CHARACTERISTIC CURVES. 



399 



generator is more complicated than in the case of the series gen- 

 erator (with given current output), inasmuch as the increase of 

 speed of a shunt generator involves an increase of field excitation. 

 The curve showing the voltage of a shunt generator as a function 



Amperes 



Ohms 



Fig. 32. 



of speed, at zero current output, may be derived from the mag- 

 netization curve of the machine as follows : 



For the purpose of this derivation it is to be noted that if both 

 the speed n and the shunt field resistance R t are imagined to be 

 changed in the same ratio, the electromotive force E a (= E x at 

 zero load) will also be changed in that ratio, inasmuch as this 

 change of p a and R t will leave the shunt field current EJR t 

 unchanged, so that <J> will be unaltered, and E a (= QZ'ri) will 

 vary as n. 



Let the curve OA, Fig. 33, be the magnetization curve of the 

 shunt machine for a given speed n. Draw the straight line OB t 

 of which the abscissas represent values of the shunt field current 

 /, and the ordinates represent values of electromotive force, 

 RJ tJ required to produce the corresponding values of /. The 

 coordinates of the point of intersection of OA and OB will 

 then represent the values of E a (= E x at zero current output) and 

 f t , when the machine is driven at the given speed n. 



