US3138773A

US3138773A - Electrical coils with permeable resin bonding permitting penetration of dielectric liquid therethrough

Info

Publication number: US3138773A
Application number: US856434A
Authority: US
Inventors: Frank S Nichols; Eugene K Steele
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1959-12-01
Filing date: 1959-12-01
Publication date: 1964-06-23
Anticipated expiration: 1981-06-23

Description

June 23, 964 F. s. NICHOLS ETAL 3,133,773

ELECTRICAL COILS WITH PERMEABLE RESIN BONDING PERMITTING PENETRATION OF DIELECTRIC LIQUID THERETHROUGH Filed Dec. 1, 1959 I (if?! 4 United States Patent g ELECTRICAL COILS WITH PERMEABLE RESIN BONDING PERMITTING PENETRATION 0F DI- ELECTRIC LIQUID THERETlmOUGH Frank S. Nichols and Eugene K. Steele, Pittslield, Mass, assignors to General Electric Company, a corporation of New York Filed Dec. 1, 1959, Ser. No. 856,434 4 Claims. (Cl. 336-405) The present invention relates to electrical coils and particularly to induction coils for electromagnetic apparatus such as transformers and the like.

In the manufacture of coils of the above type, it has been the practice to apply an insulating coating of varnish on the coil. By this means, the mechanical strength of the wound structure is increased for affording better resistance to the stresses involved in the coil handling and assembly procedures, and especially to the high mechanical forces to which the coils may be subjected during operation of the electrical apparatus. For example, under short circuit conditions, high mechanical stresses tend to separate the core windings both axially and radially and may lead to distortion or even rupture of the coil structure. In the past, varnish compositions employed for coating the coils have included alkyd resin, phenolic resin, and polyester resin varnishes, the resin material being dissolved in a suitable volatile solvent. However, it has been found that such varnish-treated coils are not fully satisfactory for use in oil-filled apparatus where the dielectric oil or other liquid is intended to impregnate the coil to provide adequate resistance to high electrical impulse conditions. It appears that the previous varnish treatments resulted in a liquid-impermeable coating being formed over the coil structure which effectively prevented the ready penetration of the dielectric liquid into the many small crevices and interstices inevitably present in the wound coil structure. As a result, there was a greater probability of failure due to impulse conditions than where such varnish treatment was omitted. On the other hand, elimination of such varnish treatment could be made only at the sacrifice of short circuit strength of the coil.

It is an object of the invention to provide an electrical coil structure which has satisfactory mechanical strength to resist short circuit conditions as well as being capable of resisting high electrical impulse conditions.

It is a specific object of the invention to provide an electrical coil of the above type having an insulating coating thereon which affords satisfactory mechanical bonding for the coil turns While permitting the dielectric liquid in which the coil is immersed to penetrate into the interstices in the coil.

It is a further object of the invention to provide a method of treating electrical coils to impart the abovementioned improvements thereto.

Other objects and advantages will become apparent from the following description and appended claims.

With the above objects in view, the present invention relates to an electrical apparatus comprising, in combination, a. container having a dielectric fluid therein, a coil in the container immersed in the dielectric fluid therein, the coil being formed by a wound conductor having an insulating coating thereon, the coil having interstices therein and having deposited thereon a film formed of a 3,138,773 Patented June 23, 1964 fused particulate resin material, the film being deposited on the coil from a suspension of polymer particles in a non-solvent liquid medium, whereby the deposited film firmly bonds the Wound coil while permitting penetration of the dielectric fluid into the interstices of the coil winding.

The invention will be better understood from the following description and accompanying drawing in which:

FIG. 1 is a perspective view partly in section of an electrical induction coil embodying the present invention; and,

FIG. 2 shows a transformer partly in section in which the FIG. 1 coil is incorporated.

Referring now to the drawing and particularly to FIG. 1 there is shown a typical transformer coil structure A comprising an inner insulating cylinder 1 of pressboard or the like on which the coil turns are wound, the coil including a low voltage winding 2 and a high voltage winding 3 which are radially separated from each other by duct forming spacers 4 such as wooden strips or the like. Each coil winding consists of a plurality of radially superposed layers of helically wound wire 5, the wire being coated with insulating enamel such as a phenolicmodified polyvinyl formal, and the layers of wire being separated from each other by cellulosic paper insulation sheets 6. In accordance with the invention, the surfaces of the coil structure are covered with a film of resin 7 deposited from a suspension of polymeric particles in a non-solvent medium such as water.

FIG. 2 shows the coil structure of FIG. 1 operatively assembled in a transformer which comprises a tank 8 closed by a cover 9 and containing a dielectric liquid 10 such as mineral oil or the like. In the transformer, the coil structure A encircles magnetic core 11, and the high and low voltage leads 12, 13 of the coil are respectively connected to corresponding bushings 14, 15 mounted on cover 9.

By virtue of the particular type of resin coating provided on coil A, it has been unexpectedly found that effective permeation of the dielectric liquid 10 into the inner structure and interstices of the coil unit is not prevented and that as a resulta substantial decrease in the number of failures on impulse testing of such coils is achieved. This benefit is obtained While still retaining a good bond between the coil turns by means of the resin coating, thus providing for adequate short circuit strength of the coil.

The material which is applied to the coil to obtain the desired resin coating may be of known types of compositions consisting essentially of suspensions of extremely small particles of solid polymers in water, the particle size being typically about 1 to 500 microns. These materials are normally made by the emulsion polymerization of one or more liquid polymerizable monomers, the emulsification of the original monomers producing the very small size particles of polymer. The monomers used may be difunctional giving rise to thermosplastic polymers, common examples used commercially being vinyl acetate and various acrylic esters. Alternatively, the monomers may be polyfunctional provided that two of the functional sites may be preferentially reacted to produce minute particles of a thermoplastic polymer which by further reaction can convert to a thermoset polymer. Materials of this latter type prepared, for instance, from acrylonitrile, butylacrylate, methacrylic acid, and glycidyl methacrylate are described in Sanders Patent 2,787,561.

A specific example of the latter composition is as follows:

Example 1 Ingredients: Parts by wt.

Polymerizable monomers- Acrylonitrile 63.0 Butyl acrylate 32.0 Methacrylic acid 3.0 Glycidyl methacrylate 2.0 Polymerization initiator Sodium bisulfite 0.1 Potassium persulfate 0.3

Dispersing agentSodium lauryl sulfate c 0.5 Telogen-Dodecyl Inercaptan 0.5 Water 200.0

Additional suitable compositions of thermosetting types are disclosed in the patent to Erickson et a1. 2,580,901, of which the following are illustrative:

Example II Parts Styrene 180 Glycidyl methacrylate 20 Dioctyl sulfo succinate 5 Water 580 Ammonium persulfate 0.1

Example III Parts Ethyl acrylate 120 Glycidyl methacrylate 80 Sodium lauryl sulfate 1.5 Water 300 Ammonium persulfate 0.1

Other types of compositions of thermoplastic nature which may be used in practicing the invention are:

Example IV Parts Water 512 Vinyl acetate 276 Ethyl acrylate 161 Butyl acrylate 23 Sodium lauryl sulfate 7.5 Ammonium persulfate 0.5 Sodium bicarbonate 0.4

Example V Parts Ethyl acrylate "29.26 Methyl methacrylate 14.41 Methacrylic acid 0.67 Soduim lauryl sulfate 0.30 Sodium bicarbonate 0.05 Ammonium persulfate (4% solution) 2.42 Sodium methabisulfite solution) 1.00

Water 40.34 Ammonium hydroxide sufiicient to bring pH of product suspension to 8.5 to 9.0.

In all cases, the product utilized in practicing the invention is a stable suspension of minute fusible particles of polymer in a non-solvent medium, preferably water. In use, this suspension is coated onto the coil and as it loses water by drying, the suspension becomes unstable and the polymer particles are deposited as a coating. The particles may subsequently be baked to fuse them together and, in the case of the thermosetting types, to complete the second stage of polymerization and cure them. If desired, small amounts of solvent for the polymer may be added to the suspension to facilitate fusion of the polymer particles Without adversely affecting the operation of the material for the purpose of the invention. We have found that if an electrical coil, particularly one insulated with cellulose material, is dipped in such a material and dried (or cured where appropriate), while water from the suspension may penetrate into the coil, the solid particles are deposited on the surface only and that while such a coating provides eX- cellent mechanical reinforcement, it surprisingly does not prevent subsequent penetration of oil into the coil.

Several hundred distribution transformer coils have been treated with a commercially available acrylic polymer suspension of the above-described type, and it has been demonstrated that this treatment substantially improved short circuit strength while satisfactory impulse tests on these coils showed that complete penetration of the dielectric liquid took place. In contrast, treatment of the coils with conventional alkyd varnish has been found in tests to result in six times as many failures of the coils in impulse testing.

Another series of tests was conducted to determine the effectiveness of the described polymer suspension material in providing adequate bonding of the coil turns to resist high electrical and mechanical stresses. In these tests, in which the tested coils were subjected to short circuit conditions, it was found that coils having installation pretreated with shellac in accordance with a commonly employed procedure withstood only 37 times the normal operating current before showing damage due to physical shifting of the turns, whereas similar coils which had been additionally treated by dipping them into an aqueous acrylic polymer suspension of the above-described type withstood 50 times the normal operating current before such damage resulted. This test thus demonstrated the excellent coil-bonding properties of the poly mer suspension coating material.

In a typical process for treating transformer coils in accordance with the invention, the coil is dipped at room temperature in a suspension of polymer material having the composition set forth in Example I above, the coil being typically formed of wire covered with a polyvinyl formal enamel and wound on a porous laminated paper or pressboard winding form. The viscosity of the suspension bath may be adjusted as desired by adding. water. The dipping period is not critical and may vary from a few seconds to several minutes. It has been found that due to the nature of the polymeric suspension even a single short dipping period is effective to provide adequate bonding of the coil turns to provide high resistance to short circuit conditions, in contrast to conventional varnishes which normally require a relatively long or repeated dipping to provide equivalent mechanical strength. After the coil is dipped, it is removed from the bath and allowed to drain for a short period. There after, the coil is allowed either to air dry and then curing is effected during the usual transformer drying cycle,

cure the polymer.

The bake cycle, as indicated above, is used either to fuse the polymeric particles (as in the case with suspensions of thermoplastic materials, e.g., polyvinyl acetate) or to fuse and cross-link the polymer chain (as in the case of thermosetting materials such as in the above-mentioned Sanders patent). Thereafter, the dried core and coil assembly is placed in a transformer tank and the tank is filled with appropriate insulating liquid.

While it appears that improved results are obtained by the present invention because the dielectric liquid of the electrical apparatus permeates the coated coil more readily than in the case of prior types of coated coils, the reason for the improved permeability is not fully understood. It may be that the particulate resin material, although fused to a practically continuous film, leaves unbridged gaps especially at the ends of the coil between layers, through which the dielectric oil may pass, whereas prior solution-types of resin coatings have formed an impervious bridge across such irregular areas or fissures and the dielectric liquid is thus elfectively prevented from entering the interior of the coil to saturate the interstices therein.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical apparatus comprising, in combination, a container having dielectric fluid therein, a coil in said container formed by a wound conductor having an insulating coating thereon, said coil having interstices therein and having deposited only on the outer surfaces thereof a film composed of a fused particulate resin material, said deposited particulate film being porous and permeable to said dielectric fluid by both radial and axial penetration through the wound coil, whereby the deposited film firmly bonds the wound coil while permitting penetration of the dielectric fluid into the interstices of the coil winding.

2. An electrical apparatus comprising, in combination, a container having a dielectric liquid therein, a coil immersed in said dielectric liquid in said container, said coil being formed by a wound conductor having an insulating coating thereon, said coil having interstices therein and having deposited only on the outer surfaces thereof a film composed of a fused particulate polymer material, said deposited particulate film being porous and permeable to said dielectric liquid by both radial and axial penetration through the wound coil, whereby the deposited film firmly bonds the wound coil while permitting penetration of said dielectric liquid into the interstices of the coil winding.

3. A coil structure permeable to dielectric liquids comprising, in combination, an elongated conductor wound into a plurality of turns, said turns being arranged in superposed layers, insulating materials separating said turns from each other, and a resin coating composed of fused particulate polymer material overlying only the surfaces of said coil for bonding the winding turns together, said particulate resin coating being porous and permeable to dielectric liquid by both radial and axial penetration through the wound coil for impregnation of the interstices of the wound coil by the dielectric liquid.

4. A coil structure permeable to dielectric liquids comprising, in combination, an elongated conductor wound into a plurality of turns, said turns being arranged in superposed layers, insulating material separating said turns from each other, cellulosic insulating sheet material separating said layers from each other, and a resin coating composed of fused particulate polymer material overlying only the surfaces of said coil for bonding the winding turns together, said particulate resin coating being porous and permeable to dielectric liquid by both radial and axial penetration through the wound coil for impregnation of the interstices of the wound coil by the dielectric liquid.

References Cited in the file of this patent UNITED STATES PATENTS 2,246,159 Work June 17, 1941 2,814,581 Flynn Nov. 26, 1957 2,867,032 Gehrke Jan. 6, 1959 2,942,217 Ford June 21, 1960

Claims

1. AN ELECTRICAL APPARATUS COMPRISING, IN COMBINATION, A CONTAINER HAVING DIELECTRIC FLUID THEREIN, A COIL IN SAID CONTAINER FORMED BY A WOUND CONDUCTOR HAVING AN INSULATING COATING THEREON, SAID COIL HAVING INTERSTICES THEREIN AND HAVING DEPOSITED ONLY ON THE OUTER SURFACES THEREOF A FILM COMPOSED OF A FUSED PARTICULATE RESIN MATERIAL, SAID DEPOSITED PARTICULATE FILM BEING POROUS AND PERMEABLE TO SAID DIELECTRIC FLUID BY BOTH RADIAL AND AXIAL PENETRATION THROUGH THE WOUND COIL, WHEREBY THE DEPOSITED FILM