136 ELEMENTS OF ELECTRICAL ENGINEERING. 



ceiving circuit. The ratio of the useful electrical power delivered 

 by a generator, to the total electrical power developed in the 

 armature, is called the electrical efficiency* of the generator. 

 That is 



Electrical efficiency = - a/a ~^~^ = jj^f (32) 



a a 



The values of A and F may be calculated with the help of 

 equations (27), (28) and (29). 



The product of the two partial efficiencies is equal to the true 

 or commercial efficiency. That is, the efficiency of conversion is 

 the fraction of the total mechanical power which is converted into 

 electrical power, and the electrical efficiency is the fractional part 

 of the latter which is delivered to the receiving circuit. 



67. Variation of the efficiency of shunt and compound generators 

 with power output. The shunt field loss and the stray power loss 

 of a shunt or compound generator driven at constant speed are ap- 

 proximately constant. On the other hand, the armature loss plus 

 the series field loss is nearly proportional to the square of the cur- 

 rent output. The efficiency of the generator is therefore very 

 small when the power output is small, inasmuch as the constant 

 losses, namely shunt field loss and stray power loss, are then large 

 as compared with the power output. The efficiency of the gener- 

 ator increases with increase of output, passes through a maximum 

 value for a certain output, and then with further increase of out- 

 put, the efficiency decreases because of the rapid increase of arma- 

 ture loss. 



The efficiencies of a shunt or compound generator may be 

 calculated for a series of assigned values of power output by the 

 method explained in Art. 65, and a curve may then be plotted 

 showing power outputs as abscissas, and efficiencies as ordinates. 

 Such a curve is called the efficiency curve of the generator. Effi- 

 ciency curves are frequently plotted showing efficiencies as ordi- 

 nates, and current outputs in amperes as abscissas. 



* Sometimes called the economic coefficient of a generator. 



