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1、The Transformer on loadIntroduction to DC MachinesThe Transformer on loadIt has been shown that a primary input voltage can be transformed to any desired open-circuit 1Vsecondary voltage by a suitable choice of turns ratio. is available for circulating a load current 2E2Eimpedance. For the moment, a
2、 lagging power factor will be considered. The secondary current and the resulting ampere-turns will change the flux, tending to demagnetize the core, reduce and 2NI mwith it . Because the primary leakage impedance drop is so low, a small alteration to will cause 1E 1Ean appreciable increase of prima
3、ry current from to a new value of equal to 0II. The extra primary current and ampere-turns nearly cancel the whole of the ijXRV11/secondary ampere-turns. This being so , the mutual flux suffers only a slight modification and requires practically the same net ampere-turns as on no load. The total pri
4、mary ampere-turns are increased 10NIby an amount necessary to neutralize the same amount of secondary ampere-turns. In the vector 2NIequation , ; alternatively, . At full load, the current is 101I 2101NII0Ionly about 5% of the full-load current and so is nearly equal to . Because in mind that 1/, th
5、e input kVA which is approximately is also approximately equal to the 211/NE 1IEoutput kVA, .IThe physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary , , is shown unchanged because the total back 1mpe.m.f.,（ ）is still e
6、qual and opposite to . However, there has been a redistribution dtNE/11Vof flux and the mutual component has fallen due to the increase of with . Although the change is 1Ismall, the secondary demand could not be met without a mutual flux and e.m.f. alteration to permit primary current to change. The
7、 net flux linking the secondary winding has been further reduced by sthe establishment of secondary leakage flux due to , and this opposes . Although and 2Imm2are indicated separately , they combine to one resultant in the core which will be downwards at the instant shown. Thus the secondary termina
8、l voltage is reduced to which can be dtNVS/2considered in two components, i.e. or vectorially dtm/2. As for the primary, is responsible for a substantially constant secondary 22IjXEV2leakage inductance . It will be noticed that the primary leakage flux is responsible 2/Nifor part of the change in th
9、e secondary terminal voltage due to its effects on the mutual flux. The two leakage fluxes are closely related; , for example, by its demagnetizing action on has caused the 2 mchanges on the primary side which led to the establishment of primary leakage flux.If a low enough leading power factor is c
10、onsidered, the total secondary flux and the mutual flux are increased causing the secondary terminal voltage to rise with load. is unchanged in magnitude pfrom the no load condition since, neglecting resistance, it still has to provide a total back e.m.f. equal to . It is virtually the same as , tho
11、ugh now produced by the combined effect of primary and 1V1secondary ampere-turns. The mutual flux must still change with load to give a change of and permit 1Emore primary current to flow. has increased this time but due to the vector combination with 1E 1Vthere is still an increase of primary curre
12、nt.Two more points should be made about the figures. Firstly, a unity turns ratio has been assumed for convenience so that . Secondly, the physical picture is drawn for a different instant of time 21from the vector diagrams which show , if the horizontal axis is taken as usual, to be the zero 0mtime
13、 reference. There are instants in the cycle when primary leakage flux is zero, when the secondary leakage flux is zero, and when primary and secondary leakage flux is zero, and when primary and secondary leakage fluxes are in the same sense.The equivalent circuit already derived for the transformer
14、with the secondary terminals open, can easily be extended to cover the loaded secondary by the addition of the secondary resistance and leakage reactance.Practically all transformers have a turns ratio different from unity although such an arrangement is sometimes employed for the purposes of electr
15、ically isolating one circuit from another operating at the same voltage. To explain the case where the reaction of the secondary will be viewed from 21Nthe primary winding. The reaction is experienced only in terms of the magnetizing force due to the secondary ampere-turns. There is no way of detect
16、ing from the primary side whether is large and 2Ismall or vice versa, it is the product of current and turns which causes the reaction. Consequently, a 2Nsecondary winding can be replaced by any number of different equivalent windings and load circuits which will give rise to an identical reaction on the primary .It is clearly convenient to change the secondary winding to an equivalent winding having the same number of turns a
