58 PRINCIPLES OF ELECTRICAL DESIGN 



the two magnet limbs should be considered as made up of two 

 parts : 



(a) The flux leakage from pole shoe to pole shoe, which is due 

 to the total m.m.f., available for the air gap. 



(6) The flux leakage between the circular cores under the 

 windings, which increases in density from the yoke to the pole 

 pieces and is equal to the average m.m.f. X the permeance of the 

 air paths between the two iron cylinders. This average m.m.f. 

 will be approximately one-half the total m.m.f. of the exciting 

 coils. 



For the permeance of the paths comprised under (a), we have: 



1. Between opposing rectangular faces, 



6.45X2X0.25 

 2.54 X 3.375 



2. Between the two pairs of faces parallel to the plane of the 

 paper (Fig. 20), by formula (10) Art. 5, 



_Z . (vt + s\ 

 P 2 = 2-loge( j 



0.25X2.54 /TX 2 + 3.375 \ 



"V" X gl \ 3.375 / 



= 0.425. 



Neglecting the flux lines that may leak out from the pole faces 

 farthest removed from each other, and also those between the 

 small ledges caused by the change from square pole piece to 

 circular section under the coil, the total leakage flux between pole 

 pieces is 



$ p = m.m.f. X (Pi+P 2 ) 

 = 0.4T X 9,300 X 0.801 

 = 9,350 maxwells. 



The permeance of the air paths comprised under (b) may be 

 calculated by applying formula (13) of Art. 5. Thus, 



TT X 11 X 2.54 



Ps 



!.31og w (- 



17875 



.5 + 1.875- V(3.5) 2 + (2 X 3.5 X 1.875) / 

 The leakage flux between magnet cores under the windings is, 



0.4rr X 10,230 



~2~ 

 = 330,000 maxwells 



