US1889497A - Vacuum fuse system - Google Patents

Description

Nov. 29, 1932. D. c. PRINCE VACUUM FUSE SYSTEM Filed Oct. 1, 1929 Fig.2.

HI I 1| J! in 1H Fig.1.

His tto-wwg Patented Nov. 29, 1932 UNITED STATES PATENT OFFICE DAVID C. PRINCE, OF SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK VACUUM FUSE SYSTEM Application filed October 1, 1929. Serial No. 396,547.

My invention relates to high tension vacuum fuses, and more particularly to the application of such vacuum fuses to either direct or alternating current circuits.

It is a principal object of my invention to provide protective means arranged in series, as vacuum fuses of the high tension type, which are particularly adapted to interrupt direct and alternating currents respectively, thereby rendering said protective means effective for use in connection with apparatus which may be connected either in a direct or alternating current system.

My invention will be more fully set forth in the following description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

This application is a continuation in part of my copending application, Serial No. 264,117, filed March 23, 1928.

Referring to the drawing, Fig. 1 is a dial grammatic illustration of an arrangement embodying my invention, and Fig. 2 diagrammatically illustrates a system embodying my invention applied to an electric railway vehicle.

In certain relations of utility, apparatus is involved which operates at times on alternating current and at other times on direct current. An example of such apparatus is an electric locomotive which is arranged so as to operate during a part of its journey from an alternating current trolley or third rail, and during other times from a direct current supply. To protect apparatus under such circumstances, itis desirable to connect two fuses in series, one fuse being of the high vacuum alternating current type and the other of the high vacuum direct current type.

Such an arrangement is diagrammatically illustrated in Fig. 1 of the drawing, and more particularly in Fig. 2, wherein an electric locomotive 1 is provided with the usual pantograph 2 for taking current from the overhead wire 3. Two fuses 4 and 5 are connected in series with each other in the supply circuit of the electric locomotive and are connected to one pole of adouble-pole, double-throw switch 6.

The fuse 4 is preferably of the high vac uum, high tension type, particularly adapted for the interruption of direct currents, and may be of the character described and claimed in a copending application, Serial No. 37 9,- 976, filed July 22, 1929 by myself and Bertram Wellman, wherein at least one of the supporting electrodes for the fusible element is composed of a refractory metal capable of thermionic emission such as tungsten, for example. It is not believed to be necessary in this application to explain in detail the specific structure and theory of operation of this type of vacuum fuse other than to mention that either or both of the supporting electrodes indicated at at, particularly the cathode, are constructed so that cathode spot emission, which may take place in the case of metals having a much lower melting point than tungsten, may not occur, thereby preventing electron flow between the supporting electrodes after rupture of the fusible element when the cathode has cooled below its emission point. The flow of electrons by thermionic emission, however, necessitates that the cathode be heated to such temper ature that electrons are emitted therefrom without regard to any cathode-spot effect. As contrasted with low melting point metals, copper for example, a refractory metal such as tungsten may be heated to such a temperature that thermionic emission therefrom occurs. However, when the tempera ture of the tungsten decreases to below this point electron emission practically immediately ceases. With the supporting electrodes 4 composed of a refractory metal, as tungsten, rupture of the fusible element between the supporting electrodes, due to excessive current, may momentarily cause the cathode to be heated. sufficiently to emit electrons thermionically so that current will continue to flow but will tend to reduce in value as the gap across which the discharge is taking place increases. The supporting electrodes, and particularly the cathode, have reasonably large heat radiating surfaces so that heat due to rupture of the fusible element may be rapidly conducted away and dissipated.

With direct current flowing, the cathode will continuously be cooled by the emission of electrons and by conduction through and radiation from the metal until the temperature is too low to emit further electrons thermionically after which the circuit will be opened. The small amount of vapor produced from the tungsten cathode is not suflicient to result in a positive ion bombardment of the cathode sufficient to restore as much heat as is conducted away from the cathode. The thermionic electron emission of course bombards the anode while the current is flowing and raises the anode to a high temperature but when the circuit is interrupted the anode cools down. Such tungsten fuse terminals are not well adapted for operation to interrupt alternating current because of the fact that the electron bombardment from the cathode to the tungsten anode raises the anode to such a high temperature that it is in a condition to emit electrons thermionically to restart the discharge when the alternating-current wave reverses so as to make the fuse terminal which formerly was the anode now the cathode. The heating of the anodes from electron bombardment is so ef' fective that the current flow may be maintained'in an alternating-current circuit after the fuse has blown by reason of the alternating heating of the electrodes and the hot cathode electron emission occurring alternately from one fuse terminal and then the other. 7

The fuse 5, on the other hand, is preferably of the high tension, high vacuum alternating current type, and may be of the character described and claimed in a copending application, Serial No. 264,113, filed March 23, 1928 by myself and Bertram Wellman, of which application, Serial No. 379,976, above referred to, is a division.

The supporting electrodes or terminals 5' of the fuse 5 are composed of a metal which has too low a melting point for thermionic emission and which is capable of cathodespot emission. Examples of such metals are copper, silver and nickel, and aluminum also may be used. The operation of high tension'fuses of this type is explained in com plete detail in the aforesaid application, and it is only necessary to mention that the com paratively soft metal electrode which happens to be the cathode at any given half cycle when the fusible element blows, permits the current to continue flowing to the other electrode or anode by reason of the cathode-spot emission until the current reaches the zero part of the cycle, at which time the electron flow ceases, and the anode, which now becomes the cathode, is incapable of producing a cathode-spot due to its lower temperature and comparatively low potential gradient at the surface thereof. Rupture of the fusible element, which interconnects the low melting point electrodes 0r terminals 5, due to excessive current, produces a very high potential gradient at that point of the cathode to which the fusible element Was attached, whereby electrons are pulled out of the soft metal terminal at a temperature much lower than that corresponding to emission from a hot cathode of refractory metal. These elec trons in turn collide with the vaporized metal in the space formerly occupied by the fu sible element to ionize some of the metal vapor, thus producing additional electrons, the positive ions traveling to the cathode to accumulate there and maintain the high potential gradient necessary for continuing the flow of electrons, and also serving to maintain the cathode spot at high temperature by their constant bombardment. Accordingly, it is seen that the flow of electrons from the cathode, once it is started, continues as long as the conditions originally present obtain. The current is accordingly interrupted at the first zero value of the cycle and is not again reestablished, since the conditions originally existing after the formation of the first cathode-spot do not obtain after a half cycle, during which there has been no flow of electrons between the fuse terminals.

The double-pole double-throw switch 6 when positioned as illustrated connects the high tension high vacuum fuses in series with the driving motors 7, so that they are adapted to operate on direct current. When the motors are to be operated on alternating current, the switch 6 is thrown over into engagement with the contacts 8, thereby connecting the transformer 9 to the driving motors and to the fuses in series.

It will be accordingly noted that in the event of excessive overload on the motors on either alternating or direct current there will always be a protective fuse connected in circuit with the motors which shall clear the circuit in a manner best adapted for the type of current used. This feature is most im pcrtant since it has been found that a single fuse of the high vacuum high tension type operates altogether differently on direct and alternating currents, as more specifically pointed out in the aforesaid applications.

Assuming,for example, that the driving motors are operating on direct current as illustrated and that an overload at which the current is to be interrupted occurs, it may happen that both the direct and alternating current fuses blow. In the alternating current fuse, however,.the creation of the cathode-spot causes electron flow to the anode which will continue as long as the polarity of the current does not reverse. It is therefore clearly apparent that the alternating current fuse by itself would be ineffective to inter iao rupt a direct current. The direct current fuse, however, would quickly clear the circuit since its cathode of refractory material would prevent the formation of a cathode spot. 5 Moreover, since the temperature of the refractory cathode is but momentarily raised by the blowing of the fusible element to a point at which thermionic emission may occur, rapid cooling thereof prevents further thermionic emission and the current between the fuse terminals stops flowing. In clearing the circuit on alternating current, the alternating current fuse performs best, since electron flow ceases immediately after the current has passed through its first zero value, and the lapse of one-half cycle is sufiicient to prevent restoration of the electron flow.

It shall be understood that my invention is not limited to the specific arrangement and system herein illustrated, but is applicable to any protective system or apparatus adapted to operate on either high tension direct or alternating currents.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a protective system for apparatus adapted to operate on either direct or alternating current, an alternating current fuse and a direct current fuse connected in series so with said apparatus.

2. In a protective system for apparatus adapted to operate on either direct or alternating current, two fuses in series in the circuit of the apparatus to be protected, one of said fuses being of the high vacuum direct current type and the other being of the high vacuum alternating current type.

3. In a protective system for apparatus adapted to operate on either direct or alternating current, a high vacuum fuse having one of its supporting electrodes composed of a refractory metal, and a second high vacuum fuse having supporting electrodes of a metal of thermionic emission, said high vacuum fuses being in series and in the circuit of the apparatus to be protected.

4. In a protective system for apparatus adapted to operate on either direct or alternating current, a high vacuum high tension fuse having one of its supporting electrodes composed of tungsten, and a second high vacuum high tension fuse having its supporting electrodes composed of nickel, said fuses being connected in series with each other and in the circuit of the apparatus to be protected.

5. The combination with an electrically propelled vehicle comprising motors adapted to operate on either alternating current or direct current, of protective means connected in series in the circuit of said motors comprising a high vacuum direct current fuse and a high vacuum alternating current fuse.

In witness whereof, I have hereto set my hand this 30th day of September, 1929. 05 DAVID C. PRINCE.

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