AIR-GAP FLUX DISTRIBUTION 



273 



by this method. It will usually suffice to make the calculations 

 for one tooth pitch in the position of greatest permeance, and 

 again in the position of least permeance. The average of these 

 calculated values, divided by the area of the tooth pitch in square 

 centimeters, will give the average value of the air-gap permeance 

 per square centimeter. 



Under this condition of constant air-gap permeance, the flux 

 distribution on open circuit will follow the shape of the m.m.f. 

 curve; but, in any case, since B m.m.f. X permeance per 

 square centimeter, the flux curve can always be obtained when 

 the m.m.f. distribution is known. Thus, if the portion W 



M.M.F. Curve 



for Field Winding only 



FIG. 105. M.M.F. over pole pitch, due to distributed field winding. 



of the pole (Fig. 105) is not slotted, the permeance curve, in- 

 stead of being a straight line of which the ordinates are of con- 

 stant value, would be generally as shown in Fig. 106. From 

 these values of the permeance per square centimeter of armature 

 surface, curves such as those of Fig. 49 (page 133) can be drawn, 

 so as to include the reluctance of the armature teeth. From all 

 points on the armature between A and B, and C and D (Fig. 

 106), the average air-gap permeance would have the value A A f . 

 Over the central portion W it would have the value EE', while, 

 in the neighborhood of the points B and C, it may be assumed 

 to have an intermediate value as indicated by the ordinate BB'. 



18 



