Detail of simple hanging lamp supported by rosette
Official Inspection
In all communities, your insurance agent must inspect and pass your wiring before you are permitted to throw the main switch and turn on the electricity. Frequently they require that the moulding be left uncapped, until they have inspected it. If you have more than 660 watts in lamps to a circuit; if your joints are not soldered and well taped; if the moulding is used in any concealed or damp place, the agent is liable to condemn your work and refuse permission to turn on the electricity. However the rules are so clearly defined that it is difficult to go wrong; and a farmer who does his own wiring and takes pride in its appearance is more apt to be right than a professional electrician who is careless at his task. After the work has been passed, tack on the moulding capping, with brads, and paint the moulding to match the woodwork.
Wooden moulding wiring is perfectly satisfactory if properly installed. It is forbiddenin many large cities, because of the liability of careless workmanship. It should never be installed in damp places, or out of sight. If the work is well done, the system leaves nothing to be desired; and it has the additional advantage of being cheap, and easily done by any farmer who can use carpenter tools. Farmers with moulding machinery can make their own moulding. The code prescribes it shall be of straight-grained wood; that the raceways for the wires shall be separated by a tongue of wood one-half inch wide; and that the backing shall be at least 3/8 inch thick. It must be covered, inside and out, with at least two coats of moisture-repellant paint. It can be had ready-made for about 2 cents a foot.
Special Heating Circuits
If one plans using electricity for heavy-duty stoves, such as ranges and radiators, it is necessary to install a separate heating circuit. This is the best procedure in any event, even when the devices are all small and suited tolamp circuits. The wire used can be determined by referring to the table for carrying capacity, under the column headed "rubber-covered." A stove or range drawing 40 amperes, would require a No. 4 wire, in moulding. A good plan is to run the heating circuit through the basement, attaching it to the rafters by means of porcelain knobs. Branches can then be run up through the floor to places where outlets are desired. Such a branch circuit should carry fuses suitable to the allowed carrying capacity of the wire.
Knob and Cleat Wiring
Knob and cleat wiring, such as is used extensively for barns and out-buildings, requires little explanation. The wires should not be closer than 2½ inches in open places, and a wider space is better. The wires should be drawn taut, and supported by cleats or knobs at least every four feet. In case of branch circuits, one wire must be protected from the other it passes by means of a porcelain tube. It should never be used in dampplaces, and should be kept clear of dust and litter, and protected from abrasion.
Knob and cleat wiring
Knob and tube wiring is frequently used in houses, being concealed between walls or flooring. In this case, the separate wires are stretched on adjoining beams or rafters, and porcelain tubes are used, in passing through cross beams. For a ceiling or wall outlet, a spliced branch is passed through the plaster by means of porcelain tubes or flexible loom.
Wires from the house to the barn should be uniform with transmission wires. At the point of entry to buildings they must be at least six inches apart, and must take the form of the "drop loop" as shown in the illustration. A double-pole entrance switch must be provided, opening downward, with a double-pole fuse. In passing over buildings wires must not come closer than 7 feet to flat roofs, or one foot to a ridge roof. Feed-wires for electric motors should be determined from the table of safe carrying capacities, and should be of liberal size.
THE ELECTRIC PLANT AT WORK
Direct-connected generating sets—Belt drive—The switchboard—Governors and voltage regulators—Methods of achieving constant pressure at all loads: Over-compounding the dynamo; A system of resistances; (A home-made electric radiator); Regulating voltage by means of the rheostat—Automatic devices—Putting the plant in operation.
Direct-connected generating sets—Belt drive—The switchboard—Governors and voltage regulators—Methods of achieving constant pressure at all loads: Over-compounding the dynamo; A system of resistances; (A home-made electric radiator); Regulating voltage by means of the rheostat—Automatic devices—Putting the plant in operation.
Dynamos may be connected to water wheels either by means of a belt, or the armature may spin on the same shaft as the water wheel itself. The latter is by far the more desirable way, as it eliminates the loss of power through shafting and belting, and does away altogether with the belts themselves as a source of trouble. An installation with the water wheel and armature on the same shaft is called a "direct-connected set" and is of almost universal use in large power plants.
To be able to use such a direct-connectedset, the dynamo must be designed to develop its full voltage when run at a speed identical with that of the water wheel. That is, if the dynamo is wound to be run at a speed of 800 revolutions per minute, it must be driven by a water wheel which runs at this speed and can be governed within narrow limits. Small impulse wheels running under great heads attain high speed, and for such wheels it is possible to obtain a suitable dynamo at low cost. For instance, a 12-inch impulse wheel, running under a 200-foot head will develop 6¾ horsepower when running at a speed of 875 revolutions per minute. A dynamo for direct coupling to such a wheel should have a rated speed within 5 per cent of 875 r.p.m.; and, as generators of this speed are to be had from the stock of almost all manufacturers, there would be no extra charge.
When it comes to the larger wheels, however, of the impulse type, or to turbines operating under their usual head the question becomes a little more difficult. In such cases, the speed of the water wheel will varyfrom 150 revolutions per minute, to 400, which is slow speed for a small dynamo. As a general rule, the higher the speed of a dynamo, the lower the cost; because, to lower the speed for a given voltage, it is necessary either to increase the number of conductors on the armature, or to increase the number of field coils, or both. That means a larger machine, and a corresponding increase in cost.
In practice, in large plants, with alternating-current machines it has become usual to mount the field magnets on the shaft, and build the armature as a stationary ring in whose air space the field coils revolve. This simplifies the construction of slow-speed, large-output dynamos. Such a machine, however, is not to be had for the modest isolated plant of the farmer with his small water-power.
Instantaneous photograph of high-pressure water jet being quenched by buckets of a tangential wheel
A tangential wheel, and a dynamo keyed to the same shaft—the ideal method for generating electricity. The centrifugal governor is included on the same base
Dynamos can be designed for almost any waterwheel speed, and, among small manufacturers especially, there is a disposition to furnish these special machines at little advance in price over their stock machines. Frequently it is merely a matter of changing the winding on a stock machine. The farmer himself, in many cases, can re-wind an old dynamo to fit the speed requirements of a direct-connected drive if the difference is not toogreat. All that would be necessary to effect this change would be to get the necessary winding data from the manufacturer himself, and proceed with the winding. This data would give the gauge of wire and the number of turns required for each spool of the field magnets; and the gauge of wire and number of turns required for each slot in the armature. The average boy who has studied electricity (and there is something about electricity that makes it closer to the boy's heart than his pet dog) could do this work. The advantages of direct drive are so many that it should be used wherever possible.
When direct drive cannot be had, a belt must be used, either from a main shaft, or a countershaft. The belt must be of liberal size, and must be of the "endless" variety—with a scarfed joint. Leather belt lacing, or even the better grades of wire lacing, unlessvery carefully used, will prove unsatisfactory. The dynamo feels every variation in speed, and this is reflected in the lights. There is nothing quite so annoying as flickering lights. Usually this can be traced to the belt connections. Leather lacing forms a knot which causes the lights to flicker at each revolution of the belt. The endless belt does away with this trouble. Most dynamos are provided with sliding bases, by which the machine can be moved one way or another a few inches, to take up slack in the belt. To take advantage of this, the belt must be run in a horizontal line, or nearly so. Vertical belting is to be avoided.
The dynamo is mounted on a wooden base, in a dry location where it is protected from the weather, or dampness from any source. It must be mounted firmly, to prevent vibration when running up to speed; and the switchboard should occupy a place within easy reach. Wires running from the dynamo to the switchboard should be protected from injury, and must be of ample size to carrythe full current of the machine without heating. A neat way is to carry them down through the flooring through porcelain tubes, thence to a point where they can be brought up at the back of the switchboard. If there is any danger of injury to these mains they may be enclosed in iron pipe. Keep the wires out of sight as much as possible, and make all connections on the back of the switchboard.
The Switchboard
Connecting switchboard instruments
The switchboard is constructed of some fireproof material, preferably slate or marble. When the cost of this material is an item to consider, build a substantial wooden frame for your switchboard. You can then screw asbestos shingles to this to hold the variousinstruments and with a little care such a switchboard can be made to look business-like, and it is fully as serviceable as the more expensive kind. The switchboard instruments have already been described briefly. They consist of a voltmeter (to measure voltage); an ammeter (to measure the strength of the current drawn, in amperes), a rheostat (to regulate the voltage of the machine to suit the individual requirements); and the usual switches and fuses. The main switch should be so wired that when open it will throw all the current off the line, but still leave the field coils, the voltmeter, and the switchboard lamp in circuit. The main-switch fuses should have a capacity about 50 per cent in excess of the full load of the dynamo. If the machine is rated for 50 amperes, 75-ampere fuses should be installed. This permits throwing on an overload in an emergency; and at the same time guards against a short circuit. If the capacity of the machine is under 30 amperes, plug fuses, costing 3 cents each, can be used. If it isabove this capacity, cartridge fuses, costing a little more, are required. A supply of these fuses should be kept handy at all times.
Governors and Voltage Regulators
A centrifugal governor(Courtesy of the C. P. Bradway Company, West Stafford, Conn.)
The necessity for water wheel governors will vary with conditions. As a general rule, it may be said that reaction turbines working under a low head with a large quantity of water do not require as much governing as the impulse wheel, working under high heads with small quantities of water. When governing is necessary at all, it is because the prime mover varies in speed from no load to full load. Planning one's plant with a liberal allowance of power—two water horsepower to one electrical horsepower is liberal—reduces the necessity of governors to aminimum. As an instance of this, the plant described in some detail in Chapters One and Six of this volume, runs without a governor.
However, a surplus of water-power is not usual. Generally plants are designed within narrow limits; and then the need of a governor becomes immediately apparent. There are many designs of governors on the market, the cheapest being of the centrifugal type, in which a pair of whirling balls are connected to the water wheel gate by means of gears, and open or close the gate as the speed lowers or rises.
Constant speed is necessary because voltage is directly dependent on speed. If the speed falls 25 per cent, the voltage falls likewise; and a plant with the voltage varying between such limits would be a constant source of annoyance, as well as expense for burned-out lamps.
Since constant voltage is the result aimed at by the use of a governor, the same result can be attained in other ways, several of which will be explained here briefly.
Over-Compounding
(1) Over-compounding the dynamo. This is simple and cheap, if one buys the right dynamo in the first instance; or if he can do the over-compounding himself, by the method described in the concluding paragraphs of Chapter Seven. If it is found that the speed of the water wheel drops 25 per cent between no load and full load, a dynamo with field coils over-compounded to this extent would give a fairly constant regulation. If you are buying a special dynamo for direct drive, your manufacturer can supply you with a machine that will maintain constant voltage under the normal variations in speed of your wheel.
A System of Resistances
(2) Constant load systems. This system provides that the dynamo shall be delivering a fixed amount of current at all times, under which circumstances the water wheel would not require regulation, as the demands on it would not vary from minute to minute or hour to hour.
This system is very simply arranged. It consists of having a set of "resistances" to throw into the circuit, in proportion to the amount of current used.
Let us say, as an example, that a 50-ampere generator is used at a pressure of 110 volts; and that it is desirable to work this plant at 80 per cent load, or 40 amperes current draft. When all the lights or appliances were in use, there would be no outside "resistance" in the circuit. When none of the lights or appliances were in use (as would be the case for many hours during the day) it would be necessary to consume this amount of current in some other way—towaste it. A resistance permitting 40 amperes of current to flow, would be necessary. Of what size should this resistance be?
The answer is had by applying Ohm's Law, explained in Chapter Five. The Law in this case, would be read R = E/C. Therefore, in this case R = 110/40 = 2¾ ohms resistance, would be required, switched across the mains, to keep the dynamo delivering its normal load.
The cheapest form of this resistance wouldbe iron wire. In place of iron wire, German silver wire could be used. German silver wire is to be had cheaply, and is manufactured in two grades, 18% and 30%, with a resistance respectively 18 and 30 times that of copper for the same gauge. Nichrome wire has a resistance 60 times that of copper; and manganin wire has a resistance 65 times that of copper, of the same gauge.
First figure the number of feet of copper wire suitable for the purpose. Allowing 500 circular mills for each ampere, the gauge of the wire should be 40 × 500 = 20,000 circular mills, or approximately No. 7 B. & S. gauge. How many feet of No. 7 copper wire would give a resistance of 2¾ ohms? Referring to the copper wire table, we find that it requires 2006.2 of No. 7 wire to make one ohm. Then 2¾ ohms would require 5,517 feet.
Since 30 per cent German silver wire is approximately 30 times the resistance of copper, a No. 7 German silver wire, for this purpose, would be 1/30 the length of the copper wire, or 186 feet. If nichrome wirewere used, it would be 1/60th the length of copper for the same gauge, or 93 feet. This resistance wire can be wound in spirals and made to occupy a very small space. As long as it is connected in circuit, the energy of the dynamo otherwise consumed as light would be wasted as heat. This heat could be utilized in the hot water boiler or stove when the lights were turned off.
In actual practice, however, the resistance necessary to keep the dynamo up to full load permanently, would not be furnished by one set of resistance coils. Each lamp circuit would have a set of resistance coils of its own. A double-throw switch would turn off the lamps and turn on the resistance coils, orvice versa.
Let us say a lamp circuit consisted of 6 carbon lamps, of 16 candlepower each. It would consume 6 × ½ ampere, or 3 amperes of current, and interpose a resistance of 36.6 ohms—say 37 ohms. Three amperes would require a wire of at least 1,500 circular mills in area for safety. This corresponds to a No. 18wire. A No. 18 copper wire interposes a resistance of one ohm, for each 156.5 feet length. For 37 ohms, 5,790 feet would be required, for copper wire, which of course would be impractical. Dividing by 30 gives 193 feet for 30% German silver wire; and dividing by 60 gives 96 feet of nichrome wire of the same gauge.
It is simple to figure each circuit in this way and to construct resistance units for each switch. Since the resistance units develop considerable heat, they must be enclosed and protected.
A Home-made Stove or Radiator
While we are on the subject of resistance coils it might be well here to describe how to make stoves for cooking, and radiators for heating the house, at small expense. These stoves consist merely of resistances which turn hot—a dull red—when the current is turned on. Iron wire, German silver wire, or the various trade brands of resistance wire, of which nichrome, calido, and manganin are samples,can be used. In buying this wire, procure the table of resistance and carrying capacity from the manufacturers. From this table you can make your own radiators to keep the house warm in winter. Iron wire has the disadvantage of oxidizing when heated to redness, so that it goes to pieces after prolonged use. It is cheap, however, and much used for resistance in electrical work.
Let us say we wish to heat a bathroom, a room 6 × 8, and 8 feet high—that is a room containing 384 cubic feet of air space. Allowing 2 watts for each cubic foot, we would require 768 watts of current, or practically 7 amperes at 110 volts. What resistance would be required to limit the current to this amount? Apply Ohm's Law, as before, and we have R equals E divided by C, or R equals 110 divided by 7, which is 15.7 ohms. Forty-two feet of No. 20 German silver wire would emit this amount of heat and limit the current output to 7 amperes. In the Far West, it is quite common, in the outlying district, to find electric radiators made out of iron pipe coveredwith asbestos, on which the requisite amount of iron wire is wound and made secure. This pipe is mounted in a metal frame. Or the frame may consist of two pipes containing heating elements; and a switch, in this case, is so arranged that either one or two heating elements may be used at one time, according to the weather. An ingenious mechanic can construct such a radiator, experimenting with the aid of an ammeter to ascertain the length of wire required for any given stove.
Regulating Voltage at Switchboards
The voltage of any given machine may be regulated, within wide limits, by means of the field rheostat on the switchboard.
A dynamo with a rated speed of 1,500 revolutions per minute, for 110 volts, will actually attain this voltage at as low as 1,200 r.p.m. if all the regulating resistance be cut out. You can test this fact with your own machine by cutting out the resistance from the shunt field entirely, and starting themachine slowly, increasing its speed gradually, until the voltmeter needle registers 110 volts. Then measure the speed. It will be far below the rated speed of your machine.
If, on the other hand, the speed of such a machine runs up to 2,500 or over—that is, an excess of 67%—the voltage would rise proportionally, unless extra resistance was cut in. By cutting in such resistance—by the simple expedient of turning the rheostat handle on the switchboard,—the field coils are so weakened that the voltage is kept at the desired point in spite of the excessive speed of the machine. Excessive speeds are to be avoided, as a rule, because of mechanical strain. But within a wide range, the switchboard rheostat can be used for voltage regulation.
As it would be a source of continual annoyance to have to run to the switchboard every time the load of the machine was varied greatly this plan would not be practical for the isolated plant, unless the rheostat could be installed,—with a voltmeter—in one'skitchen. This could be done simply by running a small third wire from the switchboard to the house. Then, when the lights became dim from excessive load, a turn of the handle would bring them back to the proper voltage; and when they flared up and burned too bright, a turn of the handle in the opposite direction would remedy matters. By this simple arrangement, any member of the family could attend to voltage regulation with a minimum of bother.
Automatic Devices
There are several automatic devices for voltage regulation at the switchboard on the market. These consist usually of vibrator magnets or solenoids, in which the strength of the current, varying with different speeds, reacts in such a way as to regulate field resistance. Such voltage regulators can be had for $40 or less, and are thoroughly reliable.
To sum up the discussion of governors and voltage regulators: If you can allow a liberalproportion of water-power, and avoid crowding your dynamo, the chances are you will not need a governor for the ordinary reaction turbine wheel. Start your plant, and let it run for a few days or a few weeks without a governor, or regulator. Then if you find the operation is unsatisfactory, decide for yourself which of the above systems is best adapted for your conditions. Economy as well as convenience will affect your decision. The plant which is most nearly automatic is the best; but by taking a little trouble and giving extra attention, a great many dollars may be saved in extras.
Starting the Dynamo
You are now ready to put your plant in operation. Your dynamo has been mounted on a wooden foundation, and belted to the countershaft, by means of an endless belt.
See that the oil cups are filled. Then throw off the main switch and the field switch at the switchboard; open the water gate slowly,and occasionally test the speed of the dynamo. When it comes up to rated speed, say 1,500 per minute, let it run for a few minutes, to be sure everything is all right.
Having assured yourself that the mechanical details are all right, now look at the voltmeter. It is probably indicating a few volts pressure, from 4 to 8 or 10 perhaps. This pressure is due to the residual magnetism in the field cores, as the field coils are not yet connected. If by any chance, the needle does not register, or is now back of 0, try changing about the connections or the voltmeter on the back of the switchboard.
Now snap on the field switch. Instantly the needle will begin to move forward, though slowly; and it will stop. Turn the rheostat handle gradually; as you advance it, the voltmeter needle will advance. Finally you will come to a point where the needle will indicate 110 volts.
If you have designed your transmission line for a drop of 5 volts at half-load, advance the rheostat handle still further, until theneedle points to 115 volts. Let the machine run this way for some time. When assured all is right, throw on the main switch, and turn on the light at the switchboard. Then go to the house and gradually turn on lights. Come back and inspect the dynamo as the load increases. It should not run hot, nor even very warm, up to full load. Its brushes should not spark, though a little sparking will do no harm.
Your plant is now ready to deliver current up to the capacity of its fuses. See that it does not lack good lubricating oil, and do not let its commutator get dirty. The commutator should assume a glossy chocolate brown color. If it becomes dirty, or the brushes spark badly, hold a piece of fine sandpaper against it. Never use emery paper! If, after years of service, it becomes roughened by wear, have it turned down in a lathe. Occasionally, every few weeks, say, take the brushes out and clean them with a cloth. They will wear out in the course of time and can be replaced for a few cents each. Thebearings may need replacing after several years' continuous use.
Otherwise your electric plant will take care of itself. Keep it up to speed, and keep it clean and well oiled. Never shut it down unless you have to. In practice, dynamos run week after week, year after year, without stopping. This one, so long as you keep it running true to form, will deliver light, heat and power to you for nothing, which your city cousin pays for at the rate of 10 cents a kilowatt-hour.
GASOLINE ENGINES, WINDMILLS, ETC. THE STORAGE BATTERIES
GASOLINE ENGINE PLANTS
The standard voltage set—Two-cycle and four-cycle gasoline engines—Horsepower, and fuel consumption—Efficiency of small engines and generators—Cost of operating a one-kilowatt plant.
The standard voltage set—Two-cycle and four-cycle gasoline engines—Horsepower, and fuel consumption—Efficiency of small engines and generators—Cost of operating a one-kilowatt plant.
Electricity is of so much value in farm operations, as well as in the farm house, that the farmer who is not fortunate enough to possess water-power of his own, or to live in a community where a coöperative hydro-electric plant may be established, should not deny himself its many conveniences. In place of the water wheel to turn the dynamo, there is the gasoline engine (or other forms of internal combustion engine using oil, gas, or alcohol as fuel); in many districts where steam engines are used for logging or other operations, electricity may be generated as a by-product; and almost any windmill capable of pumping water can be made togenerate enough electricity for lighting the farm house at small expense.
The great advantage of water-power is that the expense of maintenance—once the plant is installed—is practically nothing. This advantage is offset in some measure by the fact that other forms of power, gas, steam, or windmills, are already installed, in many instances and that their judicious use in generating electricity does not impair their usefulness for the other farm operations for which they were originally purchased. In recent years gasoline engines have come into general use on farms as a cheap dependable source of power for all operations; and windmills date from the earliest times. They may be installed and maintained cheaply, solely for generating electricity, if desired. Steam engines, however, require so much care and expert attention that their use for farm electric plants is not to be advised, except under conditions where a small portion of their power can be used to make electricity as a by-product.
There are two types of gasoline engine electric plants suitable for the farm, in general use:
First:The Standard Voltage Set, in which the engine and dynamo are mounted on one base, and the engine is kept running when current is required for any purpose. These sets are usually of the 110-volt type, and all standard appliances, such as irons, toasters, motors, etc., may be used in connection with them. Since the electricity is drawn directly from the dynamo itself, without a storage battery, it is necessary that these engines be efficient and governed as to speed within a five per cent variation from no load to full load.
Second:Storage Battery Sets, in which the dynamo is run only a few hours each week, and the electricity thus generated is "stored" by chemical means, in storage batteries, for use when required. Since, in this case, the current is drawn from the battery, instead of the dynamo, when used for lighting or other purposes, it is not necessary that aspecial type of engine be used to insure constant speed.
The Standard Voltage Set
In response to a general demand, the first type (the direct-connected standard voltage set) has been developed to a high state of efficiency recently, and is to be had in a great variety of sizes (ranging from one-quarter kilowatt to 25 kilowatts and over) from many manufacturers.
The principle of the gasoline engine as motive power is so familiar to the average farmer that it needs but a brief description here. Gasoline or other fuel (oil, gas, or alcohol) is transformed into vapor, mixed with air in correct proportions, and drawn into the engine cylinder and there exploded by means of a properly-timed electric spark.
Internal combustion engines are of two general types—four-cycle and two-cycle. The former is by far the more common. In a four-cycle engine the piston must travel twice up and down in each cylinder, to deliver onepower stroke. This results in one power impulse in each cylinder every two revolutions of the crank shaft. On its first down stroke, the piston sucks in gas. On its first up stroke, it compresses the gas. At the height of this stroke, the gas is exploded by means of the electric spark and the piston is driven down, on its power stroke. The fourth stroke is called the scavening stroke, and expels the burned gas. This completes the cycle.
A one-cylinder engine of the ordinary four-cycle type has one power stroke for every two revolutions of the fly wheel. A two-cylinder engine has one power stroke for one revolution of the fly wheel; and a four-cylinder engine has two power strokes to each revolution. The greater the number of cylinders, the more even the flow of power. In automobiles six cylinders are common, and in the last year or two, eight-cylinder engines began appearing on the market in large numbers. A twelve-cylinder engine is the prospect for the immediate future.
Since the dynamo that is to supply electriccurrent direct to lamps requires a steady flow of power, the single-cylinder gas or gasoline engine of the four-cycle type is not satisfactory as a rule. The lights will flicker with every other revolution of the fly wheel. This would be of no importance if the current was being used to charge a storage battery—and right here lies the reason why a cheaper engine may be used in connection with a storage battery than when the dynamo supplies the current direct for lighting.
A two-cylinder engine is more even in its flow of power and a four-cylinder engine still better. For this reason, standard voltage generating sets without battery are usually of two or four cylinders when of the four-cycle type. When a single-cylinder engine is used, it should be of the two-cycle type. In the two-cycle engine, there is one power stroke to each up-and-down journey of the piston. This effect is produced by having inlet and exhaust ports in the crank case, so arranged that, when the piston arrives at the bottom of the power stroke, the waste gases are pushedout, and fresh gas drawn in before the up stroke begins.
For direct lighting, the engine must be governed so as not to vary more than five per cent in speed between no load and full load. There are many makes on the market which advertise a speed variation of three per cent under normal loads. Governors are usually of the centrifugal ball type, integral with the fly wheel, regulating the amount of gas and air supplied to the cylinders in accordance with the speed. Thus, if such an engine began to slow down because of increase in load, the centrifugal balls would come closer together, and open the throttle, thus supplying more gas and air and increasing the speed. If the speed became excessive, due to sudden shutting off of lights, the centrifugal balls would fly farther apart, and the throttle would close until the speed was again adjusted to the load.
These direct-connected standard voltage sets are as a rule fitted with the 110-volt, direct current, compound type of dynamo, the duplicate in every respect of the machinedescribed in previous chapters for water-power plants. They are practically automatic in operation and will run for hours without attention, except as to oil and gasoline supply. They may be installed in the woodshed or cellar without annoyance due to noise or vibration. It is necessary to start them, of course, when light or power is desired, and to stop them when no current is being drawn. There have appeared several makes on the market in which starting and stopping are automatic. Storage batteries are used in connection with these latter plants for starting the engine. When a light is turned on, or current is drawn for any purpose, an automatic switch turns the dynamo into a motor, and it starts the engine by means of the current stored in the battery. Instantly the engine has come up to speed, the motor becomes a dynamo again and begins to deliver current. When the last light is turned off, the engine stops automatically.
Since the installation of a direct-connected standard voltage plant of this type is similarin every respect, except as to motive power, to the hydro-electric plant, its cost, with this single exception, is the same. The same lamps, wire, and devices are used.
With gasoline power, the cost of the engine offsets the cost of the water wheel. The engine is more expensive than the ordinary gasoline engine; but even this item of cost is offset by the cost of labor and materials used in installing a water wheel.
The expense of maintenance is limited to gasoline and oil. Depreciation enters in both cases; and though it may be more rapid with a gasoline engine than a water wheel, that item will not be considered here. The cost of lubricating oil is inconsiderable. It will require, when operated at from one-half load to full load, approximately one pint of gasoline to each horsepower hour. When operated at less than half-load, its efficiency lowers. Thus, for a quarter-load, an average engine of this type may require three pints of gasoline for each horsepower hour. For this reason it is well, in installing such a plant, to have it ofsuch size that it will be operating on at least three-fourths load under normal draft of current. Norman H. Schneider, in his book "Low Voltage Electric Lighting," gives the following table of proportions between the engine and dynamo:
This table is figured for an efficiency of only 40 per cent for the smaller generators, and 60 per cent for the larger. In machines from 5 to 25 kilowatts, the efficiency will run considerably higher.
To determine the expense of operating a one-kilowatt gasoline generator set of this type, as to gasoline consumption, we can assume at full load that the gasoline engine is delivering 2½ horsepower, and consuming, let us say, 1¼ pint of gasoline for each horsepower hour(to make allowance for lower efficiency in small engines). That would be 3.125 pints of gasoline per hour. Allowing a ten per cent loss of current in wiring, we have 900 watts of electricity to use, for this expenditure of gasoline. This would light 900 ÷ 25 = 36 lamps of 25 watts each, a liberal allowance for house and barn, and permitting the use of small cooking devices and other conveniences when part of the lights were not in use. With gasoline selling at 12 cents a gallon, the use of this plant for an hour at full capacity would cost $0.047. Your city cousin pays 9 cents for the same current on a basis of 10 cents per kilowatt-hour; and in smaller towns where the rate is 15 cents, he would pay 13½ cents.
Running this plant at only half-load—that is, using only 18 lights, or their equivalent—would reduce the price to about 3 cents an hour—since the efficiency decreases with smaller load. It is customary to figure an average of 3½ hours a day throughout the year, for all lights. On this basis the cost of gasoline for this one-kilowatt plant would be16½ cents a day for full load, and approximately 10½ cents a day for half-load. This is extremely favorable, as compared with the cost of electric current in our cities and towns, at the commercial rate, especially when one considers that light and power are to be had at any place or at any time on the farm simply by starting the engine. A smaller plant, operating at less cost for fuel, would furnish ample light for most farms; but it is well to remember in this connection plants smaller than one kilowatt are practical for light only, since electric irons, toasters, etc., draw from 400 to 660 watts each. Obviously a plant of 300 watts capacity would not permit the use of these instruments, although it would furnish 10 or 12 lamps of 25 watts each.
THE STORAGE BATTERY
What a storage battery does—The lead battery and the Edison battery—Economy of tungsten lamps for storage batteries—The low-voltage battery for electric light—How to figure the capacity of a battery—Table of light requirements for a farm house—Watt-hours and lamp-hours—The cost of storage battery current—How to charge a storage battery—Care of storage batteries.
What a storage battery does—The lead battery and the Edison battery—Economy of tungsten lamps for storage batteries—The low-voltage battery for electric light—How to figure the capacity of a battery—Table of light requirements for a farm house—Watt-hours and lamp-hours—The cost of storage battery current—How to charge a storage battery—Care of storage batteries.
For the man who has a small supply of water to run a water wheel a few hours at a time, or who wishes to store electricity while he is doing routine jobs with a gasoline engine or other source of power, the storage battery solves the problem. The storage battery may be likened to a tank of water which is drawn on when water is needed, and which must be re-filled when empty. A storage battery, or accumulator is a device in which a chemical action is set up when an electric current is passed through it. This is calledcharging.When such a battery is charged, it has the property of giving off an electric current by means of a reversed chemical action when a circuit is provided, through a lamp or other connection. This reversed action is calleddischarging. Such a battery will discharge nearly as much current as is required originally to bring about the first chemical action.
There are two common types of storage battery—the lead accumulator, made up of lead plates (alternately positive and negative); and the two-metal accumulator, of which the Edison battery is a representative, made up of alternate plates of iron and nickel. In the lead accumulator, the "positive" plate may be recognized by its brown color when charging, while the "negative" plate is usually light gray, or leaden in color. The action of the charging current is to form oxides of lead in the plates; the action of the discharging current is to reduce the oxides to metallic lead again. This process can be repeated over and over again during the life of the battery.
Because of the cost of the batteries themselves,it is possible (from the viewpoint of the farmer and the size of his pocketbook) to store only a relatively small amount of electric current. For this reason, the storage battery was little used for private plants, where expense is a considerable item, up to a few years ago. Carbon lamps require from 3½ to 4 watts for each candlepower of light they give out; and a lead battery capable of storing enough electricity to supply the average farm house with light by means of carbon lamps for three or four days at a time without recharging, proved too costly for private use.
The Tungsten Lamp
With the advent of the new tungsten lamp, however, reducing the current requirements for light by two-thirds, the storage battery immediately came into its own, and is now of general use.
Since incandescent lamps were first invented scientists have been trying to find some metal of high fusion to use in place of the carbon filament of the ordinary lamp. Thehigher the fusing point of this filament of wire, the more economical would be the light. Edison sought, thirty years ago, for just the qualities now found in tungsten metal. Tungsten metal was first used for incandescent lamps in the form of a paste, squirted into the shape of a thread. This proved too fragile. Later investigators devised means of drawing tungsten into wire; and it is tungsten wire that is now used so generally in lighting. A tungsten lamp has an average efficiency of 1¼ watts per candlepower, compared with 3½ to 4 watts of the old-style carbon lamp. In larger sizes the efficiency is as low as .9 watt per candlepower; and only recently it has been found that if inert nitrogen gas is used in the glass bulb, instead of using a high vacuum as is the general practice, the efficiency of the lamp becomes still higher, approaching .5 watt for each candlepower in large lamps. This new nitrogen lamp is not yet being manufactured in small domestic sizes, though it will undoubtedly be put on the market in those sizes in the near future.