DYNAMO DESIGN 



75 



This is not an empirical formula since it is based on fundamental 

 scientific principles, and it is capable of giving valuable infor- 

 mation regarding the size of the armature core, provided the 

 quantities B g , q, and r, can be correctly determined. 



The quantity B g will depend somewhat upon whether the 

 pole shoe is of cast iron or steel, also upon the flux density in the 

 armature teeth, which, in turn, depends upon the proportions 

 of the teeth and slots. If the flux density in the teeth is very 

 high, this may lead to (1) an excessive number of ampere-turns 

 on the field poles to overcome tooth reluctance, and (2) excessive 

 power loss in the teeth through hysteresis and eddy currents. 



As a guide in selecting a suitable gap density for the preliminary 

 calculations, the accompanying table may be used. The 

 column headed B g is the apparent air-gap density in gausses, 

 while B" g is the same quantity expressed approximately in lines 

 per square inch. 



APPROXIMATE VALUES OP APPARENT AIR-GAP DENSITY 



The expression "apparent gap density" means that the flux is 

 supposed to be distributed uniformly over the face of the pole 

 and the effect of "fringing" is neglected. Thus 



total flux per pole 

 area of pole face 

 It is customary to think of this as the average density over the 

 armature surface covered by the pole face, in which case 



Bn = 



___ _ 



9 ~ l a X r X r 



where 3>, k, and r, have the same meaning as in formula (41), 

 and r is the pole pitch or length of arc from center to center of 

 pole measured on the armature periphery. The lower values of 



