CHAPTER XIII 

 AIR-GAP FLUX DISTRIBUTION WAVE SHAPES 



90. Shape of Pole Face. When an alternator is provided 

 with salient poles, the open-circuit flux distribution over the 

 pole pitch can be made to approximate to a sine curve by suit- 

 ably shaping the pole face. One method of increasing the air- 

 gap reluctance from the center outward is illustrated in Fig. 102. 

 The "equivalent" air gap, d e , at center of pole face is calculated 

 as explained in Art. 36 of Chap. VII (formula 58), and the 



FIG. 102. Method of shaping pole face of salient pole alternator, 

 equivalent gap at any other point under the pole is made equal 

 where a is the angle (electrical space degrees) between 



to 



cos a 



the center of pole and the point considered. The pole face would 

 extend about 56 degrees on each side of the center, the pole tips 

 being rounded off with a small radius. In practice the curve 

 of the pole face would probably not conform exactly with this 

 cosine law; it would generally be a circular arc, not concentric 

 with the bore of the armature, but with the center displaced so 



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