128 



PRINCIPLES OF ELECTRICAL DESIGN 



ing poles is negligible. The m.m.f. between pole shoe and arma- 

 ture core being the same at all points on the armature surface, 

 it is evident that this curve of flux distribution will correctly 

 represent the variations of air-gap permeance per unit area of 

 the armature surface. Thus, the permeance per square centi- 

 meter at the point 7 cm. from center of pole is the permeance 

 of the tube GHEF in Fig. 42 divided by the area of the surface 

 EF. The permeance of the tube GHEF is exactly the same 

 as that of the tube CDAB, i.e., 0.25 per centimeter of depth 

 measured axially. The area of the surface EF is 0.5, and the 

 permeance per square centimeter at the point considered is, 

 therefore, 



0.25 



p i = oTo = - 5 - 



40. Open-circuit Flux Distribution and M.m.f. Curves. The 

 dotted curve marked "flux" in Fig. 44 has been plotted from 



M.M.F. 



M 

 Permeance 



1 2 3 4 5 6 7 8 9 10 11 12 13 



Surface of Armature Core 

 FIG. 44. Curves of permeance, flux, and m.m.f. 



Fig. 43, and shows the effect of the neighboring pole in reducing 

 the air gap flux and causing it to pass through zero value at a 

 point exactly halfway between the poles. A comparison of the 

 full line and dotted flux curves shows that, even with the greatly 

 increased air gap, the influence of the neighboring poles is not 

 appreciable except in the uncovered spaces between the pole 

 tips. The vertical dotted line in Fig. 44 shows the limit of the 

 pole arc. 



In order to find a scale for the flux curve it is necessary to 

 know either the total flux entering the armature in the space 



