GB2357372A

GB2357372A - Vacuum stage in wound capacitor manufacture.

Info

Publication number: GB2357372A
Application number: GB9930138A
Authority: GB
Inventors: Conor Gibney; Finian Carney
Original assignee: NEWS DISTRIB Ltd
Current assignee: NEWS DISTRIB Ltd
Priority date: 1999-12-16
Filing date: 1999-12-21
Publication date: 2001-06-20
Anticipated expiration: 2019-12-21
Also published as: GB9930138D0; GB2357372B; IE991054A1

Abstract

A manufacturing method includes a metal, vapour deposited onto a polypropylene, polycarbonate or polyester film; the metallised film is wound to form a capacitor section and a first vacuum of less than 4 x 10<SP>-2</SP> millibars is applied thereto for at least 6, pref. 8 hours to eliminate pockets of air or moisture, reducing the possibility of corona discharge in use and improving the capacitance per unit volume. The capacitor section is then cured in two stages at least at 85{C, pref. 95{C for 2, pref. 4 hours and then at 95{C, pref. 105{C for 8, pref. 12 hours and pre tested after application of an electrical contact medium; contacts are then applied. A second vacuum is applied to remove any moisture that may be absorbed during the manufacturing process and the capacitor is encapsulated in a resin filled housing and tested.

Description

2357372 "A METHOD FOR MANUFACTURING A WOUND FILM CAPACITOR"

Introduction

The invention relates to a method for manufacturing a wound film such as a dry film dielectric capacitor without harmful electrolytes. One such dry film capacitor is described in WO-A-9419813 and US 5614111, the entire contents of which are incorporated herein by reference.

Because of their relatively small size dry film wound capacitors offer considerable physical advantages in many different applications. However, the relatively small size of such capacitors presents serious problems in manufacturing such capacitors with high performance characteristics on an economic scale.

In conventional manufacturing processes there is a risk of moisture and/or air becoming trapped within the layers of dielectric material. This can cause a number of significant problems. The dielectric constant of air is significantly lower than that of the dielectric film and the presence of air pockets within the capacitor will therefore reduce the overall capacitance of the capacitor, due to the lower dielectric constant of air and also the area occupied by the air pockets. Any air trapped within the capacitor will also increase the risk of corona discharges occurring within the capacitor. Corona discharges take place in the tiny air pockets that are adjacent to the dielectric surface. Corona is an electrical discharge mechanism that occurs as a result of the continuous ionisation of a gas, which in this instance is the air trapped within the capacitor. Severe corona discharges can cause a rapid deterioration of the dielectric due to the concentrated heat and elevated temperatures, generated by the discharges within the air pockets. In addition, moisture that saturates the air pockets and which is absorbed by the dialectic will allow the passage of electrons through the dielectric.

This results in a reduction in the insulation resistance of the capacitor. The ability of the capacitor to store energy is inhibited, as over a period of time the capacitor will slowly discharge. Energy storage capability is a crucial requirement of the capacitor in many applications.

This invention is directed towards providing an improved method for manufacturing such capacitors.

Statements of Invention

A method for manufacturing a wound film capacitor comprising the steps of winding a metallised film to form a capacitor section-, -applying a first vacuum to the capacitor section; curing the capacitor section; applying contacts to the ends of the capacitor section; and encapsulating the wound capacitor in a housing.

In a preferred embodiment of the invention the method includes the step, prior to encapsulation, of applying a second vacuum to the capacitor.

The level of the or each vacuum applied is preferably less than 4 x 10-2 millibars.

In a pre ferred embodiment the or each vacuum is applied for a period of at least 6 hours, most preferably for a period of at least 8 hours.

In one embodiment of the invention the method includes the step of preparing a metallised film by metallising a base film. The metal is preferably aluminium which is vapour deposited onto a base film.

Preferably the method includes the step of splitting the metallised film, prior to winding.

In one aspect the method includes the steps, prior to curing, ofispraying a conducting means onto an electrical contact area of the capacitor section; first testing of the capacitor; subsequently applying contacts to the sprayed contact area of the strips; and after encapsulation, final testing of the capacitor.

In a preferred aspect the curing of the wound film strips is carried out in a first stage at a temperature of at least 8ST for a period of at least 2 hours and a subsequent second stage at a temperature of at least 9ST for a period of at least 8 hours.

Preferably the first stage curing is carried out at a temperature of approximately 95T over a period of approximately 4 hours.

Preferably the second stage curing is carried out at a temperature of approximately 105T for a period of approximately 12 hours.

Detailed Descriptio

The invention will be more clearly understood from the following description thereof given by way of example.

The first step in the method of the invention involves vapour deposition of a metal onto a dielectric. The metal is usually aluminium which is fed in the form of a wire into a boat. The boat is heated to create a pool of metal which is vapourised and the metal vapour is deposited onto a dielectric film through a shield to regulate the desired thickness of metal deposited. The dielectric may be a polypropylene, polycarbonate or polyester film. One such polyester film is available under the name Mylar which is a Trade Mark of DuPont.

The sheet of metallised film is then slit to a desired width and wound into a roll to form a capacitor section.

To eliminate tiny pockets of air or moisture which may become lodged on and adjacent to the surfaces of the capacitor section a level of vacuum of less than 4 x 10-2millibars is applied for a period of at least 6 and preferably at least 8 hours.

The capacitor section which have been vacuum treated as described above are then cured in an oven in two stages. In the first stage the wound strips are cured at a temperature of at least WC, preferably approximately WC, for a period of at least 2 hours and preferably about 4 hours. In a second stage the curing is carried out at a temperature of at least WC, typically about 105'C for a period of at least 8 hours, and preferably about 12 hours.

The problem the invention addresses is that the surface of the dielectric is not perfectly smooth and tiny pockets of air may become trapped between and adjacent to the rough surface of the dielectric, during the manufacturing process.

Eventually these air pockets will become saturated with moisture as a result of the relative humidity of the environment and the susceptibility of the dielectric to moisture absorption.

We have found that by applying a vacuum prior to curing these problems are overcome because any air or moisture that may become trapped between the metallised dielectric layers is effectively removed.

On completion of curing the ends of the capacitor section are sprayed with an electrical contact medium, typically a mixture of zinc and tin to provide an electric contact area. After spraying, the capacitor section is pre- tested for capacitance, dissipation factor, insulation resistance and equivalent series resistance.

Terminals are then typically welded to the contact areas at the ends of the capacitor. A second stage vacuum is then applied as described above for first stage vacuum. The second stage vacuum removes any moisture that may be absorbed by the dielectric during the manufacturing process. The capacitor is then scaled in a resin-filled housing.

The encapsulated capacitor is then Wy tested.

The process of the invention overcomes the potentially serious problems associated with conventional process. The process provides a capacitor in which the capacitance per unit volume is increased. This is because air pockets are removed and the area occupied by the air is reduced. The possibility of corona discharge occurring within the capacitor is decreased which improves the reliability of the capacitor. In addition, the insulation resistance of the capacitor is significantly improved which in turn results in improved performance and reliability of the capacitor and significantly reduced manufacturing losses associated with low insulation resistance. Another advantage is that the voltage capability of the metallised dielectric is enhanced because of the reduction of destructive corona at elevated voltage levels. More generally the overall manufacturing yield is improved because capacitance stability and increased insulation resistance levels are achieved.

The invention is not limited to the embodiments hereinbefore described which may be varied in detail.

Claims

Claims

I: A method for manufacturing a wound film capacitor comprising the steps of- winding a metaffised film to form a capacitor section; applying a first vacuum to the capacitor section; curing the capacitor section; applying contacts to the ends of the capacitor section; and encapsulating the wound capacitor in a housing.
2. A method as claimed in claim 1 including the step, prior to encapsulation, of applying a second vacuum to the capacitor.
3. A method as claimed in claim 1 or 2 wherein the level of the or each vacuum applied is less than 4 x 10' millibars.
4. A method as claimed in any of claims 1 to 3 wherein the or each vacuum is applied for a period of at least 6 hours.
5. A method as claimed in any of claims 1 to 4 wherein the or each vacuum is applied for a period of at least 8 hours.
6. A method as claimed in any preceding claim including the step of preparing a metallised film by metallising a base film.
7. A method as claimed in claim 6 wherein the metal is aluminium which is vapour deposited onto a base film.
8. A method as claimed in any preceding claim including the step of splitting the metallised film, prior to winding.
9. A method as claimed in any preceding claim including the steps, prior to curing, ofi- spraying a conducting means into an electrical contact area of the capacitor section; first testing of the capacitor section; subsequently applying the contacts to the sprayed contact area of the capacitor section; and after encapsulation, final testing of the capacitor.
10. A method as claimed in any preceding claim wherein the curing of the capacitor section is carried out in a first stage at a temperature of at least 85T for a period of at least 2 hours and in a subsequent second stage at a temperature of at least 9ST for a period of at least 8 hours.
11. A method as claimed in claim 10 wherein the first stage curing is carried out over a period of approximately 4 hours.
12. A method as claimed in claim 10 or 11 wherein the second stage curing is carried out at a temperature of at least 105T.
13. A method as claimed in any of claims 10 to 12 wherein the second stage curing is carried out for a period of approximately 12 hours.
14. A method for manufacturing a wound film capacitor substantially as hereinbefore described.
15. A wound film capacitor whenever manufactured by a method as claimed in any preceding claim.