Fig. 1,180.—Storage battery connected in parallel with a dynamo. This arrangement enables the dynamo to be stopped for a considerable portion of the time, and thus save labor and attention. It also acts to prevent fluctuations as in a dynamo driven by a gas engine whose speed varies periodically because of the nature of its cycle of operation.
When the load is below the average, the dynamo charges the battery, and when the load rises above the average, during the hours of maximum demand, the battery discharges into the line in parallel with the dynamo. During the hours of minimum demand the engines may be shut down and the necessary current supplied from the battery alone, thus not only increasing the efficiency of the plant, but serving to maintain a steadier pressure under fluctuating loads.
Ques. What is understood by the expression "floating the battery on the line"?
Ans. A storage battery is said to float on a line when connected across the circuit at some distance from the power station, so that a heavy load on the line, within the range of the battery influence, causes sufficient line drop to allow the battery to discharge, while with a light load on the line, the drop is small and the impressed voltage at the battery high enough to charge the battery. This usage is confined chiefly to electric railway service, where large voltage changes are permissible.
Fig. 1,181.—Diagram showing effect of storage battery in regulating the dynamo load in a combined railway and lighting plant. In this case the average and line loads are about equal and the battery covers the instantaneous fluctuations. It will be noted that while the line load fluctuations vary between 780 and 1,420 amperes, those of the dynamo load are kept at an average between 1,030 and 1,160 amperes.
Ques. When the battery is floated on the line, how may the amount of charge be made to approximately equal the amount of discharge?
Ans. By properly proportioning the number of cells in series.