86 ELEMENTS OF ELECTRICAL ENGINEERING. 



excited generator, because of the decrease of / which accompanies 

 the decrease of E x . 



(fr) An increase in current output of a shunt generator causes 

 a much greater decrease in E x than in the case of the separately 

 excited generator, because of the decrease of / which accompa- 

 nies the decrease of E x . 



The compound generator. (a) When a generator has a com- 

 pound field winding, the current output of the machine flows 

 through the series field coil,* and the field excitation of the 

 machine is therefore increased with increased current output. If 

 the series field coil has a sufficient number of turns of wire, this 

 increase of field excitation may not only counterbalance the 

 demagnetizing action of the current in the armature, but it may 

 more than counterbalance this demagnetizing action and cause <J> 

 to actually increase with increase of current output, thus increas- 

 ing the total induced electromotive force E a . 



1. This increase of E a can be made sufficient to compensate 

 for the internal drop, RJ a -\-RJ f , thus keeping E t approximately 

 constant with increase of current output. In this case the 

 machine is said to bz flat-compounded. 



2. This increase of E can be made more than sufficient to com- 



Q, 



pensate for the internal drop. In this case the voltage between 

 the terminals of the machine increases with increase of current 

 output, and the machine is said to be over-compounded. 



(U) The dependence of the voltage of a compound generator 

 upon speed is, for a given current output, nearly the same as in 

 the case of the shunt generator. 



Series generator. (a) The voltage of a series generator is zero 

 or nearly zero,f when the current output is zero, and it increases 

 rapidly with the current output. This is evident when we con- 

 sider that the field winding is in series with the external circuit. 



(b) For a given current output, the total induced voltage, E a , 



* This statement applies specifically to the short-shunt compound dynamo, 

 f Residual magnetism produces a small electromotive force even when the series 

 field current is zero. 



