THE ALTERNATING CURRENT GENERATOR 249 



in this one turn that the ampere-turns on the armature 

 are the same as with the real winding. 



Magnitude of the Effect of Armature Reaction Upon the 

 Main Field. The magnitude of the effect of the armature 

 reaction depends upon the ratio of the armature ampere- 

 turns to the field ampere-turns. A machine having many 

 ampere-turns on its field (thus saturating the field poles) 

 and but few ampere-turns on its armature is said to have 

 a stiff field. The field of such a machine is affected but 

 little by armature reaction. 



Analysis of Armature Reaction in a Two-phase Alternator 

 with Non-inductive Load. The effect of armature reaction 

 in a polyphase generator may easily be obtained by draw- 

 ing the vector showing the armature m.m.f. for each phase 

 separately and then combining them. Thus in the two- 

 phase armature the total m.m.f. may be obtained by adding 

 vectorially two chords of the circular locus at right angles 

 to one another. They are added at right angles because 

 the phase windings on the machine are 90 apart. 



Reaction Constant in a Two-phase Alternator. Fig. 153 

 shows that the m.m.f. of the armature resulting from such 

 a construction is constant in magnitude and direction. The 

 m.m.f. of phase 1-1' is shown at OB and at the same instant 

 phase 2-2' is producing the m.m.f. OA. The resultant 

 of these is OC, and no matter at what instant the armature 

 reaction is considered the resultant will always be OC, the 

 maximum value of the m.m.f. of one phase. 



Reaction with Inductive Load. Fig. 153 shows the 

 conditions for a non-inductive load. The resultant OC t 

 is seen to be entirely a cross-magnetizing effect. Now 

 if the load were inductive the proper diagram would show 

 that the armature reaction is of the same magnitude OC, 

 but that it is advanced in the direction of rotation, as OC 

 of Fig. 154; the angle of advance being equal to $, where 

 cos <j> is the power factor of the load. 



Now in the case of a load with a leading current the 



