188 PRINCIPLES OF ELECTRICAL DESIGN 



known, and a suitable leakage factor may be selected from the 

 table in the preceding article. The method of procedure is 

 exactly as explained in connection with the design of a horseshoe 

 lifting magnet (see Art. 16, page 52), and it will be again fol- 

 lowed in detail when working out a numerical example of con- 

 tinuous-current generator design. The flux path of average 

 length is indicated in Fig. 71. There may be some doubt as to 

 what is the proper value to take for the length of the path a in 

 the armature core below the teeth, because the flux density will 

 be less uniform in this part of the magnetic circuit than in the 

 poles and yoke. As a matter of fact, the ampere-turns neces- 

 sary to overcome the reluctance of the armature core (apart 

 from the teeth) are but a small percentage of the total, because 

 the flux density must necessarily be low to avoid large losses 

 due to the reversals of magnetic flux. It is, therefore, something 

 of a refinement to take account of the unequal distribution of 

 the flux in the armature core; but if the length of the path a 

 of Fig. 71 be taken as one-third of the pole pitch r, a more 

 accurate value for the ampere-turns will be obtained than if 

 the length were measured along the curved path shown in the 

 illustration. It is, of course, understood that the density to be 

 used in the calculation is the maximum flux density at the 

 section midway between the poles, on the assumption that the 

 flux is uniformly distributed over this section. Thus, if <i> 

 is the useful flux per pole entering the armature core, and R* 



is the radial depth of stampings below the teeth, the maximum 



<j> 

 density in armature core to be used in the calculations is ^ p / 



where l n stands, as before, for the net axial length of the 

 armature. 



Solid-pole shoes are rarely used in connection with armatures 

 having open slots. With semi-closed slots, or even with open 

 slots if the air gap is large, the eddy-current losses in solid-pole 

 shoes may be very small, but laminated pole pieces are now the 

 rule rather than the exception. The thickness of the steel 

 sheets used to build up the pole pieces is usually greater than 

 that of the armature punchings, a thickness of 0.025 in. being 

 fairly common. In small machines it is sometimes economical 

 to construct the complete pole of sheet-steel stampings, as this 

 dispenses with the labor cost of fitting a separate built-up pole 

 piece on the solid pole core. 



