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Publication numberUS3233311 A
Publication typeGrant
Publication dateFeb 8, 1966
Filing dateApr 6, 1964
Priority dateJun 5, 1961
Publication numberUS 3233311 A, US 3233311A, US-A-3233311, US3233311 A, US3233311A
InventorsGiegerich Bertrand V, Steele Eugene K
Original AssigneeGen Electric
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method of making encapsulated coils
US 3233311 A
Abstract  available in
Images(4)
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Claims  available in
Description  (OCR text may contain errors)

Feb. 8, 1966 B. v. Gn-:GERlcH ETAL 3,233,311

METHOD OF MAKING ENCAPSULATED COILS Original Filed June 5, 1961 4 Sheets-Sheet 1 Feb- 8, 1966 B. v. GIEGERICH ETAL 3,233,311

METHOD 0F MAKING ENCAPSULATED COILS Original Filed June 5, 1961 4 Sheets-Sheet 2 Egan@ l. .fa/f, @www Feb- 8, 1966 B. v. GIEGERICH ETAL 3,233,311

` METHOD OF MAKING ENCAPSULATED COLS Original Filed June 5, 1961 4 Sheets-Sheet 5 Feb 8, 1966 B. v. Gu-:GERlcH ETAL 3,233,311

METHOD OF MAKING ENCAPSULATED COILS Original Filed June 5, 1961 4 Sheets-Sheet 4 WIND 60H5 0N C0!! [MM5 i @wwf United States Patent C) 3,233,311 METHOD OF MAKING ENCAPSULATED COILS Bertrand V. Giegerich, Pittsfield, and Eugene K. Steele,

Dalton, Mass., assignors toy General Electric Company, a corporation of New York Original application `lune S, 1961, Ser. No. 114,747.

Divided and this application Apr. 6, 1964, Ser.

3 Claims. (Cl. 29-155.57)

This invention relates to electromagnetic induction apparatus, and more particularly to such induction apparatus in which the coils are completely encapsulated in a synthetic resinous material, and to the method of encapsulating such coils.

This application is a division of application Serial No. 114,747 for Encapsulated Coils for Electromagnetic Induction Devices and Method of Making Such Coils led lune 5, 1961 in t-he names of the present inventors and assigned to the same assignee as this invention.

As is well known to those skilled in this art, the conventional electromagnetic induction apparatus, such as a transformer, presently in use today comprises a core and coil unit encased in a steel tank. The tank is filled with a liquid dielectric, such as a mineral oil, to provide a portion of the `desired insulation for the core and coil unit. These conventional transformers provide many drawbacks to the eicent distribution of electrical energy. Among the numerous problems involved in utilizing conventional transformers in the distribution of electrical energy is the great weight of such transformers. This weight, as is well understood, is comprised not only of the core and coil unit but of the large steel tank which houses such core and coil unit, as well as the large quantity of oil or other liquid dielectric which is utilized to provide part of the desired insulation for such conventional transformers. Of course, with such weight also goes the relatively large size or bulk, which is found in such conventional transformers. The space requirement of these transformers makes it undesirable to place them in the vicinity of houses or other installations which will be utilizing the electrical energy distributed by such transformer. It is also well known that the conventional transformer, with its large bulky tank, high voltage bushings, and the like, provide an unattractive appearance.

These transformers and like inductors also provide a great maintenance problem, particularly in the oil utilized as the insulating material which becomes contaminated with use. As will be understood, the insulating properties of contaminated oil deteriorate substantially. In such instances, it becomes necessary to completely drain such transformer and either process the oil to remove such contaminants and restore the oil to somewhat of its original dielectric property, or else to replace it with new oil. It will be well recognized that the draining and re-processing of oil or lthe supplying of new oil to a transformer is a time consuming and relatively expensive maintenance problem. Further, as the transformer oil deteriorates, this tends to affect the various types of solid insulation which are used in conventional transformers, thereby leading to the deterioration of such solid insulation. Also, as is well known, in those transformers which utilize cellulosic insulation asa part4 of the solid insulation system, the operation of such transformers at high peak loads tends to unduly age such cellulosic insulation which also tends to contaminate the oil, leading to a shorter life of the oil. Of course it is well understood, that all of these problems of deterioration tend to shorten the useful life of the transformer and tend to require more maintenance of the transformer, thus, overall leading to an increase in the total cost of `the transformer due to its many puroblems of maintenance and replacement.

ice

Of course, another well-recognized problem in conventional transformers, especially such transformers as the oil filled, is the fire hazard which is present in some circumstances due to the high peak load operation of the transformer. Because of this problem o-f a fire hazard due to the oil in the transformer, which has a relatively low ash point compared to the other insulation used in the transformer, such oil filled transformers are not recommended for use close to occupied buildings or in highly explosive or other dangerous areas, where the lire from the transformer could tend to destroy property or life.

It has generally been realized that it is desirable to provide transformers and other types of electromagnetic induction apparatus which would dispense with the necessity of the tank, the oil, and the various types of cellulosic insulation, Obviously, were it possible to dispense with the oil, the tank, and the types of cellulosic insulation presently used today, many of the problems hereinbefore set forth would be negated. However, numerous problerns have arisen in attempts to provide transformers where the tank and the oil is no longer necessary. One well known manner of eliminating oil is the dry type of induction apparatus. Dry type apparatus may be classified in three broad catagories, ventilated, gas-filled and encapsulated. The ventilated type of dry induction apparatus typically has a lower dielectric strength than found in the liquid lled apparatus. This is due to the fact that in such apparatus it is -not possible to eliminate air from regions of high dielectric stress. The air in these regions cause corona and low impulse strength. As is well known, corona is the ionization of insulation or other. material surrounding a conductor. Eventually such corona can lead to breakdown of the insulation, the conductor, .and eventually the equipment subject to the Corona also leads to objectionable high frequency discharges, which are o-ften termed radio noise. To eliminate these problems of ventilated apparatus gaslled units have been provided. However, gas-filled units require a tank to enclose the gas, which leads back to the problems earlier related with respect to the liquid filled units.

It has been proposed to eliminate these problems by encapsulating the core and coil unit of an induction apparatus in a synthetic resinous material, thereby obviating both the tank and the oil. It was thought that encapulation would tend to eliminate the air from the areas of high dielectric stress. However, it has been found that the .materials used for encapsulation do `not have the ability to provide the desired dielectric strength of the insulation which is found in conventional oil-filled electromagnetic induction apparatus. This is due to voids present in areas of high dielectric stress. It is diicult to obtain and maintain void-free encapsulated structures. These voids in encapsulated transformers Vtend to develop corona in the high voltage stress areas.

From the above it will be apparent that there is presently a great need in the induction apparatus field for an electromagnetic induction apparatus that will eliminate the need for tanks and large quantities of oil and the like while at the same time having dielectric qualities comparable to liquid filled apparatus.

It is therefore an object of this invention to provide an electromagnetic induction apparatus in which the coils of such apparatus are encapsulated in a solid synthetic resinous material.

A further object of this invention is to provide an electromagnetic induction apparatus having encapsulated coils in which the coils have a very high dielectric strength comparable to the conventional oil insulated electromagnetic induction apparatus.

.tion apparatus, which provide for a high dielectric strength to the coil structure.

A further object of thisV invention is to provide an electromagnetic apparatus having encapsulated coils and a methodV of making such coils, wherein the encapsulated coils have no significant amount of corona effect at normal operating voltages.

A still further object of this invention is to provide a method of encapsulating coils for use in electromagnetic induction apparatus, wherein the coils are provided with dielectric characteristics comparable to coils of conventional oil-lled apparatus;

In carrying out this invention in one form, an electromagnetic induction apparatus is provided in which the coils of such apparatus are encapsulated in a synthetic resinous material. A small opening is provided in the material encapsulating the high voltage winding -of the coil and oil Vis added into the cavity of the high voltage winding through such opening. The electromagnetic inductionapparatus is substantially smaller and lighter than a conventional tank and oil-filled apparatus, and have substantially equivalent electrical characteristics, such as very low corona level and very high dielectric strength.

One method of making the encapsulated coil of this invention comprises the steps of winding the high and low voltage coils on winding forms, surrounding the high voltage Winding form with a sealing means to prevent the entry of the encapsulating material, providing an opening to the high voltage winding, placing the coils in a mold and encapsulating them with a synthetic resinous material, curing such resin, then vacuum irnpregnating the high voltage winding with a liquid dielectric and, finally, sealing such opening.

The subject matter which is desired to be protected will be particularly pointed out and distinctly claimed in the claims appended to this specification. However, it is believed that this invention and its objects and advantages, as well as other objects and advantages thereof, will be better understood from the following detailed description when considered in connection with the accompanying drawings, in which:

FIGURE 1 is a perspective View of an electromagnetic induction apparatus including one form of encapsulated coils according to this invention;

FIGURE 2 is a detailed perspective view, partially in section, of the encapsulated coils shown in FIG. l.;

FIGURE 3 is a detailed sectional view taken at theV arrow 3 of FIG. 2;

FIGURE 4 is a detailed sectional view similar to FIG. 3 showing another aspect of the encapsulated coil;

FIGURE 5 is a perspective view showing one form of making the coils of thisV invention; and

FIGURE 6 is a ow diagram of one method of making the encapsulated coil of this invention.

Referring now to the drawings, in which like numerals are used to indicate like parts throughout, and particularly considering FIG. 1, there is shown an electromagnetic induction apparatus in the form of a transformer 10, such transformer being provided with an encapsulated coil 12 and a pair of electromagnetic cores 14 and 16. As can be seen from the figure, the encapsulated coil 12 is provided with a window opening 18 through which the electromagnetic cores 14 and 16 may be inserted to electromagnetically connect the windings of the encapsulated coil 12, in a manner which is well known to those skilled in this art. Of course, it will be understood, that while the invention is shown in FIGURE l as applied to a single encapsulated coil having a pair of electromagnetic cores, electromagnetically connected thereto, this invention is not limited to such type of electromagnetic induction apparatus. For example, considering merely the transformer application of this invention, it will be understood that a single core element could beprovided having encapsulated coils on each winding leg thereof, or only on one leg, if desired. Further, a three-phase core could be utilized and encapsulated coils could be provided on each of the elements of the three-phase core in a manner well understood by those skilled in the art. The main point to 'oe brought out herein is that the coil of the' electromagnetic induction apparatus is encapsulated in a synthetic resinous material, and is provided with a window opening through which the electromagnetic core, associated therewith, may be inserted to provide the desired electromagnetic relationship between such core and coil. Y

Considering more particularly FIG. 2 of the drawing, which is a detailed view, showing a preferred embodiment of the coil 12 with the core elements 14 and 16 shown in phantom therein, it can be seen that the encapsulated coil 12 comprises a high voltage winding Zt) and a low voltage winding 22. As is more particularly shown in FIG. l, the high voltage leads 24 are preferably brought out through the side of the encapsulated coil, while the low voltage leads 26 are brought out through the top or bottom of the encapsulated coil. As can be seen in FIG. 2, the high voltage winding 2t) is comprised of a nurnber of turns of wire wound about a winding form 28, the winding form being shown with a central spacer 30 to separate the top and the bottom portions of the high voltage winding. Of course, it will be understood that the mineral oil used in transformers.

separator is not necessary in the winding form; however, it is utilized to reduce the stresses between the various layers of the high voitage winding, wound on the coil form 28. The low voltage winding 22 is comprised of a number of Aturns of low voltage wire which is wound about the winding form 32. As is Clearly indicated in FIG. 2, between the various layers of the low voltage winding 22 layer insulation 34 is placed, to insulate the different layers of the low voltage winding from each other, as is well understood by those skilled in the art. Of course, it will be understood that layer insulation could also be provided for the high voltage winding, if desired. In general, when a coil, such as coil 12, including the high voltage winding 20 and low voltage winding 22, is encapsulated within a synthetic resinous material, both the high voltage winding and the low voltage winding are completely surrounded and impregnated to some extent with the encapsulating material. However, it has been found that a number of problems arise, especially with reference to the high voltage winding, in that voids are found within the high voltage winding which are not filled by the encapsulating material. These voids or air spaces, in general, lead to corona due to the high voltage stresses at these points in the high voltage coil, and as is well known, this leads to objectionable radio noise in such coils. Further,'it has been discovered that, apparently where the high voltage winding and the encapsulating resin contact each other they also provide a source of corona. It has also been found that the development of voids in the encapsulated high voltage winding, as well as the development of corona due to the contact of the high voltage windings with the encapsulating material, can be avoided by impregnating the high voltage winding with a dielectric liquid, such as, for example, It has been found that only a small amount of dielectric liquid is necessary, generally in amount suicient to provide a film about each of the wires of the high voltage winding. The amount of dielectric liquid required for this invention is not of sufficient quantity to be considered a fire hazard.

Therefore, in making the encapsulated coil of this invention, after the high voltage winding is wound about tained by utilizing a continuous film about the open'portion of the winding form 28, the 4continuous film being .sufliciently flexible and impermeable to the encapsulating resin to prevent such resin from contacting the high voltage winding. As can be seen from FIG. 2, the use vof the sealant, shown as material 36, completely seals off the high voltage winding from the encapsulating material, which` is generally indicated at 38 in the drawing. Priorto the encapsulation of the coil, which is comprised of the high voltage winding 20 and the low voltage winding 22 and their winding forms 28 and 32, means are provided in the winding' form of the high voltage winding so that oil may be readily added to the cavity 29' comprised of the winding form 28 and the sealing means 36.

As more clearly shown in FIG. 3 `of the drawing, which is an exaggerated detail view taken at the arrow 3 of-FIIG.` 2, this entry means is preferably in the form of a tube 40 which is inserted into a notch 42 of the high voltage winding form 28, and is firmly secured thereto as by an adhesive or by means of the sealing member 36. As will be well understood, the tube means 40 must be of a sufficient height to extend above the top of the encapsulating material, which will be molded about the high voltage winding 20 and the low voltage winding 22. Of course, as will be understood, generally it is desired to vacuumimpregnate the winding 20 in the cavity 29 comprising the sealing means 36 and the high voltage winding form 28. Therefore, a` pair of tubes 40 will be provided on `opposite ends of the coil form 28, as is indicated in FIG. 2, where the additional tube 40 is shown at the lefthand side of the top of the coil 12 near the low voltage leads 26.

'As' will be understood, after the low voltage and high voltage winding have been encapsulated in the encapsulating material 38 then one `of the tubes `40' is connected to a vacuum pump. The cavity 29, in which the high voltage winding 20 resides, is evacuated and a di-electric liquid` is forccdinthrough the other tube 40, in a manner which will be well understood by those skilled in this art. As can be seen from FIG. 3 of the drawing, as the vacuum is pulled in the cavity 29 through one tube 48, which is not shown `in FIG. 3, the dielectric liquid is forced into the cavity through the other tube 4t). The dielectric liquid will be forced into the cavity 29 due to the vacuum in the cavity and the atmospheric pressure on such liquid. The dielectric liquid will circulate through each of the windings of the high voltage winding 20, in the manner shown by the arrows in FIG. 3, thus completely impregnating the high voltage winding 20. Dielectric liquid is forced into the cavity 29 until the cavity overiiows and then the tube means 40 are sealed off, as will be well understood. However, if desired, it would be possible to remove the tube 40 from the encapsulated coil 12 and place a plug in the opening, for example, a plug 44 as is shown in FIG. 4.

As will be clear from FIG. 4 of the drawing, the dielectric liquid in the high voltage winding completely fills all areas of the winding. In either sealing off the tube 4i) or in inserting the plug 44, the liquid is actually forced out of the high voltage winding cavity 29. This is to ensure that there will be no air present within the dielectric liquid impregnated high voltage winding.

Throughout the previous portion of the specification, it has been indicated that the high voltage winding is impregnated with a dielectric liquid, such as transformer mineral oil. Of course, it will be readily understood by those skilled in the art that approximately the same results could be obtained were the low voltage winding Vimpregnated withv oil.l Theimpregnation of the low voltage winding would provide thev oil insulation about the windings 22, separating them from the encapsulating material 38. Therefore, the tendency of7 the high voltage stressesthrough the encapsulating material 38 would be reduced in much the same manner as they are reduced in the preferred' embodiment by the impregnation of the high voltage winding. Of course, `it Will be apparent that this reduction will `not be of the same order as that obtained when the high voltage winding is impregnated with oil, since the stress in the high voltage winding is 4much greater than in the low voltage winding. Further, it will be understood that where desired, both the high voltage windingl and .the low voltage windingv could be impregnated with dielectric liquid. However, it has been found only necessary to impregnateA one `of the windings to obtain the `desired results, that is, a substantial iricrease in the dielectric strength 'of the encapsulated coil, and `a substantial decrease in any corona present within the encapsulated coil, thus substantially eliminating the severe problem of radio noise present in prior encapsulated coils. Of course, it has lbeen mentioned that the' cavity for the high voltage winding is completely filled with the dielectric liquid such as, for example, mineral oil'. However, it will be understood that under certain circumstances, when the encapsulated transformer is operating at an extremely high ambient operating temperature, that there will be a tendency for the dielectric liquid to expand and possibly such expansion could break the encapsulating material 38. One manner of eliminating `this problem would be an expansion chamber, in the form of a 'bellows device`,mprovided` over the tube 40 or within the opening. filled by the plug 44. Such expansion chamber wouldk be completely filled with oil. However, it would be a flexible material such as, for example, metallic bellows, such that any expansion ofthe dielectric liquid would be taken upl by an expansion of such bellows. Y

The method of making the encapsulated `coil oft-his invention is more clearly shown in FIGS.` 5 and 6 of the drawing. As kshown in FIG. 5, the wind-ing coil forms 28 for the high voltage winding and 32 for the low voltage windings are first wound with the wire which com-prises the high voltage winding 20 and the low voltage winding 22. As indicated in FIG'. 5, a layer insulation means 34 is provided betweenpthe various layers of the low voltage winding 22. The high voltage winding form `28 is providedvxwithtwo notches, indicated at 42, for the reception of the tube means 40, ashas ,been4 previously described. After the high voltage winding 20 and the low voltage winding 22 have been wound on their respective forms 28 and 32, the sealing means 36 is wrapped about the high voltage winding form 28 to thereby completely seal in the high voltage winding 20. At this time, the tube means 40 will be inserted within the openings 42 yon opposite sides of the high voltage winding 28 to provide the desired entry to the cavity 29 within the high voltage winding form 28.

The winding forms are then placed one within the other, as indicated in FIG. 5, and the entire coils are then placed within a mold having the desired shape. The entire unit will then be encapsulated with a synthetic resinous material such as, for example, a lled epoxy resin. After the entire unit has been encapsulated, the coils will be placed in an oven for curing of the synthetic resinous material. After curing, a vacuum is pulled on one of the tubes 40 while a dielectric liquid, such as mineral oil, is inserted through the other tube 40 into the cavity 29 containing the high voltage winding 20. As earlier indicated the high voltage winding 20 is impregnated with oil until the tubes 40 are overflowing with the oil. The tubes are then sealed 01T, thereby providing the cavity 29 containing the high voltage winding 2t) with complete impregnation with the dielectric liquid without any air spaces therein.

7 Of course it will be understood -that some of the steps of this method could be interchanged. -For example, it would be possible to irnpregnate the winding coil with a liquid dielectric after sealing of such winding and'before encapsulating. Also `it would be possible to impregnate the Winding With the dielectric liquidl after er1- capsulating, but prior to curing of the encapsulating resin. Each ofthe above order of steps has its advantages, but as earlier indicated the present preferred method is to impregnate the winding after the curing of the encapsulating resin. As earlier pointed out, either the high Voltage winding or the low voltage Winding, or both such windings may be impregnated With dielectric liquid, if such should be desired.

While an electromagnetic induction apparatus comprising an encapsulated coil impregnated with a liquid dielectric, and the method of making such encapsulated coil, has been described in detail hereinbefore in the present preferred embodiment as required by the patent statutes, it will be Well understood by those skilled in the art that various 'changes and modifications may be made therein. For example, as earlier indicated the impregnation of the 101W voltage winding may be utilized rather than the oil impregnation of the high voltage winding. Further as noted, the sealing means may be in the form of a thin film wh-ich is wrapped about the high voltage winding form 2'8, rather than utilizing a continuous lm, as indicated at 36. Obviously, other sealing means could be used. Further, it should be understood that While the winding forms are presently desired in order to make the encapsulatedv coil of this invention, it would be possible toy merely Wind either the high voltage winding or the .low voltage winding, or both such windings with a sealing means completely about it in place of the winding form and the sealing means herein'described. The winding or windings could be impregnated through such sealing means with the impregnating liquid dielectric, as earlier described. It will of course be understood that while deaereated oil is the ,preferred dielectric liquid,

means to form a cavity, and to completely enclose said high voltage Winding within said sealing means and said high voltage winding form, providing entry means into said cavity, encapsulating said high and low voltage windingswith a synthetic resinous material, curing Vsaid synthetic resinous material, vacuum impregnating said high voltage winding through said entry meansV with a liquid dielectric material and sealing said ent-,ry means.

2. A method of making an encapsulated coil for use with an electromagnetic induction apparatus comprising: winding a high voltage winding on a winding form, winding a low voltage winding on a winding form, sealing said high voltage winding form to forma cavity completely enclosing said high voltage Winding, providing entry means into said cavity, assemblingsaid high voltage Winding and said low voltage winding together to form a coil, encapsulating said assembled coil with a synthetic resinous material, curing said synthetic resinous material, impregnated said high voltage winding through said entry means with a liquid dielectric material and sealing said entry means.

3. A method of making an encapsulated coil for use With an elecromagnetic induction apparatus comprising: winding a high Voltage Winding on a winding form having a window opening therein, Winding a low voltage winding on a Winding form having a window opening therein, sealing said high voltage winding form to provide a cavity completely enclosing said high voltage winding, providing entry means into said cavity, mounting said low voltage winding form Within the Window opening orf said high Voltage winding form, encapsulating said high voltage winding and said low voltage Winding with a synthetic resinous material, curing said synthetic resinous material, impregnating said high voltage winding through said entry means With a liquid dielectric material and sealing said entry means.

References Cited by the Examiner UNITED STATES PATENTS 1,360,752 11/ 1920 Johannesen 336-60 XR 2,300,910 11/19'42 Camilli 336-199 2,464,029 3/ 1949 Ehrman 264-277 XR 2,924,264 2/ 1960 Imhof 161-184 XR 2,930,011 3/1960 Wi-gert et al 336-96 2,964,730 12/1960 Blanchard 336-96 XR WHITMORE A. WILTZ, Primary Examiner. JOHN F. CAMPBELL, Examiner.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3274320 *Jul 29, 1963Sep 20, 1966Mc Graw Edison CoMethod of encapsulating transformer
US3503797 *Sep 20, 1967Mar 31, 1970Matsushita Electric Ind Co LtdInsulating method for electrical machinery and apparatus
US3611226 *Dec 8, 1969Oct 5, 1971Westinghouse Electric CorpEncapsulated electrical windings
US3792528 *May 3, 1972Feb 19, 1974Bbc Brown Boveri & CieProcess of for manufacturing electrical apparatus such as casing-enclosed transformers and reactors
US3905001 *Oct 24, 1973Sep 9, 1975Matsushita Electric Ind Co LtdOil-filled electrical instrument
US4112041 *Apr 28, 1977Sep 5, 1978Canadian General Electric CompanyMethod for reducing corona in dynamoelectric machines
US4275319 *Jun 1, 1979Jun 23, 1981Trw Inc.Oil-filled submergible electric pump motor with improved stator winding insulation
US4663604 *Jan 14, 1986May 5, 1987General Electric CompanyCoil assembly and support system for a transformer and a transformer employing same
US5258594 *Oct 12, 1990Nov 2, 1993Pioch ReneHigh frequency heating process for impregnating or polymerization of transformer windings
US6080964 *Apr 16, 1998Jun 27, 2000Micafil Vakuumtechnik AgProcess for predrying a coil block containing at least one winding and solid insulation
US6411188 *Mar 25, 1999Jun 25, 2002Honeywell International Inc.Amorphous metal transformer having a generally rectangular coil
US6583707Apr 25, 2001Jun 24, 2003Honeywell International Inc.Apparatus and method for the manufacture of large transformers having laminated cores, particularly cores of annealed amorphous metal alloys
US6668444Apr 25, 2001Dec 30, 2003Metglas, Inc.Method for manufacturing a wound, multi-cored amorphous metal transformer core
US6765467Apr 25, 2001Jul 20, 2004Dung A. NgoCore support assembly for large wound transformer cores
US8056256 *Sep 17, 2008Nov 15, 2011Slack Associates, Inc.Method for reconditioning FCR APG-68 tactical radar units
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US20100064541 *Sep 17, 2008Mar 18, 2010Slack Howard CMethod for reconditioning fcr apg-68 tactical radar units
EP0191694A1 *Feb 5, 1986Aug 20, 1986Schlumberger Canada LimitedHigh voltage transformer and method
EP0519939A1 *Feb 12, 1991Dec 30, 1992Du PontProcess for manufacturing a polymeric encapsulated transformer.
EP0519939A4 *Feb 12, 1991Jun 30, 1993E.I. Du Pont De Nemours And CompanyProcess for manufacturing a polymeric encapsulated transformer
WO1991007764A1 *Oct 12, 1990May 30, 1991Pioch, Sylvie +HfControlled drying-impregnation-polymerization processes
WO2009138096A1 *May 13, 2008Nov 19, 2009Abb Technology AgDry-type transformer
Classifications
U.S. Classification29/605, 336/94, 264/272.19, 336/96
International ClassificationH01F27/02, H01F41/00
Cooperative ClassificationH01F41/005, H01F27/022
European ClassificationH01F27/02A, H01F41/00A