276 



PRINCIPLES OF ELECTRICAL DESIGN 



accounted for by the fact that the angular displacement between 



180 

 adjoining belts of conductors is only -= = 60 electrical degrees; 



o 



but the connections between the phase windings are so made as 

 to obtain the phase difference of 120 degrees between the re- 

 spective e.m.fs. Thus, the reversal of the vector (1) in the 



^Current= *mw. 



(1) 



(2) 



Current=I max i 



(> J 



max.OSZ), 



180 



J 



Direction of Travel of Poles 



Current= 





(2) (1) 



FIG. 107. Instantaneous values of armature m.m.f. in three-phase 



generator. 



diagram would cause it to lead vector (2) by 120 degrees instead 

 of lagging behind by 60 degrees; and the reversal of the vector (3) 

 would cause it to be 120 degrees behind (2) instead of 60 degrees 

 ahead. 



The lower diagram of Fig. 107 shows the armature m.m.f. 

 one-twelfth of a period later, i.e., when the poles have moved to 

 the left (or the conductors to the right) 30 electrical space de- 



