Part 15 (1/2)
CHAPTER XL.
TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.
At one time, soon after his arrival in America, Mr. Tesla was greatly interested in the subject of arc lighting, which then occupied public attention and readily enlisted the support of capital. He therefore worked out a system which was confided to a company formed for its exploitation, and then proceeded to devote his energies to the perfection of the details of his more celebrated ”rotary field” motor system. The Tesla arc lighting apparatus appeared at a time when a great many other lamps and machines were in the market, but it commanded notice by its ingenuity. Its chief purpose was to lessen the manufacturing cost and simplify the processes of operation.
We will take up the dynamo first. Fig. 271 is a longitudinal section, and Fig. 272 a cross section of the machine. Fig. 273 is a top view, and Fig. 274 a side view of the magnetic frame. Fig. 275 is an end view of the commutator bars, and Fig. 276 is a section of the shaft and commutator bars. Fig. 277 is a diagram ill.u.s.trating the coils of the armature and the connections to the commutator plates.
The cores c c c c of the field-magnets are tapering in both directions, as shown, for the purposes of concentrating the magnetism upon the middle of the pole-pieces.
The connecting-frame F F of the field-magnets is in the form indicated in the side view, Fig. 274, the lower part being provided with the spreading curved cast legs e e, so that the machine will rest firmly upon two base-bars, r r.
To the lower pole, S, of the field-magnet M is fastened, by means of babbitt or other fusible diamagnetic material, the base B, which is provided with bearings b for the armature-shaft H. The base B has a projection, P, which supports the brush-holders and the regulating devices, which are of a special character devised by Mr. Tesla.
The armature is constructed with the view to reduce to a minimum the loss of power due to Foucault currents and to the change of polarity, and also to shorten as much as possible the length of the inactive wire wound upon the armature core.
[Ill.u.s.tration: FIG. 271.]
It is well known that when the armature is revolved between the poles of the field-magnets, currents are generated in the iron body of the armature which develop heat, and consequently cause a waste of power. Owing to the mutual action of the lines of force, the magnetic properties of iron, and the speed of the different portions of the armature core, these currents are generated princ.i.p.ally on and near the surface of the armature core, diminis.h.i.+ng in strength gradually toward the centre of the core. Their quant.i.ty is under some conditions proportional to the length of the iron body in the direction in which these currents are generated. By subdividing the iron core electrically in this direction, the generation of these currents can be reduced to a great extent. For instance, if the length of the armature-core is twelve inches, and by a suitable construction it is subdivided electrically, so that there are in the generating direction six inches of iron and six inches of intervening air-s.p.a.ces or insulating material, the waste currents will be reduced to fifty per cent.
As shown in the diagrams, the armature is constructed of thin iron discs D D D, of various diameters, fastened upon the armature-shaft in a suitable manner and arranged according to their sizes, so that a series of iron bodies, i i i, is formed, each of which diminishes in thickness from the centre toward the periphery. At both ends of the armature the inwardly curved discs d d, of cast iron, are fastened to the armature shaft.
The armature core being constructed as shown, it will be easily seen that on those portions of the armature that are the most remote from the axis, and where the currents are princ.i.p.ally developed, the length of iron in the generating direction is only a small fraction of the total length of the armature core, and besides this the iron body is subdivided in the generating direction, and therefore the Foucault currents are greatly reduced. Another cause of heating is the s.h.i.+fting of the poles of the armature core. In consequence of the subdivision of the iron in the armature and the increased surface for radiation, the risk of heating is lessened.
The iron discs D D D are insulated or coated with some insulating-paint, a very careful insulation being unnecessary, as an electrical contact between several discs can only occur at places where the generated currents are comparatively weak. An armature core constructed in the manner described may be revolved between the poles of the field magnets without showing the slightest increase of temperature.
[Ill.u.s.tration: FIG. 272.]
[Ill.u.s.tration: FIG. 273.]
The end discs, d d, which are of sufficient thickness and, for the sake of cheapness, of cast-iron, are curved inwardly, as indicated in the drawings. The extent of the curve is dependent on the amount of wire to be wound upon the armatures. In this machine the wire is wound upon the armature in two superimposed parts, and the curve of the end discs, d d, is so calculated that the first part--that is, practically half of the wire--just fills up the hollow s.p.a.ce to the line x x; or, if the wire is wound in any other manner, the curve is such that when the whole of the wire is wound, the outside ma.s.s of wires, w, and the inside ma.s.s of wires, w', are equal at each side of the plane x x. In this case the pa.s.sive or electrically-inactive wires are of the smallest length practicable. The arrangement has further the advantage that the total lengths of the crossing wires at the two sides of the plane x x are practically equal.
[Ill.u.s.tration: FIG. 274.]
To equalize further the armature coils at both sides of the plates that are in contact with the brushes, the winding and connecting up is effected in the following manner: The whole wire is wound upon the armature-core in two superimposed parts, which are thoroughly insulated from each other. Each of these two parts is composed of three separated groups of coils. The first group of coils of the first part of wire being wound and connected to the commutator-bars in the usual manner, this group is insulated and the second group wound; but the coils of this second group, instead of being connected to the next following commutator bars, are connected to the directly opposite bars of the commutator. The second group is then insulated and the third group wound, the coils of this group being connected to those bars to which they would be connected in the usual way. The wires are then thoroughly insulated and the second part of wire is wound and connected in the same manner.
Suppose, for instance, that there are twenty-four coils--that is, twelve in each part--and consequently twenty-four commutator plates. There will be in each part three groups, each containing four coils, and the coils will be connected as follows: Groups. Commutator Bars. { First 1--5 First part of wire { Second 17--21 { Third 9--13 { First 13--17 Second part of wire { Second 5--9 { Third 21--1 In constructing the armature core and winding and connecting the coils in the manner indicated, the pa.s.sive or electrically inactive wire is reduced to a minimum, and the coils at each side of the plates that are in contact with the brushes are practically equal. In this way the electrical efficiency of the machine is increased.
[Ill.u.s.tration: FIG. 275.]
[Ill.u.s.tration: FIG. 276.]
The commutator plates t are shown as outside the bearing b of the armature shaft. The shaft H is tubular and split at the end portion, and the wires are carried through the same in the usual manner and connected to the respective commutator plates. The commutator plates are upon a cylinder, u, and insulated, and this cylinder is properly placed and then secured by expanding the split end of the shaft by a tapering screw plug, v.
[Ill.u.s.tration: FIG. 277.]
The arc lamps invented by Mr. Tesla for use on the circuits from the above described dynamo are those in which the separation and feed of the carbon electrodes or their equivalents is accomplished by means of electro-magnets or solenoids in connection with suitable clutch mechanism, and were designed for the purpose of remedying certain faults common to arc lamps.
He proposed to prevent the frequent vibrations of the movable carbon ”point” and flickering of the light arising therefrom; to prevent the falling into contact of the carbons; to dispense with the dash pot, clock work, or gearing and similar devices; to render the lamp extremely sensitive, and to feed the carbon almost imperceptibly, and thereby obtain a very steady and uniform light.
In that cla.s.s of lamps where the regulation of the arc is effected by forces acting in opposition on a free, movable rod or lever directly connected with the electrode, all or some of the forces being dependent on the strength of the current, any change in the electrical condition of the circuit causes a vibration and a corresponding flicker in the light. This difficulty is most apparent when there are only a few lamps in circuit. To lessen this difficulty lamps have been constructed in which the lever or armature, after the establis.h.i.+ng of the arc, is kept in a fixed position and cannot vibrate during the feed operation, the feed mechanism acting independently; but in these lamps, when a clamp is employed, it frequently occurs that the carbons come into contact and the light is momentarily extinguished, and frequently parts of the circuit are injured. In both these cla.s.ses of lamps it has been customary to use dash pot, clock work, or equivalent r.e.t.a.r.ding devices; but these are often unreliable and objectionable, and increase the cost of construction.
Mr. Tesla combines two electro-magnets--one of low resistance in the main or lamp circuit, and the other of comparatively high resistance in a shunt around the arc--a movable armature lever, and a special feed mechanism, the parts being arranged so that in the normal working position of the armature lever the same is kept almost rigidly in one position, and is not affected even by considerable changes in the electric circuit; but if the carbons fall into contact the armature will be actuated by the magnets so as to move the lever and start the arc, and hold the carbons until the arc lengthens and the armature lever returns to the normal position. After this the carbon rod holder is released by the action of the feed mechanism, so as to feed the carbon and restore the arc to its normal length.
Fig. 278 is an elevation of the mechanism made use of in this arc lamp. Fig. 279 is a plan view. Fig. 280 is an elevation of the balancing lever and spring; Fig. 281 is a detached plan view of the pole pieces and armatures upon the friction clamp, and Fig. 282 is a section of the clamping tube.
M is a helix of coa.r.s.e wire in a circuit from the lower carbon holder to the negative binding screw -. N is a helix of fine wire in a shunt between the positive binding screw + and the negative binding screw -. The upper carbon holder S is a parallel rod sliding through the plates S' S^{2} of the frame of the lamp, and hence the electric current pa.s.ses from the positive binding post + through the plate S^{2}, carbon holder S, and upper carbon to the lower carbon, and thence by the holder and a metallic connection to the helix M.
[Ill.u.s.tration: FIG. 278.]
[Ill.u.s.tration: FIG. 279.]
[Ill.u.s.tration: FIG. 280.]
[Ill.u.s.tration: FIG. 281.]
[Ill.u.s.tration: FIG. 282.]
The carbon holders are of the usual character, and to insure electric connections the springs l are made use of to grasp the upper carbon holding rod S, but to allow the rod to slide freely through the same. These springs l may be adjusted in their pressure by the screw m, and the spring l maybe sustained upon any suitable support. They are shown as connected with the upper end of the core of the magnet N.
Around the carbon-holding rod S, between the plates S' S^{2}, there is a tube, R, which forms a clamp. This tube is counter-bored, as seen in the section Fig. 282, so that it bears upon the rod S at its upper end and near the middle, and at the lower end of this tubular clamp R there are armature segments r of soft iron. A frame or arm, n, extending, preferably, from the core N^{2}, supports the lever A by a fulcrum-pin, o. This lever A has a hole, through which the upper end of the tubular clamp R pa.s.ses freely, and from the lever A is a link, q, to the lever t, which lever is pivoted at y to a ring upon one of the columns S^{3}. This lever t has an opening or bow surrounding the tubular clamp R, and there are pins or pivotal connections w between the lever t and this clamp R, and a spring, r^{2}, serves to support or suspend the weight of the parts and balance them, or nearly so. This spring is adjustable.
At one end of the lever A is a soft-iron armature block, a, over the core M' of the helix M, and there is a limiting screw, c, pa.s.sing through this armature block a, and at the other end of the lever A is a soft iron armature block, b, with the end tapering or wedge shaped, and the same comes close to and in line with the lateral projection eon the core N^{2}. The lower ends of the cores M' N^{2} are made with laterally projecting pole-pieces M^{3} N^{3}, respectively, and these pole-pieces are concave at their outer ends, and are at opposite sides of the armature segments r at the lower end of the tubular clamp R.
The operation of these devices is as follows: In the condition of inaction, the upper carbon rests upon the lower one, and when the electric current is turned on it pa.s.ses freely, by the frame and spring l, through the rods and carbons to the coa.r.s.e wire and helix M, and to the negative binding post V and the core M' thereby is energized. The pole piece M^{3} attracts the armature r, and by the lateral pressure causes the clamp R to grasp the rod S', and the lever A is simultaneously moved from the position shown by dotted lines, Fig. 278, to the normal position shown in full lines, and in so doing the link qand lever t are raised, lifting the clamp R and S, separating the carbons and forming the arc. The magnetism of the pole piece e tends to hold the lever A level, or nearly so, the core N^{2} being energized by the current in the shunt which contains the helix N. In this position the lever A is not moved by any ordinary variation in the current, because the armature b is strongly attracted by the magnetism of e, and these parts are close to each other, and the magnetism of e acts at right angles to the magnetism of the core M'. If, now, the arc becomes too long, the current through the helix M is lessened, and the magnetism of the core N^{3} is increased by the greater current pa.s.sing through the shunt, and this core N^{3}, attracting the segmental armature r, lessens the hold of the clamp R upon the rod S, allowing the latter to slide and lessen the length of the arc, which instantly restores the magnetic equilibrium and causes the clamp R to hold the rod S. If it happens that the carbons fall into contact, then the magnetism of N^{2} is lessened so much that the attraction of the magnet M will be sufficient to move the armature a and lever A so that the armature bpa.s.ses above the normal position, so as to separate the carbons instantly; but when the carbons burn away, a greater amount of current will pa.s.s through the shunt until the attraction of the core N^{2} will overcome the attraction of the core M' and bring the armature lever A again into the normal horizontal position, and this occurs before the feed can take place. The segmental armature pieces r are shown as nearly semicircular. They are square or of any other desired shape, the ends of the pole pieces M^{3}, N^{3} being made to correspond in shape.
In a modification of this lamp, Mr. Tesla provided means for automatically withdrawing a lamp from the circuit, or cutting it out when, from a failure of the feed, the arc reached an abnormal length; and also means for automatically reinserting such lamp in the circuit when the rod drops and the carbons come into contact.
Fig. 283 is an elevation of the lamp with the case in section. Fig. 284 is a sectional plan at the line x x. Fig. 285 is an elevation, partly in section, of the lamp at right angles to Fig. 283. Fig. 286 is a sectional plan at the line y y of Fig. 283. Fig. 287 is a section of the clamp in about full size. Fig. 288 is a detached section ill.u.s.trating the connection of the spring to the lever that carries the pivots of the clamp, and Fig. 289 is a diagram showing the circuit-connections of the lamp.
In Fig. 283, M represents the main and N the shunt magnet, both securely fastened to the base A, which with its side columns, S S, are cast in one piece of bra.s.s or other diamagnetic material. To the magnets are soldered or otherwise fastened the bra.s.s washers or discs a a a a. Similar washers, b b, of fibre or other insulating material, serve to insulate the wires from the bra.s.s washers.
The magnets M and N are made very flat, so that their width exceeds three times their thickness, or even more. In this way a comparatively small number of convolutions is sufficient to produce the required magnetism, while a greater surface is offered for cooling off the wires.
[Ill.u.s.tration: FIG. 286.]
[Ill.u.s.tration: FIG. 283.]
[Ill.u.s.tration: FIG. 285.]
[Ill.u.s.tration: FIG. 284.]
[Ill.u.s.tration: FIG. 287.]
[Ill.u.s.tration: FIG. 288.]