ALTERNATORS. 



211 



It is now necessary to explain the action of a lagging or 

 leading armature current in causing variation in the terminal 

 voltage. The action depends on the magnetic re-action of 

 the armature field upon the main field. 



Effect of Lag or Lead of Current upon Armature Reaction. 

 As being the simplest case for purposes of explanation, 

 the Fig. 98 has been drawn to represent part of a 2-pole 

 alternator with ring- wound armature. 



Case I. Current and Voltage in Phase. If current is 

 taken from the armature and supplied to a non-inductive 

 circuit, there will be no phase difference between current and 

 volts, and these quantities will pass through their maximum 



FIG. 98. DIAGRAM ILLUSTRATING EFFECT OF ARMATURE REACTIONS. 



and minimum values simultaneously if the effect of the self- 

 induction of the armature is neglected for the present. 



With the armature in the position shown, the voltage 

 will be at its maximum value. If the current is also at its 

 maximum value (i.e., if there is no difference of phase between 

 the current and volts) the current will produce a field per- 

 pendicular to the main field, as indicated by the symbols 

 n s. This armature reaction field will distort but not 

 strengthen or weaken the main field. Earlier in the revolu- 

 tion the field of the armature would be weaker, since the 

 current would be weaker, and would be directed partly 



t 



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