398 



ELEMENTS OF ELECTRICAL ENGINEERING. 



Fig. 31. 



lost in overcoming the resistance of the armature. The values of 

 f t , corresponding to different values of E xt are proportional to 

 E x (= f g R s ), and may be represented by the abscissas of the 



straight line OC\ the value 

 of RJ0 for various values of 

 I a , may be represented by the 

 ordinates of the straight line 

 OB\ and the total character- 

 istic may be constructed from 

 the external characteristic by 

 first increasing the abscissa 

 of each point P, Fig. 30, by 

 an amount equal to the cor- 

 responding abscissa of the 

 line OC, and then increasing 

 the ordinate of the point P n 

 so reached by an amount equal to the corresponding ordinate of 

 the line OB, thus reaching the point P r on the total charac- 

 teristic curve. 



The effect of residual magnetism upon the characteristic curve 

 of a shunt generator is to cause the external characteristic to 

 intersect the ampere-axis to the right of the origin, as shown in 

 Fig. 31- 



30. The ampere-ohm characteristic of the shunt generator. For 

 some purposes it is convenient to represent the current output of 

 a shunt generator as a function of the resistance of the receiving 

 circuit by plotting the values of current output as ordinates, and 

 the corresponding values of resistance of the receiving circuit as 

 abscissas. Fig. 32 shows a typical ampere-ohm characteristic 

 of a shunt generator. This curve may be derived from the volt- 

 ampere characteristic, Fig. 30, exactly as in the case of the series 

 generator, as explained in Art. 27. 



31, The voltage-speed characteristic of the shunt generator at 

 zero load. The relation between voltage and speed of a shunt 



