THE ALTERNATOR. 



II 



the field magnet of an alternator is so excited, there is a tendency 

 for the electromotive force between the terminals of the alterna- 

 tor to fall off with increase of current output, so that it is neces- 

 sary to have a switch-board attendant to adjust the exciting cur- 

 rent with changes of load on the alternator in order to keep the 

 terminal voltage of the alternator constant. 



In the case of a direct-current shunt generator, the variations 

 of terminal voltage with change of current output may be coun- 

 teracted by using an auxiliary (series) field winding through 

 which the current delivered by the generator flows, thus provid- 



Fig. 13. 



ing for an automatic increase of field excitation with increase of 

 load. A direct-current generator provided with both shunt and 

 series field windings is called a compound generator. 



In order to make use of the current delivered by an alternator 

 for automatically increasing its field excitation with increase of 

 current output, it is necessary to interpose a device for reversing 

 the connections of the series field windings at each reversal of the 

 alternating current so that the current may flow always in the 

 same direction in the series field coil and thus have the desired 



