FLUX DISTRIBUTION 



135 



usually known. The average density over the pole pitch, T, 

 as explained in Art. 41, is 



$ 



E g (average) = ^ 



where l a is the gross length of armature. 1 By drawing the dotted 

 rectangle of height B g ( average) as shown in Fig. 51, its area can 

 be compared with that of curve A by measuring with a planim- 

 eter. If these areas are not equal, the ampere-turns per field 

 pole, as represented by the curve of Fig. 50, must be altered 

 and a new flux curve plotted, of which the area must indicate the 

 required flux. 



FIG. 51. Open-circuit flux distribution. 



43. Effect of Armature Current in Modifying Flux Distribution. 



The distribution of m.m.f . over armature surface when the 

 field poles are acting alone has already been calculated and 

 plotted in Fig. 50, and the effect of the armature current in 

 modifying this distribution may be ascertained by noting that 

 the armature ampere-turns between any two points such as d 

 and d f on the armature circumference (see Fig. 52) are q(b a), 

 where q stands for the specific loading or the ampere-conductors 

 per unit length of armature circumference. 

 Let p = number of poles, 



Z = total number of conductors on armature surface, 



I c = current in each conductor, 



T = pole pit 



then q = - 

 pr 



1 The gross length l a of the armature core usually exceeds the axial length 

 of the pole shoe by an amount equal to twice the air gap, 5. Thus, by as- 

 suming the flux distribution as given by curve A of Fig. 51 to extend to the 

 extreme ends of the armature, a practical allowance is made for the fringe 

 of flux which enters the corners and ends of the armature core. 



