358 



ELECTRICAL EQUIPMENT 



exciters will, as a rule, be of a rather slow speed and thus more 

 expensive per Kw. than water-wheel-driven units, the difference, 

 however, diminishing as the head and number of units increase. 

 On the other hand, water-wheel-driven exciters involve the cost 

 of the hydraulic equipments, and besides the additional expense 

 of the building caused by the space occupied by these units. The 

 efficiency is, however, mostly in favor of direct-connected units. 



The general practice is to provide one or two sets of common 

 bus-bars to which all the exciters are connected in parallel and 



Equalizer 

 ' Volt 

 Auxiliary 



FIG. 213. System of Exciter Connections. 



from which the fields of the different generators are excited, a 

 rheostat being inserted in each field circuit. 



In the arrangement shown in Fig. 212 there are three exciters, 

 two of which are water-wheel-driven, the reserve being motor- 

 driven. Only one set of exciter bus-bars is shown, although 

 frequently an auxiliary set is also installed. The equalizer con- 

 nection and the exciter shunt fields are left out so as to simplify 

 the diagram. Means are provided for sectionalizing the bus, as 

 shown. Power for the induction motor is taken from the main 

 bus, and any number of motors can be started by one compen- 

 sator if a common running and starting bus is provided. 



Fig. 213 represents a comparatively large system with not less 

 than six direct-connected exciters operating in parallel. There 

 are two sets of bus-bars, one for excitation and the other for 

 emergency or auxiliary service, and switches are provided so that 

 the exciters can be connected to either set as desired. One exciter 



