US2387759A - Method of manufacturing electrical condensers - Google Patents

Description

@ct 30, N435.

K. W. JARVES METHOD OF MANUFACTURING ELECTRICAL CONDENSERS liled may' 15, 1940 2 Sheets-Sheet K. VL'. ARVES ct. 302 i945,

METHOD OF MANUFACTURING ELECTRICAL CONDENSERS Filed May 13, 194C 2 SheeLS-Sheez 2 Patented` Oct. 3 0, 1945 METHOD F MANUFACTURING ELECTRICAL CONDENSERS Kenneth'W. Jarvis, Winnetka, lll.

Application May 13, 1940, Serial No. 334,926

(Cl. 29-25A2) 9 Claims.

This invention relates to methods f manufacturing electrical condensers, particularly condensers of the type in which metal foils, plates or strips are insulated from but bonded to each other by a plastic dielectric material.

The invention is not limited to but is particularly adapted to the manufacture of condensers by winding two insulated metal foils into a coil. Spiral wound condensers have a low inductance and a relatively low resistance, for any given dielectric material, when the metal foils are laterally offset and the condenser terminals are welded or soldered to several points on or .continuously along the rolled edges of the foils at the opposite ends of the wound assembly. Paper is not homogeneous but reasonable insurance against voltage breakdown has beenobtained by employing two or more layers of paper between the metal foils as it is not probable that defects in adjacent paper layers, such as conductive impurities or minute air pockets, will be in alinement.

This type of `condenser must be protected against surface leakage by extending the paper beyond 'the overlapping sections of the laterally oset foils. Surface leakage is determined by the distance from the edge of one foil, within the paper layers, to the portion of the other foil that projects beyond the paper. The extension of the paper beyond the overlapping foils is usually approximately inch in the case of a low voltage condenser and may be more than 1/2 inch in the case of condensers for use at 600 to 800 volts. Where a very low leakage is necessary, and particularly in the case of condensers for use where the humidity is high, the effective area of the foils, i. e. the overlapping areas, may be less than one-half the total area of a foil. This results in an expensive and bulky construction, and even then does not eliminate the danger of surface leakage.

Some increase in protection against voltage breakdown and surface leakage may be had by impregnating the wound condenser with wax but this increases the manufacturing costs without affording real protection against damage from moisture.

It has been proposed to form condensers by coating metal foils with insulating material, and winding the coated foils into a coil, but the proposed dielectric materials were such that the processes were not practical commercially and /or would result in condensers of poorer quality than the known paper condensers that can be manufactured at less expense. 'I'he effective resistance of a condenser with paper insulation is relatively high, and it is apparent that no substantial improvement over paper condensers can be attained by employing dielectric materials having a higher conductivity and/or a; higher dielectric hysteresis than paper.

According to the present invention, the conducting plates or foils of a condenser are coated with a, moisture proof, low loss insulating material in a solvent; and the coated materials are rolled, folded or placed in contact when the coating is surface dry but contains suiilcient solvent to insure a bonding of adjacent insulating coatings to each other, thus eliminating strains that might give rise to high dielectric hysteresis and resultant voltage breakdown after a period of use.

Polystyrene, an artificial resin, is the preferred dielectric material as it has a power factor of about 0.04% while the power factors of other available insulating materials vary from about 1% for vinyl acetate to about 14% for Celluloid. It has been proposed to use polystyrene films in place of paper in spirally wound condensers, but such lms have contained air bubbles and could not be rolled into full surface contact with the metal foils. Furthermore, the lms were mechanically weak and it was therefore necessary to use lms of far greater thickness than was required for protection against voltage breakdown. The quantity of metal foil and double layer polystyrene 4films for a condenser of a given capacity was therefore far in excess of the quantity that would be necessary if the film thickness could be reduced to a value adequate for protection against voltage breakdown.

An object of the present invention is to provide methods of manufacturing electrical condensers by coating portions of the complementary metal strips or foils with a solution of a low loss dielectric material, forming the coated strips into a compact assembly when the coatings are surface dry but retain an appreciable quantity of the solvent, and maintaining the assembly under pressure until the adjacent layers coalesce by a redistribution 0f the solvent in the dielectric material. An object is to provide methods of manufacturing electrical condensers from strips of metal foil by coating portions ofthe opposite faces and one edge of the foil with a plastic dielectric material, and rolling or folding two coated foil strips into an assembly in which the uncoated edges of the respective foils project from opposite sides to receive terminal connections. An object is to provide, in the manufacture of electrical condensers, methods of reducing the size cfa condenser of a given capacity by restricting the .thickness of the dielectric material to values deJ termined by the breakdown voltage and not the mechanical strength of the dielectric material. A further object is to provide methods of manufacturing condensers from coated metal foils, the methods being characterized by a reduction of the cohesive forces between the molecules of the coating material to values below that at which the coating material shrinks away from the edges of the coated foils. v

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Fig. l is a schematic view illustrative of the conductor coating step of the invention;

Fig. 2 is a perspective View of a conducting strip coated on portions of its opposite surfaces;

Fig. 3 is a fragmentary section through a conducting strip or foil carrying a. plurality of thin insulating coatings that extend across the edge of the strip or foil; f

Fig. 4 is a perspective view, partly in section, and with the thickness of the metal foil and dielectric greatly exaggerated, of a condenser manufactured in accordance with the invention;

larretrate constant up to at least l0 million cycles and probably to considerably higher frequencies.

'Ihe thickness of the layer 3 of insulating material upon the conductive member ldepends upon the concentration of the bath 2, and, for relatively thick layers 3 that are obtained when about 30 parts of polystyrene are dissolved in 100 parts of solvents such as benzol, toluol or xylol, the cohesive forces between the molecules of the material overcome the adhesion of the insulating lm to the metal and the film ruptures at the end and edges of the metal l as the solvent evaporates. It has been proposed to maintain the continuity of a coating around the edge of a l rupture the nlm decrease approximately with reductions in the film thickness. Thin films of polystyrene, such as obtained with solutions containing about 2% by weight of solid material, adhere to thin metal foils to produce coatings that cover the edges of the foil. The films are extremely thin, possibly of the order of $55000 of an inch,

Fig. 5 is a curve sheet showing the relation,

with respect to time, of the quantityv of solvent in the insulating coating and the adhesive bond between the coating and the foil;

Fig. 6 is a curve sheet showing the distribution of the solvent throughout the insulating coating when the coating is surface dry and afterl an appreciable time, respectively;

Fig. 7 is a fragmentary perspective view showing another method of extending the insulating coating over the edges of the metal foil;

Fig. 8 is a diagrammatic view of apparatus for the manufacture of spirally wound condensers; Fig. 9 is a transverse view on line 9-9 rof trating a method of folding coated foils to form a condenser.

As shown diagrammatically in Fig. 1, the press1 ent invention contemplates the coating of portions only of the opposite surfaces, and preferably of the adjacent edges of a metal foil or strip I by immersion in a solution of an insulating material in a solvent. The preferred insulating material is a water-clear polystyrene which has a power factor of about 0.04% and is resistant to acids, alkalies and moisture, but it is to be understood that other resins, for example Vinyl acetate, or'mixtures of resins may be used when a minimum power factor is not essential. For simp iicity of description, only polystyrene will be discussed in detail in the following specification. The dielectric constant of polystyrene is about 2.6, and I have found that the value remains but continuous layers 3a of greater thickness may be obtained by repeating the coating step to build up laminations that merge to form a thick homogeneous layer, see Fig. 3. The thickness of the dielectric layer determines the breakdown voltage of the condenser and repeated coatings are required only when the condenser must stand high voltages as layers deposited from thin solutions are homogeneous and free from air bubbles.

A plurality'of metal strips l, with coatings 3a that are continuous around the sides and end of the strips, may be assembled into a condenser, as shown in Fig. 4, by arranging the bare ends of the metal strips in alternation at opposite sides of the assembly, the coated portions of the strips being superposed. The coated strips are preferably stacked as soon as the coatings are surface dry and before the solvent is completely evaporated, as I have found that the adjacent film surfaces will then soften and unite through a redistribution of the solvent within the polystyrene films.

The quantity of solvent within the polystyrene hns varies with time, as shown by curve A of Fig. 5, and an appreciable amount of the solvent is present in the polystyrene films at a time, indicated by line aa, when the surface is dry and can be touched without adhesion. The lms havev a high degree of elasticity and a high coefficient of adhesion to the foil when surface dry and for an interval up' to a time, indicated by line b-b, at which the major portion of the solvent has left the nlm. vDuring this interval, which may vary from a few seconds up to possibly 30 seconds, depending upon temperature, the thickness of the lms, the concentration of the coating bath, etc., the metal foil and its coating may be folded, bent or rolled without separating the film from the foil or breaking or cracking the illm. The condensers are therefore assembled or wound during this interval when the insulating i-llms are surface dry but still contain an appreciablequantity of solvent. The surface dry films slip readily upon each other during the stacking or rolling of the'coated metal foils, but the contacting surfaces soften quickly and unite to form a homogeneous layer of polystyrene tha-t bonds the adjacent foils to each other. 'I'he engagement of the surface dry layers does not establish any strains in the dielectric layers as the coated strips or foils may slip upon each y other during folding or rolling operations without destroying the adhesive bond between the film layers and the metal strips.

This union of the surface dry iilms is explained by the curves of Fig. 6 which show the relative quantities of solvent at diil'erent portions of the insulating layer. Curve B shows the .distribution and quantity of solvent within the layer at a time corresponding to line o-a of Fig.

`5 when the coating is surface dry, and curve C coated ioils. Rapid drying is essential when the condensers are wound by machinery, as will be described later, as the coated metal foils must be surface dry to pass freely over guide rollers.

The characteristic property o freshly prepared but surface dry polystyrene films, i. e. oi coalescing when placed in contact, may be used to er:u tend the insulation over the edges and end of the metal strip l by rolling or pressing the coated strips as soon as the coating 3 is surface dry. This step, as illustrated in Fig. 7, may 'oe effected by passing a coated strip, such as shown in 2, between rollers t in the direction of the arrow r. The surface dry, but still plastic, layers oi polystyrene 3 at opposite faces ci the metal strip l are forced outwardly to unite at the edges of the strip to lorin a continuous homogeneous coating tb.

The invention is particularly adapted to the manufacture of spirally wound ,condensers by apparatus such as shown diagrammatically in E and 9. 'il/letal foils la, lh are drawn from rolls la, lb, respectively, and pass over rubber coated guide rollers 8a. lib to dip into solutions 2 of polystyrene in tanks 9c, 9b. metal foils pass over inclinedguide rollers ita, lob in the tanks but are not completely submerged. rlhe coating bath. and the drying conditions are so regulated that the coatings are surface dry oeore reaching the guide rollers ila, tb at the exit sides ci the tanks. "vl/hen the foils are dipped a plurality of times, they are passed between sets ol inclined rollers da, b, respectively, to press the coatings towards the lower edges oi the foils prior to the iinal passage through the coating solutions. The foils ic, ib are twisted in opposite directions as they pass from the last guide rollers to the leed rollers lla, lib, respectively, to place the uncoated edges of the foils at opposite ends of the wound assembly that is to be formed on the split mandrel l2.

The end of one coated foil, for example thefoil la, is passed through the slot in the mandrel l2, `and the mandrel is rotated one or :more complete turns beiore the folded end of the other foil is inserted into the bight of the foil la on the mandrel. The coated foils are tightly wound upon each other by driving the mandrel l2 through a slip coupling that tends to overdrive the feed of the coated foils by the feed rollers lla, lib, and by a rubber coated roller I3 that is pressed against the winding foil layers by a spring I4. The folding of the ends of the coated foils is necessary to insure adequate insulation o1 the cut and ragged ends la: of the coated foils, see Fig. ll. Alternatively, a folded strip l5 of insulation may be placed over the end of the foil lb, as shown in Fig. 10.

Either one oi the foils, for example the foil lb, is cut when the Wound assembly has the desired capacity and is folded back upon itself to insulate .the raw end of the conducting strip. The other foil may be cut and folded back in the same manner but preferably, as shown in Fig. 13, the se@ ond foil is continued for some distance beyond the end of the first foil, thug providing s, shield ing for the assembly. A quick-drying adhesive 2', that may be a polystyrene solution, is applied to the end of the outer ioil to bind the outer turn to the wound assembly. The contacting nlm layers soften during the winding operation and the union of the layers starts at once under the pressure incident to the close Winding of the foils. The coalescing of the film layers doubtless continues for some time but the condenser may be removed from the machine as soon as the outer turn is pasted down.

The preferred form ci terminal for the spiral condenser is a wire it having a spirally Wound base that is secured to the end of the bare roll by dipping the terminal in molten solder and pressing it against the coiled foil. The Solder iuses to the foil before cooling, and establishes direct metallic connection between the terminal and various points along the toil edges that are forced into engagement by pressure applied by the terminal. The condenser may be finished by inserting it in a paper tube and turning down the ends or" the tube, or may used without the added paper tube.

A surface dry condition of the insulating layers before reaching the mandrel is `essential to mit the coated foils to slip upon each other as they are wound into the spiral assembly, thus nrecluding high mechanical strains that might buckle or break the coated foils, but the solvent within the surface dry layers seitens unites the contacting layers. The insulating coatings extend over the edges of the foils and is er;u eluded by the winding process. The final as= sembly is therefore air tight moisture proof, and the only .leakage paths are along the enf; terior or interior cylindrical surfaces the tribuu lar body.

The coated foils lc, ifo may be assembled in compact relation by running the strips together, as shown in Fig. l5, and folding the doubled strip bach and forth. The zig-zag folding is come mercially practical without damage to the foils or the insulating coatings as the surface dry :illrns slip readily upon each other. The coated foils are laterally offset as previously described, to superpose the coated portions While leaving the bare ends of the foil exposed to receive terminal connections.

The thickness of the polystyrene coating may be regulated, by varying the concentration in the coating solution, in accordance with the clesired breakdown voltage. This is possible as the characteristic of polystyrene of iniinite solution insures continuous homogeneous lms of minute thickness. A microscopic examination of nlms oi' the order of /wwo inch and less shows no holes, bubbles or cracks. VIllhe nlm thickness may therefore be reduced for low voltage condensers, and the quantity or length of the metal foils may be correspondingly reduced as the capacity in.. creases iwith a decrease in the spacing of the metal foils. The quantity of material in a condenser of a given capacity has not been important under prior practice, as the maior part of the total manufacturing costs has been the labor cost but, with the development of automatic machinery for winding condensers. the cost of materials is of greater importance. 1

Additions may be made to the polystyrene solution for special purposes, for example to increase the adhesion forces and decrease the cohesive forces that tend to shrink the coatings away from the edges of the foil. Such additions are generally undesirable as they increase the power factor, but small amounts of an addition agent such as a lead soap, in quantities lup to say 5%, will increase the mechanical characteristics of the insulating film without raising the power factor beyond about 0.5%. A power factor of this low magnitude is substantially better than that obtained by any previously used dielectric materials other than quartz or strips of polystyrene.

Condensers manufactured by the described processes are characterized by low power losses and freedom from damage from moisture, acids ,and alkalies. Polystyrene, either alone or mixed with other polymerized resins or addition agents, is preferred but some of the advantages of the invention are retained when less desirable dielectric materials are used. It is therefore to be -understood that various changes are possible in the described steps and materials without departure from the spirit of my invention as set forth in the following claims.

The condensers manufactured by the described processes are more particularly described and claimed in my (zo-pending application Ser. No, 334,927, filed May 13, 1940. i

I claim:

1. The process of manufacturing an electrical condenser that comprises insulating a plurality of metal strips by coating the same with polystyrene in solution in a solvent, evaporating the solvent until the coatings are surface dry, then assembling the coated strips in superposed relation with the polystyrene coatings of adjacent strips in contact, and maintaining the assembly under pressure to unite the contacting polystyrene coating through a redistribution of the solvent then present in the contacting coatings.

2. The process of manufacturing an electrical condenser that comprises coating'an edge and the adjacent surface portions of two strips of metal foil with a solution of polystyrene inA a solvent, the concentration of the solution being above the value that results in a shrinking of the polystyrene coatings from the edges of the foil strips, pressing the coated strips to displace the polystyrene coatings over and into contact at the edges of the foil strips when said coatings are surface dry and contain a substantial quantity of solvent, and assembling the strips with the coated portions thereof superposed and in contact While the solvent content of the coatings is above the value at which the contacting surfaces of adjacent coatings are softened and united by a redistribution of the solvent within the coatings.

3. The process of manufacturing electrical conaeeaese densers that comprises drawing metal foin from a pair of rolls of foil, coating one edge and adjacent-portionsof the opposite surfaces of each foil with polystyrene dissolved in a solvent, evaporating the solvent until the coatings are surface dry, rolling the coated foils upon each other with the coated portions superposed and the ,uncoated edges of the respective foils extending at op posite sides of the superposed coated portions, cutting one foil and folding it back to cover the cut end with the coating upon that foil, winding the other foil upon the rolled assembly to a point beyond the folded end of the first foil, cutting the other foil and securing the end thereof to the rolled assembly.

4. The process as claimed in claim 3, wherein the rolling of the coated foils is initiated by winding at least one full turn of one foil, folding back the end of the other foil upon itself to cover the cut end of the foil, and inserting the folded end of the second foil between the wound turn of the rst foil and the portion thereof leading to the wound turn.

5. The process as claimed in claim 3, wherein the rolling of the coated foils is initiated by winding at least one full turn of one foil, placing a folded strip of insulating material upon the cut end of the other foil, and inserting the folded strip and end ofthe second foil between the wound turn of the first foil and the portion thereof leading to the wound turn.

6. The process as claimed in claim 3, wherein the cut end of said other foil is secured to the rolled assembly by coating the cut end with an adhesive.

7. The method of manufacturing electrical condensers `from long strips of metal foil that comprises dipping an edge portion of each of a pair of foil strips in a solution of polystyrene in a solvent, whereby the opposite edge portion of each strip is left uncoated, evaporating the vsolvent until the coatings are surface dry, arranging the coated strips in contact with the coated portions alined and the uncoated portions at opposite sides of. the assembly, and folding the strips baci: and forth to interleave the strips in a zig-zag assembly, the outer ends of the uncoated portions of the foils being adapted to receive terminal connections.

8. In the manufacture of electrical condensers, the method of sealing the overlapping portions of two metal strips against the entrance of moisture that comprises coating those face and edge portions of the strips that are to overlap with polystyrene in an air-drying solvent, evaporating the solvent until the coated portions are surface dry, then arranging the strips with the said coated portions overlapping, and pressing the overlapping portions of the strips together to unite the coatings by the solvent within the surface-dry coatings.

9. In the manufacture of electrical condensers by coating the component metal strips with insulating material, the method of obtaining continuous coatings over the surfaces and edges of the strips which comprises dissolving polystyrene in a solvent, adding lead soap to the solution to reduce 'the cohesive forces developed in the coatings during evaporation of the solvent, and dipping the strips in the solution.

" KENNETH W. JARVIS.

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