【正文】 1 The Transformer on load﹠ Introduction to DC Machines The Transformer on load It has been shown that a primary input voltage 1V can be transformed to any desired opencircuit secondary voltage 2E by a suitable choice of turn’s ratio. 2E is available for circulating a load current impedance. For the moment, a lagging power factor will be considered. The secondary current and the resulting ampereturns 22NI will change the flux, tending to demagize the core, reduce m? and with it 1E . Because the primary leakage impedance drop is so low, a small alteration to 1E will cause an appreciable increase of primary current from 0I to a new value of 1I equal to ? ? ? ?ijXREV ?? 111 / . The extra primary current and ampereturns nearly cancel the whole of the secondary ampereturns. This being so, the mutual flux suffers only a slight modification and requires practically the same ampereturns 10NI as on no load. The total primary ampereturns are increased by an amount 22NI necessary to neutralize the same amount of secondary ampereturns. In the vector equation, 102211 NININI ?? 。 alternatively, 221011 NININI ?? . At full load, the current 0I is only about 5% of the fullload current and so 1I is nearly equal to 122 /NNI . Because in mind that 2121 / NNEE ? , the input kVA which is approximately 11IE is also approximately equal to the output kVA, 22IE . The physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary, 111 ??????? mp is shown unchanged because the total back ., （ dtdNE /111 ?? ） is still equal and 2 opposite to 1V . However, there has been a redistribution of flux and the mutual ponent has fallen due to the increase of 1? with 1I . Although the change is small, the secondary demand could not be met without a mutual flux and . alteration to permit primary current to change. The flux s? linking the secondary winding has been further reduced by the establishment of secondary leakage flux due to 2I , and this opposes m? . Although m? and 2? are indicated separately, they bine to one resultant in the core which will be downwards at the instant shown. Thus the secondary terminal voltage is reduced to dtdNV S /22 ??? which can be considered in two ponents, . dtdNdtdNV m // 2222 ????? or vectorially 2222 IjXEV ?? . As for the primary, 2? is responsible for a substantially constant secondary leakage inductance 222222 / ??? NiN . It will be noticed that the primary leakage flux is responsible for part of the change in the secondary terminal voltage due to its effects on the mutual flux. The two leakage fluxes are closely related。 2? , for example, by its demagizing action on m? has caused the changes on the primary side which led to the establishment of primary leakage flux. If a low enough leading power factor is considered, the total secondary flux and the mutual flux are increased causing the secondary terminal voltage to rise with load. p? is unchanged in magnitude from the no load condition since, neglecting resistance, it still has to provide a total back . equal to 1V . It is virtually the same as 11? , though now produced by the bined effect of primary and secondary ampereturns. The mutual flux must still change with load to give a change of 1E and permit more primary current to flow. 1E has increased this time 3 but due to the vector bination with 1V there is still an increase of primary current. Two more points should be made about the figures. Firstly, a unity turns ratio has been assumed for convenience so that 39。21 EE ? . Secondly, the physical picture is drawn for a different instant of time from the vector diagrams which show 0??m , if the horizontal axis is taken as usual, to be the zero time 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 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 turn’s ratio different from unity although such an arrangement is sometimes employed for the purposes of electrically isolating one circuit from another operating at the same voltage. To explain the case where 21 NN ? the reaction of the secondary will be viewed from the primary winding. The reaction is experienced only in terms of the magizing force due to the secondary ampereturns. There is no way of detecting from the primary side whether 2I is large and 2N small or vice versa, it is the product of current and turns which causes the reaction. Consequently, a secondary 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 1N as the primary. 4 With 2N changes to 1N , since the are proportional to turns, 2212 )/(39。 ENNE ? which is the same as 1E . For current, since the reaction ampere turns must be unchanged 1222 39。39。39。 NINI ? must be equal to 22NI .. 2122 )/( INNI ? . For impedance, since any secondary voltage V bees VNN )/( 21 , and secondary current I bees INN )/( 12 , then any secondary impedance, including load impedance, must bee IVNNIV /)/(39。/39。 221? . Consequently, 22212 )/(39。 RNNR ? and 22212 )/(39。 XNNX ? . If the primary turns are taken as reference turns, the process is called referring to the primary side. There are a few checks which can be made to see if the procedure outlined is valid. For example, the copper