196 ELEMENTS OF ELECTRICAL ENGINEERING. 



and /' the current demanded at b. Then A -f RI is the neces- 

 sary voltage between the a feeders at the station, and B + R' P is 

 the necessary voltage between the b feeders at the station, and, 

 since / and /' vary independently, A -\- RI cannot in general be 

 equal to B + R' I' . 



The maintenance of a prescribed constant voltage at each 

 center of distribution depends upon the proper control of the 

 voltage between each pair of feeders at the switch board. The 

 following methods of feeder control are feasible in direct-current 

 stations. Methods of feeder control employed in alternating sta- 

 tions are described in a subsequent volume of this text. 



(1) Rheostat method. One method of feeder control is to 

 insert a low resistance rheostat in each feeder circuit at the switch 

 board, to keep the voltage between the bus bars at the highest 

 value required by any of the feeders, and to lower the voltage 

 between each pair of feeders to the desired value by adjusting 

 these rheostats. 



(2) Combination three bus and rheostat method. The rheo- 

 stat method of feeder control involves a considerable loss of en- 

 ergy in the rheostats when many of the feeders require voltages 

 much below the bus bar voltage. This loss of energy may be 

 greatly reduced by operating the generators in two independent 

 sets, each set consisting of one or more generators operating in 

 parallel, so that one set may be operated at a voltage somewhat 

 higher than that of the other set. Then all of the feeders requir- 

 ing higher voltages may be supplied from the first set of gener- 

 ators, and all of the feeders requiring lower voltages may be sup- 

 plied from the second set of generators. In this arrangement the 

 controlling rheostats for adjusting the feeder voltages to the 

 exact desired values absorb but little power. 



The operation of the generators in two independent sets requires 

 only three bus bars inasmuch as one bus bar can serve as the 

 common positive (or negative) terminal of all of the generators. 



When the combination method of feeder control is used, one 

 wire of each pair of feeders is connected to the common positive 



