310 



PRINCIPLES OF ELECTRICAL DESIGN 



Let the external power factor be cos 0, and OR' (Fig. 129) the 

 constant field excitation which would, on open circuit, develop 

 the pressure E represented by R'S. The full-load-current zero- 

 power-Tactor saturation curve O'R has been drawn as previously 

 described. If then it is possible to determine the point Q on 

 the full-load saturation curve for power factor cos 6, the required 



OS 

 percentage regulation may be expressed as lOO^-pp- 



In Fig. 130 draw the right-angled triangle E t PE such that 

 PE t represents the armature resistance drop per phase with full- 

 load current, and EoP the corresponding synchronous reactance 

 drop, as given by SR in Fig. 129. From E t draw the line E t m 



O 0' E' 



FIG. 129. Method of constructing saturation curve for any load and 



power factor. 



of indefinite length and so that mEtP is the required power-factor 

 angle 0. From E as center describe the arc of a circle of radius 

 R'S (Fig. 129) equal to the open-circuit voltage, and cutting 

 mE t produced at 0. Then OE t will be the required terminal vol- 

 tage, which may be plotted as R'Q in Fig. 129. This construc- 

 tion provides for the proper angle 6 between terminal voltage 

 and current; and in regard to the relation between the terminal 

 voltage E t and the open-circuit voltage E when load is thrown 

 off, it will be seen that the total synchronous reactive drop has 



