The Illustrated Tesla
By Nikola Tesla
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Nikola Tesla
Nikola Tesla (1856–1943) was a Serbian-American inventor, writer, physicist, and engineer, best known for his work on the alternating current (AC) electricity supply system.
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The Illustrated Tesla - Nikola Tesla
Introduction
Njan poetry has so distinct a charm that Goethe is said to have learned the musical tongue in which it is written rather than lose any of its native beauty. History does not record, however, any similar instance in which the Servian language, though it be that of Boskovich, expounder of the atomic theory, has been studied for the sake of the scientific secrets that might lurk therein. The vivid imagination and ready fancy of the people have been literary in their manifestation and fruit. A great Slav orator has publicly reproached his one hundred and twenty million fellows in Eastern Europe with their utter inability to invent even a mouse-trap. They were all mere barren idealists. If this were true, to equalize matters, we might perhaps barter without loss some score of ordinary American patentees for a single singer of Illyrian love-songs. But racial conditions are hardly to be offset on any terms that do not leave genius its freedom, and once in a while Nature herself rights things by producing a man whose transcendent merit compensates his nation for the very defects to which it has long been sensitive. It does not follow that such a man shall remain in a confessedly unfavorable environment. Genius is its own passport, and has always been ready to change habitats until the natural one is found. Thus it is, perchance, that while some of our artists are impelled to set up their easels in Paris or Rome, many Europeans of mark in the fields of science and research are no less apt to adopt our nationality, of free choice. They are, indeed, Americans born in exile, and seek this country instinctively as their home, needing in reality no papers of naturalization. It was thus that we welcomed Agassiz, Ericsson, and Graham Bell. In like manner Nikola Tesla, the young Servian inventor with whose work a new age in electricity is beginning, now dwells among us in New York. Mr. Tesla’s career not only touches the two extremes of European civilization, east and west, in a very interesting way, but suggests an inquiry into the essential likeness between poet and inventor. He comes of an old Servian family whose members for centuries have kept watch and ward along the Turkish frontier, and whose blood was freely shed that our western vanguard might gain time for its advance upon these shores. Yet, remote as such people and conditions are to us, it is with apparatus based on ideas and principles originating among them that the energy from Niagara Falls is to be widely distributed by electricity, in the various forms of light, heat, and power. This, in itself, would seem enough to confer fame, but Mr. Tesla has done, and will do, much else. Could he be tamed to habits of moderation in work, it would be difficult to set limit to the solutions he might give us, through ripening years, of many deep problems; but when a man springs from a people who have a hundred words for knife and only one for bread, it is a little unreasonable to urge him to be careful even of his own life. Thirty-six years make a brief span, but when an inventor believes that creative fertility is restricted to the term of youth, it is no wonder that night and day witness his anxious activity, as of a relentless volcano, and that ideas well up like hot lava till the crater be suddenly exhausted and hushed.
A Slav of the Slays, with racial characteristics strongly stamped in look, speech, and action, Mr. Tesla is a notable exemplification of the outcropping in unwonted form of tendencies suppressed. I have never heard him speak of a picture or a piece of music, but his numerous inventions, and the noble lectures that embody his famous investigations with currents of high frequency and high potential, betray the poetic temperament throughout. One would expect the line separating fact from theory to fade at the altitudes of thought to which his later speculations reach; but this lithe, spare mountaineer is accustomed to the thin, dry air, and neither loses sharpness of sight nor breathes painfully. Has the Servian poet become inventor, or is the inventor a poet? Mr. Tesla has been held a visionary, deceived by the flash of casual shooting stars; but the growing conviction of his professional brethren is that because he saw farthest, saw first the low lights flickering on tangible new continents of science. The perceptive and imaginative qualities of the mind are not often equally marked in the same man of genius. Overplus of imagination may argue dimness of perception; an ability to dream dreams may imply a want of skill in improving reapers. Now and then the two elements combine in the creative poet of epic and drama; occasionally they give us the prolific inventor like Tesla.
Jules Breton has spoken of the history of his life as being at the same time the genesis of his art. This is true of Nikola Tesla’s evolution. His bent toward invention we may surely trace to his mother, who, as the wife of an eloquent clergyman in the Greek Church, made looms and churns for a pastoral household while her husband preached. Tesla’s electrical work started when, as a boy in the Polytechnic School at Gratz, he first saw a direct-current Gramme — machine, and was told that the commutator was a vital and necessary feature in all such apparatus. His intuitive judgment or latent spirit of invention at once challenged the statement of his instructor, and that moment began the process of reasoning and experiment which led him to his discovery of the rotating magnetic field, and to the practical polyphase motors, in which the commutator and brushes, fruitful and endless source of trouble, are absolutely done away with. These perfected inventions did not come at once; they never do. The conditions that surrounded this youth in the airy fastnesses of the Dinaric Alps all made against the hopes he nursed of becoming an electrician ; and not the least impediment was the fond wish of his parents at Siniljan Lika that he should maintain the priestly tradition, and benefit by the preferment likely to come through his uncle, now Metropolitan in Bosnia. But Tesla felt himself destined to serve at other altars than those of his ancient faith, with other means of approach to the invisible and unknown. He persevered in mathematical and mechanical studies, mastered incidentally half a dozen languages, and at last became an assistant in the Government Telegraph Engineering Department at BudaPest. His salary was small enough to please those who hold that the best endowment of genius is poverty, and he would make no appeals to his widowed mother for help. Experimenting, of course, went on all the time at this juncture it was on telephony that he wasted his meager substance in riotous invention. Desirous of going to a fete with some friends, and anxious not to spend on clothes the money that might buy magnets and batteries, the brilliant idea occurred to him to turn his only pair of trousers inside out and to disport in them on the morrow as new. He sat up all night tailoring, but the fete came and went before he could reappear in public. This episode is quite in keeping with his boyish efforts to fly from the steep roof of the house at Smiljan, using an old umbrella as arostat; or with the peculiar tests, stopped by the family doctor before the results could be determined, as to how long he could suspend the beating of his heart by will power.
Naturally enough for a young inventor seeking larger opportunity, Tesla soon drifted west- ward from Budapest. He made his way to Paris, where he quickly secured employment in electric lighting, then a new art, and encountered an observant associate of Mr. Edison. Almost before he knew it, he was on his glad voyage across the Atlantic to work in one of the Edison shops, and to enter upon a new stage of development. He had profound faith in the value of the principles first meditated in the silence of the sterile mountains that border the Adriatic, and he knew that in a country where every new invention in electricity has its chance, his turn would come also, for he now had demonstrated his theories in actual apparatus.
If anything were needed to confirm Mr. Tesla in his hopes and enthusiasm, it would have been the close relation that he was thus thrown into with the robust, compelling genius who has created so many new things in electricity. Emerson has said that steam is half an Englishman; may we not, in view of what such men as Edison have done, add that electricity is half an American? The fiery zeal with which this young recruit flung himself on the most exacting tasks matched that of his chief. It went the length of a daily breakfast of Welsh rabbit, for weeks Mr. Tesla accepting as true, in spite of protesting stomach, the jocular suggestion that it was thus that his hero fortified himself successfully for renewed effort after their long vigils of toil. Mr. Edison, like most other people, had some difficulty in finding anywhere within the pale of civilization, as marked by the boundaries of maps, the isolated region of Mr. Tesla’s birth, and once inquired seriously of his neophyte whether he had ever tasted human flesh. It was inevitable that a really delightful intimacy and apprenticeship should end. Even the most cometic genius has its orbit, and these two men are singularly representative of different kinds of training, different methods, and different aims. Mr. Tesla must needs draw apart; and stimulated by this powerful spirit, he went on his own way for his own works sake.
Of late years a sharp controversy has raged in the electrical field as to the respective advantages of the continuous and the alternating current for light and power. In bitterness and frequent descent to personalities it has resembled the polemics of the old metaphysical schoolmen, and uninstructed, plain folk have mildly wondered whether it was really worthwhile to indulge in such terrible threatening and slaughter over purely speculative topics. There is, however, a very practical aspect to the discussion, and from the first Mr. Tesla has been an advocate of the alternating current, not because he loved the direct current less, but because he knew that with the alternating he could achieve results otherwise impossible, especially in power transmission. Furthermore, all direct current generators and motors have required commutators and brushes, but Mr. Tesla, who has himself perfected many inventions based on direct currents, has shown that with the application of the rotating-field principle these elements of complication and restriction were no longer needed. This utilization of polyphase currents was a most distinct advance, made a deep imprint on the electric arts, and has been duly signalized. In America the invention found immediate sale. From Italy came an insistent cry of priority, reminding one of the anticipations that have clustered thick around the telegraph, the telephone, and the incandescent lamp. In Germany, with money raised by popular and imperial subscription, apparatus on the polyphase principle was built by which large powers were transmitted electrically more than a hundred miles from Neckar- on-the Rhine to Frankfort-on-the-Main; and now by equivalent agency Niagara is to drive the wheels of Buffalo and beyond.
So thoroughly has Mr. Tesla worked out his discovery of the rotating magnetic field, or resultant attraction, that the record of his inventions contains no fewer than twenty-four chapters on varying forms of his polyphase- current apparatus and arrangements of circuit. But ever pursuing new researches, Mr. Tesla, after the enunciation of these fundamental ideas, next brought to notice his series of even more interesting investigations on several novel groups of phenomena produced with currents of high potential and high frequency. To familiarize the American public with some of his results, he lectured upon them at Columbia College, before the American Institute of Electrical Engineers, in May, 1891. The year following, with riper results to publish, and by special invitation, he lectured twice in England, appearing before the Institution of Electrical Engineers, a (distinguished scientific body of which Professor Crookes was then president, and, later, at the Royal Institution, where the immortal Faraday lived and labored. From England he was called to France to repeat his demonstrations before the Society Internationale des Electrifies and the Society Francaise de Physique. In Germany he received the greetings of Hertz and Von Helmholtz, and from his own country came the Order of Saint Sava, conferred by the king. Since his return to this country he has lectured before the Franklin Institute at Philadelphia and the National Electric Light Association at St. Louis. But he has an intense dislike to the platform, and has returned to his laboratory with a remorseful sense of neglected work from which long months of abandonment to unremitting research will not free him.
I can only outline the vast range of the researches that these lectures, and the apparatus connected with the demonstrations, cover. Broadly stated,, Mr. Tesla has advanced the opinion, and sustained it by brilliant experiments of startling beauty and grandeur, that light and heat are produced by electrostatic forces acting between charged molecules or atoms. Perfecting a generator that would give him currents of several thousand alternations per second, and inventing his disruptive discharge coil, he has created electrostatic conditions that have already modified not a few of the accepted notions about electricity. It has been supposed that ordinary currents of one or two thousand volts potential would surely kill, but Mr. Tesla has been seen receiving through his hands currents at a potential of more than 200,000 volts, vibrating a million times per second, and manifesting themselves in dazzling streams of light. This is not a mere tour de force, but illustrates the principle that while currents of lower frequency destroy life, these are harmless. After such a striking test, which, by the way, no one has displayed a hurried inclination to repeat, Mr. Tesla’s body and clothing have continued for some time to emit fine glimmers or halos of splintered light. In fact, an actual flame is produced by this agitation of electrostatically charged molecules, and the curious spectacle can be seen of puissant, white, ethereal flames, that do not consume anything, bursting from the ends of an induction coil as though it were the bush on holy ground. With such vibrations as can be maintained by a potential of 3,000,000 volts, Mr. Tesla expects some day to envelop himself in a complete sheet of lambent fire that will leave him quite uninjured. Such currents as he now uses would, he says, keep a naked man warm at the North Pole, and their use in therapeutics is but one of the practical possibilities that has been taken up.
Utilizing similar currents and mechanism, Mr. Tesla has demonstrated the fact that electric lamps and motors can not only be made to operate on one wire, instead of using a second wire on the ground to complete the circuit, but that we can operate them even by omitting the circuit. Our Subway Boards are to find their wires and occupations gone. Electric vibrations set up at any point of the earth may by resonance at any other spot serve for the transmission of either intelligence or power. With these impulses or wave discharges, Mr. Tesla also opens up an entirely new field of electric lighting. His lamps have no filaments as ordinarily known, but contain a straight fiber, a refractory button, or nothing but a gas. Tubes or bulbs of this kind, in which the imprisoned ether or air beats the crystal walls, when carried into the area or room through which these unsuspected currents are silently vibrating, burst into sudden light. If coated inwardly with phosphorescent substances, they glow in all the splendors of the sunset and the aurora.
These are only a scant handful of ideas and discoveries from the rich mint of Mr. Tesla’s laboratory, where alone, secluded, intimates or assistants shut out, he reasons from cause to effect; and with severe, patient diligence not only elaborates his theories, but tries them by the rack and thumbscrew of experiment. He is of all men most dissatisfied with things as they are in his own field of work. Recently, the high-frequency generators with which he has done so much of this advanced work have been laid aside in discontent for an oscillator, which he thinks may not only replace the steam-engine with its ponderous fly-wheels and governors, but embodies the simplest possible form of efficient mechanical generator of electricity. He may be wrong, but misdirection will only suggest new avenues to the goal.
Mr. Tesla has often been urged to assume domestic ties, settle down, and till some corner of the new domain. But shall he farm or explore? Soon enough the proprietary fences will be set up; soon enough will the dusty, beaten highway, dotted with milestones and finger-posts, run straight ahead. If we would, we cannot leash the pioneers whose yearnings are for inner Nature, whose sense is keenest to her faint voices and odors, the quest of which lures onward through the trackless woods.
—Thomas Commerford Martin
A New System of Alternating Current Motors and Transformers
Delivered before the American Institute of Electrical Engineers, May 1888.
I desire to express my thanks to Professor Anthony for the help he has given me in this matter. I would also like to express my thanks to Mr. Pope and Mr. Martin for their aid. The notice was rather short, and I have not been able to treat the subject so extensively as I could have desired, my health not being in the best condition at present. I ask your kind indulgence, and I shall be very much gratified if the little I have done meets your approval.
In the presence of the existing diversity of opinion regarding the relative merits of the alternate and continuous current systems, great importance is attached to the question whether alternate currents can be successfully utilized in the operation of motors. The transformers, with their numerous advantages, have afforded us a relatively perfect system of distribution, and although, as in all branches of the art, many improvements are desirable, comparatively little remains to be done in this direction. The transmission of power, on the contrary, has been almost entirely confined to the use of continuous currents, and notwithstanding that many efforts have been made to utilize alternate currents for this purpose, they have, up to the present, at least as far as known, failed to give the result desired. Of the various motors adapted to be used on alternate current circuits the following have been mentioned: 1. A series motor with subdivided field. 2. An alternate current generator having its field excited by continuous currents. 3. Elihu Thomson’s motor. 4. A combined alternate and continuous current motor. Two more motors of this kind have suggested themselves to me.
1. A motor with one of its circuits in series with a transformer and the other in the secondary of the transformer. 2. A motor having its armature circuit connected to the generator and the field coils closed upon themselves. These, however, I mention only incidentally.
The subject which I now have the pleasure of bringing to your notice is a novel system of electric distribution and transmission of power by means of alternate currents, affording peculiar advantages, particularly in the way of motors, which I am confident will at once establish the superior adaptability of these currents to the transmission of power and will show that many results heretofore unattainable can be reached by their use; results which are very much desired in the practical operation of such systems and which cannot be accomplished by means of continuous currents.
Before going into a detailed description of this system, I think it necessary to make a few remarks with reference to certain conditions existing in continuous current generators and motors, which, although generally known, are frequently disregarded.
In our dynamo machines, it is well known, we generate alternate currents which we direct by means of a commutator, a complicated device and, it may be justly said, the source of most of the troubles experienced in the operation of the machines. Now, the currents so directed cannot be utilized in the motor, but they must—again by means of a similar unreliable device—be reconverted into their original state of alternate currents. The function of the commutator is entirely external, and in no way dues it affect the internal working of the machines. In reality, therefore, all machines are alternate current machines, the currents appearing as continuous only in the external circuit during their transit from generator to motor. In view simply of this fact, alternate currents would commend themselves as a more direct application of electrical energy, and the employment of continuous currents would only be justified if we had dynamos which would primarily generate, and motors which would be directly actuated by such currents.
But the operation of the commutator on a motor is twofold; firstly, it reverses the currents through the motor, and secondly, it effects, automatically, a progressive shifting of the poles of one of its magnetic constituents. Assuming, therefore, that both of the useless operations in the system, that is to say, the directing of the alternate currents on the generator and reversing the direct currents on the motor, be eliminated, it would still be necessary, in order to cause a rotation of the motor, to produce a progressive shifting of the poles of one of its elements, and the question presented itself,—How to perform this operation by the direct action of alternate currents? I will now proceed to show how this result was accomplished.
figures 1, 1a
In the first experiment a drum-armature was provided with two coils at right angles to each other, and the ends of these coils were connected to two pairs of insulated contact-rings as usual. A ring was then made of thin insulated plates of sheet-iron and wound with four coils, each two opposite coils being connected together so as to produce free poles on diametrically opposite sides of the ring. The remaining free ends of the coils were then connected to the contact-rings of the generator armature so as to form two independent circuits, as indicated in figure 9. It may now be seen what results were secured in this combination, and with this view I would refer to the diagrams, figures 1 to 8a. The field of the generator being independently excited, the rotation of the armature sets up currents in the coils C C1, varying in strength and direction in the well-known manner. In the position shown in figure 1 the current in coil C is nil while coil C1 is traversed by its maximum current, and the connections my be such that the ring is magnetized by the coils c1 c1 as indicated by the letters N S in figure 1a, the magnetizing effect of the coils c c being nil, since these coils are included in the circuit of coil C.
figures 2, 2a
In figure 2 the armature coils are shown in a more advanced position, one-eighth of one revolution being completed. Figure 2a illustrates the corresponding magnetic condition of the ring. At this moment the coil c1 generates a current of the same direction as previously, but weaker, producing the poles n1 s1 upon the ring; the coil c also generates a current of the same direction, and the connections may be such that the coils c c produce the poles n s, as shown in figure 2a. The resulting polarity is indicated by the letters N S, and it will be observed that the poles of the ring have been shifted one-eighth of the periphery of the same.
figures 3, 3a
In figure 3 the armature has completed one-quarter of one revolution. In this phase the current in coil C is maximum, and of such direction as to produce the poles N S in figure 3a, whereas the current in coil C1 is nil, this coil being at its neutral position. The poles N S in figure 3a are thus shifted one-quarter of the circumference of the ring.
figures 4, 4a
Figure 4 shows the coils C C in a still more advanced position, the armature having completed three-eighths of one revolution. At that moment the coil C still generates a current of the same direction as before, but of less strength, producing the comparatively weaker poles n s in figure 4a, The current in the coil C1 is of the same strength, but of opposite direction. Its effect is, therefore, to produce upon the ring the poles n1 and sl as indicated, and a polarity, N S, results, the poles now being shifted three-eighths of the periphery of the ring.
figures 5, 5a
In figure 5 one-half of one revolution of the armature is completed, and the resulting magnetic condition of the ring is indicated in figure 5a. Now, the current in coil C is nil, while the coil C1 yields its maximum current, which is of the same direction as previously; the magnetizing effect is, therefore, due to the coils Cl Cl alone, and, referring to figure 5a, it will be observed that the poles N S are shifted one-half of the circumference of the ring. During the next half revolution the operations are repeated, as represented in the figures 6 to 8a.
figures 6, 6a
A reference to the diagrams will make it clear that during one revolution of the armature the poles of the ring are shifted once around its periphery, and each revolution producing like effects, a rapid whirling of the poles in harmony with the rotation of the armature is the result. If the connections of either one of the circuits in the ring are reversed, the shifting of the poles is made to progress in the opposite direction, but the operation is identically the same. Instead of using four wires, with like result, three wires may be used, one forming a common return for both circuits.
figures 7, 7a
This rotation or whirling of the poles manifests itself in a series of curious phenomena. If a delicately pivoted disc of steel or other magnetic metal is approached to the ring it is set in rapid rotation, the direction of rotation varying with the position of the disc. For instance, noting the direction outside of the ring it will be found that inside the ring it turns in an opposite direction, while it is unaffected if placed in a position symmetrical to the ring. This is easily explained. Each time that a pole approaches it induces an opposite pole in the nearest point on the disc, and an attraction is produced upon that point; owing to this, as the pole is shifted further away from the disc a tangential pull is exerted upon the same, and the action being constantly repeated, a more or less rapid rotation of the disc is the result. As the pull is exerted mainly upon that part which is nearest to the ring, the rotation outside and inside, or right and left, respectively, is in opposite directions, figure 9. When placed symmetrically to the ring, the pull on opposite sides of the disc being equal, no rotation results. The action is based on the magnetic inertia of the iron; for this reason a disc of hard steel is much more affected than a disc of soft iron, the latter being capable of very rapid variations of magnetism. Such a disc has proved to be a very useful instrument in all these investigations, as it has enabled me to detect any irregularity in the action. A curious effect is also produced upon iron filings. By placing some upon a paper and holding them externally quite close to the ring they are set in a vibrating motion, remaining in the same place, although the paper may be moved back and forth; but in lifting the paper to a certain height which seems to be dependent on the intensity of the poles and the speed of rotation, they are thrown away in a direction always opposite to the supposed movement of the poles. If a paper with filings is put flat upon the ring and the current turned on suddenly; the existence of a magnetic whirl may be easily observed.
figures 8, 8a
To demonstrate the complete analogy between the ring and a revolving magnet, a strongly energized electro-magnet was rotated by mechanical power, and phenomena identical in every particular to those mentioned above were observed.
Obviously, the rotation of the poles produces corresponding inductive effects and may be utilized to generate currents in a closed conductor placed within the influence of the poles. For this purpose it is convenient to wind a ring with two sets of superimposed coils forming respectively the primary and secondary circuits, as shown in figure 10. In order to secure the most economical results the magnetic circuit should be completely closed, and with this object in view the construction may be modified at will.
The inductive effect exerted upon the secondary coils will be mainly due to the shifting or movement of the magnetic action; but there may also be currents set up in the circuits in consequence of the variations in the intensity of the poles. However, by properly designing the generator and determining the magnetizing effect of the primary coils the latter element may be made to disappear. The intensity of the poles being maintained constant, the action of the apparatus will be perfect, and the same result will be secured as though the shifting were effected by means of a commutator with an infinite number of bars. In such case the theoretical relation between the energizing effect of each set of primary coils and their resultant magnetizing effect may be expressed by the equation of a circle having its center coinciding with that of an orthogonal system of axes, and in which the radius represents the resultant and the co-ordinates both of its components. These are then respectively the sine and cosine of the angle U between the radius and one of the axes (O X). Referring to figure 11, we have r2 = x2 + y2; where x = r cos a, and y = r sin a.
Assuming the magnetizing effect of each set of coils in the transformer to be proportional to the current—which may be admitted for weak degrees of magnetization—then x = Kc and y = Kc1, where K is a constant and c and cl the current in both sets of coils respectively. Supposing, further, the field of the generator to be uniform, we have for constant speed c1 = K1 sin a and c = K1 sin (90o + a) = K1 cos a, where K1 is a constant. See figure 12. Therefore, x = Kc = K K1 cos a; y = Kcl = K K1 sin a; and K K1 = r.
That is, for a uniform field the disposition of the two coils at right angles will secure the theoretical result, and the intensity of the shifting poles will be constant. But from r2 = x2 + y2 it follows that for y = O, r = x; it follows that the joint magnetizing effect of both sets of coils should be equal to the effect of one set when at its maximum action. In transformers and in a certain class of motors the fluctuation of the poles is not of great importance, but in another class of these motors it is desirable to obtain the theoretical result.
In applying this principle to the construction of motors, two typical forms of motor have been developed. First, a form having a comparatively small rotary effort at the start, but maintaining a perfectly uniform speed at all loads, which motor has been termed synchronous. Second, a form possessing a great rotary effort at the start, the speed being dependent on the load.
These motors may be operated in three different ways: 1. By the alternate currents of the source only. 2. By a combined action of these and of induced currents. 3. By the joint action of alternate and continuous currents.
figures 9,
The simplest form of a synchronous motor is obtained by winding a laminated ring provided with pole projections with four coils, and connecting the same in the manner before indicated. An iron disc having a segment cut away on each side may be used as an armature. Such a motor is shown in figure 9. The disc being arranged to rotate freely within the ring in close proximity to the projections, it is evident that as the poles are shifted it will, owing to its tendency to place itself in such a position as to embrace the greatest number of the lines of force, closely follow the movement of the poles, and its motion will be synchronous with that of the armature of the generator; that is, in the peculiar disposition shown in figure 9, in which the armature produces by one revolution two current impulses in each of the circuits. It is evident that if, by one revolution of the armature, a greater number of impulses is produced, the speed of the motor will be correspondingly increased. Considering that the attraction exerted upon the disc is greatest when the same is in close proximity to the poles, it follows that such a motor will maintain exactly the same speed at all loads within the limits of its capacity.
Fifure10,
To facilitate the starting, the disc may be provided with a coil closed upon itself. The advantage secured by such a coil is evident. On the start the currents set up in the coil strongly energize the disc and increase the attraction exerted upon the same by the ring, and currents being generated in the coil as long as the speed of the armature is inferior to that of the poles, considerable work may be performed by such a motor even if the speed be below normal. The intensity of the poles being constant, no currents will be generated in the coil when the motor is turning at its normal speed.
figures 11, 12
Instead of closing the coil upon itself, its ends may be connected to two insulated sliding rings, and a continuous current supplied to these from a suitable generator. The proper way to start such a motor is to close the coil upon itself until the normal speed is reached, or nearly so, and then turn on the continuous current. If the disc be very strongly energized by a continuous current the motor may not be able to start, but if it be weakly energized, or generally so that the magnetizing effect of the ring is preponderating it will start and reach the normal speed. Such a motor will maintain absolutely the same speed at all loads. It has also been found that if the motive power of the generator is not excessive, by checking the motor the speed of the generator is diminished in synchronism with that of the motor. It is characteristic of this form of motor that it cannot be reversed by reversing the continuous current through the coil.
figure 13
The synchronism of these motors may be demonstrated experimentally in a variety of ways. For this purpose it is best to employ a motor consisting of a stationary field magnet and an armature arranged to rotate within the same, as indicated in figure 13. In this case the shifting of the poles of the armature produces a rotation of the latter in the opposite direction. It results therefrom that when the normal speed is reached, the poles of the armature assume fixed positions relatively to the field magnet and the same is magnetized by induction, exhibiting a distinct pole on each of the pole-pieces. If a piece of soft iron is approached to the field magnet it will at the start be attracted with a rapid vibrating motion produced by the reversals of polarity of the magnet, but as the speed of the armature increases; the vibrations become less and less frequent and finally entirely cease. Then the iron is weakly but permanently attracted, showing that the synchronism is reached and the field magnet energized by induction.
The disc may also be used for the experiment. If held quite close to the armature it will turn as long as the speed of rotation of the poles exceeds that of the armature; but when the normal speed is reached, or very nearly so; it ceases to rotate and is permanently attracted.
A crude but illustrative experiment is made with an incandescent lamp. Placing the lamp in circuit with the continuous current generator, and in series with the magnet coil, rapid fluctuations are observed in the light in consequence of the induced current set up in the coil at the start; the speed increasing, the fluctuations occur at longer intervals, until they entirely disappear, showing that the motor has attained its normal speed. A telephone receiver affords a most sensitive instrument; when connected to any circuit in the motor the synchronism may be easily detected on the disappearance of the induced currents.
In motors of the synchronous type it is desirable to maintain the quantity of the shifting magnetism constant, especially if the magnets are not properly subdivided.
To obtain a rotary effort in these motors was the subject of long thought. In order to secure this result it was necessary to make such a disposition that while the poles of one element of the motor are shifted by the alternate currents of the source, the poles produced upon the other element should always be maintained in the proper relation to the former, irrespective of the speed of the motor. Such a condition exists in a continuous current motor; but in a synchronous motor, such as described, this condition is fulfilled only when the speed is normal.
figure 14
The object has been attained by placing within the ring a properly subdivided cylindrical iron core wound with several independent coils closed upon themselves. Two coils at right angles as in figure 14, are sufficient, but greater number may he advantageously employed. It results from this disposition that when the poles of the ring are shifted, currents are generated in the closed armature coils. These currents are the most intense at or near the points of the greatest density of the lines of force, and their effect is to produce poles upon the armature at right angles to those of the ring, at least theoretically so; and since action is entirely independent of the speed—that is, as far as the location of the poles is concerned—a continuous pull is exerted upon the periphery of the armature. In many respects these motors are similar to the continuous current motors. If load is put on, the speed, and also the resistance of the motor, is diminished and more current is made to pass through the energizing coils, thus increasing the effort. Upon the load being taken off, the counter-electromotive force increases and less current passes through the primary or energizing coils. Without any load the speed is very nearly equal to that of the shifting poles of the field magnet.
It will be found that the rotary effort in these motors fully equals that of the continuous current motors. The effort seems to be greatest when both armature and field magnet are without any projections; but as in such dispositions the field cannot be very concentrated, probably the best results will be obtained by leaving pole projections on one of the elements only. Generally, it may be stated that the projections diminish the torque and produce a tendency to synchronism.
A characteristic feature of motors of this kind is their capacity of being very rapidly reversed. This follows from the peculiar action of the motor. Suppose the armature to be rotating and the direction of rotation of the poles to be reversed. The apparatus then represents a dynamo machine, the power to drive this machine being the momentum stored up in the armature and its speed being the sum of the speeds of the armature and the poles.
If we now consider that the power to drive such a dynamo would be very nearly proportional to the third power of the speed, for this reason alone the armature should be quickly reversed. But simultaneously with the reversal another element is brought into action, namely, as the movement of the poles with respect to the armature is reversed, the motor acts like a transformer in which the resistance of the secondary circuit would be abnormally diminished by producing in this circuit an additional electromotive force. Owing to these causes the reversal is instantaneous.
If it is desirable to secure a constant speed, and at the same time a certain effort at the start, this result may be easily attained in a variety of ways. For instance, two armatures, one for torque and the other for synchronism, may be fastened on the same shaft, and any desired preponderance may be given to either one, or an armature may be wound for rotary effort, but a more or less pronounced tendency to synchronism may be given to it by properly constructing the iron core; and in many other ways.
As a means of obtaining the required phase of the currents in both the circuits, the disposition of the two coils at right angles is the simplest, securing the most uniform action; but the phase may be obtained in many other ways, varying with the machine employed. Any of the dynamos at present in use may be easily adapted for this purpose by making connections to proper points of the generating coils. In closed circuit armatures, such as used in the continuous current systems, it is best to make four derivations from equi-distant points or bars of the commutator, and to connect the same to four insulated sliding rings on the shaft. In this case each of the motor circuits is connected to two diametrically opposite bars of the commutator. In such a disposition the motor may also be operated at half the potential and on the three-wire plan, by connecting the motor circuits in the proper order to three of the contact rings.
In multipolar dynamo machines, such as used in the converter systems, the phase is conveniently obtained by winding upon the armature two series of coils in such a manner that while the coils of one set or series are at their maximum production of current, the coils of the other will be at their neutral position, or nearly so, whereby both sets of coils may be subjected simultaneously or successively to the inducing action of the field magnets.
figures 15, 16, 17
Generally the circuits in the motor will be similarly disposed, and various arrangements may be made to fulfill the requirements; but the simplest and most practicable is to arrange primary circuits on stationary parts of the motor, thereby obviating, at least in certain forms, the employment of sliding contacts. In such a case the magnet coils are connected alternately in both the circuits; that is 1, 3, 5 . . . in one, and 2, 4, 6 . . . in