EXCITERS 



359 



can, if necessary, be connected to the auxiliary bus while the others 

 are operating on the field-bus. As previously stated, however, it 

 is not considered good practice to use the excitation system for 

 the auxiliary service. To provide spare capacity for the system 

 shown, a motor-driven exciter can be installed, feeding the aux- 

 iliary bus and the field switches made double-throw instead. 



In very large plants the general tendency is to so arrange the 

 system that each generating unit shall form a complete plant in 

 itself, capable of independent operation, although normally the 

 units are operated in parallel. Each generator is, therefore, 



n 



f 1 Auxiliary Generators 

 A_ with Direct 

 (J~ Conn. Exciter* 



FIG. 214. System of Exciter Connections. 



provided with its individual exciter, which may be. either direct 

 connected, as previously described, or also motor driven with 

 power from a separate source. Which system is the most eco- 

 nomical and advantageous depends entirely on the conditions. 



There are two large systems in operation which use the latter 

 scheme, differing only somewhat with respect to the power supply, 

 which, however, is entirely independent in either case. 



One of these arrangements l is illustrated by the representative 

 diagram in Fig. 214. The exciters, which have a capacity cor- 

 responding to that required by their respective generators, are 

 not operated in parallel, but have their terminals connected 

 directly to the generator fields through the collector rings. The 

 regulation is accomplished by adjusting the exciter fields (see 

 Voltage Regulation), thus eliminating large field rheostats in the 



1 Mississippi River Power Co. 



