AIR-GAP FLUX DISTRIBUTION 



277 



grees. The current in phases (1) and (2) now has the instan- 

 taneous value i = I max cos 30 = 0.866/7*; while the current 

 in (3) is zero. If several curves of this kind are drawn, it will 

 be found that the instantaneous values of m.m.f. at any point on 

 the armature periphery (considered relatively to the poles) differs 

 very little from the average value; in other words, the pulsations 

 of flux due to cyclic changes in the m.m.f. will, in a three-phase 

 machine, be negligibly small. For this reason, and also in order 

 to shorten and simplify the work, the armature m.m.f. of a 

 polyphase generator may conveniently be studied by assuming a 

 large number of conductors, and a number of phases equal to 



I 



v 



Lab of Current Behind Open Circuit E.M.P. 

 Corresponding to Brush Shift in D. 0. Machines 



FIG. 108. Method of obtaining armature m.m.f. curve from curve of 

 current distribution. 



the number of conductors in the space of one pole pitch. Thus, 

 the ordinates of curve I of Fig. 108 (assumed to be a sine curve) 

 give the value of the current in the various conductors distributed 

 over the armature surface. It is understood that the current 

 in each individual conductor varies according to the simple 

 harmonic law; but it is constant in value for any given point on 

 the armature surface considered relatively to the poles. The 

 direction of the current in the conductors between the points 

 A and B may be considered as being downward, while the direc- 

 tion of the current in the adjoining sections of width T would be 

 upward. The maximum value of the armature m.m.f., there- 

 fore, occurs at the points A and B where the current has zero 

 value, while the zero value of m.m.f. must occur at the points 

 and 0' where the current is a maximum. 



The value of the armature ampere turns per pole at any inter- 

 mediate point C, is equal to the ampere-conductors in the space 



