134 



ALTERNATING CURRENTS 



when the brushes are in the geometrical neutral (see Vol. I, page 

 268, Fig. 238) and cross-magnetization alone results. 



It will be observed in Fig. 136 that the coil in question is 

 acting principally on the interpolar space, whose reluctance is 

 high. Therefore, in this position the effect of the coil ampere- 

 turns upon the magnetic flux of the generator is a minimum. 

 This does not apply to a non-salient pole machine, where the air 

 gap is uniform. 



Figure 138 is a vector diagram 

 representing these conditions. F\ is 

 the mmf. due to the field coils, A is 

 the mmf. due to the armature coil 

 and F is the resultant of the two. 

 When the current is in phase with the 

 induced emf ., the space direction of the 

 armature mmf., A, is 90 electrical space-degrees from the resultant 

 mmf. F. It will be observed, Fig. 138, that the principal effect 

 of A is to distort the alternator flux or to change it from its 

 original position practically without altering its magnitude. 



Figure 138 is a space-vector diagram of mmfs. The resultant 

 flux is equal to the mmf. F divided by the reluctance of the mag- 

 netic circuit. In a non-salient pole machine, where the reluctance 



FIG. 138. Vector diagram 

 showing effect of armature re- 

 action when current is in phase 

 with induced e.m.f. 



\ 



Field llmf. 



\ S \ 



(a) 



Coil Mmf. 



'Current "V,^ 



FIG. 139. Armature reaction due to current lagging 90. 



of the air gap is uniform, may be found in terms of induced 

 emf. from the saturation curve. In salient-pole machines, this 

 method is only approximate, as the reluctance of the magnetic 

 circuit varies. The reluctance is a minimum when the space 

 direction of F is along the pole centers and is a maximum when 

 the space direction of F is midway between pole centers. 



Figure 139 represents the conditions when the current lags 90 



