3-B] PREDETERMINATION. 85 



From Figs. 4 and 5 it is seen that the RI drop becomes ineffec- 

 tive, being at right angles to ET, and can be neglected. Hence, 

 practically, 



ET = EQ XI, for lagging current ; 



ET = EQ + XI, for leading current. 



For this case, the various voltages are combined algebraically. 

 Practically, XI = ZI = E Z , and these expressions become 



E T = Eo E z . 



This expression, approximate for 6 = 90, would be exact for 

 a value of a little less than 90 ; so that, in Fig. 4, OB A forms 

 a straight line and tan 6 = XI-t-RI. 



22. Given the Terminal Voltage at One Poiver Factor, to 

 Determine it at Any Other Power Factor. Given ET at any 

 power factor, Eo is found by method (a) of the preceding para- 

 graphs. With EQ thus known, the value of ET is readily found 

 for any desired power factor by method (b). 



In conducting tests, it is often difficult or impossible to deter- 

 mine ET: at unity or high power factors, on account of the power 

 required. The value of ET can, however, be found by test at a 

 low power factor (52) and then determined by calculation for 

 any desired high power factor. Usually EO is found by test and 

 resistance drop is known; the reactance drop is not known. In 

 this case the procedure is as follows : 



In Fig. 4, lay off resistance drop BC\ at right angles draw the 

 indefinite line CA, the value of reactance drop being unknown. 

 At an angle 6 with *BC, lay off BO equal to the value of ET found 

 by test at power factor cos 6. Draw OA = E , as found by test, 

 cutting CA at A. The point A being located and EQ known, 

 values of ET at any power factor are determined by method (b) 

 above. 



In this manner, if the regulation is known for one power fac- 

 tor, it can be calculated for any power factor. At constant 

 terminal voltage, the locus of the point O will be the arc of a 



