210 



ALTERNATORS. 



The inductive resistance was then made less inductive, 

 and a similar series of readings taken. This was repeated 

 for five different values of the inductive resistance, giving 

 points corresponding to five different voltages on curves 

 corresponding to five different values of the power-factor. 

 These curves were plotted so as to show the variation of 

 exciting current with power-factor. The curves actually 

 obtained in this manner are shown on Fig. 96. From these 

 curves points on the magnetisation curve corresponding to 

 any required power-factor may be obtained, although actual 

 readings for the exact value of the power-factor were not 

 actually taken. 



100 



9O 



80 



7O 



1.O 1-2 1-4 1-6 1-8 2-O 2-2 24 2'6 

 Amperes, excitation. 

 FIG. 97. MAGNETISATION CUBVES AT FULL LOAD AND VARIOUS POWER FACTORS. 



Thus the actual excitation required to give respectively 

 98, 90, 80, 70, and 60 volts at the three values of the power- 

 factor were read off from the curves in Fig. 96 and the 

 complete curves obtained from these points, as shown in 

 Fig. 97.* 



With a leading, instead of a lagging, current in the circuit 

 supplied, it would be found that the terminal voltage of the 

 alternator is increased instead of decreased. Curves illus- 

 trating this are given later in connection with the deter- 

 mination of the characteristic curve of an alternator. 

 , w , 



* The speed was not exactly the same during the readings taken for 

 Figs. 94 and 97, which accounts for the slight difference between the load 

 curve in Fig. 94 and the curve marked cos <f> = 1 in Fig. 97. 



