Across the other set of bars are connected the circuits supplying the fluctuating load, and the battery is also connected directly across these power bars.
The power bars are supplied with current from the lighting bars, a non-reversible or so called constant current booster being interposed between the two as shown in [fig. 1,202]. Since this permits only a constant current to pass from the lighting bus bars, the load on the dynamo does not vary, although the load on the power busses may vary widely.
Fig. 1,203.—Hubbard's separately excited booster system (Gould Storage Battery Co.); diagram showing general arrangement.
Separately Excited Boosters.—In some forms of booster the field excitation is secured by a small exciting dynamo. An example of this class is shown in [fig. 1,203]. The exciter is provided with a single series coil, through which the station output or a proportional part thereof passes. The armature of the exciter is connected to the exciting coil on the booster, and thence across the mains as shown.
NOTE.—Reversible boosters should be used where the average total current to the fluctuating load is greater than the battery discharge current, and where the pressure of the power bus bars must not fall off with increase in load. Electric railway and lighting plants having long feeders are examples of the systems to which reversible boosters are suited. Non-reversible boosters should be used where the average total load is less than the battery discharge current, and where a drop in the voltage of the power bus bars is of advantage. Examples of such plants are hotels or apartment houses where electric elevators are operated from the lighting dynamos. Boosters are usually driven by electric motors directly connected to them, though any form of driving power may be used.
With the average current passing through the field coil or the exciter, its armature generates a voltage which is equal to that of the system, and in opposition to it. These two opposing pressures balance, and no current flows in the booster field coils.
