Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1999-09-20
2002-12-10
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S308100, C029S025420
Reexamination Certificate
active
06493206
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a wound capacitor preferably formed from layers of metalized film and metal electrodes. The capacitor has improved heat transfer performance that reduces internal thermal stress and permits increased current-carrying capacity.
BACKGROUND OF THE INVENTION
A metalized film capacitor is formed from layers of metallic film on a dielectric substrate. A suitable metal, such as aluminum, is vapor deposited on the substrate. The thickness of the metallic film is typically in the range of 100 to 400 angstroms. The dielectric substrate is generally a thermoplastic resin, such as polypropylene. The thickness of the substrate is significantly greater than the thickness of the film. Two metalized film layers are convolutely wound together to form to form he capacitor. In this configuration, the dielectric substrates separate the two metalized film layers, which form the electrodes of the capacitor.
Film and foil (film/foil) capacitors are formed from four alternating layers of a dielectric and a metallic foil. The two layers of metallic foil form the electrodes of the capacitor. The thickness of the foil is typically in the range of 0.0005 to 0.0025 centimeters (cm). In this configuration, the dielectric layers separate the two metal electrodes. By extending the longitudinal ends of the two metallic films layer beyond the ends of the dielectric layers, at opposite axial ends of the capacitor, an extended foil capacitor is formed. A conductive metal end-spray can be applied to the extended whorl of metallic foils at each axial end of the capacitor to provide for the connection of terminals to the capacitor.
A hybrid of a metalized film and a film/foil capacitor is known as a metalized film and foil (metalized film/foil) capacitor. FIGS.
1
(
a
) and
1
(
b
) show typical cross sectional layering for series-wound metalized film/foil capacitors known in the art. In FIG.
1
(
a
), a metalized film layer, composed of a metalized film
152
on a dielectric substrate
153
, is wound between a dielectric layer
130
and a layer composed of two separate strips of metallic foil
120
and
122
. The strips of metallic foil form the two metal electrodes of an extended foil capacitor. FIG.
1
(
b
) illustrates a common variation of the hybrid capacitor in FIG.
1
(
a
). For the configuration shown in FIG.
1
(
b
), two metalized film layers are wound with their metallic films in contact with each other to double the thickness of the metalized film conductor. The space between the inner longitudinal ends of the foils in FIGS.
1
(
a
) and
1
(
b
) is referred as the margin. As show in these figures, the margin has a width
190
. Minimum cross sectional margin widths are primarily driven by voltage withstand values determined from the working voltages for the wound capacitor. Typical widths for the margin at 600 volts ac are on the order of 0.4 cm.
Internal heat is generated in a capacitor in two ways, namely dielectric heating and conduction losses. Dielectric heating is linearly proportional to the operating frequency of the capacitor and proportional to the square of its operating voltage. Conductive (I
2
R) losses are proportional to the square of the current carried by the capacitor. Consequently, for high frequency and high current capacitors, the amount of internal heat generated is a significant and limiting factor for the use of a metalized film/foil capacitor. The capacitor temperature due to internal heating plus the ambient temperature should not exceed the maximum allowable operating temperature for the selected dielectric, or it is possible that the capacitor will open or, in some cases, rupture.
In metalized film/foil capacitors, the electrodes formed by the metalized film have a significant resistance in comparison with the resistance of the metallic foil metalized film. Consequently, significant conduction losses are generated in the conductor formed by the metalized film. The metallic foils have a relatively high value of thermal conductivity and serve as an efficient conductor of heat from the interior of the wound capacitor. Conversely, the dielectric substrate typically is a very poor thermal conductor. Therefore, the metallic foils play an important part in the dissipation of heat from the dielectric losses and the conduction losses in the metalized film.
In a series-wound capacitor with electrodes formed from extended metallic foils, the foils act as an effective heat sink in contact with the dielectric substrate except for the region formed by the margin. Current flows across the margin through the metalized film. Internal heat will build up in the region formed by the metalized film bridging the margin. In the prior art capacitors shown in FIGS.
1
(
a
) and
1
(
b
), the nearest surface of metallic foil from the center of the metalized film bridging the margin is equal to half of the margin's width. This is more clearly appreciated when looking at a cross section of multiple coils of the layers, such as in FIG.
1
(
c
), for two coils of the prior art capacitor in FIG.
1
(
b
). For the typical margin width of 0.4 cm noted above, the nearest distance to the surface of a metallic foil from the center of the metalized film bridging the margin will be 0.2 cm.
The present invention addresses the problem of internal heat buildup by using two parallel capacitors wound together with adjacent margins being offset so that all (including the center) of the metalized film bridging the margin is no further away than approximately the thickness of one dielectric substrate from the surface of a metallic foil. For the typical 0.001 cm thickness of a dielectric substrate, the nearest distance to the surface of a metallic foil from all of the metalized film bridging the margin will be 0.001 cm. This is a significant decrease in the distance to a metallic foil that will act as a heat sink for heat transfer from the metalized film bridging the margin. This results in a significant increase in the amount of current that can be carried by a capacitor using the same amount of materials as that for the prior art capacitors shown in FIGS.
1
(
a
) and
1
(
b
).
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention is an axially wound capacitor formed from at least two composite layers. Each composite layer is formed from component layers. In the preferred embodiment of the invention, one electrode layer and two metalized film layers form the minimum component layers. Each electrode layer is formed from two metal electrodes. There is a space, or margin, between the adjacent longitudinal ends of the two metal electrodes. The two metal electrodes have their outer longitudinal ends disposed at opposite axial ends of the capacitor. The longitudinal ends of all metal electrodes at each axial end of the capacitor are electrically connected together to form the two external electrical connection points for the capacitor. Each of the two metalized film layers of each composite layer is formed from a metalized film on a dielectric substrate. The metalized film layers for the two substrates are disposed adjacent to each other. All composite layers that make up the capacitor are similarly oriented to each other with the exception that the margins in the electrode layers of adjacent composite layers are offset from each other.
Equal widths of diagonally opposed metal electrodes in adjacent electrode layers can be provided. The width of the margins in all electrode layers can be equal, and the margins can be offset in adjacent electrode layers by the width of a margin. In alternate embodiments of the invention, a dielectric layer can replace one of the two metalized film layers in one or more of the composite layers that form the capacitor. Also, a single metalized film layer can replace the two metalized film layers in at least one composite layer of the capacitor when its metalized film is placed adjacent to a dielectric layer, or the dielectric substrate of a metalized film layer. A dielectric layer can be placed between one or more ad
Drinker Biddle & Reath LLP
High Energy Corp.
Reichard Dean A.
Thomas Eric W.
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