Fig. 1,201.—Diagram of differential booster system with compensating coil. In operation, the compensating field coil of the booster opposes the shunt coil and prevents the variation of the battery voltage disturbing the equilibrium of the system. If the battery pressure be lower than normal, it will not discharge rapidly enough to relieve the dynamo from overload fluctuations, unless the booster voltage be increased, and the dynamo will therefore have to supply a current greater than normal. If a current greater than normal flow through the compensating coil, the effect of the shunt coil opposed by the series coil is decreased, and the compensating coil, acting in the same direction as the series coil, causes a higher booster pressure tending to discharge the battery, and thus brings down the dynamo load to normal. Should the battery voltage be above its normal value, the battery would discharge too rapidly and carry more than its share of the load. In operating this system, the varying load must be beyond the booster equipment. The series and compensating coils may be temporarily short circuited so that the battery may be charged more rapidly.
Ques. For what service is the differential booster adapted?
Ans. It is suited to power and railway circuits where the load fluctuates widely and suddenly.
There are several varieties of this type of booster, and many patents have been issued covering the different methods of varying the voltage of the machine.
Constant Current Boosters.—In installations where it is desired to supply both an approximately constant load and a fluctuating load from the same dynamos (as for instance, in office buildings or hotels, where it is necessary to supply lights and elevators from the same source), the fluctuations in the power circuits must not interfere with the lighting circuits and to prevent this, two sets of bus bars are provided.
Fig. 1,202.—Diagram of non-reversible or constant current booster system. The booster armature and field are in series between one side of the lighting and power bus bars. A shunt field is also provided, which acts in opposition to the series field. This booster carries a practically unvarying current from the lighting to the power bus bars, regardless of the fluctuations of the external load, which current is equal to the average required by the fluctuating load. Except under abnormal conditions the shunt field always predominates giving a voltage which is added to that of the lighting bus bars, so that the voltage across the power busses is always higher than that across the lighting by an amount equal to the booster voltage. If an excessive load come on the power circuits, the increased excitation of the series coil, due to a slight increase in current from the lighting to the power bus bars, lowers the booster voltage and consequently reduces the voltage across the power bus bars. The battery discharges, furnishing an amount of current equal to the difference between that required by the load and the constant current through the booster. If the power load decrease below normal, the slight decrease in current in the booster series field increases the booster armature voltage and the excess current goes into the battery. The booster, therefore, does not in reality give a constant current, but by proper design the variation may be kept within a few per cent.
The dynamos are connected in the usual manner to one set of bus bars, and the lighting circuits are connected across these.