26 



PRINCIPLES OF ELECTRICAL DESIGN 



greater than the radius of the iron core (r), the permeance may 

 be expressed with sufficient accuracy for practical purposes as 

 p _ mean cross-sectional area 



length of flux path 

 _ TT (R + r)h 

 (R-r) 



Case (e). Same as Case (d) Except that Coils Occupy the 

 Whole of the Available Space. This is the more usual case, and 

 it is illustrated by Fig. 10. The leakage flux in the annular 

 space occupied by the windings will depend not only upon the 



I 



FIG. 9. Leakage paths in circu- 

 lar magnet (space not occupied by 

 copper). 



FIG. 10. Leakage paths in circu- 

 lar magnet (space entirely fitted by 

 exciting coils). 



permeance of the air path, but also upon the m.m.f. tending to 

 establish a magnetic flux. This m.m.f. has no longer a constant 

 value, but, on the assumption that the reluctance of the iron paths 

 is negligible, it will increase according to a straight-line law from 

 zero when x (see Fig. 10) to a maximum when x = h. 



Starting with the fundamental formula, $ = m.m.f. X P t 

 we have, 



~ 2irdx 



= [-*- X I] 



X 



log 



R 



whence 



or 



* "~~ 



0.4mS7 



R 



(12) 



