132 PRINCIPLES OF ELECTRICAL DESIGN 



flux per pole will be known at this stage of the work, and the 

 unknown factor will be the ampere-turns on the poles necessary 

 to produce this flux. Measure the area of the curve OA'N and 

 construct the rectangle OO'N'N of exactly the same area. The 

 height of this rectangle will be a measure of the average density 

 over the pole pitch. This is known to be 



$ 



B (average) = -- 



where $ = total flux per pole in the air gap. 

 T = pole pitch in centimeters. 

 l a = gross length of armature in centimeters. 



In this manner a scale is provided for the flux curve OA'N, 

 which should preferably be replotted. The curve of resultant 

 m.m.f. over armature surface can now be derived as explained 

 in connection with Fig. 44. 



Since the permeance curve as obtained by either of the methods 

 here explained does not take into account the reluctance of the 

 armature teeth, or indeed the reluctance of any part of the 

 magnetic circuit other than the air gap, the actual ampere-turns 

 necessary to produce the required flux will be greater than the 

 amount indicated by the maximum ordinate of the m.m.f. curve 

 of Fig. 44. The fact that the reluctance of the teeth at different 

 points under the pole face is dependent upon the flux distribu- 

 tion tends to complicate the problem, but a method of accounting 

 for this variation will be explained in the following article. 



42. Open-circuit Flux-distribution Curves, as Influenced by 

 Tooth Saturation. Before considering the effect of the armature 

 current in altering the distribution of magnetizing force over the 

 armature periphery, it will be necessary to examine briefly how 

 the degree of saturation of .the teeth may be taken into account 

 and a correct flux-distribution curve plotted. The method about 

 to be explained is due to PROFESSOR C. R. MOORE, it is probably 

 more easily applied and less tedious than an equally scientific 

 method more recently proposed by DR. ALFRED HAY. 1 



In Fig. 48 a permeance curve has been drawn. It has the 

 same meaning as the curve marked " Permeance" in Fig. 44 

 (Art. 39), and it may be obtained for any given machine in the 

 manner described in Art. 41. If this curve (Fig. 48) has been 



1 A. HAY: "Predetermining Field Distortion in Continuous-current 

 Generators," Electrician, 72, pp. 283-285, Nov. 21, 1913. 



