WO2019219986A2

WO2019219986A2 - Magnet assembly comprising magnet devices each having a focusing magnetic domain alignment pattern

Info

Publication number: WO2019219986A2
Application number: PCT/EP2019/073215
Authority: WO
Inventors: Ziad Azar; Richard Clark; Alexander Duke; Arwyn Thomas; Hans-joergen Thougaard; Adriana Cristina Urda
Original assignee: Siemens Gamesa Renewable Energy A/S
Priority date: 2019-03-11
Filing date: 2019-08-30
Publication date: 2019-11-21
Also published as: CN113785473A; WO2019219986A3

Abstract

It is described a magnet assembly (260) comprising (a) a first magnet device (362) having a first angular distribution (463) of magnetization directions resulting in a first focused magnetization producing a first magnetic focal region (463a) and (b) a second magnet device (364) having a second angular distribution (465) of magnetization directions resulting in a second focused magnetization producing a second magnetic focal region (465a). The first magnetic focal region (463a) is different from the second magnetic focal region (465a). It is further described a rotor arrangement (150) with such a magnet assembly (360, 460), an electromechanical transducer (140) with such a rotor arrangement (150), and a wind turbine (100) with such an electromechanical transducer (140).

Description

DESCRIPTION

Magnet assembly comprising magnet devices each having a fo cusing magnetic domain alignment pattern

Field of invention

The present invention relates to the technical field of per manent magnets having a non-uniform magnetic domain alignment pattern. The present invention further relates to a rotor ar rangement for an electromechanical transducer, which rotor arrangement comprises at least one of such a permanent mag net. Furthermore, the present invention relates to an elec tromechanical transducer comprising such a rotor arrangement and to a wind turbine comprising such an electromechanical transducer .

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 arrange ment with respect to a stator arrangement of the electric mo tor. In a physically complementary manner, an electric gener ator, also called a dynamo electrical machine, converts me chanical 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.

It is obvious that the efficiency of an electric generator is probably the most important factor for optimizing the produc tion of electric energy. For a PM electric generator it is essential that the magnetic flux produced by the permanent magnets is strong. Presently, this can probably best be achieved with sintered rare earth magnets, e.g. using a FeNdB material composition. However, also the spatial magnetic field distribution produced by PMs has an impact on the gen erator efficiency. In the latter case it is often of ad vantage when PM devices or PM arrangements are used which have a non-uniform magnetic domain alignment pattern result ing in an intentionally inhomogeneous magnetic field strength or magnetic flux density in particular in an air gap between a rotor arrangement and a stator arrangement.

WO 2012/141932 A2 discloses PM magnet arrangements where dif ferently magnetized PMs are combined such that a "magnetic focusing" is achieved. The differently magnetized PMs may be mounted on a common back plate made e.g. from iron.

EP 3 276 642 A1 discloses 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 manu facturing PMs, wherein during a sintering process an non- uniform external magnetic field is applied in order to mag netize different regions of a PM in different directions.

With a proper external magnetic field also magnetic domain alignment patterns can be produced which form bent magnetiza tion lines within the PM body.

WO 2009 017430 A1 discloses a magnet device having magnetic domains aligned non-isotropic in order to form a magnetic do main alignment pattern, wherein the direction of correspond ing magnetization direction varies substantially continuously across at least a part of the magnet device between its lat eral edges from at least partially radial to at least par tially tangential.

There may be a need for providing a magnet assembly which, when being used for rotor arrangement of a PM electromechani cal transducer, causes a good electromagnetic linkage between the rotor arrangement and a stator arrangement of the trans ducer such that a good the transducer efficiency can be real ized.

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 a magnet assembly comprising (a) a first magnet device having a first angular distribution of magnetization direc tions resulting in a first focused magnetization producing a first magnetic focal region; and (b) a second magnet device having a second angular distribution of magnetization direc tions resulting in a second focused magnetization producing a second magnetic focal region. A first location of the first magnetic focal region is different from a second location of the second magnetic focal region.

The described magnet assembly (MA) is based on the idea that, at least for several types of electromechanical transducers, a multiple magnetic focal region configuration increases the strength of an electromagnetic linkage between a rotor ar rangement and a stator arrangement. As a consequence, the ef ficiency of the corresponding transducer can be improved.

In a "microscopic picture" the described distribution of mag netization directions may correspond to a distribution of "magnetic domain alignment directions" within the bulk mate rial of the respective magnet device.

It is pointed out that the magnetic focusing of the respec tive magnet device may not be perfect. Hence, the distribu tion of magnetization directions may result, at least in a cross sectional view, in a focal volume having a certain spa tial extension. In case of a perfect focusing the magnetic focal region may be, at least in a cross sectional view, a magnetic focal point.

In this regard it is further mentioned that the described fo cusing may be (A) a two dimensional (2D) focusing or (B) a three dimensional (3D) focusing.

(A) In case of a 2D focusing the magnetization directions are distributed two-dimensionally . This means that all magnetiza tion vectors are oriented within or parallel to a plane being defined by an x-axis and a z-axis. Thereby, the z-axis may be associated with a thickness direction of the magnet device and the x-axis, which is perpendicular to this z-axis, may be associated with a width direction of respective magnet de vice. In the "real 3D world" a theoretically perfect focusing would result in a focal line. In the field of optics a 2D fo cusing is achieved e.g. by means of a cylindrical lens.

(B) In case of a 3D focusing the magnetization directions are distributed three-dimensionally . This means that there is not only a focusing along one direction (e.g. the above mentioned x-direction) but also along another direction being perpen dicular thereto. Specifically, this another direction may be parallel to a y-axis which is perpendicular to both the x- axis and the above mentioned z-axis. The y-axis may define a depth direction of the respective magnetic device. In the "real 3D world" a theoretically perfect 3D focusing would re sult in a focal point. In the field of optics a 3D focusing is achieved e.g. by means of a spherical lens.

According to an embodiment of the invention the first magnet device and/or the second magnet device are realized in a sym metric configuration. Thereby, the symmetric configuration is given by (i) the spatial shape and dimension of the respec tive magnet device and/or (ii) the respective angular distri bution of magnetization directions.

Configuring the first magnet device and/or the second magnet device in a symmetric manner may provide the advantage that, as compared to a non-symmetric configuration, the respective magnet device can be manufactured comparatively easy with known procedures and apparatuses for inhomogeneously magnet izing the central magnet device e.g. during a sintering pro cedure. Further, a "magnetic design" of the magnet devices and/or of the MA will be facilitated, which "magnetic design" yields a desired spatial variation of magnetic flux density.

The described symmetric configuration may be given with a "magnetic axis" representing a (mirror) symmetry axis which is defined by the shortest distance between the respective magnetic focal region and a main surface of the respective magnet device. This means that the magnetic axis is oriented perpendicular to (the plane of) a main surface of the respec tive magnet device and that the magnetic focal region is lo cated on the magnetic axis. In this respect the magnetic axis may be seen as to correspond to an optical axis of a focusing optical element, e.g. a refractive lens.

According to a further embodiment of the invention (i) a first focal distance between the first magnetic focal region and a first main surface of the first magnet device is at least approximately the same as (ii) a second focal distance between the second magnetic focal region and a second main surface of the second magnet device. This may provide the ad vantage that for realizing the described MA only one type of magnet device can be used by assembling at least two of these magnet devices. Hence, the manufacturing of the described MA will be facilitated not only in a technical but also in a lo gistic point of view.

When the two magnet devices are spatially arranged such that the first main surface and the second main surface form a common planar main surface the two magnetic focal regions will have the same focal distance with regard to this common planar main surface. This not only facilitates the magnetic design of the MA but also, in many applications, further im proves the magnetic linkage within an electromechanical transducer having a rotor arrangement with at least one de scribed MA.

According to a further embodiment of the invention the two magnet devices directly abut against each other. A MA with directly abutting magnet devices may provide the advantage that it can be realized within a compact design. A further advantage may be that at the interface between two neighbor ing magnet devices there may be, at least approximately, no distortion of magnet flux lines. Such a distortion of magnet flux lines would most probably occur if there would be a gap in between the two respective magnet pieces.

A further advantage of directly abutting magnet devices may be seen in that a common main surface can be realized. Such a common main surface may be for instance a front surface of the MA, wherein the magnet devices are attached to a ferro magnetic (iron) back plate with its respective back surface. Hence, the described MA can be realized without unnecessary rough edges, which facilitates a further handling of the MA in particular when mounting the MA, together with other as semblies, to a support structure of a rotor arrangement. In this document the term "directly abut" may mean that there is no intended gap between the two magnet devices. This means that e.g. a small layer of adhesive and/or a surface protec tion or passivation layer in between the actual magnetic ma terials of the two magnet devices does not mean that the two magnet devices do not directly abut against each other.

According to a further embodiment of the invention the first magnet device and/or the second magnet device is formed by at least two magnet pieces being attached to each other.

Descriptively speaking, in this embodiment at least one mag net device of the (at least) two magnet devices is composed of at least two single magnet pieces. This may provide the advantage that the (focusing) magnet devices can be realized by composing or assembling smaller magnet pieces. Although assembling the different magnet pieces may require some addi tional effort this additional effort will, in most cases, be overcompensated because only smaller magnet pieces have to be produced. This holds true because in order to realize a fo cusing magnet device it is often easier to manufacture two or more small focusing magnet pieces than to manufacture one larger focusing magnet piece.

It is mentioned that of course at least one of the magnet de vices can be realized with a single magnet piece. Even fur ther, also two (or more) magnet devices of the described MA can be realized as a single piece. In the context of this document the term "single piece" may particularly mean that the respective magnet device is integrally or monolithically formed by means of a single bulk magnetic material.

According to a further embodiment of the invention the magnet assembly has at least one of the following spatial of geomet ric dimensions: (a) A thickness of the magnet devices in in a range in between 10 mm and 40 mm and in particular in a range between 18 mm and 25 mm. (B) A width of the magnet assembly in a range in between 20 mm and 200 mm and in particular in a range between 50 mm and 150 mm.

With regard to the described thickness it is mentioned that depending on a thickness of a potential ferromagnetic (iron) back plate the thickness of the entire MA might be larger than the mentioned thickness values.

With regard to the described width of the MA it is mentioned that the MA of course has also a certain depth. The denomina tions "width" and "depth" of the geometry respectively of the geometric dimensions may be taken from a moving direction when the MA is mounted to a rotor arrangement of an electro mechanical transducer. Specifically, the "width" may be the extension measured along a circumference of the rotor ar rangement and the "depth" may be the extension along an axial direction. Correspondingly, the thickness may be the exten sion along a radial direction of the rotor arrangement.

In some exemplary embodiments the first magnet device has a first width, measured along a direction being perpendicular to a thickness direction of the first magnet device; and the second magnet device has a second width, measured along a di rection being perpendicular to a thickness direction of the second magnet device. The first width may be the same as the second width or the first width may be different from as the second width.

Varying the width and in particular the width ratio between the magnet devices of the MA gives the designer of the de scribed MA a further degree of freedom for realizing a de sired magnetic flux density profile within in particular an air gap between a rotor arrangement comprising the described MA and a stator arrangement.

According to a further embodiment of the invention at least one of the magnet devices comprises an aspect ratio in the range between 0.2 and 1.0, in particular between 0.4 and 1.0 and more in particular between 0.6 and 1.0. Thereby, the as pect ratio is defined by the ratio between the thickness of the magnet device and the width of the magnet device. The thickness is measured along a direction being parallel to a magnetic axis of the respective magnet device, which magnetic axis is defined by a normal direction of a main surface of the respective magnet device and the spatial position of the center of the respective focal region. The width is given by the spatial extension of the magnet device along a direction which is defined by an axis extending between (the center of the body of) the first magnet device and (the center of the body of) the second magnet device.

Put in other words, the thickness may be measured along a di rection being parallel to the magnetic axis and the width is measured along a direction being parallel to a common normal vector of the mutually facing side surfaces of the two side magnet devices.

In this respect the inventors found out that a proper aspect ratio of the focusing central magnet device may have a sig nificant effect on the magnetic flux which can be realized within an airgap of an electric generator. Specifically, by contrast to not focusing magnet devices, which are typically dimensioned by machine designers with a minimum thickness (in particular for cost reasons) , a focusing magnet device may provide a significantly bigger efficiency for producing a strong magnetic flux. This significantly bigger efficiency may be a reason for designing the focusing magnet device with a bigger magnet volume, which of course is associated with more costs or expenses for the necessary magnet material.

The width of the central magnet device may be, at least for generators being suitable for wind turbines, in a range be tween 25mm and 200 mm and in particular in a range between 50 and 100 mm. In this respect the inventors have further found out that an optimum aspect ratio may depend on the absolute value of the width. For example, for a magnet device having a width of 50 mm a beneficial aspect ratio may be in the range between 0.4 and 0.8. For a magnet device having a width of 100 mm a beneficial aspect ratio may be in the range between 0.2 and 0.6. In these considerations also the expenses for magnetic material may be taken into account.

It is mentioned that the number of magnet device of the MA is not limited to three. In principle the MA may comprise any higher number of magnet devices.

Preferably, all the magnet devices are mounted to a common ferromagnetic (iron) back plate. For many applications, in particular for a generator of a wind turbine, three, four or five focusing magnet devices are used for one MA.

According to a further embodiment of the invention the magnet assembly further comprises a third magnet device having a third angular distribution of magnetization directions re sulting in a third focused magnetization producing a third magnetic focal region. The third location of the third mag netic focal region is different both from the first magnetic focal region and from the second magnetic focal region. Fur ther, the second magnet device is located between the first magnet device and the third magnet device. Furthermore, the first magnet device has a first width, the second magnet de vice has a second width, and the third magnet device has a third width, measured along a direction being perpendicular to a thickness direction of the respective magnet device.

In this embodiment, the second magnet device may be denomi nated a central magnet device, the first magnet device may be denominated a first side magnet device, and the third magnet device may be denominated a third side magnet device.

The described MA having three focusing magnet devices with three different focal regions can be realized in a symmetric manner. Hence, not only the manufacturing of the MA but also the magnetic design of the MA can be simplified. Further, by varying the ratio between (i) the second width and (ii) the first (and second) width a benefit can be taken also from the above mentioned increased freedom of design. This means that, in other words, the MA can be "magnetically designed" in such a manner that a desired (spatial) magnetic flux density pro file or variation (in particular within an air gap) can be realized.

The third width may be the same as the first width and/or a third thickness of the third magnet device may be the same as a first thickness of the first magnet device. This may pro vide the advantage that the entire MA can be realized in a spatially symmetric manner which facilitates the (magnetic) design of the MA.

According to a further embodiment of the invention the thick ness of the second magnet device is different, in particular bigger, than the thickness of at least one of the first mag net device and the third magnet device.

In this respect the inventors found out that with a not uni form thickness of the described MA the (upper) surface of the MA can approximate a curved (sinusoidal) surface which may spatially shape respectively modify the magnetic flux density in particular within an air gap between a rotor assembly and a stator assembly in such a manner that a smooth operation of a corresponding electromechanical transducer (small cogging torque, vibrations, etc.) can be obtained.

Preferably, the thickness of the second magnet device is dif ferent from the thickness of the first and the third magnet device. This may provide the advantage that also magnet as semblies having magnet devices with different thicknesses can be realized in a spatially (mirror) symmetric shape.

According to a further embodiment of the invention the width of at least one of the first magnet device and the third mag- net device is different, in particular bigger, than the width of the second magnet device.

In this respect the inventors found out that by choosing ap propriate widths the MA can be realized with a further degree of freedom in design. Also this further degree of freedom can be exploited in order to realize, for each specific applica tion, a MA which contributes to a smooth operation of an electromechanical transducer.

Preferably, the second magnet device has a second width and both the first magnet device and the third magnet device have common first width. This may provide the advantage that also magnet assemblies having magnet devices with different widths can be realized in a spatially (mirror) symmetric shape.

According to a further embodiment of the invention at least the first magnet device and the second magnet device is a sintered magnet, in particular a sintered magnet comprising NdFeB .

Using a sintered magnet material, in particular with a rare earth material composition, may provide the advantage that a strong magnetic flux density in particular within the various focal regions can be realized.

Further, when taking into account that typically sintered magnets are very rigid and/or brittle structures such that a further processing of the respective sintered magnet is not easy, the described (multi) focusing MA having at least two separate magnet devices can be manufactured comparatively easy because only comparative small magnet devices / magnet pieces are involved. This consideration, which has already been elucidated above, may hold true in particular for mag nets comprising a typical NdFeB material composition.

By using at least two comparatively small sized sintered mag net devices or magnet pieces instead of a smaller number of larger sized sintered magnet devices or magnet pieces the risk of mechanically damaging a magnet device or magnet piece during a further processing may be significantly reduced.

Such a further processing may include for instance a proce dure of providing a protection layer at the outer surface of the magnet piece.

In order to avoid any misunderstanding with regard to the (internal) magnetization structure of the sintered magnet it is pointed out that an angular distribution of magnetization directions as described above is based on or is directly re lated with a preferred direction of grain orientations. This means that it is not necessary that all grains (contributing to a particular magnetic domain alignment direction or mag netization line) have to be oriented exactly in the same di rection. It is rather only necessary that among a certain distribution of grain orientations there is (in average) a preferred grain orientation.

According to a further embodiment of the invention the mag netization directions of at least the first angular distribu tion and the second angular distribution comprises straight lines. Preferably, the above described magnetization direc tions form respectively one straight line.

Having focusing magnetization directions along straight lines may provide the advantage that the process of manufacturing the magnet devices, e.g. during a sintering procedure, may be facilitated. This holds true in particular in view of the matter of fact that an external magnetic field having a cor responding and necessary inhomogeneity can be generated com paratively easy with a proper spatial arrangement of external magnet coils.

According to a further aspect of the invention there is pro vided a rotor arrangement for an electromechanical transduc er, in particular for a generator of a wind turbine. The pro vided rotor arrangement comprises (a) a support structure, and (b) at least one magnet assembly as described above. The magnet assembly is mounted to the support structure.

The provided rotor arrangement is based on the idea that with the above described MA assembly an electromechanical trans ducer can be built up, which in operation, due to its (multi ple) magnetic focusing, yields a high operational efficiency. In particular, unwanted effects such as e.g. cogging torque, vibrations, etc. can be reduced. Such a reduction results not only in a high efficiency factor but also in a low noise op eration of the electromechanical transducer.

According to a further aspect of the invention there is pro vided an electromechanical transducer, in particular a gener ator of a wind turbine. The provided electromechanical trans ducer comprises (a) a stator arrangement, and (b) a rotor ar rangement as described above.

The provided electromechanical transducer is based on the idea that with the above described rotor arrangement one can design a PM electromechanical transducer with which, due to the reduction of at least some unwanted effects, a high oper ational efficiency can be achieved at comparatively low manu facturing costs for the at least one PM assembly.

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 representing a generator for the wind turbine may allow for an increased power production ef ficiency and/or for a reduced operational noise while at the same time keeping the manufacturing expenses for the at least one MA small. This may contribute for improving the attrac tiveness of wind turbine technology for regenerative power production compared to other technologies such as solar plants .

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 linearized representation of the generator.

Figure 4 shows a magnet assembly with three focusing magnet devices .

Figure 5 shows a top view of the magnet assembly shown in

Figure 4.

Figure 6 shows magnetic flux density profiles for different magnets respectively magnet assemblies.

Figure 7 shows magnetic flux density profiles for magnet as semblies having three focusing magnet devices for different width of a central magnet device.

Figure 8 shows, for different magnet devices having different widths, the magnetic flux density achievable within an airgap as a function of the aspect ratio. 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.

Further, spatially relative terms, such as "front" and

"back", "above" and "below", "left" and "right", et cetera are used to describe an element's relationship to another el ement (s) as illustrated in the figures. Thus, the spatially relative terms may apply to orientations in use which differ from the orientation depicted in the figures. Obviously all such spatially relative terms refer to the orientation shown in the figures only for ease of description and are not nec essarily limiting as an apparatus according to an embodiment of the invention can assume orientations different than those illustrated in the figures when in use.

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 110a.

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 140. The electromechanical transducer is a generator 140.

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 153 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 140 comprises a stator arrangement 145 and a rotor arrangement 150. In the embodiment described here the generator 140 is realized in a so called "inner stator - out er rotor" configuration, wherein the rotor arrangement 150 surrounds the stator arrangement 145. This means that non- depicted permanent magnets respectively magnet assemblies of the rotor arrangement 150 travel around an arrangement of a plurality of non-depicted coils of the inner stator arrange ment 145 which coils produce an induced current resulting from picking up a time varying magnetic flux from the travel ing permanent magnets.

According to the embodiment described here each magnet assem bly (MA) comprises at least three sintered permanent magnet devices, which are made from a Nd-Fe-B material composition and which are further described below.

Figure 2 shows in a cross sectional view a schematic repre sentation of the generator 140. A rotational axis of the gen erator 140 is denominated with reference numeral 240a. The generator 140 comprises the stator arrangement 145 depicted already in Figure 1. The stator arrangement 145 comprises a stator support structure 247 comprising a stack of a plurali ty of lamination sheets and a plurality of stator windings 249 being accommodated within the stator support structure 247. The windings 249 are interconnected in a known manner by means of non-depicted electrical connections. The rotor arrangement 150 of the generator 140, which is sep arated from the stator arrangement 145 by an air gap ag, com prises a rotor support structure 252 providing the mechanical base for mounting a plurality of magnet assemblies 260. Ac cording to the exemplary embodiment described here each mag net assembly (MA) comprises three magnet devices, which are not shown in Figure 2 but which are presented below in de tail .

In is mentioned that in Figure 2 only one MA 260 is depicted for the sake of ease of illustration. In reality, depending on the dimension of the generator 140, a plurality of magnet assemblies 260 are mounted to the rotor support structure 252. The PM assemblies 260 are preferably arranged in a ma trix like structure around a curved surface area of the sup port structure 252 having a basically cylindrical geometry around the generator axis 240a.

As can be seen from Figure 2, the magnet assemblies 260 are not mounted directly to the rotor support structure 252. In stead, for each MA 260 there is provided a back plate 254 made from a ferromagnetic material, e.g. iron. The back plate 254 ensures a proper guidance of magnetic flux. This signifi cantly reduces in a beneficial manner the intensity of mag netic stray fields and increases the magnetic flux in the re gion of the air gap.

Figure 3 shows a linearized illustration of the generator 140. In this context "linearized" means that although both the stator arrangement 145 and the rotor arrangement 150 have a circumferential shape (around a non-depicted generator axis being perpendicular to the plane of drawing) , they are, for the easy of illustration, depicted (unrolled) as straight de vices.

Along a circumferential direction of the stator arrangement 145 there are placed a plurality of stator windings 249. Along a circumferential direction of the rotor arrangement 150 there are placed a plurality of magnet assemblies 260. According to the exemplary embodiment described here each MA is mounted at an iron back plate 254 which itself is mounted to a rotor support structure 252.

Figure 4 shows in an enlarged view of one of the magnet as semblies 260. The MA 260 is mounted to an iron back plate 254.

According to the exemplary embodiment described here the MA 260 comprises three focusing magnet devices, a first magnet device 362, a second magnet device 364, and a third magnet device 366. The second magnet device 364 is sandwiched be tween the first magnet device 362 and the third magnet device 366.

Each one of the magnet devices 362, 364, and 366 comprises an angular distribution of magnetization directions, wherein each magnetization direction follows a straight line. Specif ically, the first magnet device 362 comprises a first angular distribution of magnetization directions 463, which yields (outside of the magnet device 362) a first magnetic focal re gion 463a. In a corresponding manner, the second magnet de vice 364 comprises a second angular distribution of magneti zation directions 465 producing a second magnetic focal re gion 465a and the third magnet device 366 comprises a third angular distribution of magnetization directions 467 produc ing a third magnetic focal region 467a.

For each one of the magnet devices 362, 364, and 366 there is a magnetic axis 471 which is defined by a normal direction of a main surface 470 of the respective magnet device 362, 364, 366 and the spatial position of the center of the respective focal region 463a, 465a, 467a. According to the exemplary em bodiment described here the magnetic axis 471 is also a sym- metry axis for the geometry of the respective magnetic device 362, 364, 366.

It is mentioned that in other non-depicted embodiments the magnetization directions do not follow straight lines. Hence, for realizing a MA in accordance with the invention it is al so possible to magnetize the magnet devices in a different manner unless the magnetization is such that a magnetic fo cusing effect is achieved.

Figure 5 shows a top view of the magnet assembly 260 shown already in Figure 4. Magnetic field lines 575, which are pro duced by the magnet devices 362, 364, and 366 outside from the respective magnet body are illustrated in a three dimen sional or perspective manner.

Figure 6 shows magnetic flux density profiles for different magnet devices respectively magnet assemblies. When being mounted to a rotor arrangement of a generator the depicted magnetic flux density profiles are the profiles which are present in an air gap between the rotor arrangement and the stator arrangement. Specifically, these plots show the normal component of the magnetic flux density along a path above the magnet surface (at the center of the airgap) .

A first plot 681, depicts, just for the purpose of compari son, the magnetic flux density profile produced by a single magnet with a single magnetization direction. This means that in this magnet all magnetization directions are parallel with respect to each other.

A second plot 682, depicts, again for the purpose of compar ing, the magnetic flux density profile produced by a single focusing magnet (device) having a spatial dimension of a en tirety of three magnet devices as shown in Figure 4. The max imum value of the magnetic flux 682 is larger than the maxi- mum value of the magnetic flux 681 for a single magnet being magnetized solely in a parallel manner.

A third plot 683 depicts a magnetic flux density profile for a MA 260 with three focusing magnet devices 362, 364, 366 as shown best in Figure 4. As can be taken for Figure 4, the magnetic flux density profile 683 comprises three spatially distinct maximum values each being assigned to one focal re gion 463a, 465a, 467a.

Figure 7 shows magnetic flux density profiles for magnet as semblies each having three focusing magnet devices. The dif ference between the different profiles are caused by a dif ferent width of the central (second) magnet device being sandwiched by the other two magnet devices.

A first plot 793 depicts the magnetic flux density profile produced by the MA 260 wherein the width (along a circumfer ential direction of the rotor device) of the central or sec ond magnet device 364 is the same as the width of the two other (side) magnet devices 362 and 366. This means that the width of the central or second magnet device 364 is 1/3 of the total width of the entire MA 260.

A second plot 794 depicts the magnetic flux density profile produced by a MA wherein the width of the central magnet de vice is larger than the width of the two other (side) magnet devices. This means that the width of the central magnet de vice 364 is more than 1/3 of the total width of the entire MA. According to the exemplary embodiment described here the ratio between the width of the central magnet device and the total width of the MA is 1/2. As can be seen from Figure 7, in this embodiment the width of the central peak of the pro file 794 is larger than the width of the two outer peaks of the profile 794. A third plot 795 depicts the magnetic flux density profile produced by a MA wherein the width of the central magnet de vice is smaller than the width of the two other (side) magnet devices. This means that the width of the central magnet de vice 364 is less than 1/3 of the total width of the entire MA. According to the exemplary embodiment described here the ratio between the width of the central magnet device and the total width of the MA is 1/4. As can be seen from Figure 7, in this embodiment the width and the level of the central peak of the profile 795 is smaller than the width and the level of the two outer peaks of the profile 795.

Figure 8 shows a diagram wherein a magnetic flux density, which can produced with different magnet devices within an airgap of a generator, is depicted as a function of the as pect ratio of the respective magnet device. In this context for a focusing magnet device the aspect ratio is the ratio between a thickness and a width of the magnet device, whereby the thickness is measured along a direction being parallel to the magnetic axis and the width is given by the dimension of the magnet device along a direction being perpendicular to the thickness direction.

Within the diagram of Figure 8, reference numeral 830 points to a curve depicting, for the purpose of comparing, an airgap flux density which can be achieved with a parallel magnetized magnet device having a width of 50 mmm. Curve 832 depicts the corresponding airgap flux density which can be achieved with a focusing magnet device having the same spatial dimensions. From a comparison between the two curves 830 and 832 it can be seen that for larger aspect ratios the difference between the larger flux density produced by the focusing magnet de vice and the smaller flux density produced by the parallel magnetized magnet device is bigger. With increasing aspect ratio the curve 832 shows a significant increase starting from 0.2 up to 0.6. For aspect ratios larger than 0.8 the achievable airgap magnetic flux density increases only with a much smaller extend.

Curves 834 and 836 show the corresponding curves for magnet devices having a width of 100 mmm. Again, the difference be tween the larger flux density produced by the focusing magnet device (see curve 836) and the smaller flux density produced by the parallel magnetized magnet device (see curve 834) gets bigger as the aspect ratio increase. For the 100 mm magnet device a saturation is reached for an aspect ratio above 0.4.

It is not surprising that for lager magnet devices (here for the magnet device having a width of 100 mm) the absolute val ue for the magnetic flux density which can be achieved in an airgap is significantly larger.

From the above presented considerations it can be seen that the aspect ratio is a further parameter which can be varied in order to increase the airgap flux density. Of course, the degree of flux focusing may also be controlled by altering the location of the focal region.

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. A magnet assembly (260) comprising

a first magnet device (362) having a first angular dis tribution (463) of magnetization directions resulting in a first focused magnetization producing a first magnetic focal region (463a); and

a second magnet device (364) having a second angular distribution (465) of magnetization directions resulting in a second focused magnetization producing a second magnetic fo cal region (465a); wherein

the first magnetic focal region (463a) is different from the second magnetic focal region (465a) .

2. The magnet assembly (260) as set forth in the preceding claim, wherein

the first magnet device (362) and/or the second magnet device (364) are realized in a symmetric configuration, wherein the symmetric configuration is given by

(i) the spatial shape and dimension of the respective magnet device (362, 364) and/or

(ii) the respective angular distribution (463, 465) of mag netization directions.

3. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

a first focal distance between the first magnetic focal re gion (463a) and a first main surface (470) of the first mag net device (362) is at least approximately the same as a second focal distance between the second magnetic focal re gion (465a) and a second main surface (470) of the second magnet device (364) .

4. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

the two magnet devices (362, 364) directly abut against each other .

5. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

the first magnet device (362) and/or the second magnet device (364) is formed by at least two magnet pieces being attached to each other.

6. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

the magnet assembly has at least one of the following dimen sions :

(a) a thickness of the magnet devices (362, 364, 366) in in a range in between 10 mm and 40 mm and in particular in a range between 18 mm and 25 mm;

(b) a width of the magnet assembly (260) in a range in be tween 20 mm and 200 mm and in particular in a range between 50 mm and 150 mm.

7. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

at least one of the magnet devices comprises an aspect ratio in the range between 0.2 and 1.0, in particular between 0.4 and 1.0 and more in particular between 0.6 and 1.0, wherein the aspect ratio is defined by the ratio between the thickness of the magnet device and the width of the magnet device ;

the thickness is measured along a direction being paral lel to a magnetic axis of the respective magnet device, which magnetic axis is defined by a normal direction of a main sur face of the respective magnet device and the spatial position of the center of the respective focal region;

the width is given by the spatial extension of the mag net device along a direction which is defined by an axis ex tending between the first magnet device and the second magnet device .

8. The magnet assembly (260) as set forth in any one of the preceding claims, further comprising a third magnet device (366) having a third angular dis tribution of magnetization directions (467) resulting in a third focused magnetization producing a third magnetic focal region (467a); wherein

- the third magnetic focal region (467a) is different both from the first magnetic focal region (463a) and from the sec ond magnetic focal region (465a);

- the second magnet device (364) is located between the first magnet device (362) and the third magnet device (366);

- the first magnet device (362) has a first width, the second magnet device (364) has a second width, and the third magnet device (366) has a third width, all widths measured along a direction being perpendicular to a thickness direction of the respective magnet device (362, 364, 366).

9. The magnet assembly as set forth in the preceding claim, wherein

the thickness of the second magnet device is different, in particular bigger, than the thickness of at least one of the first magnet device and the third magnet device.

10. The magnet assembly as set forth in any one of the two preceding claims, wherein

the width of at least one of the first magnet device and the third magnet device is different, in particular bigger, than the width of the second magnet device.

11. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

at least the first magnet device (362) and the second magnet device (364) is a sintered magnet, in particular a sintered magnet comprising NdFeB.

12. The magnet assembly (260) as set forth in any one of the preceding claims, wherein

the magnetization directions of at least the first angular distribution (463) and the second angular distribution (465) comprises straight lines.

13. A rotor arrangement (150) for an electromechanical trans ducer (140), in particular for a generator (140) of a wind turbine (100), the rotor arrangement (150) comprising

a support structure (252), and

at least one magnet assembly (260) as set forth in any one of the preceding claims, wherein the magnet assembly (260) is mounted to the support structure (252) .

14. An electromechanical transducer (140), in particular a generator (140) of a wind turbine (100), the electromechani cal transducer (140) comprising

a stator arrangement (145), and

a rotor arrangement (150) as set forth in the preceding claim.

15. 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 (140) as set forth in the preceding claim, wherein the electromechanical transducer (140) is mechanically coupled with the wind rotor (110) .