250 



ALTERNATORS. 



In consequence of these influences the lines shown in 

 Fig. 119 diverge slightly as they rise, showing that the voltage 

 of the alternator when loaded does not increase with speed 

 at quite the same rate as when unloaded. The lower curve, 

 like the upper one, closely approximates to a straight line. 

 This may be explained by the fact that the chief cause which 

 produces the difference between the no-load voltage and the 

 voltage under load is due to the voltage spent in overcoming 



120 



100 



80 



I 



60 



20 









2OO 4OO 6OO 8OO 1.OOO 



Revolutions per Minute. 



1,2OO 1.4OO 



FIG. 119. KELATION BETWEEN SPEED AND VOLTAGE OF AN ALTERNATOR 

 Upper curve no load. 

 Lower curve load = 20 amperes. 

 Excitation T95 amperes. 



armature impedance. The armature current remains the 

 same, but the impedance ( ^/R 2 + (2 * n L) 2 ) varies with the 

 periodicity of the current. The value of R, the armature 

 resistance, is small compared with the reactance, 2 * n L, 

 and hence the impedance varies almost in direct proportion 

 with the value of the periodicity n, and thus the loss of 

 voltage due to this cause increases also in direct proportion 



