132 



ALTERNATING CURRENTS 



63. Armature Reaction. In direct-current machines, the arma- 

 ture ampere-turns act on the magnetic circuit of the machine in 

 such a way as to distort and to change the magnitude of the air- 

 gap flux. For a given armature .current, the direction and mag- 

 nitude of this armature reaction depend on the position of the 

 brushes. In an alternator practically the same conditions exist. 

 For a given armature current, the magnitude and direction of the 

 armature reaction cannot depend upon brush position, but do 



Fio. 136. Distortion of alternator flux by an in-phase current. 



depend on the phase relation existing between the current and 

 the voltage and hence on the power-factor of the load. 



Figure 136 (a) shows the position of an armature coil whose 

 sides are directly under the pole centers. At this instant the 

 induced voltage in the coil is at its maximum value. If the 

 current is in phase with this induced voltage, corresponding 

 approximately to a load of unity power-factor, the current is at 

 its maximum value at this same instant, and flows in the direction 



