3-B] 



PREDETERMINATION. 



regulation is 9 per cent.; at power factor 0.5 (lagging current), 

 it is 37 per cent. ; at power factor o.o, it is 40 per cent. At high 

 power factors, it is seen that a small change in power factor 

 causes a marked change in regulation ; while at lower power fac- 

 tors the regulation is nearly constant. The reason for this will 

 appear from a consideration of the construction in Figs. 4 and 5. 

 This fact is made use of in 52. 



kO. 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 



POWER. FACTOR 



FIG. 6. Curves showing no-load voltage corresponding to a constant full-load 

 voltage (575) for full-load current (43.4 amperes) at different power factors, 



26. Predetermination of External Characteristics. For a 

 definite open-circuit voltage EQ and various power factors, com- 

 pute (by method (b) of 17-20) the terminal voltage ET, for 

 different load currents. Armature XI drop and RI drop are to 

 be taken as proportional to current ; i. e., X and R are taken as 

 constant.* Data are thus obtained for plotting the complete ex- 

 ternal characteristic, from open circuit to short circuit, for differ- 

 ent power factors. 



* (26a). In 24, 26 and 32, the same constant values of X and Z are 

 to be used. In 26 it is proper that X and Z be considered constant for 

 the reason that field excitation is constant. In 24 the armature current 

 is constant, but not the field, and strictly speaking X and Z might not 

 remain constant, although for simplicity and for ease in comparison they 

 are so taken. In the case of 32, X and Z should only be taken as con- 

 stant for a certain range, and for very high saturations should be taken as 

 variable as in 33. 



