Fig. 1,201.—Diagram of differential booster system with compensating coil. In operation, the compensating field coil of the booster opposes the shunt coil and prevents the variation of the battery voltage disturbing the equilibrium of the system. If the battery pressure be lower than normal, it will not discharge rapidly enough to relieve the dynamo from overload fluctuations, unless the booster voltage be increased, and the dynamo will therefore have to supply a current greater than normal. If a current greater than normal flow through the compensating coil, the effect of the shunt coil opposed by the series coil is decreased, and the compensating coil, acting in the same direction as the series coil, causes a higher booster pressure tending to discharge the battery, and thus brings down the dynamo load to normal. Should the battery voltage be above its normal value, the battery would discharge too rapidly and carry more than its share of the load. In operating this system, the varying load must be beyond the booster equipment. The series and compensating coils may be temporarily short circuited so that the battery may be charged more rapidly.
Ques. For what service is the differential booster adapted?
Ans. It is suited to power and railway circuits where the load fluctuates widely and suddenly.
There are several varieties of this type of booster, and many patents have been issued covering the different methods of varying the voltage of the machine.
Constant Current Boosters.—In installations where it is desired to supply both an approximately constant load and a fluctuating load from the same dynamos (as for instance, in office buildings or hotels, where it is necessary to supply lights and elevators from the same source), the fluctuations in the power circuits must not interfere with the lighting circuits and to prevent this, two sets of bus bars are provided.
Fig. 1,202.—Diagram of non-reversible or constant current booster system. The booster armature and field are in series between one side of the lighting and power bus bars. A shunt field is also provided, which acts in opposition to the series field. This booster carries a practically unvarying current from the lighting to the power bus bars, regardless of the fluctuations of the external load, which current is equal to the average required by the fluctuating load. Except under abnormal conditions the shunt field always predominates giving a voltage which is added to that of the lighting bus bars, so that the voltage across the power busses is always higher than that across the lighting by an amount equal to the booster voltage. If an excessive load come on the power circuits, the increased excitation of the series coil, due to a slight increase in current from the lighting to the power bus bars, lowers the booster voltage and consequently reduces the voltage across the power bus bars. The battery discharges, furnishing an amount of current equal to the difference between that required by the load and the constant current through the booster. If the power load decrease below normal, the slight decrease in current in the booster series field increases the booster armature voltage and the excess current goes into the battery. The booster, therefore, does not in reality give a constant current, but by proper design the variation may be kept within a few per cent.
The dynamos are connected in the usual manner to one set of bus bars, and the lighting circuits are connected across these.
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.
Fig. 1,204.—Battery system with regulation for long feeders, for installing where it is desirable to locate the battery at a point remote from the station and avoid any equipment requiring constant attention at the battery end. The compound wound motor, constant current booster is used and keeps constant the current flowing through the feeder, the battery taking up all load fluctuations.
With an increase in external load above the average, the tendency is for an increase to take place through the exciter series coil, augmenting its field strength and consequently the exciter armature voltage. This latter now being higher than that of the line, causes current to flow in the booster field coil in such a direction as to produce a pressure in the booster armature which assists the battery to discharge, and is of a magnitude to compensate for the battery drop occasioned thereby.
Fig. 1,205.—Diagram illustrating storage battery system, as applied to an automobile for lighting.
When the load decreases below the normal, the current in the exciter field is decreased, and its armature voltage falls below that of the system. Current will now flow in an opposite direction in the booster field coil, generating a voltage in the booster armature to assist charge. Since the exciter always generates a voltage in opposition to that of the line, this system is known commercially as the counter pressure system.
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ELECTRICAL GUIDE, NO. 1
Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.
ELECTRICAL GUIDE, NO. 2
The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.
ELECTRICAL GUIDE, NO. 3
Electrical Instruments, Testing, Practical Management of Dynamos and Motors.
ELECTRICAL GUIDE, NO. 4
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.
ELECTRICAL GUIDE, NO. 5
Principles of Alternating Currents and Alternators.
ELECTRICAL GUIDE, NO. 6
Alternating Current Motors, Transformers, Converters, Rectifiers.
ELECTRICAL GUIDE, NO. 7
Alternating Current Systems, Circuit Breakers, Measuring Instruments.
ELECTRICAL GUIDE, NO. 8
Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.
ELECTRICAL GUIDE, NO. 9
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
ELECTRICAL GUIDE, NO. 10
Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.
Theo. Audel & Co., Publishers 72 FIFTH AVENUE.
1 NOTE.—The term impedance means the total opposition in an electric circuit to the flow of an alternating current, being made up of the actual or ohmic resistance and the apparent resistance due to self-induction, or if the circuit contain also capacity, the resultant apparent resistance due to self-induction and capacity.
2 NOTE.—The international ohm ÷ B. A. ohm = 1 ÷ .9866. The B. A. ohm ÷ International ohm = 1 ÷ 1.0136. Hence, to reduce British Association ohms to International ohms, divide by 1.0136. or multiply by .9866.
3 This size can be used only in the shape of flexible cord.
4 CAUTION.—The size thus obtained should be compared with the table of carrying capacity of wires as given on page 731 to see if the wires would have to carry more than the allowable current.
5 NOTE.—In case a larger loss than any given in the table is required, proceed as follows:—Divide the ampere feet by 10 and then refer to column of Actual Volts Lost divided by 10, from which the size of wire is found as before.
6 NOTE.—Specific gravity is the weight of a given substance relative to an equal bulk of some other substance which is taken as a standard of comparison. Water is the standard for liquids. In the laboratory the specific gravity bottle is often used in determining the specific gravity of a liquid. The capacity of the bottle is 1,000 grains of pure water. When it is filled with spirits of wine and weighed in a balance (together with a counterpoise for the weight of the bottle, which of course is constant), it will weigh considerably less than 1,000 grains; in fact, the bottle will contain only about 917 grains of proof spirit; therefore, taking the specific gravity of water as unity, 1 or 1.000, the specific gravity of spirits of wine is 0.917. If, on the other hand, the bottle be filled with sulphuric acid, it will weigh about 1,850 grains; hence, the specific gravity of sulphuric acid is said to be 1.850. A more convenient method for the automobilist is by the use of the hydrometer.
7 NOTE.—If the active material in the negative plates extend beyond the ribs of the grid (the supporting frame), it should be at once pressed back into place, care being taken to prevent the plates drying before this is done. The most suitable and convenient method for pressing, is to place between the plates smooth boards of a thickness equal to the distance between the plates and then put the groups under pressure.
8 NOTE.—The voltage increase or decrease with change in current is practically constant in a given type of cell for any size of cell when the current is referred to a given time rate of charge or discharge; that is, the drop in a large cell or in a small cell, when each is discharged at its four, six or eight hour rate, will be the same. The drop varies somewhat for the condition of the battery charge. For batteries which are one-third discharged, the temperature 60° Fahr., and plates in good condition, the changes in pressure which may be expected between open circuit voltage and the voltage on charge or discharge are given in the above table.
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