186 



ELECTRICAL ENGINEERING 



Thus armature reaction is due to the m.m.f. of the armature 

 currents and causes a decrease of the flux crossing the air gap and 

 therefore a decrease in the generated e.m.f. The armature m.m.f. 

 and the drop in e.m.f. caused by it increase as the load current 

 increases. 



14 Load 



i/ 2 Load 34 Load Full Load 1*4 Load 



Load Current 



Source of. 

 Power < 



Load 



FIG. 149. Voltage characteristics of a separately excited generator. 



(b) It is necessary to distinguish between the e.m.f. gen- 

 erated in the armature and the terminal e.m.f. E. At no load 

 they are the same but when current flows in the armature part of 

 the generated e.m.f. is consumed in driving the armature current 

 7 through the resistance of the armature winding and brushes r. 

 This resistance drop is Ir and increases directly with the current. 



The terminal e.m.f. is E = & - Ir. . . (211) 



