Abstract:
A magnet for a generator is provided. The magnet includes a base magnet with a main surface having a length and a width and a skewed magnet module arranged at the main surface. A rotor of a generator including the magnet is provided as well as a generator with a stator and rotor including the magnet.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 10192739.0 EP filed Nov. 26, 2010, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates in general to a magnet for a generator. In particular, the present invention is directed to a magnet for a generator, a rotor, a stator and a generator comprising the magnet. 
       BACKGROUND OF INVENTION 
       [0003]    Due to variation in magnetic forces within rotating permanent magnet machines like generators or motors pulsations of torque occur. The torque pulsations occur at no-load and load situations, hereafter denoted cogging torque and ripple torque. These torque pulsations may lead to vibrations which can be harmful to the rotating machine and interconnected parts. Furthermore, the torque pulsations can create acoustic noise of low frequency which can disturb the environment. This problem can be even more pronounced in direct drive wind generators. 
         [0004]    Reduction of torque can be achieved with different methods like optimizing the 2-dimensional shape of the magnet, shaping the stator/rotor tooth tips, displacing the magnets, conventional skew of the magnets or active damping via injection of current harmonics. Each method has its own drawbacks e.g. by magnet displacement, the magnet leakage flux is increased and thereby the output torque is reduced. Conventional magnet shaping in 2-dimension increases cost of manufacturing compared to conventional skew and needs accurate modelling tools like numerical FEM for optimizing the shape of the magnet. 
         [0005]    Skew of the magnets is simple and inexpensive in general. However, conventional skew has several drawbacks like the distribution of magnet flux and thereby reduction of the magnet flux fundamental component and thereby reduction of the output average torque of the generator. Furthermore, for simplicity reasons of the magnet shape and fitting the magnet on a curved surface of the rotor, the skew is usually done in finite and few numbers of steps. As a result, more magnet flux leaks in the regions where two blocks are distant from one another, which is depicted by flux leakage vectors in  FIG. 1 . Consequently, more volume of magnets is required to provide the same average torque (i.e. magnets are not used efficiently). 
         [0006]    Furthermore, since skewing in many steps may correlate with higher flux leakage, the skew is normally done in few numbers of steps and thereby the ripple or cogging torque minimization may not be fully effective for some designs. 
       SUMMARY OF INVENTION 
       [0007]    It is therefore an object of the present invention to reduce torque pulsations in permanent magnet machines. 
         [0008]    This object is solved by the features of the claims. The dependent claims offer further details and advantages of the invention. 
         [0009]    In one aspect the invention is directed to a magnet for a generator, comprising a base magnet with a main surface having a length and a width and a skewed magnet module arranged at the main surface. The cogging and ripple torque, thereby vibration and mechanical fatigue, in the generator are minimized using this novel skewing technique. The proposed magnet is more effective than existing alternatives e.g. conventional skew in the manner of cogging and ripple torque minimization while it does not have the drawbacks of the latter like reduction of average torque and thereby the output power. Unlike the conventional skew in which the whole magnet pole is skewed, it is proposed to introduce an asymmetrical magnet shape e.g. by adding an extra magnet on a surface of the base magnet and only skew this skewed magnet module. 
         [0010]    The analytical and FE calculation has proved that the novel magnet does minimize the ripple and cogging torque effectively. Furthermore, it does not have the drawbacks of the conventional skew. In the new technique, there is no extra leakage flux due to skewing, simply because the base magnet has not been displaced, so the flux of the skewed module has to follow the same magnetization pattern of the dominating base magnet. Another advantage of the proposed magnet is that the skewed magnet module makes it easier to have a continuous skew or since there is no extra flux leakage, it is possible to do the skew in any number of steps and thereby minimizing the ripple torque more effectively. 
         [0011]    The proposed magnet is very simple in implementation and thereby results in a cheaper product like a direct drive generator or a rotor or stator of it with improved performance. 
         [0012]    Finally, since the proposed skew reduces the magnet flux harmonics in the air gap of the generator, it is also an effective technique to minimize the loss in the generator and improve the efficiency. 
         [0013]    The base magnet may have a rectangular shape. This shape is easy to manufacture and to assemble. 
         [0014]    The base magnet and the skewed magnet module may be integrally formed. It is also possible that the base magnet and the skewed magnet module can be composed of several pieces and put together in axial direction, or the base magnet can be separate from the skewed magnet module and they can be fit together by using a fitting cover or glue. 
         [0015]    The skewed magnet module may comprise several magnet segments arranged along the length of the magnet and the magnet segments may comprise an offset in width direction. This arrangement is adapted to the standard configuration of a permanent magnet machine and can therefore be widely used. 
         [0016]    The width of the base magnet may equal substantially to two times the tooth pitch of the generator and the width of a magnet segment may equal substantially to the tooth pitch of the generator. The presence of the skewed magnet module allows a continuously width of the base magnet of around two times the tooth pitch. The segments may be skewed by the span of one tooth pitch. The non-skewed volume disappears which increases the effectiveness of the skewing by a great margin. 
         [0017]    The width of the base magnet may equal substantially to two times and two thirds the tooth pitch of the generator, two skewed magnet modules may be arranged at the main surface, the width of a magnet segment of one skewed magnet module may equal substantially to the tooth pitch of the generator and the width of a magnet segment of the other skewed magnet module may equal substantially to one third the tooth pitch of the generator. This solution keeps the advantages described above while allowing a finer adaptation to the permanent magnet machine. 
         [0018]    The skewed magnet module may comprise a recess in the main surface, wherein the recess may be arranged at an angle with respect to the length of the main surface. An alternative approach to fulfil the same advantages as previously described is to create a skewed-like trail of recess or cavity in the base magnet. The manufacturing cost will be less. The technique may be implemented by simple form of machining and trimming the specified skew-like corridor away from the rectangular base magnet. 
         [0019]    The skewed magnet module may face teeth of the generator. This arrangement improves the reduction of magnet flux harmonics in the air gap which is an effective technique to minimize the loss in the generator and improve the efficiency. 
         [0020]    The thickness of the skewed magnet module may be less than or equal to half of the thickness of the base magnet. Due to the fact that only a small fraction of the magnet flux is distributed (i.e. the fundamental flux is almost the same as the case with no skew due to the fact that the skewed part is much thinner than the dominant base magnet), the reduction of the fundamental component of the magnet flux is almost negligible, so is the reduction of average torque. 
         [0021]    The skewed magnet module may extend over the complete length of the magnet, the skewed magnet module may comprise several magnet segments and each magnet segment may have the same length. This uses the available space for skewing resulting in higher effectiveness of the skewing. 
         [0022]    In a second aspect the invention is directed to a rotor of a generator comprising a magnet as described above. The same advantages and modifications as described above apply here. This rotor is suitable for a permanent magnetic machine having the windings in a stator. 
         [0023]    In a third aspect the invention is directed to a stator of a generator comprising a magnet as described above. The same advantages and modifications as described above apply here. This rotor is suitable for a permanent magnetic machine having the windings in a rotor. 
         [0024]    In a further aspect the invention is directed to a generator with a stator and a rotor, comprising a magnet as described above. The same advantages and modifications as described above apply here. 
         [0025]    The generator may comprise an outer rotor configuration for a wind turbine. The proposed magnet is very suitable for the large direct drive generators like in wind power application. It effectively serves to reduce noise, increases reliability, increases efficiency or reduces the weight of the generator for the same efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The accompanying drawings are included to provide a further understanding of embodiments. Other embodiments and many of the intended advantages will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings do not necessarily scale to each other. Like reference numbers designate corresponding similar parts. 
           [0027]      FIG. 1  illustrates a schematic view of a magnet and teeth of a known permanent magnet machine. 
           [0028]      FIG. 2  illustrates a schematic cross-sectional view of a magnet and teeth of a permanent magnet machine according to the invention. 
           [0029]      FIG. 3  illustrates a schematic bottom view of the magnet together with the magnetic field according to the invention. 
           [0030]      FIG. 4  illustrates a schematic bottom view of another magnet according to the invention. 
           [0031]      FIG. 5  illustrates a schematic bottom view of another magnet according to the invention. 
           [0032]      FIG. 6  shows a diagram of the magnetic flux density in the air gap. 
           [0033]      FIG. 7  shows a diagram of the fundamental component of the magnetic flux density of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0034]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof and in which are shown by way of illustration specific embodiments in which the invention may be practised. In this regard, directional terminology, such as “top” or “bottom” etc. is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
         [0035]      FIG. 1  shows part of a known permanent magnet machine for example a generator  1 . With the known generator  1  the basic structure and function of the generator  1  is explained in preparation for the proposed magnet and permanent magnet machine as shown in  FIGS. 2 to 4 . 
         [0036]    The generator  1  has a stator  2  with teeth  3 . The teeth  3  are arranged with a distance called tooth pitch τ tooth  to each other. Between two teeth  3  a groove  4  for accommodating windings is arranged. The tooth pitch τ tooth  shown reaches from the middle of a groove  4  to the middle of a neighbouring groove  4  as this indicates a vibration domain. 
         [0037]    A magnet  5  which is attached to a rotor (not shown) of the generator  1  is located above the teeth  3 . Above means in radial direction r. Between the magnet  5  and the teeth  3  is an air gap  6  of the generator  1 . 
         [0038]    In an alternative design the teeth can be part of the rotor while the magnet can be arranged at the stator. 
         [0039]    The conventional magnet  5  is skewed in two steps which results in three modules or parts of the magnet  5 . The modules are skewed by a margin of half the tooth pitch τ tooth . Note that the skew could also be done with a spatial span of one tooth pitch. 
         [0040]    The hatching illustrates the un-skewed volume of the skewed magnet modules. The skewing pitch, i.e. the tooth pitch τ tooth , is often smaller, about half of the magnet width. From tooth perspective, this region is not skewed and this is one of the reasons why the effectiveness of conventional skew is usually not 100%. 
         [0041]    Further, the additional leakage flux leaving the modules is shown at those parts of a front surface (in direction z) which are not covered by a neighbouring module. The flux leakages reduce the average torque of the Generator  1  and are therefore not desired. 
         [0042]      FIG. 2  shows a permanent magnetic machine like a motor or generator  1  according to the invention. The structure shown is the same as the one of  FIG. 1  despite the magnet. The explanation above (except for the magnet) holds true for  FIG. 2 . 
         [0043]    The magnet  10  has a base magnet  11  with a width w PM  of two times the tooth pitch τ tooth . The width w PM  can be exactly two times the tooth pitch τ tooth  or it can be close to it, e.g. +/−10%. The width w PM  extends in a circumferential direction θ. The base magnet  11  has a length L in the direction of z. The width w PM  and the length L span a main surface  12 . Actually, the base magnet  11  has two main surfaces, one facing the stator  2  and its teeth  3  and another one being opposite. Later on, the term main surface refers to the first one. 
         [0044]    At the main surface  12  a skewed magnet module  13  is arranged. The skewed magnet module  13  comprises three magnet segments  14  which can be identical. Each magnet segment  14  has a width w PMM  of one time the tooth pitch τ tooth . The width w PMM  can be exactly one time the tooth pitch τ tooth  or it can be close to it, e.g. +/−10%. The length of each magnet segment  14  equals to L/m wherein m is the number of magnet segments  14 . In this case, the length of a magnet segment  14  amounts to a third of the length L of the base magnet  11 . The three magnet segments  14  are arranged in the direction of the length L of the base magnet  11 . 
         [0045]    The magnet segments  14  comprise an offset or skew in direction of the width w PM  of the base magnet  11  or the width w PMM  of the magnet segment  14  which is in this case the same direction. The offset is one third of the tooth pitch τ tooth  and the offset of the first and the last magnet segment  14  amounts to the tooth pitch τ tooth . 
         [0046]    The hatching illustrates the un-skewed volume of base magnet  11 . The width of the un-skewed volume is only one third of the tooth pitch τ tooth . 
         [0047]      FIG. 3  shows the magnet  10  from the bottom side. The base magnet  11  has a height h PM  in radial direction r while the magnet segment  14  has a height h PMM . The height h PMM  of the magnet segment  14  is smaller than the height h PM  of the base magnet  11 . The height h PMM  of the magnet segment  14  can be half, a third or even smaller than height h PM  of the base magnet  11 . 
         [0048]    Further,  FIG. 3  shows the magnetic field indicated by the arrows running in radial direction. There is no extra leakage flux due to skewing because the base magnet  11  has not been displaced. So the flux of the skewed magnet module  13  has to follow the same magnetization pattern of the dominating base magnet  11 . 
         [0049]      FIG. 4  shows another magnet  10  with a base magnet  11  as shown in  FIG. 3  and described in conjunction with  FIG. 3 . Here, two skewed magnet modules are attached to the main surface  12  of the main magnet  11 . A first skewed magnet module  15  has three magnet segments  16  which are skewed. The magnet segment  16  has a width of one tooth pitch τ tooth  and are set off by the width of the magnet segment  16 . A second skewed magnet module  17  has three magnet segments  18  which are skewed. The magnet segment  18  has a width w PMM2  of one third of the tooth pitch τ tooth  and are set off by one third of the width of the magnet segment  18 . 
         [0050]    The two skewed magnet modules  15 ,  17  are arranged such that at the main surface  12  that in width direction every part of the base magnet  11  is covered by at least one magnet segment  16  or  18 . Because of this arrangement there is no un-skewed volume of base magnet  11 . 
         [0051]    Fabrication of the magnet  10  is easy as only rectangular shapes need to be produced an fit together e.g. by means of glue. 
         [0052]      FIG. 5  shows a further magnet  10  with a base magnet  11  similar to the ones described in conjunction with  FIGS. 2 to 4 . Here, the skewed magnet module  19  is realised by a recess or cavity  20 . The recess  20  is arranged in a diagonal manner in the main surface  12 . The angle between the length L of the main surface  12  and the recess  20  can be the same under which the magnet segments  14  of  FIG. 2  are arranged. It is also possible to arrange the recess  20  in such a manner that it stretches diagonally from one side of the main surface  12  or the base magnet  11  to the other side. 
         [0053]    With respect to the dimensions of the recess  20  like the width w PMM  or the height the same dimensioning rules apply as for  FIGS. 2 to 4 . 
         [0054]      FIG. 6  shows the magnet flux density in the air gap  6 . The curve for conventional skew shows large increases in the density leading to reduced torque. The skew according to the invention reduces the flux density only by a small margin as shown by the respective curve. 
         [0055]      FIG. 7  shows the fundamental component of the magnet flux. The situation is the same as in  FIG. 6 , the skew according to the invention reduces the fundamental component only by a small margin. 
         [0056]    Due to the fact that only a small fraction of the magnet flux is distributed the reduction of the fundamental component of the magnet flux is almost negligible, so is the reduction of average torque. 
         [0057]    With the inventive skew it is possible to do the skew in any number of steps which minimizes the ripples more effectively.