' 3 -Bl PREDETERMINATION. 75 



7. It is thus seen that there are two somewhat different 

 effects produced by the armature current: the first (a) is a 

 magnetomotive force, which reduces the field flux and so reduces 

 the generated voltage; the second (b) is an electromotive force, 

 which is subtracted from the generated electromotive force (in 

 the proper phase) so as to give a lower terminal voltage. 



These two effects operate simultaneously to lower the terminal 

 voltage, the relative amounts of the two varying according to 

 details of design, saturations, air-gap, shape of slots, etc. To 

 take full and accurate account of the two effects treating one 

 as a magnetomotive force and the other as an electromotive 

 force is difficult* and will not be undertaken here. 



8. We may, however, instead of treating the two effects 

 separately, treat them combined, following either one of two 

 methods : 



(a) The magnetomotive force or ampere-turn method, which 

 assumes that all the effect is magnetomotive force ; or, 



(b) The electromotive force or reactance method, which as- 

 sumes that all the effect is electromotive force. 



If the saturation curve were a straight line, the two methods 

 would be identical ;f for, magnetomotive force would produce a 

 proportional electromotive force. With the saturation curve, 

 however, not a straight line, a given increase or decrease in mag- 

 netomotive force will cause a less than proportional change in 

 electromotive force. 



Hence, if we consider that all the effect of armature flux is 

 a magnetomotive force, we will have a less drop in terminal 

 voltage than if we consider that all the effect is an electromotive 

 force. The magnetomotive force method is, accordingly, opti- 

 mistic (Behrend) and gives the generator a better regulation 



* See Appendix II. 



f This would be true if the details of the two methods were in all 

 respects the same. Differences in the details of the two methods, as 

 usually applied, cause differences in the results, even though the saturation 

 curve is straight. 



