3-B] PREDETERMINATION. 9 



characteristic for zero power factor is a little higher than the 

 straight line for the limiting case ; the difference, however, is in- 

 appreciable. 



When the scale used is such that the ordinate on open circuit 

 is equal to the abscissa on short circuit, the characteristics are 

 ellipses with a 45 line as axis (Steinmetz, Alternating Current 

 Phenomena, 3d ed., p. 304). 



In any alternator, armature resistance is small and armature 

 reactance relatively large, so that the armature impedance is 

 practically all reactance; this gives curves as in Fig. 7. If the 

 conditions were reversed, resistance being large and reactance 

 negligible, the curves for cos 6 = I and cos 6 = o would have to 

 be interchanged. Unity power factor would give the poorest 

 regulation and the straight line characteristic now obtained for 

 zero power factor ; for, with reactance zero, Ei = Eo RI, in 

 place of ET = EQ XI, as in 21. 



29. Predetermination of Full-load Saturation Curve from 

 No-load Saturation Curve. By method (b) of 1720, com- 

 pute the terminal voltage ET corresponding to the different open- 

 circuit voltages of the no-load saturation curve; this is to be 

 done* for full-load current at unity power factor and at zero 

 power factor, current lagging. In this manner, full-load satura- 

 tion curves are plotted for unity power factor (Fig. 2, Exp. 3~A) 

 and for zero power factor (Fig. i of this experiment). 



30. The interpretation of the full-load saturation curve for 

 unity power factor is given in 10, Exp. 3~A. The curve for 

 zero power factor is capable of similar interpretation. It is seen 

 that, for the same terminal voltage, the excitation must be much 

 greater at zero than at unity power factor; or, for the same 

 excitation, the terminal voltage is much lower. 



31. In determining the full-load saturation curves for any 

 power factor, X and Z can be taken as they are (somewhat vari- 



*It is unnecessary to construct intermediate curves for part load and 

 for other power factors, unless a special study is to be made. 



