272 



METHODS OF CALCULATION 



KNL,V 2 



O' 



Ampere-turns 



FIG. 20. 



B a perpendicular BC is let fall upon the straight line OAi, and the 

 condition is such as if the alternator were delivering power to an 

 inductive system according to the total characteristic of excitation 

 (Fig. 20). The current 7 is formed of two components: one an 

 active component Oa, equal to the projection of 7 upon OA, and 



which gives the transverse reac- 

 tion BC equal to coLI w ; the 

 other component is a reactive 

 or reactive component ab, which 

 gives the fall of potential to be 

 calculated on the characteristic. 

 Let us lay off on this charac- 

 teristic (Fig. 20) an ordinate 

 equal to OC, and upon the 

 latter a segment CD equal to 

 the e.m.f. lost by the stray 



field. From the point D lay off horizontally the counter ampere- 

 turns of the armature, and thus will be obtained on the abscissa 

 O'P the total ampere-turns necessary for the excitation. The 

 ordinate PP' corresponding thereto will 

 E on open circuit necessary for alternator 

 in general, different from the length of OA\ 

 in Fig. 19. 



In the same way the e.m.f. is determined which is necessary 

 for the alternator OA^ observing that for the latter the sign of the 

 reactive armature-reaction is changed, as well as the sign of E=A 2 O, 

 and that, consequently, the armature reaction remains demagnetizing, 

 so that the geometrical construction is identical. It is, therefore, 

 easy to recognize in advance the equal excitations to be given to the 

 two alternators, in order to satisfy the desired conditions. It should 

 also be determined at the time of the test, by means of the wattmeter, 

 that there is no sensible difference of phase between the current and 

 the e.m.f. Inversely, if this condition were directly realized by 

 adjusting the excitations, it would be possible to deduce from an 

 examination of the diagram the total armature-reactions represented 

 by the abscissa OP, indicating the total fall of excitation between 

 the open circuit e.m.f. and the e.m.f. under load. In the latter 

 case, the fall due to the stray field is not separated from that due to 

 armature reaction. 



The same diagram gives immediately the value of the transverse 



represent the e.m.f. 

 AI and which will be, 

 which was represented 



