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1、The transformer on load and Introduction to DC MachinesIt has been shown that a primary input voltage V can be transformed to any desired open-circuit secondary 1voltage E by a suitable choice of turns ratio. E is available for circulating a load current impedance. For the 22moment,a lagging power f
2、actor will be considered. The secondary current and the resulting ampere-turns I N will 22change the flux, tending to demagnetize the core, reduce and with it E . Because the primary leakage impedance w1drop is so low, a small alteration to E will cause an appreciable increase of primary current fro
3、m I to a new value 10of I equal to （V + E ）/（R +jX ）. The extra primary current and ampere-turns nearly cancel the whole of 11111the secondary ampere-turns. This being so, the mutual flux suffers only a slight modification and requires practically the same net ampere-turns I N as on no load. The tot
4、al primary ampere-turns are increased by an amount I N 0122necessary to neutralize the same amount of secondary ampere-turns. In the vector equation, I N + I N =I N ; 112201alternatively, I N = I N - I N .At full load , the current I is only about 5% of the full-load current and so I is 11012201equa
5、l to I N / N . Because in mind that E =E N / N ,the input KVA which is approximately E I is also 221121211approximately equal to the output KVA , E N .22The physical current has increased, and with in the primary leakage flux to which it is proportional. The total flux linking the primary , ,is show
6、n unchanged because the total back e.m.f(E - N d/dt)is still 111wp111equal and opposite to V ,however, there has been a redistribution of flux and the mutual component has fallen due to 1the increase of with I ,Although the change is small, the secondary demand could not be met without a mutual 11fl
7、ux and e.m.f alteration to permit primary current to change, The net flux linking the secondary winding has been sfurther reduced by the establishment of secondary leakage flux due to I ,and this opposes ,Although and 2ww2are indicated separately, they combine to one resultant in the core which will
8、 be downwards at the instant shown. Thus the secondary terminal voltage is reduced to V = -N/ dt which can be considered in two components, i.e 2sd2V or vectorially V.As for the primary, is responsible for a dtdNdtdNw/22222222IjXE 2substantially constant secondary leakage inductance N.It will be not
9、iced that the primary leakage 22 2222/ANi 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;, for example, by its demagnetizing action on has caused the changes 2won the primary side which le
10、d 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 . is unchanged in magnitude from the no load condition psince, neglecting resistance
11、, it still has to provide a total back e.m.f.equal to V . It is virtually the same as , 111though now produced by the combined effect of primary and secondary ampere-turns. The mutual flux must still change with load to give a change of E and permit more primary current to flow. E has increased this
12、 time but due 11to the vector combination with V there is still an increase of primary current.1Two more points should be made about the figures. Firstly，a unity turns ratio has been assumed for convenience so that E =E . Secondly, the physical picture is drawn for a different instant of time from t
13、he vector 1 2diagrams which show =0, if the horizontal axis is taken as usual, to be the zero time reference. There are instants min cycle when primary leakage flux is zero, and when primary and secondary leakage fluxes are in the same sense. The equivalent circuit already derived for the transforme
14、r 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 ele
15、ctrically isolating one circuit from another operating at the same voltage, To explain the case where N the reaction of the secondary will be viewed from the primary winding. The reaction is 21Nexperienced only in terms of the magnetizing force due to the secondary ampere-turns. There is no way of d
16、etecting from the primary side whether I is large and N small or vice versa, it is the product of current and turns which 22causes 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 N a
