A booster is a d.c. generator whose function is to inject or add certain voltage into a circuit so as to compensate the IR drop in the feeders etc.
A booster is essentially a series d.c. generator of large current capacity and is connected in series with the feeder whose voltage drop is to be compensated as shown in Fig. 1. It is driven at constant speed by a shunt motor working from the bus-bars. As the booster is a series generator, therefore, voltage generated by it is directly proportional to the field current which is here the feeder current. When the feeder current increases, the voltage drop in the feeder also increases. But increased feeder current results in greater field excitation of booster which injects higher voltage into the feeder to compensate the voltage drop. For exact compensation of voltage drop, the booster must be marked on the straight or linear portion of its voltage-current characteristics. It might be suggested to compensate the voltage drop in the feeder by over compounding the generators instead of using a booster. Such a method is not practicable for feeders of different lengths because it will disturb the voltage of other feeders. The advantage of using a booster is that each feeder can be regulated independently — a great advantage if the feeders are of different lengths.