EP3939060A2

EP3939060A2 - Manufacturing a sintered flux focusing permanent magnet with an apparatus having asymmetrically formed magnetic devices

Info

Publication number: EP3939060A2
Application number: EP19769050.6A
Authority: EP
Inventors: Ziad Azar; Qingfang HUANG; Hans-joergen Thougaard; Qingkun TONG; Adriana Cristina Urda
Original assignee: Siemens Gamesa Renewable Energy AS
Current assignee: Siemens Gamesa Renewable Energy AS
Priority date: 2019-04-23
Filing date: 2019-08-29
Publication date: 2022-01-19
Also published as: WO2019219984A3; CN111834117A; CN111834117B; WO2019219984A2

Abstract

It is described an apparatus (460, 560) and a method for manufacturing a sintered flux focusing permanent magnet (250, 350). The apparatus comprises (a) a mold (470, 570) having a molding chamber (472, 572) for receiving a powder of a permanent magnet material (495, 595); and (b) a first magnetic device (461, 561-1, 561-2) and a second magnetic device (464, 564) both for generating a magnetic field for magnetizing the powder (495, 595) being accommodated within the molding chamber (472, 572). The first magnetic device (461; 561-1, 561-2) comprises a first yoke structure (463; 563-1, 563-2) having a first shape and the second magnetic device (464, 564) comprises a second yoke structure (466, 566) having a second shape. The second shape is spatially different from the first shape. Further, the spatial difference in the shapes of the two yoke structures (463, 466; 563-1, 563-2, 566) causes the magnetic field to be inhomogeneous in such a manner that the magnetic field within the molding chamber (472, 572) is associated with a spread angular distribution of magnetic flux lines. Further described is a Sintered flux focusing permanent magnet (250, 350) which has been produced with the described method and an electromechanical transducer (140) as well as a wind turbine (100) comprising a sintered flux focusing permanent magnet (250, 350).

Description

DESCRIPTION

Manufacturing a sintered flux focusing permanent magnet with an apparatus having asymmetrically formed magnetic devices

Field of invention

The present invention relates to an apparatus and to a method for manufacturing a sintered flux focusing permanent magnet. Further, the present invention relates to a sintered flux fo cusing permanent magnet being produced with the described method and to an electromechanical transducer and a wind tur bine comprising at least one of such a sintered magnet.

Art Background

Permanent magnetic materials are used in a plurality of dif ferent fields of application. Probably the technically and economically most important field of applications are elec tromechanical transducers, i.e. electric motors and electric generators. An electric motor being equipped with at least one permanent magnet (PM) converts electric energy into me chanical energy by producing a temporary varying magnetic field by means of windings or coils. This temporary varying magnetic field interacts with the magnetic field of the PM resulting e.g. in a rotational movement of a rotor assembly with respect to a stator assembly of the electric motor. In a physically complementary manner, an electric generator con verts mechanical energy into electric energy.

An electric generator is a core component of any power plant for generating electric energy. This holds true for power plants which directly capture mechanical energy, e.g. hydroe lectric power installations, tidal power installations, and wind power installations also denominated wind turbines. How ever, this also holds true for power plants which (i) first use chemical energy e.g. from burning fossil fuel or from nu clear energy in order to generate thermal energy and which (ii) second convert the generated thermal energy into mechan ical energy by means of appropriate thermodynamic processes.

The efficiency of an electric generator is probably the most important factor for optimizing the production of electric energy. For a PM electric generator it is essential that the magnetic flux produced by the permanent magnets (PMs) is strong. This can probably best be achieved with sintered rare earth magnets, e.g. using a NdFeB material composition. How ever, also the spatial magnetic field distribution produced by PMs has an impact on the generator efficiency. In the lat ter case it is often of advantage when PM devices or PM ar rangements are used which have a non-uniform magnetic domain alignment pattern resulting in an intentionally inhomogeneous magnetic field strength or magnetic flux density in particu lar in an air gap between a rotor assembly and a stator as sembly.

It is known to configure a non-uniform magnetic domain align ment pattern in PM in order to achieve a so called "magnetic flux focusing". WO 2012/141932 A2 discloses PM magnet ar rangements where differently magnetized PMs are combined such that a "magnetic focusing" is achieved. EP 3 276 642 A1 dis closes a sintered rare earth PM having a focusing magnetic alignment pattern with an integrally formed or single piece PM body. EP 2 762 838 A2 discloses apparatuses and methods for manufacturing PMs, wherein during a sintering process an non-uniform external magnetic field is applied in order to magnetize different regions of a PM in different directions. In this documents a flux focusing permanent magnet is abbre viated a FFPM.

For manufacturing a FFPM an apparatus for generating a proper inhomogeneous magnetic field being present within a molding chamber is needed in order to magnetize magnetic powder being compressed within the molding chamber. Summary of the Invention

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

According to a first aspect of the invention there is provid ed an apparatus for manufacturing a sintered flux focusing permanent magnet. The provided apparatus comprises (a) a mold having a molding chamber for receiving a powder of a perma nent magnet material; and (b) a first magnetic device and a second magnetic device both for generating a magnetic field for magnetizing the powder being accommodated within the molding chamber. The first magnetic device comprises a first (magnetic) yoke structure having a first shape and the second magnetic device comprises a second (magnetic) yoke structure having a second shape, the second shape being spatially dif ferent from the first shape. Further, the spatial difference in the shapes of the two yoke structures causes the magnetic field to be inhomogeneous in such a manner that the magnetic field within the molding chamber is associated with a spread angular distribution of magnetic flux lines.

The described apparatus is based on the idea that by choosing a spatially asymmetric design of a magnet system which com prises the two magnetic devices and which is used for magnet izing the (compressed) powder being present within the mold ing chamber it is very easy to generate a proper inhomogene ous magnetic field at least within the region of the molding chamber .

The described spread angular distribution of magnetic flux lines may be used in particular for causing, within the flux focusing permanent magnet (FFPM) to be produced, a spread an gular distribution of magnetic domain alignment directions which result in a focused magnetization of the sintered block. Thereby, outside from the bulk of the FFPM a magnetic focal point or at least a magnetic focal region may be de fined. In this point or in this region the magnetic flux den sity caused by the respective FFPM is increased as compared to points or regions outside from the focal point respective ly the focal region.

By designing the shape and/or the geometry of the magnetic yoke structures in a proper (asymmetric) manner a spread an gular distribution of magnetic flux lines can be produced which yields a desired focused flux magnetization design.

The magnetic yoke structure, which may also be denominated a pole piece, may be made from a ferromagnetic material, in particular from iron or cobalt iron for higher saturation. By contrast thereto, the mold may be made from a non-magnetic and in particular from a non-ferromagnetic material. A (cur rently) preferred material for the mold is stainless steel. However, also other mold materials may be used provided that are mechanical stiff.

According to an embodiment of the invention (a) at least one of the two magnetic devices comprises at least one electro magnetic coil for producing 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 being pro duced by the respective electromagnetic coil.

Using at least one electromagnetic coil being supported by a proper magnetic yoke structure in producing the magnetic field may provide the advantage that the magnetic field re spectively the magnetic flux (density) at least within se lected regions of the molding chamber can be increased sig nificantly.

By setting a current flowing through the respective coil the strength of a magnetic field generated by each magnetic de vice can be adjusted properly. By independently varying the current flowing through each one of the coils not only the strength but also the directions of the magnetic field lines can be changed towards a desired characteristic of the gener ated magnetic field.

According to a further embodiment of the invention the appa ratus comprises the following features:

(A) With regard to the molding chamber the first yoke struc ture and the second yoke structure are located at opposing sides, and (B) the first yoke structure has a first outer yoke surface facing the molding chamber and the second yoke structure has a second outer yoke surface facing the molding chamber, wherein the first outer yoke surface has a different curvature than the second outer yoke surface.

By designing the respective outer yoke surface is an appro priate manner the direction the magnetic field lines emerging from the respective yoke can be selected. Hence, the extent of the spread of the angular distribution of magnetic flux lines can be adjusted in an easy and effective manner.

According to a further embodiment of the invention the first outer yoke surface has (at least within a portion of the first outer yoke surface) a first curvature radius and the second outer yoke surface has (at least within a portion of the second outer yoke surface) a second curvature radius be ing different from the first curvature radius.

V: This may provide the advantage that higher alignment angle of the magnetic domain alignment directions at and with re gard to side edges of the FFPM can be realized.

According to a further embodiment of the invention the mold ing chamber comprises, with regard to the two outer yoke sur faces one of the following features:

(A) the first outer yoke surface (463a) is convex and the second outer yoke surface (466a) 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 selecting an appropriate curvature for the two outer yoke surfaces a proper and well defined spread angular distribu tion of magnetic flux lines can be generated in an easy and effective manner. Depending on the specific application the bending of the outer yoke surfaces may be regular (without any corners and edges ("lumps and bumps") or irregular.

At least one of the two outer yoke surfaces may have an at least approximately cylindrical shape. This causes a one di mensional (ID) magnetic flux focusing which results in a lin early extended focal region. Such a magnetic focusing corre sponds to an optical focusing by means of a cylindrical opti cal lens. Alternatively, at least one of the outer yoke sur faces may have an at least approximately spherical shape.

This causes a two dimensional (2D) magnetic flux focusing which results at least approximately in a focal point. Such a magnetic focusing corresponds to an optical focusing by means of a spherical optical lens.

According to a further embodiment of the invention the first yoke structure comprises a first yoke sub-structure and a second yoke sub-structure, the two yoke substructures being spatially separated from each other. In this configuration of the apparatus the above specified feature according to which the first shape is different from the second shape is real ized by two sub-shapes, which are spatially separated from each other.

Using, in addition to the two yoke structures at least three yoke structures by "splitting" the first yoke structure in two yoke sub-structures may allow to generate a highly spa tially inhomogeneous magnetic field within the molding cham ber. This may be in particular of advantage when a sintered magnet block for a FFPM is produced, which FFPM has a small focal length. Thereby, the focal length may be defined as the distance between the focal point or the focal region to a main surface of the FFPM.

It is mentioned that there is no principal limit regarding the number of sub-structures. It is also possible to subdi vide the second yoke sub-structure into at least two sub structures .

It is further mentioned that the yoke structure (s) and/or the yoke sub-structures have the same (outer) shape or at least two of them have a different (outer) shape. In this respect it is pointed out that when all outer shapes are geometrical ly the same the above defined feature according to which the (entire) first outer yoke surface has a different curvature than the second outer yoke surface is realized by splitting the (entire) first outer yoke surface in two sub-surfaces each being assigned to one of the yoke sub-structures.

In case at least one of the now at least three yoke (sub-) structures is magnetized by means of a magnetic field which is generated by means of an electromagnetic coil there is given the opportunity to modify (the in-homogeneity of) the overall magnetic field by selecting an appropriate electric current flowing through this electromagnetic coil. This may allow the described apparatus to be used for manufacturing FFPMs with different flux focusing characteristics.

According to a further aspect of the invention there is pro vided a method for manufacturing a sintered flux focusing permanent magnet by means of an apparatus as described above. The provided method comprises (a) filling a powder of perma nent magnet material into the molding chamber of the mold;

(b) generating the magnetic field for magnetizing the power being accommodated within the molding chamber by means of the first magnetic device and the second magnetic device; (c) compacting the powder being accommodated within the molding chamber by means of a die, the magnetizing and compacting re sulting in a magnetized compacted block of the powder; (d) sintering the magnetized compacted block in a sintering oven resulting in the flux focusing permanent magnet; and (e) re moving the flux focusing permanent magnet from the sintering oven .

Also then described method is based on the idea that with a proper spatially asymmetric design of a magnet system com prising the two magnetic devices and which is used for mag netizing the (compressed) powder it is very easy to produce a FFPM with a desired spread angular distribution of magnetic domain alignment directions.

It is mentioned that the step of generating the magnetic field in order to obtain a magnetic alignment and the step of compacting the powder are typically accomplished at least partially at the same time.

It is further mentioned that after the step of sintering there may be carried out a further or post processing in or der to end up with a FFPM piece which can be used e.g. for a rotor assembly of an electric generator. Such a post pro cessing may include e.g. a surface finishing smoothing the surface of the sintered material and/or applying an outer surface layer which causes the surface of the FFPM piece to be less sensitive to external impacts. Further, the post pro cessing may include a shaping of the sintered FFPM in order to have a desired geometry. Such a shaping can be realized e.g. by a conventional milling.

According to a further embodiment of the invention the perma nent magnet material comprises a rare earth material, in par ticular NdFeB. This may provide the advantage that very strong PMs can be manufactured without the need to produce a lot of wastage (of the typically very expensive rare earth material) for achieving a desired PM geometry. It is mentioned that other compositions of the permanent mag net material may include ferrite and/or SmCo .

According to a further aspect of the invention there is pro vided a sintered flux focusing permanent magnet being pro duced by carrying out the method as described above.

According to a further aspect of the invention there is pro vided an electromechanical transducer, in particular an elec tric generator. The provided electromechanical transducer comprises a stator assembly and a rotor assembly. The rotor assembly comprises a support structure and at least one sin tered flux focusing permanent magnet as described above, wherein the flux focusing permanent magnet is mounted to the support structure.

By designing an electric generator in such a manner that the focal point respectively the focal region is located within an airgap between a stator assembly and a rotor assembly the electric power which can be produced by the generator can be increased significantly.

According to a further aspect of the invention there is pro vided a wind turbine for generating electrical power. The provided wind turbine comprises (a) a tower; (b) a wind ro tor, which is arranged at a top portion of the tower and which comprises at least one blade; and (c) an electromechan ical transducer as described above. The electromechanical transducer is mechanically coupled with the wind rotor.

The provided wind turbine, also denominated a wind energy in stallation, is based on the idea that the above described electromechanical transducer allows to realize a wind turbine having an improved efficiency with respect to the conversion of mechanical power into electrical power which conversion is accomplished by the generator comprising the FFPMs . This may contribute for improving the attractiveness of wind turbine technology for regenerative power production as compared to other technologies such as solar plants.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to method type claims whereas other embodiments have been described with reference to apparatus type claims. How ever, 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 re lating to different subject matters, in particular between features of the method type claims and features of the appa ratus type claims is considered as to be disclosed with this document .

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Brief Description of the Drawing

Figure 1 shows a wind turbine in accordance with an embodi ment of the present invention.

Figure 2 shows in a schematic representation the generator of the wind turbine of Figure 1.

Figure 3 shows a Flux Focusing Permanent Magnet (FFPM) pro duced in accordance with an embodiment of the inven tion.

Figure 4 shows an apparatus for manufacturing a FFPM, the ap paratus having two magnetic devices which have dif ferently shaped yoke structures. Figure 5 shows an apparatus for manufacturing FFPM, the appa ratus comprising three magnetic devices for inhomo- geneously magnetizing a powder of magnetic material being within a molding chamber.

Detailed Description

The illustration in the drawing is schematic. 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 are different from the corresponding reference signs only within the first digit. In order to avoid unnecessary repetitions elements or features which have already been elucidated with respect to a previously de scribed embodiment are not elucidated again at a later posi tion of the description.

Figure 1 shows a wind turbine 100 according to an embodiment of the invention. The wind turbine 100 comprises a tower 120 which is mounted on a non-depicted fundament. On top of the tower 120 there is arranged a nacelle 122. In between the tower 120 and the nacelle 122 there is provided a yaw angle adjustment device 121 which is capable of rotating the na celle 122 around a non-depicted vertical axis being aligned with the longitudinal extension of the tower 120. By control ling the yaw angle adjustment device 121 in an appropriate manner it can be made sure that during a normal operation of the wind turbine 100 the nacelle 122 is always properly aligned with the current wind direction.

The wind turbine 100 further comprises a wind rotor 110 hav ing three blades 114. In the perspective of Figure 1 only two blades 114 are visible. The rotor 110 is rotatable around a rotational axis 110a. The blades 114, which are mounted at a hub 112, extend radially with respect to the rotational axis In between the hub 112 and a blade 114 there is respectively provided a blade angle adjustment device 116 in order to ad just the blade pitch angle of each blade 114 by rotating the respective blade 114 around a non-depicted axis being aligned substantially parallel with the longitudinal extension of the respective blade 114. By controlling the blade angle adjust ment device 116 the blade pitch angle of the respective blade 114 can be adjusted in such a manner that at least when the wind is not too strong a maximum wind power can be retrieved from the available mechanical power of the wind driving the wind rotor 110.

As can be seen from Figure 1, within the nacelle 122 there is provided a gear box 124. The gear box 124 is used to convert the number of revolutions of the rotor 110 into a higher num ber of revolutions of a shaft 125, which is coupled in a known manner to an electromechanical transducer 130. The electromechanical transducer is a generator 130.

At this point it is pointed out that the gear box 124 is op tional and that the generator 140 may also be directly cou pled to the rotor 110 by the shaft 125 without changing the numbers of revolutions. In this case the wind turbine is a so caller Direct Drive (DD) wind turbine.

Further, a brake 126 is provided in order to stop the opera tion of the wind turbine 100 or in order to reduce the rota tional speed of the rotor 110 for instance in case of emer gency .

The wind turbine 100 further comprises a control system 143 for operating the wind turbine 100 in a highly efficient man ner. Apart from controlling for instance the yaw angle ad justment device 121 the depicted control system 153 is also used for adjusting the blade pitch angle of the rotor blades 114 in an optimized manner. In accordance with basic principles of electrical engineering the generator 130 comprises a stator assembly 135 and a rotor assembly 140. In the embodiment described here the generator 130 is realized in a so called "inner stator - outer rotor" configuration, wherein the rotor assembly 140 surrounds the stator assembly 135. This means that non-depicted permanent magnets respectively magnet assemblies of the rotor assembly 140 travel around an arrangement of a plurality of non- depicted coils of the inner stator assembly 135 which coils produce an induced current resulting from picking up a time varying magnetic flux from the traveling permanent magnets.

According to the embodiment described here each permanent magnet (PM) assembly comprises at least three sintered Flux Focusing Permanent Magnets (FFPMs) which are made from a Nd- Fe-B material composition.

Figure 2 shows in a cross sectional view a schematic repre sentation of the generator 130. The generator 130 comprises a stator assembly 135. The stator assembly 135 comprises a sta tor support structure 237 comprising a stack of a plurality of lamination sheets and a plurality of stator windings 239 being accommodated within the stator support structure 237. The windings 239 are interconnected in a known manner by means of non-depicted electrical connections.

A rotor assembly 140 of the generator 130, which is separated from the stator assembly 135 by an air gap ag, comprises a rotor support structure 242 providing the mechanical base for mounting a plurality of FFPMs 250. In Figure 2 the rotational axis of the rotor assembly 140 is denominated with reference numeral 230a.

In the exemplary embodiment described here at each angular position of the rotor assembly 140 there are arranged three FFPMs arranged next to each other. It is mentioned that in Figure 2 only three FFPMs 250 being assigned to one angular position are depicted for the sake of ease of illustration.

In reality, depending on the dimension of the generator 130, a plurality of FFPMs 250 are mounted to the rotor support structure 242. The FFPMs 250 are preferably arranged in a ma trix like structure around a curved surface area of the sup port structure 242 having a basically cylindrical geometry around the generator axis 240a.

As can be seen from Figure 2, the FFPMs 250 are not mounted directly to the rotor support structure 242. Instead, there is provided a back plate 244 made from a ferromagnetic mate rial, e.g. iron. The back plate 244 is provided in order to ensure a proper guidance of magnetic flux. This significantly reduces in a beneficial manner the intensity of magnetic stray fields.

Figure 3 shows in more detail a Flux Focusing Permanent Mag net (FFPM) 350 produced in accordance with an embodiment of the invention.

The FFPM 350 is magnetized in such a manner that there is given a spread angular distribution of magnetic domain align ment directions 352. According to the embodiment described here each magnetic domain alignment direction 352 follows a straight magnetization line. The straight lines are angled or inclined with respect to each other in a fan like manner. Specifically, the spread angular distribution of the straight magnetization lines produces, in the region above a main sur face 350a of the FFPM 350, a focal point 354 being character ized by a local maximum of the magnetic field respectively the magnetic flux density produced by the FFPM 350.

According to the exemplary embodiment described here the de picted magnetic domain alignment pattern is symmetric with respect to a symmetry axis 354a. In this document the sym metry axis 354a is also denominated magnetic axis. The mag- netic axis 354a is a normal axis to the main surface 350a, which runs through the focal point 354.

Figure 4 shows an apparatus 460 for manufacturing a block in the form of pressed magnet powder that can be sintered in an oven in order to become a FFPM. Specifically, the apparatus 460 is used for magnetizing and compacting a powder of mag netic material 495. A subsequent sintering of a resulting magnetized compacted block is carried out in a non-depicted sintering oven. The apparatus 460 comprises a mold 470 within which a molding chamber 472 is formed. The molding chamber 472 can be closed by a non-depicted die, which is used for compacting the powder of magnetic material 495, which in ac cordance with usual procedures for manufacturing sintered magnets has to be filled into the molding chamber 472. The non-depicted die performs a movement along a direction being perpendicular to the plane of drawing.

The apparatus 460 further comprises means for producing a magnetic field which is applied to the compacted powder 495 during the sintering process. These magnetic field production means include a first magnetic device 461 and a second mag netic device 464. In the embodiment shown in Figure 4, the first magnetic device 461 produces a magnetic North pole N and the second magnetic device 464 produces a magnetic South pole S. In accordance with known apparatuses the first mag netic device 461 comprises (i) a first electromagnetic coil 462 for generating a magnetic field and (ii) a first magnetic yoke structure 463 for guiding and/or for shaping the magnet ic field (lines) being present within the molding chamber 472. Correspondingly, the second magnetic device 464 compris es (i) a second electromagnetic coil 465 and (ii) a second magnetic yoke structure 466.

According to the exemplary embodiment described here the first magnetic yoke structure 463 being assigned to the North pole and the second magnetic yoke structure 466 being as- signed to the South pole have a different geometry. Specifi cally, the bending radii of the outer surfaces of the two magnetic yokes structures 463, 466 are different from each other. As can be seen from Figure 4, the first magnetic yoke structure 463 comprises a curved first outer yoke surface 463a which (in a portion of the surface 463a) has a first curvature radius R1. Accordingly, the second magnetic yoke structure 466 comprises a curved second outer yoke surface 466a which (in a portion of the surface 466a) has a second curvature radius R2 being different from the first curvature radius R1.

Figure 5 shows an apparatus 560 for manufacturing a FFPM block from magnet powder ready for sintering in an oven in accordance with a further embodiment of the invention.

In accordance with the embodiment shown in Figure 4 the appa ratus 560 comprises a mold 570 having a molding chamber 572 for accommodating a powder of magnetic material 595. Again, a non-depicted die is used for compacting powder of magnetic material 595 filled into the molding chamber 572. The move ment of the die is along a direction perpendicular to the plane of drawing.

The apparatus 560 differs from the apparatus 460 shown in Figure 4 in that a spatially inhomogeneous magnetic field / flux within the molding chamber 572 is not only produced with two but with three magnetic devices. Thereby, two magnetic device are realized by splitting the first magnetic device 461 into two magnetic sub-devices, a first magnetic sub device 561-1 and a second magnetic sub-device 561-2. In the embodiment shown in Figure 5, the first magnetic sub-device

561-1 comprises an electromagnetic coil 562-1 and a first magnetic yoke sub-structure 563-1. Accordingly, the second magnetic sub-device 561-2 comprises an electromagnetic coil

562-2 and a second magnetic yoke sub-structure 563-2. Both magnetic yoke sub-structures 561-1 and 561-2 define, from the perspective of the molding chamber 572 a magnetic North pole.

The magnetic South pole is produced with a second magnetic device 564 comprising an electromagnetic coil 565 and a sec ond magnetic yoke structure 566.

It is mentioned that in the embodiment shown in Figure 5 all outer yoke surfaces facing the molding chamber 572 have the same convex curvature. However, this is not necessary. Proper inhomogeneous magnetic fields within the molding chamber 572 can also be realized with differently shaped or curved outer yoke surfaces.

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

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 molding chamber (472, 572) for receiving a powder of a permanent magnet material (495, 595) ; and

a first magnetic device (461; 561-1, 561-2) and a second magnetic device (464, 564) both for generating a magnetic field for magnetizing the powder (495, 595) being accommodat ed within the molding chamber (472, 572); wherein

the first magnetic device (461) comprises a first yoke struc ture (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

the spatial difference in the shapes of the two yoke struc tures (463, 466; 563-1, 563-2, 566) causes the magnetic field to be inhomogeneous in such a manner that the magnetic field within the molding chamber (472, 572) is associated with a spread angular distribution of magnetic flux lines.

2. The apparatus (460, 560) as set forth in the preceding claim, wherein

at least one of the two magnetic devices (461, 464; 561-1, 561-2; 564) comprises at least one electromagnetic coil (462, 465; 562-1, 562-2, 565) for producing 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 being produced by the respective electromagnetic coil (462, 465; 562-1, 562-2, 565) .

3. The apparatus (460) as set forth in any one of the preced ing claims, wherein

with regard to the molding chamber (472) the first yoke structure (463) and the second yoke structure (466) are lo cated at opposing sides, and

the first yoke structure (463) has a first outer yoke surface (463a) facing the molding chamber (472) and the sec ond yoke structure (466) has a second outer yoke surface (466a) facing the molding chamber (472), wherein

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

4. The apparatus (460) as set forth in the preceding claim, wherein

the first outer yoke surface (463a) has a first curvature ra dius (Rl) and the second outer yoke surface (466a) has a sec ond curvature radius (R2) being different from the first cur vature radius (Rl) .

5. The apparatus (460) as set forth in any one of the two preceding claims, wherein

with regard to the molding chamber (472) the two outer yoke surfaces (463a, 466a) comprise one of the following features: the first outer yoke surface (463a) is convex and the second outer yoke surface (466a) 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) as set forth in any one of the preced ing claims, wherein

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

7. A method for manufacturing a sintered flux focusing perma nent magnet (250, 350) by means of an apparatus as set forth in any one of the preceding claims, the method comprising

filling a powder of permanent magnet material (495, 595) into the molding chamber (472, 572) of the mold (470, 570); generating the magnetic field for magnetizing the power (495, 595) being accommodated within the molding chamber (472, 572) by means of the first magnetic device (461, 464) and the second magnetic device (561, 564);

compacting the powder (495, 595) being accommodated within the molding chamber (472, 572) by means of a die, the magnetizing and compacting resulting in a magnetized com pacted block of the powder (495, 595);

sintering the magnetized compacted block in a sintering oven resulting in the flux focusing permanent magnet (250,

350 ) ; and

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

8. The method as set forth in 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) being produced by carrying out a method as set forth in the preced ing claim.

10. An electromechanical transducer (140), in particular an electric generator (130), the electromechanical transducer (130) comprising

a stator assembly (135) and

a rotor assembly (140) comprising

a support structure (242) and

at least one sintered flux focusing permanent mag net (250, 350) as set forth in 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) ;

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

an electromechanical transducer (130) as set forth in the preceding claim, wherein the electromechanical transducer (130) is mechanically coupled with the wind rotor (110) .