【導(dǎo)讀】對供電電源進(jìn)行電壓等級變換，應(yīng)對電能進(jìn)行。重新分配的場所稱為變電所。建筑變電所是供配電系統(tǒng)的樞紐，供電電源由電網(wǎng)引到變電所，在。要從遠(yuǎn)端發(fā)電廠送出電能，必須應(yīng)用高壓輸電。變壓器能使電力系統(tǒng)各個部分運行在電壓不同的等級。原則和電力變壓器的應(yīng)用。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。它沒有旋轉(zhuǎn)損失，因此在電壓等級轉(zhuǎn)換的過程中，能量損失比較少。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。磁通連接的第二個繞組被稱。磁通是變化的；因此依據(jù)楞次定律，電磁感應(yīng)在。當(dāng)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。這兩種相關(guān)的損耗被稱為鐵芯損耗。應(yīng)注意的是空載電流是畸變和非正弦形的。電壓Es可以表示出來。相對于滿載電流是極其小的。

　　

【正文】 m e which is Faraday’s law applied to a finite time interval. It follows that Eavg = N21/(2 )mf? = 4fNφm which N is the number of turns on the winding. Form ac circuit theory, the effective or rootmeansquare (rms) voltage for a sine wave is times the average voltage。 thus E = Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence Ep = and Es = where Ep and Es are the number of turn on the primary and secondary windings, respectively. The ratio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a, it is seen that a = psEE = psNN Assume that the output power 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 Pm = Pout or VpIp primary PF = VsIs secondary PF where PF is the power factor. For the abovestated assumption it means that the power factor on primary and secondary sides are equal。 therefore VpIp = VsIs from which is obtained psVV = psII ≌ psEE ≌ a 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 ratio. 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. The ratio 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 fullload condition. When the secondary voltage Vs is reduced pared to the primary voltage, the transformation is said to be a stepdown transformer: conversely, if this voltage is raised, it is called a stepup transformer. In a stepdown transformer the transformation ratio a is greater than unity (a), while for a stepup transformer it is smaller than unity (a). 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 magic 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 psVV = psII ≌ psEE ≌ a , we have Vp = aVs and Ip = Is/ terms of Vs and Is the ratio of Vp to Ip is ppVI = /ssaVIa = 2ssaVI But Vs / Is is the load impedance ZL thus we can say that Zm (primary) = a2ZL This 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 impedancematching applications. 4. TRANSFORMERS UNDER LOAD The 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. 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 demagizing 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