OPERATION OF THE DYNAMO AS A GENERATOR. 85 

 In case of the long shunt compound generator 



/.-'.-/i + >. 



as is evident when we consider, referring to Fig. 29, that the 

 whole armature current flows through the series field winding, 

 and then divides between the shunt field winding and the external 

 circuit. 



In case of the short-shunt compound generator 



as is evident when we consider, referring to Fig. 30, that the 

 armature current divides at the brushes, flowing partly through 

 the shunt field winding, and partly through the series field wind- 

 ing and the external circuit. 



47. Comparison of separately excited generator, shunt generator, 

 compound generator and series generator in regard to variations of 

 terminal voltage. Separately excited generator. The field cur- 

 rent of a separately excited generator is constant, so that the 

 armature flux <J> is constant except for the comparatively slight 

 decrease due to armature reaction when the machine is delivering 

 current. Therefore : 



(a) The total induced voltage E a , of a separately excited 

 generator falls off in proportion to the speed at which the machine 

 is driven, and the terminal voltage, E x which is always approxi- 

 mately equal to E a , likewise falls off nearly in proportion to the 

 speed. This is evident from the fundamental equation E a = QZ'n. 



(b) The external voltage E , of a separately excited generator 

 falls off with increase of current, on account of internal drop 

 RJ0 and on account of the lessening of <l> by armature reaction. 



Shunt generator. The field current of a shunt generator falls 

 off when the terminal voltage, E x , decreases, inasmuch as the 

 shunt field windings are connected between the terminals of the 

 machine. Therefore : 



(a) A decrease in speed of a shunt generator causes a much 

 greater decrease in E a and .. than in the case of the separately 



