268 METHODS OF CALCULATION 



increasing the flux-density, an e.m.f. is obtained equal to that in 

 long bobbins, and the direct reaction remains in the same ratio with 

 respect to the e.m.f. while the transverse reaction is reduced. The 

 winding of Fig. 13, with a flux having the value of two-thirds of the 

 pitch, is then that which for a given ratio of counter-ampere-turns 

 of the armature to the ampere-turns of excitation, produces the smallest 

 transverse reaction. This reduction is of considerable importance. 



(B) As to the methods of reducing reactions, it results from the 



jr 



preceding that there is no means of reducing the ratio . The 



only means of improving the regulation of alternators are therefore, 

 first, to saturate the field-magnet circuit (which augments the m.m.f. 

 and reduces the variations of e.m.f. at the armature terminals as a 

 function of the reactive current); second, to increase the air-gap, 

 which is less effective; and third, to reduce the transverse reaction, 

 which has the effect of diminishing the dephasing of the diagrams, 

 and consequently the direct reaction, which is proportional to the reac- 

 tive current. An alternator which would not have transverse reaction, 

 would have nothing to fear from direct reaction, even if it were 

 enormous. 



The methods of reducing the transverse reactions alone are: First, 

 the reduction of the breadth of the flux accompanied by an augmen- 

 tation of the flux-density in the air-gap, re-establishing the same 

 field-magnet flux at the expense of the increase of the flux-density; 

 but in that case the same result would be obtained by a simple 

 increase of the air-gap, without an increase in the loss of energy 

 in the teeth; second, the saturation of the polar horns when the pole 

 pieces possess them; third, the addition of longitudinal slots in the 

 field-magnets, as in direct-current dynamos. This last method is 

 very effective when the field magnets are saturated and the slots 

 occupy their entire length and are continued partially into the yoke; 

 the reduction of distortion thus obtained involves generally a reduction 

 of the total flux, because the mean permeability of the field magnet 

 is reduced by the inequality of the m.m.f.'s established between its 

 two halves; but the augmentation of excitation which results is neg- 

 ligible in comparison with the diminution obtained in the reactive 

 current by the reduction of dephasing. 



(C) As to the comparison between single-phase and polyphase 

 alternators, it is seen that not only do single-phase alternators utilize 

 less effectively their materials for the production of energy, because 



