DIRECT-CURRENT MACHINERY 221 



Batteries are also installed in electric railway substations to 

 prevent large variations of the load on the feeders supplying them 

 and so regulate the substation voltage. 



A third very important application of storage batteries is in 

 alternating-current power stations where they provide an auxiliary 

 supply of direct current in case of a breakdown of the exciters and 

 may thus prevent a shutdown of the whole system. 



Fig. 186 shows a battery connected across the terminals of a 

 shunt generator. At normal load the battery voltage and the gen- 

 erator voltage are equal and the battery floats on the line neither 

 giving nor receiving power. If, however, the load increases the 

 generator voltage falls and the battery discharges and supplies 



"=~ Battery 



Variable Load 



FIG: 186. Battery with shunt generator. 



part of the extra load and so relieves the generator and prevents 

 any large drop in its terminal voltage. The battery in this way 

 takes care both of sudden overloads and continuous overloads. 

 During periods of light load the generator voltage rises above the 

 battery voltage and the battery charges. 



135. Boosters. Boosters are direct-current generators con- 

 nected in series with the line to raise or lower the voltage; they 

 are used very extensively to regulate the charge and discharge of 

 storage batteries without changing the generator voltage. They 

 may be either shunt, series or compound wound. 



The shunt booster, Fig. 187, is an ordinary generator with its 

 field connected across the station bus bars and its armature con- 

 nected in series with the main generator. Its function is to raise 

 the voltage impressed on the battery in order to send current into 

 it to charge it. The voltage of the booster is controlled by a field 

 rheostat. 



Compound boosters are automatic in their action and are di- 

 vided into two classes, non-reversible and reversible, depending 

 on the relative strengths of their shunt and series windings. 



