EXCITERS 



357 



Exciters direct connected to each of the main units are, as a 

 rule, used in plants having a small number of units. They are, of 

 course, affected by the speed fluctuations of the main units, and 

 at runaway speeds they may cause over-voltages amounting to 

 two or three times the normal voltage, thus greatly endangering 

 the apparatus on the system. Such over-voltages must, there- 

 fore, be guarded against either by providing means for artificially 

 loading the system, should the outside load drop, or, preferably, 

 by providing high-voltage cut-out relays, which will automatically 

 insert resistance in the exciter field circuits and thus prevent an 

 excess voltage rise. 



Where two or three units are used, each exciter should have a 

 capacity sufficient to excite two generators, while with four or 



I Main 

 x^'Generatori 



Running Bus 

 Starting Bus 



FIG. 212. System of Exciter Connections. 



more units it will undoubtedly be more advantageous to make the 

 capacity of each exciter correspond to the excitation require- 

 ments of one generator and provide a motor-driven exciter for 

 spare. This may then have the same capacity as one of the direct- 

 connected exciters or, for larger stations, it may have twice the 

 capacity or two sets may be installed. 



The economical question should, of course, also be consid- 

 ered in deciding between the two systems. Direct-connected 



