US3009041A - Arc-extinguishing device for direct current arcs - Google Patents

Description

Nov. 14, 1961 J. E. ZLUPKO ARC-EXTINGUISHING DEVICE FOR DIRECT CURRENT ARCS Filed Sept. 25, 1959 F lgJ.

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His Attorney.

United States Patent ()fiice 3,009,041 Patented Nov. 14, 1961 3,009,041 ARC-EXTINGUISHING DEVICE FOR DIRECT CURRENT ARCS John E. Zlupko, Philadelphia, Pa., assignor to General Electric Company, a corporation of New York Filed Sept. 25, 1959, Ser. No. 842,271 6 Claims. (Cl. 200-144) This invention relates to an arc-extinguishing device for direct current arcs and, more particularly, to an arcextinguishing device of this type which has its arc-cooling portions formed of an insulating material comprising a mixture of Portland cement and asbestos.

One type arc-extinguishing device to which this invention particularly relates is the arc-chute disclosed in U.S. Patent 2,460,727, Atwood et al., assigned to the assignee of the present invention. In this arc-chute, a direct current arc is initiated at one end of the chute and is thereafter driven into the interior of the chute along suitable arc-runners which serve to lengthen the arc. As the arc is lengthened, it is also cooled through contact with cooling plates extending parallel to the plane of the arc. This simultaneous lengthening and cooling eventually builds up suflicient arc voltage to effect extinction of the arc.

A material which has been found to be particularly suited for these cooling plates is a mixture comprising a Portland cement binder and asbestos. An arc-chute of the type disclosed in the Atwood patent having its cooling plates made from this material has been found capable o f interrupting heavy unidirectional currents having direct voltages on the order of 1300 volts, providing its surfaces have first been conditioned 'by series of initial operations at lower voltages. The first several of these initial operations must be performed at voltages several hundred volts below this value, or else a failure of the chute is likely to occur.

This conditioning of the surfaces has also been found necessary where the interrupter is to be applied to circuits more highly inductive than the usual rectifier circuit. Such highly inductive circuits impose upon the breaker the requirement that it be capable Olf interrupting short circuit currents involving greater amounts of transient energy than would be the case with the usual rectifier circuit.

Failure of such an arc-chute during its initial field operations can be prevented by performing these initial operations prior to actual field installation, but this approach is unduly expensive. Flame treating of the cooling plates by exposing their surfaces to an oxyacetylene or similar flame has been proposed as a substitute method of conditioning the cooling plates to render the chute capable of handling the high voltages, but this approach also has certain drawbacks. The major drawback is that the thin cooling plates tend to become warped by the heat of the flame and such warpage frequently renders them unusable.

An object of the present invention is to provide an arc-chute of this general type which is capable of handling relatively high levels of direct voltage during its initial operations but yet is constructed in such a manner that its cooling plates are not subjected to objectionable warpage during fabrication.

In carrying out my invention in one form, I provide an arc-extinguishing device for direct current arcs which comprises an arc-chute having cooling structure which is arranged to be exposed to said arcs. This cooling structure comprises a base formed of a mixture of Portland cement and asbestos. Bonded to this base material along its arcing-exposed surfaces is a flame-sprayed refractory oxide coating. This coating has a laminar structure made up of flattened interlocked particles and includes as its major portion a crystalline metallic oxide.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view partly in section showing a circuit interrupter embodying one form of my invention.

FIG. 2 is a cross-sectional view along the line 2--2 of FIG. 1.

Referring now to FIG. 1, I have shown a circuit interrupter 11 which is intended to be used for direct current applications and is particularly well-suited for use as a cathode breaker in a power rectifier system. In one such application, the interrupter may be called upon to interrupt unidirectional fault currents which can rise to very large magnitudes on the order of 100,000 or more amperes. It is important to interrupt this fault current at a relatively low magnitude before it reaches its available peak, and therefore-the arc-extinguishing process must be completed in an extremely short time. The interrupter of this particular application must also be capable of interrupting unidirectional currents at potentials as high as 1300 volts.

The interrupter of FIG. 1 includes an upper terminal stud 12 on which a stationary arcing contact 14 is mounted and a lower terminal stud 16 on which a movable conductive arm 17 carrying an arcing contact 18 is pivotally mounted. In the closed position shown, the conductive arm 17 conductively bridges the gap between the terminals 12 and 16. Circuit interruption is efiected by driving the movable arm 17 clockwise about its pivot by suitable operating means comprising a link 19 to estab lish a circuit interrupting are between the arcing contacts 14- and 18. It is to be understood that the contact structure has been shown in simplified form to facilitate an understanding of the invention. For a more detailed disclosure of suitable contact structure for use in this type of breaker, reference may be had to Patent No. 2,329,003, Seeman, assigned to the assignee of the present invention.

For extinguishing the are drawn between the contacts that is initiated between the contacts 14 and 18. These splitter plates 25, which are preferably about A inch in thickness, are maintained in spaced-apart relationship to each other and to the side walls by means of insulating bus-hing 26 disposed between these parts. These bushings 26 are mounted on suitable bolts 28 which are used for clamping the various components of the arc-chute together. The arc-chute is opened at its upper end 2,9,

and this end constitutes an outlet through which the hot gases generated by, arcing are exhausted.

For facilitating entry of the arc into the arc-chute 20, a pair of conductive arc- runners 30 and 31 extending divergently from the arc-initiation region into the interior of the chute toward the discharge end 29 of the chute are provided. The arc-runner 30 is electrically connected to the upper terminal 12 of the breaker adjacent the lower end of the arc-runner 30, and the arc-runner 31 is electrically connected to the lower terminal 16 of the breaker adjacent the lower end of the arc-runner 31 by suitable conductive means (not shown).

When the movable contact 18 is separated from stationary contact 14, the are that is drawn between the contacts quickly transfers to the arc- runners 30 and 31. One terminal of the arc moves rapidly away from the arc-initiation region along the runner 30, whereas the other terminal moves rapidly away from the arc-initiation region along the runner 31. Transfer of the are to the runners and movement along the runners is produced by the thermal effect of the hot gases rising from the arcinitiation region and also by the magnetic effect resulting from the loop-shaped configuration of the circuit through the interrupter.

As the are is driven outwardly along the arc- runners 30 and 31, it is cooled through contact with the side walls 21 and 22 and the splitter plates 25. Under heavy current conditions, the original arc is believed to be split or divided into a plurality of relatively thin parallel arcs in the narrow slots defined between the adjacent splitter plates 25 'and between. the splitter plates 25 and side walls 21 and 22. Formation of these parallel arcs is facilitated by the presence in the splitter plates of openings 34 interconnecting the adjacent slots. These openings allow the ionized gases developed in one of the slots to communicate with the other slots to initiate breakdown and corresponding arcs in these other slots. These parallel arcs and their arcing products are cooled through contact with splitter plates 25 and the side Walls, and this cooling action contributes to eventual extinction of the arcs. Splitting the original arc into parallel smaller arcs is desirable inasmuch as the smaller arcs can be cooled more rapidly than the original arc.

The above described cooling action combined with the arc lengthening action resulting from movement of the arc terminals along the divergent arc-runners 30* and 31 eventually builds up suflicient arc voltage to effect extinction of the arcs and corresponding interruption of the circuit.

The splitter plates 25 and the side walls 21 and 22 of the arc-chute 20 are preferably formed of an insulating material comprising a mixture of Portland cement and asbestos. This material has been found to be particularly suited for direct current circuit inter'rupters of the general type described hereinabove. For example, an arc-chute of this general type constructed of a Portland cement and asbestos mixture has been found capable of interrupting unidirectional voltages on the order of 1300 volts under short circuit conditions, providing its surfaces have first been conditioned by a series of initial interruptions at lower voltages. The first several of these initial interruptions must be performed at voltages several hundred volts less than 1300 volts, or else a failure of the chute is likely to occur during an initial operation.

Failure of such "an arc-chute during its initial field operations can be prevented by performing these initial operations prior to actual field installation, but this appreach is so unduly expensive as not to be practical. Flame treating of the cooling plates by exposing their surfaces to an oxyacetylene or similar flame in order to vitrirfy these surfaces has been proposed as a substitute method of conditioning the cooling plates to render the chute capable of handling the high voltages, but this approach also has certain drawbacks. One of these is that the cooling plates tend to become severely warped by the heat of the flame, and such warpage fre quently renders them unusable. Because of this tendency to produce warpage, it has been found impractical to flame treat insulating plates having a thickness less than about /2 inch.

I have been able to greatly improve the initial voltage interrupting ability of arc-chutes such as described hereinabove, yet without causing war-page of the splitter plates 25, by flame-spraying the splitter plates 25 with a refractory metallic oxide, such as alumina. This flame-spraying, which takes place before the splitter plates are assembled, can be performed with any conventional flamespraying equipment, but I prefer to rely upon a spray gun in which powdered ceramic is fed into the tip of an oxyacetylene flame. As the ceramic particles are melted by the flame, they are converted into a cloud of tiny molten droplets of ceramic by a stream of gas which projects the droplets from the gun onto the surface of the splitters 25. After striking the surface of the splitters, the particles flatten and freeze into an adherent coating.

Cross-sectional photomicrographs of coatings produced in this manner show that the coatings are laminar in structure, having been built up by the flattened droplets of ceramic as they struck the surface, these flattened panticles interlocking as the coating builds up. Although the coating need not be formed from a pure metallic oxide, it should include as its major component a metallic oxide. Refractory metallic oxides such as I use for flamespraying the splitters 25 are crystalline in the flamesprayed coating. I prefer to use coatings having a thickness of about three to seven mils.

By providing the splitters 25 with a flame-sprayed alumina coating of this nature, I have been able to raise the initial voltage interrupting capacity of the hereinabove-described arc-chute by several hundred volts to a value of about l300 volts DC. In addition, the flamesprayed refractory oxide coating has rendered the interrupter capable of being successfully applied in more highly inductive circuits, where it would be called upon to interrupt short circuit currents involving greater amounts of transient energy. All of this has been accomplished without significant warpage of the splitter inasmuch as the flame-spraying process heats the splitters to only a minor extent.

With respect to the absence of warpage, I maintain the splitter that is being flame-sprayed at a relative low temperature during the flame-spraying process by forcing a stream of air across the rear surface of the splitter during the time the opposite surface is being flame-sprayed. I also hold the nozzle of the gun at a comparatively large distance from the surface being flamesprayed, for example, about five inches. By following this procedure, the temperature of the splitter is limited to a maximum of about 700 R, which is very much lower than the temperature required to fuse or vitrify the surface, the vitrification temperature being in excess of about 2000 F. Had the surface been vitnified, the vitrified coating, upon contraction in response to cooling, would have warped the splitter into an unusable condition due to the unequal coefficients of thermal expansion of the vitreous coating and the base material and due to the rigid bond between the vitreous coating and the base material. The sprayed metal oxide coating, however, has a sponginess due to its porosity which allows the coating to yield in such a manner as to preclude significant Warpage upon cooling. The fact that the temperature change during cooling is relatively slight also lessens the tendency to produce warpage.

As was mentioned hereinabove, a flame-sprayed refractory metal oxide coating is quite porous and, in this respect, is significantly different from a vitrified or glazed coating, which is usually non-porous. It is realized that such non-porous, or vitrified, coatings have been proposed for sealing the surface of arc-chute base materials, but such proposals would not suggest that significant improvements in voltage-interrupting capacity could be derived from porous coatings, such as my flame-sprayed coatings. Moreover, such vitreous coatings cannot readily be applied to very thin parts without producing warpage, as was described hereinabove.

While I have shown and described a particular embodiment of my invention, it Will be obvious to those skilled in the art that various changes and modifications may be made Without departing from my invention in its broader aspects and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention,

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

1. In an arc-extinguishing device for direct current arcs, an arc-chute comprising arc-cooling structure which is exposed to said direct current arcs, said cooling structure comprising a base for-med of a mixture of Portland cement and asbestos and a flame-sprayed refractory oxide coating bonded to said base, said oxide coating having an interlocking laminar structure and including as its major portion a crystalline metal oxide.

2. The arc-extinguishing device of claim 1 in which said metal oxide is alumina.

3. In an arc-extinguishing device for direct current arcs, an arc-chute comprising arc-cooling structure which is exposed to said direct current arcs said cooling structuire comprising a base formed of a mixture of Portland cement and asbestos and a flame-sprayed refiractory oxide coating bonded to said base, said oxide coating having an interlocking laminar structure and including as its major portion a crystalline metal oxide, the interface between said base and said coating being ree of vitrified base material.

4. In an arcextinguishing device for direct current arcs, an arc-chute comprising thin arc-cooling plates which are exposed to said direct current arcs, at least one of said plates comprising a base formed on a mixture of Portland cement and asbestos and a flame-sprayed refractory oxide coating bonded to said base said oxide coating having an interlocking laminar structure and including as its major portion :a crystalline metal oxide.

5. The arc-extinguishing device of claim 4 in which said metal oxide is alumina.

6. The arc-extinguishing device of claim 4 in which the interface between said base and said coating is free of vitrified base material.

References Cited in the file of this patent UNITED STATES PATENTS 2,270,723 Boehne Jan. 20, 1942 2,460,727 Atwood et :al Feb. 1, 1949 2,707,691 Whei ldon May 3, 1955 2,801,672 Baldwin et a1. Aug. 6, 1957

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