SYNCHRONOUS GENERATORS 



305 



ited by the generator impedance at triple frequency; while, if 

 the triple-frequency e.m.f.'s in the two generators were equal and 

 exactly in phase, there could be no neutral potential or current. 

 Owing, however, to the difference in the angular velocity of the 

 machines, different wave forms, different excitation, etc., this 

 condition never exists; and a triple-frequency current, therefore, 

 always flows between the neutrals, if interconnected. This cur- 



No, i 



No. 2 



-No. 3 



No. 4 



I Resistance 



FIG. 182. System of Grounding Generator Neutrals. 



FIG. 183. System of Grounding Generator Neutrals. (Only one neutral 

 can be grounded at one time.) 



rent may be of considerable magnitude with low reactance ma- 

 chines, and, if excessive, precautions must be taken for preventing 

 it. This can be done by grounding only one generator at a time, 

 if the generators are to be grounded, leaving the neutrals of the 

 other machines isolated. Arrangements must then also neces- 

 sarily be made so that any one of the generator neutrals can be 

 grounded, as shown in Figs. 182 and 183. 



