278 



SYNCHRONOUS MOTORS. 



of current necessary to maintain the rotation of the shaft 

 increases for a double reason : (1) It has to increase in 

 order to make up for the weakening of the field by the 

 lessened excitation ; (2) the total current supplied must 

 increase in a greater ratio than the weakening of the field, 

 because only the portion C cos </> is active, and <f> increases 

 as the current lags behind the back electromotive force. 



VO 1-5 



Amperes, Excitation. 



2-O 



2-5 



FIG. 130. VARIATION OF ARMATURE CURRENT WITH EXCITATION IN SYNCHRONOUS 



MOTOR. 



I. = Motor running light. 

 II. = Output '38 h.p. 

 III. = -81 h.p. 



The curve consequently rises to the left of the 

 minimum point due to these causes. 



The curve also rises, although rather less steeply, for 

 values of the excitation higher than 1'47 amps. This is 

 due to the fact that the armature current in this case leads 



