Fig. 1,193.—Diagram of Joseph Bijur's storage battery system (General Storage Battery Co.). The booster field winding has one terminal connected to the middle point of the battery and the other terminal, to the wire joining the resistances A and B. A lever, pivoted at L, carries at either end a number of contact points which dip into troughs of mercury when one end of the lever moves upward or downward. These points are connected to corresponding points on their respective resistances, and therefore all of the resistances connected to contact points which are immersed in the mercury are short circuited. The points are of various lengths, so that when the lever operates, they contact progressively with the mercury. If more of the A points than the B points be immersed in the mercury, the resistance of B is less than that of A, more sections of it being short circuited. Current will therefore flow from the middle point of the battery, through the booster field, and through B to the negative side of the system, exciting the booster field and producing a booster voltage to charge the battery. Again, if more of the A points be immersed, the A resistance becomes the smaller, and current then flows from the positive side of the system through resistance A, through the booster field to the middle point of the battery, the field excitation and the booster pressure produced being in a direction opposite to the first described, and tending to discharge the battery. When the resistances A and B are equal, there is no pressure to send current in either direction through the booster field coil. When the load on the external circuit is normal, the lever is in a horizontal position, A and B being equal, no current flows through the booster field hence, no current passes into or out of the battery. With increase of external load, the pull of the solenoid is strengthened by a small increase in dynamo current passing through the winding. This draws down the left end of the lever producing a current in the booster field such as to discharge the battery and assist the dynamo to supply the load demand. A decrease in external load is attended by a slight diminution in dynamo current, the solenoid is weakened and the pull of the spring predominates. This results in a downward movement of the right side of the lever causing excitation of the booster field to produce a pressure to send charge into the battery.
Ques. Describe some characteristics of the series booster.
Ans. It is automatic and adjusts its voltage to produce the proper ratio of charge or discharge with varying external load, and it also tends to maintain a constant voltage across the line, under all conditions of change in circuit.
Fig. 1,194.—Load diagram, showing kind of service to which the shunt booster is adapted.
Shunt Boosters.—This type of machine is simply a shunt dynamo, having its armature circuit in series with the line from the main dynamo to the battery. A rheostat controls the field excitation. Its function is to send charge into the battery. It is used in plants where the battery is not designed to take up load fluctuations, but is in service only to carry the peak of the load, being charged during periods of light loads and discharged in parallel with the dynamo.
The shunt booster acts to increase the voltage applied to the battery so that the charging current will flow into the latter.
Ques. How is a battery used with a shunt booster proportioned?
Ans. Usually sufficient battery is provided to carry the entire load during the light load period.