188 



ELECTRICAL ENGINEERING 



If od represents full-load current, gd is the terminal voltage at 

 full load and bg is the rise in voltage that would occur if the load 



were removed; therefore the regulation is -~ 100 per cent. 



So far only the separately excited generator has been considered. 



In the shunt generator, armature reaction and armature resist- 

 ance cause a decrease in terminal e.m.f. under load, but a third 

 condition must also be taken into account. The field circuit is 

 connected across the armature terminals and the current in it is 

 proportional to the terminal e.m.f. Thus, when the terminal 

 e.m.f. decreases due to armature reaction and armature resistance, 

 the field current also decreases and causes a further decrease in 

 the flux and therefore the terminal e.m.f. of a shunt-excited gen- 

 erator is less than it would be if the machine were separately ex- 

 cited. (Fig. 150.) 



The load current can be increased by decreasing the resistance 

 in the load circuit up to the point M in Fig. 151 which is the com- 



3 x Full Load 



FIG. 151. Voltage characteristic of a shunt generator. 



S Full Load 2 x Full Load 



Amperes Load I 



plete voltage characteristic of a shunt generator, om is the max- 

 imum current output. If the load resistance is decreased further, 

 the armature current increases for an instant and then decreases 

 as its m.m.f. wipes out part of the flux and causes the generated 

 e.m.f. to decrease. Finally when the load resistance is zero and 



