Abstract:
An Nd—Fe—B permanent magnet is disclosed. The Nd—Fe—B permanent magnet includes 28-34 weight % of rare earth elements, where the content of Dy is smaller than 0.1 weight % and the Nd—Fe—B permanent magnet has a spatial extension parallel to a magnetization direction of the Nd—Fe—B permanent magnet which is larger than 30 mm. Further described is a rotor assembly for an electro-mechanical transducer. The rotor assembly includes at least one of such an Nd—Fe—B permanent magnet. Further described is an electromechanical transducer including such a rotor assembly and a wind turbine including said electromechanical transducer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to European Patent Office application No. 12189269.9 EP filed Oct. 19, 2012, the entire content of which is hereby incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to the technical field of rare earth permanent magnets of the Nd—Fe—B type. The present invention further relates to a rotor assembly for an electromechanical transducer, which rotor assembly comprises at least one of such an Nd—Fe—B permanent magnet. Furthermore, the present invention relates to an electromechanical transducer comprising such a rotor assembly and to a wind turbine comprising such an electromechanical transducer. 
       ART BACKGROUND 
       [0003]    Electromechanical transducers are machines, which convert electrical energy into mechanical energy or vice versa. An electric motor is an electromechanical transducer that converts electrical energy into mechanical energy using magnetic field linkage. An electric generator is an electromechanical transducer that converts mechanical energy into electrical energy also using magnetic field linkage. 
         [0004]    An electromechanical transducer comprises a stator and a rotor. The stator is an assembly, which represents the stationary part of an electromechanical transducer. The rotor is an assembly, which represents the moving part of an electromechanical transducer. The above mentioned magnetic field linkage couples the stator with the rotor. 
         [0005]    In recent years, especially since the introduction of rare-earth magnetic materials, permanent magnet (PM) electro-mechanical transducers have become popular since they eliminate the need for commutators and brushes, which are commonly used with conventional Direct Current electromechanical transducer. The absence of an external electrical rotor excitation eliminates losses on the rotor side and makes PM electro-mechanical transducers more efficient. Further, the brushless design of a PM electromechanical transducer allows conductor coils to be located exclusively in the stationary stator. In this respect it is mentioned that non-PM electromechanical transducers, which are equipped with commutators and brushes, are susceptible to significantly higher maintenance costs. 
         [0006]    PM electromechanical transducers are also known for their durability, controllability, and absence of electrical sparking Thanks to their advantages PM electromechanical transducers are widely used in many applications such as electric vehicles (i.e. the electromechanical transducer is a motor) or in power generation systems (i.e. the electro-mechanical transducer is a generator) such as for instance in wind turbines. PM electromechanical transducers are used in particular in so called Direct Drive (DD) wind turbines, wherein the generator is directly connected via a main shaft to (a hub of) a rotor of the respective DD wind turbine. 
         [0007]    Nowadays, highly efficient PM electromechanical transducers are often equipped with rare earth PM of the Nd—Fe—B type, which produce a strong magnetic field and which, as a consequence, guarantee for a strong magnetic field linkage between rotor and stator. This holds also for the generator of a modern wind turbine and in particular for the generator of a DD wind turbine. Therefore, for realizing a (direct drive) wind turbine generator with a permanent excited synchronous generator the PM material Nd—Fe—B is used. 
         [0008]    In order to increase the ability of an Nd—Fe—B PM to resist against a demagnetization field it is known to add the element Dysprosium (Dy) to the magnet grade. In other words, by adding Dy the so called intrinsic coercivity HcJ of the Nd—Fe—B PM is increased. 
         [0009]    Due to a high demand and poor resources the element Dy became quite expensive in recent time. At the moment the cost level of Dy is challenging in particular for producers of wind turbines. In addition all new mines which have opened worldwide cannot supply Dy. They can only supply the element Neodymium (Nd). Therefore, expert prediction is that in the next years with the increasing demand on Nd—Fe—B PM, the price for Dy will further increase and the price for Nd will most likely decrease. 
         [0010]    A further increase in Dy prices would increase the PM cost for a direct drive generator and would result in a high overall cost of a wind turbine. 
         [0011]    There may be a need for providing a Nd—Fe—B permanent magnet which despite of a small or even vanished content of Dy has a sufficiently high intrinsic coercivity HcJ such that the magnet can be used under temperature conditions which allow the use of the magnet for electric generators of wind turbines. 
       SUMMARY OF THE INVENTION 
       [0012]    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. 
         [0013]    According to a first aspect of the invention there is provided a Nd—Fe—B permanent magnet. The provided Nd—Fe—B permanent magnet comprises 28-34 weight % of rare earth elements, wherein the content of the rare earth element Dy is smaller than 0.1 weight %. Further, the Nd—Fe—B permanent magnet has a spatial extension parallel to a magnetization direction of the Nd—Fe—B permanent magnet which is larger than 30 mm. 
         [0014]    The described Nd—Fe—B permanent magnet (PM) is based on the idea that by adapting the geometry of the PM towards a shape having at least a certain thickness parallel to the magnetization of the PM the content of the element Dy of the Nd—Fe—B PM can be significantly reduced while maintaining acceptable values for demagnetization. Hence, the described PM shows a sufficient ability to resist against a demagnetization field. 
         [0015]    In other words, the inventors of the described Nd—Fe—B PM have found out that the geometry and in particular the geometric dimensions of a Nd—Fe—B PM has/have an impact on the ability to resist against a demagnetization. Specifically, the inventors found out that in order to have a sufficient high immunity or resistance against an unwanted demagnetization even when Dy is significantly reduced as compared to a known Nd—Fe—B PM the spatial dimension, e.g. the height, of the PM parallel to the magnetization direction of the PM must exceed a certain threshold value of 30 mm. 
         [0016]    By reducing the amount of Dy (a known Nd—Fe—B PM being used for a wind turbine generator has a Dy content of e.g. up to 15 weight %) the overall cost of the PM can be reduced even though by increasing the height of the Nd—Fe—B PM the overall volume and also the overall mass of the PM magnet will be increased. This means that for realizing the described Nd—Fe—B PM a larger amount of Nd is necessary. However, in view of the large and expected further increase of the price for Dy and the comparatively small and expected further decrease of the price for Nd the described Nd—Fe—B PM may be produced much cheaper than a known Nd—Fe—B PM having a Dy content in the range of up to 15 weight %. 
         [0017]    Apart from neodymium (Nd) the described rare earth elements may in particular include at least one or the following rare earth elements: Praseodymium (Pr), Gadolinium (Gd), Terbium (Tb) Dysprosium (Dy), Holmium (Ho). 
         [0018]    According to a further embodiment of the invention the content of Dy is smaller than 0.01 weight %. This may provide the advantage that the content of Dy is even further reduced such that in view of the extreme high price for the element Dy the overall cost for realizing the described Nd—Fe—B PM can be further reduced. 
         [0019]    It is mentioned that the described Nd—Fe—B PM can be realized even with a zero content of Dy, wherein unintentional impurities are not taken into account. Such impurities may be possible since due to its similar chemical behavior different rare earth element can typically not be separated with a reliability of 100%. 
         [0020]    The relationship between (i) the height parallel to the magnetization direction of the PM and (ii) the intrinsic coercivity HcJ of the PM for a Nd—Fe—B PM having a zero content of Dy and a Nd—Fe—B PM having a certain weight content of Dy can be described by the following phenomenological equation (I): 
         [0000]      HcJ(0% Dy)·h_Mag(0% Dy)□HcJ(x% Dy)·h_Mag(x% Dy)  (I)
 
         [0021]    Thereby, HcJ(0% Dy) is the magnetic intrinsic coercivity for a magnet grade of 0% Dy and a rated magnet temperature, which, depending on the specific cooling concept of the respective generator may be within the range of e.g. 50° C. to 140° C. The term h_Mag(0% Dy) is the height of the PM containing no Dy parallel to its magnetization direction. Correspondingly, HcJ(x% Dy) is the magnetic intrinsic coercivity for a magnet grade having x% Dy and the same rated magnet temperature. Further, the term h_Mag(x% Dy) is the height of the PM containing Dy direction. Again, the height h_Mag(x% Dy) is measured parallel to the magnetization direction. 
         [0022]    This means that the multiplication of the magnetic intrinsic coercivity HcJ with the magnet height h_Mag is a measure of the strength of the magnet as well as a measure against demagnetization. According to the embodiment described here this measure is for the bigger magnet with 0% Dy higher than for the lower magnet with a certain (x) weight % of the element Dy. Of course, this measure has to be higher at the maximum rated operational temperature of the respective magnet which for typical wind turbine generator applications is between e.g. 50° C. to 140° C. 
         [0023]    Descriptive speaking: Compared to known Nd—Fe—B PM the magnet height in magnetization direction is increased so that the resulting bigger PM with 0% Dy has the same behavior against demagnetization, e.g. in a generator short circuit case, at rated operational temperature as the known PM with a certain percentage amount of Dy. 
         [0024]    According to a further embodiment of the invention the spatial extension parallel to the magnetization direction of the Nd—Fe—B permanent magnet is larger than 35 mm. This may provide the advantage that due to the increased height parallel to the magnetization direction the ability of the described PM to withstand an unwanted demagnetization is further increased. 
         [0025]    According to a further embodiment of the invention a further spatial extension perpendicular to the magnetization direction of the Nd—Fe—B permanent magnet is at least 80 mm. This may provide the advantage that the described PM having an extreme small or even a zero content of Dy can be realized with geometrical dimensions which make the PM suitable for wind turbine generator applications. 
         [0026]    Specifically, the described further spatial extension which may be called the width of the described PM is such that a magnet pole of the corresponding generator can consist of one or more PM pieces in a circumferential, radial and axial direction of the generator. In this respect it is mentioned that for a typical generator of a modern direct drive wind turbine the rotor assembly where the PM are mounted may have a diameter of about 3 to 8 m and a length of about 1 to 3 m. This results in a certain area A (=2□·diameter/2·length) which has to be covered with an array of a plurality of PM. 
         [0027]    According to a further embodiment of the invention the intrinsic coercivity HcJ of the Nd—Fe—B permanent magnet at a temperature of 20° C. is less than 1200 kA/m and in particular less than 1000 kA/m. 
         [0028]    According to a further embodiment of the invention the intrinsic coercivity HcJ of the Nd—Fe—B permanent magnet at a temperature of 60° C. is less than 800 kA/m and in particular less than 600 kA/m. 
         [0029]    In this respect it is mentioned that the described Nd—Fe—B PM having an extreme small or even a zero content of Dy differs from a known Nd—Fe—B PM having a certain Dy content by a smaller intrinsic coercivity HcJ. However, this smaller intrinsic coercivity HcJ is still large enough to use the described Nd—Fe—B PM for a rotor of a generator of a (direct drive) wind turbine. 
         [0030]    According to a further aspect of the invention there is described a rotor assembly for an electromechanical transducer, in particular for a generator of a wind turbine. The described rotor assembly comprises (a) a support structure, and (b) at least one Nd—Fe—B permanent magnet as described above. The Nd—Fe—B permanent magnet is mounted to the support structure. 
         [0031]    V: The described rotor assembly is based on the idea that with the above described Nd—Fe—B permanent magnet a rotor assembly for an electromechanical transducer can be built up, which despite of a small or even zero content of Dy allows for a reliable operation by avoiding an unwanted demagnetization of the Nd—Fe—B PM. 
         [0032]    The mentioned support structure may be realized for instance by a so called rotor housing which, when being installed in a PM electromechanical transducer, is rotatable around a rotational axis of the PM electromechanical transducer. 
         [0033]    According to a further aspect of the invention there is described an electromechanical transducer, in particular a generator of a wind turbine. The described electromechanical transducer comprises (a) a stator assembly, and (b) a rotor assembly as described above. 
         [0034]    The described electromechanical transducer is based on the idea that with the above described rotor assembly one can design a PM electromechanical transducer wherein despite having only a reduced amount of Dy the ability to resist against a demagnetization field is sufficiently high such that a reliable operation of the electromechanical transducer can be ensured. 
         [0035]    It is mentioned that the provided electromechanical transducer may be realized with different designs. A common design option is characterized by the matter of fact that the rotor assembly is located within the stator assembly. A less common design option is characterized by the matter of fact that the rotor assembly is located outside from the stator assembly. In the latter option during operation of the PM electromechanical transducer the magnetic flux interacting between the stator coils and the Nd—Fe—B magnet components may be directed radially outwards from the stator. 
         [0036]    According to a further aspect of the invention there is provided a wind turbine for generating electrical power. The provided wind turbine comprises (a) a tower, (b) a rotor, which is arranged at a top portion of the tower and which comprises at least one blade, and (c) an electromechanical transducer as described above, wherein the electromechanical transducer is mechanically coupled with the rotor. 
         [0037]    The described wind turbine is based on the idea that the above described electromechanical transducer representing a generator for the wind turbine may allow for realizing the wind turbine in a less expensive way because due to the defined geometric dimensions of the PM being used for the rotor assembly of the generator compared to known PM a large amount of the very expensive element Dy can be saved. 
         [0038]    It has to be noted that embodiments of the invention have been described with reference to different subject matters. 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 is considered as to be disclosed within this document. 
         [0039]    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 embodiment but to which the invention is not limited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0040]      FIG. 1  shows a wind turbine according to an embodiment of the present invention. 
           [0041]      FIGS. 2 and 3  show a comparison between a known electro-mechanical transducer comprising a rotor assembly being equipped with Nd—Fe—B magnets having a certain content of Dy and an electro-mechanical transducer in accordance with an embodiment of the invention wherein Nd—Fe—B magnets are used which are free from Dy. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    The illustration in the drawing is schematically. 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 described embodiment are not elucidated again at a later position of the description. 
         [0043]      FIG. 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 nacelle  122  around a non depicted vertical axis being aligned with the longitudinal extension of the tower  120 . By controlling 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. However, the yaw angle adjustment device  121  can also be used to adjust the yaw angle to a position, wherein the nacelle  122  is intentionally not perfectly aligned with the current wind direction. 
         [0044]    The wind turbine  100  further comprises a rotor  110  having three blades  114 . In the perspective of  FIG. 1  only two blades  114  are visible. The rotor  110  is rotatable around a rotational axis  110   a.  The blades  114 , which are mounted at a hub  112 , extend radially with respect to the rotational axis  110   a.    
         [0045]    In between the hub  112  and a blade  114  there is respectively provided a blade adjustment device  116  in order to adjust 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 adjustment 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 wind power. However, the blade pitch angle can also be intentionally adjusted to a position, in which only a reduced wind power can be captured. 
         [0046]    As can be seen from  FIG. 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 number of revolutions of a shaft  125 , which is coupled in a known manner to an electromechanical transducer  140 . The electro-mechanical transducer is a generator  140 . 
         [0047]    At this point it is pointed out that the gear box  124  is optional and that the generator  140  may also be directly coupled 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. 
         [0048]    Further, 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 instance (a) in case of an emergency, (b) in case of too strong wind conditions which might harm the wind turbine  100 , and/or (c) in case of an intentional saving of the consumed fatigue life time and/or the fatigue life time consumption rate of at least one structural component of the wind turbine  100 . 
         [0049]    The wind turbine  100  further comprises a control system  153  for operating the wind turbine  100  in a highly efficient manner. Apart from controlling for instance the yaw angle adjustment 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. 
         [0050]    In accordance with basic principles of electrical engineering the generator  140  comprises a stator assembly  145  and a rotor assembly  150 . The stator assembly  145  comprises a plurality of not depicted coils for generating electrical current in response to a time alternating magnetic flux. The rotor assembly  150  comprises a plurality of Nd—Fe—B permanent magnet pieces which are arranged in an array being arranged around a curved surface area of the rotor assembly  150 . In accordance with embodiments of the invention the Nd—Fe—B permanent magnet pieces do not contain the element Dy. In order to ensure that they nevertheless have a sufficient high intrinsic coercivity these Nd—Fe—B permanent magnet pieces have a comparatively large physical extension along a direction being parallel with the magnetization direction. Specifically, this physical extension, which in this document is also called the height of the PM pieces, is larger than 30 mm. Specifically, according to the concrete embodiment described here this height is approximately 37 mm. 
         [0051]    It is mentioned that it is not necessary that the rotor assembly  150  is arranged within the stator assembly  145 . It is rather also possible that the rotor assembly surrounds the stator assembly. 
         [0052]      FIGS. 2 and 3  show in a cross sectional view a comparison between (a) a known electromechanical transducer  240  comprising a rotor assembly  250  being equipped with Nd—Fe—B magnet pieces  254  having a certain content of Dy and (b) an electromechanical transducer  340  in accordance with an embodiment of the invention wherein Nd—Fe—B magnet pieces  354  are used which have a composition being free from the (very expensive) element Dy. 
         [0053]    Both the known electromechanical transducer  240  shown in  FIG. 2  and the electromechanical transducer  340  in accordance with an embodiment of the invention shown in  FIG. 3  have the same stator assembly  245 . As can be seen from the Figures the stator assembly  245  has a stator support structure  247  comprising a stack of a plurality 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. 
         [0054]    The rotor assembly  250  of the known electromechanical transducer  240 , which is separated from the stator assembly  245  by an air gap ag, comprises a rotor support structure  252  providing the mechanical base for mounting the Nd—Fe—B PM pieces  254 . The support structure  252  comprises a magnetic material in order to optimize the magnetic field generated by the Nd—Fe—B PM piece  254  and other Nd—Fe—B PM pieces not shown in  FIG. 2 . Thereby, in accordance with known principles in the design of electromechanical transducers the magnetic field is optimized in such a manner that the magnetic field linkage between the stator assembly  245  and the rotor assembly  250  is maximized. 
         [0055]    A rotor assembly  350  of the electromechanical transducer  340  comprises a rotor support structure  352  providing the mechanical base for mounting the Nd—Fe—B PM pieces  354 . Again, the support structure  352  comprises a magnetic material in order to optimize the magnetic field linkage between the stator assembly  245  and the rotor assembly  350 . 
         [0056]    According to the embodiment described here the air gap ag between the rotor assembly  350  and the stator assembly  245  can be used for guiding a cooling fluid through the electro-mechanical transducer  340  in order to remove waste heat which during operation is generated in particular by the stator coils  249  of the stator assembly  245 . 
         [0057]    It is mentioned that a plurality of PM pieces can be mounted to the support structure  352 . The PM pieces are preferably arranged in a matrix like structure around a curved surface area of the support structure  352  having a basically cylindrical geometry. In  FIG. 3  for the sake of clarity only one PM piece  254  is depicted. 
         [0058]    As can be seen from a comparison between the  FIGS. 2 and 3 , the height h of the Nd—Fe—B PM piece  354  with 0% Dy is approximately larger by a factor of two compared to the height of the Nd—Fe—B PM piece  254  with x% Dy. Thereby, the height which in  FIGS. 2 and 3  extends along the vertical direction is oriented parallel to the magnetization direction of the Nd—Fe—B PM pieces  254 ,  354 . By increasing the magnet height along the magnetization direction and by eliminating Dy from the magnets composition it is possible to keep the same or at least similar demagnetization properties as compared to the magnet with a smaller height and a certain content of Dy. 
         [0059]    In other words, by changing both the composition and the size of the Nd—Fe—B PM the magnetic behavior can be kept at least approximately the same. When reducing the amount of Dy in the magnets composition the magnet height parallel to its magnetization direction has to be increased to end up with at least approximately the same magnet piece properties. 
         [0060]    In view of the high price for the element Dy by reducing the amount of Dy the overall cost of the PM can be reduced even though by increasing the height of the Nd—Fe—B PM the overall volume and, as a consequence, also the overall mass of the PM magnet and the needed amount of Nd will be increased. 
         [0061]    It should be noted that the term “comprising” does not exclude 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.