Showing the analogy of water to volts and amperes of electricity
The same is true in figuring the power of electricity. We multiply theamperesby the number of electric impulses that are created in the wire in the course of one second. The unit of velocity, or pressure of the electric current is called avolt. Voltage is the pressure which causes electricity to flow. A volt may be likened to the velocity in feet per second of water in falling past a certain point. If youthink a moment you will see that this has nothing to do with quantity. A pin-hole stream of water under 40 pounds pressure has the same velocity as water coming from a nozzle as big as a barrel, under the same pressure. So with electricity under the pressure of one volt or one hundred volts.
One volt is said to consist of a succession of impulses caused byone wire cutting 100,000,000 lines of magnetic force in one second. Thus, if the strength of a magnet consisted of one line of force, to create the pressure of one volt we would have to "cut" that line of force 100,000,000 times a second, with one wire; or 100,000 times a second with one thousand wires. Or, if a magnet could be made with 100,000,000 lines of force, a single wire cutting those lines once in a second would create one volt pressure. In actual practice, field magnets of dynamos are worked at densities up to and over 100,000 lines of force to the square inch, and armatures contain several hundred conductors to "cut" these magnetic lines. The voltage then depends on the speed atwhich the armature is driven. In machines for isolated plants, it will be found that the speed varies from 400 revolutions per minute, to 1,800, according to the design of dynamo used.
Pressure determines volume of flow in a given time
Multiplying amperes (strength) by volts (pressure), gives uswatts(power). Seven hundred and forty-six watts of electrical energy is equal to one horsepower of mechanical energy—will do the same work. Thus an electric current under a pressure of 100 volts, and a density of 7.46 amperes, is one horsepower; as is 74.6 amperes, at 10 volts pressure; or 746 amperes at one volt pressure. For convenience (as a watt is a small quantity) electricity is measured inkilowatts, or 1,000 watts. Since 746 watts is one horsepower, 1,000 watts or one kilowatt is 1.34 horsepower. The work of such a current for one hour is called akilowatt-hour, and in our cities, where electricity is generated from steam, the retailprice of a kilowatt-hour varies from 10 to 15 cents.
Now as to how electricity may be controlled, so that a dynamo will not burn itself up when it begins to generate.
Again we come back to the analogy of water. The amount of water that passes through a pipe in any given time, depends on the size of the pipe, if the pressure is maintained uniform. In other words theresistanceof the pipe to the flow of water determines the amount. If the pipe be the size of a pin-hole, a very small amount of water will escape. If the pipe is as big around as a barrel, a large amount will force its way through. So with electricity. Resistance, introduced in the electric circuit, controls the amount of current that flows. A wire as fine as a hair will permit only a small quantity to pass, under a given pressure. A wire as big as one's thumb will permit a correspondingly greater quantity to pass, the pressure remaining the same. The unit of electrical resistance is called theohm—named after a man, as are all electrical units.
Ohm's Law
Theohmis that amount ofresistancethat will permit the passage ofone ampere, under the pressure ofone volt. It would take two volts to force two amperes through one ohm; or 100 volts to force 100 amperes through the resistance of one ohm. From this we have Ohm's Law, a simple formula which is the beginning and end of all electric computations the farmer will have to make in installing his water-power electric plant. Ohm's Law tells us that the density of current (amperes) that can pass through a given resistance in ohms (a wire, a lamp, or an electric stove) equalsvoltsdivided byohms—orpressuredivided byresistance. This formula may be written in three ways, thus:
C = E/R, or R = E/C or, E = C × R.
Or to express the same thing in words,currentequalsvoltsdivided byohms;ohmsequalsvoltsdivided bycurrent; orvoltsequalscurrentmultiplied byohms. So, with any two of these three determining factors known, we can find the third.As we have said, this simple law is the beginning and end of ordinary calculations as to electric current, and it should be thoroughly understood by any farmer who essays to be his own electrical engineer. Once understood and applied, the problem of the control of the electric current becomes simple a b c.
Examples of Ohm's Law
Let us illustrate its application by an example. The water wheel is started and is spinning the dynamo at its rated speed, say 1,500 r.p.m. Two heavy wires, leading from brushes which collect electricity from the revolving armature, are led, by suitable insulated supports to the switchboard, and fastened there. They do not touch each other. Dynamo mains must not be permitted to touch each otherunder any conditions. They are separated by say four inches of air. Dry air is a very poor conductor of electricity. Let us say, for the example, that dry air has a resistance to the flow of an electric current, of 1,000,000 ohms to the inch—that would be4,000,000 ohms. How much electricity is being permitted to escape from the armature of this 110-volt dynamo, when the mains are separated by four inches of dry air? Apply Ohm's law, C equals E divided by R. E, in this case is 110; R is 4,000,000; therefore C (amperes) equals 110/4,000,000—an infinitesimal amount—about .0000277 ampere.
Let us say that instead of separating these two mains by air we separated them by the human body—that a man took hold of the bare wires, one in each hand. The resistance of the human body varies from 5,000 to 10,000 ohms. In that case C (amperes) equals 110/5,000, or 110/10,000—about 1/50th, or 1/100th of an ampere. This illustrates why an electric current of 110 volts pressure is not fatal to human beings, under ordinary circumstances. The body offers too much resistance. But, if the volts were 1,100 instead of the usual 110 used in commercial and private plants for domestic use, the value of C, by this formula at 5,000 ohms, would be nearly 1/5th ampere. To drive 1/5thampere of electricity through the human body would be fatal in many instances. The higher the voltage, the more dangerous the current. In large water-power installations in the Far West, where the current must be transmitted over long distances to the spot where it is to be used, it is occasionally generated at a pressure of 150,000 volts. Needless to say, contact with such wires means instant death. Before being used for commercial or domestic purposes, in such cases, the voltage is "stepped down" to safe pressures—to 110, or to 220, or to 550 volts—always depending on the use made of it.
Now, if instead of interposing four inches of air, or the human body, between the mains of our 110-volt dynamo, we connected an incandescent lamp across the mains, how much electricity would flow from the generator? An incandescent lamp consists of a vacuum bulb of glass, in which is mounted a slender thread of carbonized fibre, or fine tungsten wire. To complete a circuit, the current must flow through this wire or filament.In flowing through it, the electric current turns the wire or filament white hot—incandescent—and thus turns electricity back into light, with a small loss in heat. In an ordinary 16 candlepower carbon lamp, the resistance of this filament is 220 ohms. Therefore the amount of current that a 110-volt generator can force through that filament is 110/220, or ½ ampere.
Armature and field coils of a direct current dynamo
One hundred lamps would provide 100 paths of 220 ohms resistance each to carry current, and the amount required to light 100 such lamps would be 100 × ½ or 50 amperes. Every electrical device—a lamp,a stove, an iron, a motor, etc.,—must, by regulations of the Fire Underwriters' Board be plainly marked with the voltage of the current for which it is designed and the amount of current it will consume. This is usually done by indicating its capacity in watts, which as we have seen, means volts times amperes, and from this one can figure ohms, by the above formulas.
A Short Circuit
We said a few paragraphs back that under no conditions must two bare wires leading from electric mains be permitted to touch each other, without some form of resistance being interposed in the form of lamps, or other devices. Let us see what would happen if two such bare wires did touch each other. Our dynamo as we discover by reading its plate, is rated to deliver 50 amperes, let us say, at 110 volts pressure. Modern dynamos are rated liberally, and can stand 100% overload for short periods of time, without dangerous overheating. Let us say that themains conveying current from the armature to the switchboard are five feet long, and of No. 2 B. & S. gauge copper wire, a size which will carry 50 amperes without heating appreciably. The resistance of this 10 feet of No. 2 copper wire, is, as we find by consulting a wire table, .001560 ohms. If we touch the ends of these two five-foot wires together, we instantly open a clear path for the flow of electric current, limited only by the carrying capacity of the wire and the back pressure of .001560 ohms resistance. Using Ohm's Law, C equals E divided by R, we find that C (amperes) equals 110/.001560 or70,515 amperes!
A direct current dynamo
Unless this dynamo were properly protected, the effect of such a catastrophe would be immediate and probably irreparable. In effect, it would be suddenly exerting a force of nearly 10,000 horsepower against the little 10 horsepower water wheel that is driving thisdynamo. The mildest thing that could happen would be to melt the feed-wire or to snap the driving belt, in which latter case the dynamo would come to a stop. If by any chance the little water wheel was given a chance to maintain itself against the blow for an instant, the dynamo, rated at 50 amperes, would do its best to deliver the 70,515 amperes you called for—and the result would be a puff of smoke, and a ruined dynamo. This is called a "short circuit"—one of the first "don'ts" in handling electricity.
As a matter of fact every dynamo is protected against such a calamity by means of safety devices, which will be described in a later chapter—because no matter how careful a person may be, a partial short circuit is apt to occur. Happily, guarding against its disastrous effects is one of the simplest problems in connection with the electric plant.
Direct Current and Alternating Current
When one has mastered the simple Ohm's Law of the electric circuit, the next step is todetermine what type of electrical generator is best suited to the requirements of a farm plant.
In the first place, electric current is divided into two classes of interest here—alternating, anddirect.
We have seen that when a wire is moved through the field of a magnet, there is induced in it two pulsations—first in one direction, then in another. This is analternatingcurrent, so called because it changes its direction. If, with our armature containing hundreds of wires to "cut" the lines of force of a group of magnets, we connected the beginning of each wire with one copper ring, and the end of each wire with another copper ring, we would have what is called analternating-currentdynamo. Simply by pressing a strap of flexible copper against each revolving copper ring, we would gather the sum of the current of these conductors. Its course would be represented by the curved line in the diagram, one loop on each side of the middle line (which represents time) would be acycle. The numberofcyclesto the second depends on the speed of the armature; in ordinary practice it is usually twenty-five or sixty. Alternating current has many advantages, which however, do not concern us here. Except under very rare conditions, a farmer installing his own plant should not use this type of machine.
Diagram of alternating and direct current
If, however, instead of gathering all the current with brushes bearing on two copper rings, we collected all the current traveling in one direction, on one set of brushes—and all the current traveling in the other direction on another set of brushes,—we would straighten out this current, make it all travel in one direction. Then we would have adirect current. A direct current dynamo, the type generally used in private plants, doesthis. Instead of having two copper rings for collecting the current, it has a single ring, made up of segments of copper bound together, but insulated from each other, one segment for each set of conductors on the armature. This ring of many segments, is called acommutator, because it commutates, or changes, the direction of the electric impulses, and delivers them all in one direction. In effect, it is like the connecting rod of a steam engine that straightens out the back-and-forth motion of the piston in the steam cylinder and delivers the motion to a wheel running in one direction.
Such a current, flowing through a coil of wire would make a magnet, one end of which would always be the north end, and the other end the south end. An alternating current, on the other hand, flowing through a coil of wire, would make a magnet that changed its poles with each half-cycle. It would no sooner begin to pull another magnet to it, than it would change about and push the other magnet away from it, and so on, as long as it continued to flow. Thisis one reason why a direct current dynamo is used for small plants. Alternating current will light the same lamps and heat the same irons as a direct current; but for electric power it requires a different type of motor.
Types of Direct Current Dynamos
Just as electrical generators are divided into two classes, alternating and direct, so direct current machines are divided into three classes, according to the manner in which their output, in amperes and volts, is regulated. They differ as to the manner in which their field magnets (in whose field of force the armature spins) are excited, or made magnetic. They are calledseries,shunt, andcompoundmachines.
The Series Dynamo
By referring to the diagram, it will be seen that the current of aseriesdynamo issues from the armature mains, and passes through the coils of the field magnets before passing into the external circuit to do its work. Theresidual magnetism, or the magnetism left in the iron cores of the field magnets from its last charge, provides the initial excitation, when the machine is started. As the resistance of the external circuit is lowered, by turning on more and more lights, more and more current flows from the armature, through the field magnets. Each time the resistance is lowered, therefore, the current passing through the field magnets becomes more dense in amperes, and makes the field magnets correspondingly stronger.
We have seen that the voltage depends on the number of lines of magnetic force cut by the armature conductors in a given time. If the speed remains constant then, and the magnets grow stronger and stronger, the voltage will rise in a straight line. When no current is drawn, it is 0; at full load, it may be 100 volts, or 500, or 1,000 according to the machine. This type of machine is used only in street lighting, in cities, with the lights connected in "series," or one after another on the same wire, the last lamp finally returningthe wire to the machine to complete the circuit. This type of dynamo has gained the name for itself of "mankiller," as its voltage becomes enormous at full load. It is unsuitable, in every respect, for the farm plant. Its field coils consist of a few turns of very heavy wire, enough to carry all the current of the external circuit, without heating.
Connections of a series dynamo
The Shunt Dynamo
The shunt dynamo, on the other hand, has field coils connected directlyacrossthe circuit, from one wire to another, instead of in "series." These coils consist of a great many turns of very fine wire, thus introducingresistanceinto the circuit, which limits the amount of current (amperes) that can be forced through them at any given voltage. As a shunt dynamo is brought up to its rated speed, its voltage gradually rises until a conditionof balance occurs between the field coils and the armature. There it remains constant. When resistance on the external circuit is lowered, by means of turning on lamps or other devices, the current from the armature increases in working power, by increasing its amperes. Its voltage remains stationary; and, since the resistance of its field coils never changes, the magnets do not vary in strength.
Connections of a shunt dynamo
The objection to this type of machine for a farm plant is that, in practice, the armature begins to exercise a de-magnetizing effect on the field magnets after a certain point is reached—weakens them; consequently the voltage begins to fall. The voltage of a shunt dynamo begins to fall after half-load is reached; and at full load, it has fallen possibly 20 per cent. A rheostat, or resistancebox on the switchboard, makes it possible to cut out or switch in additional resistance in the field coils, thus varying the strength of the field coils, within a limit of say 15 per cent, to keep the voltage constant. This, however, requires a constant attendance on the machine. If the voltage were set right for 10 lights, the lights would grow dim when 50 lights were turned on; and if it were adjusted for 50 lights, the voltage would be too high for only ten lights—would cause them to "burn out."
Shunt dynamos are used for charging storage batteries, and are satisfactory for direct service only when an attendant is constantly at hand to regulate them.
The Compound Dynamo
The ideal between these two conditions would be a compromise, which included the characteristics of bothseriesandshunteffects. That is exactly what thecompounddynamo effects.
A compound dynamo is a shunt dynamowith just enough series turns on its field coils, to counteract the de-magnetizing effect of the armature at full load. A machine can be designed to make the voltage rise gradually, or swiftly, by combining the two systems. For country homes, the best combination is a machine that will keep the voltage constant from no load to full load. A so-calledflat-compoundedmachine does this. In actual practice, this voltage rises slightly at the half-load line—only two or three volts, which will not damage the lamps in a 110-volt circuit.
The compound dynamo is therefore self-regulating, and requires no attention, except as to lubrication, and the incidental care given to any piece of machinery. Any shunt dynamo can be made into a compound dynamo, by winding a few turns of heavy insulated wire around the shunt coils, and connecting them in "series" with the external circuit. How many turns are necessary depends on conditions. Three or four turns to each coil usually are sufficient for "flat compounding." If the generating plant is a longdistance from the farm house where the light, heat, and power are to be used, the voltage drops at full load, due to resistance of the transmission wires. To overcome this, enough turns can be wound on top of the shunt coils to cause the voltage to rise at the switchboard, but remain stationary at the spot where the current is used. The usual so-called flat-compounded dynamo, turned out by manufacturers, provides for constant voltage at the switchboard. Such a dynamo is eminently fitted for the farm electric plant. Any other type of machine is bound to cause constant trouble and annoyance.
Connections of a compound dynamo
WHAT SIZE PLANT TO INSTALL
The farmer's wife his partner—Little and big plants—Limiting factors—Fluctuations in water supply—The average plant—The actual plant—Amount of current required for various operations—Standard voltage—A specimen allowance for electric light—Heating and cooking by electricity—Electric power: the electric motor.
The farmer's wife his partner—Little and big plants—Limiting factors—Fluctuations in water supply—The average plant—The actual plant—Amount of current required for various operations—Standard voltage—A specimen allowance for electric light—Heating and cooking by electricity—Electric power: the electric motor.
The farmer's wife becomes his partner when he has concluded the preliminary measurements and surveys for building his water-power electric plant. Now the question is, how big a plant is necessary, or how small a plant can he get along with. Electricity may be used for a multitude of purposes on the farm, in its sphere of furnishing portable light, heat and power; but when this multitude of uses has been enumerated, it will be found that the wife shares in the benefits no less than the farmer himself. The greatest dividend of all,whether dividends are counted in dollars or happiness, is that electricity takes the drudgery out of housework. Here, the work of the farmer himself ends when he has brought electricity to the house, just as his share in housework ends when he has brought in the kerosene, and filled the woodbox. Of the light and heat, she will use the lion's share; and for the power, she will discover heretofore undreamed-of uses. So she must be a full partner when it comes to deciding how much electricity they need.
How much electricity, in terms of light, heat, and power, will the farmer and his wife have use for? How big a plant should be installed to meet the needs of keeping house and running the farm?
The answer hangs mainly on how much water-power there is available, through all the seasons of the year, with which to generate electricity. Beyond that, it is merely a question of the farmer's pocketbook. How much money does he care to spend? Electricity is a cumulative "poison." The more one usesit, the more he wants to use it. After a plant has been in operation a year, the family have discovered uses for electricity which they did not think of in the beginning. For this reason, it is well to put in a plant larger than the needs of the moment seem to require. An electrical horsepower or two one way or another will not greatly change the first cost, and you will always find use for any excess.
Once for all, to settle the question of water-power, the water wheel should be twice the normal capacity of the dynamo it drives, in terms of power. This allows for overload, which is bound to occur occasionally; and it also insures smooth running, easy governing, and the highest efficiency. Since the electric current, once the plant is installed, will cost practically nothing, the farmer can afford to ignore the power going to waste, and consider only how to get the best service.
The Two Extremes
The amount of water to be had to be turned into electricity, will vary with location, andwith the season. It may be only enough, the greater part of the year, for a "toy" plant—a very practical toy, by the way—one that will keep half a dozen lights burning in the house and barn at one time; under some conditions water may be so scarce that it must be stored for three or four days to get enough power to charge a storage battery for these six or eight lights. A one-quarter, or a one-half kilowatt electrical generator, with a one horsepower (or smaller) wheel, will light a farmstead very satisfactorily—much better than kerosene lamps.
On the other hand, the driving power of your wheel may be sufficient to furnish 50 or 100 lights for the house, barn, and out-buildings, and barn-yard and drives; to provide ample current for irons, toasters, vacuum cleaners, electric fans, etc.; to do all the cooking and baking and keep the kitchen boiler hot; and to heat the house in the coldest weather with a dry clean heat that does not vitiate the air, with no ashes, smoke or dust or woodchopping—nothing but an electric switchto turn on and off; and to provide power for motors ranging from tiny ones to run the sewing machine, to one of 15 horsepower to do the threshing. A plant capable of developing from 30 to 50 kilowatts of electricity, and requiring from 50 to 100 horsepower at the water wheel, would do all this, depending on the size of the farmstead. One hundred horsepower is a very small water project, in a commercial way; and there are thousands of farms possessing streams of this capacity.
Fluctuations in Water Supply
It would be only during the winter months that such a plant would be driven to its full capacity; and since water is normally plentiful during these months, the problem of power would be greatly simplified. The heaviest draft on such a plant in summer would be during harvesting; otherwise it would be confined to light, small power for routine work, and cooking. Thus, a plant capable of meeting all the ordinary requirements of the four dry months of summer, when water is apt tobe scarce, doubles or quadruples its capacity during the winter months, to meet the necessities of heat for the house.
A dynamo requires only as much power to drive it, at any given time, as is being used in terms of electricity. There is some small loss through friction, of course, but aside from this the power required of the prime mover (the water wheel) is always in proportion to the amount of current flowing. When water is scarce, and the demands for current for heating are low, it is good practice to close a portion of the buckets of the turbine wheel with wooden blocks provided for this purpose. It is necessary to keep the speed of the dynamo uniform under all water conditions; and where there is a great fluctuation between high and low water periods, it is frequently necessary to have a separate set of pulleys for full gate and for half-gate. The head must remain the same, under all conditions. Changing the gate is in effect choking or opening the nozzle supplying the wheel, to cut down or increase its consumption of water.
The Average Plant
It will be the exceptional plant, however, among the hundreds of thousands to be had on our farms, which will banish not only the oil lamp and kitchen stove, but all coal or wood burning stoves as well—which will heat the house in below-zero weather, and provide power for the heavier operations of the farm. Also, on the other hand, it will be the exceptional plant whose capacity is limited to furnishing a half-dozen lights and no more.
A happy medium between these two conditions is the plant large enough to supply between five and ten electrical horsepower, in all seasons. Such a plant will meet the needs of the average farm, outside of winter heating and large power operations, and will provide an excess on which to draw in emergencies, or to pass round to one's neighbors. It is such a plant that we refer to when we say that (not counting labor) its cost, under ordinary conditions should not greatly exceed the price of one sound young horse for farm work.
Since the plant we described briefly in the first chapter, meets the requirements of this "average plant" let us inquire a little more fully into its installation, maintenance, and cost.
An Actual Plant
In this instance, the water-power was already installed, running to waste, in fact. The wheel consists of the so-called thirty-six inch vertical turbine, using 185 square inches of water, under a 14-foot head. Water is supplied to this wheel by a wooden penstock 33 inches square, inside measurements, and sloping at an angle of 30° from the flume to the wheel.
Details of voltmeter or ammeter
This wheel, under a 14-foot head, takes 2,312 cubic feet of water a minute; and it develops 46.98 actual horsepower (as may be figured by using the formulas of Chapter III). The water supply is provided by a small mountain river. The dam is 10 feet high, and the race, which feeds the flume from the mill pond is 75 yards long. The race has two spillways, one near the dam, and the second at the flume itself, to maintain an even head of water at all times.
Half-Gate
Since the water supply varies with the seasons, it has been found practical to run the wheel at half-gate—that is, with the gate only half-open. A set of bevel gears work the main shaft, which runs at approximately 200 revolutions per minute; and the dynamo is worked up to its required speed of 1,500 revolutions per minute through a countershaft.
The dynamo is a modern four-pole machine, compound-wound, with a rated output of 46 amperes, at 125 volts—in other words a dynamo of 5.75 kilowatts capacity, or 7.7 electrical horsepower. At full load this dynamo would require a driving power of 10 horsepower, counting it as 75 per cent efficient; and, to conform to our rule of two water horsepower to one electrical horsepower, the wheel should be capable of developing 20 horsepower. As a matter of fact, in thisparticular instance, shutting down the wheel to half-gate more than halves the rated power of the wheel, and little more than 15 horsepower is available. This allowance has proved ample, under all conditions met with, in this plant.
A switchboard and its connections:G.Dynamo;A.Shunt field coils;B.Series coils;DD.Fuses;FF.Main switch;F.Field switch;C.Ammeter;V.Voltmeter;E.Lamp;R.Rheostat. Dotted lines show connections on back of board
The dynamo is mounted on a firm floor foundation; and it is belted from the countershaft by an endless belt running diagonally. A horizontal belt drive is the best. Vertical drive should be avoided wherever possible.
The Switchboard
The switchboard originally consisted of a wooden frame on which were screwed ordinary asbestos shingles, and the instruments were mounted on these. Later, a sheet of electric insulating fibre was substituted, for look's sake. The main requisite is something substantial—and fireproof. The switchboard instruments consist of a voltmeter, with a range of from 0 to 150 volts; an ammeter, with a range, 0 to 75 amperes; a field regulating rheostat (which came with the dynamo); a main switch, withcartridge fuses protecting the machine against a draft of current over 60 amperes; and two line switches for the two owners, one fuse at 20 amperes, and the other at 40 amperes. Electric fuses are either cartridges or plugs, enclosing lead wire of a size corresponding to their rating. All the current of the line they protect passes through this lead wire. If the current drawn exceeds the capacity of the lead wire, it melts from the heat, and thus opens the circuit, and cuts off the current.
Items of Cost
This water wheel would cost $250 new. There is a duplicate in the neighborhood bought at second-hand, for $125. Thedynamo cost $90, and was picked up second-hand in New York City. New it would cost $150. The voltmeter cost $7, and the ammeter $10; and the switches and fuses could be had for $5. A wheel one-half the size, using one-half the amount of water at full gate, would do the work required, and the cost would be correspondingly less.
Capacity
This plant supplies two farms with electric light. One farm (that of the owner of the wheel) has 30 lamps, of 16 candlepower each, and two barn-yard lamps of 92 candlepower each. His wife has an electric iron and an electric water heater. Needless to say, all these lamps, and the iron and water heater are not in use at one time.
Carbon Lamps Gem Type (¼ scale)
The partner who owns the electric part of the plant has 30 lamps in his house and barn,many of them being 25 watt tungsten, which give more light for less power, but cost more to buy. They are not all in use at one time, though (since the current costs nothing) the inclination is to turn them on at night and let them burn. In his kitchen he has an electric range, and a water heater for the 40 gallon boiler. In addition to this he has all sorts of appliances,—irons, toasters, grills, a vacuum cleaner, a vibrator, etc. Naturally all these appliances are not in use at one time, else the draft on the plant would be such as to "blow" the fuses. For instance, all the baking is done in daylight; and when the oven is used after dark, they are careful to turn off all lights not needed. An ideal plant, of course, would be a plant big enough to take care of the sum of lamps and handy devices used at one time.
To make this plant ideal, (for, being an actual affair, it has developed some short-comings, with the extension of the use of electricity) it would require a dynamo whose capacity can be figured, from the following:
Thus, in this plant, if every electrical device were turned on at once, the demand on the dynamo would be for 12.1 kilowatts, or an overload of over 100 per cent. The main-switch fuse, being for 60 amperes, would "blow" or melt, and cut off all current for themoment. To repair the damage would be merely the work of a second—and at a cost of a few cents—simply insert a new fuse, of which there must be a supply on hand at all times. Or, if either owner exceeded his capacity, the line fuses (one for 20 amperes, and the other for 40 amperes) would instantly cut off all current from the greedy one.
25 and 40 watt Mazda tungsten lamps (¼ scale)
Lessons From This Plant
The story of this plant illustrates two things which the farmer and his wife must take into account when they are figuring how much electricity they require. First, it illustrates how one uses more and more current, as he finds it so serviceable and labor-saving, and at the same time free. The electric range and the water boiler, in the above instance, were later acquisitions not counted on in figuring the original installation. Second, it illustrates,that while the normal load of this generator is5.75kilowatts, one does not have to limit the electrical conveniences in the home to this amount. True, he cannot use more electricity than his plant will produceat any one time,—but it is only by a stretch of the imagination that one may conceive the necessity of using them all at once. Ironing, baking, and the use of small power are usually limited to daylight hours when no lights are burning.
As a matter of fact, this plant has proved satisfactory in every way; and only on one or two occasions have fuses been "blown", and then it was due to carelessness. A modern dynamo is rated liberally. It will stand an overload of as much as 100 per cent for a short time—half an hour or so. The danger from overloading is from heating. When the machine grows too hot for the hand, it is beginning to char its insulation, to continue which, of course would ruin it. The best plant is that which works under one-half or three-quarters load, under normal demands.
Standard Voltage
We are assuming the farmer's plant to be, in 99 cases out of 100, the standard 110-volt, direct current type. Such a plant allows for at least a 10 per cent regulation, in voltage, up or down the scale; supplies for this voltage are to be had without delay in even the more remote parts of the country, and (being sold in greater volume) they are cheaper than those for other voltages.
There are two general exceptions to this rule as to 110-volt plants: (1) If the plant is located at a distance greater than a quarter of a mile from the house, it will be found cheaper (in cost of transmission line, as will be shown later) to adopt the 220-volt plant; (2), If the water supply is so meagre that it must be stored for many hours at a time, and then used for charging storage batteries, it will be found most economical to use a 30-volt plant. A storage battery is made up of cells of approximately 2 volts each; and, since more than 55 such cells would be required for a110-volt installation, its cost would be prohibitive, with many farmers.
So we will assume that this plant is a 110-volt plant, to be run without storage battery. It will be well to make a chart, dividing the farm requirements into three heads—light, heat, and power.
Light
60 and 100 watt Mazda tungsten lamp. These lamps may be had in sizes from 10 to 500 watts (¼ scale)
The lamp of the future. A 1000 watt Mazda nitrogen lamp, giving 2000 candlepower (¼ scale)
Light is obtained by means of incandescent lamps. There are two styles in common use, the carbon and the tungsten lamp. It requires 3.5 to 4 watts of electricity to produce one candlepower in a carbon lamp. It requiresfrom 1 to 1.25 watt to produce one candlepower in the tungsten lamp. The new nitrogen lamp, not yet in general use, requires only ½ watt to the candlepower. Since tungsten lamps give three times the light of the carbon lamp, they are the most economical to use in the city or town where one is paying for commercial current. But, in the country where water-power furnishes current for nothing, it will be found most economical to use the carbon lamp, since its cost at retail is 16 cents, as compared with 30 cents for a corresponding size in tungsten. A 60 watt carbon lamp, of 16 candlepower; or a 25 watt tungsten lamp, of 20candlepower, are the sizes to use. In hanging lamps, as over the dining room table, a 100 watt tungsten lamp, costing 70 cents, and giving 92 candlepower light is very desirable; and for lighting the barn-yard, these 100 watt tungsten lamps should be used. For reading lamps, the tungsten style, of 40 or 60 watt capacity, will be found best. Otherwise, in all locations use the cheaper carbon lamp. Both styles have a rated life of 1,000 hours, after which they begin to fall off in efficiency. Here again, the farmer need not worry over lack of highest efficiency, as a lamp giving only 80 per cent of its rated candlepower is still serviceable when he is not paying for the current. With care not to use them at voltages beyond their ratings, lamps will last for years.
A Specimen Light Allowance
Below is a typical table of lights for a large farm house, the barns and barn-yard. It is given merely as a guide, to be varied for each individual case:
This provides for 44 lights, an extremely liberal allowance. How many of these lights will be burning at any one time? Probably not one-half of them; yet the ideal plant is that which permits all fixtures to be in service at one time on the rare occasions when necessary. Thus, for lighting only, 2,800watts maximum service would require a 4 kilowatt generator, and 10 water horsepower, on the liberal rating of two to one. A 3 kilowatt generator would take care of these lights, with a 30 per cent overload (which is not excessive) for maximum service. The above liberal allowance of lights may be cut in two, or four—or even eight—and still throw a kerosene lamp in shadow. It all depends on the number of lights one wants burning at one time; and the power of the water wheel.
If the 36 carbon lights in the above table were replaced by 25 watt tungsten lights, the saving in power would be 35 watts each, or 1,260 watts, nearly two electrical horsepower; while the added first cost would be 14 cents a light, or $5.04. A generator of 2 kilowatt capacity would take care of all these lights then, with 460 watts to spare.
Heating
Electric heating and cooking is in its infancy, due to the prohibitive cost of commercialcurrent in our cities. Here the farmer has the advantage again, with his cheap current.
For heating the house, it is calculated that 2 watts is required for each cubic foot of air space in a room, during ordinary winter weather. Thus, a room 10 × 12, and 8 feet high, would contain 960 cubic feet, and would require 1,820 watts energy to heat it in cold weather. Five such rooms would require 9.1 kilowatts; and 10 such rooms, or their equivalent, would require 18.2 kilowatts.
Electric heating devices are divided into two classes: (1) those which can be used on lamp circuits,and do not draw more than 660 watts each; and (2) those which draw more than 660, thereforerequire special wiring. The capacity of these devices is approximately as follows:
Lamp circuit devices:
Lamp circuit devices:
Special circuit devices:
The only device in the above list which is connected continuously, is the hot water boiler, and this can be credited with at least one electrical horsepower 24 hours a day. It is a small contrivance, not much bigger than a quart can, attached to the back of the kitchen boiler, and it keeps the water hot throughout the house at all hours. Its cost will vary with the make, ranging from $8 to $15; and since it is one of the real blessings of the farm kitchen and bathroom, it should be included in all installations where power permits. Electric radiators will be used 24 hours a day in winter, and not at all in summer. They are portable, and canbe moved from room to room, and only such rooms as are in actual use need be heated. The other devices are for intermittent service, many of them (like the iron) for only a few hours each week.
The grill, chafing dish, coffee percolator, etc., which are used on the dining room table while the family is at meals, each draw an equivalent of from 6 to 10 carbon lights. By keeping this in view and turning off spare lights, one can have the use of them, with even a small plant. Thus, a one kilowatt plant permits the use of any one of these lamp circuit devices at a time, with a few lights in addition.
Power
Electric power is to be had through motors. A direct current dynamo and a direct current motor are identical in construction. That is, a motor becomes a generator if belted to power; and a generator becomes a motor, if connected to electric mains. This is best illustrated by citing the instance of a trans-continentalrailroad which crosses the Bitter Root Mountains by means of electric power. Running 200 miles up a 2 per cent grade, it is drawnby its motors. Coasting 200 miles down the 2 per cent grade on the other side of the mountains, its motors become generators. They act as brakes, and at the same time they pump the power of the coasting weight of this train back into the wires to help a train coming up the other side of the mountains.