變壓器-外文翻譯

1、<p>　　譯自<<科技英語>></p><p><b>　　變壓器</b></p><p><b>　　1. 介紹</b></p><p>　　要從遠端發(fā)電廠送出電能，必須應(yīng)用高壓輸電。因為最終的負荷，在一些點高電壓必須降低。變壓器能使電力系統(tǒng)各個部分運行在電壓不同的等級。本文我們討論的

2、原則和電力變壓器的應(yīng)用。</p><p><b>　　2. 雙繞組變壓器</b></p><p>　　變壓器的最簡單形式包括兩個磁通相互耦合的固定線圈。兩個線圈之所以相互耦合，是因為它們連接著共同的磁通。</p><p>　　在電力應(yīng)用中，使用層式鐵芯變壓器(本文中提到的)。變壓器是高效率的，因為它沒有旋轉(zhuǎn)損失，因此在電壓等級轉(zhuǎn)換的過程中，能量

3、損失比較少。典型的效率范圍在92到99%，上限值適用于大功率變壓器。</p><p>　　從交流電源流入電流的一側(cè)被稱為變壓器的一次側(cè)繞組或者是原邊。它在鐵圈中建立了磁通φ，它的幅值和方向都會發(fā)生周期性的變化。磁通連接的第二個繞組被稱為變壓器的二次側(cè)繞組或者是副邊。磁通是變化的；因此依據(jù)楞次定律，電磁感應(yīng)在二次側(cè)產(chǎn)生了電壓。變壓器在原邊接收電能的同時也在向副邊所帶的負荷輸送電能。這就是變壓器的作用。</p&

4、gt;<p>　　3. 變壓器的工作原理</p><p>　　當二次側(cè)電路開路是，即使原邊被施以正弦電壓Vp，也是沒有能量轉(zhuǎn)移的。外加電壓在一次側(cè)繞組中產(chǎn)生一個小電流Iθ。這個空載電流有兩項功能：（1）在鐵芯中產(chǎn)生電磁通，該磁通在零和φm之間做正弦變化，φm是鐵芯磁通的最大值；（2）它的一個分量說明了鐵芯中的渦流和磁滯損耗。這兩種相關(guān)的損耗被稱為鐵芯損耗。</p><p>　

5、　變壓器空載電流Iθ一般大約只有滿載電流的2%—5%。因為在空載時，原邊繞組中的鐵芯相當于一個很大的電抗，空載電流的相位大約將滯后于原邊電壓相位90º。顯然可見電流分量Im= I0sinθ0，被稱做勵磁電流，它在相位上滯后于原邊電壓VP 90º。就是這個分量在鐵芯中建立了磁通；因此磁通φ與Im同相。</p><p>　　第二個分量Ie=I0sinθ0，與原邊電壓同相。這個電流分量向鐵芯提供用于

6、損耗的電流。兩個相量的分量和代表空載電流，即</p><p>　　I0 = Im+ Ie</p><p>　　應(yīng)注意的是空載電流是畸變和非正弦形的。這種情況是非線性鐵芯材料造成的。</p><p>　　如果假定變壓器中沒有其他的電能損耗一次側(cè)的感應(yīng)電動勢Ep和二次側(cè)的感應(yīng)電壓Es可以表示出來。因為一次側(cè)繞組中的磁通會通過二次繞組，依據(jù)法拉第電磁感應(yīng)定律，二次側(cè)繞組中

7、將產(chǎn)生一個電動勢E，即E=NΔφ/Δt。相同的磁通會通過原邊自身，產(chǎn)生一個電動勢Ep。正如前文中討論到的，所產(chǎn)生的電壓必定滯后于磁通90º，因此，它于施加的電壓有180º的相位差。因為沒有電流流過二次側(cè)繞組，Es=Vs。一次側(cè)空載電流很小，僅為滿載電流的百分之幾。因此原邊電壓很小，并且Vp的值近乎等于Ep。原邊的電壓和它產(chǎn)生的磁通波形是正弦形的；因此產(chǎn)生電動勢Ep和Es的值是做正弦變化的。產(chǎn)生電壓的平均值如下<

8、/p><p>　　Eavg = turns×</p><p>　　即是法拉第定律在瞬時時間里的應(yīng)用。它遵循</p><p>　　Eavg = N = 4fNφm</p><p>　　其中N是指線圈的匝數(shù)。從交流電原理可知，有效值是一個正弦波，其值為平均電壓的1.11倍；因此</p><p>　　E = 4.44f

9、Nφm</p><p>　　因為一次側(cè)繞組和二次側(cè)繞組的磁通相等，所以繞組中每匝的電壓也相同。因此</p><p>　　Ep = 4.44fNpφm</p><p><b>　　并且</b></p><p>　　Es = 4.44fNsφm</p><p>　　其中Np和Es是一次側(cè)繞組和二次側(cè)繞

10、組的匝數(shù)。一次側(cè)和二次側(cè)電壓增長的比率稱做變比。用字母a來表示這個比率，如下式</p><p><b>　　a = = </b></p><p>　　假設(shè)變壓器輸出電能等于其輸入電能——這個假設(shè)適用于高效率的變壓器。實際上我們是考慮一臺理想狀態(tài)下的變壓器；這意味著它沒有任何損耗。因此</p><p><b>　　Pm = Pout&

11、lt;/b></p><p><b>　　或者</b></p><p>　　VpIp × primary PF = VsIs × secondary PF</p><p>　　這里PF代表功率因素。在上面公式中一次側(cè)和二次側(cè)的功率因素是相等的；因此</p><p>　　VpIp = VsIs&l

12、t;/p><p><b>　　從上式我們可以得知</b></p><p><b>　　= ≌ ≌ a</b></p><p>　　它表明端電壓比等于匝數(shù)比，換句話說，一次側(cè)和二次側(cè)電流比與匝數(shù)比成反比。匝數(shù)比可以衡量二次側(cè)電壓相對于一次惻電壓是升高或者是降低。為了計算電壓，我們需要更多數(shù)據(jù)。</p><

13、p>　　終端電壓的比率變化有些根據(jù)負載和它的功率因素。實際上, 變比從標識牌數(shù)據(jù)獲得, 列出在滿載情況下原邊和副邊電壓。</p><p>　　當副邊電壓Vs相對于原邊電壓減小時，這個變壓器就叫做降壓變壓器。如果這個電壓是升高的，它就是一個升壓變壓器。在一個降壓變壓器中傳輸變比a遠大于1(a>1.0)，同樣的，一個升壓變壓器的變比小于1(a<1.0)。當a=1時，變壓器的二次側(cè)電壓就等于起一次側(cè)電

14、壓。這是一種特殊類型的變壓器，可被應(yīng)用于當一次側(cè)和二次側(cè)需要相互絕緣以維持相同的電壓等級的狀況下。因此，我們把這種類型的變壓器稱為絕緣型變壓器。</p><p>　　顯然，鐵芯中的電磁通形成了連接原邊和副邊的回路。在第四部分我們會了解到當變壓器帶負荷運行時一次側(cè)繞組電流是如何隨著二次側(cè)負荷電流變化而變化的。</p><p>　　從電源側(cè)來看變壓器，其阻抗可認為等于Vp / Ip。從等式

15、= ≌ ≌ a中我們可知Vp = aVs并且Ip = Is/a。根據(jù)Vs和Is，可得Vp和Ip的比例是</p><p><b>　　= = </b></p><p>　　但是Vs / Is 負荷阻抗ZL，因此我們可以這樣表示</p><p>　　Zm (primary) = a2ZL</p><p>　　這個等式表

16、明二次側(cè)連接的阻抗折算到電源側(cè)，其值為原來的a2倍。我們把這種折算方式稱為負載阻抗向一次側(cè)的折算。這個公式應(yīng)用于變壓器的阻抗匹配。</p><p>　　4. 有載情況下的變壓器</p><p>　　一次側(cè)電壓和二次側(cè)電壓有著相同的極性，一般習慣上用點記號表示。如果點號同在線圈的上端，就意味著它們的極性相同。因此當二次側(cè)連接著一個負載時，在瞬間就有一個負荷電流沿著這個方向產(chǎn)生。換句話說，極性

17、的標注可以表明當電流流過兩側(cè)的線圈時，線圈中的磁動勢會增加。</p><p>　　因為二次側(cè)電壓的大小取決于鐵芯磁通大小φ0，所以很顯然當正常情況下負載電勢Es沒有變化時，二次側(cè)電壓也不會有明顯的變化。當變壓器帶負荷運行時，將有電流Is流過二次側(cè)，因為Es產(chǎn)生的感應(yīng)電動勢相當于一個電壓源。二次側(cè)電流產(chǎn)生的磁動勢NsIs會產(chǎn)生一個勵磁。這個磁通的方向在任何一個時刻都和主磁通反向。當然，這是楞次定律的體現(xiàn)。因此，Ns

18、Is所產(chǎn)生的磁動勢會使主磁通φ0減小。這意味著一次側(cè)線圈中的磁通減少，因而它的電壓Ep將會增大。感應(yīng)電壓的減小將使外施電壓和感應(yīng)電動勢之間的差值更大，它將使初級線圈中流過更大的電流。初級線圈中的電流Ip的增大，意味著前面所說明的兩個條件都滿足：（1）輸出功率將隨著輸出功率的增加而增加（2）初級線圈中的磁動勢將增加，以此來抵消二次側(cè)中的磁動勢減小磁通的趨勢。</p><p>　　總的來說，變壓器為了保持磁通是常數(shù)，

19、對磁通變化的響應(yīng)是瞬時的。更重要的是，在空載和滿載時，主磁通φ0的降落是很少的（一般在）1至3%。其需要的條件是E降落很多來使電流Ip增加。</p><p>　　在一次側(cè)，電流Ip’在一次側(cè)流過以平衡Is產(chǎn)生的影響。它的磁動勢NpIp’只停留在一次側(cè)。因為鐵芯的磁通φ0保持不變，變壓器空載時空載電流I0必定會為其提供能量。故一次側(cè)電流Ip是電流Ip’與I0’的和。</p><p>　　因為

20、空載電流相對較小，那么一次側(cè)的安匝數(shù)與二次側(cè)的安匝數(shù)相等的假設(shè)是成立的。因為在這種狀況下鐵芯的磁通是恒定的。因此我們?nèi)耘f可以認定空載電流I0相對于滿載電流是極其小的。</p><p>　　當一個電流流過二次側(cè)繞組，它的磁動勢（NsIs）將產(chǎn)生一個磁通，于空載電流I0產(chǎn)生的磁通φ0不同，它只停留在二次側(cè)繞組中。因為這個磁通不流過一次側(cè)繞組，所以它不是一個公共磁通。</p><p>　　另外，

21、流過一次側(cè)繞組的負載電流只在一次側(cè)繞組中產(chǎn)生磁通，這個磁通被稱為一次側(cè)的漏磁。二次側(cè)漏磁將使電壓增大以保持兩側(cè)電壓的平衡。一次側(cè)漏磁也一樣。因此，這兩個增大的電壓具有電壓降的性質(zhì)，總稱為漏電抗電壓降。另外，兩側(cè)繞組同樣具有阻抗，這也將產(chǎn)生一個電阻壓降。把這些附加的電壓降也考慮在內(nèi)，這樣一個實際的變壓器的等值電路圖就完成了。由于分支勵磁體現(xiàn)在電流里，為了分析我們可以將它忽略。這就符我們前面計算中可以忽略空載電流的假設(shè)。這證明了它對我們分析

22、變壓器時所產(chǎn)生的影響微乎其微。因為電壓降與負載電流成比例關(guān)系，這就意味著空載情況下一次側(cè)和二次側(cè)繞組的電壓降都為零。</p><p>　　TRANSFORMER</p><p>　　1. INTRODUCTION</p><p>　　The high-voltage transmission was need for the case electrical powe

23、r is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a

24、power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications.</p><p>　　2. TOW-WINDING TRANSFORMERS</p><p>　　A transformer in its si

25、mplest form consists of two stationary coils coupled by a mutual magnetic flux. The coils are said to be mutually coupled because they link a common flux.</p><p>　　In power applications, laminated steel core

26、 transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transform

27、ing power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers.</p><p>　　The current flowing in the coil connected t

28、o the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary wind

29、ing or simply secondary. The flux is changing; therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenz’s law. Thus the primary receives its power from the source while the se

30、condary supplies </p><p>　　3. TRANSFORMER PRINCIPLES</p><p>　　When a sinusoidal voltage Vp is applied to the primary with the secondary open-circuited, there will be no energy transfer. The impr

31、essed voltage causes a small current Iθ to flow in the primary winding. This no-load current has two functions: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and φm, where φm is t

32、he maximum value of the core flux; and (2) it provides a component to account for the hysteresis and eddy current losses in the core. There combined</p><p>　　The no-load current Iθ is usually few percent of

33、the rated full-load current of the transformer (about 2 to 5%). Since at no-load the primary winding acts as a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90º. It

34、 is readily seen that the current component Im= I0sinθ0, called the magnetizing current, is 90º in phase behind the primary voltage VP. It is this component that sets up the flux in the core; φ is therefore in phase

35、 with Im.</p><p>　　The second component, Ie=I0sinθ0, is in phase with the primary voltage. It is the current component that supplies the core losses. The phasor sum of these two components represents the no-

36、load current, or</p><p>　　I0 = Im+ Ie</p><p>　　It should be noted that the no-load current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material.</

37、p><p>　　If it is assumed that there are no other losses in the transformer, the induced voltage In the primary, Ep and that in the secondary, Es can be shown. Since the magnetic flux set up by the primary windi

38、ng，there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, E=NΔφ/Δt. This same flux also links the primary itself, inducing in it an EMF, Ep. As discussed earlier, the induced vo

39、ltage must lag the flux by 90º, therefore, they are 180º out of phase with the</p><p>　　Eavg = turns×</p><p>　　which is Faraday’s law applied to a finite time interval. It follows

40、 that</p><p>　　Eavg = N = 4fNφm</p><p>　　which N is the number of turns on the winding. Form ac circuit theory, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the

41、average voltage; thus</p><p>　　E = 4.44fNφm</p><p>　　Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence</p><p&g

42、t;　　Ep = 4.44fNpφm</p><p><b>　　and</b></p><p>　　Es = 4.44fNsφm</p><p>　　where Ep and Es are the number of turn on the primary and secondary windings, respectively. The r

43、atio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a, it is seen that</p><p><b>　　a = = </b></p><p>　　Assume that the output pow

44、er of a transformer equals its input power, not a bad sumption in practice considering the high efficiencies. What we really are saying is that we are dealing with an ideal transformer; that is, it has no losses. Thus<

45、;/p><p><b>　　Pm = Pout</b></p><p><b>　　or</b></p><p>　　VpIp × primary PF = VsIs × secondary PF</p><p>　　where PF is the power factor. For

46、 the above-stated assumption it means that the power factor on primary and secondary sides are equal; therefore</p><p>　　VpIp = VsIs</p><p>　　from which is obtained</p><p><b>

47、　　= ≌ ≌ a</b></p><p>　　It shows that as an approximation the terminal voltage ratio equals the turns ratio. The primary and secondary current, on the other hand, are inversely related to the turns ra

48、tio. The turns ratio gives a measure of how much the secondary voltage is raised or lowered in relation to the primary voltage. To calculate the voltage regulation, we need more information.</p><p>　　The rat

49、io of the terminal voltage varies somewhat depending on the load and its power factor. In practice, the transformation ratio is obtained from the nameplate data, which list the primary and secondary voltage under full-lo

50、ad condition.</p><p>　　When the secondary voltage Vs is reduced compared to the primary voltage, the transformation is said to be a step-down transformer: conversely, if this voltage is raised, it is called

51、a step-up transformer. In a step-down transformer the transformation ratio a is greater than unity (a>1.0), while for a step-up transformer it is smaller than unity (a<1.0). In the event that a=1, the transformer s

52、econdary voltage equals the primary voltage. This is a special type of transformer used in instances w</p><p>　　As is apparent, it is the magnetic flux in the core that forms the connecting link between prim

53、ary and secondary circuit. In section 4 it is shown how the primary winding current adjusts itself to the secondary load current when the transformer supplies a load.</p><p>　　Looking into the transformer te

54、rminals from the source, an impedance is seen which by definition equals Vp / Ip. From = ≌ ≌ a , we have Vp = aVs and Ip = Is/a.In terms of Vs and Is the ratio of Vp to Ip is</p><p><b>　　= = </b

55、></p><p>　　But Vs / Is is the load impedance ZL thus we can say that</p><p>　　Zm (primary) = a2ZL</p><p>　　This equation tells us that when an impedance is connected to the secon

56、dary side, it appears from the source as an impedance having a magnitude that is a2 times its actual value. We say that the load impedance is reflected or referred to the primary. It is this property of transformers that

57、 is used in impedance-matching applications.</p><p>　　4. TRANSFORMERS UNDER LOAD</p><p>　　The primary and secondary voltages shown have similar polarities, as indicated by the “dot-making” conve

58、ntion. The dots near the upper ends of the windings have the same meaning as in circuit theory; the marked terminals have the same polarity. Thus when a load is connected to the secondary, the instantaneous load current

59、is in the direction shown. In other words, the polarity markings signify that when positive current enters both windings at the marked terminals, the MMFs of the two windings a</p><p>　　Since the secondary v

60、oltage depends on the core flux φ0, it must be clear that the flux should not change appreciably if Es is to remain essentially constant under normal loading conditions. With the load connected, a current Is will flow in

61、 the secondary circuit, because the induced EMF Es will act as a voltage source. The secondary current produces an MMF NsIs that creates a flux. This flux has such a direction that at any instant in time it opposes the m

62、ain flux that created it in the first p</p><p>　　In general, it will be found that the transformer reacts almost instantaneously to keep the resultant core flux essentially constant. Moreover, the core flux

63、φ0 drops very slightly between n o load and full load (about 1 to 3%), a necessary condition if Ep is to fall sufficiently to allow an increase in Ip.</p><p>　　On the primary side, Ip’ is the current that fl

64、ows in the primary to balance the demagnetizing effect of Is. Its MMF NpIp’ sets up a flux linking the primary only. Since the core flux φ0 remains constant. I0 must be the same current that energizes the transformer at

65、no load. The primary current Ip is therefore the sum of the current Ip’ and I0.</p><p>　　Because the no-load current is relatively small, it is correct to assume that the primary ampere-turns equal the secon

66、dary ampere-turns, since it is under this condition that the core flux is essentially constant. Thus we will assume that I0 is negligible, as it is only a small component of the full-load current.</p><p>　　W

67、hen a current flows in the secondary winding, the resulting MMF (NsIs) creates a separate flux, apart from the flux φ0 produced by I0, which links the secondary winding only. This flux does no link with the primary windi

68、ng and is therefore not a mutual flux.</p><p>　　In addition, the load current that flows through the primary winding creates a flux that links with the primary winding only; it is called the primary leakage

69、flux. The secondary- leakage flux gives rise to an induced voltage that is not counter balanced by an equivalent induced voltage in the primary. Similarly, the voltage induced in the primary is not counterbalanced in the