【正文】 NDER LOADThe primary and secondary voltages shown have similar polarities, as indicated by the “dotmaking” convention. 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 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 add.Since the secondary voltage 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 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 main flux that created it in the first place. Of course, this is Lenz’s law in action. Thus the MMF represented by NsIs tends to reduce the core flux φ0. This means that the flux linking the primary winding reduces and consequently the primary induced voltage Ep, This reduction in induced voltage causes a greater difference between the impressed voltage and the counter induced EMF, thereby allowing more current to flow in the primary. The fact that primary current Ip increases means that the two conditions stated earlier are fulfilled: (1) the power input increases to match the power output, and (2) the primary MMF increases to offset the tendency of the secondary MMF to reduce the flux.In general, it will be found that the transformer reacts almost instantaneously to keep the resultant core flux essentially constant. Moreover, the core flux 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.On the primary side, Ip’ is the current that flows 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 no load. The primary current Ip is therefore the sum of the current Ip’ and I0.Because the noload current is relatively small, it is correct to assume that the primary ampereturns equal the secondary ampereturns, 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 ponent of the fullload current.When 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 winding and is therefore not a mutual flux.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 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 secondary winding. Consequently, these two induced voltages behave like voltage drops, generally called leakage reactance voltage drops. Furthermore, each winding has some resistance, which produces a resistive voltage drop. When taken into account, these additional voltage drops would plete the equivalent circuit diagram of a practical transformer. Note that the magnetizing branch is shown in this circuit, which for our purposes will be disregarded. This follows our earlier assumption that the noload current is assumed negligible in our calculations. This is further justified in that it is rarely necessary to predict transformer performance to such accuracies. Since the voltage drops are all directly proportional to the load current, it means that at noload conditions there will be no voltage drops in either winding.2譯文變壓器1. 介紹要從遠(yuǎn)端發(fā)電廠送出電能，必須應(yīng)用高壓輸電。因?yàn)樽罱K的負(fù)荷，在一些點(diǎn)高電壓必須降低。變壓器能使電力系統(tǒng)各個(gè)部分運(yùn)行在電壓不同的等級(jí)。本文我們討論的原則和電力變壓器的應(yīng)用。2. 雙繞組變壓器變壓器的最簡(jiǎn)單形式包括兩個(gè)磁通相互耦合的固定線圈。兩個(gè)線圈之所以相互耦合，是因?yàn)樗鼈冞B接著共同的磁通。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?yàn)樗鼪](méi)有旋轉(zhuǎn)損失，因此在電壓等級(jí)轉(zhuǎn)換的過(guò)程中，能量損失比較少。典型的效率范圍在92到99%，上限值適用于大功率變壓器。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(huì)發(fā)生周期性的變化。磁通連接的第二個(gè)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(shí)也在向副邊所帶的負(fù)荷輸送電能。這就是變壓器的作用。3. 變壓器的工作原理當(dāng)二次側(cè)電路開(kāi)路是，即使原邊被施以正弦電壓Vp，也是沒(méi)有能量轉(zhuǎn)移的。外加電壓在一次側(cè)繞組中產(chǎn)生一個(gè)小電流Iθ。這個(gè)空載電流有兩項(xiàng)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(gè)分量說(shuō)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。變壓器空載電流Iθ一般大約只有滿載電流的2%—5%。因?yàn)樵诳蛰d時(shí)，原邊繞組中的鐵芯相當(dāng)于一個(gè)很大的電抗，空載電流的相位大約將滯后于原邊電壓相位90186。顯然可見(jiàn)電流分量Im= I0sinθ0，被稱做勵(lì)磁電流，它在相位上滯后于原邊電壓VP 90186。就是這個(gè)分量在鐵芯中建立了磁通；因此磁通φ與Im同相。第二個(gè)分量Ie=I0sinθ0，與原邊電壓同相。這個(gè)電流分量向鐵芯提供用于損耗的電流。兩個(gè)相量的分量和代表空載電流，即I0 = Im+ Ie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。如果假定變壓器中沒(méi)有其他的電能損耗一次側(cè)的感應(yīng)電動(dòng)勢(shì)Ep和二次側(cè)的感應(yīng)電壓Es可以表示出來(lái)。因?yàn)橐淮蝹?cè)繞組中的磁通會(huì)通過(guò)二次繞組，依據(jù)法拉第電磁感應(yīng)定律，二次側(cè)繞組中將產(chǎn)生一個(gè)電動(dòng)勢(shì)E，即E=NΔφ/Δt。相同的磁通會(huì)通過(guò)原邊自身，產(chǎn)生一個(gè)電動(dòng)勢(shì)Ep。正如前文中討論到的，所產(chǎn)生的電壓必定滯后于磁通90186。，因此，它于施加的電壓有180186。的相位差。因?yàn)闆](méi)有電流流過(guò)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(dòng)勢(shì)Ep和Es的值是做正弦變化的。產(chǎn)生電壓的平均值如下Eavg = turns即是法拉第定律在瞬時(shí)時(shí)間里的應(yīng)用。它遵循Eavg = N = 4fNφm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(gè)正弦波，；因此E = 因?yàn)橐淮蝹?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Ep = 并且Es = 其中Np和Es是一次側(cè)繞組和二次側(cè)繞組的匝數(shù)。一次側(cè)和二次側(cè)電壓增長(zhǎng)的比率稱做變比。用字母a來(lái)表示這個(gè)比率，如下式a = = 假設(shè)變壓器輸出電能等于其輸入電能——這個(gè)假設(shè)適用于高效率的變壓器。實(shí)際上我們是考慮一臺(tái)理想狀態(tài)下的變壓器；這意味著它沒(méi)有任何損耗。因此Pm = Pout或者VpIp primary PF = VsIs secondary PF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VpIp = VsIs從上式我們可以得知 = ≌ ≌ a它表明端電壓比等于匝數(shù)比，換句話說(shuō)，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(duì)于一次惻電壓是升高或者是降低。為了計(jì)算電壓，我們需要更多數(shù)據(jù)。 終端電壓的比率變化有些根據(jù)負(fù)載和它的功率因素。實(shí)際上, 變比從標(biāo)識(shí)牌數(shù)據(jù)獲得, 列出在滿載情況下原邊和副邊電壓。 當(dāng)副邊電壓Vs相對(duì)于原邊電壓減小時(shí)，這個(gè)變壓器就叫做降壓變壓器。如果這個(gè)電壓是升高的，它就是一個(gè)升壓變壓器。在一個(gè)降壓變壓器中傳輸變比a遠(yuǎn)大于1(a)，同樣的，一個(gè)升壓變壓器的變比小于1(a)。當(dāng)a=1時(shí)，變壓器的二次側(cè)電壓就等于起一次側(cè)電壓。這是一種特殊類型的變壓器，可被應(yīng)用于當(dāng)一次側(cè)和二次側(cè)需要相互絕緣