Electricity compared to the heat and light of the Sun—The simple dynamo—The amount of electric energy a dynamo will generate—The modern dynamo—Measuring power in terms of electricity—The volt—The ampere—The ohm—The watt and the kilowatt—Ohm's Law of the electric circuit, and some examples of its application—Direct current, and alternating current—Three types of direct-current dynamos: series, shunt, a
What a farmer really does in generating electricity from water that would otherwise run to waste in his brook, is to install a private Sun of his own—which is on duty not merely in daylight, but twenty-four hours a day; a private Sun which is under such simple control that it shines or provides heat and power, when and where wanted, simply by touching a button.
This is not a mere fanciful statement. When you come to look into it you find that electricity actually is the life-giving power of the Sun's rays, so transformed that it can be handily conveyed from place to place by means of wires, and controlled by mechanical devices as simple as the spigot that drains a cask.
Nature has the habit of traveling in circles. Sometimes these circles are so big that the part of them we see looks like a straight line, but it is not. Even parallel lines, according to the mathematicians, "meet in infinity." Take the instance of the water wheel which the farmer has installed under the fall of his brook. The power which turns the wheel has the strength of many horses. It is there in a handy place for use, because the Sun brought it there. The Sun, by its heat, lifted the water from sea-level, to the pond where we find it—and we cannot get any more power out of this water by means of a turbine using its pressure and momentum in falling, than the Sun itself expended in raising the water against the force of gravity.
Once we have installed the wheel to change the energy of falling water into mechanical power, the task of the dynamo is to turn this mechanical power into another mode of motion—electricity. And the task of electricity is to change this mode of motion back into the original heat and light of the Sun—which started the circle in the beginning.
Astronomers refer to the Sun as "he" and "him" and they spell his name with a capital letter, to show that he occupies the center of our small neighborhood of the universe at all times.
Magnets and Magnetism
The dynamo is a mechanical engine, like the steam engine, the water turbine or the gas engine; and it converts the mechanical motion of the driven wheel into electrical motion, with the aid of a magnet. Many scientists say that the full circle of energy that keeps the world spinning, grows crops, and paints the sky with the Aurora Borealis, begins and ends with magnetism—that the sun's rays are magnetic rays. Magnetism is the force that keeps the compass needle pointing north and south. Take a steel rod and hold it along the north and south line, slightly inclined towards the earth, and strike it a sharp blow with a hammer, and it becomes a magnet—feeble, it is true, but still a magnet.
Take a wire connected with a common dry battery and hold a compass needle under it and the needle will immediately turn around and point directly across the wire, showing that the wire possesses magnetism encircling it in invisible lines, stronger than the magnetism of the earth.
Insulate this wire by covering it with cotton thread, and wind it closely on a spool. Connect the two loose ends to a dry battery, and you will find that you have multiplied the magnetic strength of a single loop of wire by the number of turns on the spool—concentrated all the magnetism of the length of that wire into a small space. Put an iron core in the middle of this spool and the magnet seems still more powerful. Lines of force which otherwise would escape in great circles into space, are now concentrated in the iron. The iron core is a magnet. Shut off the current from the battery and the iron is still a magnet—weak, true, but it will always retain a small portion of its magnetism. Soft iron retains very little of its magnetism. Hard steel retains a great deal, and for this reason steel is used for permanent magnets, of the horseshoe type so familiar.
A Simple Dynamo
A dynamo consists, first, of a number of such magnets, wound with insulated wire. Their iron cores point towards the center of a circle like the spokes of a wheel; and their curved inner faces form a circle in which a spool, wound with wire in another way, may be spun by the water wheel.
Now take a piece of copper wire and make a loop of it. Pass one side of this loop in front of an electric magnet.
As the wire you hold in your hands passes the iron face of the magnet, a wave of energy that is called electricity flows around this loop at the rate of 186,000 miles a second—the same speed as light comes to us from the sun. As you move the wire away from the magnet, a second wave starts through the wire, flowing in the opposite direction. You can prove this by holding a compass needle under the wire and see it wag first in one direction, then in another.
This is a simple dynamo. A wire "cutting" the invisible lines of force, that a magnet is spraying out into the air, becomes "electrified." Why this is true, no one has ever been able to explain.
The amount of electricity—its capacity for work—which you have generated with the magnet and wire, does not depend alone on the pulling power of that simple magnet. Let us say the magnet is very weak—has not enough power to lift one ounce of iron. Nevertheless, if you possessed the strength of Hercules, and could pass that wire through the field of force of the magnet many thousands of times a second, you would generate enough electricity in the wire to cause the wire to melt in your hands from heat.
This experiment gives the theory of the dynamo. Instead of passing only one wire through the field of force of a magnet, we have hundreds bound lengthwise on a revolving drum called an armature. Instead of one magnetic pole in a dynamo we have two, or four, or twenty according to the work the machine is designed for—always in pairs, a North pole next to a South pole, so that the lines of force may flow out of one and into another, instead of escaping in the surrounding air. If you could see these lines of force, they would appear in countless numbers issuing from each pole face of the field magnets, pressing against the revolving drum like hair brush bristles—trying to hold it back. This drum, in practice, is built up of discs of annealed steel, and the wires extending lengthwise on its face are held in place by slots to prevent them from flying off when the drum is whirled at high speed. The drum does not touch the face of the magnets, but revolves in an air space. If we give the electric impulses generated in these wires a chance to flow in a circuit—flow out of one end of the wires, and in at the other, the drum will require more and more power to turn it, in proportion to the amount of electricity we permit to flow. Thus, if one electric light is turned on, the drum will press back with a certain strength on the water wheel; if one hundred lights are turned on it will press back one hundred times as much. Providing there is enough power in the water wheel to continue turning the drum at its predetermined speed, the dynamo will keep on giving more and more electricity if asked to, until it finally destroys itself by fire. You cannot take more power, in terms of electricity, out of a dynamo that you put into it, in terms of mechanical motion. In fact, to insure flexibility and constant speed at all loads, it is customary to provide twice as much water wheel, or engine, power as the electrical rating of the dynamo.
We have seen that a water wheel is 85 per cent efficient under ideal conditions. A dynamo's efficiency in translating mechanical motion into electricity, varies with the type of machine and its size. The largest machines attain as high as 90 per cent efficiency; the smallest ones run as low as 40 per cent.
Measuring Electric Power
The amount of electricity any given dynamo can generate depends, generally speaking, on two factors, i. e., (1) the power of the water wheel, or other mechanical engine that turns the armature; and (2) the size (carrying capacity) of the wires on this drum.
Strength, of electricity, is measured in amperes. An ampere of electricity is the unit of the rate of flow and may be likened to a gallon of water per minute.
In surveying for water-power, in Chapter III, we found that the number of gallons or cubic feet of water alone did not determine the amount of power. We found that the number of gallons or cubic feet multiplied by the distance in feet it falls in a given time, was the determining factor—pounds (quantity) multiplied by feet per second—(velocity).
The same is true in figuring the power of electricity. We multiply the amperes by 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 a volt. 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 you think 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 by one 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 at which 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.
Multiplying amperes (strength) by volts (pressure), gives us watts (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 in kilowatts, 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 a kilowatt-hour, and in our cities, where electricity is generated from steam, the retail price 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 the resistance of 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 the ohm—named after a man, as are all electrical units.
The ohm is that amount of resistance that will permit the passage of one ampere, under the pressure of one 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) equals volts divided by ohms—or pressure divided by resistance. This formula may be written in three ways, thus:
Or to express the same thing in words, current equals volts divided by ohms; ohms equals volts divided by current; or volts equals current multiplied by ohms. 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 other under 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 be 4,000,000 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 1/5th ampere 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.
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 the mains 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 or 70,515 amperes!
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 this dynamo. 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 to determine 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, and direct.
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 an alternating current, 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 an alternating-current dynamo. 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 a cycle. The number of cycles to 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.
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 a direct current. A direct current dynamo, the type generally used in private plants, does this. 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 a commutator, 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. This is 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 called series, shunt, and compound machines.
The Series Dynamo
By referring to the diagram, it will be seen that the current of a series dynamo issues from the armature mains, and passes through the coils of the field magnets before passing into the external circuit to do its work. The residual 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 returning the 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.
The Shunt Dynamo
The shunt dynamo, on the other hand, has field coils connected directly across the circuit, from one wire to another, instead of in "series." These coils consist of a great many turns of very fine wire, thus introducing resistance into 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 condition of 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.
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 resistance box 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 both series and shunt effects. That is exactly what the compound dynamo effects.
A compound dynamo is a shunt dynamo with 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-called flat-compounded machine 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 long distance 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.