US3469221A - Transformer core - Google Patents

Description

- PRIOR ART Sept. 23, 1969 w. OLSEN 3,

- TRANSFORMER cqnn Filed May 19, 1967 3 Sheets-Sheet 1 WILLY OLSEN INVENTOR ATTORNEYS Sept. 23, 1969 w. OLSEN 3,469,221

TRANSFORMER 001m Filed May 19, 1967 a sheets-sheet z INVENTOR WILLY OLSEN TOTAL MAGNETI ZING VOLTAMPS ssnvso'lm swswao m 1V3d momvs- Se t. 23, 1969 w. OLSEN 3,469,221

TRANSFORMER CORE I Filed May 19, 1967 3 Sheets-Sheet 5 I169 I EIGJO WILLY OLSEN ATTORNEYS INVEN'I'OR United States Patent US. Cl. 336-212 Claims ABSTRACT OF THE DISCLOSURE A C-type cut transformer core has the ends of the laminations of each part of the core staggered relative to one another and assembly of the completed core with its coil is facilitated by coating the cut ends of the lamination so as to cover the metal burrs formed during the operation of cutting of the lamination.

The method employed in initially fabricating the cores is also important from a speed of manufacture consideration. The method employs several novel techniques relating to magnetic separation of laminations after initial cutting and subsequent assembly of the laminations with staggered joints on a magnetic jig.

This application is a continuation-in-part of my application Ser. No. 568,285, filed July 27, 1966, now abandoned, and entitled Transformer Core.

In the industry, a C-type cut core is a core used generally for small transformers, in which the ends of the legs of two C-shaped members are butted together to form a generally rectangular core having a centrally located hollow window adapted to receive the coil of the transformer structure. The magnetic performance of such cores is not as gOOd as in many magnetic structures and much of the difliculty is found in the fact that, when the two parts of the core are butted, an average air gap remains which contributes to v.-a. losses. Further, the two halves of the core are usually bonded to one another by an epoxy resin which in itself results in degradation of the electrical properties of the core. A related problem is that high temperatures degrade the resin and permit the legs of the core to move outwardly under expansion due to heat, resulting in a tapered and thus enlarged air gap.

It is an object of the present invention to provide a C-type cut core having superior magnetic properties rela tive to the conventional C-type cut core described above.

It is a further object of the invention to provide a method of assembly of a C-type cut core having staggered joints which method permits economical manufacture of the core while maintaining improved magnetic properties.

The core of the present invention provides zig-zag joints between the two halves of the core, the zig-zag joint extending over a relatively great length of the legs of the core. The zig-zag joint structure has a two-fold effect upon the magnetic properties of the core. Specifically, the zig-zag structure reduces leakage flux at the joint due to the interleaving of the lamination of the core. Further, the interleaving of the laminations on a lamination-by-lamination basis prevents undue spreading of the legs of the core and thus, reduces disengagement of various laminations from one another. Further, since the laminations overlap at the joints, slight disengagement of one lamination from its aligned lamination has little effect upon the magnetic characteristic of the core.

It would be expected that certain difliculties would be encountered in fabricating such a core due to the thinness of each of the laminations, and the difliculty of obtaining precise registration of the laminations on a lamination-by-lamination basis at each of the joints. This problem has been completely overcome in accordance with the present invention.

Specifically, after the strip material from which the core is to be made is wound, heat treated to relieve stresses, and cut in half along its diameter to provide two half groups of laminations, each half group of laminations is applied to a magnetic separator to loosen the individual metal strips from one another. Prior to cutting of the laminations, the coiled metal may be permitted to slip to provide a space factor of percent to 92 percent. The laminations are then assembled about a nonmagnetic hollow cylinder having a strong, generally circumferential magnetic field developed thereabout. The inner half lamination is taken from each group and the adjacent ends of the half laminations are abutted. The magnetic field causes the half laminations to be pulled very tightly about the cylinder so that the other ends of the half laminations also abut. Each lamination is thus assembled from half laminations until the buildup is complete. The laminations are then tightly bound, the magnetic field turned off and the core removed for subsequent forming and heat treating.

It was believed that such great difliculty would be encountered in attempting to assemble a C-type core with staggered joints about an electric coil, as to render the entire operation impractical. However, by employing a lamination space factor of not greater than 90 percent to 92 percent and, prior to final assembly of the core with its electrical coil, spraying the ends of the lamination with a tough, low-friction plastic or like material, it is found that the two halves of the cores slip into place without any difiiculty.

The above and still further objects, features and ad vantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment of the invention, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is an illustration of a prior art C-type cut core with one of the joints illustrated somewhat enlarged;

FIGURE 2 is an illustration of the C-type cut core of the present invention;

FIGURE 3 is an enlarged view of one of the joints of the core of FIGURE 2;

FIGURE 4 is a graph of the magnetic performance of prior art cores and cores of the present invention;

FIGURES 5-10 illustrate various steps in the manufacture and assembly of the cores of the present invention;

FIGURE 11 is an enlarged view of the ends of the laminations of a core illustrating their sharp edges;

FIGURE 12 is an enlarged view of the ends of the laminations of a core after coating; and

FIGURE 13 illustrates an E-type core employing the principles of the present invention.

Referring specifically to FIGURE 1 of the accompanying drawings, there is illustrated a conventional C-type cut core generally designated by the reference numeral 1. The core is formed of two C- shaped elements 3 and 4 comprising individual laminations 4 of grain-oriented magnetic material. The two halves of the core 2 and 3, or more properly, the two parts of the core, since each part may not constitute a full half of the core, are butted to provide joints 6 and 7 in the opposed long legs of the core. The laminations of the butted joints, since the metal is somewhat ragged after the cutting operation, do not seat tightly against one another and an average air gap is developed at both joints between the two halves of the core. Further, when the transformer final assembly is subjected to heat during use, the corners between the yokes and legs of the core tend to straighten so that the legs want to move outwardly. The mechanical strength of the joints results entirely from the bonding material employed and this material is often damaged by heat. Thus the legs of the core are permitted to expand outwardly limited only by the bands used to hold the cores in place.

The cumulative result of the above is a wedge shaped air gap as illustrated in FIGURE 1 and particularly at the joint 7 of the core. The joint 7 is greatly enlarged relative to the remainder of the illustration so as to illustrate the effects of heat on the core. As indicated above, the abutting legs of the core sections 2 and 3 tend to spread outwardly forming a wedge-shaped gap between the two sections of the cores, adversely affecting the magnetic properties of the apparatus. Further, even if the two abutting faces were substantially parallel, it would not be possible to maintain the ends of all the laminations precisely aligned so that small gaps would occur between the ends of the various laminations thus providing a relatively large leakage at each of the joints.

Referring now specifically to FIGURE 2 of the accompanying drawings, there is illustrated the C-type cut core of the present invention. The core comprises two C- shaped sections 8 and 9 having joints 11 and 12 formed in parallel legs. The joints 11 and 12 as illustrated are zig-zag joints so that laminations of the two sections 8 and 9 (see FIGURE 3) are interleaved providing mechanical strength substantially greater than provided in the prior art core of FIGURE 1. The interleaving of the laminations reduces flux leakage due to the fact that leakage flux at each of the small individual gaps finds a low reluctance path in adjacent laminations. It can be seen that such interleaving also tends to prevent outward deflection of the legs of the core since a certain amount of mechanical strength is added to the joint due to the compressive forces between adjacent laminations.

The improvement in magnetic properties achieved with the present core relative to prior art C-type cut cores becomes apparent by a reference to FIGURE 4 of the accompanying drawings which is a graph comparing cores of the present invention with the average of the performance of four standard quality C-type cut cores. The graph is a plot of the flux density in kilogausses as a function of the total magnetizing volt-amperes. Curve A of FIGURE 4 is based on tests of the type of core illustrated in FIGURE 2 with the distribution of the gap over a one-half inch length of the central area of the legs of a core having a window length of approximately two and one-half inches. Curve B is a graph of the core of FIGURE 2 in which the gap distribution is off-center, or more particularly, runs diagonally of the leg of the core in a zig-zag pattern and extended over one and one-fourth inches of the two and one-half inch core. Curve C is a graph of the performance of a core which is substantially identical with that of the core of curve A except that the gap distribution extended over one and one-fourth inches of the leg of the two and one-half inch long core. Curve D is a graph of the average performance of four standard quality C-type cut cores of the type illustrated in FIGURE 1.

It can readily be seen that the performance of the C-core of the present invention, particularly of the core of curve C, is considerably better than that obtained with prior art C-type cut cores. For instance, at three voltamps, the core of curve C establishes a flux density of 16,000 kilogausses whereas the standard C-core establishes a flux density of only 13,000 kilogausses The percentage of improvement in the core of the present invention is thus approximately 23 percent. Tests on the same cores for iron losses show that the standard C-core has a loss of one watt at 13.1 kilogausses whereas the same Wattage loss was suffered by the core of graph C only upon obtaining 15.5 kilogausses of flux density. Thus, the core of the present invention not only provides higher flux densities per volt-amp, but also has lower losses per volt-amp.

Referring for a moment to FIGURES 2 and 3, it can be seen that one might anticipate some difliculty in assembling the final core of the present invention due to the very small spacing between the laminations which would result in adjacent laminations catching on one another and not permitting proper assembly and abutting of each lamination with the lamina-tion with which it is to eventually be aligned. The magnitude of the probelm can best be appreciated when it is realized that the individual laminations 13 are, in the cores tested for FIG- URE 4, no more than a few thousandths of an inch thick. This feature of assembly is quite important since the fact that one core has better magnetic properties than another, is not sufiicient to insure commercial utilization of the improved core. For instance, a 23 percent increase in kilogausses can be achieved with the present core over the conventional C-core and a reduction of losses of 30 percent may be realized. Such improvements will support a small increase in the price of the core, but if the manufacturing time of the core is greatly increased, such a core will remain a laboratory item until such time as practical methods of manufacture make it economically practical. Thus, the method of manufacture of the core of the present invention becomes important in considering the overall commercial applicability of the invention.

FIGURES 5-10 illustrate various steps employed in the actual method of manufacture and final assembly of the core of the present invention. Referring specifically to FIGURE 5 of the accompanying drawing, the initial step in forming the core is to wind a round core from a suitable strip of grain-oriented magnetic material. The core may initially be wound quite tight or it may be wound with suflicient looseness to provide the percent to 92 percent space factor which is preferred in the core of the invention. If, of course, the core is initially wound quite tightly, then after the winding operation, it is permitted to slip to the extent necessary to obtain the desired space factor. After the winding operation the round core is annealed to relieve stresses.

Referring now specifically to FIGURE 6, the core is now cut at locations 16 and 17 along a common diameter of the core to provide two semi-circular half stacks of laminations 18 and 19 of the core, each of the half groups eventually being formed into C-shaped members in the step illustrated in FIGURE 10. After the cutting operation, the short inner and outer pieces 15, constituting less than half turns of the core are thrown away so that all of the remaining laminations may be made to abut against one another.

As will be described subsequently, in order to produce theassembly of FIGURE 9, the laminations are formed by individual assembly of each half lamination from one s tack"with the corresponding half lamination from the other stack. Thus each half lamination must be easily separated from each other half lamination. The laminations,"however, tend to stick together and according to one step of the method of the invention separation of the lamination is effected magnetically.

Referring specifically to FIGURE 7 of the accompanying drawing, one half group of laminations is placed against a magnet having a north pole 20 and a south pole 21. Each lamination abutting the south pole becomes a north pole and each lamination abutting the south pole becomes a north pole. In consequence each lamination is magnetically repelled by each adjacent lamination and the laminations are separated so that they may be easily handled during subsequent operations.

Informing the staggered joint, the inner half lamination from each half stack is placed around a hollow cylindrical nonmagnetic mandrel 22 having an external diameter equal to the internal diameter of the core.

In performing the assembly operation, one end of each lamination is abutted against the adjacent end of the corresponding lamination of the other half stack. A strong magnetic field is developed about the mandrel, as will be described, and the half laminations are sufficiently tightly drawn about the mandrel so that the other ends are brought into abutting relationship. Successive pairs of laminations from the two stacks are placed around the mandrel with the joints of each pair offset from the prior pairs as illustrated in FIGURE 8. After all laminations have been assembled the outside laminations are taped, the magnetic field is discontinued, and the core removed from the mandrel and transferred to the forming press.

The magnetic field developed about the mandrel 22 is derived from an electric current passed through wires located internally of the mandrel. In one form the apparatus comprises four parallel wires 23 each carrying about eighty amperes of direct current.

The core is now pressed to a rectangular shape, as illustrated in FIGURE by means of a suitably driven member or piston 21 which presses the core against a base member 22. In order to produce accurate alignment of the legs and the accurate rectangular shape, a mandrel 23 is inserted in window 24 of the core so that each of the legs of the core is individually pressed to proper shape. Accurate alignment of the legs in the region of the joints is highly desirable to facilitate subsequent assembly.

The core is again annealed to relieve stresses. The steps of the operation thus far may be rapidly performed by'a, skilled mechanical in this field.

Difficulties arise when the core must be disassembled; that is, the two halves of the core pulled apart, so that one leg of a coil may be placed in the window of the core and then the core closed. During this operation, there is no automatic alignment of the ends of the cores and considerable difficulties was experienced in this operation. In accordance with the present invention, this difficulty is overcome by spraying the ends of the laminations forming the joints of the core with a relatively low friction plastic material. Specifically, it has been determined that considerable difficulty is normally experienced in attempting to concurrently assemble the two joints comprising staggered laminations, due to the small spaces involved, the sharpness of the edge of each of the laminations and various very small burrs formed thereon during cutting operation. It has been determined, however, that if the ends of the laminations are sprayed with plastic a relatively smooth tough coating of relatively lowfriction material is formed over the ends of each of the laminations and when the joints are brought together, they slip right into place without any manipulation of the parts by the operator.

Referring specifically to FIGURES 10 and 11, the former illustrates the sharp catching edges of the laminations and FIGURE 11 illustrates the coating effect of the plastic material. The coating material, which bears reference numeral 26 provides a smooth rounded profile at every sharp edge or junction in the joint area thus covering all of the surfaces of the laminated material that could catch on an adjacent surface of adjacent laminations of the core during assembly. The coating provided is sui'ficiently thin that it has no detectable effect upon the magnetic properties of the core, and specifically, the tests indicate that within measureable tolerances, the cores perform magnetically exactly the same before as after the coating process.

The specific material employed to coat the laminations is not critical, it is only necessary that the material, in effect, cover the sharp edges of the laminations with a relatively low friction and relatively tough material that will not be pushed aside as a core is assembled. One particular material that has been employed for this purpose is a commercially available spray can plastic which is normally used by housewives to coat brass articles after they have been polished so as to reduce the tarnishing effect of air. Other suitable materialsobviously may be employed. Also, if the particular coating material used does not have a low enough coefficient of friction, oil may be used as a lubricant over the plastic to provide the necessary low friction.

One additional step may be employed if desired. The ends of the laminations at the time of final assembly with a coil may be covered with a conventional core shellac so that the two halves of the cores are effectively cemented together after assembly to provide additional strength to the overall structure. The interleaving of laminations produces large surface areas of contact between adjacent overlapped laminations of the two core sections.

A still further step may be employed if desired. After annealing of the cores, and prior to disassembly of the core, the edges of the core are sprayed with a bonding material and the material allowed to dry. The two halves of the core are broken apart, turned over, carefully reassembled and sprayed again. The two halves are again broken apart and the abutting edges of the core then treated as indicated previously. The resulting product is two rigid half cores that can be assembled with one another with no difiiculty. The lubricant employed in the process of the invention is preferably a silicon base resin or plastic, these being known to have slippery, i.e., waxlike, surface characteristics.

The present invention, although originally intended for the manufacture of C-type, cut cores, has been found to be equally applicable to E-type cores such as illustrated in FIGURE 12. Such a core may be composed of two C- type, cut cores 27 and 28, individually manufactured and assembled and a further C-type cut core 29 large enough to snugly receive the cores 27 and 28 arranged side by side. All of the cores 27-29 have staggered joints according to the present invention.

Although the invention has primarily employed zig-zag joints as illustrated in FIGURE 2, other staggered joints may be employed. Such a useful configuration is one in which the joint pattern is a W with the outer legs about half the length of the inner legs. Also, it should be noted from FIGURE 3 that the joints are staggered relative to one another, i.e., the joints above and below the joint at the farthest right location are not aligned perpendicular to the length of the leg. Outside of magnetic benefits, the joint slips together more easily. Also, as pointed out in my prior patent application Ser. No. 443,929, now Patent No. 3,328,737, filed on Mar. 30, 1965, the joints should extend over as long a length of the legs as practical since the larger the ratio of joint length to leg length the better the core performance.

It has been found that smaller size cores are more easily wound if the strip is preheated to about C. This treatment prevents cracking of the strip since the heat reduces the stiffness of the strip.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A C-type core comprising first and second generally U-shaped sections each including an equal plurality of laminations, each U-shaped section having a pair of spaced-apart parallel legs joined by a yoke, the ends of said laminations of said first section being offset relative to one another to provide a stepped relationship between the ends of adjacent ones of said laminations, the ends of said laminations of said second section being offset relative to one another in a pattern complementary to that of the ends of said laminations of said first section such that when said sections are in abutting relationship to provide a complete core with two joints, the end of said laminations of one of said sections abut the ends of the corresponding laminations of the other of said sections on a one-for-one basis, and a tough, slippery, continuous coating over the ends and adjacent regions of said laminations of at least one section at each of said joints of a suflicient thickness generally to coat metal burrs subsisting on said laminations in the regions of the ends of said laminations.

2. The combination according to claim 1 wherein said Offset ends define a zig-zag configuration.

3. The combination according to claim 1 wherein all of said ends of said laminations are coated with said coating.

4. The combination according to claim 3 wherein said coating on said ends is coated with a cement.

5. An E-type core comprising a pair of substantially identical cores as defined in claim 1, said cores being arranged side-by-side with said legs parallel and in contact, a third C-type core of claim 1 surrounding and snugly receiving said pair of cores within the space defined by said legs and yokes of said third core.

6. The combination according to claim 1 wherein said laminations have a space factor of 90 percent to 92 percent at the junction of the yokes and the legs.

7. The combination according to claim 1 wherein said joints lie centrally of said legs and exend over approxi mately one-half of the length of the innermost lamina- =tion of said legs.

8. A C-type core according to claim 1 material is a silicone base resin.

9. A C-type core according to claim 1 further comprising an adhesive coating applied to the ends and adjacent areas of said laminations to bind adjacent laminations of each of said sections to the other laminations 0f the same section whereby each U-shaped section comprises a unitary structure, said adhesive coating lying under said slippery coating.

10. A C-type core comprising first and second generally U-shaped sections each including an equal plurality of laminations, each U-shaped section having a pair of wherein said spaced-apart parallel legs joined by a yoke, the ends of said laminations of said first section being offset relative 'to one another to provide a stepped relationship between the ends of adjacent ones of said laminations, the ends of said laminations of said second section being offset relative to One another in a pattern complementary to that of the ends of said laminations of said first section such that when said sections are in abutting relationship to provide a complete core with two joints, the ends of said laminations of one of said sections abut the ends of the corresponding laminations of the other of said sections on a one-for-one basis, and a tough, low. friction, continuous coating over the ends and adjacent regions of said laminations of at least one section at each of said joints of a sufiicient thickness generally to coat metal burrs subsisting on said laminations in the region of the ends of said laminations.

References Cited UNITED STATES PATENTS 2,387,943 10/1945 Putman 336-219 XR 2,477,350 7/ 1949 Somerville 336217 2,484,215 10/1949 Foster 336219 XR 2,488,391 11/1949 Ford et al. 336216 XR 2,523,071 9/1950 Somerville 336-217 XR 2,548,624 4/1951 Sclater 336-217 XR 2,931,993 4/1960 Dornbush 336217 3,222,626 12/1965 Feinberg et al. 336-219 XR 3,223,955 12/1965 Olsen et al 336-217 XR 3,328,737 6/1967 Olsen 3362l7 XR LEWIS H. MYERS, Primary Examiner T. I. KOZMA, Assistant Examiner US. Cl. X.R.

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