134 TREATISE ON ALTERNATING CURRENTS. 



ARMATURE EEACTIOX. 



88. We have hitherto supposed that the E.M.F. of the 

 generator, and the counter E.M.F. of the motor, are due simply 

 to the exciting current which passes round their respective field 

 coils, or that the excitations of the fields are fixed by the field 

 currents. Now, this is not quite true, since the magnetization of 

 the fields depends upon all the magnetizing forces in the neigh- 

 bourhood, and consequently upon the combined effects of the 

 currents in both field and armature coils. 



The effect of the armature current in modifying the field is 

 usually called armature reaction. 



The whole question of armature reaction turns upon the phase 

 relationships of the current and the E.M.F.s of generator and 



FIG. 45. 



motor. Our investigation will include both generator and motor, 

 because both questions are of importance when the whole plant 

 is taken into consideration. 



Let Fig. 45 be a diagrammatic representation of an alternating- 

 current machine running as a generator, the direction of rotation 

 being clockwise. If the armature is wound as in figure, the E.M.F. 

 will be a maximum when each armature coil is midway between 

 two consecutive field poles, since the number of lines of force of 



