GB2083950A - Capacitor - Google Patents

Abstract

One electrode 12 of a wound capacitor 8 comprises a dielectric strip metallized on both sides and along one edge while the other electrode 10 consists of a metal foil. Two dielectric layers 14 are interleaved with the electrodes. The capacitor has a heat dissipating core. <IMAGE>

Description

SPECIFICATION Capacitor This invention relates to capacitors formed from wound electrode and dielectric stips.

Wound capacitors commonly include electrodes comprising metal foil strips, metallized dielectric strips, or both, with dielectric layers between electrodes. For example, Kalstein U.S.

Patent No. 3,705,336, hereby incorporated by reference, discloses a wound capacitor having two dielectric layers and two electrodes, whereby one electrode is a metal foil and the other is a metallized layer vacuum deposited on one of the dielectric layers.

The wound capacitor of Grahame U.S. Patent No. 3,522,496 also includes a metal strip and a one-side metallized strip; these electrodes are interleaved with porous paper dielectric layers.

The present invention seeks to provide an improved wound capacitor of novel construction.

In accordance with the present invention there is provided a wound capacitor comprising: an elongate metal foil electrode strip; a first layer of dielectric material associated with said metal strip; adjacent said first layer of dielectric material, an elongate dielectric strip having a metallized layer deposited on both of its surfaces, said metallized layer covering both of said surfaces to at least one lateral edge of said elongate dielectric strip; a second layer of dielectric material associated with said metallized dielectric strip; and electrically connecting means connected to said metal foil electrode at one end of said capacitor; and the opposite end of said capacitor having a metal coating to electrically connect the edges of said metallized elongate strip.

In preferred embodiments, the metallized dielectric is a porous, liquid dielectric-impregnated material such as paper; the capacitor is wound such that a wound lateral edge of the metallized dielectric extends beyond the other wound elements at one end of the capacitor; the metal foil electrode is narrower than the other wound strips; and the capacitor includes a heat-dissipating core.

Capacitance is generated between each surface of the metallized dielectric and the metal foil which is separated from it by a dielectric strip, but is not generated between the two metallized surfaces, which are shorted out by the metal coating electrically connecting their edges.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a somewhat diagrammatic perspective view of an embodiment of capacitor in accordance with this invention, partially unwound; and Fig. 2 is a diagrammatic sectional view of said embodment taken along line 2-2 of Fig. 1.

There is shown in Fig. 1 a preferred capacitor 8, comprising two polypropylene dielectric strips 14 interleaving aluminium foil electrode strip 10 and porous paper dielectric strip 12, both sides of which are completely coated with a thin layer of aluminium.

As is shown in Fig. 2, metal foil strip 10 is narrower than the other wound components and as shown in Fig. 1, overlaps them at their ends.

Metallizedpaper strip 12 is the same width as polypropylene strips 14, but is wound such that one lateral edge extends beyond the other wound elements at one end of the capacitor. This extended metallized edge is spray-coated with zinc to form terminal 17.

At the opposite end of the capacitor protrude copper tabs 16, staked to aluminium extension tabs 1 8. There is provided one such tab/extension pair for about every 1 ,000 inches of foil electrode.

These tabs are all electrically connected to form the second capacitor terminal (not shown).

Core 19, which is wound with the rest of the capacitor, includes a corrugated paper insert 20, about which is wound a strip of paper/foil laminate 1 9.

To raise the corona inception voltage, the capacitor is impregnated with a dielectric fluid by, the standard method, involving first removing moisture in a vacuum tank and then flooding the tank with dielectric fluid while varying tank pressure. Dielectric fluid enters the capacitor via the porous metallized paper, which acts as a wick, and whose aluminium coating is thin enough to itself be somewhat porous.

Capacitance is generated between both metallized surfaces of paper strip 12 and the windings of foil strip 10, separated from those surfaces by high-dielectric strength polypropylene strips 14. The two metallized surfaces, which are separated by the poor-dielectric strength paper, do not generate capacitance between them because they are taken out of the circuit by zinc coating 1 7.

The positive qualities of the paper'its porosity (wicking capability), thinness, and light weight-are thus taken advantage of, while paper's major disadvantage-poor dielectric strength-- is neutralized.

Aluminium extension tabs 18, as well as aiding in securing copper tabs 1 6 to foil 10, improve the electrical qualities of the capacitor because of their added surface area and low resistivity.

The extreme heat which can be generated under high-voltage operating conditions is in part dissipated both by the windings of the core 1 9, and by the corrugated core insert 20, whose open spaces are particularly advantageous.

The above described embodiment is that which we currently prefer; but other embodiments are envisaged within the broad scope of the invention.

For example, although the materials cited are preferred, any dielectric material metallized on both sides can be used, and strips of any standard dielectric material can separate the metallized layers from the foil electrode. | The foil can be of any suitable conducting material, and is not necessarily narrower than the other capacitor components.

One embodiment which can employ foil which is as wide as or wider than the other wound elements is a capacitor having the same structure as the capacitor of Fig. 1, except that the foil strip is wound such that its wound edge, on the end of the capacitor opposite the extended metallized paper edge, extends beyond the edges of the other wound elements, facilitating heat dissipation. In still another embodiment, this extended wound foil edge is spray-coated with zinc, eliminating the need for tab terminal connections.

Although a core is preferred, particularly if there are many turns in the winding and operating voltages are high, the capacitor can be constructed without a core. The core can be of any material which dissipates heat, is not necessarily wound, and need not include a corrugated insert.

The conducting extension tabs do not require corresponding separate tabs on the foil, and may be attached to the foil by a variety of convenient methods, such as soldering. If surface area is desired to be increased, the extension tabs can be made wider at the end attached to the foil.

Claims (12)

1. A wound capacitor comprising: an elongate metal foil electrode strip; a first layer of dielectric material associated with said metal strip; adjacent said first layer of dielectric material, an elongate dielectric strip having a metallized layer deposited on both of its surfaces, said metallized layer covering both of said surfaces to at least one lateral edge of said elongate dielectric strip; a second layer of dielectric material associated with said metallized dielectric strip; and electrically connecting means connected to said metal foil electrode at one end of said capacitor; and the opposite end of said capacitor having a metal coating to electrically connect the edges of said metallized elongate strip.

2. A wound capacitor according to Claim 1, wherein said metallized dielectric strip comprises a porous material impregnated with a dielectric fluid.

3. A wound capacitor according to Claim 1 or Claim 2, wherein a lateral edge of said dielectric strip having said metallized layers extending to it extends beyond the edges of said foil strip and said first and second layers of dielectric material to expose a wound portion of said metallized strip at one end of said capacitor.

4. A wound capacitor according to any preceding claim, further comprising a heatdissipating core.

5. A wound capacitor according to Claim 4, wherein said core comprises a strip of non-heat conductive material wound in the same direction as the rest of said capacitor.

6. A wound capacitor according to Claim 5, further comprising a heat conductive strip associated with and wound in the same direction as said core non-heat conductive strip.

7. A wound capacitor according to any of Claims 4 to 6, further comprising a corrugated piece of non-heat conductive material inserted into said core.

8. A wound capacitor according to any preceding claim, wherein said electrically connecting means connected to said metal foil electrode comprises at least one tab of electrically conducting material secured at one end to said metal foil electrode and protruding from one end of said capacitor.

9. A wound capacitor according to Claim 8, wherein each said tab of conducting material is attached to a piece of conducting material which is attached to said foil and overlaps a portion of said tab.

10. A wound capacitor according to any preceding claim, wherein, on the end of said capacitor having said electrically connecting means connected to said metal foil, the lateral edge of said metal foil, extends beyond the edges of said first and second layers of dielectric material and said metallized dielectric strip a wound portion of said metal foil.

1 A wound capacitor according to Claim 10, wherein said exposed wound portion of said foil strip is coated with conducting material, and wherein said electrically connecting means connected to said metal foil comprises said conducting metal coating.

12. A wound capacitor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.