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The Distribution of Power by Direct Currents

( Originally Published Early 1900's )



IT IS part of the programme for this evening that I shall advocate the use of direct currents for long distance power transmission. I have taken this task with pleasure, believing that I have a strong case to present to you, but I beg you will remember that what I may say in favor of direct or against alternating methods is said from the standpoint of the engineer, and that as an engineer I must recognize the necessity to the world of each and every line of development now open to electricity. There is no electrical phenomenon which we can afford to say is trifling and not worthy of careful investigation. But a few years ago the alternating current trans-former was merely a scientific toy; today it is as practical a piece of engineering as the locomotive. Until now the condenser has had but a limited application in telegraphic work. Today it promises to become as important to industrial electricity as the transformer. I do not know but that the next few years may see Holtz or Wimshurst static machines built on as large a scale and used as extensively as magneto-electric machines are now.

Therefore, I must consider the power transmission problem as it stands this evening, and leave to those who are prophets and sons of prophets the prediction of what its future will be. And if I, in my remarks, appear to indulge in predictions, it is only guessing, not prophecy. I am entitled to guess, although not New England born, but I "don't never prophesy unless I know." Physical Properties of Alternating Current

Of the physical properties of the alternating current as they affect its use in power transmission, the most prominent are the following: First, its production by the generator and delivery to the motor without the use of a commutator. Second, the facility with which it can be transformed in its E. M. F., either upward or downward, by the use of simple apparatus. These points are in its favor. We notice, third, that inasmuch as an alternating current has its periods of zero E. M. F., its mean effective value is not the same as its maximum value. It follows that insulation of apparatus to use alternating currents must be calculated to withstand a higher potential difference than that available for virtual work. Fourth, we observe that the alternating current in its relation to conductors does not conform to Ohm's law in the elementary form, but to that law as amplified by the introduction of factors of periodicity, range of pulsation and inductance. The practical result of this is that to convey, under similar conditions, an equal amount of energy, the alternating current requires more copper than the direct current. This fact is not appreciated by the public, although it is familiar to engineers. Before the public the alternating current has been advertised as the current requiring little copper, whereas it may and does sometimes require, under practical conditions, 40 per cent more copper than the direct current. The fifth physical property which we notice is nearly allied to the fourth. It is merely the fourth property in a special application. It is said that in circuits of great inductance, such as the winding of electromagnets, the alternating current is retarded in its development to such an extent that a great potential difference or a large current is required to produce magnetic effects which could be produced by direct current with much less apparent expenditure of energy. It is quite true that the expenditure of energy is apparent only, but that does not affect the fact that the windings, if the potential difference is to be kept low, must be of a conductor large enough to carry an enormous current without heating; or that, if the current is to be small, the insulation must be designed to withstand a great potential difference. A parallel observation is that the variable state of an electromagnet, which only exists at the moments of charge and discharge in a magnet excited by direct current, is continuous when the magnet is excited by an alternating current, and that the loss of power by hysteresis goes on continually in such a magnet.

Transmission for Incandescent Lighting

Now, we must consider in the planning of any given transmission whether the good points of the alternating current do not outweigh the others which are to its disadvantage. In incandescent lighting this question has been answered in practice, and every day continues to be answered. We know that the answer given by practice is sometimes wrong, else we would not see direct current lighting stations located on the outskirts of towns and attempting to light their area at the cost of a large annual interest on copper; neither would we see alternating stations located in the middle of densely populated districts, and sacrificing efficiency on the greater part of their work in order that they may secure, with the same system, the revenue from a few distant lights. Still the problem is being solved, as such problems must finally be, by the daily progress of the world.

Transmission of Power

But the conditions of lighting do not correspond with those of power transmission. The advantages which outweigh the disadvantages of the alternating current for lighting purposes are not of so much value under other conditions. Thus, dispensing with the commutator on the alternator in a lighting station and substituting therefor a small self-exciting dynamo to charge the field, is a method both convenient and efficient. Then the lighting transformer, even at its lowest efficiency, is not such a wasteful device that it has not its proper use. But when we come to consider the transmission of power to a distant motor, the problem of how we are to excite the motor is a serious one; and when we look at the combination of alternator, exciter, step-up transformer at the generating station, plus step-down transformer, exciter and alternator, and some means of giving the alternator field its initial charge, all of which must be provided at the motor station, we are compelled to consider whether the simple, self-exciting direct current generator and the equally simple direct current motor are not to be preferred, even with their commutators and their disadvantage that the E. M. F. to be used on the line must be employed also in the generator and the motor. And it is just those considerations that have so far retarded the development of alternating power transmission. For while the transmission of power by direct current has grown up, parallel with the development of direct current lighting, to tremendous proportions (as witness the use of motors on the circuits of all direct current stations, the magnificent developments of the electrical street car system and the transmissions of power to be found in all manufacturing and mining districts), there is, so far as my knowledge goes, only one alternating current trans-mission of power in the United States which can be considered to have passed the experimental stage. That is a transmission in Colorado by the synchronous method, which is doing the work it was set to do, well and satisfactorily. Perhaps my statement that there is only one transmission is too sweeping. I make it subject to correction. I once incurred the wrath of a whole community in Nebraska by thoughtlessly saying in public that there was only one tree in town. Whereas it was well known to all reputable residents that besides the three which were in evidence on the Court House Square, there were also two cottonwoods growing out on section 35. I don't wish again to cause similar offense by unqualified statements.

The Synchronous Method of Transmission

There is nothing wrong with the synchronous method of transmitting power by alternating currents. True, it is necessary to start the motor by some exterior power, but if proper arrangements are made, the starting is no more difficult or tedious an operation than the starting up of a large Corliss steam plant. I think that the fifteen minutes before seven o'clock, which is usually devoted by a mill engineer to getting his engine and shafting into proper motion for the day's work, would be sufficient, if proper arrangements were made, to start up a synchronous motor. True it is, also, that the synchronous motor when overloaded "lies down" and declines to do any work at all until the load is reduced to what it considers its capacity. The same statement is correct, as every Sunday school child knows, regarding the model of animal power, the camel, and as any old soldier can tell concerning that much maligned carrier, the army mule; but it has not been imputed unto those animals for wickedness that they would not pull more than they thought they should; it has merely passed into proverb as "the nature of the beast." Then the synchronous alternating motor has an excellent quality in which it is only equaled by the plain shunt wound motor of the everyday electric light station: It regulates perfectly as to speed, keeping in step, within the limits of its capacity, with the generator which drives it.

And the synchronous motor is not the patented property of one inventor or corporation : It has belonged to the world ever since Dr. Hopkinson's experiments at North Foreland. There is not a company manufacturing alternate current machinery which would not gladly take an order for any number of synchronous power transmissions. The reason that there is not an indefinite number of such transmissions in use is that the direct current motor stands ready to do the work under all conditions that have yet come up in practice. There is no long felt want waiting to be filled by a new method. The owners of water powers today who are considering electric transmissions can have their wants supplied by machinery of a type with which the electrical world is familiar and whose capabilities have been proven by long experience. Not that the synchronous method will not have its uses. It certainly will. Its possibilities are recognized by all engineers, and its day will come; but remember, the direct current motor has not reached its highest development. Its future is in good hands, and it will always, even under conditions most favorable to the alternator, be a competitor that cannot be disregarded.

Polyphase Transmission

There is another system of alternating current power trans-mission being developed, which has been aptly named the "multi-phase method." It is a beautiful application of electrical laws, interesting to the lover of mathematics as much as to the engineer, for it has vast possibilities for analysis, and the production of graphic representations. It is not entirely new to the practical electric community. It has been applied to meters and to certain small motors.

The motors designed for multiphase currents are self-starting, self-exciting and not necessarily synchronous, so that they are free from difficulties which attend the synchronous method. They still are vexed with impedance and hysteresis, twin enemies of alternating apparatus, but not without hope. The lines along which the development of these motors must proceed are plainly marked out and they are certain to have their place in the future of power transmission. Today they have no place. The brilliant experiment at Frankfort has not given us a commercially valuable method.

The absence of detailed data regarding all apparatus of this class limits me to some general observations, the first of which is that the weight efficiency of the class appears to be low—that it takes a multiphase machine much larger than the direct current machine required to do equal work. This we would anticipate from the use of the multiphase current in field magnets at low E. M. F. While the actual power absorbed is not excessive, the current out of step with the E. M. F. is very large.

The next observation is that some multiphase designers are working on the problem of conversion of multiphase to direct currents. This seems a complicated process, but may have a commercial value, and indicates recognition of the desirability of direct currents.

The last observation is that the multiphase motors, and other motors excited by alternating currents, are apparently going to make their next step toward practicability by the assistance of condensers. This will cause a development of the condenser along commercial lines, akin to that which has produced the lighting transformers of today from the laboratory induction coil of yesterday. And this will be a benefit to electrical apparatus at large.

High Voltage Commutation

Now to examine the difficulties under which direct current machines are said to labor. First of all there is the old story, the commutator, which it is said is a source of trouble at high potentials. I have made it my business to inquire regarding the behavior of every high potential commutator which I could hear of. The list, outside of the commutators of arc machines working up to 3,000 volts or so, is a small one, but I have heard of at least two commutators working with 5,000 volts and 10 amperes which gave no trouble, of an indefinite number of commutators working at 1,500 volts and 20 amperes which give no trouble, and of about a dozen working at 1,500 to 2,000 volts and 40 amperes which give no trouble. The record appeared to be the same all through. The commutator did not appear to be the weak part of the machine. You see none of these machines is a very large one. The reason I got no data about machines with outputs greater than 75 kilowatts is that I could not find any such machines working at high voltages. It is only lately that there have been large dynamos put in use in this country for any purpose, though there are now many at 500 and 1,000 volts. When a power transmission needs large machines of higher E. M. F., the machines will be ready, and the commutators will not give trouble.

Copper Lines

Then we have the objection that high potential differences must be handled in direct current machines if economy of copper is to be secured on the lines. The alternating current transformer has the advantage in this respect, in that it is a stationary piece of apparatus whose construction admits of high insulation without material reduction of efficiency. And therein lies the certainty of the future development of alternating methods of transmission. There will come a few cases when the copper in the line will be a large factor in the total cost of the plant. These cases are far between. I have ascertained the proportion that copper bears to the total cost of line construction in a number of plants employing such E. M. F. as is common in direct current work. I am speaking of overhead lines now, the cheapest method of construction, and I find the percentage of copper cost is very small. Thus, in the cost of a mile of pole line, built in a workmanlike manner, with ordinary insulation, two wires being strung, the proportion of copper was 30 per cent. The remaining 70 per cent, represented poles, fittings, insulation and labor of construction. The proportion which this 30 per cent of cost of pole line bore to the whole cost of the plant was very small. I have looked over the bills of actual expenditures on a number of pole lines, and over the estimates made by responsible contractors for many others, and I find that where a good line is built according to the idea of line construction generally prevalent in the United States, the copper is a comparatively small item of the line, and proportionately less in the whole cost of plant. But we cannot figure in power transmission work on using overhead lines, not even in the mountains, where there is no aesthetic sentiment to force the wires below the ground. The telegraph lines through the mining districts and others where water powers abound break down every winter under pressure of storms.

Commercial Limitations of Transmission Distance

I need not show this assembly the visionary nature of schemes to carry the water power of Niagara to New York or Chicago. We will, any of us, undertake to do this if we receive the order from a responsible person, but we will each and all of us abstain from taking stock in the manufactories which depend on the transmitted power, and we will do this because we know that power transmitted means power lost in transmission, and that a difference of even 3 or 4 or 5 per cent in the cost of power means the control or the loss of the market for manufactured goods in these days of sharp competition. The same commercial necessities which built up in the wilderness Rochester, Minneapolis and the towns on our own Fox River,, away from the markets of their product but near to cheap power—that same commercial necessity will limit the distance to which electrical power will ever be transmitted. It is not a question of engineering. It is a question of dollars and cents to the manufacturer.



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