So SYNCHRONOUS ALTERNATORS. [Exp. 



taking, from Fig. I, corresponding values for EQ and /s. 

 Eventually there is a tendency for the curve to bend, although 

 in this instance there is none within the range for which Fig. 2 

 is drawn. The ratio of any ordinate to the corresponding 

 abscissa gives the value of the synchronous impedance; thus, in 

 Fig. 2, the impedance drop is 234 volts for a full-load current of 

 43.4 amperes, and the impedance is, therefore, 234-^-43.4 = 5.4 

 ohms. The normal full-load voltage of this machine is 575 ; the 

 impedance drop is, accordingly, 40.7 per cent. This is called* 

 the impedance ratio. An open-circuit voltage of 627 is seen to 

 give a short-circuit current of 116 amperes, as already seen in 

 Fig. i. 



14. Resistance drop is plotted as a straight line, Fig. 2. The 

 resistance, found by the fall-of-potential method, is 0.17 ohms; 

 the resistance drop, for 43.4 amperes, is 0.17 X 434 = 7-4 volts. 

 15. The reactance drop is x=Vz 2 R Z ; or, for 43.4 

 amperes, reactance drop = V 2 34 2 74 2 = 2 33-9 volts. Usu- 

 ally, as in this case, resistance is small so that there is little differ- 

 ence between the values of synchronous impedance and synchro- 

 nous reactance. It is common, therefore, not to calculate the 

 value of reactance drop, but to use the value of impedance drop 

 in its place. 



Synchronous reactance is proportional to speed; hence, syn- 

 chronous impedance is practically proportional to speed. 



Synchronous impedance and synchronous reactance are ficti- 

 tious quantities, comprising not only the real impedance and re- 

 actance of the armature, but also including the effect of arma- 

 ture reactions. 



It is instructive to compare the curves of Fig. 2 with similar 

 curves for a transformer ; see Fig. 7, Exp. 5-B. 



1 6. Electromotive Force Method. Aside from its usefulness 

 in predetermining the performance of alternators, this method 

 serves as an excellent illustration of the use of vector diagrams 



* Standardization Rule, 208. 



