MAGNKTU' DKSUiX OF KKLAVS o3 



atid ^^;lx\v('l^s appi'oxiniat ion for coil ol fcctaiiiiular section : 



a(log|-2) 



where R = geometric mean distance of coil cross-section. 



The examples (l(>scribed in the present section cover those procedures 

 which are used most frequently in estimating leakage reluctances. For a 

 more extended treatment of this subject, reference may be made to S. 

 Evershed and H. C. Roters.^ 



6 MAGNETIC CIRCUIT EVALUATION 



In the discussion of the magnetic circuit concept in Section 3, it was 

 noted that the accuracy of the representation varies with the extent to 

 which the network of tubes of induction is sub-di\'ided to correspond to 

 the distributed nature of the actual field. The effect of sub-division upon 

 the accuracy is greater in the high density region, where the reluctance 

 of the iron parts is variable and increasing, than in the low density 

 region, where the iron reluctance is small and approximately constant. 

 Hence the choice of an adequate network is largely contingent upon the 

 location of the iron parts having the highest flux density. Incipient 

 saturation affects the reluctance of these parts, and thus th(> pattern of 

 the field, while parts of lower density remain of low and approximately 

 constant reluctance. 



In ordinary electromagnets the core is the part of highest density, in 

 which saturation limits the attainable field. It is good design practice to 

 limit the core section to as small a \'alue as is consistent with satisfactory 

 performance, as this results in a minimum inside coil diameter. This is 

 advantageous with respect to the coil constant, as shown bv e(|uation 



(1). 



In the special case ot high speed relays, it is advantageous to minimize 

 the mass of the armature and hence its cross-section. In such relays arma- 

 ture saturation ma}^ control, or occur concurrently with core saturation. 

 Saturation elsewhere than in the core or armatun^ is of intei'cst only in 

 the diagnosis of faulty design, since the return members should have a 

 section adequate to carr^' the maximum field at densities well below 

 saturation. 



Thus in most electromag?iets, saturation occurs in the core, and in- 

 cipient saturation affects its reluctance and the pattern of the associated 

 field. The magnetomotive force \-aries along the length of the coil, and 

 the core is therefore subject to variations along its length in magnetic 



