The Inventions, Researches and Writings of Nikola Tesla 44 – Martin · Verses Across
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The Inventions, Researches and Writings of Nikola Tesla 44Part 3 · Auxiliary Brush Regulation of Direct Current Dynamos
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Literature · Technology
The Inventions, Researches and Writings of Nikola Tesla 44
Part 3 · Auxiliary Brush Regulation of Direct Current Dynamos
MartinEnglish
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.
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 Tesla, that a great deal of work in this interesting line is unpublished.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 shifted, 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 passed 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 illustrating 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 illustrating the electric devices that may be employed for automatically adjusting the brushes, and Fig. 258 is a diagram illustrating 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.
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 c is 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 x and 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-space 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 space between the brushes a and c is 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.
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 shifted, 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 shifting 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 passing through the neutralizing helix.
The relative positions of the respective brushes may be varied by moving the auxiliary brush, or the brush c may remain stationary and the core P be connected to the main-brush holder A, so as to adjust the brushes a b in their relation to the brush c. If, however, an adjustment is applied to all the brushes, as seen in Fig. 257, the solenoid should be connected to both a and c, so as to move them toward or away from each other.
There are several known devices for giving motion in proportion to an electric current. In Figs. 256 and 257 the moving cores are shown as convenient devices for obtaining the required extent of motion with very slight changes in the current passing through the helices. It is understood that the adjustment of the main brushes causes variations in the strength of the current independently of the relative position of those brushes to the auxiliary brush. In all cases the adjustment should be such that no current flows over the auxiliary brush when the dynamo is running with its normal load.
In Figs. 256 and 257 A A indicate the main-brush holder, carrying the main brushes, and C the auxiliary-brush holder, carrying the auxiliary brush. These brush-holders are movable in arcs concentric with the centre of the commutator-shaft. An iron piston, P, of the solenoid S, Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is effected by means of a spring and screw or tightener.
In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is shown. The piston of the coil is attached to both brush-holders A A and C. When the brushes are moved directly by electrical devices, as shown in Figs. 256 and 257, these are so constructed that the force exerted for adjusting is practically uniform through the whole length of motion.
It is true that auxiliary brushes have been used in connection with the helices of the field-wire; but in these instances the helices receive the entire current through the auxiliary brush or brushes, and these brushes could not be taken off without breaking the circuit through the field. These brushes cause, moreover, heavy sparking at the commutator. In the present case the auxiliary brush causes very little or no sparking, and can be taken off without breaking the circuit through the field-helices. The arrangement has, besides, the advantage of facilitating the self-excitation of the machine in all cases where the resistance of the field-wire is very great comparatively to the resistance of the main circuit at the start--for instance, on arc-light machines. In this case the auxiliary brush c is placed near to, or better still in contact with, the brush b, as shown in Fig. 258. In this manner the part M' is completely cut out, and as the part M has a considerably smaller resistance than the whole length of the field-wire the machine excites itself, whereupon the auxiliary brush is shifted automatically to its normal position.
In a further method devised by Mr. Tesla, one or more auxiliary brushes are employed, by means of which a portion or the whole of the field coils is shunted. According to the relative position upon the commutator of the respective brushes more or less current is caused to pass through the helices of the field, and the current developed by the machine can be varied at will by varying the relative positions of the brushes.
In Fig. 259, a and b are the positive and negative brushes of the main circuit, and c an auxiliary brush. The main circuit D extends from the brushes a and b, as usual, and contains the helices M of the field wire and the electric lamps or other working devices. The auxiliary brush c is connected to the point x of the main circuit by means of the wire c'. H is a commutator of ordinary construction. It will have been seen from what was said already that when the electro-motive force between the brushes a and c is to the electromotive 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 C B c c' D, the potentials of the points x and y will be equal, and no current will pass over the auxiliary brush c; but if that brush occupies a different position relatively to the main brushes the electric condition is disturbed, and current will flow either from y to x or from x to y, according to the relative position of the brushes. In the first case the current through the field-helices will be partly neutralized and the magnetism of the field magnets will be diminished. In the second case the current will be increased and the magnets gain strength. By combining with the brushes at a b c any automatic regulating mechanism, the current developed can be regulated automatically in proportion to the demands of the working circuit.
In Figs. 264 and 265 some of the automatic means are represented that maybe used for moving the brushes. The core P, Fig. 264, of the solenoid-helix S is connected with the brush a to move the same, and in Fig. 265 the core P is shown as within the helix S, and connected with brushes a and c, so as to move the same toward or from each other, according to the strength of the current in the helix, the helix being within an iron tube, S', that becomes magnetized and increases the action of the solenoid.
In practice it is sufficient to move only the auxiliary brush, as shown in Fig. 264, as the regulation is very sensitive to the slightest changes; but the relative position of the auxiliary brush to the main brushes may be varied by moving the main brushes, or both main and auxiliary brushes may be moved, as illustrated in Fig. 265. In the latter two cases, it will be understood, the motion of the main brushes relatively to the neutral line of the machine causes variations in the strength of the current independently of their relative position to the auxiliary brush. In all cases the adjustment may be such that when the machine is running with the ordinary load, no current flows over the auxiliary brush.
The field helices may be connected, as shown in Fig. 259, or a part of the field helices may be in the outgoing and the other part in the return circuit, and two auxiliary brushes may be employed as shown in Figs. 261 and 262. Instead of shunting the whole of the field helices, a portion only of such helices may be shunted, as shown in Figs. 260 and 262.
The arrangement shown in Fig. 262 is advantageous, as it diminishes the sparking upon the commutator, the main circuit being closed through the auxiliary brushes at the moment of the break of the circuit at the main brushes.
The field helices may be wound in the same direction, or a part may be wound in opposite directions.
The connection between the helices and the auxiliary brush or brushes may be made by a wire of small resistance, or a resistance may be interposed (R, Fig. 263,) between the point x and the auxiliary brush or brushes to divide the sensitiveness when the brushes are adjusted.
The accompanying sketches also illustrate improvements made by Mr. Tesla in the mechanical devices used to effect the shifting of the brushes, in the use of an auxiliary brush. Fig. 266 is an elevation of the regulator with the frame partly in section; and Fig. 267 is a section at the line x x, Fig. 266. C is the commutator; B and B', the brush-holders, B carrying the main brushes a a', and B' the auxiliary or shunt brushes b b. The axis of the brush-holder B is supported by two pivot-screws, p p. The other brush-holder, B', has a sleeve, d, and is movable around the axis of the brush-holder B. In this way both brush-holders can turn very freely, the friction of the parts being reduced to a minimum. Over the brush-holders is mounted the solenoid S, which rests upon a forked column, c. This column also affords a support for the pivots p p, and is fastened upon a solid bracket or projection, P, which extends from the base of the machine, and is cast in one piece with the same. The brush-holders B B' are connected by means of the links e e and the cross-piece F to the iron core I, which slides freely in the tube T of the solenoid. The iron core I has a screw, s, by means of which it can be raised and adjusted in its position relatively to the solenoid, so that the pull exerted upon it by the solenoid is practically uniform through the whole length of motion which is required to effect the regulation. In order to effect the adjustment with greater precision, the core I is provided with a small iron screw, s'. The core being first brought very nearly in the required position relatively to the solenoid by means of the screw s, the small screw s' is then adjusted until the magnetic attraction upon the core is the same when the core is in any position. A convenient stop, t, serves to limit the upward movement of the iron core.
To check somewhat the movement of the core I, a dash-pot, K, is used. The piston L of the dash-pot is provided with a valve, V, which opens by a downward pressure and allows an easy downward movement of the iron core I, but closes and checks the movement of the core when it is pulled up by the action of the solenoid.
To balance the opposing forces, the weight of the moving parts, and the pull exerted by the solenoid upon the iron core, the weights W W may be used. The adjustment is such that when the solenoid is traversed by the normal current it is just strong enough to balance the downward pull of the parts.
The electrical circuit-connections are substantially the same as indicated in the previous diagrams, the solenoid being in series with the circuit when the translating devices are in series, and in shunt when the devices are in multiple arc. The operation of the device is as follows: When upon a decrease of the resistance of the circuit or for some other reason, the current is increased, the solenoid S gains in strength and pulls up the iron core I, thus shifting the main brushes in the direction of rotation and the auxiliary brushes in the opposite way. This diminishes the strength of the current until the opposing forces are balanced and the solenoid is traversed by the normal current; but if from any cause the current in the circuit is diminished, then the weight of the moving parts overcomes the pull of the solenoid, the iron core I descends, thus shifting the brushes the opposite way and increasing the current to the normal strength. The dash-pot connected to the iron core I may be of ordinary construction; but it is better, especially in machines for arc lights, to provide the piston of the dash-pot with a valve, as indicated in the diagrams. This valve permits a comparatively easy downward movement of the iron core, but checks its movement when it is drawn up by the solenoid. Such an arrangement has the advantage that a great number of lights may be put on without diminishing the light-power of the lamps in the circuit, as the brushes assume at once the proper position. When lights are cut out, the dash-pot acts to retard the movement; but if the current is considerably increased the solenoid gets abnormally strong and the brushes are shifted instantly. The regulator being properly adjusted, lights or other devices may be put on or out with scarcely any perceptible difference. It is obvious that instead of the dash-pot any other retarding device may be used.
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The Inventions, Researches and Writings of Nikola Tesla 45
