagnetic torque This a countcnorque opposing　rotation．and hence the voltage vary widely with load．As a consequlnce，series generators are not often u

49、sed．The voltage of shunt generators drops of somewhat with load．Compound generators are normally connected so that the　m．m．f．of the series winding aids that of the</p><p>　　shunt winding．The advantage is th

50、at through the action of the series winding the flux perpole can increase with load，resulting in a voltage output which is nearIy usually,contains many turns of relatively small wire.the series winding,wound on the ou

51、tside，consists of a few turns of comparatively heavy conductor because it must carry the full armture cufrent of the machine．The voltage of both shunt gnd compound generators can be controlled over reasonable Limits by

52、 means of rheostats in the</p><p><b>　??；</b></p><p>　　where Ia is now the armature current inPut.the generated e．m．f．Ea is now smaller than　the terminal voltage Vt，the afmature curre

53、nt is in the opposite direction to that in a generator　and the electromagnetic torque is in the direction to sustain of the armature．</p><p>　　In shunt and separately excited motors the field flux is nearly

54、constant．Conscequently，incrsead torque must be accompanied by a very nearly proportionincrease in armature　current and hence by a small decrease in couter e．m．f． to allow this increased current　through the smallarmatur

55、e re5istance．5incl 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 Percent <

56、/p><p>　　An outstanding advantage of the shunt motor is ease of speed contol．With a rheostat in the shunt—field circuit ,the field current and flux per pole can be varied at will，and varlation of flux causes th

57、e inverse variation of speed to maintain counter e．m．f．approximately equal to the impressed terminal voltage．A maximum speed range of about 4 to 5 can be obtained by this method，the limitation being commutating condt

58、ions．By variation of the impressed armature voltage，very wide speed ranges can </p><p>　　In the series motor，increase in load is accompanied by incrlase in the armature current and m．m．f． and the stator fie

59、ld flux(provided the iron is not completely saturated)．Because flux increases with load，speed must drop in order to maintain the balance belween impressed voltage and counter e. m．f．; more over，the increase in armature c

60、urrcnt caused by increas torque is smaller than in the shunt motor bccause of the increased flux．The series motor is therefore a varying -peed motor with a marked</p><p>　　In the compound motor the series fi

61、eld may be connected either cumulatively，so that its m．m．f．adds to that of the shunt field，or differentially so that it oppose6d．The differencial connection is very rarely used．A cumulatively compounded motor has speed一

62、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 sefies fields．it does not have the disadvantage

63、 of very high light—load </p><p>　　The application advantages of DC machines lie in the vafiety of performance characteristics offered by the possibilities 0f shunt,series，and compound excitaion . Some of

64、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 obtained from the commutotor．Thus the versatili

65、ty of DC machine systems and their adaptability to control，both manual and automatic，are their outsanding fe</p><p><b>　　直流調速系統(tǒng)概述</b></p><p>　　直流調速是指人為地或自動地改變直流電動機的轉速，以滿足工作機械的要求。從機械特

66、性上看，就是通過改變電動機的參數或外加工電壓等方法來改變電動機的機械特性，從而改變電動機機械特性和工作特性機械特性的交點，使電動機的穩(wěn)定運轉速度發(fā)生變化。調速通常通過給定環(huán)節(jié)，中間放大環(huán)節(jié)，校正環(huán)節(jié)，反饋環(huán)節(jié)和保護環(huán)節(jié)等來實現(xiàn)。電動機的轉速不能自動校正與給定轉速的偏差的調速系統(tǒng)稱為開環(huán)控制系統(tǒng)。這種調速系統(tǒng)的電動機的轉速要受到負載波動及電源電壓波動等外界擾動的影響。電動機的轉速能自動的校正與給定轉速的偏差，不受負載及電網電壓波動等外界擾

67、動的影響，使電動機的轉速始終與給定轉速保持一致的調速系統(tǒng)稱為閉環(huán)控制系統(tǒng)。這是由于閉環(huán)控制系統(tǒng)具有反饋環(huán)節(jié)。電氣傳動控制系統(tǒng)通常由電動機、控制裝置和信息裝置三部分組成。它能按照規(guī)定的指令，及時的控制電動機的啟動、制動、運轉方向、位置、速度和加速度等，以滿足工作機械及生產過程的要求。隨著電機、傳感器、控制器件、變流技術和控制理論的發(fā)展，電氣傳動控制系統(tǒng)也得到了很大的發(fā)展。目前，所用電機的單機容量從幾百瓦發(fā)展到數萬千瓦，變流設備從旋轉式電機

68、變流機組發(fā)展到大功率晶閘管靜止變流裝置，中</p><p><b>　　直流調速的優(yōu)點</b></p><p>　　直流電動機具有良好的起、制動性能，宜于在廣泛范圍內平滑調速，在軋鋼機、礦井卷揚機、挖掘機、海洋鉆機、金屬切削機床、造紙機、高層電梯等需要高性能可控電力拖動的領域中得到了廣泛的應用。近年來，交流調速系統(tǒng)發(fā)展很快，然而直流拖動系統(tǒng)無論在理論上和實踐上都比較成

69、熟，并且從反饋閉環(huán)控制的角度來看，它又是交流拖動控制系統(tǒng)的基礎，所以直流調速系統(tǒng)在生產生活中有著舉足輕重的作用。直流調速系統(tǒng)，特別是雙閉環(huán)直流調速系統(tǒng)是工業(yè)生產過程中應用最廣的電氣傳動裝置之一。廣泛地應用于軋鋼機、冶金、印刷、金屬切削機床等許多領域的自動控制系統(tǒng)中。它通常采用三相全控橋式整流電路對電動機進行供電，從而控制電動機的轉速，傳統(tǒng)的控制系統(tǒng)采用模擬元件，如晶體管、各種線性運算電路等，雖在一定程度上滿足了生產要求，但是因為元件容易

70、老化和在使用中易受外界干擾影響，并且線路復雜、通用性差，控制效果受到器件性能、溫度等因素的影響，從而致使系統(tǒng)的運行特性也隨之變化，故系統(tǒng)的運行可靠性及準確性得不到保證，甚至出現(xiàn)事故。</p><p>　　雙閉環(huán)直流調速系統(tǒng)的組成</p><p>　　轉速、電流雙閉環(huán)調速系統(tǒng)原理如圖2-1。由圖可見，該系統(tǒng)有兩個反饋回路，構成兩個閉環(huán)回路，（故稱雙閉環(huán)）。其中一個是由電流調節(jié)器LT 和電流檢

71、測—反饋環(huán)節(jié)構成的電流環(huán)，另一個是由速度調節(jié)器ST 轉速檢測—反饋環(huán)節(jié)構成的速度環(huán)。由于速度環(huán)包圍電流環(huán)，因此稱電流環(huán)為內環(huán)，（又稱副環(huán)），稱速度環(huán)為外環(huán)（又有稱主環(huán)）。在電路中，ST 和LT 實行串級聯(lián)接，即由ST 去“驅動”LT，再由“LT”去“控制”觸發(fā)電路。圖中速度調節(jié)器和電流調節(jié)器均為比例加積分（ＰＩ）調節(jié)器，其輸入和輸出均設有限副電流。轉速環(huán)要求電流迅速響應轉速的變化而變化，而電流環(huán)則要求維持電流不變。這種性能會不利于電流對

72、轉速變化的響應，有使靜特性變軟的趨勢。但由于轉速環(huán)是外環(huán)，電流環(huán)的作用只相當轉速環(huán)內部一種擾動而已，不起主導作用。只要轉速環(huán)的開環(huán)放大倍數足夠大，最后仍能靠ST 的積分作用。消除轉速偏差。</p><p>　　雙閉環(huán)調速系統(tǒng)的機械特性</p><p>　　由于ST 為PI 調節(jié)器，系統(tǒng)為無靜差，穩(wěn)態(tài)誤差小，一般講來，大多</p><p>　　能滿足生產上的要求。其機

73、械特性近似為一水平線，如圖2—2a 所示。</p><p>　　當電動機發(fā)生嚴重過載，并當I＞I 時，電流調節(jié)器將使整流裝置輸出</p><p>　　電壓明顯降低，這一方面限制了電流繼續(xù)增長，另一方面將使轉速迅速下降，出現(xiàn)了很陡的下垂特性，見圖2—2b 段。</p><p>　　直流調速系統(tǒng)的數字化</p><p>　　隨著微電子技術、微處理

74、機以及計算機軟件的發(fā)展，使調速控制的各種功能幾乎均可通過微處理機，借助軟件來實現(xiàn)。即從過去的模擬控制向模擬-數字混合控制發(fā)展，最后實現(xiàn)全數字化。在數字化系統(tǒng)中，除具有常規(guī)的調速功能外，還具有故障報警、診斷及顯示等功能，同時，數字系統(tǒng)通常具有較強的通信能力，通過選配適當的通信接口模板，可方便地實現(xiàn)主站（如上一級PLC 或計算機系統(tǒng)）和從站（單機交、直流傳動控制裝置）間的數字通信，組成分級多機的自動化系統(tǒng)。為易于調試，數字系統(tǒng)的軟件，一般設

75、計有調節(jié)器參數的自動化優(yōu)化，通過啟動優(yōu)化程序，實現(xiàn)自動尋優(yōu)和確定系統(tǒng)的動態(tài)參數，以及實現(xiàn)如直流電動機磁化特性曲線的自動測試等，有利于縮短調試時間和提高控制性能。國外一些電氣公司都有成系列的與模擬調整系統(tǒng)相對應的全數字交、直流調速裝置產品可供選用，新開發(fā)的調速系統(tǒng)幾乎全是數字式的。與模擬系統(tǒng)類似，全數字調速系統(tǒng)已發(fā)展成為緊湊式和模塊式兩大類。但全數字調速系統(tǒng)還是有模擬調速系統(tǒng)無法比擬的優(yōu)點，技術更先進，操作方便。數字調速系統(tǒng)與模擬調速系統(tǒng)

76、相對比，技術性能有如下優(yōu)點：</p><p>　?。?）態(tài)精度高且能長期保持。</p><p>　?。?）動態(tài)性能好，動態(tài)參數調整方便，借助于豐富的軟件，易于實現(xiàn)各類自適應和復合控制。</p><p><b>　　（3）調速范圍寬</b></p><p><b>　?。?）電壓波動小</b><

77、/p><p>　?。?）參數實現(xiàn)軟件化，無漂移影響</p><p>　?。?）所用元件數量少，不易失效</p><p>　?。?）設定值量化程度高，且狀態(tài)重復率好</p><p>　?。?）放大器和級間耦合噪聲很小，電磁干擾小</p><p>　?。?）調試即投產靈活方便，易于設計和修改設計</p><

78、p>　　（10）標準及通用化程度高，除主CPU 模塊外，僅數種附加模塊</p><p>　　可實現(xiàn)，包括工藝參數在內的多元閉環(huán)控制</p><p>　?。?1）適用范圍廣，可實現(xiàn)各類變速控制及易于實現(xiàn)與PLC 系統(tǒng)通信</p><p>　　（12）常規(guī)的晶閘管直流調速系統(tǒng)中大量硬件可用軟件代替，從而簡化系統(tǒng)結構，減少了電子元件虛焊、接觸不良和漂移等引起的一些故

79、障，而且維修方便。</p><p>　　國內外直流調速系統(tǒng)發(fā)展 現(xiàn)狀</p><p>　　近30多年來，由于以下原因，推動了電力拖動控制系統(tǒng)的迅速發(fā)展，第一是控制理論的發(fā)展，出現(xiàn)了最優(yōu)控制，智能控制，自適應控制，智能控制等。相應的拖動控制系統(tǒng)也在實踐中逐步形成。其二是電子器件的發(fā)展，帶來了拖動控制系統(tǒng)組成結構的重大變化。過去采用旋轉變流機組（交流電動機帶動直流電動機）來實現(xiàn)直流電

80、動機的平滑調速，而直流發(fā)動機的勵磁又采用交磁功率放大機，磁放大器進行控制；到了六十年代，出現(xiàn)了晶閘管整流裝置，不僅在經濟上和可靠性上有所提高，而且在技術上也顯示了很大的優(yōu)越性。別外集成運算放大器和眾多的電子模塊的出現(xiàn)，不斷促進了控制結構的變化。其三是八十年代計算機技術和通信技術的發(fā)展，開創(chuàng)了拖動控制系統(tǒng)蓬勃發(fā)展的新年代，8位-32位單片機的相繼出現(xiàn)并應用于控制系統(tǒng)，使其結構更加簡單，功能更強，可靠性更高。隨著計算機技術的發(fā)展，無論是生產

81、還是生活當中，人民對數字化信息的依賴程度越來越高。如果說計算機是大腦，網絡是神經，那么電機傳動系統(tǒng)就是骨骼和肌肉。它們之間的完美結合才是現(xiàn)代產業(yè)發(fā)展方向。為了使直流調速系統(tǒng)與信息系統(tǒng)緊密結合，同時也為了提高直流調速系統(tǒng)自身的性能，必須使直流調速系統(tǒng)實現(xiàn)全數字化控制。單</p><p><b>　　展望 </b></p><p>　　從傳統(tǒng)系統(tǒng)來說，雖然近幾年

82、來交流電機調速技術迅速發(fā)展，在許多方面正向直流電機調速技術領域擴展，在直流調速的研究與制造過程中，硬件的設計與組裝占了相當大的比重。電機制造以及調速裝置的制造需要大批的技術熟練工人，對人員的素質有一定要求。而國外相關產業(yè)的人工成本相對較高，在近十年內，直流調速的制造業(yè)有可能向發(fā)展中國家轉移。對中國來說，這也是一個機遇，如果我們抓住這個機會，再利用本身的市場有利條件，有可能在我國形成直流調速系統(tǒng)的制造業(yè)中心，使我國工業(yè)上一個新的臺階。需要

83、注意的是發(fā)達國家在高技術領域是不會輕易放棄的，他們非常注意核心技術及軟件的保護和保密，為此，必須加大該領域的科研與開發(fā)的力度。</p><p><b>　　直流電機</b></p><p>　　直流電機以其多功用性而形成了鮮明的特征。通過并勵、串勵和他勵繞組的各種不同組合，直流電機可設計成在動態(tài)和穩(wěn)態(tài)運行時呈現(xiàn)出寬廣范圍變化的伏—安或速度—轉矩特性。由于直流電機易于控

84、制，因此該系統(tǒng)用于要求電動機轉速變化范圍寬或能精確控制電機輸出的場合。</p><p>　　直流電機的總貌如圖所示。定于上有凸極，由一個或一個以上勵磁線圈勵磁。勵磁繞組產生的氣隙磁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，每個旋轉的電樞繞組中產生的交流電壓，經由一與電樞聯(lián)接的旋轉的換向器和靜止的電刷，在電樞繞組出線端轉換成直流電壓。換向器——

85、電刷的組合構成機械整流器，它產生一直流電樞電壓和一在空間固定的電樞磁勢波形。電刷的放置應使換向線圈也處于磁極中性區(qū)，即兩磁極之間。這樣，龜樞磁勢波形的軸線與滋極軸線相差90”電角度，即位于交軸上。在示意圖中，電刷位于交軸上，因為此處正是與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。(在實際電機中，電刷的幾何位置大約偏移圖例中所示位置90電角度，這是因為元件的末端形狀構成圖示結果與換向器相連。)</p>

86、;<p>　　電刷上的電磁轉矩和速度電壓與磁通分布的空間波形無關；為了方便起見，我們假設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矩。</p><p>　　轉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l，則對于一臺P極電機</p><p>　　式中由于轉短的正方向可以根據物理概念的

87、推斷確定，因此負號已經去掉的空間基波是峰值的。上式變換后有</p><p>　　式中 ＝電樞外部電路中的電流；</p><p>　　Ca＝電樞繞組中的總導體數；</p><p>　　m＝通過繞組的并聯(lián)支路數；</p><p>　　其為一個由繞組設計而確定的常數。</p><p>　　簡單的單個線圈的電樞中的整流電壓前

88、面已經討論過了。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，在因中由標以ea的波線表示。當每極有大約十幾個換向器片，波線的波動變得非常小，從電刷端觀察到的平</p><p>　　均電壓等于線圈整流電壓平均值之和。電刷間的整流電壓ea即速度電壓，為</p><p>　　

89、式中Ka為設計常數。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大大減小。</p><p>　　將上述幾式中的所有變量用SI單位制表達，有</p><p>　　這個等式簡單地說明與速度電壓有關的瞬時功率等于與磁場轉矩有關的瞬時機械功率，能量的流向取決于這臺電機是電動機還是發(fā)電機。</p><p>　　直軸氣隙磁通由勵磁繞組的合成磁勢產生

90、，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，因為電樞磁勢波的軸線與磁場軸線垂直，因此假定電樞磁勢對直軸磁通不產生作用。這種修設有必要在后述部分加以驗證，屆時飽和效應會深入研究。因為電樞電勢與磁通時速成正比，所以通常用恒定轉速。下的電樞電勢來表示磁化曲線更為方便。任意轉速wn時，任一給定磁通下的電壓與轉速成正比，即</p><p>　　在一個相當寬的勵磁范圍內，鐵磁材料部分的磁阻

91、與氣隙磁阻相比可以忽略不計，在此范圍內磁通與勵磁繞組總磁勢呈線性比例，比例常數便是直軸氣隙磁導率。</p><p>　　直流電機的突出優(yōu)點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行特性。勵磁繞組可以由外部直流電源單獨激磁，或者也可自勵，即電機提供自身的勵磁。</p><p>　　勵磁方法不僅極大地影響控制系統(tǒng)中電機的靜態(tài)特性，而且影響其動態(tài)運行。</p>&

92、lt;p>　　他勵發(fā)電機的連接圖已經給出，所需勵磁電流是額定電樞電流的很小一部分。勵磁電路中很小數量的功率可以控制電樞電路中相對很大數量的功率，也就是說發(fā)電機是一種功率放大器。當需要在很大范圍內控制電樞電壓時，他勵發(fā)電機常常用于反饋控制系統(tǒng)中。自勵發(fā)電機的勵磁繞組可以有三種不同的供電方式。勵磁繞組可以與電樞串聯(lián)起來，這便形成了</p><p>　　串勵發(fā)電機；勵磁繞組可以與電樞并聯(lián)在一起，這使形成了并勵發(fā)電

93、機；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)，這便形成復勵發(fā)電機。為了引起自勵過程，在白勵發(fā)電機中必須存在剩磁。</p><p>　　在典型的靜態(tài)伏-安特性中，假定原動機恒速運行，穩(wěn)態(tài)電勢Ea和端電壓Vf關系為</p><p>　　其中Ia為電樞輸出電流，Rd為電樞回路電阻。在發(fā)電機中，Ea比Vt大、電磁轉矩T是阻轉矩。</p><p>　　

94、他勵發(fā)電機的端電壓隨著負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)電機中的勵磁電流與負載電流相同，這樣，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，但在許多應用場合，這并不影響使用。復勵發(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)點是通過串勵繞組的作用。每極磁通隨著負載增加，從而產生一個隨負載增加近似為常數或稍稍增大的輸出電壓。通常，并勵繞組匝數多，導線細；而繞在

95、外部的串勵繞組由于它必須承載電機的整個電樞電流，所以其構成的導線相對較粗。不論是并勵還是復勵發(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得到調節(jié)。</p><p>　　任何用于發(fā)電機的勵磁方法都可用于電動機。在電動機典型的靜態(tài)轉速—轉矩特性中，假設電動機兩端由一個恒壓源供電。在電動機電樞中感應的電勢與端電壓Vt間的關系為</p><p>　　式Ia此時為輸入的電樞電流。電勢Ea此時

96、比端電壓小，電樞電流與發(fā)電機中的方向相反，且電磁轉矩與電樞旋轉方向相同。</p><p>　　在并勵和他勵電動機中磁場磁通近似為常數，因此轉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速，因此，轉速必須稍稍降低。與鼠籠式感應電動機相類似，并勵電動機實際上是一種從空載到滿載速降僅約為5％的恒速電動機。起動轉矩和最大轉矩受到能成功換<

97、/p><p>　　向的電樞電流的限制。</p><p>　　并勵電動機的突出優(yōu)點是易于調運。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，而磁通的變化導致轉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速范圍為4或5比1，最高轉速同樣受到換向條件的限制。通過改變外施電樞電壓，可以獲得很寬的調運范圍。</p><p>　　在串勵電動機中，電

98、樞電流、電樞電勢和定于磁場磁通隨負載增加而增加(假設鐵芯不完全飽和)。因為磁通隨負載增大，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，所以轉矩增大所引起的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速-負載特性的變速電動機。對于要求轉矩過載很多的應用場合，由于對應的過載功率隨相應的轉速下降而維持在一個合理的范圍內，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

99、還帶來非常有用的起動特性。</p><p>　　在復勵電動機中，串勵磁場可以連接成積復勵式，使其磁勢與并勵磁場相加；也可以連接成差復勵式，兩磁場方向相反。差復勵連接很少使用。積復勵電動機具有界于并勵和串勵電動機之間的速度-負載特性，轉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。這種電動機沒有像串勵電動機那樣輕載高轉速的缺點，但它在相當的程度上保持著串勵方式的優(yōu)點。</p><p>