Abstract:
A direct drive generator for a wind turbine is provided. The direct drive generator includes a stator arrangement and a rotor arrangement. The stator arrangement and/or the rotor arrangement include an at least partly flexible front and/or rear endplate. The endplate is at least partly made of fiberglass.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2009/057977 filed Jun. 25, 2009, and claims the benefit thereof. The International Application claims the benefits of European Patent Application No. 08012250.0 EP filed Jul. 7, 2008. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a direct drive generator for a wind turbine and a wind turbine comprising such a direct drive generator. 
     BACKGROUND OF INVENTION 
     In principle there are two main types of wind turbines in view of the drive configuration of a wind turbine. The first type of a wind turbine is the more classical type of a wind turbine comprising a gearbox arranged between the main shaft and a generator of the wind turbine. The second type of a wind turbine is a gearless type comprising a direct drive or a directly driven generator. Such a direct drive generator can be made as a synchronous generator with winded rotor or with permanent magnets attached to the rotor, or it can be designed as an alternative type of a generator. Independently from the type of a direct drive generator it is desirable that the width of the air gap between the rotor and the stator of the generator is preferably maintained constantly or at least within certain tolerances during the operation of the wind turbine and the direct drive generator respectively even when the arrangement of wind turbine rotor, main shaft and direct drive generator is subject to loads. 
     Therefore different bearing arrangements for a drive configuration of a wind turbine comprising a direct drive generator were developed. Up to now the classical bearing arrangement of a direct drive generator is a two-bearing arrangement. Thereby the rotor of the generator which is connected to the wind turbine rotor is supported with two bearings towards a stationary inner shaft or a fixed inner shaft. The stator is on one side attached to the stationary inner shaft. Thus the rotor can turn relatively to the stator around the stationary inner shaft. Wind turbines with such a design are e.g. described in EP 1 641 102 A1 and U.S. Pat. No. 6,483,199 B2. The drawback of such a design is that the one-side support of the stator makes it difficult to maintain the width of the air gap at least substantially constant at the unsupported side of the stator in particular when the entire generator structure is not only subject to gravity and mass inertia but also to unbalanced magnetic pull. In order to reduce this drawback a direct drive generator with such a two-bearing arrangement needs a large and heavy stator support structure capable of absorbing relatively large bending moments of the stator. Such a stator support structure is e.g. described in WO 02/05408 A1 wherein the stator support structure comprises a support construction having a plenty of support arms. 
     In an alternative design the two-bearing arrangement is replaced with a single bearing with a stationary inner bearing part attached to a stationary inner shaft and a rotating outer bearing part supporting the rotor of the direct drive generator. Wind turbines comprising a direct drive generator with a single bearing are disclosed in US 2006/0152014 A1 and WO 02/057624 A1. But the replacement of the two bearings with a single bearing does not substantially change the drawback of the unilaterally supported stator structure. 
     In some further solutions the stationary inner shaft concept is replaced with a rotating shaft concept. Since the stator of the generator is supported on both sides according to the rotating shaft concept, it is easier to maintain the width of the air gap between the rotor and the stator of the generator at least substantially constantly. There are two known variants of the rotating shaft concept, one with a two-bearing arrangement and one with a four-bearing arrangement. 
     According to the two-bearing arrangement the bearings of the generator act as bearings of a main shaft for the wind turbine which main shaft is connected to the wind turbine rotor. The stator structure is supported towards the main shaft and attached to a bedplate of the wind turbine. Wind turbines having such a design are disclosed in U.S. Pat. No. 7,119,453 B2 and WO 03/023943 A2. A drawback of this design is that the stator structure needs to be dimensioned to absorb and transfer all wind turbine rotor loads, i.e. the weight of the wind turbine rotor and all asymmetric aerodynamic loads to maintain the width of the air gap within the necessary tolerances. On large wind turbines this leads to very heavy and expensive stator structures. 
     In the four-bearing arrangement the main shaft of the wind turbine which is connected to the wind turbine rotor on its one end is supported by its own two bearings and carries at its other end the direct drive generator. The direct drive generator has a two-bearing arrangement for the centering of the rotor inside the stator. An example of such a wind turbine is described in U.S. Pat. No. 6,781,276 B1. In this main shaft mounted arrangement the generator stator is carried by the generator rotor and torque is transferred from the generator to the wind turbine bedplate through a torque aim arrangement. The torque arm arrangement needs to comprise some kind of flexibility, e.g. implemented with rubber elements, to allow for minor misalignments between the main shaft—generator structure and the turbine bedplate. The bilateral support of the stator on the rotor enables for a relatively lightweight stator structure. The main drawback of this design is that a total of four bearings are required, and that the full torque has at least partially to pass through these bearings. For large wind turbines this means comparatively large and expensive bearings. Furthermore, for large wind turbines the torque arm arrangement becomes a comparatively substantial and heavy structure. 
     In U.S. Pat. No. 4,291,235 a further bearing arrangement for a wind turbine is described. The wind turbine comprises a stationary shaft as well as a direct drive generator having an inner stator and an outer rotor. The inner stator is arranged on the stationary shaft. The outer rotor is connected to the hub of the wind turbine, on the front side connected to the stationary shaft by a front bearing and on the rear side connected to the stationary shaft by rear bearing. Thereby the bearing arrangement is not optimal in relation to the load capacity. 
     Furthermore, in EP 2 014 917 A1 a wind turbine comprising a direct drive generator having a rotor and a stator is described. The rotor of the direct drive generator is attached to the rear side of a main shaft which main shaft is supported by two main bearings. The stator is substantially arranged around the rotor and comprises a front endplate, a rear endplate and a casing element connecting the front and the rear endplate to each other. The rear endplate of the stator is supported on the rotor or the main shaft by a third bearing. The front endplate of the stator is at least indirectly attached to a stationary part of the wind turbine. Therefore, in the EP 2 014 917 A1, a three bearing arrangement is described. 
     Such a three bearing structure can be sometimes statically undetermined. In this case any misalignments due to mounting tolerances or any deformations arising as a result of gravity or external loads to the main shaft and/or the direct drive generator could potentially lead to an uneven load distribution between the three bearings of the wind turbine which in turn could cause a premature bearing failure. The front endplate of the stator is therefore at least partially in a certain adequate extent flexible in the directions of a centre axis of the main shaft. 
     Thus the potential problem of a static indeterminacy of the three bearing arrangement is eliminated by establishing a sufficient flexibility of the front endplate in the directions of the centre axis of the main shaft. The front endplate acts like a membrane supporting the stator substantially firmly in the radial direction so as to maintain the air gap, but flexing readily so as to enable e.g. a bending of the main shaft with no major resistance. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to indicate an appropriate material for a flexible endplate of a direct drive generator of a wind turbine to preferably guarantee a sufficient flexibility of the endplate. 
     This object is inventively achieved by a direct drive generator for a wind turbine comprising a stator arrangement and a rotor arrangement, wherein the stator arrangement and/or the rotor arrangement comprises at least one at least partly flexible front and/or rear endplate, wherein the at least one endplate is at least partly, preferably completely made of fibreglass. Fibreglass comprises a relatively low module of elasticity and is therefore an appropriate material for the at least one endplate in view of the required flexibility. Additionally fibreglass comprises excellent corrosion behaviour, even in an aggressive environment like an offshore application of a wind turbine. A further advantage of fibreglass as material for a generator is the well electrical isolation. A direct drive generator comprising at least one such an endplate made of fibreglass is therefore well suitably for a wind turbine comprising a drive configuration which could be sometimes statically undetermined, e.g. a drive configuration with a three bearing structure. 
     According to an embodiment of the invention the direct drive generator comprises a centre axis, wherein the at least one endplate is at least partially in a certain extent flexible in the directions of the centre axis. Thus the potential problem of a static indeterminacy e.g. of a three bearing arrangement is eliminated by establishing a sufficient flexibility of the at least one endplate in the directions of the centre axis of the direct drive generator. When required the at least one endplate flexing readily so as to enable e.g. a bending of a respective component of the drive configuration of the wind turbine with no major resistance so that there occurs substantially no uneven load distribution between e.g. the three bearings of the drive configuration of the wind turbine. 
     According to a variant of the invention the at least one endplate is a ring-shaped endplate. According to a further variant of the invention the at least one endplate has a thickness of approximately 15-50 mm, preferably 20-30 mm in the direction of the centre axis. Thus the at least one endplate has such dimensions e.g. in dependence of the structure of the at least one endplate that the at least one endplate has a comparatively little bending stiffness. 
     In an embodiment of the invention the stator arrangement is substantially arranged around the rotor arrangement, comprises a front endplate, a rear endplate and a hollow cylindrical stator element connecting the front endplate and the rear endplate with each other. Preferably the hollow cylindrical stator element comprises on the inside at least one lamination stack provided with at least one winding, typically with a plurality of windings. 
     In a further embodiment of the invention the rotor arrangement is substantially arranged inside the stator arrangement. In particular the rotor arrangement comprises a front endplate, a rear endplate and a hollow cylindrical rotor element connecting the front endplate and the rear endplate with each other. Preferably the hollow cylindrical rotor element comprises on the outside at least one permanent magnet arranged oppositely to the lamination stack with windings for power generation. As a rule a plurality of permanent magnets are arranged on the outer surface of the hollow cylindrical rotor element. 
     In another embodiment of the invention the rotor arrangement is substantially arranged around the stator arrangement, comprises a front endplate, a rear endplate and a hollow cylindrical rotor element connecting the front endplate and the rear endplate with each other. Preferably the hollow cylindrical rotor element comprises at least one permanent magnet on the inside. As a rule a plurality of permanent magnets are arranged on the inner surface of the hollow cylindrical rotor element. 
     According to a variant of the invention the stator arrangement is substantially arranged inside the rotor arrangement and comprises a stator support structure. Preferably at least one lamination stack provided with at least one winding, typically with a plurality of windings, is arranged on the stator support structure oppositely to the permanent magnets for power generation. 
     The object of the invention is also achieved by a wind turbine comprising a direct drive generator as previously disclosed and a drive configuration for turning the rotor arrangement relatively to the stator arrangement comprising at least three bearings or two bearings wherein at least one of the two bearings is a four-point bearing which typically shows the behaviour of two bearings so that the two bearing arrangement with at least one four-point bearing behaves like a three bearing structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will in the following be explained in more detail with reference to the schematic drawing, wherein 
         FIG. 1  shows a first type of a wind turbine comprising a direct drive generator having a flexible endplate, 
         FIG. 2  shows a second type of a wind turbine comprising a direct drive generator having two flexible endplates, 
         FIG. 3  shows a third type of a wind turbine comprising a direct drive generator having a flexible endplate and 
         FIG. 4  shows a fourth type of a wind turbine comprising a direct drive generator having a flexible endplate. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows schematically a first embodiment of a wind turbine  1  comprising a direct drive generator  2  which is arranged on the downwind side of a tower  3  of the wind turbine  1 . 
     A tower flange  4  is arranged on the top of the tower  3 . A bedplate  5  is attached to the tower flange  4 . The wind turbine  1  comprises in a not explicitly shown manner a yaw system for turning the bedplate  5  of the wind turbine around the axis Y together with the other components of the wind turbine  1  which are directly or indirectly attached to the bedplate  5 . 
     A bearing housing  6  is firmly arranged on the bedplate  5 . The bearing housing  6  comprises two main bearings  7  and  8 . Whilst each fixed part of the main bearings  7 ,  8  is attached to the bearing housing  6 , each rotating part of the main bearings  7 ,  8  is attached to a main shaft  9 . On the front end the main shaft  9  is connected to a hub  11  of the wind turbine  1 . The hub  11  comprises three mounting devices  12  for three not shown, but well known wind turbine rotor blades. 
     On the rear end the main shaft  9  comprises a ring like flange  13  firmly connected to the main shaft  9 . Also the bearing housing  6  comprises on the rear end a ring like flange  14 . A tube like connection piece  15  with a first flange  16  on its front side and with a second flange  17  on its rear side is bolted with the ring like flange  14  of the bearing housing  6 . 
     The direct drive generator  2  is arranged on the main shaft  9  and on the bearing housing  6  and thus on the bedplate  5 . The main shaft  9  and the direct drive generator  2  comprise a joint centre axis A. The direct drive generator  2  is one unit comprising a rotor arrangement  18  and a stator arrangement  19 . 
     The rotor arrangement  18  comprises on the front side a ring like flange  20 . The ring like flange  20  is bolted with the ring like flange  13  of the main shaft  9 . Not in detail shown permanent magnets  24  are mounted on the outer surface of a ring element  23  of the rotor arrangement  18 . On the rear side the rotor arrangement  18  comprises a lug  25 . A tube like projection  26  is attached to the lug  25  by bolts. A third bearing  27  is arranged on the tube like projection  26 . 
     The stator arrangement  19  comprises a rear ring-shaped stator endplate  28  attached to the substantially stationary part or housing of the third bearing  27 . 
     Furthermore the stator arrangement  19  comprises a front ring-shaped stator endplate  29  having a ring like flange  30 . The ring like flange  30  of the front endplate  29  and the ring like flange  17  of the tube like connection piece  15  are bolted together. A hollow cylindrical stator element  32  connects the front ring-shaped endplate  29  and the rear ring-shaped endplate  29  to each other. The hollow cylindrical stator element  32  comprises on the inside and the inner surface respectively a lamination stack  33  with windings. 
     The lamination stack  33  with windings of the stator arrangement  19  and the permanent magnets  24  of the rotor arrangement  18  are arranged oppositely to each other with an intermediate ring-shaped air gap  34  of approximately 5 mm. Thus the rotor arrangement  18  can turn together with the main shaft  9  relatively to the stator arrangement  19  around the centre axis A. 
     According to the described three-bearing arrangement in particular comprising the main shaft  9  and the direct drive generator  2  the stator arrangement  19  is supported on the rear side with the rear endplate  28  on the rotor arrangement  18  via the third bearing  27  and is supported on the front side with the front endplate  29  via the bearing housing  6  on the fixed structure of the wind turbine bedplate  5  close to the main bearing  8 . 
     For avoiding situations in which the three-bearing arrangement is statically undetermined the front endplate  29  of the stator arrangement  19  which is supported on the fixed structure of the bedplate  5  is made of fibreglass and thus comprises sufficient flexibility in the directions of the centre axis A. Thereby the front endplate  29  acts like a membrane supporting the stator arrangement  19  substantially firmly in the radial direction so as to maintain the width of the air gap  34  substantially constant, but flexing readily so as to allow e.g. a bending of the main shaft  9  with no major resistance. The front endplate  29  has thereby a thickness of approximately 15-50 mm, preferably 20-30 mm and has a comparatively little bending stiffness. It simply flexes passively in the directions of the axis A when e.g. the main shaft  9  is shifted a bit by deflection. Thus when a bending of the main shaft  9  to which the rotor arrangement  18  is attached occurs the front endplate  29  bends in the directions of the centre axis A wherein the width of the air gap  34  is maintained substantially constant or within the required tolerances. Moreover an uneven load distribution between the three bearings of the wind turbine is avoided. 
       FIG. 2  shows schematically a second embodiment of a wind turbine  100  comprising a direct drive generator  102  which is arranged on the upwind side of a tower  103  of the wind turbine  100 . 
     A retaining arm  106  is on its base side directly arranged on a bedplate  105 . On the other side the retaining atm  106  comprises a flange  108 . A stationary shaft  107  is attached to the flange  108  with a flange  109 . 
     A main shaft  110  is supported towards the stationary shaft  107  by means of a first main bearing  111  and a second main bearing  112 . On the front end the main shaft  110  is connected to a hub  114  of the wind turbine  100 . The hub  114  comprises three mounting devices  115  for three not explicitly shown, but well known wind turbine rotor blades. 
     The direct drive generator  102  is substantially arranged around the main shaft  110  and comprises a rotor arrangement  116  and a stator arrangement  117 . 
     The rotor arrangement  116  comprises a front ring-shaped rotor endplate  118 , a rear ring-shaped rotor endplate  119  and a hollow cylindrical rotor element  120  connecting the front  118  and the rear  119  rotor endplate with each other. The hollow cylindrical rotor element  120  carries around its outer perimeter not in detail shown permanent magnets  125 . 
     The stator arrangement  117  comprises a front ring-shaped stator endplate  126 , a rear ring-shaped stator endplate  127  and a hollow cylindrical stator element  128  connecting the front  126  and the rear  127  stator endplate with each other. The hollow cylindrical stator element  128  carries around its inner perimeter a not in detail shown lamination stack  133  with windings. 
     The lamination stack  133  with windings and the permanent magnets  125  are arranged oppositely to each other for power generation with an intermediate ring-shaped air gap  134  of approximately 5 mm. The width of the air gap  134  is so small that the air gap  134  is not explicitly visible in  FIG. 2 . 
     In order that the rotor arrangement  116  can turn together with the main shaft  110  around a centre axis A of the direct drive generator  102  and relatively to the stator arrangement  117  the wind turbine  100  in particular the direct drive generator  102  comprise a third or front generator bearing  135  and a fourth or rear generator bearing  136 . 
     The third bearing  135  is in case of the present embodiment of the invention attached to a flange  137  of the main shaft  110 . More precisely the inner bearing shell of the third bearing  135  is firmly attached to the flange  137  of the main shaft  110 . The inner bearing shell of the third bearing  135  is furthermore firmly attached to the front rotor endplate  118 , which supports the front part of the rotor arrangement  116 . The outer bearing shell of the third bearing  135  is firmly connected to the front stator endplate  126 , which supports the front part of the stator arrangement  117 . 
     The rear part of the stator arrangement  117  is supported by the rear stator endplate  127 , which is firmly connected to the flange  109  of the stationary shaft  107  and thus to the retaining arrangement. In case of the present embodiment of the invention the inner bearing shell of the fourth bearing  136  is firmly attached to the rear stator endplate  127  and the rear rotor endplate  119  supporting the rear part of the rotor arrangement  116  is firmly connected to the outer bearing shell of the fourth bearing  136 . 
     Based on the described arrangement comprising the main shaft  110 , the first main bearing  111 , the second main bearing  112 , the rotor arrangement  116 , the stator arrangement  117 , the third bearing  135  and the fourth bearing  136  the main shaft  110  turns in operation of the wind turbine  100  together with the rotor arrangement  116  relatively to the stator arrangement  117 . 
     For avoiding situations in which this four bearing arrangement is statically undetermined in case of the present embodiment of the invention the front rotor endplate  118  firmly supported on the main shaft  110  and the rear stator endplate  127  firmly supported on the retaining arrangement are made of fibreglass and thus comprise a certain and sufficient extent of flexibility in the directions of the centre axis A. The front rotor endplate  118  and the rear stator endplate  127  have thereby a thickness of approximately 15-50 mm, preferably 20-30 mm and a comparatively little bending stiffness. Thereby these end plates  118 ,  127  act like membranes supporting the rotor arrangement  116  and the stator arrangement  117  substantially firmly in the radial direction so as to maintain the width of the air gap  134 , but flexing readily so as to allow e.g. a bending of the main shaft  110  with no major resistance. The end plates  118 ,  127  simply flex passively when e.g. the main shaft  110  is shifted a bit by deflection. Thus when a bending of the main shaft  110  occurs to which the rotor arrangement  116  and the stator arrangement  117  are connected the front rotor endplate  118  and the rear stator endplate  127  bend in substantially a respective way in the directions of the centre axis A wherein the width of the air gap  134  is maintained substantially constant or within required tolerances. Further more an uneven load distribution between the four bearings of the wind turbine is avoided. 
       FIG. 3  shows schematically a third embodiment of a wind turbine  200  comprising a direct drive generator  202  having a centre axis A which is arranged on the upwind side of a tower  203  of the wind turbine  200 . 
     The wind turbine  200  comprises a stationary outer shaft  206 . The rear side of the stationary outer shaft  206  is attached to a retaining arrangement  207  which is attached to a bedplate  205 . On the front side of the stationary outer shaft  206  a stator arrangement  208  of the direct drive generator  202  is arranged. The stator arrangement  208  comprises a stator support structure  209  and a lamination stack  210  with windings  211 . The stator support structure  209  comprises two support elements  212  for a two side support of the lamination stack  210 . In case of the present embodiment of the invention the support elements  212  are ring-shaped support elements  212  attached, e.g. bolted, to the outside of the stationary outer shaft  206 . The ring-shaped support elements  212  are able to be compact or are able to comprise spokes or a spoke structure. A kind of hollow cylindrical support element  213  is attached to the outer ends of the ring-shaped support elements  212 . The hollow cylindrical support element  213  carries the ring-shaped lamination stack  210  with windings  211 . The lamination stack  210  is able to comprise ring segment shaped lamination stack segments each having at least one winding  211  which segments build in their entirety the lamination stack  210 . 
     A rotatable inner shaft  214  is arranged inside the stationary outer shaft  206  and rotatably supported by two main bearings  215 ,  216  against the stationary outer shaft  206 . In case of the present embodiment of the invention the centre axis A is a joint centre axis A of the direct drive generator  202 , the rotatable inner shaft  206  and the stationary outer shaft  214 . A hub  217  is connected to the front end of the rotatable inner shaft  214  e.g. by means of bolts. The hub  217  comprises three mounting devices  218  for three not shown, but well known wind turbine rotor blades. Thus the rotatable inner shaft  214  can turn together with the hub  217 . 
     A rotor arrangement  219  is substantially arranged around the stator arrangement  208 . In case of the present embodiment of the invention the rotor arrangement  219  comprises a front ring-shaped endplate  220 , a rear ring-shaped endplate  221  and a hollow cylindrical rotor element  222  connecting the front ring-shaped endplate  220  and the rear ring-shaped endplate  221  to each other. On the inside the hollow cylindrical rotor element  222  comprises a plurality of permanent magnets  223  substantially arranged oppositely to the lamination stack  210 . An air gap  224  having a width of approximately 5 mm is located between the permanent magnets  223  and the lamination stack  210 . 
     In case of the present embodiment of the invention the front ring-shaped endplate  220  is connected e.g. bolted to the hub  217  and the rotatable inner shaft  214 . The rear ring-shaped endplate  221  is connected to the stationary outer shaft  206  by means of a third bearing, a so called support bearing  225 . Thus also the rotor arrangement  219  has a two side support. Moreover the rotor arrangement  219  can turn together with the hub  217  and the rotatable inner shaft  214 , wherein in particular the permanent magnets  223  turn relatively to the lamination stack  210  for power generation. 
     For avoiding situations in which the three-bearing arrangement comprising the two main bearings  215 ,  216  and the support bearing  225  is statically undetermined the front endplate  220  of the rotor arrangement  219  comprises an adequate flexibility in the directions of the centre axis A. Thereby the front endplate  220  acts like a membrane supporting the rotor arrangement  219  substantially firmly in the radial direction so as to maintain the width of the air gap  224 , but flexing readily so as to allow e.g. a bending of the rotatable inner shaft  214  and the stationary outer shaft  206  with no major resistance. The front endplate  220  has such dimensions that it has a comparatively little bending stiffness. It simply flexes passively when e.g. the rotatable inner shaft  214  is shifted a bit by deflection. Thus when a bending of the rotatable inner shaft  214  and the stationary outer shaft  206  which are connected to each other by means of the main bearings  215 ,  216  occurs the front endplate  220  bends in the directions of the centre axis A wherein the width of the air gap  224  is maintained substantially constant or within required tolerances. Again an uneven load distribution between the three bearings is avoided. 
     The front endplate  220  is made of fibreglass and has a thickness of approximately 15-55 mm, preferably of 20-30 mm. The rear endplate  221  of the rotor arrangement  219  is also able to be made of fibreglass or steel. Normally the rear endplate  221  has a higher thickness as the front endplate  220 . 
     In an embodiment of the invention the support bearing is a four-point bearing capable of transmitting high axial loads in both directions of the centre axis A. 
       FIG. 4  shows schematically a fourth embodiment of a wind turbine  300  comprising a direct drive generator  302  having a centre axis A which is arranged on the upwind side of a tower  303  of the wind turbine  300 . 
     The wind turbine  300  comprises a stationary shaft  306 . The rear side of the stationary shaft  306  is attached to a retaining arrangement  307  which is attached to a bedplate  305 . On the front side of the stationary shaft  306  a stator arrangement  308  of the direct drive generator  302  is arranged. The stator arrangement  308  comprises a stator support structure  309  and a lamination stack  310  with windings  311 . The stator support structure  309  comprises two support elements  312  for a two side support of the lamination stack  310 . The support elements  312  are ring-shaped support elements  312  attached, e.g. bolted, to the outside of the stationary shaft  306 . The ring-shaped support elements  312  are able to be compact or are able to comprise spokes or a spoke structure. A kind of hollow cylindrical support element  313  is attached to the outer ends of the ring-shaped support elements  312 . The hollow cylindrical support element  313  carries the ring-shaped lamination stack  310  with windings  311 . The lamination stack  310  is able to comprise ring segment shaped lamination stack segments each having at least one winding  311  which segments build in their entirety the lamination stack  310 . 
     A rotor arrangement  314  is substantially arranged around the stator arrangement  308 . In case of the present embodiment of the invention the rotor arrangement  314  comprises a front ring-shaped endplate  315 , a rear ring-shaped endplate  316  and a hollow cylindrical rotor element  317  connecting the front ring-shaped endplate  315  and the rear ring-shaped endplate  316  to each other. On the inside the hollow cylindrical rotor element  317  comprises a plurality of permanent magnets  318  substantially arranged oppositely to the lamination stack  310 . An air gap  319  having a width of approximately 5 mm is located between the permanent magnets  318  and the lamination stack  310 . 
     In case of the present embodiment of the invention the front ring-shaped endplate  315  is indirectly arranged on the stationary shaft  306  by a four-point bearing  320  capable of transmitting high axial loads in both directions of the centre axis A. An appropriate four-point bearing is e.g. disclosed in DE 201 16 649 U1. The stationary part  321  of the four-point bearing  320  is attached to the stationary shaft  306 . The rotating part  322  of the four-point bearing  320  is in case of the present embodiment of the invention connected to a mounting ring  323 . The front endplate  315  as well as the hub  324  of the wind turbine  300  are attached e.g. bolted to the mounting ring  323 . By the way the hub  324  comprises three mounting devices  325  for three not shown, but well known wind turbine rotor blades. 
     The rear ring-shaped endplate  316  is connected to the stationary shaft  306  by means of another four-point bearing  326 , a so called support bearing  326 . Thus also the rotor arrangement  314  has a two side support. Moreover the rotor arrangement  314  can turn together with the hub  324  relatively to the stator arrangement  308 , wherein in particular the permanent magnets  318  turn relatively to the lamination stack  310  for power generation. 
     For avoiding situations in which the bearing arrangement comprising the two four-point bearings  320  and  326  is statically undetermined the front endplate  315  of the rotor arrangement  314  comprises and adequate flexibility in the directions of the centre axis A. Thereby the front endplate  315  acts like a membrane supporting the rotor arrangement  314  substantially firmly in the radial direction so as to maintain the width of the air gap  319 , but flexing readily so as to allow e.g. a bending of the stationary shaft  306  with no major resistance. The front endplate  315  has such dimensions that it has a comparatively little bending stiffness. It simply flexes passively when e.g. the stationary shaft  306  is shifted a bit by deflection. Thus when a bending of the stationary shaft  306  occurs the front endplate  315  bends in the directions of the centre axis A wherein the width of the air gap  319  is maintained substantially constant or within required tolerances. As previously mentioned an uneven load distribution between the bearings is avoided. 
     The front endplate  315  is made of fibreglass and has a thickness of approximately 15-55 mm, preferably of 20-30 mm. The rear endplate  316  of the rotor arrangement  314  is also able to be made of fibreglass or steel. Normally the rear endplate  316  has a higher thickness as the front endplate  315 . 
     Moreover it is not necessary that both bearings, the main bearing  320  and the support bearing  326  are four-point bearings. It is also possible that only the main bearing  320  or only the support bearing  326  is a four-point bearing. 
     A ring-shaped endplate having a certain flexibility need not to have this flexibility in the whole endplate. Thus the ring-shaped endplate is able to have different areas. The ring-shaped endplate may have e.g. a comparatively rigid area e.g. for the attachment to a component of the wind turbine and an area having the mentioned flexibility in the directions of the centre axis A. 
     Furthermore the respective endplate need not to be completely made of fibreglass. Thus the endplate may have an area made of fibreglass and comprising the required flexibility and an area made of another material e.g. steel for mounting reasons.