Ques. Explain the use of the rheostat controlling the field excitation.
Ans. It is used to vary the booster voltage so as to hasten the charging of the battery if desired.
Fig. 1,195.—Entz' carbon pile booster system (Electric Storage Battery Co.). The booster field winding is connected at one end to the middle point of the battery. The other end is connected to the upper contact points of two carbon pile resistances, A and B. The lower end of A is connected to the negative side of the battery, and the corresponding end of B, to the positive side. This arrangement constitutes in effect a potentiometer. If the resistance of A be equal to that of B, there is no pressure in the booster field to establish current through it. The drop through A + B is equal to the total battery voltage, and if A = B, the drop from either side of the battery through A or B is one-half the total drop, hence the end of the booster field winding, connected to the upper ends of A and B is also at the pressure of the middle point of the battery which is likewise the pressure of the other side of the booster field coil. Accordingly when A = B, there can be no current through the coil. When the two resistances are unequal, there will be current through the booster field, its direction depending on which of the resistances is the less, and its magnitude will be proportional to the difference between the two resistances. Variations in the pressure on a carbon pile causes variations in its resistance and the solenoid, M, opposed by spring S, both pulling on lever L which rests on the two piles A and B, controls the relative resistances of the two piles to cause charge and discharge of the battery. The solenoid winding is in series with the dynamo circuit and when the load is normal, the spring pull is just equal to the magnet pull, and the resistance of A and B are equal. When external load varies, a small but proportional variation in the pull of P charges the relative resistances of the piles and the booster field is energized to produce a voltage to cause battery charge or discharge.
Ques. For what service is the shunt booster not suited?
Ans. It is not adapted to circuits where there are sudden fluctuations that are great compared with the capacity of the dynamo.
Ques. What is its action in changing from charge to discharge?
Ans. It is not automatic, the switching must be done by hand.