PROCEDURE IN DESIGN OF D.C. GENERATOR 213 



this will be helpful in deciding upon the most suitable shape of 

 the pole shoe. The full-load flux per pole (item (51)) is 6,970,000 

 maxwells; and if we assume a leakage coefficient of 1.2 (Art. 

 56, page 187), the flux to be carried by the pole core is 8,370,000 

 maxwells, approximately. The density in the core may be as 

 high as 16,000 gausses, and the cross-section is therefore 

 8,370,000 



16,000 X 6.45 

 cross-section, 



81 sq. in. If the pole core is made of circular 



Diameter = 



(say) 10 in. 



A laminated pole shoe of the shape shown in Fig. 78 will be 

 suitable. The stampings would be riveted together and at- 



FIG. 78. Graphical construction for calculating permeance of fiux paths. 



tached by screws to the face of the cylindrical pole core. The 

 distance aO, measured on the armature surface, is one-half 

 of item (21), or 5.5 in.; while al is one-half of the pole pitch T 

 (item (20)), and measures 7.67 in. The tip of the pole is shaped 

 so that the air gap increases from the point / outward, being M 

 in. greater at the ends of the pole arc than at the center. The 

 extreme point of the pole shoe is rounded off with a iKe-in. 

 radius. The reference points on the armature surface have been 

 chosen for convenience at intervals of 10, except in the neighbor- 

 hood of the pole tip, where the selected points are only 5 apart. 



