US2638494A - Electrical condenser - Google Patents

Description

Patented May 12, 1953 ELECTRICAL CONDENSER Albert Liecliti, Zurich, Switzerland, assigner to Micafl A.G., Zurich, Switzerland Application March 20, 1950, Serial No. 150,581 In Switzerland April 4, 1949 12 Claims.

The invention relates to electrical condensers, and particularly to condensers in which the dielectric material between the electrodes is built up of a number of layers of insulating material impregnated with an insulating substance.

it is already known that a limit is set to the loading capacity of these condensers in that, due to ionizati n phenomena in the impregnating substance o the insulating layers, leakage occurs between the electrodes. According to experience, such disturbances proceed in the first place from the edges of the electrodes located inside the. dielectric material. A substantial increase in the critical voltage which the condenserv can withstand without damage is obtained with new constructions in which the lines of force issuing from the electrode layer at the critical places first' pass directly through a strip or layer'oi insulation and enter the impregnating substance only after leaving this insulating layer. This insulating layer is to hereinafter as the insulating envelope to distinguish it from the other layers of insulation which form the conventional dielectric between the electrode layers.

Objects of invention are to provide electrical condensers of the impregnated type in which at least edge portions of electrode layers within the impregnated insulation are reinforced against break-down by a U-haped stripv of insulating material having a coating of conductive material at its inner su face, the conducting material being elec ical contact with the adjacent edge portion of the electrode layer. An object is to provide elect condensers in which the edges of within the impregnated insulaelectrode if tion are enclosed within insulating envelopes he ig conducting material at their inner suiiaees contacting at least the edge portions ci the associated electrode layers. Other objects are to provide condensers of the type stated in which insulating envelopes extend over at least surface of the active portion ofA each electrode layer and are so arranged that at least one layer of conductive material of the insulating envelope is located between and co-extensive with the opposed active surfaces of adjacent electrode layers. A further object is to provide electrical condensers of the type stated in which insulating envelopes with linings of conductive material completely enclose the portions of the electrodes within the impregnated insulating material.

These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accom'-v panying drawings in which:

Figs. 1 to 8 inclusive are transverse sections throng-h unit layers, comprising a pair of electrodes of opposite polarity and associated elements, of stacked or wound condensers constituting` diierent embodiments of the invention; and

Fia. 9 is a fragmentary transverse section through one edge portion of a unit layer of a stacked or Wound condenser constituting a further embodiment of the invention.

It is to be noted that the thickness of the several layers is greatly exaggerated in the drawings for clarity of illustration, and that the actual thickness of the several layers of a condenser as manufactured on a Winding machine may be from about a few mils to about mils.

In Fig. l of the drawings, the reference characters A and B identify the electrodes of opposite polarity which may be metal foils or thin iilms of other conducting material. The electrodes are insulated. from each other by layers D of insulation, usually paper, which are thoroughly impregnated with oil, wax or other insulating inaterial. The insulation layers D project beyond the oppositey edges of the electrodes in this form of the invention, and terminal connections are made tol the electrodes in any desired manner, not shown.

In accordance with the invention, each edge of each electrode is enclosed within an insulating envelope comprising a strip I of insulating ma... terial whichv is folded upon itself into Uderm and which has a coating or layer E5 of conducting material on the inner side of the fold and in electrical contact with the adjacent edge portions of the associated electrode. The conductive layer S preferably, as shown., terminates some distance from the edges of the insulating strip i, and the conductive layer S is applied directly to and united to the insulating strip I to avoid air pockets or discontinuities into which the impregnating material may enter.

The insulating strips l may be paper of the same kind as that usually employed' for the insulation layers D but it is preferable to use strips l of a paper which is more highly sized than the paper' o layers D therefore is not so thoroughlyA impregnated by the oil, wax or other impregnating material. The conductive layers S be sprayed on the insulating material strips l, or they may be metallic coatings applied to paper strips by known processes as now employed in the manufacturey of metallized paper which is used for Wrappings or decorative purposes. Anotherinethod of forming the insulating envelopes with conductive inner surfaces is to cement a metal foil to a paper strip. When the cement or adhesive material has a higher electrical strength than the impregnating material, it is the layer of cement which constitutes the insulating envelope, and the paper backing strip for the metal foil may th-en be of the same kind as the insulation layers D.

In the winding of the condenser, or the stacking of superposed unit layers to form the condenser, the conductive layers S are pressed so firmly upon the electrodes as to have the same potential as the associated electrodes, wherebythere is no eld gradient across any gaps H which result from a failure of a con-ductive layer S to maintain continuous surface contact with the edge of the associated electrode. The lines of force E issuing from the fold of the conductive layer S pass through the insulating strip I and, only after passing through the strip 1', pass into the impregnating material which saturates the insulation layers D .and lls the spaces F between the outer edge portions of the insulating layers.

As shown in Fig. 2, insulating envelopes I, S as above described may be arranged at the inner edges of electrodes AI, Bl of another known type of condenser in which the electrodes are offset with edges projecting from opposite sides of the insulation layers D for connection to terminals, not shown.

As shown in Fig. 3, the insulating envelopes may be of J-shape in transverse cross-section with one short leg and a long leg which extends to the edge of the insulating layers D. The conductive layer or film SI extends to the outer edge of the long leg of the insulating strip Il and terminates short of the end of the short leg of the insulating strip. The insulating envelopes of J-shape in cross-section are positioned in the same sense, although reversed from right to left as shown in Fig. 3, so that a conductive layer of film Si is located between adjacent electrodes AI, Bl. This construction is especially advantageous when the electrodes are very thin conductive layers which may have minute holes or cracks where ionization elects would develop in the insulating material.

Maximum protection against damage from over voltages is afforded by a condenser construction, as illustrated in Fig. 4, with insulating envelopes comprising folded strips 12 of insulating material with conductive coatings or layers S2 at their inner surface, the envelopes completely enclosing the portions of electrodes Al, Bl within the insulating layers D.

Other embodiments of the invention, as illustrated in Figs. to 7, including paper strips I2 of U-shape in cross-section and provided with conductive coatings or layers which are coextensive with more or less of the inner surface of the folded insulating strip. Each of these embodiments of the invention is characterized by a uniform winding thickness of the unit layers of the condenser.

As illustrated in Fig. 5, the conductive layers are of J-shape in cross-section with a long leg S3 coextensive with the active surface of the electrodes and a short leg S4 of a length substantially one-half the active length of the electrode surfaces. The shorter sections S4 of conducting coatings are opposed to each other to provide a single thickness of conductive material between the electrodes Al, Bl of the unit winding layer and a double thickness of conductive layers S3 between electrodes of adjacent unit winding layers.

It is not essential that the shorter legs of the insulating layers terminate at the transverse center of the condenser and, as illustrated in Fig. 6, the end of long leg S5 of conductive coating on folded insulating strip I2 may terminate in substantial transverse alinement with the end of the shorter leg S6 of the insulating envelope associated with an adjacent electrode layer.

As shown in Fig. 7, the insulating envelopes may have conductive layers S1 of U-form with edges of adjacent conductive layers in transverse alinement at the center or another intermediate point of the condenser,

Uniform winding thickness is a characteristic of the condenser layers shown in Figs. 5 to 7 since the insulating strips I2 are of U-shape with edges extending to the sides of the insulating layers D, and the total thickness of the conducting coatings is uniform from side to side of the unit winding layer. It will be apparent that the uncoated portions of the insulating strips could be omitted, as in Fig. 3, without loss of the advantage of uniform winding thickness.

Another construction for `a condenser with electrodes A, B completely enclosed by the dielectric D, as in Fig. l, is illustrated in Fig. 8. The insulating envelopes comprise paper strips I3 with conducting coatings S8 which extend continuously along one surface of the associated electrode and have their edge portions folded over to enclose the electrode edges. The folded edges of the strips I3 abut, as illustrated, when uniform winding thickness is desired, but adequate protection against break-down is had when the conductive coatings SB of the folded edges extend at least to the lines a-a and :2V-a respectively, and the edges of the strips I3 extend to lines A-A and A'-A respectively.

Another construction is shown in Fig. 9 for a condenser of the type in which the electrodes A', B' are staggered and each have one edge projecting beyond the impregnated insulating layers D. In this form of the invention, the insulating strip is of J-shape in cross-section with a long leg I4 which extends over the electrode Bl and a short leg I5 which is alined with electrode Bl but separated from it by a small space L. The conductive layer S8 terminates short of the longer leg I4 of the insulating strip but preferably extends to the end of the shorter leg I5. A slight separation of the layers is shown for clarity but it is to be understood that these gaps close when the condenser is wound under tension, i. e. the upper layer of conductive material is pressed tightly against the edge of the electrode Bl and the lower conductive layer. The point of greatest electrical stress is no longer the edge of the electrode Bl, since the space L is bridged over by the conductive layer S9, but is the fold K of the conductive layer. This conductive fold or edge is Within the fold of the insulating strip, and the lines of force must pass through the insulating strip before they enter the impregnating material and layers D.

For approximately constant winding thickness, the leg Ill of the insulating strip could be extended, either with or without an extension of the conductive coating S9, towards or to the other edge of the electrode B l.

The several illustrated embodiments of the invention are indicative of the wide latitude in the construction of insulating envelopes with interior conductive coatings which may be employed in condensers which are contemplated by and fall within lthe scope of the invention as set forth in the following claims.

I claim:

1. A condenser of the type including electrodes insulated from each other by layers of insulating material impregnated with an insulating substance, characterized by the fact that a folded strip of insulation of greater electrical strength than said impregnating material extends continuously along each electrode edge within the impregnated insulating material, and conductive coatings are on and united to the inner surfaces of said folded strip of insulation and in electrical contact with the edge portions of the adjacent electrodes; whereby electrical field lines terminate on said conductive coatings and traverse said insulating strips before entering said insulating substance and impregnated layers of insulating material.

2. A condenser as recited in claim l, wherein one of said folded strips of insulation is of J -shape in cross-section, the longer leg of the folded strip and its conductive coating extending over the edge portion of the adjacent electrode, and the shorter leg of the folded strip being alined laterally with and spaced from the said electrode.

3. A condenser as recited in claim 1, wherein the edges of said electrodes with the impregnated insulating material extend within and are enveloped by the adjacent folded strips of insulation.

4. A condenser as recited in claim 1, wherein said folded strips of insulation each have an edge extending along the surface of the associated electrode, and said conductive coatings terminate short of the said edges of the folded strips.

5. A condenser as recited in claim l, wherein each folded strip and its associated conductive coating extend substantially completely over one surface of the active area of vthe associated electrode.

6. A condenser as recited in claim 1, wherein each folded strip and its associated conductive coating extend over one surface of the active area of the associated electrode and over a portion only of the other active surface of the electrode.

7. A condenser as recited in claim 1, wherein the conductive coating of each folded strip extends over one surface of the active area of the associated electrode and over a portion only of the other active surface of the electrode.

8. A condenser as recited in claim 7, wherein the overall thicknesses of said folded strips and of said conductive coatings are each constant across the active electrode areas of a unit layer of the condenser.

9. A condenser as recited in claim 1, wherein each folded strip and its associated conductive coating extend over both surfaces of the active area of the associated electrode.

l0. A condenser as recited in claim 1, wherein one outer edge of each strip of insulation and its conductive coating extend to an edge of said im- References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,279,941 Thomas Sept. 24, 1918 1,945,917 Scarpa Feb. 6, 1934 2,244,090 Traub June 3, 1941 2,559,141 Williams July 3, 1951 FOREIGN PATENTS Number Country Date 365,023 Great Britain Jan. 14, 1932 536,668 Great Britain May 22, 1941

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