132 



ELECTRICAL MACHINERY 



load. When the load current is OA we would expect the 

 generated voltage to be equal to AD. But the demagnet- 

 izing effect of the armature neutralizes part of the field 

 m.m.f. The strength of this armature m.m.f. is taken 

 as equal to A B and hence the effective current in the series 

 field is really OB and not OA. The current OB gives a 

 voltage on the magnetization curve of BF, and AC is 

 equal to BF. So C is one point on the curve of generated 



C G^Armature IB drop 



C D=Armaturereactit>n"drop f >' 



+ effect of decreased 

 field current 



Field Current 

 Load Current 



FIG. 84. External Characteristic of a Shunt Generator. 



e.m.f. and other points can be found in similar manner. 

 The armature demagnetizing effect (AB) is of course pro- 

 portional to the load. 



Shunt Generator. The shunt wound generator has an 

 external characteristic that falls off more rapidly than 

 does that of the separately excited machine because in the 

 shunt generator there are three effects tending to make the 

 terminal voltage fall as the load is increased. We have 



