74 PRINCIPLES OF ELECTRICAL DESIGN 



The voltage per conductor is 



3>pN 

 Ec = WX 10* 



where the unknown quantity <f> may be expressed in terms of flux 

 density and armature dimensions. Thus 



$>p = QA5B g l a TrDr (41) 



where B g = average flux density in the air gap under the pole 



face. (Gausses.) 



l a = gross length of armature core, in inches. 

 D = diameter of armature core, in inches, 

 pole arc 



= the ratl r^itch' 



It will be seen that the quantity l a X irDr is the area in square 

 inches of the armature surface covered by the pole shoes; while 

 6.455<, is the flux in the air gap per square inch of polar surface. 



The pole pitch is usually thought of as the distance from center 

 to center of pole measured on the armature surface; and the 

 ratio r is therefore a factor by which the total cylindrical surface 

 of the armature must be multiplied to obtain the area covered 

 by the pole shoes the effect of "fringing" at the pole tips being 

 neglected. 



Substituting for $p in equation (40) its value as given by equa- 

 tion (41), and putting this value of E c in equation (39), we 

 have 



_ QA5B g l a 7rDrNZI c 



60 X 10 8 



from which it is necessary to eliminate Z and I c if the formula 

 is to have any practical value. 



A quantity which does not vary very much, whatever the 

 number of poles or diameter of armature, is the specific loading, 

 which is defined as the ampere-conductors per inch of armature 

 periphery. It will be represented by the symbol q. Thus 



ZI C 



= ^D 

 whence 



Zl c = qirD 



Substituting in equation (42), we have 



w = i** (43) 



