302 PRINCIPLES OF ELECTRICAL DESIGN 



that the probable short-circuit current may be estimated, not 

 only after the armature ampere-turns have had time to react 

 upon the exciting field, but also at the instant when the impedance 

 of the armature windings alone limits the current. 



The usual methods of predetermining the regulation of alter- 

 nators involve almost invariably the use of vectors or vector 

 algebra. This is convenient, and to some extent helpful, be- 

 cause the problem is thus presented in its simplest aspect: the 

 very fact that vectors are used assumes the sinusoidal variation 

 of the alternating quantities, or the substitution of so-called 



equivalent sine-wave forms for 

 the actual wave shapes, thus 

 eliminating the less easily cal- 

 culated effects caused by cross- 

 magnetization and the conse- 

 quent distortion of the wave 

 shapes. On the other hand, the 

 omission of these factors, espe- 

 cially when departures are made 

 from standard types, may lead 

 to incorrect conclusions, and in 

 any case the plotting of the ac- 

 tual flux and e.m.f. curves is 

 of great value to the designer. 



Ampere-turns on f.dd """ It is therefore proposed to con- 



FIG. 124. Inherent regulation ob- sider in the first place what is 

 tained from open-circuit and load xi^ v . jv. pfln ^P Hrmp with 

 saturation curves. 



the aid of vectors on the usual 



assumption of sine-wave form, and afterward show how a greater 

 degree of accuracy Can be attained by using curves representing 

 the actual flux distribution in the air gap, corresponding to the 

 required load conditions. 



The curves in Fig. 124 may be considered as having been 

 plotted from actual test data. The upper curve is the open- 

 circuit saturation characteristic, giving the relation between the 

 number of ampere-turns of field excitation per pole and the pres- 

 sure at the terminals, which in this case is the same as the electro- 

 motive force actually developed in the armature windings. The 

 lower curve is the load characteristic corresponding to a given 

 armature current and a given external power factor. The in- 

 herent regulation when the field excitation is of the constant 



