WO2008104636A1

WO2008104636A1 - Method for manufacturing a magnetic core piece and a magnetic core piece

Info

Publication number: WO2008104636A1
Application number: PCT/FI2008/050086
Authority: WO
Inventors: Jarkko SALOMÄKI
Original assignee: Salomaeki Jarkko
Priority date: 2007-02-26
Filing date: 2008-02-25
Publication date: 2008-09-04
Also published as: FI20070160A0

Abstract

The invention relates to the manufacture of inductive components and electrical energy filters, particularly it relates to an inductive component core piece and the manufacture of said core piece. By the method according to the invention it is possible to effectively and flexibly manufacture various types of inductive components and compact filters they comprise.

Description

METHOD FOR MANUFACTURING A MAGNETIC CORE PIECE AND A MAGNETIC CORE PIECE

This invention relates to a method for manufacturing a magnetic core piece, specified in the preamble of claim 1 , and to a magnetic core piece, specified in the preamble of claim 10.

Inductive components, such as chokes and transformers, are used in the storage of energy (chokes) and for the transmission of energy over galvanic isolation (transformer) using a magnetic field. Inductive components are comprised of a coil and a core, of which there may be one or more, and which are in direct contact with one another. Voltage applied to the coil produces in the core a changing magnetic field that is capable of storing energy. In more specific terms, energy is stored in the magnetic flux that flows in the core. Transformer iron plates, iron powder, ferrite and amorphic metals, among others, are used as the core material in inductive components. Copper wire, aluminium wire, circuit board, foils, among others, are used as the coil for inductive components. Some of the most common component shapes used in inductive components are: toroid, E-core and a so- called cup core. Due to their mechanical properties, different types of magnetic core materials are always best suited to just a specific geometry. For example, it is difficult to make a cup core out of transformer iron plate, which is why strips of transformer iron plate are used in the manufacture of toroid cores.

Of particular interest are magnetic cores compressed from magnetic powder by pressing and sintering processes; these cores are mechanically strong and geometric details can be effectively joined to them. They are manufactured by compressing and heat-treating insulated magnetic powder, typically iron powder. Furthermore, iron in the powder may contain coagulants, such as silicon.

The size of magnetic cores manufactured by means of powder compression is limited to the pressing tool's maximum pressing area, which is determined according to the maximum pressure output of the pressing tool. Typically, the pressure can be, for example, 500 tonnes, and the largest pressing tools may have a pressure of, for example, 1000 tonnes. Furthermore, the performance of a magnetic core manufactured with powder is limited by the fact that the magnetic and mechanical properties of the core are identical throughout the entire structure, even in cases where an optimisation of the design would require the specification of different properties for different parts of the structure. For example, the core central hub, around which the coil is wound, is in a hotter environment than the return route of the magnetic flux, which is usually in direct contact with surrounding air.

It can be generally stated that known magnetic core manufacturing methods cannot optimise the price/performance capacity within the boundary conditions of the process and raw materials.

The purpose of the invention being presented here is to improve the manufacture and performance of inductive component cores. The method according to the invention is characterized by what is disclosed in the characterization part of claim 1 and the magnetic core piece according to the invention is characterized by what is disclosed in the characterization part of claim 10. Other embodiments of the invention are characterized by what is disclosed in the other claims.

The advantage of the solution according to the invention is that the method according to the invention can be applied to effectively and flexibly manufacture different types of magnetic core pieces, such as cores for chokes and transformers. A magnetic core piece according to the invention manufactured using the method can also be optimised in relation to manufacturing costs and performance.

The invention is described below in greater detail using various examples of embodiments, with reference made to the enclosed drawings, wherein:

Figure 1 presents an overhead view of a magnetic cup core according to prior art, Figure 2 presents a side view of the cup core shown in Figure 1 , with a cross-section along the A-A line in Figure 1 , Figure 3 presents an overhead view of a square shaped magnetic cup core according to prior art,

Figure 4 presents an overhead view of a core according to the invention, Figure 5 presents an overhead view of another advantageous core according to the invention, Figure 6 presents a side view of the core according to the invention shown in Figure

5, Figure 7 presents an overhead view of a third advantageous core according to the invention, Figure 8 presents an overhead view of yet another advantageous core according to the invention, Figure 9 presents a side view of a choke, with a coil removed, accomplished by the solution according to the invention, Figure 10 presents a side view of another choke, with a cross-section of the coil, accomplished by the solution according to the invention, and

Figure 11 presents a side view of a third choke, with the coil removed, accomplished by the solution according to the invention

The figure descriptions mention the term "core piece", which can refer to either the whole core or a core half, whereupon the two halves, placed one upon the other and one against the other, form a single whole core.

Figure 1 and 2 show a common shape for a magnetic core in accordance with known technology, i.e. a so-called cup core, seen from overhead and the side. The core is comprised of a so-called central hub 200 and magnetic flux return route base 201 as well as a ring 202 and a space 203 between the ring 202 and central hub 200 for the coil. The core structure shown in Figures 1 and 2 enables the effective manufacture of an inductive component, when the coil is placed between the two cup core halves placed on top of one another and opposite one another. The magnetic flux therefore runs through a whole core consisting of the two core halves shown in Figures 1 and 2 placed on top of one another: first in the central hub 200, moving in, for example, a downward direction; then radially out along the core base 201 towards the outer edge; up the ring 202 that forms the return route; and again in the second core half from the ring 202 along the base 201 towards the central hub 200.

The largest size of this type of a core is limited by the total surface area of the core, because, as stated above, the pressing tool has a limited pressing area.

Figure 3 shows a square shaped variation of the cup core according to known technology; this core has the same operational parts as the cup core shown in Figures 1 and 2. The parts are here: central hub 200, magnetic flux return route base 201, return route legs 204 and coil area 203. This shape can be integrated more effectively inside electro technical devices, which are generally rectangular in shape, than the round profile of the core shown in Figures 1 and 2. Also in this case the largest possible size is limited by the surface area of the core being compressed and available pressing capacity. The central hub 200 could, of course, be other than round in shape. For example, it can be oval, rectangular or another suitable shape. In any case, however, the round shape offers a minimal circumference in relation to the forming surface area, which is generally a desirable attribute.

Figure 4 shows a core structure according to the invention. This structure has also a central hub 1, return route legs 2 forming a return route for the flux, and route members 3 forming a base, as well as a space between the central hub 1 and return route legs 2, i.e. the coil window 4 for the coil. It must, however, be noted that the base is not a solid piece, but the outer ends of the route members 3 and return route legs 2 have empty spaces 5 between them, which contain no material. The empty spaces 5 are widest at the outer edge of the core piece and narrow as they approach the central hub 1. These areas can be removed, because the perpendicular line of the route members 3 between the central hub 1 and return route legs 2 provide an adequate path for the magnetic flux, as the smallest combined cross-sectional area of these four route members 3, which are situated perpendicularly between the central hub 1 and the return route legs 2, and which essentially form a letter x, is essentially the same as the smallest cross-sectional area of the central hub 1. In this case the required surface area being compressed in the manufacture of the core piece decreases and the same pressing tool can be used to manufacture a larger magnetic core piece, which is generally both cost-effective and desirable. The number of return route legs 2 could be also other than four legs, as shown in Figure 4. Due to the interrelated dimensions of the route members 3 and central hub 1, the magnetic flux density remains stable and essentially constant. The central hub 1 is made either by compressing it together with the other parts of the core piece or the central hub 1 is a separate piece, which is joined to the centre part of the route members 3, serving as a base, by means of gluing or another suitable method of joining.

Figures 5 and 6 show an overhead and side view of an advantageous magnetic core piece according to the invention. It should be noted that the outer circumference Ia of the core piece's central hub 1 is round. This minimizes the resistance of the coil. Furthermore, the overhead and side profile of the core piece, i.e. the contours, form a rectangle, whose sides are essentially straight and one side is essentially perpendicular to the adjacent side. In this case, for example, the cross-section at the plane perpendicular to the centre axis of the central hub 1 on the core piece body, i.e. the core piece contour, is essentially square in shape. The side view of the profile can also be square in shape, or another rectangular shape.

These profile shapes make it easy to integrate the component together with other, similar components, such as choke components or together with other electronics. In addition, the outer edge 4a of the coil window 4 facing the return route legs 2 is round, i.e. the inner edge of each of the return route legs 2 as seen from above is round, thus forming an arc section of the circle, which is concentric with the central hub 1. This facilitates the easy manufacturing of the coil by enabling the axial winding. Base areas 5 unnecessary for the flow of the magnetic flux are left out. This reduces the overall projection of the component, i.e. the total cross-sectional area of the component material, and, in turn, the amount of force required by the pressing tool when making the pressing. Mounting holes 6 equivalent to the height of the core piece are made in the return route legs 2. These mounting holes 6 enable the joining of one core piece to another, both side-by-side and on top of one another. The mounting holes 6 have slot openings 7, equivalent to the height of the core piece, opening out of the outer corners of the core piece. These openings facilitate the manufacture of the core piece and serve to enhance the pressing process. It should also be noted that the outer circumference of the return route legs 2 is comprised of five different parts, i.e. the arc section of the circle forming the outer edge 4a of the coil window 4, two straight outer edge sections 8, which are perpendicular to each another, and two short, inwardly oriented straight sections 9. This type of shape promotes optimisation of the magnetic circuit.

When the central hub 1 is made shorter than the height of the return route legs 2, an air gap 10 will be formed between the top of the central hub 1 and the upper surface of the core piece. The central hub 1 can also be made essentially equal in height to the height of the return route legs 2, or higher. During the manufacture of the core piece, the desired magnetic circuit properties can be set by adjusting the size of the air gap 10 formed by the central hub 1. The size of the air gap can be flexibly adjusted by altering the motion of the hydraulic pressing tool. Thus, the air gap of the central hub 1 can be, if necessary, set individually for each piece being compressed of the powder in the pressing process. This enables the flexible manufacture of magnetic cores for different purposes using the same mould in a continuous process. One advantage of the method and finished core structure is that also the coil wire can be run from the projection through the missing base sections 5 without obstruction, which facilitates winding and enhances the fill degree of the coil.

Because this type of core piece is mechanically compact, it can be effectively integrated inside other electronics. This solution is very suitable for use for example in, motor drives or UPS units ensuring power supply.

Alternatively, other electronics can be effectively integrated inside this type of structure, as it forms effective mechanics, which, in this context, refers to mechanical support and mounting platforms. For example, the electronics required in a motor drive or UPS unit could be integrated inside the core. One possibility is to integrate various sensors inside the core, such as heat and current sensors, whose readings could be used to, for example, adjust a motor drive, or the data could be used in remote diagnostics.

Furthermore, channels for liquid-cooling elements can be directly integrated in the core. An exceptionally effective is to make a groove or grooves on the surface of the core, through which the coolant runs. Thus placing two of these types of parts against one another or placing a cover plate on the surface of the core will form a liquid cooling system. It would be much more difficult to drill or otherwise machine a liquid cooling channel running through the piece.

An important application area for inductive components is various types of filters. Different types of filters can be effectively manufactured using the method according to the invention. For example, the capacitors used in filters can be integrated with chokes to form a single mechanical unit.

The costs of a magnetic component are primarily formed of coil and core material costs. The required amount of coil and core material is determined by the current density, magnetic flux density, i.e. saturation and magnetic core dissipation power density. The latter two of these are related to the core material properties. In general magnetic components are designed to be either saturation or core dissipation power density limited. In order to ensure maximum utilisation of the magnetic core material, the component should be designed in such a way that both these boundary conditions are simultaneously limiting the designing. In other words, the component will simultaneously make maximal use of the available magnetic flux density, which is typically 0.4 T as well as the core material capacity to withstand variations in high frequency magnetic flux, which is typically 0.05 T at a typical operating frequency of 10kHz, which therefore generates magnetic dissipation, i.e. heat. In order to make this type of component cost-effective, the coil and core material costs must also be of the same order of magnitude. Using the method according to the invention, it is possible to design and manufacture the above- described optimal magnetic component effectively in such a manner that the component shall be simultaneously saturation-limited and core dissipation power density-limited.

Figure 7 shows yet another solution for improving the core's efficiency. In the method according to the invention it is possible to increase the size of the air gap in a magnetic core to a dimension equal to that of the entire central hub 1, i.e. the original central hub 1 would be completely omitted. Thus the original central hub 1 can be replaced by a central hub 11 made from a different material, which is, for example, magnetically effective, but mechanically fragile or expensive, thereby preventing its use for the manufacture of the whole core. Thus the original material used in the return route legs 2 provides mechanical strength and lower total costs, as it can now be more modest magnetic material. The reason for this is that, in designing an inductive component, the core surface remaining inside the coil, i.e. the central hub 11, is essentially included in the designing. For example, when the central hub 11 material has endurance to a higher magnetic flux density, i.e. the material has a higher maximum flux density or the material has an AC component withstanding higher switching frequency flux density, the cross-sectional area of the central hub 11 can be reduced. Thus the distance required for the coil to go around it is also reduced, which reduces the resistance, i.e. dissipation and the cost of the coil. All the surface area required by the return routes, formed by the return route legs 2 and route members 3, can be utilised, as the coil is not wound around them, i.e. increasing their surface area to reduce flux density will not affect the length of the coil, i.e. the resistance and dissipation, whereupon it can be made in the scope of the freely available total surface area. A central hub 11 made of another material is joined to its base, for example, by gluing or another suitable joining method. A central hub made of another material can be made, for example, by pressing either at a different time and using a different tool than the other parts of the core piece, or essentially at the same time using the same tool as the other parts of the core piece. A central hub made of another material can also be made by means of a method other than pressing.

Figure 8 shows yet another core according to the invention, whose size could be increased further during the manufacture by the pressing tool. In this version the core projection is divided into two parts 12 and 13 by a division line 14. Manufacturing the core with smaller parts such as these enables the use of a smaller pressing tool in their manufacture. There may be more parts used than the two shown in the example in Figure 8. These types of separate core parts are joined to each another by various joining methods, such as gluing, welding, part-mounted couplers or another suitable joining method.

Figure 9 shows a schematic side view of a choke with the coil removed, which applies the solution according to the invention, and in which the choke's air gap is divided into several smaller parts 15 using shorter central hub pieces 16 in the central hub. Air gaps 15 are made, for example, using thin insulators that are not shown in the Figure. The central hub pieces 16 and insulators are glued or joined by means of another suitable method to each other and to the base platform. This solution produces numerous benefits, the most important of which is that the distension in the magnetic flux caused by the air gap and the extra coil dissipation resulting from this are reduced. Furthermore, the desired air gap is achieved by combining suitable central hub pieces 16.

Figure 10 shows a schematic side view and coil cross-section of an alternative method for minimising additional coil dissipation resulting from the dispersion field generated by the air gap. The choke's central hub 1 is fitted so that its top end is inserted into the cavity 3b in the bottom of the cover plate 3a, thus forming an air gap and its dispersion field. This prevents the dispersion field from affecting the coils 17.

Figure 11 shows a schematic side view of one advantageous solution, with the coil removed, to reduce additional core dissipation caused by variations in flux density when using various core materials. In this instance the choke central hub is made of two different materials. The inner central hub 18 is made of a high flux density enduring material, thus enabling a reduction in its size. Because the flux density in this material is higher than in the material used in, for example, the cover and base plates 20, significant additional core dissipation would occur at the junction if the materials contacted directly, as the flux density decreases to a value appropriate to the cover and base plates 20 only a short distance away from said junction. It is for this reason that the transition from a material with a high flux density 18 to the normal material used in the cover and base plates 20 is made gradually, using a transition piece 19, in which the flux density is transformed to a value appropriate to different materials, between the central hub 18 and the cover and base plates 20. Adding suitable air gaps between different pieces 18, 19, 20 can enhance the effect even further. These types of air gaps can be made in all or some of the spaces. They could, however, also be omitted altogether.

In the primary advantageous embodiment of the invention the magnetic core piece is made powder metallurgically of magnetic powder by compressing in essentially one pressing pass, so that the central hub 1 of the core piece is compressed into an essentially round cross-section, and the cross-section of other parts in the core piece body, being perpendicular to the centre axis of the central hub 1 , i.e. the core piece contour, as seen from above, is compressed during the same pressing pass to essentially form a square. Furthermore, during the same pressing pass, the return route legs 2 on the corners of the core piece body are compressed in such a manner that the outer edge 4a of the coil window 4 facing the return route legs 2 are compressed into a round cross-section, as above. Furthermore, the magnetic flux route members 3 are compressed during the same pressing pass to form the base of the core piece body, thus joining the core piece central hub 1 to the return route legs 2 in such a manner that areas 5, which are not directly on optimal magnetic flux route, at the outer ends of the route members 3 are left free of material. Yet, in the same pressing pass, mounting holes 6 are made in the outer corners of the return route legs 2 in such a manner that each of the mounting holes has an opening 7 facing outside of the core piece. It is in this manner that all exterior side and base contours of the core piece are, as seen from the side and above, essentially straight, thus giving the mechanical structure of the core piece an essentially rectangular profile. Due to the rectangular profile, core pieces can be easily placed on top of one another or adjacent to one another when assembling inductive components. Thus the exterior surfaces of the core pieces placed opposite one another are always planar or straight in their plane.

To those skilled in the art it is clear that the invention is not exclusively limited to the examples specified above, but can be varied within the scope of the patent claims listed below. Thus, for example, the detailed structure of the core piece can differ from the examples presented above.

Claims

1. A method by means of which a magnetic core piece equipped at least with magnetic flux route members (3) and return route legs (2) located at the outer ends of said members is manufactured out of a powder by pressing and heat treating, characterized in that said core piece is manufactured essentially using a pressing tool in such a manner that at the same time at least the route members (3) forming the base of the core piece and the return route legs (2) at the outer ends of said members are compressed, and open areas (5) free of material between the outer ends of the route members (3) are formed, and the core piece profile is made essentially rectangular in shape.

2. The method according to claim 1, characterized in that essentially at the same time in connection with the pressing of the core piece return route legs (2) and the route members (3) the core piece central hub (1) is pressed essentially round in its cross-sectional shape.

3. The method according to claim 1 or 2, characterized in that the cross-sectional area of the route members (3) in the core piece base are made in such a manner that the combined cross-sectional area of the route members (3) is essentially equal in size to the cross- sectional area of the core piece central hub (1, 11).

4. The method according to claim 1, 2 or 3, characterized in that, in connection with the pressing of the core piece, mounting holes (6) are made in the outer corners of the return route legs (2) essentially by the same pressing as the other core piece parts, in such a manner, that each mounting hole has an opening (7) facing outside of the core piece.

5. The method according to any of the claims above, characterized in that the core piece central hub (1, 11) is formed of a material possessing different properties than the material used in the return route legs (2) and route members (3).

6. The method according to claim 5, characterized in that the core piece central hub (1, 11) is formed of a material possessing a higher maximum flux density and/or lower dissipation than the material used in the return route legs (2) and route members (3).

7. The method according to any of the claims above, characterized in that liquid-cooling channels are made on the surface of the core piece in connection with the pressing of said core piece.

8. The method according any of the claims above, characterized in that the size of the air gap (10) in the central hub (1) is varied during the pressing process using the same pressing tool.

9. The method according to any of the claims above, characterized in that in connection with the pressing of the core piece, the core piece is made simultaneously saturation- and dissipation-limited in its dimensions and form.

10. A magnetic core piece manufactured of a powder by pressing and heat treating, which comprises at least magnetic flux route members (3) and return route legs (2) at the outer ends of said members, characterized in that the side, base and top surfaces of the core piece profile are fitted essentially rectangular in shape in such a manner that the outer surfaces of the core pieces placed opposite one another are always essentially straight in their plane.

11. The magnetic core piece according to claim 10, characterized in that the core piece contains an essentially round central hub (1, 11), which is made of either the same material as or a different material than used in the magnetic flux route members (3) and the return route legs (2).

12. The magnetic core piece according to claim 11, characterized in that the central hub (1, 11) of said core piece is comprised of a material possessing a higher maximum flux density and/or lower dissipation than the material used in the magnetic flux route members (3) and the return route legs (2).

13. The magnetic core piece according to claim 11 or 12, characterized in that the central hub (1, 11) is a separate component comprised of one or several parts and joined to its base by means of gluing or another suitable joining method.

14. The magnetic core piece according to claim 11, 12 or 13, characterized in that the spaces between route members (3) in the base of said core piece have areas (5) free of material, which are made so large that the combined cross-sectional area of the route members (3) is essentially equal in size to the cross-sectional area of the central hub (1, 11).

15. The magnetic core piece according to any of claims 10-14 above, characterized in that liquid-cooling channels are integrated in said core piece.

16. The magnetic core piece according to any of claims 10-15 above, characterized in that said core piece is comprised of several separately compressed parts.