60 PRINCIPLES OF ELECTRICAL DESIGN 



The total leakage flux will be approximately 



= 100,000 maxwells. 



This calculated value of the leakage flux should be slightly 

 increased because the total permeance between the two magnet 

 limbs will actually be greater than as calculated by the con- 

 ventional formulas. Let us assume the total flux to be 125,000 

 maxwells. This makes the value of the leakage factor. 



_ 170,000 + 125,000 _ 

 170,000 



The maximum value of the density in the iron cores will be 



170,000 + 125,000 



j = 73,600 lines per square inch. 



Closer Estimate of Exciting Ampere Turns. The modified 

 magnet will now be generally as shown in Fig. 21. The ampere 

 turns required to overcome the reluctance of the two air gaps 

 have already been calculated; the remaining parts of the mag- 

 netic circuit consist of the two magnet limbs under the windings, 

 together with the yoke and the armature. If we know the 

 amount of the flux through the iron portions of the circuit we can 

 readily calculate the flux density, and then ascertain the neces- 

 sary m.m.f. to produce this density, by referring to the B-H 

 curves of the material used in the magnet. 



In the magnet cores under the coils, the flux density varies 

 from a minimum value near the poles to a maximum value near 

 the yoke; and as the leakage flux is not uniformly distributed 

 over the length l c (Fig. 21), it would not be correct to base re- 

 luctance calculations upon the arithmetical average of the two 

 extreme densities, even if the flux density were below the "knee" 

 of the B-H curve, with the permeability, p, approximately 

 constant. With high values of B, the length of the magnetic 

 core should be divided into a number of sections, and each section 

 treated separately in calculating the required ampere-turns. 

 With comparatively low densities, as in this example, the 

 calculation can be made on the assumption of an average density 

 in the magnet cores, the value of which is 



B c = n ' 



