202 ELEMENTS OF ELECTRICAL ENGINEERING. 



98. The actual transformer and the ideal transformer. The 



action of a transformer would be very simple (a) if the coils of 

 wire had zero resistance, (&) if there were no magnetic leakage, 

 that is, if all of the magnetic flux which links with one coil also 

 linked with the other coil, and (c) if the magnetic reluctance of 

 the iron core were zero. A transformer satisfying these condi- 

 tions would be called an ideal transformer. In an actual trans- 

 former, however, the coils always have more or less resistance ; 

 some of the magnetic flux, the so-called leakage flux, links with 

 one coil and does not link with the other ; and a certain amount 

 of magnetomotive force (ampere-turns) is necessary to force the 

 magnetic flux through the transformer core. The effects of these 

 things on the action of the transformer is quite complicated, 

 but in many respects the behavior of the actual transformer ap- 

 proximates closely to the behavior of the ideal transformer. All 

 operating engineers in dealing with transformers, make use of the 

 very simple theory of the ideal transformer, whereas designing 

 engineers find it necessary to take account of the effects of coil 

 resistance, magnetic leakage, and core reluctance. 



Transformer action. In the following discussion N f repre- 

 sents the number of turns of wire in the primary coil and N" 

 represents the number of turns of wire in the secondary coil, the 

 coils are assumed to have negligible resistance, and all of the 

 flux which passes through one coil is assumed to pass through 

 the other coil also. The transformer core, however, is not 

 assumed to have zero reluctance. 



Ratio of primary current to secondary current. Aside from 

 resistance, the only thing which opposes the flow of current 

 through the primary coil is the reacting electromotive force in- 

 duced in the primary coil by the reversals of magnetization of the 

 core. The greater the range of this magnetization the greater 

 the value of the reacting electromotive force. The combined mag- 

 netizing action of the primary and secondary coils is always such 

 as to magnetize the core to that degree which will make the react- 

 ing electromotive force in the primary coil equal to the electromotive 



