134 



PRINCIPLES OF ELECTRICAL DESIGN 



against the corresponding values of B g , and the resulting curve 

 shows the excitation required to overcome tooth reluctance for 

 all values of the air-gap density. The curves for the air gap 

 proper will all be straight lines when plotted in Fig. 49. Let 

 OR be the curve for the point a at center of pole. Add the 

 ampere-turns required for the teeth, and obtain curve (a), 

 which gives directly the ampere-turns required to overcome 

 reluctance of air gap, teeth, and slots, for all values of the air- 

 gap density. It will be understood that the ordinates represent 

 the average value of the air-gap density at armature surface 



\a b c d e 



FIG. 50. Open-circuit m.m.f. curve. 



\ 



over a slot pitch. Any other curve, such as (d), is obtained 

 by first drawing a straight line OQ such that PQ bears to PR 

 the same relation as the ordinate at d in Fig. 48 bears to the 

 ordinate at a, and then adding thereto the ampere-turns for the 

 teeth, as already obtained. The curves of Fig. 49 should in- 

 clude a sufficient number of points on the armature surface; and 

 when the resultant m.m.f. between armature points and pole is also 

 known, the correct flux-distribution curves can readily be plotted. 

 Let the curve of Fig. 50 represent the distribution of the re- 

 sultant field m.m.f. on open circuit obtained as explained in Art. 

 40 (Fig. 44) ; then, at any point such as e, the m.m.f. is given by 

 the length of the ordinate ee'. Find this value on the horizontal 

 scale of Fig. 49, and the height of the ordinate at this point, 

 where it meets curve e (which is not drawn in Fig. 49), is the 

 flux density, which can be plotted as ee' in Fig. 51. In this 

 manner the flux curve A of Fig. 51 is obtained. The area 

 of this curve, taken between any given points on the armature 

 periphery, is obviously a measure of the total flux entering the 

 armature between those points. The required flux per pole is 



