In discharging, the current passes through the series field of the booster and produces a proportional pressure acting with the shunt field to raise the voltage of the booster, thus increasing the battery discharge and shifting more of the load from the dynamo, until the system becomes balanced.
Fig. 1,199.—Diagram showing method of charging a storage battery at one voltage and supplying lights at a different voltage. As may be seen, two end cell switches are required. The voltage of the supply current is adjusted by the number of cells in series on switch S', while switch S is moved to cut out cells as they become fully charged. In this instance the end cells included between the contact arms of the two end cell switches must be of sufficient size to receive the charging current, plus the current to the supply circuit. If the battery can be charged at times when the dynamo is supplying no other load, only one end cell switch is required.
If the load on the external circuit be small, the dynamo voltage rises and current flows into the battery. In this case the series field acts against the shunt field and decreases the booster voltage so that the pressure at the dynamo is greater than booster and battery voltage combined, thus increasing the rate of charge of the battery until the load causes the dynamo voltage to drop to normal and the system is again balanced.
The battery and booster can be placed at the power house or where the greatest drop is likely to occur. As this system, like the series booster, depends for its action upon the drop of voltage with increase of load, it is only adapted to shunt wound dynamos.
From the foregoing description it will be seen that the compound booster is automatic within certain limits of battery charge. Any marked change of battery voltage will be followed by a corresponding change in dynamo current, unless the rheostat be manipulated to bring battery voltage + booster voltage back to normal.
While the theoretical dynamo current variation is small for a given change of load, there is always a sudden, momentary, current rush from the dynamo on increase of load, the duration of which is equal to the time lag of magnetization of the booster field.
Lights on a circuit with variable load will "wink" on sudden changes of load. In this respect the compound booster is not so satisfactory as the constant current booster, as in the latter all dynamo current passes through the series fields, which, by reason of their self-induction, oppose and check any sudden current rush, giving the booster field time to change its magnetization to the proper degree.
Fig. 1,200.—Diagram of connection of one form of differential booster. In operation, the dynamo current passes through the series winding of the booster, and the current in this winding is to remain practically constant. The shunt coil produces a field which opposes the field produced by the series coil, the resulting magnetization being, in direction and amount, the resultant of the two field strengths. The adjustments are so made that when the normal dynamo current is passing through the series coil, the shunt field just neutralizes its effect, and the resultant magnetization is zero. Since the open current voltage of the battery is equal to that of the system, neither charge nor discharge takes place. With increased demand on the line, the slight increase in dynamo current in the series coil overpowers the shunt field, and causes a pressure in the booster armature in such direction as to assist discharge. If the external load fall below the average demand, the current in the series coil decreases slightly so that the shunt field predominates, producing a booster armature pressure in a direction to assist charge. Although the voltage of the battery falls while discharging by an amount proportional to the outflowing current the increased excitation due to this current through the series coil is also proportional to it, and the booster voltage rises as that of the battery falls, their sum being always equal to that of the system. In other words, the booster serves to compound the battery for constant pressure.
Differential Boosters.—In this type of booster, a series coil energized from the main current, tends to discharge the battery, and a shunt coil, excited from the battery, tends to charge the cells. These two coils are opposed to one another, and the difference in their respective strengths represents the net strength available for boosting. In order to produce quicker reversal, additional compound coils are sometimes added.