THE CRAVITY ALARM BATTERY.
CONTEND that the only way to instal dynamos for automatic fire alarm service is to have one dynamo for each circuit. With two or more circuits supplied from one dynamo, the grounding of one threatens the efficiency of the other, while, if two or more are groundded, the efficiency of the whole is impaired and possibly destroyed. In case one of the group should be grounded (and they often are) it should be at once disconnected and connected to a generator by itself until the trouble is removed In a central office system where operators are on duty at all times, it is possible to detect grounded circuits and disconnect them from the others before any harm results; but even then I can conceive of conditions under which it would be difficult, if not impossible to detect a leak from one circuit to the other, such a one might prevent the operation of alj the circuits on that particular dynamo at a most critical time. When more than one circuit is supplied from the same source, it is necessary to hold several dynamos in reserve in order to supply the circuits that are temporarily “grounded,” If dynamos are to be used in connection with the automatic repeater system of fire alarm, under no circumstances should more than one circuit be supplied by a dynamo unless an operator is present at all times for the purpose of testing for “grounds”and adjusting the “dead” resistance placed iu the circuits in case one or more is accidentally grounded or opened. The energy required for the longest circuit of the Boston fire alarm, ⅛ 28 watts or 245,280 watts per year of 365 days of 24 hours. The cost of producing this by means of a gravity battery would be $70. At 8 cents per thousand watts,the commercial rate for electric power, the cost would be $19.62. The result would be obtained only where the current was taken from the power circuits direct without any translating device such as dynamos or motors. Electrical energy cannot be transformed without a certain amount of loss, and such loss occurs in dynamos, motors, and accumulators. The type of motor-generator decided on for the Boston fire alarm service has a capacity of 250 watts, and its efficiency is 65 per cent., or, in other words, 65 per cent. Of the electrical energy supplied from the generating station is available for effective work. This would be 163 watts. There are 28 closed circuits iu the system; allowing 2.8 watts for each of them, which is not the case, the energy required would be 78.4 watts. All this could be supplied by one of these motor dynamos at 8 cents per k.w. The cost for power would be $175.20. To produce the same result from the battery would cost $1,950.00, about 89 per cent, more than the cost of dynamo current. This result is obtained with all the circuits connected to one dynamo, and only 31 per cent, of the energy received performing useful work.
If enough circuits were connected to require the total output of the dynamo —163 watts—we should obtain the following results. The cost at 8 cents per k. w. would remain the same—$175.20; but all the energy would now disappear in the circuits, as none of it would be dissipated in the form of heat in the resistance coils; but the cost for the same amount of energy produced from the battery would now be $4,075. The motor-generator has now obtained its maximum efficiency, and is producing the same amount of electrical energy at 4 per cent, of the cost of the battery. Under present existing circumstances such a result as this cannot be attained; but were the circuits ail placed underground and properly insulated there would be no difficulty iu doing so.
I will now go to the other extreme and attach but one dynamo to a circuit. In that case the cost of the battery would be $70; cost of dynamo $175.20 as before, but the cost of battery is only 40 per cent, of dynamo cost. Cost of battery for two circuits is $140.80 per cent, of the cost of the dynamo circuit. With three circuits the cost of battery would be $210 or $120 per cent, of cost of dynamo circuit.. It will be seen from this that the cost of producing electrical energy from the motor-generator is but 4 per cent, of the cost of producing the same amount front the battery, providing the dynamo is carrying its maximum load; but when carrying only 33 per cent, of that load, this order of things is reversed and the cost is 120 per cent, more than that of the battery.
There are many things to commend the dynamo for fire alarm besides the cost. It takes up less space; it is under complete control; the load can be increased or decreased at will; it responds instantly to changed conditions, and when properly proportioned and well constructed, it is perfectly reliable, but there are only a very few cities that can avail themselves of its benefits, and none of its full benefits,owning, first, to their inability to procure an unfailing supply of power; second, owing to the lack of perfect insulation of the fire alarm circuits and freedom from interruption by others—for to connect fire alarm circuits in multiple,multiple-series,or series-multiple, unless they are at all times free from “grounds” and kept so, is to invite disaster. It may be said, Why not use smaller dynamos—say 50 or 100 watts? The answer to that is that the smaller dynamos are very much less efficient and it would be “difficult to make the various parts sufficiently strong to meet the requirements of the continuous duty of fire alarm service; but it is possible to reduce their size somewhat. Bury the wires beneath the surface of the ground and you can then reap the full benefit of this new source of power.
The next possible source of supply is the storage battery or accumulator. This, like the motor dynamo, depends on the steam engine and dynamo-electric generator for the energy it supplies. It requires fully as much space as the gravity battery and double the amount of cells that is required to do the work must be secured, as it is necessary to charge one while discharging the other into the different circuits. It must be kept at an even temperature and in a clean, dry place. It should not be removed to the place of charging, but wires should be led into it from the power circuits from which the charging current is received.
The opportunities for receiving satisfactory data as regards the practical workings of the accumulators in Connection with fire alarm circuits are very limited indeed; but I have obtained what information 1 could on the subject from those having them in use. The city of Hartford, Conn., has been using the storage battery for several months. The power necessary for charging the same is obtained from the Hartford Electric Light Company and the amount required is 66 watts or a total of 578,160 watts per year. The cost per annum for this power is $63.00 oi .062 cents per kilowatt (a very low rate indeed). Estimating the output of a gravity cell at .04 watts,the number of cells necessary would be 16,50. The cost per annum of this would be $1650.00; at .08 watts the cost per annum would be $825.00; .08 watts per cell is the amount of energy required to operate the Gamewell Automatic Fire Alarm Telegraph. This would make the cost of producing 66 watts by means of the storage battery only about 4 per cent of that of the same amount from the gravity. But I find that the number of gravity cells displaced was but 300, at .08 watts, their total energy would be 24 watts, about 36 per cent, of the energy supplied to the storage battery,showing that the efficiency of the storage battery is but 36 per cent. At $1.00 per year per cell, the cost of the gravity battery is $300.00, or $1,43 perk, w., (a kilowatt is 1000 watts.) The cost of 578.160 watts,the amount required per annum,is $30.00 or but 12 per cent of the gravity battery; but something must be added for depreciation, and I will call it 6 per cent; this would make the annual cost of 578,160 watts at .062 per k. w., $65.70. at .10 per k. w., $87.57. at .15 per k. w.. $116.42: at .20 per k. w., $145.48, or 22, 29. 39,and 48 per cent of the cost of producing the game amount of energy from the gravity battery. The amount of energy delivered to the storage battery is66 watts, and the amount required to operate the fire alarm circuits is but 24 watts or about 36 percent of the energy received,about 64 watts being expended in the battery or between the battery and dynamo.
In making these estimates I have taken the dynamo plant of the Boston Fire Alarm System and the storage battery plant of the Hartford, Conn., as typical installations. As far as the experience of both has extended, I believe the figures are correct; the crucial test of time Back of this, and between the two engine runs, are the may necessitate a change therein. From the data obtain able, I am convinced that the cost, of producing electric energy for fire alarm purposes is much cheaper by the storage battery, and very much cheaper by the dynamo, provided it is operated under the most favorable circumstances than when produced by the gravity battery. In installing a storage battery, the watt 01 ampere capacity should be ample, to provide against any interruption of the power service. The price paid for electric power as applied to motors or for charging storage batteries ranges from 6 to 20 cents per k. w.
Hence result the following conclusions; When properly installed there is nothing equal to the motor-generator for a general office fire alarm service. With an automatic testing apparatus to test for “grounds” or other escapes and to switch defective circuits on to a spare dynamo to remain thereon until the defect was removed, the dynamo system could be operated at its highest rate of efficiency, reducing the cost of power to the figures given before, or lower, depending on the rate charged per k.w. by the power company. To operate a plant of this kind by small steam engine would not be advisable, as the cost would be greatly increased, due to the fact that small steam engines are very inefficient, and small dynamos also. With an automatic, device, such as described above, the motor-dynamo could safely be applied to the Gamewell automatic system of fire alarm so universally in use. The storage battery is within the reach of nearly every town and city that possesses a fire alarm telegraph system, as nearly all such have either electric light or railway plants, either of which can furnish the necessary power for charging, and, with a week’s supply of energy stored, any ordinary interruption of the power service need not be feared. The question of cost needs further consideration; its reliability and uniformity of action is fully established in other fields.