276 



ELECTRICAL ENGINEERING 



1GO 



150 

 110 

 130 

 120 

 110 

 100 



I 80 



| 



$ 7 



Jj 60 



a 50 



40 



20 



10 



100 200 300 400 500 600 700 800 900 1000 

 Amperes Load Current I 



FIG. 254. Compounding curves of a synchronous motor. 



Fig. 255 represents the load characteristics for a given value of 

 field current //. Since // is constant EQ is constant. 7 is varied 

 and the corresponding values of cos $ are obtained from equation 

 264. These values are substituted in equation 267 and the values 

 of I and cos <j> are plotted on a base of power output. 



At light loads the power factor is low and leading and the motor 

 is over excited; as the load is increased the power factor increases 

 until it reaches 100 per cent at a value of load depending on the 

 field excitation ; beyond this point the power factor decreases again 

 and becomes lagging. The current increases continually but is 

 finally limited by the synchronous impedance of the armature. 



