222 



ELECTRICAL ENGINEERING 



Fig. 188 shows a non-reversible automatic booster. The shunt 

 field / is connected across the station bus bars. The series field s 

 carries the load current of the generator and it opposes the shunt 

 field but has at all times a smaller m.m.f. and thus the booster 

 voltage is always in the direction of the generator voltage. 



!=: Battery 



Load 



FlG. 187. 



Booster 



Shunt booster. 



When the load current increases an increase of current through 

 s decreases the booster voltage and allows the battery to discharge; 

 when the load decreases the booster voltage rises and causes the 

 battery to charge. The current from the generator remains prac- 

 tically constant regardless of the fluctuations of load. 



Load 



FIG. 188. Compound booster. 



The reversible booster is similar in construction to the non- 

 reversible booster but has a stronger series field. At normal load 

 the shunt and series fields are of equal strength and the booster 

 voltage is zero. The battery then floats on the line and neither 

 charges nor discharges. An increase of load above normal in- 

 creases the strength of s and overpowers / and so discharges the 

 battery. When the load decreases / becomes stronger than s and 

 causes the battery to charge. 



