POWER LOSSES IN GENERATORS AND MOTORS. 137 



The full line curve in Fig. 95 is a typical efficiency curve of a 

 shunt generator. The efficiency curve of a compound generator 

 is very similar in form to the efficiency curve of the shunt gener- 

 ator. A generator is usually designed to give its maximum 

 efficiency at its rated full load output. The ordinate of the hori- 

 zontal dotted curve represents the approximately constant loss due 

 to stray power and RJ* loss in the shunt field winding. The 

 ordinates of the curved dotted line, measured from the horizontal 

 dotted line, represent the RJ* loss in the armature. 



J. 



Z 



fclf 



25 SO 75 100 13.5 



Ataperes output in % of full rated output 



Fig. 95. Efficiency and loss curves of a shunt generator. 



The maximum efficiency of a shunt or compound generator 

 occurs for that particular current output for which the sum of the 

 variable losses (series field loss and armature loss) is equal to the 

 sum of the constant losses (shunt field loss and stray power loss). 



Proof. Let L be the sum of the constant losses, and let E x be the terminal volt- 

 age of the machine. For the sake of simplicity let us assume I a = I c = I x and let 

 us represent this common current by /. This assumption is approximately true, inas- 

 much as the shunt field current is small as compared with the current output, except 

 when the current output is very small. Then we have : 



Power output = E X I. 



Armature loss -f- series field loss = (fi a -\- R c ] I* = RI 2 , where R is written 

 for J? a -4- R c . 



