Part 14 (1/2)
G is a water-supply pipe, and H is the steam-exhaust pipe, which communicates with all the tubes C in the armature B, so that steam escaping from the boiler will pa.s.s through the tubes.
In the steam-exhaust pipe H is a valve V, to which is connected the lever I, by the movement of which the valve is opened or closed. In such a case as this the heat of the fire may be utilized for other purposes after as much of it as may be needed has been applied to heating the core B. There are special advantages in the employment of a cooling device, in that the metal of the core B is not so quickly oxidized. Moreover, the difference between the temperature of the applied heat and of the steam, air, or whatever gas or fluid be applied as the cooling medium, may be increased or decreased at will, whereby the rapidity of the magnetic changes or fluctuations may be regulated.
CHAPTER x.x.xVII.
ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.
In direct current dynamos of great electromotive force--such, for instance, as those used for arc lighting--when one commutator bar or plate comes out of contact with the collecting-brush a spark is apt to appear on the commutator. This spark may be due to the break of the complete circuit, or to a shunt of low resistance formed by the brush between two or more commutator-bars. In the first case the spark is more apparent, as there is at the moment when the circuit is broken a discharge of the magnets through the field helices, producing a great spark or flash which causes an unsteady current, rapid wear of the commutator bars and brushes, and waste of power. The sparking may be reduced by various devices, such as providing a path for the current at the moment when the commutator segment or bar leaves the brush, by short-circuiting the field-helices, by increasing the number of the commutator-bars, or by other similar means; but all these devices are expensive or not fully available, and seldom attain the object desired.
To prevent this sparking in a simple manner, Mr. Tesla some years ago employed with the commutator-bars and intervening insulating material, mica, asbestos paper or other insulating and incombustible material, arranged to bear on the surface of the commutator, near to and behind the brush.
In the drawings, Fig. 244 is a section of a commutator with an asbestos insulating device; and Fig. 245 is a similar view, representing two plates of mica upon the back of the brush.
In 244, C represents the commutator and intervening insulating material; B B, the brushes. d d are sheets of asbestos paper or other suitable non-conducting material. f f are springs, the pressure of which may be adjusted by means of the screws g g.
In Fig. 245 a simple arrangement is shown with two plates of mica or other material. It will be seen that whenever one commutator segment pa.s.ses out of contact with the brush, the formation of the arc will be prevented by the intervening insulating material coming in contact with the insulating material on the brush.
[Ill.u.s.tration: FIG. 244.]
[Ill.u.s.tration: FIG. 245.]
Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia, or other suitable material, may be used, as the paper and cloth are soft, and serve at the same time to wipe and polish the commutator; but mica or any other suitable material can be employed, provided the material be an insulator or a bad conductor of electricity.
A few years later Mr. Tesla turned his attention again to the same subject, as, perhaps, was very natural in view of the fact that the commutator had always been prominent in his thoughts, and that so much of his work was even aimed at dispensing with it entirely as an objectionable and unnecessary part of dynamos and motors. In these later efforts to remedy commutator troubles, Mr. Tesla constructs a commutator and the collectors therefor in two parts mutually adapted to one another, and, so far as the essential features are concerned, alike in mechanical structure. Selecting as an ill.u.s.tration a commutator of two segments adapted for use with an armature the coils or coil of which have but two free ends, connected respectively to the segments, the bearing-surface is the face of a disc, and is formed of two metallic quadrant segments and two insulating segments of the same dimensions, and the face of the disc is smoothed off, so that the metal and insulating segments are flush. The part which takes the place of the usual brushes, or the ”collector,” is a disc of the same character as the commutator and has a surface similarly formed with two insulating and two metallic segments. These two parts are mounted with their faces in contact and in such manner that the rotation of the armature causes the commutator to turn upon the collector, whereby the currents induced in the coils are taken off by the collector segments and thence conveyed off by suitable conductors leading from the collector segments. This is the general plan of the construction adopted. Aside from certain adjuncts, the nature and functions of which are set forth later, this means of commutation will be seen to possess many important advantages. In the first place the short-circuiting and the breaking of the armature coil connected to the commutator-segments occur at the same instant, and from the nature of the construction this will be done with the greatest precision; secondly, the duration of both the break and of the short circuit will be reduced to a minimum. The first results in a reduction which amounts practically to a suppression of the spark, since the break and the short circuit produce opposite effects in the armature-coil. The second has the effect of diminis.h.i.+ng the destructive effect of a spark, since this would be in a measure proportional to the duration of the spark; while lessening the duration of the short circuit obviously increases the efficiency of the machine.
[Ill.u.s.tration: FIG. 246.]
[Ill.u.s.tration: FIG. 247.]
The mechanical advantages will be better understood by referring to the accompanying diagrams, in which Fig. 246 is a central longitudinal section of the end of a shaft with the improved commutator carried thereon. Fig. 247 is a view of the inner or bearing face of the collector. Fig. 248 is an end view from the armature side of a modified form of commutator. Figs. 249 and 250 are views of details of Fig. 248. Fig. 251 is a longitudinal central section of another modification, and Fig. 252 is a sectional view of the same. A is the end of the armature-shaft of a dynamo-electric machine or motor. A' is a sleeve of insulating material around the shaft, secured in place by a screw, a'.
[Ill.u.s.tration: FIG. 248.]
[Ill.u.s.tration: FIG. 249.]
[Ill.u.s.tration: FIG. 250.]
The commutator proper is in the form of a disc which is made up of four segments D D' G G', similar to those shown in Fig. 248. Two of these segments, as D D', are of metal and are in electrical connection with the ends of the coils on the armature. The other two segments are of insulating material. The segments are held in place by a band, B, of insulating material. The disc is held in place by friction or by screws, g' g', Fig. 248, which secure the disc firmly to the sleeve A'.
The collector is made in the same form as the commutator. It is composed of the two metallic segments E E' and the two insulating segments F F', bound together by a band, C. The metallic segments E E' are of the same or practically the same width or extent as the insulating segments or s.p.a.ces of the commutator. The collector is secured to a sleeve, B', by screws g g, and the sleeve is arranged to turn freely on the shaft A. The end of the sleeve B' is closed by a plate, f, upon which presses a pivot-pointed screw, h, adjustable in a spring, H, which acts to maintain the collector in close contact with the commutator and to compensate for the play of the shaft. The collector is so fixed that it cannot turn with the shaft. For example, the diagram shows a slotted plate, K, which is designed to be attached to a stationary support, and an arm extending from the collector and carrying a clamping screw, L, by which the collector may be adjusted and set to the desired position.
Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit the insulating segments of both commutator and collector loosely and to provide some means--as, for example, light springs, e e, secured to the bands A' B', respectively, and bearing against the segments--to exert a light pressure upon them and keep them in close contact and to compensate for wear. The metal segments of the commutator may be moved forward by loosening the screw a'.
The line wires are fed from the metal segments of the collector, being secured thereto in any convenient manner, the plan of connections being shown as applied to a modified form of the commutator in Fig. 251. The commutator and the collector in thus presenting two flat and smooth bearing surfaces prevent most effectually by mechanical action the occurrence of sparks.
The insulating segments are made of some hard material capable of being polished and formed with sharp edges. Such materials as gla.s.s, marble, or soapstone may be advantageously used. The metal segments are preferably of copper or bra.s.s; but they may have a facing or edge of durable material--such as platinum or the like--where the sparks are liable to occur.
[Ill.u.s.tration: FIG. 251.]
[Ill.u.s.tration: FIG. 252.]
In Fig. 248 a somewhat modified form of the invention is shown, a form designed to facilitate the construction and replacing of the parts. In this modification the commutator and collector are made in substantially the same manner as previously described, except that the bands B C are omitted. The four segments of each part, however, are secured to their respective sleeves by screws g' g', and one edge of each segment is cut away, so that small plates a b may be slipped into the s.p.a.ces thus formed. Of these plates a a are of metal, and are in contact with the metal segments D D', respectively. The other two, b b, are of gla.s.s or marble, and they are all better square, as shown in Figs. 249 and 250, so that they may be turned to present new edges should any edge become worn by use. Light springs d bear upon these plates and press those in the commutator toward those in the collector, and insulating strips c c are secured to the periphery of the discs to prevent the blocks from being thrown out by centrifugal action. These plates are, of course, useful at those edges of the segments only where sparks are liable to occur, and, as they are easily replaced, they are of great advantage. It is considered best to coat them with platinum or silver.
In Figs. 251 and 252 is shown a construction where, instead of solid segments, a fluid is employed. In this case the commutator and collector are made of two insulating discs, S T, and in lieu of the metal segments a s.p.a.ce is cut out of each part, as at R R', corresponding in shape and size to a metal segment. The two parts are fitted smoothly and the collector T held by the screw h and spring H against the commutator S. As in the other cases, the commutator revolves while the collector remains stationary. The ends of the coils are connected to binding-posts s s, which are in electrical connection with metal plates t t within the recesses in the two parts S T. These chambers or recesses are filled with mercury, and in the collector part are tubes W W, with screws w w, carrying springs X and pistons X', which compensate for the expansion and contraction of the mercury under varying temperatures, but which are sufficiently strong not to yield to the pressure of the fluid due to centrifugal action, and which serve as binding-posts.
In all the above cases the commutators are adapted for a single coil, and the device is particularly suited to such purposes. The number of segments may be increased, however, or more than one commutator used with a single armature. Although the bearing-surfaces are shown as planes at right angles to the shaft or axis, it is evident that in this particular the construction may be very greatly modified.
CHAPTER x.x.xVIII.
AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.
An interesting method devised by Mr. Tesla for the regulation of direct current dynamos, is that which has come to be known as the ”third brush” method. In machines of this type, devised by him as far back as 1885, he makes use of two main brushes to which the ends of the field magnet coils are connected, an auxiliary brush, and a branch or shunt connection from an intermediate point of the field wire to the auxiliary brush.[14]
[14] The compiler has learned partially from statements made on several occasions in journals and partially by personal inquiry of Mr. Tesla, that a great deal of work in this interesting line is unpublished. In these inventions as will be seen, the brushes are automatically s.h.i.+fted, but in the broad method barely suggested here the regulation is effected without any change in the position of the brushes. This auxiliary brush invention, it will be remembered, was very much discussed a few years ago, and it may be of interest that this work of Mr. Tesla, then unknown in this field, is now brought to light.
The relative positions of the respective brushes are varied, either automatically or by hand, so that the shunt becomes inoperative when the auxiliary brush has a certain position upon the commutator; but when the auxiliary brush is moved in its relation to the main brushes, or the latter are moved in their relation to the auxiliary brush, the electric condition is disturbed and more or less of the current through the field-helices is diverted through the shunt or a current is pa.s.sed over the shunt to the field-helices. By varying the relative position upon the commutator of the respective brushes automatically in proportion to the varying electrical conditions of the working-circuit, the current developed can be regulated in proportion to the demands in the working-circuit.
Fig. 253 is a diagram ill.u.s.trating the invention, showing one core of the field-magnets with one helix wound in the same direction throughout. Figs. 254 and 255 are diagrams showing one core of the field-magnets with a portion of the helices wound in opposite directions. Figs. 256 and 257 are diagrams ill.u.s.trating the electric devices that may be employed for automatically adjusting the brushes, and Fig. 258 is a diagram ill.u.s.trating the positions of the brushes when the machine is being energized at the start.
a and b are the positive and negative brushes of the main or working-circuit, and c the auxiliary brush. The working-circuit D extends from the brushes a and b, as usual, and contains electric lamps or other devices, D', either in series or in multiple arc.
M M' represent the field-helices, the ends of which are connected to the main brushes a and b. The branch or shunt wire c' extends from the auxiliary brush c to the circuit of the field-helices, and is connected to the same at an intermediate point, x.
[Ill.u.s.tration: FIG. 253.]
H represents the commutator, with the plates of ordinary construction. When the auxiliary brush c occupies such a position upon the commutator that the electro-motive force between the brushes a and cis to the electro-motive force between the brushes c and b as the resistance of the circuit a M c' c A is to the resistance of the circuit b M' c' c B, the potentials of the points x and Y will be equal, and no current will flow over the auxiliary brush; but when the brush c occupies a different position the potentials of the points xand Y will be different, and a current will flow over the auxiliary brush to and from the commutator, according to the relative position of the brushes. If, for instance, the commutator-s.p.a.ce between the brushes a and c, when the latter is at the neutral point, is diminished, a current will flow from the point Y over the shunt c to the brush b, thus strengthening the current in the part M', and partly neutralizing the current in part M; but if the s.p.a.ce between the brushes a and cis increased, the current will flow over the auxiliary brush in an opposite direction, and the current in M will be strengthened, and in M', partly neutralized.
By combining with the brushes a, b, and c any usual automatic regulating mechanism, the current developed can be regulated in proportion to the demands in the working circuit. The parts M and M' of the field wire may be wound in the same direction. In this case they are arranged as shown in Fig. 253; or the part M may be wound in the opposite direction, as shown in Figs. 254 and 255.
[Ill.u.s.tration: FIG. 254.]
It will be apparent that the respective cores of the field-magnets are subjected to neutralizing or intensifying effects of the current in the shunt through c', and the magnetism of the cores will be partially neutralized, or the points of greatest magnetism s.h.i.+fted, so that it will be more or less remote from or approaching to the armature, and hence the aggregate energizing actions of the field magnets on the armature will be correspondingly varied.
In the form indicated in Fig. 253 the regulation is effected by s.h.i.+fting the point of greatest magnetism, and in Figs. 254 and 255 the same effect is produced by the action of the current in the shunt pa.s.sing through the neutralizing helix.