Electricity for the farm Part 6

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 (quant.i.ty) multiplied by feet per second--(velocity).

[Ill.u.s.tration: Showing the a.n.a.logy of water to volts and amperes of electricity]

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 quant.i.ty.

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.

[Ill.u.s.tration: Pressure determines volume of flow in a given time]

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 quant.i.ty) 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 a.n.a.logy of water. The amount of water that pa.s.ses 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 quant.i.ty to pa.s.s, under a given pressure. A wire as big as one's thumb will permit a correspondingly greater quant.i.ty to pa.s.s, the pressure remaining the same. The unit of electrical resistance is called the _ohm_--named after a man, as are all electrical units.

_Ohm's Law_

The _ohm_ is that amount of _resistance_ that will permit the pa.s.sage 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 pa.s.s 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:

C = E/R, or R = E/C or, E = C R. 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 ill.u.s.trate 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 ill.u.s.trates why an electric current of 110 volts pressure is not fatal to human beings, under ordinary circ.u.mstances. 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 gla.s.s, 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 1/2 ampere.

[Ill.u.s.tration: 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 1/2 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_!

[Ill.u.s.tration: 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 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 cla.s.ses 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.

[Ill.u.s.tration: 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 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 cla.s.ses, alternating and direct, so direct current machines are divided into three cla.s.ses, 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 pa.s.ses through the coils of the field magnets before pa.s.sing 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 pa.s.sing 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.

[Ill.u.s.tration: Connections of a series dynamo]

_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.

[Ill.u.s.tration: 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 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.