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Publication numberUS2411374 A
Publication typeGrant
Publication date19 Nov 1946
Filing date7 Jan 1943
Priority date7 Jan 1943
Publication numberUS 2411374 A, US 2411374A, US-A-2411374, US2411374 A, US2411374A
InventorsHorstman Clifford C
Original AssigneeWestinghouse Electric Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Magnetic core structure for threephase transformers
US 2411374 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

1946. c. c. HQRSTMAN 2,411,374

MAGNETIC CORE STRUCTURE FOR THREE PHASE TRANSFORMERS Filed Jan. 7, 1943 WITNESSES: INVENTOR ATTO P aten ted Nov. 19, 19.46

MAGNETIC CORE STRUCTURE FOR PHASE TRANSFORMERS Clifford C. Horstman,

THREE- Sharpsville, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 7, 1943, Serial No. 471,538

- 8 Claims. 1

My invention relates to electrical induction apparatus such as transformers, and particularly to the core or magnetic circuit structure thereof and the method of making the same.

Recent developments in the manufacture of electric steel have produced steel having a preferred orientation of the grains in the direction of rolling. By properly cold rolling and annealing silicon steel, the crystals of the steel can be oriented in such manner that substantially all the crystals present a tube edge parallel to the direction of rolling. When the lines of magnetic flux pass through the steel in the direction of rolling, the permeability of such steel is considume or unit weight at operating densities-is lower than for commercial grades of hot rolled same density when magnetized in of rolling. To properly take advantage of the properties of this'improved steel, it is necessary that the steel be so used that the direction of magnetization of the steel coincide substantially with the grain of the steel, or directionoi rolling, so that the lines of'magnetic flux shall not pass through the steel at an appreciable angle from the direction of rolling.

There are many advantages in the manufacture of transformer structures in utilizingpreformed current conducting coils for the windings among which are the application of high frequency testing and the inspection of the individual unassembled coils. These coils are wound on separate temporary transformers or mandrels and are then assembled together with a magnetic core structure of the transformer or other induction apparatus with required insulation and spacers.

It has been the usual practice in building core structures for use in distribution or power transformers to employ stacks of layers of thin sheets or laminations of magnetic material shaped as L plates. These L plates are stamped or punched from sheets of silicon steel, and it is necessary that a compromise in the direction of cutting be made which gives poorer magnetic qualities than when the lines of magnetic flux pass in the optimum direction. When a core structure is built up from L-shaped or I-shaped plates, the individual sheets are placed layer upon layer to form the core; those in the Winding leg extending through the opening in the winding structure. The joints between the successive layers are member to form the well formed by overlapping the ends of the sheets forming one core member with the ends of the sheets in an adjacent layer forming another core known lapped joint. This type of core structure does not lend itself well to the use of the high permeability low watts loss steel which must be used in such manner that the flux passes through the sheets of steel in the direction of rolling.

Magnetic core structures having butt joints instead of the lapped joints have been characterized in general by high iron core losses and high excitation currents which prevent their success ful operation at high magnetic flux densities such as are otherwise possible when the improved magnetic steels having preferred orientation are used. Recently ways and means have been discovered to provide low low loss butt joints which will enable the building of core structures from stacks or bundles of laminations of predetermined shapes. 2,293,951 issued to J. B. Seastone and myself on August 25, 1942, for induction apparatus and method of core construction therefor, and assigned to the same assignee as this application is directed to a magnetic core and method of making it in which low loss butt joints are em- Played.

It is an object of the present invention to provide magnetic core structures for electrical in duction apparatus in which the leg and yoke portions of the core are separately formed and are so arranged that the lines of magnetic flux pass through all parts of the core in the direction of rolling of the steel sheets of which it is composed.

It is a further object of the invention to provide a magnetic core structure of the above indicated character in which the parts of the core are formed of groups of laminations bonded together as separate units and in which low loss butt joints are provided between the leg and yoke members of the core.

The invention accordingly is disclosed in the embodiments thereof shown in the accompanying drawing and comprises the features of con; struction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth and the scope of the application of which will be indicated in the'claims.

For a fuller understanding of the nature and objects of the invention; reference should be had to the following detailed description taken in connection with. the accompanying drawing, in which;

reluctance and yoke members 3 Figure 1 is a diagrammatic perspective view of a single phase core type transformer having core structure built in accordance with the teachin s of the invention,

Fig. 2 is a diagrammatic perspective view of a three-phase transformer having a core structure built in accordance with the teachings of the invention,

Fig. 3 is a perspective view of a wound coil or loop of magnetic material employed to form the yoke portions of the core in Fig. 1,

Fig. 4 is a perspective view of a wound coil or loop assembly of magnetic material used to form the yoke portions of the core in Fig. 2,'and

Fig. 5 is a cross sectional view taken along the line VV of Fig. 1.

Referring to the drawing, Fig. 1 shows a core structure having two winding legs are similar in construction, are placed parallel to each other, and are joined at opposite ends by and 4. The assembly of the two leg members and two yoke members are shown as held in place by a banding strap 5 and fastener 5 in a known manner, and the entire core assem-v bly may be mounted in end frames for supporting it in place inside of a transformertank in -a well known manner. Windings 8 are shown diagrammatically about the leg members i and 2 of the core and would in practice consist of inductively related primary and secondary windings in accordancewith well known transformer practice.

Each piece of thecore structure is assembled with magnetizable laminations, as it in Fig. 5, and H2 in Fig. 3, electrically insulated from each other and solidly held together with intervening bonding layers of an adherent insulating bonding material between the laminations. This bonding material is preferably a chemically inert material such, for example, as one of the well known resinous products which may be applied 'to the surfaces of the laminations ii and it.

The core legs 6 and 2 may be made cruciform in cross section as shown in Figs. .1 .and 5 with the laminations running in a plane from the front to the rear of the core structure as viewed in Fig. 1. Certain of the laminations adjacent two opposite sides of the leg have a width or, certain others a width b, and others the full width 0 of the leg which corresponds to the width of the laminations forming the yoke portions 3 and ii. The laminations or sheets of steel extend from the front to the back of the core in the direction of dimension lines a, b and c as distinguished from the direction the dimension line of line d which extends across the stack of laminations. The cruciform shape of the winding legs is desirable from a coil designstandpoint in order to permit the more efiicient use of round coils.

The sheets or laminations comprising the leg members are coated with a bonding material such as a toluol-alcohol solution of alvar, Vinylite and Bakelite. In practice it has been found satisfactory to utilize a resinous product such as acondensation product of the phenol aldehyde type. To meet difierent conditions, the phenol aldehyde type resin may be modified by contacts well known in the art. After the bonding materialhas been applied to the sheets of steel, it is permitted to dry and after drying the punchings are stacked in a jig and heated while pressure is being applied to the stack. The assembly is then cooled under pressure resulting in the punchings being solidly bonded together. When a chemically inert material is applied in this I and 2 which manner, it strongly adheres to bond the laminations between which it is disposed whereby the bundle of laminations is solidly held together in a. rigid piece or bonding material applied to them.

In order to provide accurately smooth faces on the ends of: the leg members land 2 which will cooperate with the corresponding faces on the adjoining portions of the yoke members 3 and l to give the desired closely fitting butt joints, the ends of the leg members are either machined or ground or otherwisemechanically worked to present an accurately smooth surface and to provide that the two legs shall be of exactly the same length. It has been found that during the machining or grinding operations to provide the smooth faces, burrs are formed. These burrs are tiny slivers of magnetizable material which may span the bonding layers between adjacent layers of the magnetizable material at the mechanically worked surface, thereby electrically connecting or short-circuiting the adjacent laminations. This increases both the iron losses and the magnetizing current of the electrical apparatus if not removed.

It is, therefore, desirable to remove these burrs in order to provide a commercially acceptable device. The burrs may be removed by applying an etching solution to the faces in any suitable manner. Many different kinds of etching solutions may be employed successfully. It has been found in practice that a 30% nitric acid solution is highly satisfactory. The etching solution may be applied in any suitable manner such as by dipping the face of the worked surface to be etched in the nitric acid solution for about 30 seconds. When the faces are withdrawn from the nitric acid solution, it is desirable to apply sodium carbonate to neutralize the acid. When the acid remaining on the stack has been neutralized, water is applied to remove the particles resulting from the neutralizing process. The above described lightly etched treatment of the working faces removes the burrs without substantially roughening the accurately smooth planed face resulting from the machining or grinding operation.

Structurally the yoke members 3 and ii consist of arch shaped groups of laminations bonded together by an adherent bonding material between them in the same manner as are the laminations of the leg members i and 2 and having similarly worked faces forming accurately smooth surfaces cooperating with the ends of the leg members i and 2 to form highly efficient butt joints.

The yoke members 3 and Li may be formed by winding a continuous strip of oriented grain steel 0 under tension about a mandrel of suitable di= mension forming a loop 7, as shown in Fig. 3. Various widths of strip may be used so as to build up a cruciform section if desired, corresponding to the cruciform section of the winding legs i and In the embodiment shown, the use of a single width'strip vantages that it is somewhat simpler to wind and provides widening or an increased cross section of the yoke members over that of the leg mem- The width of the strip 62 used would correspond to the width 0 of the widest laminations ii used in the core leg and the number of turns or laminations used in the yoke would be that required to provide a surface on the yoke having the same dimension d as the wide part of the unit of the core structure, the laminations of the unit being united by the l is illustrated and has the adby a loop 33 having core leg. After the loop I has been wound to the shape shown in Fig. 3, it is then out along the lines l3 on two opposite sides of the loop to form two U-shaped members 3 and 4. Prior to the cutting of the core members the several layers of the strip |2 have been bonded together in a manner generally similar to that described above with respect to the laminations forming the core legs I and 2, so that when the loop 1 is out along the line I3, two separate yoke-units are formed each having an arched or U-shape. The U- shaped members 3 and 4 are in efl'ect stacks or bundles of laminations solidly bonded together similar to the laminations in the leg members I and 2 and the cut surfaces will be worked and treated in exactly the same manner. After the faces of the U-shaped members have been worked and treated and the preformed windings shown diagrammatically at 8 have been placed about the leg members I and 2, the leg and core members will be assembledto form the unit shown in Fig. 1.

Referring particularly to Figs. 2 and 4, a threephase transformer core is shown having three winding legs 2|, 22 and 23 which are each similar in construction to the leg I and 2 in Fig. 1, and each of which accommodate or extend through the primary and secondary windings comprising the windings of one of the three phases. of the electrical circuit in a well known manner. The yoke members are constructed similar in principle to the yoke members 3 and 4 shown in Fig. 1, but differ therefrom in that three arch shaped structures are provided both above and below the winding legs. An arch shaped member 24 engages the upper ends of the leg members 2| and 22, an arch-shaped yoke member 25 engages the upper ends of the leg members 22 and 23 and an arch-shaped yoke member 28 engages the upper ends of the leg members.2l and 23. In a similar manner, the arch-shaped member 21 engages the lower ends of the leg members 2| and 22-, an archshaped member 28 engages the lower ends of the leg members 22 and 23 and an arch-shaped member 23 engages the lower ends of the leg members 2| and 23. The cross sectional area of the several arch-shaped members of the yoke should correspond, to provide three separate paths for the magnetic flux between the three winding legs of the core at each end thereof that are alike in cross sectional area, so that the flux passing between any two winding legsmay complete its circuits through corresponding upper and lower yoke arch members 24 and 21, 25 and 28 or 26 and 23.

In forming the yoke members used in the threephase transformer core shown in Fig. 2, three separate core loops are wound, the loops 3| and 32 being wound about mandrels in the same manner as the loop 1 shown in Fig. 3 to the proper dimension. The two core loops 3| and 32 which are formed of an equal number of turns of the strip of magnetic material are then surrounded an equal number of turns or steel. The three core loops, are then out at three places plane to provide upper and lower yoke members having the same dimensions. The individual turns of all three loops are bonded together in the same manner as in the loop 7 formed in Fig. 3, and the several surfaces formed by cutting along the lines 34 are worked in the same manner to provide cooperating accurately smooth surfaces engaging corresponding accurately smooth surfaces on the ends of the leg member 2|, 22 and 22, the entire assembly being laminations of sheet as shown in Fig. 4, 34 along the same held together by a band 35 and mounted within any suitable frame structure for positioning it in the transformer tank.

In the form of core construction disclosed, the high density flux passing through the winding leg and the yoke portion of the core does not have to pass across the grain of the steel but coincides with the direction of rolling or grain direction of the steel at all points, thus maintaining the lowest watts losses and highest permeability characteristics of the core structure possible. There are no areas adjacent any of the joints in which flux is required to flow across the grain of the steel for even a short distance. The construction lends itself to a ready widening of the cross section of the yoke members to provide a greater cross section in the yoke members than in the leg member-s if desired. The construction lends itself to the employment of winding legs having cruciform section, thus providing the most economical use of core material with round coils, together with a rectangular cross section for the yoke portions of the core, together with the use of high eiiiciency butt joints between the core All magnetic material used is active material, thu reducing the required amount of magnetic material to a minimum with a saving of core weight. The core legs are batch stacked, that is stacked in a bundle rather than placed in position within the preformed conducting coils one sheet at a time as in building up a core from L-shaped punchings, and the yoke portions are wound from a continuous strip, thus reducing the labor required in forming the core below that required for the usual type of built-up core stacks oflaminations. The coils may be readily removed from the core for repair since the disassembling of the core structure merely means separating the yoke and leg portions at the butt joints'rather than the disassembling of the core sheet by sheet, as in the built-up core type of structure.

Since modifications may be made in the structures illustrated and described within the spirit of my invention, I do not wish to be limited otherwise than by the scope of the appended claims.

I claim as my invention:-

1. In a magnetic core structure for electrical induction apparatus, in combination, a plurality of leg members positioned in parallel relation and having cruciform cross-section and yoke members connecting the corresponding ends'of the leg members, the leg members comprising core units formed of stacks of laminations of magnetic material, the laminations of each unit being united by bonding nations and having faces worked on the core leg for making butt joints, certain of the laminations having a different width than others to provide a cruciform shaped core leg, the yoke members comprising portions of a magnetic loop structure formed of magnetic sheet material wound flatwise, the sheet material having the same width throughout the yoke member to provide a greater cross section in the yoke members than in the leg members, the successive turns being bonded together by bonding material applied to the sheet material, and cut to provide two yoke member structures for joining the leg members of the core and having faces worked thereon to provide elements for closely adjoining the cooperating faces of the core leg members to form low loss butt joints.

2, In a magnetic core structure for electrical induction apparatus, in combination, a plurality of leg members positioned in parallel relation and material applied to the lami-' yoke members connecting the end portions of the leg members together to complete the magnetic circuit, the leg members comprising core units formed of stacks of laminations of magnetic material, the laminations of each unit being united by bonding material applied to the laminations, and faces worked on the opposite ends of the units to provide elements for making butt joints, certain of the laminations of the leg members having a different width than others to provide leg members having cruciform-shaped cross sections, the yoke members comprising arched structures formed of superimposed layers of sheet material bent flatwise to conform to the shape of the arch and bonded together by bonding matemembers positioned in parallel relation and yoke members connecting the end portions of the leg members together to complete the magnetic circuit, the leg members comprising core units formed of stacks of laminations of magnetic material,the laminationsof each unit being united by bonding material applied to the laminations, the three winding legs being arranged in a row and having faces worked on the opposite ends of the units to provide elements for making butt joints,

the yoke members comprising arched structures yoke member is held solidly together to constitute one" of the plurality of separate yoke pieces of the core, the arch shaped yoke members bein formed by winding a continuous strip of magnetic material in a plurality of turns, filling the spaces between the several turns with an adherent insulating bonding layer to provide a solid loop structure, cutting and mechanically working the wound and filled core loop to divide it-into two yoke members having accurately smooth butt joint surfaces on each piece for closely fitting the matching surfaces of the leg members to provide smooth low loss butt joints between the leg and yoke members of the core, and in assembling the yoke and leg members in final position.

5. A method of making a magnetic core for an induction device having a plurality of core leg members arranged in parallel relation, and yoke members connecting the corresponding ends of the leg members, said method comprising the steps of building separate leg member pieces each as a solidly held bundle of laminations of magnetizable material with interveningbonding-layers of an adherent insulating bond between the laminations, each bonding-layer adhering to both of the laminations between which it is disposed whereby the bundle of laminations is solidly held together to constitute one of the plurality of separate leg pieces of the core, mechanically working a plurality of accurately smooth butt-joint surfaces on each piece so that each of said buttjoint surfaces comprises a plurality of strata consisting of smooth-surfaced lamination-ends sep-.

formed of superimposed layers of sheet material bent fiatwise to conform to the shape of the arch and bonded together, by bonding material api plied to the sheet material, the arched structures having faces worked thereon for cooperating with the faces on the units comprising the leg members to form low loss butt joints, the yoke members at each end of the leg members comprising three such arch structures, two of the three arch structures being of lesser span than the third and positioned to span from the first to the second and from the second to the third of the three leg members, respectively, and the third one of the three arch structures being dimensioned and positioned to extend from the first to the third of the three leg members.

4. A method of makinga magnetic core for an induction device having a plurality of core leg members arranged in parallel relation, and yoke members connecting the corresponding ends of the leg memberssaid method comprising the steps of building separate leg member pieces each as .a solidly held bundle of laminations of magnetizable material with intervening bonding layersof an adherent insulating bond between the laminations, each bonding layer adhering to both of the laminations between which it is disposed whereby the bundle of laminations. is solidly held together to constitute one of the plurality of separate leg pieces of the core, mechanically working a plurality of accurately smooth butt joint surfaces on each piece, and in building separate arch shaped-yoke members each as a solidly held bundle of laminations of magnetizable material with intervening bonding layers of an adherent insulating bnd between the laminations, each bonding layer adhering to both of the laminations between which it is disposed whereby the bundle of laminations forming the arch shaped arated by insulating adherent bonding layers, said mechanical working operation being performed in such manner and under such conditions as to produce short-circuiting burrs of the magnetizable material spanning the bondinglayers at each butt-joint surface, and subsequently etching away substantially all of said shortcircuiting burrs without substantially roughening the smooth-surfaced lamination ends, and in building separate arch shaped yoke members each as a solidly held bundle of laminations of magnetizable material by winding a continuous strip of magnetic material in a plurality of turns, filling the spaces between the turns with bonding layers of an adherent insulating bonding material to provide a solid loop structure, cutting and mechanically working the wound and filled core a loop to divide it into two yoke members having accurately smooth butt-joint surfaces on each piece so that each of said butt-joint surfaces comprises a plurality of strata consisting of smoothsurfaced lamination-ends separated by insulating adherent bonding layers, said mechanical working operation being performed in such manner and under such conditions as-to produce shortcircuiting burrs of the magnetizable material spanning the bonding-layers at each butt-joint surface, and subsequently etching away substantially all of said short-circuiting burrs without substantially roughening the smooth-surfaced lamination ends, and 'in building separate arch shaped yoke members each as a solidly held bundle of laminations of magnetizable material by winding a continuous strip of magnetic material in a plurality of turns, filling the spaces between the turns with bonding layers of an adherent insulating bonding material, and in assembling the leg and yoke members in final position. r

6. In a magnetic core structure for electrical induction apparatus, in combination, a plurality of leg members positioned in parallel relation and yoke members connecting the corresponding ends of the leg members, the leg members comprising core units formed of stacks of lamlnations of magnetic material, the laminations of each unit being united by bonding material applied to the laminations and having faces worked on the core leg to provide elements for making butt- ,ioints, certain of the laminations of the leg members having a diilerent width than others to pro= vide leg members having cruciiorm shaped cross sections, the yoke members comprising portions of a magnetic loop structure formed or" magnetic sheet material wound flatwise, the successive turns being bonded together by bonding material applied to the sheet material, and cut to provide two yoke member structures for joining the leg members of thecor and having faces worked thereon to provide elements for closely adjoining the cooperating faces or the core leg members to form low loss butt Joints, the sheet material forming the yoke members having the same width throughout to provide a greater cross section in the yoke'members than in the leg members, the leg members of the core being placed in the assembled structure in such position 1 that the edges of the laminations comprising the leg members and the edges of the lamlnations comprising the yoke members extend in the same direction in the adjacent faces cooperating to form the butt Joints between the leg and yoke members of the core.

7. Ina magnetic core structure for electrical induction apparatus, in combination, a plurality of leg members positioned in parallel relation and yoke members connecting the end portions of the leg members together to complete the magnetic circuit, the leg members comprising core units formed of stacks of laminations of magnetic material, the iaininations of each unit being united by bonding material applied to the laminations, and faces worked on the opposite ends of the units to provide elements for making butt joints, certain of the iaminations'or the leg members having a diflerent width than others to provide leg members having cruciform-shaped cross sec tions, the yoke members comprising arched structures formed of superimposed layers of sheet material bent fiatwise to conform to the shape of the arch and bonded together by bonding material applied to the sheet material, the arched structures having faces worked thereon for cooperating with the faces on the units comprising the 1Q leg members to form low loss butt joints, the sheet material forming the yoke members having the same width throughout to provide a greater cross section in the yoke members than in the leg members, the leg members of the core being placed in the assembled structure in such position that the edges of the laminations comprising the leg members and the edges of the laminations comprising the yoke members both extend from the front to the back or", the assembled core structure in the adjacent faces cooperating to form the butt joints between the leg and yoke members of the core.

8. In a three-phase core structure for electrical induction apparatus, in combination, three leg members positioned in parallel relation and yoke members connecting the end portions of the leg members together to complete the magnetic circuit, the leg members comprising core units formed of stacks of laminations of magnetic material, the laminations of each unit being united by bonding material applied to the laminations, the three winding legs being arranged in a row and having faces worked on the opposite ends of the units to provide elements for making butt joints, the yoke members comprising arched structures formed of superimposed layers of sheet material bent flatwise to conform to the shape of the arch and bonded together by bonding material applied to the sheet material, the arched structures having faces worked thereon for cooperating with the faces on the units comprising the leg members to form low loss butt joints, the yoke members at each end of the leg members comprising three such arch structures, two of the three arch structures being of lesser span than the third anclpositicned to span from the first to the second and from the second to the third of the three leg members, and the third one of the three arch structures being dimensioned and positioned-to extend from the first to the third of the three leg members, the leg members of the core being placed in the assembled structure in such position that the edges of the laminations comprising the leg members and the edges of the laminations comprising the yoke members both extend from the front to the back of, the assembled core structure in the adjacent faces cooperating to form the butt joints between the leg and yoke members of the core.

CLIFFORD C. HORSTMAN.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2498702 *15 Sep 194428 Feb 1950Gustave NahmanCylindrical magnetic core
US2548624 *5 Feb 194610 Apr 1951Gen ElectricElectric induction apparatus
US2550500 *24 Sep 194824 Apr 1951Gen ElectricLow yoke transformer core
US2586320 *13 Apr 194819 Feb 1952Westinghouse Electric CorpMethod of making core and coil assemblies
US2903537 *30 Jul 19568 Sep 1959Allen Bradley CoTwin coil electromagnetic actuator
US2909742 *1 Sep 195320 Oct 1959Gen ElectricMachine wound magnetic core
US2937352 *23 Apr 195317 May 1960Gen ElectricMagnetic core structure
US2963776 *18 Apr 195613 Dec 1960Mc Graw Edison CoMethod of construction for magnetic core
US5625370 *25 Jul 199429 Apr 1997Texas Instruments IncorporatedIdentification system antenna with impedance transformer
US938961929 Jul 201312 Jul 2016The Boeing CompanyTransformer core flux control for power management
US945508428 Mar 201427 Sep 2016The Boeing CompanyVariable core electromagnetic device
US94729468 Aug 201318 Oct 2016The Boeing CompanyElectrical power distribution network monitoring and control
US956856321 Feb 201314 Feb 2017The Boeing CompanyMagnetic core flux sensor
US963377625 Aug 201625 Apr 2017The Boeing CompanyVariable core electromagnetic device
US9651633 *28 Mar 201416 May 2017The Boeing CompanyMagnetic core flux sensor
US20140232384 *28 Mar 201421 Aug 2014The Boeing CompanyMagnetic core flux sensor
EP2769389A4 *18 Oct 201217 Jun 2015Keith D EarhartWound transformer core and method of manufacture
WO2013058808A3 *18 Oct 20126 Mar 2014Earhart Keith DWound transformer core and method of manufacture
Classifications
U.S. Classification336/213, 336/219, 29/609, 336/215, 336/218, 336/216
International ClassificationH01F27/245
Cooperative ClassificationH01F27/245
European ClassificationH01F27/245