US2655717A

US2655717A - Method of forming wound magnetic cores

Info

Publication number: US2655717A
Application number: US736688A
Authority: US
Inventors: Ulysses S Dunn
Original assignee: Individual
Current assignee: Individual
Priority date: 1947-03-24
Filing date: 1947-03-24
Publication date: 1953-10-20
Anticipated expiration: 1970-10-20

Description

Oct. 20, U s DUNN METHOD OF FORMING WOUND MAGNETIC CORE-S 4 Sheet s-Sheet 1 Filed March 24, 1947 INVENTOR. 00055.: J. DUNN ATTORNEYS U. S. DUNN METHOD OF FORMING WOUND MAGNETIC CORES Oct. 20, 1953 4 Sheets-Sheet 2 Filed March 24, 1947 j INVENTOR.

U1. r5555 5. 0u/v/v W Wm A TTORNE Y5 Oct. 20, 5 DUNN METHOD OF FORMING WOUND MAGNETIC COREIS Filed March 24, 1947 4 e s- 3 INVENTOR.

- U1. V6566 5. DUN/v Oct. 20, U. S. DUNN METHOD OF FORMING WOUND MAGNETIC CORES 4 Sheets-Sheet 4 Filed March 24, 1947 a 1 a U C; 1/ I INVENTOR. ULYSSEJ 5. Dumv M 'fl-Wr M ArroR/vns A I/I a UNITED STATES PATENT OFFICE 2-,'s'55,71-i here-ob, oe .rloamiso wouso MAGNETIC corms Ulysses s. mmh; Clii'cai'o', in. Application March 24, 1946, Serial l lo. z'o'ieimez (01. 29-15558) This invention relates to the winding of mag: netic' strip material into cores for transformers and other electrical apparatus and is particularly' concerned with novel methods of winding the core and assembly or the wound core with e wound coil or coils in the manufacture of eieo-' trio'al equipment. v

The invention 1h as srererreo embodiment comprises the winding of strip steel or use metal of high magnetic permeability under tension into a suitable core having the form of a closed figure, cutting' the core open at ne place for as;-- ser'nbling it with re -wound coil which also is in the form or a noted fl'urear'id 1s uncut, and restoring the core to its original wound shape interlinked with the eon.

It has heretofore been piopo sed to provide as-' sem'biie's' of wound cores or strip ih'e'tei and associated coils in transformers and like electrical instruments in merry dlfi're'ii't simmers. A widely used commercial method or assembly has been to Wind the core W'itli 8'. bonding material between the winding layers to prevent the core from falling apart when out in two places, and cut the wound core into two separable pieces which are then reassembled to enclose the cut wound coil, the core thus having two b'utt Joints in the finished roduct. I

A diniculty with this method of assembly is that it is necessary to provide special bonding material that is not adverselyafiec'ted by the high annealing teniperatur'e's'jt'o' which the core is later subjected. In rhy inventi n I are able to dispense with the use of such bonding" r'n'at'eifiall Another objection to this" method of assembly has been that the provision of two butt joints in the core provi'des' double opportunity for undesired air gaps in the magnetic circ'uit, thereby reducing the efliclehy of the magnetic circuit.

A further difficulty is that quote inherent limitations in eccureoy'm cutting the oore into two parts it is always necessary to keep the corresponding severed coreparts matched together" for subsequent reassembly because parts of two different cores are usually not interchangeable.

Besides, there are difficulties of assembly in aligning the adjacent ends of matched core parts, the core bond may be fractured during handling or assembly in which case thecore' parts will not snugly fit together again so as toallow the butt joints to close adequately, and there is always danger of fracture of the core at the butt Joints during incorporation of the coreandcoil assembly into an instrument or duringuse of the instrument.

sieseht i vehafoh relates generally to this type of assembly but, instead of having to out the core so as to produce two butt joints, I am enabled to assemble eel'o's'eu wound core with a cllis''dwdund 6011 while making only one butt joint in the sore, this joint being so located on the core as to enable the wound core to be sprung' apart for insertion of the coil on one leg th ereofwithout damagin the coil and withooth'eh eheht y defiir'fiiiii fhe core, and be refor'r'ned with so efficiently closed butt joint.

With the abovein mind, it is a major object of invention tb'provide a novel method of winding magnetic strip material into a core in such a manner that only a single out need be made 11; the o'rfe for interimrihig assembly with a c osed wound coil; and t e novel core so wound.

It is e furltheif obieot ofniyinVention to provide a novel method ofwin'dir'ig' magnetic strip material un er ension iiit'da closed core where mthe winding t'eheioh' is' iiiedueuy reduced dur ing thewinding operation, and the novel core thus formed which may out through to provide a single bu'tt J'Q hflfha't closes fliciently' through the e hr'e petrr or the out.

It is a further object or invention to provide e novel method of winding magnetic strip lrrateria l into' a closed one for transformers or like electrical eq'ulpmen't' wherein the core is wound on a rotating 'form under gradually and 'co nstantl cli'anjgiin'g strip tension which lessens as the" s 2e of the wound core increases. Pursuantto this object provision may be made for automatically decreasing the winding tension, as by decreasing it in sillta'bl' increments as each turn is wound on the core.

A, furtherobject of my invention is to provide a novel wound ooreoi' magnetic strip material adapted for association with a closed pre-wound coil whe'r el'n hee'ojre ha only a single butt Joint located at an' o'othhuiq position for assembly with the coil without injuriously deforming th core that is deforming or bending the core pqd. he iel point A further object 01 myv invention is to provide s ovel assembly of a generally oblong wound strip steel core with a closed form wound coil wherein the core has onlyia single butt joint located at a" corner thereof in predetermined relation to the legon whieh the coil is mounted It is a further object of my invention to provide a novel method of cutting a closed wound core. in such a manner 8 to prevent separation of the turns d rin cutt so as to prevent formation of burrs between turnsand to provide a" comparatively smooth cut to give a joint of high magnetic efficiency. and apparatus holding it during cutting.

A further object of my invention is to provide a novel method of winding a core of strip magnetic material which is to be cut at one portion to provide a single butt joint when assembled with a form wound coil, wherein the core is initially wound with an oversized dimension at said portion so as to compensate for the change in dimension in the closed core after material cut out to make the butt joint is removed from the core.

A further object of my invention is to provide a novel method and apparatus for preheating strip metal as it is wound upon a form to make a closed core to eliminate subsequent annealing of the wound core.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawlngs wherein:

Figure l is a front elevation of a simple transformer core and coil assembly having a core wound according to a preferred embodiment of the invention;

Figure 2 is a side elevation of the assembly of Figure 1;

Figure 3 is a diagrammatic representation of the preferred method of winding the core of Figures l and 2;

Figure 4 is an elevation partly in section illustrating an annealing furnace for a plurality of wound cores prior to cutting;

Figure 5 is a plan view illustrating a clamping fixture for holding and cutting the wound core;

Figure 6 is a side elevation of the wound core of Figure 1 after cutting at the corner;

Figure '7 is a diagrammatic elevation illustrating separation of the outer turns of a core wound under constant tension;

Figure 8 is a side elevation illustrating how the core is sprung to receive the coil during assembly;

Figure 9 is a side elevation illustrating the use of a wedge-shaped winding form which is enough wider at the end where the cut is made to compensate for the width of the cut whereby the finished core after cutting is the same width at both ends;

Figure 10 illustrates in elevation a choke coil reactor containing a core made according to the invention;

Figure 11 is a side elevation, partly in section, of a "shell" type transformer having a core embodying two core parts each wound according to the invention; and

Figure 12 is a side elevation in section diagrammatically illustrating a-further embodiment of the invention wherein the strip is wound hot, a process by which the subsequent annealing operation may be eliminated.

Figures 1 and 2 illustrate a typical core and coil assembly made according to the invention. The assembly comprises a closed oblong core I I wound of magnetic strip material such as high silicon electrical steel, a closed form wound electrical coil l2 which is mounted on the long leg of core I i surrounding and enclosing it, and a core clamp comprising a thin flexible metal band [3 having its opposite ends tightly drawn together by suitable fastening means such as indicated at H for drawing the core ends together to tightly close the butt joint [5 at the upper left corner of the core in Figure 1. Above and below the coil [2 are preferably provided protective strips of insu- 4 lation or like material l6 and I! which also serve as spacers and to hold the coil against undesired movement along the side leg of the core. Insulation strip l6 also serves to hold the joint (5 in alignment as band 13 is tightened during assembly.

Core II is wound from continuous strip electrical steel that is of high silicon content, preferably in the present commercial range of about 3 to 6 percent silicon, which is of a requisite quality and possesses special magnetic characteristics commonly known to such core steel in the art. Strip steel of this kind having a non-conducting surface coating to reduce interlamination eddy currents and processed to have a desired grain orientation so that it has high magnetic permeability in one direction lengthwise of the strip may be obtained as stock material from several of the large steel companies, the characteristics varying according to the treatment given the sheet by the different manufacturers. It is believed sufiicient for purpose of this invention to state that preferably core H is made of winding continuous strip I of electrical steel having desired magnetic properties.

Referring now to Figure 3, the core I l is wound on a form block 18 which is removably secured to a rotating head [9 that revolves about a center 2| being driven by a suitable lathe motor or the like (not shown). Suitable dowels 20 prevent relative rotation of the block and head l9 and help support the block on the head. The peripheral shape of block l8 of course determines the shape of the wound core. In the embodiment selected for illustration, as for winding the core of Figure 1, block [8 is rectangular, having slightly rounded corners to facilitate the winding of the steel which winds smoother and tighter around rounded corners. In practice the corner where the core is to be cut may be fairly sharp and the other corners may have a much larger radius especially the corners opposite the coil, and the block is provided with suitable slot or other means on its periphery for anchoring the 7 leading end of the strip material 22 to be wound thereon.

Material 22 is uncoiled from a large roll of strip material 23 suitably mounted on a conventional expansible reel 24, and strip 22 passes in turn through an adjustable tension device 25 provided with a manual control 28 later to be explained, and a pair of hardened

pressure rollers

21 and 28, the upper of which is mounted in a slidable support whereby it may be moved toward or from the strip 22. Preferably friction device 25 comprises a stationary block and a block 30' movable toward and away from strip 22 by manipulation of control 26, adjacent faces of the

blocks

30 and 30 being provided with friction coverings engaging opposite sides of the strip. The purpose of

rollers

21 and 28 is to roll out any burr or imperfection on the edges of the strip formed by slitting of the relatively narrow strip 22 from wider rolled sheet material.

Strip 22 then passes through a set of straightening rolls 29 which also serve as guide rolls for preparing the strip for entry to the winding apparatus in alignment with the rotating block [8.

Instead of using a form block [8 as in Figure 3, a two jaw chuck may be used and the coil wound around the jaws. The exterior of the jaws are contoured so that when expanded to the required size they will make a form providing a window in the coil and havingthe required characteristlcs described in connection with the block [8. around the jaws to form the core until the desired number of turns are obtained. The same principle of holding the free end of the coil, after the strip is severed is used. The chuck jaws are then retracted and the wound co're removed. Due to the tension in the windings the core will bulge outwardly so that the window is shortened. Before annealing, which will be described, a block or pair of tapered blocks are inserted into the window to return the core to the required size and also to provide a means for pressing the core for its annealing operation. By this method a block i8 is not needed for each core as it is wound, and only enough of the described tapered blocks are required as are needed for the number of cores being annealed at one time.

A suitable adjustable friction brake (not shown) is provided on the reel 24 to prevent uncoiling of the strip from the reel except when drawn therefrom by the pull of winding block I8. This may be of the usual type of unreeling apparatus. Strip tension control device 28 frictionally grips strip 22 in opposition to the pull exerted by rotating winding block l8, thereby controlling the winding tension. This winding tension may be varied at will by the control 26, and reliable and predictable control of this winding tension during the winding operation is a very important feature of the invention, as will presently appear. If desired additional strip tension control could be obtained by adjusting the brake at reel 24. In practice the tension at reel '24 is comparatively small as compared to the maximum tension at tension device 25 in order to permit a high ratio of total tension variation.

In practicing the invention, I prefer to decrease the winding tension of strip 22 gradually as the core turns increase in number on block i8. This is preferably done in the following manner. When winding of the core is started with the leading and of strip 22 anchored on block (0, I adjust the frictional gripping force at device 26 by manipulation of handle 26 to a practical limit which is usually determined by the tendency of strip 22 to pull loose from the point of attachment to block I 8. In other words, I initially provide substantially maximum tension on the strip permissible with winding of the strip. Then rotation of block 18 is started and as the core turns are wound onto block i 8, the tension on the strip exerted by device 25 is gradually reduced. This may be done either manually, as by manipulation of handle 26, or, if desired, a suitable automatic means may be provided operating in timed relation with the rotation of block ill for releasing tension of the strip as the core turns accumulate. Where an automatic tension releasing device is provided at 25, it can be timed to release a predetermined increment of tension for each wound turn of the core. The initial and final tension will vary and are related to the core size as determined by trial during the winding, annealing, and cutting operations of the first cores. In general however, -I have found that this gradual release in tension of the strip during winding is of such degree that tension exerted on the strip when the core has been finally wound on block i8 is roughly about one-fourth as great as the initial tension when the winding was started. The advantage of winding the strip in this manner with constantly decreasing tension will be de- The chuck is rotated and the strip wound scribed later on when its assembly with the coil is described.

After a suitable length of strip material 22 has been wound on block I 8, strip 22 is severed and the block is pulled axially oil the lathe head l9, the free end of strip 22 of the wound core H being bound tightly against the core body by a twisted wire loop H (see Figure 4) that extends throlugh a suitable notch 32 in one end of the bloc The wound core is then ready for annealing. In annealing the core, it is placed in a suitable clamping frame which tends to hold it in the shape which it is to assume in the finished product, and the wound core ll may be annealed either separately, or in a group as in Figure 4. In Figure 4, a plurality of the wound cores, still on their blocks 18 as removed from the lathe head of Figure 3, and with their outer ends tied by wire loops 3!, are held together between two

compression jacks

33 and 34 within a suitable furnace 35 wherein annealingheat is applied to the wound cores in a suitable manner. As illustrated in Figure 4,

Jacks

33 and 34 are mounted for adjustment towards and away from each other whereby the series of wound cores being annealed may be rigidly clamped, the adjacent side of each forming a support for the other, whereby the core shape is maintained during the annealing process and given a permanent set by reason of the annealing process.

The purpose of the annealing under pressure is chiefly to remove strains set up in the strip material by the slitting, winding and other mechanical operations which have been performed on the material so that it is given a permanent set to the required shape and also improve the magnetic properties of the finished core. Preferably the annealing furnace 35 has a temperature range of about 1500 to 1700 degrees F., and for most steel wound into core form, I prefer to anneal the cores by heating them within the furnace to about 1700 F. for one hour, and then cooling them slowly to below 500 F. while still maintaining them clamped in the

jacks

33 and 34 before removing them from the furnace. The exact annealing temperature will vary somewhat with the different grades of steel as may be recommended by experience or the steel manufacturers. The annealing and cooling time varies with the size of the core and would be greater for a larger core, and are usually determined best by trial.

Annealing procedures have been largely standardized for various grades of electrical steel and it is necessary only to briefly discuss conditions of this practical step. Where the core is wound of the standard grades of hot rolled silicon steel. the furnace atmosphere is not critical. Neutral to slightly oxidizing atmospheres give very satisfactory results. A carbonizing atmosphere is to be avoided as an increase in carbon content increases the magnetic losses. Certain grades of high silicon steels, especially those cold rolled to produce high magnetic permeability parallel to the length 0'1 the strip have inherent characteristics which may be adversely affected by excessive annealing temperatures or an incompatible furnace atmosphere. For example, a highly oxidizing atmosphere, especially when combined with temperature above 1700' F. increases the magnetic losses and produces a heavy oxide on the edges of the core layers so as to cause the'core to be bonded between layers by the oxide which makes the core so stiif that it is 'dimcult to spring apart for assembly of the coil in the process later to be described and equally difficult to close again with an eilicient butt Joint. If the atmosphere is excessively reducing, such may cause the edges of the wound sheet steel to adhere metal to metal between layers so as to cause the same difficulties in opening and closing the core, with the further difficulty that magnetic losses due to eddy currents would be greatly increased. Hence it is most practical for all grades of steel to main taln a completely neutral atmosphere around the cores while annealing. A mixture of 90 percent nitrogen and 10 percent hydrogen flowing through the heating chamber gives a practically neutral atmosphere even with incidental leakage of free air. A vacuum in the annealing chamber gives a strictly neutral condition. Hence it is the optimum condition of annealing at 1500 F. to l700 F. in a neutral atmosphere that is recommended for practice of the invention.

The annealed core removed from the furnace now has its permanent shape and the wire loops 3| may be removed from the ends thereof without the strip 22 uncoiling. The problem now is to assemble the closed wound core with a prewound closed annular coil of wire which, in the preferred embodiment of the invention, is intended to surround long leg 36 of core II. In order to do this, I cut the wound core entirely through at the corner adjacent one end of leg 38 for the purpose of springing leg 36 out in order to slip the annular coil 12 thereover during assembly.

Figure 5 illustrates a Jig for holding the wound core while cutting through the core corner for this purpose. This jig comprises a base 31 which is a steel plate of required size. Core H is mounted on base 31 for sawing, being rigidly held in place on block 31 by bolts 38 that pass through the dowel pin holes of winding block l8 and are threaded at their lower ends in plate 31. While I prefer to employ the original block l8 for holding the core rigidly in sawing position, any equivalent mounting arrangement may be provided on plate 31, as for example a block integral with or permanently secured to base 3'! and shaped like block [8 to be received by the wound core. At one corner, base 31 is diagonally slotted at 31 to allow a saw to enter and be correctly located and guided with respect to the cut to be made in the corner of core ii. At the same corner of the base are a pair of side by side

Jaws comprising blocks

39 and 4| slidable parallel to each other and slot 31' for engaging the outer periphery of core H at its diagonal corner and on opposite sides of and very close to the desired saw cut location. The dimension of

blocks

39 and 4| normal to the plane of the paper is preferably at least equal to the width of strip 22.

Blocks

38 and 4! are arcuate at their inner ends to fit snugly with the wound core periphery. Adjustment screws 42 and 43 are threadedly mounted in stationary supports on base 31 and rotatably connected to

blocks

39 and 4|, whereby when the screws are turned in the same direction, blocks 38 and 4! will be moved inwardly against the periphery of wound core I I as illustrated in Figure 5. The purpose of blocks 38 and 41 is to clamp the core windings very tightly on opposie sides of the line where it is to be cut and as near as possible thereto. This is important in order to reduce the formation of burrs and irregular edges at the cut portion.

Besides being the optimum place for cutting the 8 require a minimum distortion of leg 36 to receive the coil, the corner has been found to be the best place to cut an oblong core because it is there that the material can be clamped under maximum core for threading the coil onto leg 38 so as to tension for cutting, and this the place that the Joint closes with the least movement of the two intersecting sides as determined by the geometric proportions of the core. An analytical consideration of the process of winding with decreasing tension followed by clamping and annealing under pressure shows that the corner has an inherent advantage as the place to make a cut that will close again to give a satisfactory closed butt Joint. The process of winding with decreasing tension provides a minute amount of space or slack between turns which increases toward the outside layer. When the core is clamped and annealed under pressure as shown in Figure 4 this graduated space or slack between adjacent turns is forced entirely into the end sections. This is demonstrated by the fact that the dimensions 13 of Figure 6 are always greater than dimension A in the annealed wound core. The circumferential clamping action of band i3 displaces this excess material in the end portions, especially in the end that is cut, so as to tightly close the joint at IS. The method of assembling the coil onto the core, the method of clamping and sawing, the natural geometric proportion of the core, and the spacing of the turns at the end due to gradual winding tension followed by clamping and annealing all indicate that these steps result in a wound core of the most practical form and high efficiency when the cut is located at the corner as shown. Hence, in the preferred embodiment of the invention, I make a saw cut exaggeratedly indicated at 44 in Figure 5 through the entire wound core at the selected corner. In oblong core II, the cut 44 is preferably at 45 to the straight sides of the wound core. As this saw cut 44 is indicated enlarged for purposes of clarity in Figure 5, it will be noted that it is disposed between the pressure blocks 39 and 4!, whereby the cut core winding ends on opposite sides of the saw cut will be held under such considerable pressure that there will be little or no relative movement between the core winding layers during the cutting, and it is possible to make a sharp smooth out which will provide a butt joint having an optimum chance of closing tightly when the core is clamped together in the complete assembly.

It is immaterial what type of instrument is used to make the cut for the core at 44. Chiefly I have found it advisable to use a metal band saw which gives a joint that is good enough for most practical purposes, although it may be an abradant saw, friction saw, or any other saw or like cutter. The chief thing to observe is that the out should remove as little material as possible because it will be appreciated that such will change the dimension of the core and thereby reduce the chances of getting an efilcient butt Joint when the core is clamped together in the assembly. A manner of compensating for excess material removed by such a cut will be described in connection with Figure 9.

Figure 6 illustrates the core after it has been cut and removed from the fixture of Figure 5. As a result of the core being wound under gradually reducing tension, it will be noted that the butt Joint I8 is capable of closing substantially completely and is magnetically very efllcient. Had the core been wound on form I8 in the proc- 688 0! ure 3 with constant tension on the strip material at all times as has heretofore 9 been accepted practice, the inner lamlnations of such a core H would be much more loosely wound than the outer laminations due to accumulating tension in the strip as the core turns accumulated on block i8 whereby, after the corner cut was made in the fixture of Figure and the core removed therefrom, the parts would tend to assume the position illustrated in Figure 7 wherein the more highly tensioned outer laminations of the core would tend to relax and the fact that the outer turns have an ever increasing corner radius increases the tendency for the outer turns to relax or straighten at the corners and therefore decrease in length more than the inner turns which are held in plaoe'by the more nearly square corners as well as the combined pressures of the outer turns whereby the resulting non-uniform wedge shaped gap at 44 could never be completely closed as a butt joint. Hence the gap 44' would remain partly open, thereby providing an air gap in the magnetic circuit and a consequent loss of efllciency in the transformer due to a greatly increased exciting current.

By providing constantly decreasing tension during the winding operation as above explained, I insure that after the cut in the core has been made and the laminations allowed to assume their final positions, there is no such relative shrinking oi the different laminations as in Figure 7 but instead there is excess material in the end portions represented by minute space between turns which space increases towards the outer layers in such a manner that this excess material is displaced by its inherent pressure and by the pressure or the clamping device I3 so that the ends of the respective layers are readily closed to form a smooth butt Joint as in Figure 6. This is an important feature of the invention as has been confirmed by alternating current magnetization tests on various sizes and grades of cores throughout the useful range of magnetic densities. These tests show that cores made in the manner described and cut with an ordinary metal band saw have a total core loss in watts per pound not greater than and often less than the steel manuiacwrers' currently published guarantees based on the ideal conditions of the Epstein test. The exciting current in volt-amperes per pound is less than the manufacturers currently published data based on Epstein tests especially above a density of 10,000 gausses. This is a measure of the practicability oi this invention which is technically and commercially competitive with wound cores made with two or more butt joints or with uncut cores which are annealed and then uncolled and rewound to thread them through a form wound coil turn by turn.

The wound, annealed and cut core i l of Figure 6 is now assembled with the transformer or other coil i2. This assembly is accomplished according to the method illustrated best in Fig- 11-. 8 wherein the leg 36 is simply sprung out- Wind to provide a gap 45 at the corner large enough to enable coil I! to be siidubly inserted onto the end oi leg 88 and pushed: therealong until. when leg 38 is released the parts spring back to approximately the positional Figure 1. Then to hold the core: closed a suitable steel strap or like clamp I3 is mounted about the com. and fastening, means. i4 manipulated to draw it as tight as possible to secure maximum closure.- and efficiency: of the butt Joint it. The insulation strips, [8 and H may be inserted at any time during this operation.

- vibrations.

10 While the above described saw cut is described 'only as being made at the optimum position of 45 degrees to the straight sides of the core, it will be appreciated that as a practical matter this angle may be varied somewhat within certain practical limits for obtaining equivalent results without departing from the spirit of the invention. For example, referring to Figure 6, I may advantageously cut the core anywhere within the 45 degree angle between the optimum 45 degree illustrated Joint l5 and the dotted line 45 parallel to leg 36 and intersecting the inner corner. -yond line 48 across the short leg of the core, as at the dotted line 41. because such would make the end of leg 88 larger than the cross section of the leg and too large to accommodate the It is preierable not to cut the core becoil. It is also preferable not to cut the core across the long leg 38 as at dotted line 48 because suoh would require leg 36 to be bent too far outwardly to clear the short leg and receive the coil. Ii the cut exceeds an angle of 45 to the inner surface 01' leg 88 it is necessary (or the end portion of the core to bend downward in order to close the butt Joint, whereas it the cut is made preferably at line I l but not beyond line 48 the entire deflection necessary to close the Joint comes in the longer legs of the core.

I have investigated the efiect oi cutting core H at the various lines ll, 48, 41 and 48 as well as across line 49 similar to line 48 but on the opposite side of the corner parallel to the short 1 leg of the core.

that some oi them, particularly the inner turns, were loose and subject to vibration. Where the major clamping force exerted by strap is was normal to the butt joint line in such unequalized pressure Joints, the joints were tighter than in other methods of clamping but there was much tendency toward vibration.

Hence as a practical matter and a result of observation I prefer to make the saw out exactly diagonally of the corner or the coil and ii that is not possible somewhere between the diagonal and a lin 48 parallel to the long leg of the core and substantially coincident with the inner periphery of the lon leg It will be understood that these limits are by no means restrictive or the invention but they are described herein to inform as to the best practical aspects of the invention which applicant has found from experienc and tests.

As motioned above, the amount oi material which is removed at the saw cut 44 is important as a practical matter in that it may change the dimensions of the core, and this change of dimension becomes evident where a wide blade saw is used that removes as much as up to a onemtoenth to one-eighth of an inch wide section across the core. In such cases I have iound it advisable to employ a suitably shaped winding block N, which is trapezoidal in Figure 9, being wider at the short end adlacent which the out 44 is to'bemade. As winding of the strip 21 prograsseson such a block the core is deformed so asiidbe-wider at cneend than the other, but this deformity is corrected after the saw cut has been made and the butt joint formed, because now the leg 36 and the adjacent short leg of the core approach each other to such extent that the opposite legs of the core become substantially parallel in the final assembly. The difference in length between the short sides of the block 51 is therefore a function of the width of cut 44.

Figure illustrates an electrical instrument made according to the invention wherein a coil 52 is mounted on the long leg of a wound core 53 which is cut at its corner at 54 according to the above method, but instead of being forced together by a non-magnetic strap clamp l3, a suitable non-magnetic spacer 55 is placed in the saw cut to interrupt the magnetic circuit by an air gap. This is a choke coil reactor of common type.

Figure 11 illustrates a further transformer core of the shell type wherein the common coil 56 is mounted to surround the adjacent contiguous legs 51 and 58 of separate wound cores l I. In this transformer the adiacent inner corners have diagonal butt joints at 59 and GI. During assembly the legs 51 and 58 are held tightly together while the remaining parts of the two core ele ments are spread apart to permit the insertion of coil 56 over the two legs. Then the whole assembly is clamped together by the steel strap 62 and fastening means 63. Suitable upper and lower metal blocks 64 contoured to strap 62 and the arcuate corners of the core elements are inserted between strap 62 and the core. The block 54 at the top of the transformer core bridges the open ends of both the butt Joints 59 and 6| and in cooperation with the clamp strap 62 helps to maintain the butt joints more compactly and tightly in the assembly.

The method of winding the core illustrated in Figure 12 is essentially the same as in Figure 3 except that the winding head I9 is disposed in a suitable furnace 65 having an entrance throat 66 wide enough to accommodate strip 22 during winding. The purpose of this furnace is to heat strip 22 prior to and while winding on block l8 so that the core turns may be given a permanent shape concommitantly with the winding operation, whereby the separate annealing operation may be eliminated.

A long gas burner or equivalent heater 61 is mounted to heat the strip 22 as it enters the fur-- nace so that it is wound hot about block l8 and therefore readily bends and conforms itself to the arcuate corners of the wound core. If desired burner 61 may be replaced by an electrical induction heater. Preferably I maintain a temperature of about 1700" F. in the zone of heater 61, but this must be correlated with strip winding speed to obtain the optimum condition in each furnace. Preferably a pair of further burners or like heaters 68 are provided for applying local heat to the core during winding, the most heat being applied to the lower corners when the core is in and near the position of Figure 12. These heaters 68 prevent the core from chilling too swiftly and insure that each turn at the corner is heated to 1700 F. while conforming to the contour of the core corner thus effectively reproducing conditions in the separate annealing furnace.

ment of the invention it is particularly essential that the winding tension be progressively decreased as the core turns accumulate because, as the succeeding hot wound interior turns cool. they contract and build up increasing tension in the succeeding turns which must be compensated by a corresponding correlated decrease in winding tension. Thu th strip temperature, winding speed and tension must all be correlated in this embodiment to obtain optimum results.

My invention provides a wound core of magnetic strip material that is cut at only one place during assembly with a coacting coil so that it contains only one smoothly and tightly closing butt Joint with pressure on the lamination ends at the joint being substantially equally distributed for producing a minimum of vibration of the laminations and minimum exciting current losses. The efficiency of my core in the final assembly compares favorably with uncut cores, the losses and exciting current in my core being no more than 10 percent greater than in uncut cores.

My improved method of winding the core wherein the winding tension is gradually reduced as the core turns build up enables the manufacture of a practical wound core that is less expensive and represents a saving in material and labor over punching and other core forming methods. In my method the reduction of winding tension with the accompanying reduction of inner surface pressures between the accumulating turns insures against contraction or relative slipping of the outer layers, particularly at the rounded corners, after the core is out.

If desired two or more strips of the same or different width or magnetic qualities may be simultaneously or successively wound on form i8. For example successively wound strips of different width may be employed to produce a cruxiform core usable in some transformers. Each strip so wound is controlled as above explained.

The preferred embodiment of the invention illustrates a straight sided oblong core to which the invention is peculiarly adapted, especially where the coil window is long. The invention is particularly advantageous in core and coil assemblies of high electrical efficiency which demand a coil window length at least four times the core thickness and a coil window width of the order of twice the core thickness, usually not more unless the window is proportionally longer. As a practical consideration of both good transformer design as well as the mechanical characteristics of this core, the window should be at least three times as long as it is wide. When the core is closed and clamped tight all the ends of the respective turns are held tightly closed at the joint except the first turn on the inside and the second turn which sometimes fail to lock securely against the adjacent end in which case they must be supported and restrained from vi- I have discovered that the winding tension of strip 22, in the method illustrated in Figure l2. must be maintained relatively low during the entire operation so as not to stretch the hot steel strip or reduce its cross section.

In m d embOd brating by pressure of the coil assembly and the insulating strip [8 of Figure 1. In case the coil and insulation strip do not clamp this inner layer, its vibration may be urther prevented by providing a suitable thin metal clip 40 extending laterally across the inner turn of the core adjacent the inner end of joint i5 as illustrated in Figure 6. Clip 40 has its opposite ends inturned and interleaved with the inner several turns so that the latter are bound together and given such mass that they do not vibrate in normal operation. No clip is needed in the short 13 side laminations because of their low leverage with respect to the adjacent corner.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. The method of forming a hollow wound core of magnetic strip material comprising the steps 01, winding a length of said strip material on a form having two substantially parallel straight sides, with the tension in said strip prior to its disposition on the already accumulated turns on said form decreasing gradually as the turns accumulate, clamping the wound length of strip tightly along said two sides, annealing said clamped wound strip, clamping one of said straight sides and the contiguous side at their juncture to tightly compact said wound strips thereat, and cutting said wound strips at said juncture between said clamped portions.

2. The method of forming a magnetic core 30 and coil comprising the steps of, winding a length oi strip material under gradually decreasing tension on a form having two substantially parallel straight sides, clamping the wound length of strip tightly along said two sides, annealing said clamped wound strip, clamping one of said straight sides and the contiguous side at their juncture to tightly compact said wound strip thereat, cutting said wound strip at said juncture between the clamped portions, springing said straight side away from said contiguous side, placing a coil over said straight side, and applying ressure completely around the periphery of said wound strip and assembled coil to close said out substantially across its full area.

ULYSSES S. DUNN.

References Cited in the tile of this patent UNITED STATES PATENTS Number Name Date 1,935,426 Acly Nov. 14, 1933 2,264,800 Horstman Dec. 2, 1941 2,313,306 Wiegand Mar. 9, 1943 2,387,099 Vienneau Oct. 16, 1945 2,393,439 White et a1 Jan. 22, 1946 2,408,211 Hodnette Sept. 24, 1946 2,416,989 Gauthier Mar. 4, 1947 2,478,029 Vienneau Aug. 2, i949 FOREIGN PATENTS Number Country Date 7,856 Great Britain Mar. 15, 1890