GB2341979A - Improvements relating to vacuum switching devices - Google Patents

Abstract

A vacuum switching device comprises an evacuated envelope formed by coaxial aligned insulating cylinder members 22, 24 having end walls 25, 26. A tubular shield 18 is disposed adjacent the inner wall surface of the cylinder. Peripheral portions 28, 30, 32 of the shield and the end walls engage the cylinder. The shield and end walls are provided with a set of at least two angularly spaced apart tabs 34, 36, 38 extending from their peripheries and oriented to lie against an outer wall of the cylinder, the protrusions holding the components centred with respect to the insulating cylinder. The shield 18 is held within the insulating cylinder by means of a peripheral flange 30 held between confronting ends of the first and second cylinder members. Outwardly protruding domes 40 centre the shield within the first cylinder 22 and tabs 36 centre the shield within the second cylinder 24, whereby the second cylinder is centred with respect to the shield and the first cylinder.

Description

2341979 VACUUM SWITCHING DEVICES The present invention relates to

improvements in vacuum switching devices, for example vacuum interrupters.

Known vacuum switching devices comprise an evacuated envelope formed by an insulating cylinder sealed at opposite ends by end-wall components, the switching device having at least one electrically conductive, tubular, shield component disposed adjacent the inner wall of the insulating cylinder, the end wall and shield components engaging the cylinder at their respective peripheries. Examples of such switching devices are shown in patent publications EP 0 780 868 and GB 1, 134,487.

It is known to assemble such vacuum switching devices in a single brazing process, in which the various components are positioned as required in an external jig, with brazing discs between those items to be brazed. The whole jigged arrangement is heated up to a temperature at which the brazing agent will flow and the temperature is then reduced so that the brazing agent solidifies, the components of the switching device being then set together to form a complete unit.

One problem with the known brazing procedure is the need for bulky external jigs to hold all the components in alignment with each other. The jigs take up space within the brazing furnace which could otherwise be devoted to production of more switching devices.

A flu-ther problem with this known procedure is that internal items such as the shield are not normally alignable using the jig, so that during brazing, because the brazing agent is a liquid, these non-alignable components can move relative to the jig and assume non-central positions within the switching device. Where this occurs with the shield component, problems can occur. For example unbalanced electrical stresses may occur on the insulator.

Yet another problem arises in connection with accurate centring of the end wall and shield components with respect to the insulating cylinder. It is known to centre these components by providing them with circular flanges which co-operate with and engage the outer wall surface of the insulating cylinder. However, such flanges cannot readily be removed after brazing has been accomplished, and therefore are inconvenient to use if a smooth outer surface of the device is desired.

According to the present invention, a vacuum switching device comprises an evacuated envelope formed by an insulating cylinder sealed at opposite ends by end wall components, the switching device having at least one electrically conductive, tubular shield component disposed adjacent the inner wall of the insulating cylinder, the end wall and shield components engaging the cylinder at their respective peripheries, the shield component andlor at least one of the end wall components being provided with a set of at least two angularly spaced apart protrusions extending from the periphery of said component and adapted to engage an external wall surface of the cylinder, said protrusions being effiective to hold the shield component and/or at least one of the end waft components centred with respect to the insulating cylinder.

The protrusions may comprise a tab or tag of material, possibly substantially rectangular, and may be integral with the component itself, Such protrusions help to align the components during their assembly and brazing together into a unit so that the problems of components moving relative to each other during brazing can be alleviated. Furthermore, the use of such protrusions enable the external jigs which hold the components aligned with each other to be eliminated, or at least reduced in size, so that less space is taken up within the furnace, in turn making for a more efficient and profitable fabrication process.

The provision of protrusions or tags which engage the external wall surface of the insulating cylinder has advantages in comparison with the use of circular flanges because it is cheaper to make the components with tags instead of circular flanges. Moreover, less processing time is required, better productivity is achieved for evacuation of the assembly, and the tags are easily removable after the brazing process which seals the assembly.

Preferably, there are three protrusions in the or each set thereof, and these should be equi-spaced around the periphery of the or each component.

In the preferred embodiment, the insulating cylinder comprises first and second coa.Nially aligned cylinder members and the shield component is held within the insulating cylinder by means of a peripheral flange, which flange is held between confronting ends of the first and second cylinder members. Advantageously, the shield component has first and second sets of protrusions, the first set of protrusions being adapted to engage the internal wall surface of the first cylinder, thereby to centre the shield component within the first cylinder, and the second set of protrusions being adapted to engage the external wall surface of the second cylinder, thereby to centre the sffield component within the second cylinder, whereby the first and second cylinders are held centred with respect to each other.

Preferably, at least one of the end wall components is provided with a set of protrusions adapted to engage the external wall surface of the cylinder so as to align the component with the cylinder. Preferably each end wall has a set of such protrusions.

Providing protrusions on all of the components has the advantage that all the components can be aligned axially and can be concentrically arranged with respect to the cylinder.

Each of the end wall and shield components, including the protrusions, may be formed by a pressing or stamping process from a sheet material, with the protrusions modified by subsequent deformation. It will be appreciated that if the components are formed by pressing, the protrusion may be formed as an integral part of the 5 component during the pressing process.

The first set of protrusions on the shield component may comprise radially outwardly protruding domes as disclosed in EP 0 780 868. The contents of EP 0 780 868 relating to the outwardly protruding domes is incorporated by reference and the skilled person is directed to read EP 0 780 868.

One embodiment of our invention is illustrated in the accompanying drawings in which:

Figure 1 shows a perspective and partially cut away sketch of a typical vacuum interrupter; Figure 2 shows a longitudinal section through a vacuum interrupter according to the present invention; and Figures 3 and 4 show enlargements of certain aspects of the invention as shown in Figure 2.

Vacuum interrupters are commonly used in electrical equipment for interrupting an AC supply in the event of a fault, such as a short-circuit on a power line. A typical vacuum interrupter is shown in Figure 1. The interrupter comprises an outer insulating cylindrical housing 10, normally of ceran- dc, sealed at both ends to provide an evacuated envelope. Housing 10 contains two electrically conductive contacts 11, 12, attached to respective conductive stems 13,14, the stems terminating in endportions 15,16 outside the housing for connection to further electrical equipment (not shown). The end-portions 15, 16 may have external or internal threads (not shown) for effecting the connections.

A bellows unit 17 and a shield component 18 are also contained within housing 10.

The bellows unit 17 seals the stem 14 to the housing 10 while allowing axial movement of the stem 14 with respect to the housing to selectively make and break electrical contact between the contacts 11 and 12, contact 11 and stem 13 being fixed with respect to the housing and stem 13 being sealed thereto. When the contacts are separated while current is flowing through the device, an arc is drawn between the contacts and metal from the contacts is vaporised by the are. The shield 18, which is also electrically conductive, acts to prevent the arc striking the housing and to impede deposition of the metal vapour on the housing.

Figure 2 shows a vacuum interrupter 20 having basic parts similar to the interrupter of Figure 1, corresponding reference numerals being applied to corresponding parts.

The electrically conductive contacts 11, 12 are housed within the evacuated envelope.

The cylinder of the interrupter 20 comprises first and second co-axially aligned cylinder members 22, 24, as opposed to the single unitary cylinder 10 shown in Figure 1. The end portions IS, 16 of the stems 13,14 are each provided with an internally threaded bore (not shown) by means of which external connections can be made.

Substantially circular end wall components or plates 25, 26 seal the outer ends of respective cylinder members 24, 22.

Also as seen in Figure 2, small radially outwardly protruding dome-shaped protrusions 40 are formed in the cylindrical shield component 18. The domes 40 are impressed into the shield wall itself and form dimple-like structures viewed from inside the shield component 18 and roughly hernispherical protrusions when viewed from outside the shield member. Their purpose, as will be explained in more detail below, is to accurately locate the shield 18 by centring it within the first, lower, cylinder 22. Shield 18 is further provided with an annular peripheral flange 30 which is sandwiched between the confronting end faces of the lower and upper cylinders 22, 24.

As will be appreciated by the skilled person, both end faces of both cylinder members 22, 24 have formed thereon metallic rings (not shown) by means of which brazing can be performed to seal the end walls 25, 26 and the shield 18 to the cylinder members. Specifically, the periphery 28 of end wall component 26 contacts and is brazed to a metallic ring on the bottom end face of the cylinder member 22; the periphery 32 of end wall component 25 contacts and is brazed to a metallic ring on the upper end face of the cylinder member 24; and flange 30 of shield component 18 is brazed to metallic rings on the confronting end faces of the two cylinder members 22, 24. Hence, when the interrupter is in an assembled state, each component 18, 25 and 26 contacts at least one end face of the cylinders 22, 24. The brazing of the components to the cylinder members 22, 24 seals the interrupter and allows the inside of the cylinder to 15 be evacuated to form the evacuated envelope. Each of the end wall components 25, 26 and the flange 30 of shield component 18 are provided on their outer peripheries with three protrusions 34, 36, 38 spaced at substantially 120 intervals around the periphery. Each tab 34, 36, 38 is bent at 20 substantially 900 relative to the part of the component from which it projects, so that it is oriented to He alongside and engage an external wall surface of one of the cylinders 22, 24. It may be possible to have only two protrusions instead of three, but to be efFective in centring the shield and end-wall components with respect to the insulating cylinder, the two protrusions should plainly not be spaced either at or very 25 near to 180' apart, or very close together. Between 90 and (say) 170' apart seems reasonable. The protrusions comprise substantially rectangular tags or tabs of material integral with the components. In the section of Figure 2, only a single protrusion can be shown on each flange 28, 30, 32.

The tabs 34, 38 of the end wall components 26, 25, respectively, each engage only one of the cylinders (i.e., the cylinder on which the end wall component is provided), thereby centring themselves with respect to that cylinder. However, shield 18 has two sets of protrusions 36 and 40 which act to centre shield 18 with respect to both cylinders simultaneously that is, the shield is centred within the first cylinder 22 by means of the domes 40 engaging the first cylinder's internal wall surface and within the second cylinder 24 by means of tags 36 engaging the second cylinder's external wall surface. In fact, it is accurate to say that the first and second cylinders 22, 24 are centred with respect to each other by means of the domes 40 and tags 36 of the shield 18.

To assemble the vacuum interrupter a single brazing step is utilised, in which the various components are positioned as required in an external jig, with brazing discs between those items to be brazed, the whole jigged arrangement being heated up to a temperature at which the brazing agent will flow and the temperature then reduced so that the brazing agent sofidifies, the components of the switching device being then set together to form a complete unit.

It is difficult to use a jig to align all the components simultaneously and previous to the present invention the end caps 25, 26 and the shield 18 in particular could move out of alignment with the cylinders 22, 24 during a single step brazing process.

During the jigging and brazing process, the protrusions 34, 36, 38, 40 each ensure that the associated component is correctly aligned with the cylinders 22, 24.

Furthermore, the combination of protrusions 36 and 40 on the shield 18 also ensure that the cylinders 22, 24 are correctly aligned with each other. Hence even though all the components cannot be jigged they remain aligned with each other.

As explained in EP 0 780 868, the height of the domes 40 from the wall of the shield portion 30 is such that the outside diameter of the shield member is, when the assembly is cold and in an unbrazed state, slightly less than the inside diameter of the cylinders 22, 24 and significantly less than the outside diameters of each of the components 18, 25 and 26.

An enlargement of the protrusions 36 can be seen in Figure 3. If a smooth outer surface to the interrupter is desired the protrusions 36 can be removed once the interrupter has been fabricated and this can be seen in Figure 4. Protrusions 34 and 38 may be similarly removed. This contrasts with the known use of fiffly circular flanges to engage the sides of the cylinders, such flanges not being so readily removable as small tags. Since the tags enable accurate location of all the components relative to each other even when a smooth outer surface of the interrupter is required, the support jig can be simplified and does not take up as much room in the brazing furnace as known jigs.

The vacuum interrupter of the present invention is also advantageous in that it allows 15 the interrupter to be constructed and sealed in a single brazing step, thereby reducing costs by reducing the need for separate brazing of sub-assemblies using separate jigs.

Claims (13)

1. A vacuum switching device comprising an evacuated envelope formed by an insulating cylinder scaled at opposite ends by end wall components, the switching device having at least one electrically conductive, tubular shield component disposed adjacent the inner wall of the insulating cylinder, the end wall and shield components engaging the cylinder at their respective peripheries, the shield component andlor at least one of the end wall components being provided with a set of at least two angularly spaced apart protrusions extending from the periphery of said component and adapted to engage an external wall surface of the cylinder, said protrusions being effective to hold the shield component andfor at least one of the end wall components centred with respect to the insulating cylinder.

2. A device according to claim 1, in which there are three protrusions in the or each set thereof

3. A device according to claim 2, wherein the protrusions are equi-spaced around the periphery of the or each component.

4. A device according to any preceding claim wherein the insulating cylinder comprises first and second co-axially aligned cylinder members and the shield component is held within the insulating cylinder by means of a peripheral flange, which flange is held between confronting ends of the first and second cylinder members.

5. A device according to claim 4, wherein the shield component has first and second sets of protrusions, the first set of protrusions being adapted to engage the internal wall surface of the first cylinder, thereby to centre the shield component within the first cylinder, and the second set of protrusions being adapted to engage the external wall surface of the second cylinder, thereby to centre the shield _10component within the second cylinder, whereby the first and second cylinders are held centred with respect to each other.

6. A device according to any preceding claim wherein at least one of the end 5 wall components is provided with a set of protrusions.

7. A device according to claim 6 wherein both of the end wall components are provided with a set of protrusions.

8. A device according to any preceding claim, wherein protrusions which engage the outer wall surface of the insulating cylinder are removable after assembly of the vacuum switching device.

9. A device according to any preceding claim wherein the shield component 15 and/or at least one of the end wall components is a pressing or stamping.

10. A device according to claim 9 as dependent on claim 5, wherein the first set of protrusions comprise radially outwardly protruding portions of the shield component.

11. A device according to claim 10, wherein the protrusions in the first set of protrusions are dome-shaped.

12. A device according to any preceding claim wherein the vacuum switching device is a vacuum interrupter.

13. A vacuum switching device substantially as described herein with reference to Figures 2 to 4 of the accompanying drawings.