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.
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-connected set, 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 vary from 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 too great. 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, unless very 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 carry the 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.
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 various instruments 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 is above 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 a minimum. 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.
(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—to waste 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 would be 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 wire were 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, or vice 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. 18 wire. 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 covered with 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 the machine 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's kitchen. 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.
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 liberal proportion 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 the needle 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. The bearings 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.
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