GB2575990A

GB2575990A - Improvements to AMF contacts in vacuum switching devices

Info

Publication number: GB2575990A
Application number: GB1812409.9A
Authority: GB
Inventors: Thomas Falkingham Leslie
Original assignee: S&C Electric Co
Current assignee: S&C Electric Co
Priority date: 2018-07-30
Filing date: 2018-07-30
Publication date: 2020-02-05
Also published as: GB201812409D0; GB2577376B; GB2577376A; GB201910881D0

Abstract

A contact for an axial field vacuum switching device comprises a stem conductively bonded to the base of a cup-shaped support, the open end of the cup being closed by a disc of contact material and the sides of the cup having angled slots cut through to produce an axial magnetic field. Rings or short tubes are placed within the cup-shaped support and are of a suitable dimension to provide support to the cup against forces pushing inwards. The rings or tubes fit sufficiently well into the cup that they keep each other in position. The number or rings or tubes may range from two to seven. The rings or tubes may be made of a strong material with relatively poor electrical conductivity, such as stainless steel, so that very little current passes to the contact disc through them. Each ring may be provided with one or more holes or notches to allow the escape of gas during vacuum pumping. The rings or tubes may prevent the inward dishing of the contact disc due to the axial load when the contacts are closed or the collapse of the cup walls due to the structural weakness caused by the presence of angled slots.

Description

This invention relates to improvements in vacuum switching devices, which may include for example vacuum interrupters or vacuum switches.

Atypical idealised construction of a vacuum switching device is shown in figure 1. These devices generally consist of an evacuated envelope which includes an insulating component (a) and end plates (b), together with a fixed electrode (c) and a moving electrode (d). The electrodes can be engaged and disengaged mechanically to perform the switching function. Normally this movement is permitted without breaking the seal of the evacuated envelope by means of a bellows or diaphragm arrangement (e). Each electrode (c or d) consists of a conducting rod (f) and a contact or contact assembly (g).

Vacuum switching devices that are designed to switch large currents have been produced for many years. These designs use self-generated magnetic fields to control electrical arcs that arise when the contacts are opened. A number of different designs of contact assemblies have been proposed for use under high current conditions. This invention concerns a type known as an Axial Magnetic Field contact assembly, in which the contact geometry produces a magnetic field that is substantially aligned with the axis of the contacts. With axial magnetic field geometries the contact faces are usually in the form of discs, and the effect of the axial field is to diffuse the current arc fairly evenly over the contact surfaces, thereby preventing overheating of points on these surfaces. Examples of such designs may be seen in EP 0349303, DE 3915519, DE 3610241, and GB 238111A.

A number of different designs of axial field contacts and contact assemblies have been proposed to switch the current on or off under load and fault current conditions. Examples of such designs may be seen in EP 0349303, DE 3915519, DE 3610241, and GB 238111 A.

One form of axial field contact assembly is shown by US 6479779 Bl and shown in figure 2. The end of the contact stem (f) connects to a conical base component (g) which forms a closure to a cup shaped contact support component (h). These parts are made of a material of good electrical conductivity such as copper. Conductively bonded to the base of the cup is a contact disc (i) made of a layer of a material such as a copper-chromium material, which does not have such good electrical conductivity as copper, but does more effectively resist erosion by the electrical arc that forms during switching. Angled slots (j) are cut through the sides and base of the contact cup which force current passing from the contact stem to the contact disc to follow a path towards the contact edge which produces the desired form of magnetic field.

This design of contact has been made in large numbers over a number of years, but when many switching operations have been performed two undesirable effects occur:

a) The contact discs, together with the cup bases to which they are bonded, become dished inwards, as shown in figure 3, so that they are further apart towards the centre. This causes the diffused arc to concentrate at the edges of the contact discs, which increases the current density there, and so increases the rate of wear of the contacts at the edges, thus reducing the current carrying capacity and working life of the contacts.

b) Mechanical forces when the contacts are rapidly closed may also cause the slots in the side of the contact cup to become distorted to the detriment of the control of the current.

The invention mitigates these problems by providing mechanical support to the inside of the base of the cup to oppose the forces which damage the contact. According to the invention there are provided a number of rings, or short tubes (hereby referred to as rings), which fill or partly fill the interior of the cup and which are of such a diameter that they almost touch each other and the inside of the cup wall, and of such a length that they span the distance from the face of the conical base component to the inner face of the base of the cup. The case where there are three rings is illustrated in figure 3, which is a sectioned view of a complete pair of contacts, and also as (n) in figure 4. It can be seen that the rings are self locating because they keep each other in position well enough if they are a little loose but not too loose.

The rings are made of a strong material which is of relatively poor electrical conductivity, such as stainless steel, so that very little current passes directly to the contact face through the rings. The rings may be made cheaply by cutting lengths from suitable tube. Each ring may be provided with one or more holes (1) or notches to facilitate the escape of gas during vacuum pumping. The rings may be made to the precise length needed to fit closely between the base of the cup and the underside of the contact disc, or they may be shorter so that less precision is needed in manufacture, which will permit a small amount of distortion of the contact disc.

The rings can be made fairly precisely to such a diameter that they fit closely into the space available, in which case they will provide support to each other against buckling under the axial load which causes the contacts to dish. This would allow them to be manufactured with thinner walls which in turn would increase the electrical resistance of the rings. Alternatively they can be a loose fit which reduces cost and assembly problems, but also reduces the mutual support.

In figure 5 are illustrated the cases where there are (m) two rings, (n) three rings, (o) four rings, (p) five rings, (q) six rings and (r) seven rings. The rings are shown fitted within the slotted sides of the contact cup, with other parts omitted. The number of rings determines the degree to which support is provided at the centre of the contact disc and across its area, and the degree to which support is provided close to the inside of the slotted cup wall to oppose crushing of the slots.

Patent application GB2552839, illustrated in figure 6, shows a different form of axial field contact assembly in which the contact stem is joined to the base of a contact cup, and the open end of the cup is bridged by a disc (f) of contact material. The sides of the contact cup have angled slots (j) cut through them. In this type of contact assembly there is no copper layer under the contact disc (s). GB2552839 proposes a support in the form of a thin-walled cone of poorly conducting material located centrally within the contact cup to oppose forces acting to dish the contact disc. This form of support is effective but has the disadvantages that a means to centralise the cone must be provided, and it only provides support in the central area of the contact disc. The present invention, with its advantages of being self locating and of providing support more broadly across the rear area of the contact cup, provides an alternative solution to the problem in this type of contact.

Patent US 6479779 Bl shows a form of support to prevent compression of the slots in the side of the contact cup, whereby a ring of strong metal, for example stainless steel, is placed against the inside of the slotted wall of the cup. This invention is effective, but requires an accurately made component and does not provide support to the centre of the contact disc. The present invention can cooperate with this previous invention, or may replace it.

One skilled in the art will appreciate that other numbers of rings may be used, and that this form of contact disc support need not be limited to cylindrical rings, but might be in the form for example of hexagonal tubes, structures in the form of spiral springs, or flat metal strip formed into shapes such as a cross or a honeycomb.

Claims

Claims:

1. A contact for an axial field vacuum switching device as generally described in US 6479779 Bl where rings or short tubes of a suitable length to provide support to the inside of the base of the cup if forces push the cup base inwards are placed within the cup-shaped component, and the rings fitting sufficiently well into the space so that they touch or almost touch in order to keep each other in position.

2. A contact for an axial field vacuum switching device as generally described in US 6479779 Bl where the large support ring is removed and replaced by rings or short tubes of a suitable length to provide support to the inside of the base of the cup if forces push the cup base inwards are placed within the cupshaped component, and the rings fitting sufficiently well into the space so that they touch or almost touch in order to keep each other in position.

3. A contact for an axial field vacuum switching device which comprises a stem conductively bonded to the base of a cup-shaped support component, the open end of the cup being closed by a disc of contact material and the sides of the cup having angled slot cut through and rings or short tubes placed within the cup-shaped component of a suitable length to provide support to the inside of the base of the cup if forces push the cup base inwards, and the rings fitting sufficiently well into the space that they keep each other in position.

4. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is two.

5. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is three.

6. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is four.

7. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is five.

8. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is six.

9. A contact as in claim 1 or 2 or 3 in which the number of rings or tubes is seven.