GB2182804A

GB2182804A - Casing of vacuum interrupters

Info

Publication number: GB2182804A
Application number: GB08622951A
Authority: GB
Inventors: Joseph Carl Sofianek; Jr George Polinko
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1985-11-08
Filing date: 1986-09-24
Publication date: 1987-05-20
Also published as: DE3636966A1; GB8622951D0; CN86106540A; JPS62150620A

Abstract

A vacuum interrupter has one- piece cup-shaped metallic end members (13, 14) brazed to the opposing metallized ends of a cylindrical ceramic casing (12). A portion (56) of the cylindrical wall of the end members, adjacent to the interface with the casing, is thinned. The thinned portion deforms during brazing to minimize stresses from the rigid one-piece end member at the interface which could result in rupture of the sealed envelope. The end members are formed from annealed metal sheet and are then further annealed. <IMAGE>

Description

SPECIFICATION Vacuum interrupters This invention relates to a vacuum interrupter and, more particularly, to the assembly of envelope components of such an interrupter.

Vacuum interrupters comprise an evacuated envelope of substantially cylindrical configuration and opposing end portions. The end members support, respectively, a movable rod and a fixed rod, each of which supports a contact. Movements of the movable rod provides for contact closure and opening. The envelope comprises metallic members and insulating members for electrically insulating the movable and fixed rods from one another.

These members must be secured to provide an envelope structure of high mechanical strength and high heat resistance that maintains its hermetic seals so that the internal vacuum is maintained. Envelope components are commonly joined in face to face contact with the end surface of one member being secured, i.e., brazed, to the end surface of another member. The joint between components must be adequately sealed, i.e., brazed to prevent leaks. Even small leaks can result in failure of the device. Thus, it is desirable to minimize the number of joined envelope components and thus the number of brazed envelope joints so as to reduce the risk of leaks and failures.

The envelope components are secured, i.e., brazed at high temperatures, in the approximate range of 800 to 1050"C and thus are subject to thermal expansion during their assembly. If dissimilar materials, i.e., an insulating member and a metallic member, having different thermal expansion coefficients are secured, the differential thermal expansion and subsequent differential thermal contraction is likely to produce stresses that weaken the joint and result in eventual rupture of the seal.

Even a miniscule rupture in a joint can destroy the seal and thus can destroy operation of the interrupter. It would be desirable for all materials of the envelope to have identical thermal coefficients so as to prevent differential expansion and contraction. The envelope components must, however, be selected to meet a plurality of characteristics and performance requirements. Therefore, it is difficult to obtain satisfactory insulating and metallic members that additionally have substantially identical thermal expansion coefficients.

Accordingly, different solutions are required to prevent rupture causing stresses. These should neither unduly complicate the methods of assembling the interrupter, nor require the interposition of extra members in the envelope structure. The number of envelope components and thus the number of joints between components should be minimized to reduce the possibility of leaks.

In the following described example of the present invention there is provided an interrupter whose envelope is of adequate strength and rigidity to withstand atmospheric forces and mechanical forces applied during operation of the contacts, and which is designed to minimize rupture causing stresses.

Advantageously the envelope has metallic and insulating components joined face to face in a relatively low stress joint.

In particular the envelope has a central cylindrical insulating member and one-piece metallic end members brazed thereto at each of its opposing ends in a substantially low stress manner.

In accordance with the invention, a cylindrical casing of insulating material has first and second metallic end members joined thereto on its opposing sides. The movable and fixed contact rods extend through and are supported, respectively, by the end members. At least one of the end members is a unitary structure integrally formed to comprise an end portion that supports one of the rods, and a cylindrical sidewall whose end surface is secured, e.g., brazed, to an end surface of the casing. This unitary structure must be sufficiently rigid to withstand atmospheric and mechanical operating forces.A cylindrical portion of the sidewall extending from the end surface toward the end portion has a wall thickness reduced substantially with respect to the wall thickness of the other portions of the end member to provide for differential thermal deformation between the cylindrical wall and the insulated casing. In a preferred embodiment, both end members have the above described configuration.

In the accompanying drawings, by way of example only: Figure 1 is a cross sectional view of a vacuum interrupter incorporating the invention; Figure 2 is an exploded end view of an end bell and of an end shield including a brazing ring that may be used in securing these components; Figure 3 is a cross sectional exploded view of portions of the end bell, the insulated casing, and of the brazing ring.

This example of the invention is described with reference to vacuum interrupters of the general type shown in Fig. 1. The illustrated vacuum interrupter comprises a highly evacuated envelope 11 comprising a tubular casing 12 of electrical insulating material and two metal end bells 13 and 14. Located within the envelope are two relatively movable contacts 15 and 16.

Contact 15 is a stationary contact carried by stationary contact rod 1 7 which extends in sealed relationship through the upper end bell 13. Contact 16 is a movable contact carried by movable contact rod 18 which extends freely through a central opening 19 in the lower end bell 14.

A flexible metal bellows 22 provides a seal about the movable contact rod 18. Bellows 22 has its upper end 23 joined to the movable contact rod 18 and its lower end 24 joined to the periphery of the lower end bell that defines the central opening 19.

The interrupter is shown in its open position, i.e., with its contacts 15 and 16 displaced from one another. Closure is effected by driving the movable contact rod 18 upward so as to engage the contacts. Opening, i.e., circuit interruption, results from lowering the movable contact rod 18 to the illustrated position.

Contact opening, i.e., separation, establishes an arc between the contacts which generally persists approximately until a natural current zero is reached. At such time, interruption is completed. The arcing produces metal vapors that deposit on surrounding surfaces.

A bellows shield 29 is provided to protect bellows 22 from the metal vapors.

It is obviously essential to prevent metal vapor deposition on the insulated casing 12. A tubular metal main shield 26 provides primary protection against such deposition. Main shield 26 is concentrically secured to the inner periphery of insulating casing 12. This may, for example, be done with metal clips 27 that are secured, e.g., by spot welding, at points equidistant about the outer periphery of shield 26 and at its longitudinal center. Clips 27 engage protrusions 28 extending inwardly from casing 12.

Insulated casing 12 is further protected from metal vapor deposition by end shields 32 and 33. Upper metal end shield 32 and lower metal end shield 33 are secured, respectively, to upper and lower end bells 13 and 14 and extend concentrically between insulated casing 12 and main shield 26. Thus, metal vapors escaping from the ends of the main shield 26 are prevented by end shields 32 and 33 from reaching the insulated casing 12. Copending Patent Application Serial No. 8623188, in the names of Sofianek et al, is assigned to the assignee of the subject application and is related thereto. This copending applications discloses arrangements for securing the end shield 32 and 33 to the envelope of the vacuum interrupter.

The envelope structure of the interrupter comprises insulated casing 12 and end bells 13 and 14 which are sealed to opposing ends of the casing. In one exemplary embodiment, insulated cylindrical casing 12 is made of a dense, impervious, low loss, vacuum tight, high alumina ceramic that is glazed. The opposing end surfaces 53 of casing 12 have a thin metal coating 54 (Figs. 2 and 3) to facilitate brazing. This may comprise a refractory, i.e., sintered, metal coating of approximately 1 mil in thickness and a thinner electroplated and sintered nickel coating on its exterior. The ceramic, glaze and metal coating must withstand temperatures up to approximately 1000"C without deterioration. In a preferred embodiment, casing 12 has a height of about four inches, an inside diameter of about 4.6 inches, and a wall thickness of about .25 inches.

The end bells, i.e., end members, 13 and 14 comprise a unitary, substantially bell or cup-shaped structure comprising end portion 51 and integral and contiguous therewith cylindrical sidewall 38. The end portion has a central aperture 19 that accomodates a contact rod and has curved segments 52 that flare into the sidewall 38. In the above referenced embodiment, the end members have a height of approximately 2 inches and the cylindrical sidewall has an inner diameter of approximately 4.8 inches. The end members are formed of metallic sheet stock, such as &num;304 sheet steel. In the above referenced embodiment, the sheet stock has a thickness of about 0.06 inches. This thickness is selected to that the end member has sufficient strength, i.e., to support the contact rod and contact assembly and to withstand atmospheric pressure.The diameters of the cylindrical sidewall 38 and of the insulated casing 12 are dimensioned so that the end surface 55 of sidewall 38 abuts the end surface 53, i.e., metal coating 54, of casing 12. The wall of casing 12 is substantially thicker than the wall of the end members. Preferably, the interface between the casing and each end member is at approximately the midpoint between the inner and outer wall surface of insulated casing 12.

The above designated type of sheet steel is non-magnetic. Magnetic flux is generated during operating of the vacuum interrupter. This flux passes through the end plates. If the end members were made of magnetizable material, eddy currents would be generated. These eddy currents would produce undesirable temperature increases in the end members.

The insulated casing 12 and the end members 13 and 14 have different thermal expansion coefficients. The coefficient of linear expansion of casing 12, in the temperature range of interest, is 7.5 to 9.0x 10-6 inches per inch per degree centigrade. The coefficient of linear expansion of &num;304 sheet steel of the end members is greater, i.e., approximately 17.3x10-6 inches per inch per degree centigrade. During interrupter envelope assembly, i.e., brazing of the end surfaces of end members 13 and 14 to the end surface of casing 12, the envelope components may be heated to a temperature of approximately 1000"C.

This results in a substantial differential expansion. Specifically, as the abutting casing and end member are heated, the portion of the cylindrical sidewall 38 adjacent to the interface expands radially. Thus, end surface 55 of the sidewall 38 migrates along end surface 53, i.e., metal coating 54, of casing 12 toward the outside wall surface of the casing. As the envelope assembly is cooled, the molten brazing alloy at the interface between the end member, and the casing cools and solidifies.

Thus, end surface 55 of the end member is brazed to the end surface and metal coating of casing 12. During cooling, there is differential contraction between casing 12 and sidewall 38 of the end member. However, the interface between the respective end surfaces 53 and 55 is now brazed and fixed. Thus, the end surface 55 of the end member can not migrate, radially inward along the end surface of casing 12. Accordingly, the sidewall portion adjacent to end surface 55 deforms inward toward the center axis of the interrupter. This creates substantial stress at the interface between the end member and the casing. This residual stress can rupture the brazed seal between the end member and the casing. Even minute ruptures can destroy the vacuum seal and thus the operability of the vacuum interrupter.

The wall thickness of the cylindrical sidewall 38 is reduced sufficiently to minimize the stress at the interface. The wall thickness is reduced to permit the wall to flex sufficiently to minimize the stress that would otherwise result during heating and cooling during enveiope assembly. However, the wall thickness of the entire end member can not be reduced sufficiently. Such reduction would unduly weaken the end member. Further, it is undesirable to provide a separate, e.g., cylindrical, member of reduced wall thickness intermediate the end member and the casing. This would require at least one additional circular joint at each end of the envelope assembly.

Such joints are always subject to imperfect seals and vacuum seal ruptures, in addition to requiring additional cost and labor.

Instead, we have reduced the wall thickness of a cylindrical portion 56 of sidewall 38 that extends from end surface 55 toward the end portion 51 of the end member. In the above referenced embodiment, the outer wall was reduced so that cylindrical wall portion 56 has a wall thickness of .02+ inches. Its wall thickness thus is no more than about half that of the remaining wall portion of the end member.

In the above referenced embodiment, cylindrical wall portion 56 extends for a length of approximately 0.3" from end surface 55.

Thus, the thin-walled cylindrical portion 56 extends for only a very small portion of the overall, e.g., 2-inch, height of the end member. Cylindrical portion 56 is sufficiently thin and sufficiently long to provide sufficient flexure to minimize residual stress at the joint between the end member and the casing.

Conversely, we have found that the reduced wall thickness of cylindrical portion 56 is sufficiently short and of sufficient wall thickness so as not to impair the structural integrity of the envelope structure of the interrupter.

The end members are preferably secured by a braze alloy to insulated casing 12. For this purpose, a braze ring 57 can be inserted intermediate the metallic end surfaces of the end member and of the insulated casing. Braze ring 57 is preferably prepositioned prior to brazing, such as by being spot welded to the end surface 55 of the end member. The end member with its braze ring can thus be positioned adjacent to casing 12 for brazing. The braze alloy utilized in a preferred embodiment has a melting temperature of 925"C and a solidification temperature of 880"C.

Assembly of the vacuum interrupter may be accomplished as follows. The end members are made of metal sheet stock so as to be free of rupture inducing stress. The sheet stock, e.g., &num;304 steel, is first annealed, i.e., softened. The end member is then die-formed into the described cup or bell-shaped configuration comprising end portion 51 and cylindrical wall 38. Wall 38 is then cut to accurately form pianar end surface 55. Subsequently, a sidewall portion 56 is formed by cutting away, i.e., reducing the thickness of, a portion of cylindrical wall 38 as described. The end member is re-annealed prior to completion of these steps to assure that the resulting end member is stress-free.

Subsequent to manufacture of the end members, the internal components are assembled utilizing conventional techniques. For example, the components of the stationary end and the components of the movable, i.e., operating, end may each be separately assembled with suitable braze rings and shims. If desired, an end shield may be inserted into the end bell as described in the above referenced copending application. The stationary and movable assemblies may then be separately brazed. The mainshield 26 can be inserted into casing 12 and can be retained thereto, such as by clips, i.e., tabs, 27.

Finally, the stationary and movable operating assemblies may be secured to insulated casing 12 in the manner described so that the interrupter is brazed and evacuated.

Claims

1. A vacuum type circuit interrupter comprising: (a) a cylindrical casing of insulating material and first and second metallic end members joined thereto on opposing ends in a sealed relationship; (b) movable and fixed contact rods extending through and being supported by said first and second end members, respectively; (c) at least one of said end members comprising-a unitary structure of an integrally formed end portion and a cylindrical sidewall; (d) said end portion supporting one of said contact rods; (e) said cylindrical sidewall having an end surface secured to one of the opposing ends of the cylindrical casing;; (f) a cylindrical portion of said sidewall extending from the end surface toward the end portion having a wall thickness reduced substantially with respect to the wall thickness of other portions of said cylindrical sidewall and of said end portion to provide for differential thermal deformation between the cylindrical wall and the insulated casing.

2. The vacuum interrupter of Claim 1 wherein said one of the end members is substantially cup-shaped.

3. The vacuum interrupter of Claim 1 or 2 wherein the wall thickness of the cylindrical portion is less than sixty percent of the wall thickness of the other portions of the cylindrical sidewall and of the end portion.

4. The vacuum interrupter of Claim 3 wherein the wall thickness of the cylindrical portion is less than 0.05 inches.

5. The vacuum interrupter of Claim 3 wherein the height of the cylindrical portion of reduced thickness is in the range of 0.1 to 0.5 inches.

6. The vacuum interrupter of Claims 1 or 2 wherein the opposing ends of the cylindrical casing have a metalized layer, the end surface of the cylindrical sidewall is substantially flat and is brazed to the metalized layer on one of the opposing ends of the cylindrical casing.

7. The vacuum interrupter of Claim 1 wherein said at least one of the end members is formed of metallic sheet stock of substantially uniform wall thickness and the wall thickness of said cylindrical portion is reduced with respect to the uniform wall thickness.

8. The vacuum interrupter of Claim 7 wherein the wall of said cylindrical portion is reduced about its outer periphery.

9. A vacuum type circuit interrupter comprising: (a) a cylindrical casing of insulating material and first and second metallic end members joined thereto on opposing ends in a sealed relationship; (b) each of said end members comprising a unitary structure of an integrally formed end portion and a cylindrical sidewall; (c) the end portion of said first and second end members supporting, respectively, a movable and a fixed contact rod coaxially with the longitudinal center axis of the interrupter; (d) the cylindrical sidewall of each of said end members having: (1) an end surface secured to one of the opposing ends of the cylindrical casing; and (2) a cylindrical portion extending from the end surface toward the end portion having a wall thickness reduced substantially with respect to the wall thickness of other portions of said cylindrical sidewall and of said end portion to provide for thermal deformation between the cylindrical wall and the insulated casing.

10. The interrupter of Claim 9 wherein said end members are formed of metallic sheet stock of substantially uniform wall thickness except for the reduced wall thickness of said cylindrical portion.

11. The interrupter of Claim 10 wherein the end members are substantially cupshaped.

12. The interrupter of Claims 9 or 10 further comprising a mainshield concentrically positioned about said movable and said fixed contact rod and secured to the cylindrical casing of insulating material.

13. The interrupter of Claim 12 wherein said cylindrical casing comprises at least one interior protrusion and said mainshield comprises locking tabs clamped to said at least one interior protrusion.

14. The interrupter of Claim 12 further comprising first and second end shields of substantially cone-shaped configuration secured, respectively, to the cylindrical sidewall of said first and second end members.

15. The process of making a vacuum interrupter whose envelope has metallic end members brazed to a cylindrical casing of insulating material and wherein the end members support contact bearing rods, comprising the steps of: (a) annealing sheet stock of predetermined wall thickness; (b) forming the sheet stock into a substantially cup-shaped end member having an apertured end portion and integral therewith a cylindrical wall that terminates in an end surface; (c) reducing the wall thickness of a portion of the cylindrical wall extending from the end surface toward the end portion; (d) annealing the cup-shaped end member to relieve stresses therein; (e) inserting and brazing a contact bearing rod assembly into the apertured end member; (f) providing a cylindrical insulating casing having a metalized coating on at least one of its ends; ; (g) brazing the end surface of the end member to the metallized coating of the cylindrical casing whereby the portion of the cylindrical wall having a reduced wall surface deforms to relieve residual stresses at the interface of the end member and of the insulated casing.

16. A vacuum type circuit interrupter substantially as herein described with reference to the accompanying drawings.