DIRECT-CURRENT MACHINERY 



189 



the generator is short circuited, the flux is reduced to such a value 

 that the generated e.m.f. is only large enough to supply the re- 

 sistance drop in the armature. The armature current is then os 

 and the terminal e.m.f. is zero. 



The maximum current om is many times full-load current and 

 can only be reached with small machines of bad regulation. 



115. Field Characteristic. The field characteristic of a gen- 

 erator is the curve showing the variation of field current with load 

 current to maintain a constant terminal voltage at constant speed. 



In a self -excited generator, Fig. 152, the terminal voltage can 

 be maintained constant as the load current increases if the field 



^T Field Rheostat 



FIG. 152. Shunt generator. 



current is increased to such a value that the increase in field 

 m.m.f. will not only overcome the effect of armature m.m.f. but 

 will produce an increase in flux to provide the extra e.m.f. to sup- 

 ply the armature resistance drop. 



In Fig. 153 ad is the field characteristic of the generator, Fig. 

 152, oa is the field current required to produce the rated terminal 

 voltage E at no load. When a load current og is supplied by the 

 generator a larger field current oc is required. The increase in field 

 current i = ac exerts a m.m.f. which overcomes the effect of the 

 armature m.m.f. and produces an increase in the flux to provide 

 for the armature resistance drop IT. The generated voltage is 



E + Ir. 



(212) 



Fig. 154 shows the saturation curve of the generator, od is the 

 e.m.f. generated at no load by the field current // = oa and it is 



