9 SYNCHRONOUS ALTERNATORS. [Exp. 



able, 33) or they can be assumed constant,* 32. The com- 

 putations can be readily made by either method ; it is only above 

 saturation that the results differ. This will be discussed in 

 greater detail in the case of zero power factor. 



32. For zero power factor, the terminal voltage (21) is 

 ET = EQ Ez\ that is, the impedance drop, Ez is subtracted 

 arithmetically from E. 



In Fig. i, if impedance drop Ez is taken as constant, we obtain 

 Curve (4) differing from Curve (i) by a constant distance (Ez) 

 vertically.f This is satisfactory below saturation, but above 

 saturation is too pessimistic. 



33. If we wish to extend the curve above saturation, it is 

 better to take a variable value, Z = EQ -~ /s, computed from 

 Curves (i) and (2), Fig. i, for each value of EQ, that is, for 

 each excitation. This gives a decreasing value for Z and results 

 in Curve (5) instead of (4). Instead of subtracting from Curve 

 (i) a constant Ez, we now subtract 



Ez = ZI = ~E . 

 Is 



Here 7 is full-load current (43.4 amp.) ; E is taken from 

 Curve ( i ) and /s is the corresponding short-circuit current from 

 Curve (2). The formula can be interpreted thus: if a current 

 7s uses up in the armature a voltage EQ, a current 7 will use up 

 a proportional voltage, Ez= (7-=-7s)o- 



*See 26a. 



fBy the magnetomotive force method (Appendix I.), Curve (6) differs 

 from Curve (i) by a constant distance (Mz) horizontally; at high satu- 

 rations this is too optimistic. 



