變壓器外文翻譯.doc

外文文獻譯文變壓器1. 介紹要從遠端發(fā)電廠送出電能，必須應用高壓輸電。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。本文我們討論的原則和電力變壓器的應用。2. 雙繞組變壓器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。在電力應用中，使用層式鐵芯變壓器(本文中提到的)。變壓器是高效率的，因為它沒有旋轉損失，因此在電壓等級轉換的過程中，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被稱為變壓器的一次側繞組或者是原邊。它在鐵圈中建立了磁通，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，電磁感應在二次側產(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。3. 變壓器的工作原理當二次側電路開路是，即使原邊被施以正弦電壓Vp，也是沒有能量轉移的。外加電壓在一次側繞組中產(chǎn)生一個小電流。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m之間做正弦變化，m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。這兩種相關的損耗被稱為鐵芯損耗。變壓器空載電流I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90。顯然可見電流分量Im= I0sin0，被稱做勵磁電流，它在相位上滯后于原邊電壓VP 90。就是這個分量在鐵芯中建立了磁通；因此磁通與Im同相。第二個分量Ie=I0sin0，與原邊電壓同相。這個電流分量向鐵芯提供用于損耗的電流。兩個相量的分量和代表空載電流，即：I0 = Im+ Ie應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有其他的電能損耗一次側的感應電動勢Ep和二次側的感應電壓Es可以表示出來。因為一次側繞組中的磁通會通過二次繞組，依據(jù)法拉第電磁感應定律，二次側繞組中將產(chǎn)生一個電動勢E，即E=N/t。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90，因此，它于施加的電壓有180的相位差。因為沒有電流流過二次側繞組，Es=Vs。一次側空載電流很小，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：Eavg = turns即是法拉第定律在瞬時時間里的應用。它遵循：Eavg = N = 4fNm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：E = 4.44fNm因為一次側繞組和二次側繞組的磁通相等，所以繞組中每匝的電壓也相同。因此：Ep = 4.44fNpm并且：Es = 4.44fNsm其中Np和Es是一次側繞組和二次側繞組的匝數(shù)。一次側和二次側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：a = = 假設變壓器輸出電能等于其輸入電能這個假設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：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和二次側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，一次側和二次側電流比與匝數(shù)比成反比。匝數(shù)比可以衡量二次側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)。 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。 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(a1.0)，同樣的，一個升壓變壓器的變比小于1(a1.0), while for a step-up transformer it is smaller than unity (a1.0). In the event that a=1, the transformer secondary voltage equals the primary voltage. This is a special type of transformer used in instances where electrical isolation is required between the primary and secondary circuit while maintaining the same voltage level. Therefore, this transformer is generally knows as an isolation transformer.As is apparent, it is the magnetic flux in the core that forms the connecting link between primary 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.Looking into the transformer terminals 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 = = But Vs / Is is the load impedance ZL thus we can say thatZm (primary) = a2ZLThis equation tells us that when an impedance is connected to the secondary 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 is used in impedance-matching applications.4. TRANSFORMERS UNDER LOADThe primary and secondary voltages shown have similar polarities, as indicated by the “dot-making” 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 0, 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 Lenzs law in action. Thus the MMF represented by NsIs tends to reduce the core flux 0. 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 0 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 0 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 no-load current is relatively small, it is correct to assume that the primary ampere-turns equal the secondary 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.When a current flows in the secondary winding, the resulting MMF (NsIs) creates a separate flux, apart from the flux 0 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