SYNCHRONOUS GENERATORS 



289 



excitation, the reluctance of the air gap is very high and that of 

 the iron very low, and, therefore, the former may be considered 

 as constituting the entire reluctance of the magnetic circuit. 

 Since the reluctance of air is constant regardless of the flux density, 

 at small excitations the flux will be proportional to the mag- 

 neto-motive force, and, therefore, the open circuit voltage is pro- 

 portional to the field current, hence the curve is straight. As 

 the field becomes stronger, however, the proportion of the air-gap 

 reluctance to the entire reluctance decreases because the per- 

 meability of iron decreases with increased flux density, and, 

 therefore, the e.m.f . increases less rapidly with increased excitation. 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 

 Ampere Turns per Inch 



FIG. 160. Saturation Curves, 



It was pointed out in the previous section that when a current 

 is flowing in the armature circuit, i.e., under load, the field ampere- 

 turns required to maintain normal terminal voltage, exceed the 

 no-load ampere-turns required for normal voltage, due to the 

 resistance and the synchronous reactance of the armature circuit. 

 A number of more or less accurate methods have been proposed 

 for calculation of the above components, and thus determining the 

 required field excitation at full load. Knowing the resistance 

 and the leakage reactance or self-induction of the armature, the 

 voltage drop caused by these is added vectorically to the terminal 

 voltage, this giving the voltage which must be induced (see Figs. 



