218 PRINCIPLES OF ELECTRICAL DESIGN 



0.466 + (0.11 X 0.943) = 0.57; and the density at this point is 

 B w = 22,100 X ^^ = 18,100 



At the halfway section, B m = g?A C[) + 18 > lc g = 2 Q,100 



The corresponding values of H, as read off the B-H curves, Fig. 80 

 and Fig. 2, are: 



At bottom, H n = 700 



At middle, H m = 310 



At top, H w = 144. 

 By formula (64), we have, 



700 , 2 X 310 , 144 

 Average H = -y H --- ^ - + -g- = 347 



which is appreciably higher than the value of H at the section 

 halfway between the two extremes. This difference will, how- 

 ever, hardly be noticeable on low values of tooth density; and 

 indeed the somewhat tedious work involved in the above calcu- 

 lation is quite unnecessary with small values of tooth density, 

 because the ampere-turns required to overcome tooth reluctance 

 are then, in any case, but a small percentage of the air-gap 

 ampere-turns. The values of H, in the above table, for B g = 

 8,000 and B g = 6,000, are those corresponding to the average 

 values of the tooth density (B m ). 



Having plotted in Fig. 82 the curve for the teeth only, the 

 straight line for the air-gap proper can now be drawn for the 

 points under the center of the pole face where the equivalent 

 air gap is d e = 0.307 in. Obviously, since HI = QAirSI, and 

 B has the same value as H in air, we may write, 



.307 X 2.54 



/0. 



(SI) =B g ( 



This gives us the line marked A in Fig. 82. The addition to this 

 curve of the ampere-turns for the teeth and slots, results in the 

 curve marked a, 6, c, d, e, /, which may be used for all points 

 under the pole where the permeance has the same value as at a. 

 The curves for the other points on the armature surface may now 

 be drawn as explained in Art. 42. 



Items (72) to (76): Flux Distribution on Open Circuit. Refer 

 Arts. 40, 41, and 42. From the point R on the permeance curve 



