THE DESIGN OF ELECTROMAGNETS 



57 



considerable reluctance, t will be small in comparison with I. 

 The approximate value of A will then be, 



A (approx.) = 



2T X 60 -. 

 l"t Xsf Xh 

 I 2X40X 1,273,000 



(36) 



180 X 1 X 0.75 X 0.5 

 = (say) 1,250 



Length of Winding Space. The ampere-turns required for the 

 double air-gap only^'.e., not including those required to overcome 

 the reluctance of the iron portions of the magnetic circuit, will be, 

 by formula (5) of Art. 4, 



OS/), = 2.02 X 21, 



= 2.02 X 6,570 X 2 X 0.35 

 = 9,300. 



The ampere-turns for the iron part of the magnetic circuit cannot 

 be calculated accurately until the length I and the actual leakage 

 factor have been determined; but, since the air-gap, in this case, 

 offers far more reluctance than the remaining portions of the 

 magnetic circuit, we shall assume the iron portions to require 

 only one-tenth of the air-gap ampere-turns. Thus, 



total SI (both spools) = 10,230 approx. 



We are now able to solve for the length of the winding space, 

 which is 



SI total 



2*A X */ 



10,230 



~ 2 X 0.75 X 1,250 X 0.5 

 = 10.9 or (say) 11 in. 



Before proceeding further with the design, it will be well to see 

 whether the long magnet limbs will not be the cause of too great a 

 leakage flux. If the leakage factor is much in excess of the 

 assumed value (1.5) there is danger of saturating the cores under 

 the windings, and so limiting the useful flux available for drawing 

 up the armature Let the distance between the windings be 2 in., 

 as indicated on Fig. 20; this gives all necessary dimensions for 

 calculating the permeance of the leakage paths. 



Calculation of Leakage Flux The total leakage flux between 



