SYNCHRONOUS MACHINERY 



277 



The power output cannot increase indefinitely but reaches a 

 maximum when the decrease in power factor overcomes the 

 increase in current and then the motor becomes unstable and falls 

 out of synchronism and stops. 



The maximum power input occurs when 



PI = El cos 4> is maximum. 

 The maximum power output occurs when 



P 2 = El cos Pr constant losses is maximum. 



These maximum values of input and output are far beyond the 

 heating limits of the motor. 



20 



50 75 



Percent Load 



150 



FIG. 255. Load characteristics of a synchronous motor. 



The torque developed by the motor at any output is directly 

 proportional to the output. If T is the torque in foot pounds 

 developed in the armature, the mechanical power developed is 



p = 



X 746 watts = El cos < - Pr, 



33,000 



where N is the synchronous speed in revs, per min. ; thus 

 El cos <j> - Pr 33,000 



2irN 

 and the torque available is 



746 







IDS. 



El cos Pr constant losses 33,000 

 2wN ~746~' 



