88 



SYNCHRONOUS ALTERNATORS. 



[Exp. 



27. Fig. 4, Exp. 3-A, shows the characteristic for unity 

 power factor. Power is zero on open circuit and on short cir- 

 cuit. Maximum power is, in this case, obtained at about twice 

 full-load current ; at short circuit, the current is about 2\ times 

 full-load current. A small short-circuit current* is an element 



10 20 30 40 50 60 70 80 90 100 110 120 130 



AMPERES ARMATURE 

 FIG. 7. External characteristics at different power factors. 



of safety, obtained, however, by large impedance drop and poor 

 regulation. Compare 243, Exp. 5-C. 



28. External characteristics for different power factors, with 

 current lagging and leading, should be plotted as in Fig. 7. The 

 lowest possible characteristic is a straight line ; it is obtained for 

 a power factor (cos 0) of such a value that tan 6= (armature 

 reactance-drop) -+- (armature resistance-drop). See 21. The 



*(27a). This is the working part of the characteristic for constant 

 current operation, see 8, Exp. 3-A. The armature should have a high 

 reactance for constant current and low reactance for constant potential. 



