CHAPTER VI 



EXCITING AMPERE-TURNS IN ELECTRICAL 

 MACHINERY (Continued) 



38. The Ampere- turns Required for Saturated Teeth. The teeth 

 and the slots of an armature, under the poles, are magnetically in 

 parallel (Fig. 24) ; hence, part of the flux passes from the pole into 

 the armature core through the slots between the teeth. But, with 

 a moderate saturation in the teeth, say below 18 kilolines per 

 square centimeter, the amount of the flux which passes through the 

 slots is altogether negligible. If the taper of the teeth is slight, 

 the required ampere-turns are found for the average flux density 

 in the tooth, taking the value of H from the curves in Fig. 3. 



Should the taper of the teeth be considerable, as is the case in 

 revolving armatures of small diameter, the flux density should be 

 determined in say three places along the tooth, viz., at the root, 

 in the middle part, and at the crown. Let the corresponding 

 values of magnetic intensity from the magnetization curve of the 

 material be H 0) H m , and H\. Assuming H to vary along the tooth 

 according to a parabolic law, we have, according to Simpson's 

 rule in the first approximation, that the average intensity over the 

 tooth is 



(52) 



If a greater accuracy is desired, the values of H can be determined 

 for more than three cross-sections of the tooth and Simpson's 

 rule applied. 1 For instance, let the length be divided into n equal 



1 A designer who has to calculate ampere-turns for teeth frequently 

 will save time by plotting curves for the average H against the flux density 

 B at the root of the teeth. Each curve would be for one taper, and these 

 curves would cover the usual range of taper in the teeth. See A. Miller Gray 

 "Magnetomotive Force in Non-uniform Magnetic Paths," Electrical World, 

 Vol. 57 (1911), p. 111. 



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