ORIENTED MAGNETIC CORE LAMINATION AND METHOD OF MANUFACTURE

Abstract

An oriented magnetic core lamination technique and a method of producing circular lamination cores. The method includes cutting rectangular strips with teeth pointing in a single direction (may not be the traverse or rolling direction) from the steel sheet plane, as opposed to directly punching circular laminates from the steel sheet with the teeth pointing in all directions. The strips are cut in such a way that the short side is aligned to the direction that has the best magnetic properties. The strips can then be bent into a donut or toroidal shape, either inwardly (with teeth pointing to the circle center) or outwardly (with teeth pointing out of the center) depending on the design of the lamination core. The direction with the best magnetic properties may be determined by non-destructive methods such as magnetic Barkhausen noise (MBN) analysis, x-ray diffraction (XRD), or electron backscatter diffraction (EBSD).

Claims

1. A method for producing laminates suitable for magnetic cores, said laminates being produced from steel sheets, the method comprising: a) determining a preferred crystal direction on said steel sheets; b) cutting strips from said steel sheets, said strips being suitable for manufacture of laminates having teeth such that said teeth are aligned in said preferred crystal direction; c) executing steps d1)-d4) or steps e1)-e3); wherein steps d1)-d4) comprise: d1) removing excess material from said strips; d2) forming a toroidal shape from said strips by bending said strips and joining ends of said strip to one another wherein excess material removed in step d1) facilitates said bending; d3) removing further material from said toroidal shape to form teeth, thereby resulting in said laminate having teeth; d4) ending said method; and wherein steps e1)-e3) comprise: e1) removing material from said strips to form teeth; e2) bending said strips into said toroidal shape and joining ends of said strip to one another to thereby result in said laminate having teeth; and e3) ending said method.

2. The method according to claim 1 wherein step a) is executed using at least one of: magnetic Barkhausen noise (MBN) analysis; x-ray diffraction (XRD); electron backscatter diffraction (EBSD); Epstein frame testing; single sheet testing; and neutron diffraction.

3. The method according to claim 1 wherein step a) is executed using one of: a destructive testing method; and a non-destructive testing method.

4. The method according to claim 1 wherein removing material in said method is executed by punching using a die.

5. The method according to claim 1 wherein said teeth in said laminate are facing away from a center of said toroidal shape.

6. The method according to claim 1 wherein said teeth in said laminate are facing towards a center of said toroidal shape.

7. The method according to claim 1 wherein, in step e1), said material removed to form teeth includes material such that slots are formed on both sides of a yoke where bending occurs when said strip is bent into said toroidal shape.

8. The method according to claim 1 wherein step e2) is executed using two rotating plates with applied pressure such that only in-plane deformation is allowed.

9. The method according to claim 1 wherein said excess material removed in step d1) is removed such that slots are formed on both sides of a yoke part where bending occurs when said strip is bent into said toroidal shape.

10. The method according to claim 1 wherein ends of said strip are joined to one another using at least one of: welding; friction stir welding; and resistant welding.

11. The method according to claim 1 wherein said further material removed in step d3) is removed such that both yoke and teeth are formed.

12. The method according to claim 1 wherein laminates formed by said method are stackable to form a magnetic core for an electric motor.

13. The method according to claim 2 wherein said preferred direction is a direction with the largest MBN energy.

14. The method according to claim 1 wherein said preferred direction is a direction with the lowest core loss.

15. The method according to claim 1 wherein said preferred crystal direction is a direction with a largest <100> intensity.

16. A method for producing lamination cores, the method comprising: cutting rectangular strips with teeth pointing in a single direction from steel sheets such that the teeth formed are aligned in a direction that has the preferred magnetic properties.

17. The method according to claim 16 wherein said teeth are punched from a donut-shape preform bent from said rectangular strips along a long side.

18. A method for producing laminates suitable for magnetic cores, the method comprising: a) determining a preferred direction having the strongest <100> crystal orientation, or a direction having preferred magnetic properties on said steel sheets; b) cutting rectangular strips from said steel sheets, said rectangular strips being suitable for a manufacture of circular laminates and having a short side aligned in said preferred direction; c) executing steps d1)-d4) or steps e1)-e3); wherein said laminates are produced from said rectangular strips cut from steel sheets; wherein steps d1)-d4) comprises: d1) removing excess material from said strips; d2) forming a toroidal shape from said strips by bending said strips along a long side and joining ends of said strip to one another wherein the excess material removed in step d1) facilitates said bending; d3) removing further material from said toroidal shape to form teeth, thereby resulting in said laminate having teeth; d4) ending said method; and wherein steps e1)-e3) comprises: e1) removing material from said strips to form teeth; e2) bending said strips into said toroidal shape and joining ends of said strip to one another to thereby result in said laminate having teeth; e3) ending said method.

19. The method according to claim 18 wherein step a) is executed using at least one of: magnetic Barkhausen noise (MBN) analysis; x-ray diffraction (XRD); electron backscatter diffraction (EBSD); Epstein frame testing; single sheet testing; and neutron diffraction.

20. The method according to claim 18 wherein step a) is executed using one of: a destructive testing method; and a non-destructive testing method.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which:

[0038] FIG. 1 is a flow chart showing the lamination process for non-oriented electrical steels according to the prior art;

[0039] FIG. 2 shows a determination of the direction with optimum magnetic properties using MBN and the alignment of the teeth in this direction;

[0040] FIG. 3 shows a determination of the optimum direction by texture measurement: grain-oriented steel and the alignment of the teeth to the RD;

[0041] FIG. 4 shows a determination of the optimum direction by texture measurement: non-oriented electrical steel with a rotated Goss texture and the alignment of the teeth to the TD;

[0042] FIG. 5 shows a determination of the optimum direction by texture measurement: non-oriented electrical steel with a <001>//ND texture and the alignment of the teeth to an angle 45°-φ.sub.1 from RD;

[0043] FIG. 6A shows inward bending of the strips into circular laminates;

[0044] FIG. 6B shows outward bending of the strips into circular laminates;

[0045] FIG. 7 shows cutting slots in the bending locations to minimize the bulging during in-plane bending;

[0046] FIG. 8 shows bending the strip in between two rotating plates with applied pressure on the surfaces of the bending plates to ensure in-plane bending;

[0047] FIG. 9 shows the spring back of the bent strip and the formation of a complete circular laminate;

[0048] FIG. 10A shows double strip bending: two strips bent within a pair of mandrels;

[0049] FIG. 10B shows double strip bending: retracting the mandrels and bending wheel to remove the bent strips;

[0050] FIG. 11 shows the misalignment of teeth between laminates caused by the mis-oriented teeth generated in the bending process;

[0051] FIG. 12 shows the cutting and inward bending preform strip to form a donut-shape sheet;

[0052] FIG. 13 shows punching of the circular donut-shaped sheet to form the final laminate with accurate teeth dimensions and inner and outer diameters (inward bending case);

[0053] FIG. 14 shows the cutting and outward bending preform strip to form a donut-shape sheet;

[0054] FIG. 15 shows punching of the circular donut-shaped sheet to form the final laminate with accurate teeth dimensions and inner and outer diameters (outward bending case); and

[0055] FIG. 16 shows an oriented magnetic core stacked from individual laminates.

DETAILED DESCRIPTION

[0056] The present invention provides a new lamination method where, instead of directly punching circular laminates from the steel sheet with the teeth pointing in all directions, rectangular strips with teeth pointing in a single direction (may not be the TD or RD) are first cut from the steel sheets. The strips are cut in such a way that the teeth are aligned to the direction (in the sheet plane) that has the best magnetic properties. The direction with the best magnetic properties is determined by using a non-destructive evaluation method, i.e. magnetic Barkhausen noise (MBN) analysis or conventional magnetic property measurement methods such as Epstein frame testing, single sheet testing, or other non-destructive evaluation methods. Alternatively, conventional texture analysis methods such as x-ray diffraction (XRD), electron backscatter diffraction (EBSD), and neutron diffraction methods can be used to determine the preferred direction (i.e., with the strongest <100>) through texture analysis of small samples cut from the steel sheets.

[0057] For example, for grain-oriented electrical steel, the texture is Goss ({110}<001>) which has the <001> crystal direction in the rolling direction, the alignment of the teeth in this direction will allow the magnetization in the easy axis of the material and reduce the core loss while improving the magnetic flux density. In this case, the RD should be the teeth direction. The MBN angular measurement on this steel sheet indicates that the largest MBN energy (or MBN root mean square, rms) is in the RD. If the measured texture has a rotated Goss ({110}<110>) which has the <001> direction in the transverse direction, the teeth will be aligned to the TD, i.e. again in the crystal <100> direction (the highest MBN energy). Different from the slinky lamination method where the strip is continuously bent to form a helical core, the new method uses individual laminates to stack up to form the magnetic core. This is to reduce the eddy current loss by using thin steel laminates. In one alternative, the individual laminates have an insulating coating on both sides, similar to traditional laminates that are cut from coated steel sheets.

[0058] More generally, by measuring angular MBN energy (or by measuring magnetic properties using the Epstein frame method or using a single sheet tester) on the steel sheet or measuring the crystallographic texture of cut samples from the sheet, one can identify the direction with the best magnetic properties in the sheet plane and thus align the teeth of the strips in this direction. The strips with unidirectional teeth are then bent either inward or outward (depending on the configuration of the rotator or stator) to form a circular lamination. The two ends of the strip are joined or attached to form a complete circle, forming a conventional laminate. In this way, the teeth (along which the magnetization is applied) will be aligned in the direction having the best magnetic properties, while the waste of material is also minimized.

[0059] The determination of the best magnetic direction in the sheet plane is preferably done by non-destructive MBN by means of angular measurements (0°-360° from the RD, with a fixed interval, e.g. 15°). The direction at which the MBN has the highest energy or rms value is the direction for the magnetization. The MBN measurements can be conducted online during the sheet production, or offline after the sheet has been produced. An example of the MBN angular measurements of the magnetization direction on steel sheet is shown in FIG. 2. Alternatively, the best magnetic direction in the sheet plane can also be determined by conducting conventional (destructive) testing (e.g. Epstein frame method or single sheet testing) using cut samples from the steel sheets.

[0060] Alternatively, the optimum direction can also be determined using conventional texture analysis (destructive) methods, e.g. texture measurements by XRD or EBSD from samples cut from the sheet. In this case, the magnetization direction (the teeth) should be aligned to the direction where the <100> has the largest intensity. A few examples of aligning the strip teeth direction to the preferred crystal directions are shown in FIGS. 3-5.

[0061] The strips cut from the steel sheets are then individually bent to form circular laminates as in conventional lamination processes. This can be performed either inwardly (with teeth pointing to the circle center) or outwardly (with teeth pointing out of the center), depending on the design of the lamination core. FIGS. 6A and 6B shows the bending of individual strips into circular laminates.

[0062] Compared to the conventional stamping and stacking process, the strip cutting and bending process of the present invention may impose challenges to the quality and efficiency of the final manufactured laminates. To overcome the issues facing the new process, the following methods may be used:

[0063] To facilitate the in-plane bending of the strips and to minimize the deformation (bulging) in the normal direction of the strips, slots are cut on both sides of the yoke where bending occurs, to reduce the deformation needed and to localize the bending to the designated area (FIG. 7).

[0064] In one alternative, to maintain a flat surface at the bending area, the bending deformation of the strip is constrained in between two rotating plates with applied pressure to allow only in-plane deformation (FIG. 8). This is to ensure that the bent strips have a flat circular surface.

[0065] The pressure applied to the rotating plates may be divided into two parts. The part of the pressure applied to the center of the plates (P1) will be kept evenly and constantly during the bending process to ensure that, an even slot (with a space equal to the thickness of the strip) between the two plates is maintained. The part of the pressure applied on the periphery of the plates (P2) will be maintained only near the region where the bending deformation occurs so that the bent strip can be released and can spring back out of the mandrel, which will give space for the formation of a complete circle at the end, as shown in FIG. 9. The periphery pressure (P2) is intended to generate appropriate friction force between the strip and the two plates so that the strip is driven by the rotating plates into the slot and deformed.

[0066] To ensure that the bent strip can be easily released from the mandrel between the two plates, the mandrel can be manufactured on one plate only while the other plate has a flat surface (FIG. 8). When pressed together, the slot between the two plates provides the space for in-plane deformation. When the plate with the mandrel is retracted, the bent strip can be freely removed and transferred to the next working location.

[0067] To compensate for the spring back, the mandrel diameter is preferably smaller than the required inner diameter of the core (FIG. 9). The amount of reduction in diameter is determined by the mechanical properties of the material to be bent.

[0068] The rotating plates and the mandrel may be designed in such a way that two strips can be bent in the same time to increase the efficiency (FIG. 10). To remove the bent strips, the mandrels and the passive bending wheel are retracted to free up the bent strips.

[0069] One potential issue of the lamination process of the present invention is the poor accuracy of the teeth (orientation and dimension) generated by the bending process (instead of the punching process). Due to material non-uniformity, the bending process to form the center-pointing or radial-pointing teeth from straight teeth may inevitably induce inaccuracy to the teeth direction. This inaccuracy will induce difficulties in the following assembling process since it will be challenging to obtain accurately aligned teeth for all the laminates (FIG. 11). The inner and outer diameters of the assembled lamination core will not be accurate either, due to the mis-oriented teeth. It is well known that the performance of the motor is closely related to the air gap between the rotor and stator, and it is generally preferable to have a smaller air gap to render a better performance. The dimension inaccuracy of the magnetic core produced using this new method will inevitably affect the motor performance. Thus, a new process, as explained below, is provided to overcome these potential issues.

[0070] In FIG. 11, a small mis-orientation angle δ may induce a large inaccuracy in the diameter of the core and the space for coils.

[0071] The method according to one aspect of the present invention is schematically illustrated in FIGS. 12 to 15. Instead of cutting out the strips with the exact teeth geometry and dimensions, a preform strip is first cut from the steel sheet. Slots are cut on the preform strip at locations where bending occurs to facilitate the bending process. The preform strip is then bent to form a donut-shaped sheet (FIGS. 12 and 14) using the same bending process as described above. The ends are then joined or attached using a conventional method (e.g. resistant welding, friction stir welding or other welding techniques). The circular sheet is then punched using a die (FIGS. 13 and 15) to form the final laminate with accurate teeth dimensions and core diameters (similar to what is used in conventional punching and stacking processes). In this manner, the inaccuracy issue associated with the novel bending process is avoided. FIG. 16 shows the final magnetic core stacked from the laminates manufactured using this novel method.

[0072] The lamination process described above has several advantages over the traditional punching and stacking or the slinky lamination methods: [0073] The waste of the material can be significantly reduced as compared to the traditional punching and stacking method. [0074] It is possible to use existing high-quality grain-oriented electrical steel to manufacture electric motor cores with excellent magnetic properties (comparable to the transformers) in all the directions. [0075] It is able to take the advantages of preferred orientations in the non-oriented electrical steel sheets where the easy <100> direction can be aligned to the magnetization direction. [0076] It can significantly reduce the eddy current loss as compared to the slinky lamination method, in which all the layers of the lamination are physically connected, which will result in much higher losses than the proposed directional lamination method (the laminates are insulated). [0077] The dimension accuracy is much better than the slinky lamination method and is comparable to the traditional punching and stacking method.

[0078] In one aspect, the current invention provides a novel lamination technique so that the magnetization directions of the laminates can be precisely aligned to the direction in which the magnetic properties of a textured steel sheet are the best. This method can be used to manufacture magnetic cores from both grain-orientated and non-oriented electrical steel sheets to optimize the efficiency of the magnetic core while significantly reducing the material waste.

[0079] The present invention may be used in any electric motor lamination core manufacturing, including traction motors for electric vehicles, electric motors for general machines and all kinds of generators including windmills.

[0080] The present invention has a number of differences with known techniques and technologies. Specifically: [0081] The alignment of the teeth to the crystal <001> direction of electrical steel sheet is different from the conventional method that punches the circular laminate directly from the sheet and pointing the teeth to all the directions. [0082] Determining the crystal easy <100> direction before stamping the steel sheets is unique, as this will enable the alignment of the magnetization direction to the easy axis of the material, which will improve the efficiency of the magnetic core. [0083] The use of non-destructive magnetic Barkhausen noise (MBN) analysis to determine the easy magnetization axis of the material is different as compared to the traditional magnetic property or texture measurements, which are destructive. [0084] Using grain-oriented electrical steel to produce high performance electric motor core is different, as this steel is normally used in transformers only. [0085] The way the individual sheet is bent in between two plates to ensure in-plane deformation of the strip thus avoiding bulging during bending is novel. [0086] The way multiple strips can be bent together is unique. [0087] The way that the sheet was cut into preform strips and bent to form donut-shaped sheets is novel. [0088] The way that the donut-shaped sheets were further punched to form the final accurate laminates is novel.

[0089] A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.