GB2106715A

GB2106715A - Disconnector device and capacitor incorporating it

Info

Publication number: GB2106715A
Application number: GB08226862A
Authority: GB
Inventors: Leonard Joseph Hall
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-09-21
Filing date: 1982-09-21
Publication date: 1983-04-13
Also published as: GB2106715B

Abstract

A wire forming an over pressure cut-out in a capacitor is locally weakened by first forming a bend 2 in it and then compressing the bend 5 to produce work-hardening. The breaking force acts at least partly in shear on the work-hardened bend. In some cases a reduction in tensile strength by a factor of the order of 25 can be achieved without reducing current-carrying capacity by as much as half. <IMAGE>

Description

SPECIFICATION Disconnecting device and capacitor incorporating it This invention relates to electric capacitors having pressure-tight containers and more especially (but not exclusively) to capacitors for use in power circuits, e.g. in power factor correction or in circuits supplying fluorescent lamps or electric motors.

Under certain fault conditions, discharges may be set up within the capacitor element and the resultant rise in pressure in the container may cause leakage of impregnant (in the case of capacitors impregnated with insulating liquids or low-melting solids) qr of molten plastics material (in the case of unimpregnated plastics film capacitors) or even explosion of the capacitor.

In order to avoid damage arising in this way it has been proposed to make the container of the capacitor capable of controlled expansion so that if excess pressure should develop a portion of the container will move relative to another part thereof, and to make use of such movement operate a cut-out device to disconnect the capacitor from the associted circuit.

Various methods of obtaining the required expansion of the container have been proposed, but the most practical methods are to corrugate the wall of the container or to provide a closure member in the form of a laminate consisting of a relatively rigid sheet with a movable central zone and a distensible sheet bonded to the relatively rigid sheet but peelable from it on movement of the central zone.

Both these techniques are described and the latter is claimed in the specification of our British Patent No.

1539194.

As described therein, the cut-out may comprise (inter alia) a wire that is locally weakened, by notching for instance or that is tensioned across a sharp edge so that it will be broken when expansion of the container puts it under tension.

These techniques are satisfactory for relatively small capacitors (say up to about 1.5 kVAR for which the wire would be about 0.35 mm in diameter) but when used with larger sizes it becomes difficult to accommodate the extension of the wire before fracture unless it is so reduced in cross-section in forming a weak spot that serious overheating risking premature failure will occur.

In accordance with the invention, a cut-out device for a capacitor comprising a wire locally weakened to define a fracture point and means for applying a breaking force to the fracture point when required is characterised by the fact that the wire is weakened by first forming a bend in it and then compressing the bend to produce work hardening, and that the breaking force acts at least partly in shear on the work-hardened bend.

The invention includes a capacitor incorporating the cut-out device defined.

The bend preferably has a radius smaller than the wire diameter. In most cases we prefer a tight bend of 1800 (preferably with the runs of wire adjacent the bend touching or nearly so), but smaller angles, down to 900 at least and possibly lower, can be used.

The direction of compression may be in the plane öf the bend, or at right angies to it, or in any other direction found convenient.

In the accompanying drawings, Figures 1 - 3 illustrate a preferred form of cut-out device in accordance with the invention, Figure 1 showing an intermediate stage in its manufacture and Figures 2 and 3 being mutually perpendicular views of the finished device; and Figures 4 & 5, 6 & 7 and 8 & 9 are mutually perpendicular views of three alternative forms of cut-out device in accordance with the invention.

In making the device shown in Figures 1 - 3, a round wire 1 is first shaped to form a bend 2 of 1800 bounded by short parallel and closely-spaced limbs 3, 4which are perpendicular to the direction of the end-parts of the wire. The bend 2 is now compressed between a flat anvil and a narrow angular tool to form in it a short work-hardened zone 5.

By way of example, a cut-out device was made from tinned copper wire 0.90 mm diameter bent and compressed to the form shown in Figures 2 - 3. The limbs 3 and 4were one millimetre apart before compression and the zone 5 was reduced to a thickness in the range 0.45 to 0.50 mm using a tool 0.9 mm wide. In the compression operation, the zone 5 was enlarged (as seen in Figure 2) to the extent that the limbs 3 and 4 were now about 1.3 mm apart and the zone was about 1.15 mm wide at its widest point.

The device is found to fracture at a load in the approximate range 20 - 30 N (4.5 to 6.5 Ibf) compared with a tensile strength in the initial wire of about 300 N (66 Ibf); the current-carrying capacity of the wire, on the other hand, is reduced by less than half.

In the further examples illustrated by Figures 4 & 5 and Figures 6 & 7 the bend is of 1800 with a radius substantially the same as the radius of the wire so that the limbs 3 and 4 touch; in Figure 3, the bend is of 90" with about the same radius. In Figures 4 - 5 and 8 - 9 flattening is by tools moving perpendicular to the plane of the bend, whereas in Figures 6 - 7 the compressing tools move in the plane of the bend and parallel to the projecting wire ends. In all these cases the tools are alike, round and wide compared with the wire diameter. Broken lines in the Figures indicate the shape before compression.

By way of a further example, we have found that a cut-out device made from tinned copper wire 1.1 mm in diameter bent and compressed to the form shown in Figure 1 with a thickness in the flattened area of about 0.35 mm fractures on applying a load of about 16.7N (3.75 lbf) compared with a tensile strength of the order of 445N (100 Ibf); the currentcarrying capacity of the wire, on the other hand, is reduced by less than half.

1. A cut-out device for a capacitor comprising a wire locally weakened to define a fracture point and means for applying a breaking force to the fracture point when required, characterised by the fact that the wire is weakened by first forming a bend in it and then compressing the bend to produce workhardening, and that the breaking force acts at least

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Disconnecting device and capacitor incorporating it This invention relates to electric capacitors having pressure-tight containers and more especially (but not exclusively) to capacitors for use in power circuits, e.g. in power factor correction or in circuits supplying fluorescent lamps or electric motors. Under certain fault conditions, discharges may be set up within the capacitor element and the resultant rise in pressure in the container may cause leakage of impregnant (in the case of capacitors impregnated with insulating liquids or low-melting solids) qr of molten plastics material (in the case of unimpregnated plastics film capacitors) or even explosion of the capacitor. In order to avoid damage arising in this way it has been proposed to make the container of the capacitor capable of controlled expansion so that if excess pressure should develop a portion of the container will move relative to another part thereof, and to make use of such movement operate a cut-out device to disconnect the capacitor from the associted circuit. Various methods of obtaining the required expansion of the container have been proposed, but the most practical methods are to corrugate the wall of the container or to provide a closure member in the form of a laminate consisting of a relatively rigid sheet with a movable central zone and a distensible sheet bonded to the relatively rigid sheet but peelable from it on movement of the central zone. Both these techniques are described and the latter is claimed in the specification of our British Patent No. 1539194. As described therein, the cut-out may comprise (inter alia) a wire that is locally weakened, by notching for instance or that is tensioned across a sharp edge so that it will be broken when expansion of the container puts it under tension. These techniques are satisfactory for relatively small capacitors (say up to about 1.5 kVAR for which the wire would be about 0.35 mm in diameter) but when used with larger sizes it becomes difficult to accommodate the extension of the wire before fracture unless it is so reduced in cross-section in forming a weak spot that serious overheating risking premature failure will occur. In accordance with the invention, a cut-out device for a capacitor comprising a wire locally weakened to define a fracture point and means for applying a breaking force to the fracture point when required is characterised by the fact that the wire is weakened by first forming a bend in it and then compressing the bend to produce work hardening, and that the breaking force acts at least partly in shear on the work-hardened bend. The invention includes a capacitor incorporating the cut-out device defined. The bend preferably has a radius smaller than the wire diameter. In most cases we prefer a tight bend of 1800 (preferably with the runs of wire adjacent the bend touching or nearly so), but smaller angles, down to 900 at least and possibly lower, can be used. The direction of compression may be in the plane öf the bend, or at right angies to it, or in any other direction found convenient. In the accompanying drawings, Figures 1 - 3 illustrate a preferred form of cut-out device in accordance with the invention, Figure 1 showing an intermediate stage in its manufacture and Figures 2 and 3 being mutually perpendicular views of the finished device; and Figures 4 & 5, 6 & 7 and 8 & 9 are mutually perpendicular views of three alternative forms of cut-out device in accordance with the invention. In making the device shown in Figures 1 - 3, a round wire 1 is first shaped to form a bend 2 of 1800 bounded by short parallel and closely-spaced limbs 3, 4which are perpendicular to the direction of the end-parts of the wire. The bend 2 is now compressed between a flat anvil and a narrow angular tool to form in it a short work-hardened zone 5. By way of example, a cut-out device was made from tinned copper wire 0.90 mm diameter bent and compressed to the form shown in Figures 2 - 3. The limbs 3 and 4were one millimetre apart before compression and the zone 5 was reduced to a thickness in the range 0.45 to 0.50 mm using a tool 0.9 mm wide. In the compression operation, the zone 5 was enlarged (as seen in Figure 2) to the extent that the limbs 3 and 4 were now about 1.3 mm apart and the zone was about 1.15 mm wide at its widest point. The device is found to fracture at a load in the approximate range 20 - 30 N (4.5 to 6.5 Ibf) compared with a tensile strength in the initial wire of about 300 N (66 Ibf); the current-carrying capacity of the wire, on the other hand, is reduced by less than half. In the further examples illustrated by Figures 4 & 5 and Figures 6 & 7 the bend is of 1800 with a radius substantially the same as the radius of the wire so that the limbs 3 and 4 touch; in Figure 3, the bend is of 90" with about the same radius. In Figures 4 - 5 and 8 - 9 flattening is by tools moving perpendicular to the plane of the bend, whereas in Figures 6 - 7 the compressing tools move in the plane of the bend and parallel to the projecting wire ends. In all these cases the tools are alike, round and wide compared with the wire diameter. Broken lines in the Figures indicate the shape before compression. By way of a further example, we have found that a cut-out device made from tinned copper wire 1.1 mm in diameter bent and compressed to the form shown in Figure 1 with a thickness in the flattened area of about 0.35 mm fractures on applying a load of about 16.7N (3.75 lbf) compared with a tensile strength of the order of 445N (100 Ibf); the currentcarrying capacity of the wire, on the other hand, is reduced by less than half. CLAIMS

1. A cut-out device for a capacitor comprising a wire locally weakened to define a fracture point and means for applying a breaking force to the fracture point when required, characterised by the fact that the wire is weakened by first forming a bend in it and then compressing the bend to produce workhardening, and that the breaking force acts at least partly in shear on the work-hardened bend.

2. A device as claimed in Claim 1 characterised in that the radius of the bend is smaller than the wire diameter.

3. A device as claimed in Claim 1 or Claim 2 in which the bend is a tight bend of 180 .

4. A cut-out device substantially as described with reference to and as shown in any one Figure of the drawings.

5. A capacitor including the cut-out device claimed in any one of the preceding claims.