124 



PRINCIPLES OF ELECTRICAL DESIGN 



point on the periphery shall be the same as the average reluctance 

 per unit area taken over the slot pitch. 



In Figs. 39 and 40 an attempt has been made to represent 

 the actual distribution of flux lines (1) for the condition of one 

 pole acting alone without interference from neighboring poles, 

 and (2) for the practical condition of neighboring poles of equal 

 strength and opposite polarity. The machine to which these 

 diagrams apply is a continuous-current dynamo of pole pitch 

 37 cm., pole arc 27 cm., and equivalent air gap of 0.8 cm. at 

 center of pole face. The air gap is of uniform length except 

 near the pole tips, where it is slightly increased, as indicated 



A N 



FIG. 40. Flux lines in air gap of dynamo. (Effect of neighboring poles.) 



on the drawings. With a little practice, unlimited patience, and 

 ample time in which to perform the work, diagrams of flux dis- 

 tribution such as those of Figs. 39 and 40 can be drawn, and 

 they will indicate accurately the actual arrangement of the 

 flux lines. The method is one of trial and gradual elimination 

 of errors, based on the well-known principle that the space 

 distribution of the flux lines will be such as to correspond with 

 maximum total permeance, or, in other words, such as will 

 produce the maximum flux with a given m.m.f. 



Probably one of the most practical and at the same time 

 most accurate methods of procedure is that proposed by DR. 

 L/EHMANN 1 and followed in preparing the flux diagrams (Figs. 

 39, 40, 42 and 43). A section perpendicular to the shaft through 

 the pole shoe and armature is considered, and all flux lines in 



1 "Graphische Methode zur Bestimmung des Kraftlinienverlaufes in der 

 Luft," Elektrotechnische Zeitschrift, vol. 30 (1909), p. 995. 



