CHARACTERISTIC CURVES. 



397 



erator to build up in the reverse direction. During this time the 

 auxiliary machine is acting as a generator, so that it quickly comes 

 to rest as the kinetic energy of its rotating armature is exhausted, 

 and the reversed current, which is now maintained by the reversed 

 series generator, starts the auxiliary machine in the reverse 

 direction as a motor. The auxiliary machine then speeds up 

 until the line PPP falls below the point Q h ', Fig. 29, when 

 the series generator again loses its field magnetism, which leads 

 to another reversal as before, and so on. 



29, Characteristics of the shunt generator. The experimental 

 determination of the characteristic curve of the shunt generator 

 is accomplished in essentially the same manner as in the case of 

 the series generator. 



The full line curve, Fig. 30, is a typical external characteristic 

 of a shunt generator ; the abscissas represent values of current 

 delivered to the receiving cir- 

 cuit, and the ordinates repre- 

 sent the corresponding values 

 of the electromotive force be- 

 tween the generator ter- 

 minals. The upper portion 

 cc is the part of the charac- 

 teristic on which the machine 

 usually operates in practice. 



The internal or total char- 

 acteristic is the curve of which 

 the abscissas represent the 

 total current / in the arma- 



a 



ture, and of which the ordinates represent the corresponding 

 values of induced electromotive force E a (= < Z'ii). 



Consider a point P on the external characteristic. The cor- 

 responding point P' on the total characteristic is found by increas- 

 ing the abscissa by /, the value of the shunt field current, and 

 increasing the ordinate by R a l a , which is the electromotive force 



Fig. 30. 



