FLUX DISTRIBUTION 



129 



of a pole pitch or the m.m.f. between pole shoe and armature. 

 If the resultant m.m.f. tending to send flux from the pole to any 

 point on the armature is known, the flux density can be calculated 

 because, B = flux per square centimeter = (m.m.f.) X permeance 

 per square centimeter. 



Thus, if the m.m.f. necessary to overcome air-gap reluctance 

 at center of the pole is known, the curve of resultant m.m.f. 

 at all points on the armature can be plotted. This has been 



done in Fig. 44, where the ordinate of the m.m.f. curve at any 



i n 7? 



point, such as 10, is simply 10 M = ^ p> where the or- 

 dinate 10 B of the flux curve must be expressed in gausses. 



41. Practical Method of Predetermining Flux Distribution. 

 Although the method outlined above, gives excellent results 

 in the hands of an experienced designer who can afford the time 

 required to m&p out the actual paths of the flux lines, it is not 

 suitable for general use in actual designing work. By adopting 

 a simple construction which assumes a certain flux distribution 

 and avoids the drawing of equipotential lines, results of sufficient 

 accuracy for practical purposes can very quickly be obtained. 



A BC ODEF G H 



FIG. 45. Approximate flux paths between pole and armature. 



This construction is indicated in Fig. 45. A section through 

 one-half of the pole shoe, showing the air gap in its proper pro- 

 portion, is drawn to a sufficiently large scale, preferably full 

 size. The " developed" armature surface may be used, in 

 which case the construction is a little simpler because radial 

 lines may be shown as perpendiculars erected on the horizontal 

 datum Kne representing the armature surface. The distance 

 AM is the "equivalent" air gap, as calculated for the center 

 of the pole face by using formula (58). Draw the perpendicular 

 OR tangent to the pole tip, and, through the first point of 

 tangency Q, draw the semicircle BQF with its center at 0. 

 Bisect QO at the point P and draw PD at an angle of 30 degrees 



