Field of the disclosure The disclosure relates to a core element of a magnetic component. The core element can be, for example but not necessarily, a stator or rotor core of an axial magnetic bearing, a stator or rotor core of a combined axial and radial magnetic bearing, a transformer or filter core, or a stator or rotor core of an electric machine. Furthermore, the disclosure relates to a method for manufacturing a core element of a magnetic component.

Background

In many cases, a core element of a magnetic component is implemented as a laminated structure, i.e. a stacked sheet structure, so that the magnetic flux is conducted along the sheets and not through the sheets. The advantage of the laminated structure is that a changing magnetic flux causes significantly less eddy currents than in a corresponding core element made of solid steel. The core element can be, for example but not necessarily, a stator or rotor core of an axial magnetic bearing, a stator or rotor core of a radial magnetic bearing, a stator or rotor core of a combined axial and radial magnetic bearing, a transformer or filter core, a stator or rotor core of an electric machine, or a stator core of a bearingless electric machine where the stator core is used not only for torque production but also for magnetic levitation of the rotor.

An inherent drawback of a laminated structure is that it can be cumbersome to construct core elements where the ferromagnetic sheets need to be curved in mutually intersecting directions in order to achieve a situation where a magnetic flux can flow along the sheets without a need to flow through the sheets. It is straightforward to construct e.g. a stator or rotor core of a radial-flux electric machine by using planar ferromagnetic sheets that are stacked on each other in the axial direction of the electric machine. As well, it is straightforward to construct a core element for a toroidal coil by rolling an elongated ferromagnetic sheet in the same way as a tape is rolled on a roll of tape. However, it is significantly more challenging to construct e.g. a laminated stator core element for an axial magnetic bearing so that a magnetic flux does not need to flow through the ferromagnetic sheets. The stator core element of an axial magnetic bearing is usually a rotationally symmetric element that comprises an annular groove for a coil for conducting electric current. Thus, the ferromagnetic sheets of the above-mentioned core element would have to be curved in mutually intersecting directions in order to achieve a situation where the magnetic flux does not need to flow through the sheets. In many cases, it is not possible or at least it is not cost effective to make laminated core elements in which ferromagnetic sheets that are originally planar need to be shaped in complex ways.

Summary

The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In this document, the word "geometric" when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric line or axis, a geometric plane, a non-planar geometric surface, a geometric room, or any other geometric entity that is one, two, or three dimensional.

In accordance with the invention, there is provided a new method for manufacturing a core element for a magnetic component.

A method according to the invention comprises producing, by three-dimensional "3D" printing, a ferromagnetic structure comprising a plurality of ferromagnetic sections where adjacent ones of the ferromagnetic sections are connected to each other with ferromagnetic isthmuses, where the ferromagnetic structure constitutes at least a part of the core element. As the above-mentioned ferromagnetic structure is manufactured by the three- dimensional printing, it is possible to make core elements which are not possible or at least not cost effective to be made by shaping ferromagnetic sheets which are originally planar. A method according to an exemplifying and non-limiting embodiment of the invention further comprises casting electrically insulating material into the gaps between the ferromagnetic sections. A method according to an exemplifying and non-limiting embodiment of the invention further comprises, subsequently to the casting, cutting openings in the ferromagnetic structure so that at least a part of the ferromagnetic isthmuses are removed. A method according to an exemplifying and non-limiting embodiment of the invention further comprises, subsequently to the cutting, casting electrically insulating material into the openings.

In accordance with the invention, there is provided also a new core element for a magnetic component. The core element can be, for example but not necessarily, a stator or rotor core of an axial magnetic bearing, a stator or rotor core of a radial magnetic bearing, a stator or rotor core of a combined axial and radial magnetic bearing, a transformer or filter core, a stator or rotor core of an electric machine, or a stator core of a bearingless electric machine where the stator core is used not only for torque production but also for magnetic levitation of the rotor. A core element according to the invention comprises a plurality of ferromagnetic sections for conducting magnetic flux, wherein:

- adjacent ones of the ferromagnetic sections are connected to each other with ferromagnetic isthmuses arranged to keep the adjacent ones of the ferromagnetic sections a distance apart from each other, and/or - gaps between the ferromagnetic sections are filled with electrically insulating solid material and adjacent ones of the gaps are connected to each other via openings through the ferromagnetic sections, the openings being filled with electrically insulating solid material. A magnetic component according to the invention comprises a core element according to the invention and at least one winding for conducting electric current so as to generate a magnetic flux conducted by the core element.

A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of un-recited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures

Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 shows a flowchart illustrating methods according to exemplifying and non- limiting embodiments of the invention for manufacturing a core element for a magnetic component. figures 2a, 2b, 2c, 2d, 2e, and 2f illustrate a method for manufacturing a core element according to an exemplifying and non-limiting embodiment of the invention. figures 3a and 3b illustrate a detail of a core element according to an exemplifying and non-limiting embodiment of the invention, figures 4a and 4b illustrate core elements according to an exemplifying and non- limiting embodiment of the invention, figures 5a, 5b, and 5c illustrate core elements according to an exemplifying and non- limiting embodiment of the invention, figures 6a and 6b illustrate a core element according to an exemplifying and non- limiting embodiment of the invention, figures 7a and 7b illustrate a core element according to an exemplifying and non- limiting embodiment of the invention, and figures 8a and 8b illustrate core elements according to an exemplifying and non- limiting embodiment of the invention.

Description of exemplifying and non-limiting embodiments The specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given below are not exhaustive unless otherwise explicitly stated.

Figure 1 shows a flowchart illustrating methods according to exemplifying and non- limiting embodiments of the invention for manufacturing a core element for a magnetic component. An action 101 of the method comprises producing a ferromagnetic structure by three-dimensional "3D" printing. The ferromagnetic structure comprises a plurality of ferromagnetic sections where adjacent ones of the ferromagnetic sections are connected to each other with ferromagnetic isthmuses keeping the adjacent ones of the ferromagnetic sections a distance apart from each other. The ferromagnetic isthmuses and the ferromagnetic sections may constitute a single piece of ferromagnetic material, e.g. steel, so that detaching the ferromagnetic isthmuses from the ferromagnetic sections requires material breaking. The ferromagnetic sections can constitute e.g. a stack of ferromagnetic sheets or a bundle of ferromagnetic filaments. The fact that the ferromagnetic sections are kept apart from each other reduces eddy currents caused by a changing magnetic flux in the ferromagnetic structure.

In a method according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned ferromagnetic structure constitutes the core element. In this exemplifying case, the gaps between the ferromagnetic sections are filled by ambient air or by other gas or liquid surrounding the core element.

A method according to an exemplifying and non-limiting embodiment of the invention further comprises an optional action 102 in which first electrically insulating material is cast into the gaps between the ferromagnetic sections. The first electrically insulating material filling the gaps improves the mechanical strength of the core element. The first electrically insulating material can be for example resin or plastics.

A method according to an exemplifying and non-limiting embodiment of the invention further comprises an optional action 103 in which openings are cut in the ferromagnetic structure so that at least a part of the ferromagnetic isthmuses are removed. This further reduces eddy currents caused by a changing magnetic flux in the core element.

A method according to an exemplifying and non-limiting embodiment of the invention further comprises an optional action 104 in which second electrically insulating material is cast into the above-mentioned openings. The second electrically insulating material filling the openings improves the mechanical strength of the core element. The second electrically insulating material can be for example resin or plastics. The second electrically insulating material can be the same as the above-mentioned first electrically insulating material.

Figures 2a, 2b, 2c, 2d, 2e, and 2f illustrate a method for manufacturing a core element in an exemplifying case where the method comprises the above-mentioned actions 101 -104 shown in the flowchart of figure 1 . The resulting core element is denoted with a reference 21 1 in figure 2f. Figures 2a and 2b illustrate the ferromagnetic structure made by the 3D printing in the action 101 . Figure 2a shows a section view of a part of the ferromagnetic structure so that the section is taken along a geometric line A2-A2 shown in figure 2b and the geometric section plane is parallel with the xz-plane of a coordinate system 290. Figure 2b shows a section view so that the section is taken along a geometric line A1 -A1 shown in figure 2a and the geometric section plane is parallel with the xy-plane of the coordinate system 290. The ferromagnetic structure comprises a plurality of ferromagnetic sections that are connected to each other with ferromagnetic isthmuses that keep the adjacent ones of the ferromagnetic sections a distance apart from each other. In figure 2a, four of the ferromagnetic sections are denoted with references 216, 217, 218, and 219 and two of the ferromagnetic isthmuses are denoted with references 220 and 221 . Figure 2b shows a part of the ferromagnetic section 217 and cross-sections of four of the ferromagnetic isthmuses. As can be seen from figures 2a and 2b, the ferromagnetic sections are ferromagnetic sheets that are kept apart from each other by the ferromagnetic isthmuses so that there are gaps between adjacent ones of the ferromagnetic sheets. In figure 2a, one of the gaps is denoted with a reference 222.

Figure 2c illustrates the result of the action 102 shown in the flowchart of figure 1 . Figure 2c shows a section view in the same way as figure 2a. In figure 2c, the first electrically insulating material which has been cast into the gaps between the ferromagnetic sections is denoted with a reference 223. Figures 2d and 2e illustrate the result of the action 103 shown in the flowchart of figure 1 . Figure 2d shows a view of a section taken along a geometric line A4-A4 shown in figure 2e, where the geometric section plane is parallel with the xz-plane of the coordinate system 290. Figure 2e shows a view of a section taken along a geometric line A3-A3 shown in figure 2d, where the geometric section plane is parallel with the xy-plane of the coordinate system 290.

In figures 2d and 2e, one of the openings that have been cut in the ferromagnetic structure is denoted with a reference 224. In this exemplifying case, as can be seen from figure 2a and 2d, the ferromagnetic isthmus 220 has been removed by cutting whereas the ferromagnetic isthmus 221 has been left unremoved. The openings can be cut for example by drilling, laser cutting, water jet cutting, and/or with any other suitable cutting method.

Figures 2f illustrates the result of the action 104 shown in the flowchart of figure 1 . Figure 2f shows a section view in the same way as figure 2d. In figure 2f, the second electrically insulating material which has been cast into the above-mentioned openings is denoted with a reference 225. The second electrically insulating material 225 cast into the cut openings can be the same as the first electrically insulating material 223 that has been cast into the gaps between the ferromagnetic sections.

For the sake of illustrative purposes, the z-directional widths of the gaps between the ferromagnetic sheets are exaggerated in figures 2a-2f with respect to the thicknesses of the ferromagnetic sheets. In practical cases, the thicknesses of the ferromagnetic sheets are advantageously significantly greater than the widths of the gaps between the ferromagnetic sheets. Additionally, the thickness of the gaps between the ferromagnetic sheets and/or the thickness of the ferromagnetic sheets can vary at different points on the cross-section plane and at different cross-section planes.

Figures 3a and 3b illustrate a detail of a core element 31 1 according to an exemplifying and non-limiting embodiment of the invention. Figure 3a shows a section view of a part of the core element 31 1 so that the section is taken along a geometric line A2-A2 shown in figure 3b and the geometric section plane is parallel with the xz-plane of a coordinate system 390. Figure 3b shows a section view so that the section is taken along a geometric line A1 -A1 shown in figure 3a and the geometric section plane is parallel with the yz-plane of the coordinate system 390. The ferromagnetic structure of the core element 31 1 comprises a plurality of ferromagnetic sections that are connected to each other with ferromagnetic isthmuses arranged to keep the adjacent ones of the ferromagnetic sections a distance apart from each other. In figure 3a, four of the ferromagnetic sections are denoted with references 316, 317, 318, and 319. In this exemplifying case, the ferromagnetic sections are ferromagnetic filaments arranged to constitute a bundle of filaments. The gaps between the ferromagnetic filaments are filled with electrically insulating material 323. The ferromagnetic filaments are capable conducting a magnetic flux along the ferromagnetic filaments. The ferromagnetic filaments may have curved shapes for implementing core elements having desired shapes. Furthermore, the cross-sectional area and/or the cross-sectional shape of a ferromagnetic filament may vary along the longitudinal direction of the ferromagnetic filament under consideration. For the sake of illustrative purposes, the y- and z- directional widths of the gaps between the ferromagnetic filaments are exaggerated in figures 3a and 3b with respect to the y- and z-directional thicknesses of the ferromagnetic filaments. In practical cases, the thicknesses of the ferromagnetic filaments are advantageously significantly greater than the widths of the gaps between the ferromagnetic filaments.

Core elements that are manufactured by the above-illustrated methods can be used in many different applications. Some exemplifying and non-limiting applications are presented below with reference to figures 4a, 4b, 5a, 5b, 5c, 6a, 6b, 7a, and 7b. It is, however, to be noted that core elements manufactured by the above-illustrated methods can be used in many other applications, too.

Figures 4a and 4b illustrate core elements 41 1 , 412, 413, and 414 according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the core elements 41 1 and 414 constitute a stator core of an axial magnetic bearing and the core elements 412 and 413 constitute a rotor core of the axial magnetic bearing. The axial direction of the axial magnetic bearing is parallel with the z-axis of a coordinate system 490. Figure 4b shows a section view of the axial magnetic bearing. The section is taken along a geometric line A-A shown in figure 4a and the geometric section plane is parallel with the xz-plane of the coordinate system 490. As can be seen from figure 4b, the ferromagnetic sections of the core elements 41 1 -414 are rotationally symmetric ferromagnetic sheets which have substantially U-shaped cross-sections on geometric planes which coincide with the geometric rotational axis of a shaft 450. The core elements 412 and 413 are connected to the shaft 450 with the aid of a disc 434. The axial magnetic bearing comprises annular windings 426 and 427 having coil turns surrounding the shaft 450. As can be understood from figure 4b, the magnetic fluxes generated by circumferential electric currents flowing in the annular windings 426 and 427 can flow along the above-mentioned ferromagnetic sheets and the magnetic fluxes do not need to flow through the ferromagnetic sheets. Correspondingly, the magnetic fluxes do not need to substantially flow through solid iron or other materials separated from, and being in a different construction than, the above-mentioned ferromagnetic sheets. Figures 5a, 5b, and 5c illustrate core elements 51 1 , 512, 513, and 514 according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the core elements 51 1 and 514 constitute a stator core of a magnetic bearing that is a combined radial and axial magnetic bearing, and the core elements 512 and 513 constitute a rotor core of the magnetic bearing. The core elements 512 and 513 are attached on a shaft 550. The axial direction of the magnetic bearing is parallel with the z-axis of a coordinate system 590. Figure 5b shows a section view of the magnetic bearing so that the section is taken along a geometric line A1 -A1 shown in figure 5a and the geometric section plane is parallel with the xz-plane of the coordinate system 590. Figure 5c shows a section view of the magnetic bearing so that the section is taken along a geometric line A2-A2 shown in figure 5b and the geometric section plane is parallel with the xy-plane of the coordinate system 590. In figure 5c, an axially directed air-gap surface of the core element 512 is denoted with a reference 535.

As can be seen from figures 5b and 5c, the ferromagnetic sections of the core elements 512 and 513 of the rotor side are rotationally symmetric ferromagnetic sheets which have substantially L-shaped cross-sections on geometric planes which coincide with the geometric rotational axis of the shaft 550. As can be seen from figures 5b and 5c, the ferromagnetic sections of the core elements 51 1 and 514 of the stator side are ferromagnetic sheets that are otherwise rotationally symmetric except that the core elements 51 1 and 514 are segmented into four segments and that the core elements 51 1 and 514 comprise radially directed pole-portions for pole windings 528, 529, 530, and 531 . It is also possible that the number of segments of the kind mentioned above is different from four. For example, the number of the segments can be three, six, eight, or some other suitable number. The pole windings 528-531 are suitable for directing radial magnetic forces to the core elements 512 and 513 of the rotor side. The magnetic bearing comprises annular windings 526 and 527 having coil turns surrounding the shaft 550. The annular windings 526 and 527 are suitable for directing axial magnetic forces to the core elements 512 and 513 of the rotor side. As can be understood from figures 5b and 5c, the magnetic fluxes generated by circumferential electric currents flowing in the annular windings 526 and 527 can flow along the ferromagnetic sheets of the core elements 51 1 -514 and the magnetic fluxes do not need to flow through the ferromagnetic sheets. Correspondingly, the magnetic fluxes generated by electric currents flowing in the pole windings 528-531 can flow along the ferromagnetic sheets of the core elements 51 1 -514 and the magnetic fluxes do not need to flow through the ferromagnetic sheets. It is worth noting that the above-mentioned annular windings 526 and 527 are not the only possible choice for directing axial magnetic forces to the core elements 512 and 513 of the rotor side. For example, the annular windings 526 and 527 can be replaced with a combination of segment-specific windings each being wound around one segment of the respective core element so that the segment- specific windings protrude through the gaps between adjacent segments.

Figures 6a and 6b illustrate a core element 61 1 according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the core element 61 1 constitutes a rotor core of a magnetic bearing that is a combined radial and axial magnetic bearing. The axial direction of the magnetic bearing is parallel with the z-axis of a coordinate system 690. Figure 6a shows a section view of the magnetic bearing so that the section is taken along a geometric line A2-A2 shown in figure 6b and the geometric section plane is parallel with the yz-plane of the coordinate system 690. Figure 6b shows a section view of the magnetic bearing so that the section is taken along a geometric line A1 -A1 shown in figure 6a and the geometric section plane is parallel with the xy-plane of the coordinate system 690. The magnetic bearing is capable of directing an axial magnetic force to the core element 61 1 only in the positive z-direction of the coordinate system 690. Thus, there is a need for an arrangement such that an axial force is directed to the core element 61 1 in the negative z-direction, too.

The magnetic bearing comprises windings for generating magnetic fluxes that flow in stator core elements 612, 613, 614, and 615 of the magnetic bearing and in the core element 61 1 of the rotor side. In figures 6a and 6b, one of the windings is denoted with a reference 627. The stator core elements 612-615 can be constructed for example by stacking electrically insulated planar ferromagnetic sheets on each other. It is, however, also possible that the stator core elements 612-615 are manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. The magnetic bearing comprises permanent magnets for generating bias magnetic fluxes for linearizing the control of the magnetic bearing. In figures 6a and 6b, two of the permanent magnets are denoted with references 636 and 637. In figure 6a, two of the bias magnetic fluxes are depicted with dashed lines.

As illustrated in figure 6b, the ferromagnetic sections of the core element 61 1 are ferromagnetic sheets which are parallel with geometric planes coinciding with the geometric rotation axis of a shaft 650. The ferromagnetic sheets of the core element 61 1 are wedge-shaped so that the thicknesses of the ferromagnetic sheets decrease towards the geometric rotation axis of the shaft 650. As can be understood on the basis of figures 6a and 6b, the magnetic fluxes generated by the permanent magnets and by electric currents flowing in the windings can flow along the ferromagnetic sheets of the stator core elements 612-615 and along the ferromagnetic sheets of the core element 61 1 of the rotor side, and the magnetic fluxes do not need to flow through the ferromagnetic sheets. The core element 61 1 can be provided with a support ring so as to improve the mechanical strength of the core element. In figure 6a, the support ring is denoted with a reference 638. For the sake of clarity, the support ring is not shown in figure 6b. The support ring 638 may comprise for example metal, glass fiber reinforced resin or plastic, carbon fiber reinforced resin or plastic, or some other suitable material capable of withstanding mechanical stress. In the exemplifying case illustrated in figures 6a and 6b, the number of the stator core elements 612-615 is four but it is also possible that the number of corresponding stator core elements is for example three, six, or some other suitable number.

Figures 7a and 7b illustrate a detail of a core element 71 1 according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the core element 71 1 is a stator core of a radial-flux electric machine. The axial direction of the radial-flux electric machine is parallel with the z-axis of a coordinate system 790. Figure 7a shows a section view of a part of the core element 71 1 so that the section is taken along a geometric line A2-A2 shown in figure 7b and the geometric section plane is parallel with the xy-plane of the coordinate system 790. Figure 7b shows a section view of the above-mentioned part of the core element 71 1 so that the section is taken along a geometric line A1 -A1 shown in figure 7a and the geometric section plane is parallel with the yz-plane of the coordinate system 790. The core element 71 1 comprises teeth for conducting magnetic fluxes in radial directions and a yoke connecting the teeth to each other. Adjacent ones of the teeth constitute grooves for coil sides of the windings of the electric machine. Figure 7a shows a section view of one of the above-mentioned teeth. Furthermore, figure 7a shows section views of parts of coil sides 726 and 727. Figure 7b shows a section view of the above-mentioned tooth, a part of the coil side 726, and a section view of an end-winding 740.

As illustrated in figure 7b, the ferromagnetic sections of the core element 71 1 are ferromagnetic sheets which are stacked in the axial direction of the electric machine. As illustrated in figure 7b, the ferromagnetic sheets which constitute axial end- regions of the core element 71 1 are thicker on the tooth-tip regions 733 of the teeth of the core element 71 1 than elsewhere within the core element so that tooth-tips of the teeth protrude axially at the axial end-regions of the core element. Thus, the tooth-tips are extended not only circumferentially as shown in figure 7a but also in the axial directions as illustrated in figure 7b. Thus, the magnetic flux density in the air-gap where the magnetic flux harmonics are strongest can be reduced. As a corollary, the iron losses taking place in the tooth-tips and other areas in the vicinity of the air-gap can be reduced.

The principle described above with reference to figures 7a and 7b is as well applicable in a rotor core of a radial-flux electric machine. Figures 8a and 8b illustrate core elements 812, 813, 814, and 815 according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the core elements 812-815 constitute a stator core of a magnetic bearing that is a combined radial and axial magnetic bearing. A rotor core element 81 1 can be similar to the core element 61 1 illustrated in figures 6a and 6b. The axial direction of the magnetic bearing is parallel with the z-axis of a coordinate system 890. Figure 8a shows a section view of the magnetic bearing so that the section is taken along a geometric line A2-A2 shown in figure 8b and the geometric section plane is parallel with the yz-plane of the coordinate system 890. Figure 8b shows a section view of the magnetic bearing so that the section is taken along a geometric line A1 -A1 shown in figure 8a and the geometric section plane is parallel with the xy-plane of the coordinate system 890. The magnetic bearing is capable of directing an axial magnetic force to the rotor core element 81 1 only in the positive z-direction of the coordinate system 890. Thus, there is a need for an arrangement such that an axial force is directed to the rotor core element 81 1 in the negative z-direction, too.

The magnetic bearing comprises windings for generating magnetic fluxes that flow in the core elements 812-815 of the magnetic bearing and in the rotor core element 81 1 . In figures 8a and 8b, one of the windings is denoted with a reference 827. The magnetic bearing further comprises permanent magnets for generating bias magnetic fluxes for linearizing the control of the magnetic bearing. In figure 8a two of the permanent magnets are denoted with references 836 and 837. In figure 8a, two of the bias magnetic fluxes are depicted with dashed lines.

As illustrated in figure 8b, the ferromagnetic sections of the core elements 812-815 are ferromagnetic sheets which are substantially U-shaped when seen along the z- axis of the coordinate system 890. The magnetic flux components created by electric currents of the windings flow mostly through the radial paths of the yoke sections of the core elements 812-815 because of the higher reluctance of the permanent magnets on the axial paths of the yoke sections of the core elements 812-815. The rotor core element 81 1 can be provided with a support ring so as to improve the mechanical strength of the core element. In figure 8a, the support ring is denoted with a reference 838. For the sake of clarity, the support ring is not shown in figure 8b. The support ring 838 may comprise for example metal, glass fiber reinforced resin or plastic, carbon fiber reinforced resin or plastic, or some other suitable material capable of withstanding mechanical stress. In the exemplifying case illustrated in figures 8a and 8b, the number of the core elements 812-815 is four but it is also possible that the number of corresponding core elements is for example three, six, or some other suitable number.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.