168 



THE MAGNETIC CIRCUIT 



[ART. 53 



additional ampere-turns are estimated empirically, on the basis 

 of one's previous experience, because the amount is not large, and 

 a correct computation is rather tedious. 1 



The theoretical relation between the distorting ampere-turns 

 and the field ampere-turns required for their compensation is shown 

 in Fig. 43. Let OX represent the no-load saturation curve of the 

 machine for its active layer only, that is, for the air-gap, the teeth, 

 and the pole shoe, if the latter is sufficiently saturated. The 

 ordinates represent the induced e.m.f . between the brushes, or, to 

 another scale, the useful flux per pole ; the abscissa? give the corre- 

 sponding values of the field ampere-turns per pole, disregarding 

 the reluctance of the field poles, field yoke, and armature core. 



Ampere-turns and pole face areas 



FIG. 43. A construction for determining the field m.m.f. needed for the 

 compensation of the transverse reaction. 



In other words, the abscissae give the values of the difference of 

 magnetic potential across the active layer of the machine, at no 

 load. It is proper to consider here the m.m.fs. across the active 

 layer only, because the distorting action of the armature extends 

 only over this layer. No matter how irregular the flux distribu- 

 tion in the air-gap and in the teeth may be, the flux density in the 

 pole cores and in the yoke is practically uniform (compare Fig. 36). 



1 For Hobart's empirical curves for estimating the field excitation required 

 for overcoming the armature distortion see the Standard Handbook, under 

 ' Generators, direct-current, ampere-turns, estimate." 



