COMMUTATION 



173 



assuming the total slot flux to pass outward from the armature 

 core through the teeth, is, therefore ,by formula (77), 



whence, 



3s 



1.67T X 1.5 X 4 X 65 X 11 X 2.54 



3 X 0.39 

 = 47,000 maxwells, 



this being the equivalent slot flux taken over the full length of 

 the armature. 



Turning now to the flux leaving the armature in that part of 

 the commutating belt which is not covered by the interpole, let 

 the curve Fig. 66 represent the full-load flux distribution over 



FIG. 66. Flux curve showing density on geometric neutral line, 

 lated without interpoles.) 



(Calcu- 



armature surface, calculated on the assumption that there are 

 no interpoles. This is the flux curve C, derived in the manner 

 outlined in Art. 43, Chap. VII, but with the brushes left in the 

 no-load position. There will be no directly demagnetizing effect 

 due to brush lead since the brushes will remain on the geometric 

 neutral line. The flux density at a point midway between the 

 two poles has the value B, which for the purpose of this example 

 may be assumed to be 1,400 gausses. 



If lp = axial length of interpole (not yet determined), the total 

 flux which must pass from interpole into armature teeth is 



BW a (l a - lp) 



