FLUX DISTRIBUTION 



119 



equivalent air gap for various values of the flux density would 

 be unpractical, and, since no exact method is ever likely to be 

 developed, some sort of compromise must be made. 



Length of Air Gap. The air-gap clearance 5 must, of course, 

 be decided upon before the calculation of tooth and slot reluctance 

 can be made. The controlling factor in determining this clear- 

 ance is the armature strength or the ampere-turns per pole of 

 the armature. If the m.m.f . due to the armature greatly exceeds 

 the excitation on the field poles, there will be trouble due to field 

 distortion under load, which will lead to poor regulation and 

 commutation difficulties. The field ampere-turns at full load 

 should be greater than the armature ampere-turns. A safe rule 

 is to provide an air gap such that the open-circuit ampere-turns 

 required for the air gap alone assuming a smooth core and no 

 added reluctance due to slots would be equal to the ampere- 

 turns on the armature at full load. Thus if (SI) g are the ampere- 

 turns per field pole required to overcome the reluctance of an 

 air gap of length 5, we may write (SI) g = (SI) a where (SI) a 

 stands for the armature ampere-turns as calculated by formula 

 (48) page 80. This gives for 5 the value: 



QSJ), X 0.47T 

 2.54 fl 



in. 



or, approximately, 



where B g may be taken as the apparent flux density in the 

 air gap under the pole face on the assumption that there is no 

 fringing. (For approximate values of B g , refer to the table on 

 page 75.) 



The length of air gap may be somewhat reduced if com- 

 mutating interpoles are provided, especially if pole-face windings 

 (see Art. 50, p. 170, are used. It might be 25 per cent, smaller 

 if interpoles or pole-face windings are used. 



Another factor which may influence the air-gap clearance is 

 the possibility of unbalanced magnetic pull due to slight decen- 

 tralization of the armature. This becomes of importance only 

 in machines of large diameter with many poles and rarely 

 necessitates a clearance greater than that obtained by applying 

 the above rule. 



37. Actual Tooth Density in Terms of Air-gap Density. 

 It is convenient to think of the reluctance of air gap, teeth, and 



