444 



ELECTRICAL MACHINERY 



field circuit enough current to excite the field sufficiently 

 to produce in the armature the voltage considered. Hence, 

 with this value for the field circuit resistance, the generator 

 could not build up. 



But suppose that the field rheostat is all cut out so that 

 resistance of the field circuit becomes equal to tan fo. 

 Now when the generator has a field current equal to OD 

 it generates a voltage DE. But to force the current OD 

 through the field circuit requires only the voltage DB. 



W D H I 



Field current In amperes 



FIG. 286. Diagram to Show Why a Generator Refuses to Build up 

 if There is too Much Resistance in its Shunt Field Circuit. 



Hence the voltage DE forces through the field circuit a 

 current OH, which in turn makes the armature generate 

 the voltage HG, which again increases the field current. 

 This process continues until the point K is reached where 

 the generated voltage is just sufficient to force through 

 the field circuit the current at 01. 



Therefore, in starting a shunt generator, it is necessary 

 to properly reduce the resistance in the field rheostat. 



18. A dirty commutator produces the same effects as 

 the field rheostat resistance. The high contact-resistance 



