126 



PRINCIPLES OF ELECTRICAL DESIGN 



The flux density at all points on the armature periphery is 

 easily calculated when the flux lines have been drawn. Thus, 

 since each tube of induction encloses the same number of mag- 

 netic lines, exactly the same amount of flux will enter the arma- 

 ture in the space EF (Fig. 39) as in the space AB. If Bab is 

 the flux density in the tube CDAB at the center of the pole face, 

 the average density over the space EF will be 



B e f = Bab X J?p' 



Thus curves of flux distribution such as Fig. 41 can readily 

 be drawn. It will be seen that the dotted curve, giving actual 

 distribution of flux for the case of Fig. 40, does not differ from 



Flux Distribution, 

 One Pole only 



O Surf ace of Armature Core N 



FIG. 41. Curve of flux distribution over armature surface. 



the full-line curve (case of Fig. 39, with no interference from 

 neighboring poles) except in the interpolar space where the de- 

 magnetizing effect of the opposite polarity is appreciable, and 

 causes the flux to diminish rapidly until it reaches zero value on 

 the geometric neutral (the point N), where its direction re- 

 verses. This is what one would expect to find, because, although 

 the magnetic action of any one pole considered alone will ex- 

 tend far beyond each pole tip, this action will not be appreciable 

 beyond the interpolar space, on account of the shading effect 

 of the neighboring poles. In order to ascertain how far the 

 demagnetizing effect of neighboring poles is likely to extend 

 when the air gap is not constant but increases appreciably in 



