CN111834117A

CN111834117A - Sintered flux-focused permanent magnet manufacturing with apparatus having asymmetrically formed magnetic devices

Info

Publication number: CN111834117A
Application number: CN201910328872.3A
Authority: CN
Inventors: Z.阿扎尔; 黄清芳; H-J.图加尔德; 童庆坤; A.C.乌尔达
Original assignee: Siemens Gamesa Renewable Energy AS
Current assignee: Siemens Gamesa Renewable Energy AS
Priority date: 2019-04-23
Filing date: 2019-04-23
Publication date: 2020-10-27
Also published as: EP3939060A2; WO2019219984A2; WO2019219984A3

Abstract

An apparatus (460, 560) and method for manufacturing a sintered flux-focusing permanent magnet (250, 350) are described. The apparatus includes (a) a mold (470, 570); and (b) a first magnetic device (461, 561-1, 561-2) and a second magnetic device (464, 564). The first magnetic means (461; 561-1, 561-2) comprises a first yoke structure (463; 563-1, 563-2) having a first shape and the second magnetic means (464, 564) comprises a second yoke structure (466, 566) having a second shape. The second shape is spatially different from the first shape. Furthermore, the spatial difference in the shape of the two yoke structures (463, 466; 563-1, 563-2, 566) causes a non-uniform magnetic field, such that the magnetic field within the mold cavity (472, 572) is associated with a spread angle distribution of the flux lines. Further described are sintered flux-focusing permanent magnets (250, 350) and electromechanical transducers (140) and wind turbines (100) comprising sintered flux-focusing permanent magnets (250, 350) manufactured with the method.

Description

Sintered flux-focused permanent magnet manufacturing with apparatus having asymmetrically formed magnetic devices

Technical Field

The invention relates to an apparatus and a method for manufacturing a sintered flux-focusing permanent magnet. Furthermore, the invention relates to a sintered flux-focusing permanent magnet manufactured with said method, as well as an electromechanical transducer and a wind turbine comprising at least one such sintered magnet.

Background

Permanent magnetic materials are used in a number of different fields of application. Perhaps the most technically and economically important application areas are electromechanical transducers, i.e. motors and generators. An electric motor equipped with at least one Permanent Magnet (PM) converts electrical energy into mechanical energy by generating a temporarily varying magnetic field by means of windings or coils. This temporarily varying magnetic field interacts with the magnetic field of the PM, which is generated, for example, in the rotational movement of the rotor assembly relative to the stator assembly of the electric motor. In a physically complementary manner, the generator converts mechanical energy into electrical energy.

A generator is a core component of any power plant for producing electrical energy. This applies to power plants that capture mechanical energy directly, for example hydroelectric power plants, tidal power plants and wind power plants also known as wind turbines. However, this also applies to power plants which (i) firstly use chemical energy, for example from burning fossil fuels or from nuclear energy, in order to generate thermal energy and (ii) secondly convert the generated thermal energy into mechanical energy by means of suitable thermodynamic processes.

The efficiency of the generator is probably the most important factor for optimizing the production of electrical energy. For PM generators, it is necessary that the magnetic flux generated by the Permanent Magnets (PM) is strong. This can be best achieved with sintered rare earth magnets, for example, using NdFeB material compositions. However, the spatial magnetic field distribution produced by the PM also has an effect on the generator efficiency. In the latter case, it is often advantageous when using a PM device or PM arrangement with a non-uniform magnetic domain alignment pattern, which intentionally creates a non-uniform magnetic field strength or flux density, particularly in the air gap between the rotor assembly and the stator assembly.

It is known to arrange a non-uniform magnetic domain arrangement pattern in the PM in order to achieve so-called "magnetic flux focusing". WO2012/141932a2 discloses a PM magnet arrangement in which differently magnetized PMs are combined such that "magnetic focusing" is achieved. EP 3276642 a1 discloses a sintered rare earth PM having a focused magnetic alignment pattern with an integrally formed or one-piece PM body. EP 2762838 a2 discloses an apparatus and a method for manufacturing PMs, wherein an inhomogeneous external magnetic field is applied during the sintering process in order to magnetize different regions of the PM in different directions. In this document, the flux focusing permanent magnet is abbreviated FFPM.

In order to manufacture the FFPM, an apparatus for generating a suitable non-uniform magnetic field existing within the cavity so as to magnetize the magnetic powder compressed within the cavity is required.

Disclosure of Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the invention are described by the dependent claims.

According to a first aspect of the present invention, there is provided an apparatus for manufacturing a sintered flux-focusing permanent magnet. The provided apparatus includes: (a) a die having a die cavity for receiving a powder of permanent magnet material; and (b) first and second magnetic means for generating a magnetic field for magnetizing the powder contained within the die cavity. The first magnetic means comprises a first (magnetic) yoke structure having a first shape and the second magnetic means comprises a second (magnetic) yoke structure having a second shape which is spatially different from the first shape. Furthermore, the spatial difference in the shape of the two yoke structures causes the magnetic field to be non-uniform, such that the magnetic field within the die cavity is associated with an angular spread distribution of the flux lines.

The described device is based on the following idea: by choosing a spatially asymmetric design of the magnet system which comprises two magnetic means and which serves to magnetize the (compressed) powder present in the mould cavity, it is easy to generate a suitable non-uniform magnetic field at least in the region of the mould cavity.

The described spread angle distribution of magnetic flux lines can be used in particular to produce a spread angle distribution of magnetic domain alignment directions within a Flux Focusing Permanent Magnet (FFPM) to be produced, which spread angle distribution produces focused magnetization of the sintered compact. Thus, outside a substantial part of the FFPM, a magnetic focus or at least a magnetic focusing area may be defined. In this point or in this region, the magnetic flux density caused by the respective FFPM is increased as compared to a point or region from the focal point and outside the respective focusing region.

By designing the shape and/or geometry of the yoke structure in a suitable (asymmetric) way, a spread angle distribution of flux lines can be generated, which results in a desired focused flux magnetization design.

The yoke structure, which may also be referred to as a pole piece, may be made of a ferromagnetic material, in particular iron or cobalt iron, for higher saturation. In contrast, the mold may be made of a non-magnetic material, and in particular a non-ferromagnetic material. The (currently) preferred mold material is stainless steel. However, other mold materials provided with mechanical rigidity may also be used.

According to an embodiment of the invention, (a) at least one of the two magnetic devices comprises at least one electromagnetic coil for generating at least a part of the magnetic field, and (b) the respective yoke structure is configured for guiding and/or for shaping the magnetic field generated by the respective electromagnetic coil.

The use of at least one electromagnetic coil supported by a suitable yoke structure to generate the magnetic field may provide the advantage that the magnetic field and the corresponding magnetic flux (density) may be significantly increased at least in selected regions of the mould cavity.

By setting the current through the respective coils, the strength of the magnetic field generated by each magnetic device can be adjusted appropriately. By independently varying the current flowing through each coil, not only the strength but also the direction of the magnetic field lines may be varied towards the desired characteristics of the generated magnetic field.

According to another embodiment of the invention, the device comprises the following features:

(A) with respect to the mold cavity, the first yoke structure and the second yoke structure are located at opposite sides, and (B) the first yoke structure has a first outer yoke surface facing the mold cavity, and the second yoke structure has a second outer surface facing the mold cavity, wherein the first outer yoke surface has a different curvature than the second outer yoke surface.

By designing the respective outer yoke surfaces in a suitable manner, the direction of the magnetic field lines emerging from the respective yoke can be selected. Thus, the degree of expansion of the angular distribution of the flux lines can be adjusted in an easy and efficient manner.

According to another embodiment of the invention, the first outer yoke surface has (at least in a portion of the first outer yoke surface) a first radius of curvature and the second outer yoke surface has (at least in a portion of the second outer yoke surface) a second radius of curvature different from the first radius of curvature.

V: this may provide the advantage that: a higher alignment angle at the side edges of the FFPM and with respect to the magnetic domain alignment direction of the side edges of the FFPM can be achieved.

According to another embodiment of the invention, the mold cavity comprises one of the following features with respect to the two outer yoke surfaces:

(A) the first outer yoke surface (463 a) is convex and the second outer yoke surface (466 a) is convex;

(B) the first outer yoke surface is convex and the second outer yoke surface is concave;

(C) the first outer yoke surface is concave and the second outer yoke surface is convex; and

(D) the first outer yoke surface is concave and the second outer yoke surface is concave.

By choosing a suitable curvature for the two outer yoke surfaces, a suitably and well-defined spread angle distribution of the magnetic flux lines can be generated in a simple and efficient manner. The curvature of the outer yoke surface may be regular (without any corners and edges ("nubs and bumps") or irregular, depending on the particular application.

At least one of the two outer yoke surfaces may have an at least approximately cylindrical shape. This produces a one-dimensional (1D) flux focus, which produces a linearly extending focal region. This magnetic focusing corresponds to optical focusing by means of a cylindrical optical lens. Alternatively, at least one of the outer yoke surfaces may have an at least approximately spherical shape. This causes a two-dimensional (2D) magnetic flux focus, which produces at least an approximate focus. This magnetic focusing corresponds to optical focusing by means of a spherical optical lens.

According to another embodiment of the invention, the first yoke structure comprises a first yoke structure and a second yoke structure, the two yoke structures being spatially separated from each other. In this configuration of the above-mentioned specific features of the device, according to which the first shape differs from the second shape is realized by two sub-shapes, which are spatially separated from each other.

The use of at least three yoke structures, in addition to two yoke structures, may allow for the generation of a highly spatially inhomogeneous magnetic field within the mold cavity by "splitting" the first yoke structure into two yoke structures. This may be particularly advantageous when producing sintered magnet blocks for FFPM having a small focal length. Thus, the focal length may be defined as the distance between the focal point or area and the major surface of the FFPM.

It is to be mentioned that there is no major limitation on the number of sub-structures. The second yoke structure may also be subdivided into at least two sub-structures.

It is further to be mentioned that the yoke structure and/or the yoke structure have the same (outer) shape or that at least two of them have different (outer) shapes. In this respect it is to be noted that the above defined feature is achieved by dividing the (entire) first outer yoke surface, which according to the above defined feature has a different curvature than the second outer yoke surface, into two sub-surfaces, each sub-surface being assigned to one yoke structure, when all outer shapes are geometrically identical.

In case now at least one of the at least three yoke (sub-) structures is magnetized by means of a magnetic field generated by an electromagnetic coil, there is an opportunity to modify (the inhomogeneity of) the overall magnetic field by selecting an appropriate current flowing through the electromagnetic coil. This may allow the described apparatus to be used to manufacture FFPMs having different flux focusing characteristics.

According to another aspect of the invention, a method for manufacturing a sintered flux-focusing permanent magnet by means of the above-described apparatus is provided. The provided method comprises the following steps: (a) filling permanent magnet material powder into a die cavity of a die; (b) generating a magnetic field for magnetizing the powder contained in the die cavity by means of a first magnetic means and a second magnetic means; (c) compacting the powder contained in the mould cavity by means of a mould, magnetizing and compacting to produce a magnetized compacted mass of powder; (d) sintering the magnetized compacted mass in a sintering furnace that produces a flux-focused permanent magnet; and (e) removing the flux-focusing permanent magnet from the sintering furnace.

Furthermore, the described method is based on the idea that: with a suitable spatially asymmetric design of the magnet system, which comprises two magnetic means and is used for magnetizing the (compressed) powder, it is very easy to produce FFPM with a desired diffusion angle distribution of the domain alignment direction.

It is to be mentioned that the step of generating a magnetic field in order to obtain the magnetic alignment and the step of compacting the powder are usually at least partially done simultaneously.

It is further mentioned that after the sintering step further or post-treatment may be performed to end up with FFPM pieces, which may be used for e.g. rotor assemblies of electrical generators. Such post-processing may include, for example, surface finishing to smooth the surface of the sintered material and/or applying an outer surface layer, which makes the surface of the FFPM sheet less sensitive to external impacts. Further, the post-processing may include shaping of the sintered FFPM so as to have a desired geometry. Such shaping may be achieved, for example, by conventional milling.

According to another embodiment of the invention, the permanent magnet material comprises a rare earth material, in particular NdFeB. This may provide the following advantages: very strong PMs can be manufactured without the need to generate large amounts of waste (rare earth materials are typically very expensive) for achieving the desired PM geometry.

It is to be mentioned that other components of the permanent magnet material may comprise ferrite and/or SmCo.

According to another aspect of the present invention, there is provided a sintered flux-focusing permanent magnet manufactured by implementing the above-described method.

According to another aspect of the invention, an electromechanical transducer, in particular a generator, is provided. An electromechanical transducer is provided that includes a stator assembly and a rotor assembly. The rotor assembly comprises a support structure and at least one sintered flux focusing permanent magnet, as described above, wherein the flux focusing permanent magnet is mounted to the support structure.

By designing the generator in such a way that the focal point and the corresponding focal zone are located within the air gap between the stator and rotor assemblies, the power that can be generated by the generator can be significantly increased.

According to another aspect of the invention, a wind turbine for generating electrical power is provided. A wind turbine is provided comprising (a) a tower, (b) a wind rotor arranged at a top portion of the tower and comprising at least one blade; and (c) an electromechanical transducer as described above. The electromechanical transducer is mechanically coupled to the wind rotor.

The provided wind turbine, also called wind energy installation, is based on the following idea: the electromechanical transducer described above allows to realize a wind turbine with improved efficiency in the conversion of mechanical power into electrical power, which conversion is done by a generator comprising FFPM. This may help to increase the attractiveness of wind turbine technology for regenerative power production compared to other technologies, such as solar power plants.

It is noted that embodiments of the present invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims, whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject-matter also any combination between features relating to different subject-matters, in particular between features of the method type claims and features of the apparatus type claims, is considered to be disclosed with this document.

The above and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Drawings

FIG. 1 shows a wind turbine according to an embodiment of the invention.

Fig. 2 shows a generator of the wind turbine of fig. 1 in a schematic view.

FIG. 3 illustrates a Flux Focusing Permanent Magnet (FFPM) made according to an embodiment of the present invention.

Fig. 4 shows an apparatus for manufacturing FFPM having two magnetic devices with differently shaped yoke structures.

Fig. 5 shows an apparatus for manufacturing FFPM, which includes three magnetic devices for unevenly magnetizing the magnetic material powder in the cavity.

Detailed Description

The figures in the accompanying drawings are schematic representations. It is noted that in different figures, similar or identical elements or features are provided with the same reference signs or with reference signs, which differ from the corresponding reference signs only within a first digit. In order to avoid unnecessary repetition, elements or features that have been elucidated with respect to the previously described embodiments are not again elucidated at a later position in the description.

FIG. 1 shows a wind turbine 100 according to an embodiment of the invention. The wind turbine 100 comprises a tower 120, the tower 120 being mounted on a foundation, not shown. On top of the tower 120 a nacelle 122 is arranged. Between the tower 120 and the nacelle 122 a yaw angle adjusting device 121 is arranged, which yaw angle adjusting device 121 is capable of rotating the nacelle 122 about a not shown vertical axis aligned with the longitudinal extension of the tower 120. By controlling the yaw angle adjustment arrangement 121 in a suitable manner, it may be ensured that during normal operation of the wind turbine 100 the nacelle 122 is always properly aligned with the current wind direction.

Wind turbine 100 further includes a wind rotor 110 having three blades 114. In the perspective view of fig. 1, only two blades 114 are visible. The rotor 110 is rotatable about a rotation axis 110 a. Blades 114 mounted at hub 112 extend radially with respect to rotational axis 110 a.

Between the hub 112 and the blades 114, blade angle adjustment means 116 are provided, respectively, for adjusting the blade pitch angle of each blade 114 by rotating the respective blade 114 about a not shown axis, which is aligned substantially parallel to the longitudinal extension of the respective blade 114. By controlling the blade angle adjustment devices 116, the blade pitch angle of the respective blades 114 may be adjusted such that the maximum wind power may be extracted from the available mechanical power of the wind driven wind rotor 110, at least when the wind is not too strong.

As can be seen in fig. 1, a gear box 124 is provided within nacelle 122. The gearbox 124 serves to convert the number of revolutions of the rotor 110 into a higher number of revolutions of the shaft 125, the shaft 125 being coupled to an electromechanical transducer 130 in a known manner. The electromechanical transducer is a generator 130.

At this point, it is noted that the gearbox 124 is optional and that the generator 140 may also be coupled directly to the rotor 110 via the shaft 125 without changing the number of revolutions. In this case, the wind turbine is a so-called Direct Drive (DD) wind turbine.

Furthermore, a brake 126 is provided in order to stop the operation of the wind turbine 100 or in order to reduce the rotational speed of the rotor 110, for example in case of an emergency.

The wind turbine 100 further comprises a control system 143 for operating the wind turbine 100 in an efficient manner. In addition to controlling, for example, the yaw angle adjusting device 121, the depicted control system 153 serves to adjust the blade pitch angle of the rotor blades 114 in an optimized manner.

According to the basic principles of electrical engineering, the generator 130 includes a stator assembly 135 and a rotor assembly 140. In the embodiment described herein, the generator 130 is implemented in a so-called "inner stator-outer rotor" configuration, wherein the rotor assembly 140 surrounds the stator assembly 135. This means that the not shown permanent magnets of the rotor assembly 140 and the corresponding magnet assembly travel around an arrangement of a plurality of not shown coils of the inner stator assembly 135 which generate induced currents resulting from picking up time varying magnetic flux from the traveling permanent magnets.

According to embodiments described herein, each Permanent Magnet (PM) assembly includes at least three sintered Flux Focusing Permanent Magnets (FFPMs) made of a Nd-Fe-B material composition.

Fig. 2 shows a schematic view of the generator 130 in a sectional view. The generator 130 includes a stator assembly 135. The stator assembly 135 includes a stator support structure 237, the stator support structure 237 including a stack of a plurality of laminations and a plurality of stator windings 239 housed within the stator support structure 237. The windings 239 are interconnected in a known manner by means of electrical connections not shown.

The rotor assembly 140 of the generator 130 is separated from the stator assembly 135 by an air gap ag, the rotor assembly 140 including a rotor support structure 242, the rotor support structure 242 providing a mechanical base for mounting a plurality of FFPMs 250. In fig. 2, the axis of rotation of the rotor assembly 140 is indicated by reference numeral 230 a.

In the exemplary embodiment described herein, three FFPMs are disposed adjacent to each other at each angular position of the rotor assembly 140. It is noted that in fig. 2, for ease of illustration, only three FFPMs 250 are shown assigned to one angular position. In practice, a plurality of FFPMs 250 are mounted to the rotor support structure 242 depending on the size of the generator 130. The FFPMs 250 are preferably arranged in a matrix-like configuration around a curved surface area of the support structure 242 having a substantially cylindrical geometry about the generator axis 240 a.

As can be seen in fig. 2, FFPM250 is not mounted directly to rotor support structure 242. Instead, a back plate 244 made of a ferromagnetic material, such as iron, is provided. A back plate 244 is provided to ensure proper flux guidance. This significantly reduces the strength of the magnetic stray field in an advantageous manner.

Fig. 3 illustrates in more detail a Flux Focusing Permanent Magnet (FFPM) 350 made in accordance with an embodiment of the present invention.

The FFPM 350 is magnetized in a manner that gives a spread angle distribution of the magnetic domain alignment direction 352. According to the embodiment described herein, each magnetic domain alignment direction 352 follows a straight magnetization line. The lines are angled or inclined relative to each other in a fan-like manner. Specifically, the spread angle distribution of the straight magnetization lines produces a focal point 354 in a region above the major surface 350a of the FFPM 350, the focal point 354 characterized by a magnetic field produced by the FFPM 350 and a corresponding local maximum in magnetic flux density.

According to the exemplary embodiments described herein, the depicted magnetic domain alignment pattern is symmetric with respect to the axis of symmetry 354 a. In this document, the axis of symmetry 354a is also referred to as the magnetic axis. Magnetic axis 354a is the normal axis to major surface 350a, which travels through focal point 354.

Fig. 4 shows an apparatus 460 for manufacturing a block in the form of a pressed magnet powder that can be sintered in a furnace to become FFPM. Specifically, the apparatus 460 is used to magnetize and compact the magnetic material powder 495. The subsequent sintering of the resulting magnetized compacted mass is carried out in a sintering furnace, not shown. Apparatus 460 includes a mold 470, and a mold cavity 472 is formed within mold 470. The mold cavity 472 may be closed by a mold, not shown, for compacting the magnetic material powder 495, and the magnetic material powder 495 must be filled in the mold cavity 472 according to a general process for manufacturing a sintered magnet. The not depicted module performs a movement in a direction perpendicular to the plane of the drawing.

The apparatus 460 further comprises means for generating a magnetic field that is applied to the compacted powder 495 during the sintering process. These magnetic field generating means comprise a first magnetic means 461 and a second magnetic means 464. In the embodiment shown in fig. 4, the first magnetic device 461 produces a magnetic north pole N and the second magnetic device 464 produces a magnetic south pole S. According to a known device, the first magnetic means 461 comprises (i) a first electromagnetic coil 462 for generating a magnetic field, and (ii) a first yoke structure 463 for guiding and/or shaping the magnetic field (lines) present in the mold cavity 472. Accordingly, the second magnetic arrangement 464 includes (i) a second electromagnetic coil 465 and (ii) a second yoke structure 466.

According to the exemplary embodiments described herein, the first yoke structure 463 assigned to the north pole and the second yoke structure 466 assigned to the south pole have different geometries. In particular, the radii of curvature of the outer surfaces of the two

yoke structures

463, 466 are different from each other. As can be seen in fig. 4, the first yoke structure 463 includes a curved first outer yoke surface 463a having (in a portion of the surface 463 a) a first radius of curvature R1. Accordingly, the second yoke structure 466 includes a curved second outer yoke surface 466a having (in a portion of the surface 466 a) a second radius of curvature R2 that is different than the first radius of curvature R1.

Fig. 5 shows an apparatus 560 for manufacturing FFPM blocks from magnet powder ready for sintering in a furnace according to another embodiment of the invention.

According to the embodiment shown in fig. 4, the apparatus 560 comprises a mold 570, the mold 570 having a mold cavity 572 for containing a magnetic material powder 595. Further, a mold member, not shown, is used to compact the magnetic material powder 595 filled in the mold cavity 572. The movement of the modules is in a direction perpendicular to the plane of the drawing.

Apparatus 560 differs from apparatus 460 shown in FIG. 4 in that the spatially non-uniform magnetic field/flux within mold cavity 572 is generated not only with two magnetic devices, but also with three magnetic devices. Thus, the two magnetic means are realized by dividing the first magnetic means 461 into two magnetic sub-means (a first magnetic sub-means 561-1 and a second magnetic sub-means 561-2). In the embodiment shown in FIG. 5, first magnetic subassembly 561-1 includes an electromagnetic coil 562-1 and a first yoke substructure 563-1. Thus, the second magnetic sub-assembly 561-2 includes an electromagnetic coil 562-2 and a second yoke sub-structure 563-2. Both yoke structures 561-1 and 561-2 define magnetic north poles from the perspective of mold cavity 572.

A magnetic south pole is generated by second magnetic device 564, which second magnetic device 564 comprises an electromagnetic coil 565 and a second magnetic yoke structure 566.

It is noted that in the embodiment shown in FIG. 5, all of the outer yoke surfaces facing the mold cavity 572 have the same convex curvature. However, this is not essential. A suitable non-uniform magnetic field within mold cavity 572 may also be achieved with a differently shaped or curved outer yoke surface.

It is noted that the term "comprising" does not exclude other elements or steps and the use of the article "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It is also noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. An apparatus (460, 560) for manufacturing a sintered flux-focusing permanent magnet (250, 350), the apparatus (460, 560) comprising:

a mold (470, 570) having a mold cavity (472, 572) for receiving a powder (495, 595) of a permanent magnet material; and

a first magnetic means (461; 561-1, 561-2) and a second magnetic means (464, 564) each for generating a magnetic field for magnetizing a powder (495, 595) contained in the mold cavity (472, 572); wherein,

the first magnetic device (461) comprises a first yoke structure (463; 563-1, 563-2) having a first shape, and the second magnetic device (464) comprises a second yoke structure (466, 566) having a second shape, the second shape being spatially different from the first shape and being spatially different from the first shape

The spatial difference in the shape of the two yoke structures (463, 466; 563-1, 563-2, 566) causes the magnetic field to be inhomogeneous such that the magnetic field within the mold cavity (472, 572) is associated with an angular spread distribution of flux lines.

2. The apparatus (460, 560) according to the preceding claim,

at least one of the two magnetic means (461, 464; 561-1, 561-2; 564) comprises at least one electromagnetic coil (462, 465; 562-1, 562-2, 565) for generating at least a part of the magnetic field, and

the respective yoke structure (463, 466; 563-1, 563-2, 566) is configured for guiding and/or for shaping the magnetic field generated by the respective electromagnetic coil (462, 465; 562-1, 562-2, 565).

3. The apparatus (460) of any preceding claim,

with respect to the mold cavity (472), the first yoke structure (463) and the second yoke structure (466) are located at opposite sides, an

The first yoke structure (463) having a first outer yoke surface (463 a) facing the mold cavity (472) and the second yoke structure (466) having a second outer yoke surface (466 a) facing the mold cavity (472), wherein,

the first outer yoke surface (463 a) has a different curvature than the second outer yoke surface (466 a).

4. The apparatus (460) of the preceding claim,

the first outer yoke surface (463 a) has a first radius of curvature (R1), and the second outer yoke surface (466 a) has a second radius of curvature (R2) that is different from the first radius of curvature (R1).

5. The apparatus (460) of either of the preceding claims,

with respect to the mold cavity (472), the two outer yoke surfaces (463 a, 466 a) include one of the following features:

the first outer yoke surface (463 a) is convex and the second outer yoke surface (466 a) is convex;

the first outer yoke surface is convex and the second outer yoke surface is concave;

the first outer yoke surface is concave and the second outer yoke surface is convex; and

the first outer yoke surface is concave and the second outer yoke surface is concave.

6. The apparatus (560) according to any one of the preceding claims,

the first yoke structure comprises a first yoke structure (563-1) and a second yoke structure (563-2), the two yoke structures (563-1, 563-2) being spatially separated from each other.

7. A method of manufacturing a sintered flux-focusing permanent magnet (250, 350) by means of the apparatus of any one of the preceding claims, the method comprising:

filling permanent magnet material powder (495, 595) into the mold cavity (472, 572) of the mold (470, 570);

-generating said magnetic field for magnetizing a powder (495, 595) contained in said moulding cavity (472, 572) by means of said first magnetic means (461, 464) and said second magnetic means (561, 564);

compacting said powder (495, 595) contained in said mold cavity (472, 572) by means of a mold, said magnetizing and compacting producing a magnetized compacted mass of said powder (495, 595);

sintering the magnetized compacted mass in a sintering furnace, thereby producing the flux-focusing permanent magnet (250, 350); and

removing the flux-focusing permanent magnet (250, 350) from the sintering furnace.

8. The method according to the preceding claim, wherein,

the permanent magnet material (495, 595) comprises a rare earth material, in particular NdFeB.

9. Sintered flux-focusing permanent magnet (250, 350) produced by performing the method described in the preceding claims.

10. An electromechanical transducer (140), in particular a generator (130), the electromechanical transducer (130) comprising:

a stator assembly (135), and

the rotor assembly (140) comprises

A support structure (242) and

the at least one sintered flux-focusing permanent magnet (250, 350) of the preceding claim, wherein the flux-focusing permanent magnet (250, 350) is mounted to the support structure (242).

11. A wind turbine (100) for generating electrical power, the wind turbine (100) comprising:

a tower (120) having a plurality of towers,

a wind rotor (110) arranged at a top portion of the tower (120) and comprising at least one blade (114); and

the electromechanical transducer (130) according to the preceding claim, wherein the electromechanical transducer (130) is mechanically coupled with the wind rotor (110).