Direct drive generator and wind turbine

The invention concerns a direct drive or directly driven generator for a wind turbine comprising a stator and a rotor, wherein the stator comprises a stator ring comprising several ring-segment-shaped stator segments each having at least one stator element for the power generation and wherein the rotor comprises a rotor ring pivotable around a centre axis of the generator, the rotor ring comprises several ring-segment-shaped rotor segments each having at least one rotor element for the power generation. Furthermore the invention concerns a wind turbine comprising such a direct drive generator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of European application No. 07022881.2 filed Nov. 26, 2007, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to a direct drive or directly driven generator as well as to a wind turbine comprising a direct drive generator.

BACKGROUND OF THE 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 a main shaft and a generator of the wind turbine. The second type of a wind turbine is a gearless type, whereat the gearbox and the conventional generator are substituted by a multipolar generator, a so called direct drive or 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.

Common to direct driven generators is that their physical dimensions are relatively large. At a typical air gap diameter of approximately 5 m for a multi-megawatt direct drive generator the outer diameter is on the order of approximately 6 m or even more. The large outer diameter makes the transport of the direct drive generator difficultly and the heavy dead load of the direct drive generator involves further difficulties e.g. concerning the replacement for repair by occurred breakdowns.

A further difficulty arises in the normal configuration of a wind turbine with a direct drive generator, where the direct drive generator is arranged between the wind turbine rotor and the tower in order to yield a compact machine construction. In this case it will be necessary to dismantle the whole wind turbine rotor by a required dismantling of the direct drive generator.

To overcome these problems at least partially there were some suggestions to divide parts of the generator.

In WO 98/20595 A1 a stator for a rotating electric machine is disclosed comprising a stator core and a winding. The stator core is provided with stator teeth extending radially inwards towards a rotor. Each stator tooth is configured as a number of tooth sections joined axially into a stator tooth plank. That stator tooth planks are fitted together side by side thus forming a section of the stator core. This construction makes the transport of parts of the rotating electric machine to the site of erection partially easier, because the stator can be assembled on site. However, this construction requires a stator housing as such having relatively large outer dimensions.

From U.S. Pat. No. 4,594,552 an armature winding of a split stator is known. The split stator has a slotted core divided by at least two circumferentially-spaced split lines to facilitate the assembly and the disassembly of the split stator. The armature winding comprises armature coils in the slots of the stator core connected to provide poles and arranged to provide a plurality of armature coils divided at the split lines. Connecting and disconnecting means are provided to connect and disconnect the armature coils when the split stator is assembled and disassembled, respectively. This construction, however, also requires a stator housing as such having relatively large outer dimensions.

U.S. Pat. No. 5,844,341 describes an electric generator to be driven by a low speed device such as wind turbine. The generator consists of one or more rotor rings of many permanent magnets of alternating polarity and coaxial stator rings of many laminated yokes, each yoke defining slots to locate coils. The yokes and coils form modules which are supported by beams relatively to the rotor rings. The drawback of this configuration is that the electromechanical properties in this form of modular construction with single polar pairs separated by air gaps may be disadvantageous, and that a possible dismantling of a single stator module can involve that the whole generator has to be opened in situ implying risk of humidity, dirt etc., and that it may be cumbersome if the stator module has to be taken out in a disadvantageous direction.

U.S. Pat. No. 6,781,276 B1 describes a generator for a wind turbine comprising a stator and a rotor. The stator has a number of stator modules that are individual and which may be installed, repaired and dismantled individually and independently of each other. This generator has no part larger than the air gap diameter. But even if no part is larger than the air gap diameter, the largest element to be transported still has a substantial size, given that the rotor is a single piece. In its completed form this rotor is fitted with strong permanent magnets and needs to be covered by a nonmagnetic layer, e.g. wood or polystyrene of a certain thickness during transportation, and while the dimensions of the rotor are smaller than the dimensions of the finished generator, it is still at maybe 5 m diameter and 1.5 m length a very substantial piece of equipment to transport.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a generator as initially mentioned in such a way, that in particular the assembly and/or the disassembly of the generator on the site of erection is simplified. It is a further object of the invention to indicate a wind turbine comprising a respective generator.

This object is inventively achieved by a direct drive or directly driven generator for a wind turbine comprising a stator and a rotor, wherein the stator comprises a stator ring comprising several ring-segment-shaped stator segments each having at least one stator element operative for the electrical power generation and wherein the rotor comprises a rotor ring pivotable around a centre axis of the generator and relatively to the stator, the rotor ring comprises several ring-segment-shaped rotor segments each having at least one rotor element operative for the electrical power generation. According to the invention not only the stator but also the rotor of the generator is at least partially segmented. The rotor ring is able to be segmented into a number of ring-segment-shaped rotor segments which is equal to or is a multiple of the number of the ring-segment-shaped stator segments. Thus the generator as a whole comprises a plurality of single manageable parts and segments each having lesser dimensions and a lower dead load than the generator in the assembled status. As a consequence the transport of the generator in form of the single parts and segments is simplified. Also the assembly and/or the disassembly of the generator on the site of erection are simplified. In case of a wind turbine not the whole mounted generator has to be carried into the nacelle using e.g. a crane. In fact the significantly lighter single manageable parts and segments of the generator are able to be carried into the nacelle where the generator is able to be assembled, repaired or disassembled.

According to an embodiment of the invention the stator comprises a front and a rear ring-shaped supporting element to which supporting elements of the stator the ring-segment-shaped stator segments are attached by means of a non-destructively detachable connection. Preferably the ring-shaped supporting elements of the stator are one-piece elements ensuring particularly a sufficient roundness. Thus after the arrangement of the ring-segment-shaped stator segments on the front and the rear ring-shaped supporting elements of the stator the stator has substantially a hollow-cylindrical shape.

According to another embodiment of the invention also the rotor comprises a front and a rear ring-shaped supporting element to which supporting elements of the rotor the ring-segment-shaped rotor segments are attached by means of a non-destructively detachable connection. As in case of the stator the ring-shaped supporting elements of the rotor are one-piece elements ensuring preferably a sufficient roundness. According to this after the arrangement of the ring-segment-shaped rotor segments on the front and the rear ring-shaped supporting elements of the rotor the rotor has substantially a hollow-cylindrical shape, whereat in one embodiment of the invention the rotor is substantially arranged inside the stator.

In a further development of the invention the ring-segment-shaped stator segments and/or the ring-segment-shaped rotor segments are designed in such a way that the junctions between the ring-shaped supporting elements of the stator and the ring-segment-shaped stator segments and/or the junctions between the ring-shaped supporting elements of the rotor and the ring-segment-shaped rotor segments are located substantially at a radius in relation to the centre axis of the generator which is equal or smaller than the radius of the air gap between the stator elements and the rotor elements. Preferably the junctions are located at a radius which is smaller than the radius of the air gap. In this way the outer diameters of the ring-shaped supporting elements of the stator and the rotor are able to be significantly smaller than the diameter of the air gap of the generator, thereby reducing the maximum dimensions of the ring-shaped supporting elements of the stator and the rotor in particular for transportation. Thereby the diameters or the maximal dimensions of the ring-shaped supporting elements of the stator and the rotor can differ from each other.

According to an embodiment of the invention a ring-segment-shaped stator segment comprises an exterior, ring-segment-shaped stator supporting element, a radially inwardly directed front ring-segment-shaped stator connection element arranged on the front side of the exterior, ring-segment-shaped stator supporting element and a radially inwardly directed rear ring-segment-shaped stator connection element arranged on the rear side of the exterior, ring-segment-shaped stator supporting element for establishing an inwardly open, substantially U-shaped, ring-segment-shaped stator segment, wherein at least one stator element is arranged on the inside of the exterior ring-segment-shaped stator supporting element. Thereby the expression substantially U-shaped shall also cover other comparable forms such as V-shaped etc.

In a comparable way a ring-segment-shaped rotor segment comprises an exterior, ring-segment-shaped rotor supporting element, a radially inwardly directed front ring-segment-shaped rotor connection element arranged on the front side of the exterior, ring-segment-shaped rotor supporting element and a radially inwardly directed rear ring-segment-shaped rotor connection element arranged on the rear side of the exterior, ring-segment-shaped rotor supporting element for establishing a inwardly open, substantially U-shaped ring-segment-shaped rotor segment, wherein at least one rotor element is arranged on the outside of the exterior ring-segment-shaped rotor supporting element. Thereby the expression substantially U-shaped shall again also cover other comparable forms such as V-shaped etc.

In an embodiment of the invention each ring-segment-shaped rotor segment is at least partially arranged inside a ring-segment-shaped stator segment. It is also possible that two or more ring-segment-shaped rotor segments are at least partially arranged inside a ring-segment-shaped stator segment. Thereby the stator elements and the rotor elements for the power generation are arranged oppositely to each other with an intermediate air gap.

In a further development of the invention a ring-segment-shaped stator segment and at least one ring-segment-shaped rotor segment are able to be at least temporarily supported against each other. Preferably each ring-segment-shaped stator segment comprises first supporting projections and each ring-segment-shaped rotor segment comprises second supporting projections, wherein the first supporting projections of a first ring-segment-shaped stator segment and the second supporting projections of a corresponding first ring-segment-shaped rotor segment are able to be at least temporarily supported against each other. Preferably each ring-segment-shaped stator connection element of a ring-segment-shaped stator segment comprises at least one first supporting projection and each ring-segment-shaped rotor connection element of a ring-segment-shaped rotor segment comprises at least one second supporting projection. By means of the supporting projections a ring-segment-shaped rotor segment is able to rest on a ring-segment-shaped stator segment in particular when the generator and the ring-segment-shaped segments respectively are transported, assembled or disassembled. Thereby an air gap remains between the stator elements for the power generation and the rotor elements for the power generation. Since in this way the magnetic circuits concerning the stator and rotor elements for the power generation are closed, normally no specific protection against undesired magnetic pull is needed, in particular when the generator comprises permanent magnets. Further on, since during assembly and disassembly of the generator a ring-segment-shaped rotor segment is allowed to rest on a ring-segment-shaped stator segment, any crane lift is as a rule disconnected from magnetic pull. As a consequence the transport, assembly and disassembly are simplified.

Therefore according to a further embodiment of the invention a ring-segment-shaped stator segment and at least one ring-segment-shaped rotor segment are able to build at least temporarily a unit, more precisely a stator/rotor segment unit.

In a further development of the invention the width of the air gap between a stator element and a rotor element is adjustable. Thereby the junctions between the ring-shaped supporting elements of the stator and the ring-segment-shaped stator segments and/or the junctions between the ring-shaped supporting elements of the rotor and the ring-segment-shaped rotor segments preferably comprise adjusting means for the adjustment of the width of the air gap. In one embodiment of the invention the adjusting means comprise at least one shim. Thus the desired or required width of the air gap between the stator and rotor elements for the power generation can be adjusted in a relatively simple way.

According to another embodiment of the invention a ring-segment-shaped stator segment comprises at least one winding form with a winding as a stator element and/or a ring-segment-shaped rotor segment comprises at least one permanent magnet as a rotor element.

In an embodiment of the invention the ring-segment-shaped stator segments and the ring-shaped supporting elements of the stator and/or the ring-segment-shaped rotor segments and the ring-shaped supporting elements of the rotor comprise axial and/or radial extending flanges for the mounting. Thereby the axial extending flanges extend preferably substantially in the directions of the centre axis A of the main shaft and the radial extending flanges extend preferably substantially perpendicularly in relation to the centre axis A of the main shaft. In this way the ring-segment-shaped segments can be comparatively simply attached to the respective ring-shaped supporting elements.

According to a further embodiment of the invention at least one of the ring-shaped supporting elements of the rotor and/or at least one of the ring-shaped supporting elements of the stator comprise at least one man hole for providing access to the internals of the generator. Thus at least one of the front or rear ring-shaped supporting elements can have one or more man holes which are preferably closable.

The further object of the invention is inventively achieved by a wind turbine comprising a generator as described afore.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows schematically a first embodiment of an inventive wind turbine1comprising an inventive direct drive or directly driven generator2which is arranged on the upwind side of a tower3of the wind turbine1.

A tower flange4is arranged on the top of the tower3. A retaining arrangement is arranged on the tower flange4comprising in case of the present embodiment of the invention a bedplate5, a retaining frame in form of a retaining arm6and a stationary or fixed hollow shaft7. The bedplate5is attached to the tower flange4. The wind turbine1comprises in a not explicitly shown manner a yaw system for turning the bedplate5of the wind turbine1around the centre axis Y of the tower3together with the other components of the wind turbine1which are directly or indirectly attached to the bedplate5.

The retaining arm6is on its base side directly arranged on the bedplate5. On the other side the retaining arm6comprises a flange8. The stationary shaft7is attached to the flange8with a flange9. The ring-shaped flange8of the retaining arm6and the ring-shaped flange9of the stationary shaft7are bolted together with a plurality of bolts arranged around the ring shaped flanges.

A main shaft10or a main rotor pipe10is pivoted on the stationary shaft7by means of a first main bearing11and a second main bearing12. Each main bearing11,12supported by the stationary shaft7comprises an inner and an outer bearing shell. The inner bearing shells of the both main bearings11,12are mounted on the stationary shaft7, whilst the outer bearing shells of the both main bearings11,12are fitted inside the main shaft10.

On the front end the main shaft10comprises a ring-shaped flange13. The ring-shaped flange13is firmly, but detachably connected to a hub14of the wind turbine1. The hub14comprises three mounting devices15for three not explicitly shown, but well known wind rotor blades.

In case of the present embodiment of the invention the mentioned direct drive or directly driven generator2is substantially arranged around the main shaft10. The direct drive generator2comprises a rotor16or a rotor arrangement16and a stator17or a stator arrangement17.

The rotor16comprises in case of the present embodiment of the invention a first supporting element18in form of a front ring-shaped rotor end plate18, a second supporting element19in form of a rear ring-shaped rotor end plate19and a plurality of ring-segment-shaped rotor segments20attached to the front ring-shaped rotor end plate18and the rear ring-shaped rotor end plate19. In case of the present embodiment of the invention the rotor16comprises six ring-segment-shaped rotor segments20building a rotor ring when the six ring-segment-shaped rotor segments20are attached to the preferably one-piece front and rear ring-shaped rotor end plates18,19.

The stator17comprises in case of the present embodiment of the invention a first supporting element26in form of a front ring-shaped stator end plate26, a second supporting element27in form of a rear ring-shaped stator end plate27and a plurality of ring-segment-shaped stator segments28attached to the front ring-shaped stator end plate26and the rear ring-shaped stator end plate27. In case of the present embodiment of the invention the stator17comprises also six ring-segment-shaped stator segments28(cp.FIG. 4) building a stator ring when the six ring-segment-shaped stator segments28are attached to the preferably one-piece front and rear ring-shaped stator end plates26,27.

In case of the present embodiment of the invention the ring-segment-shaped stator segments28and the ring-segment-shaped rotor segments20are designed in such a way that the junctions50,51between the ring-shaped stator end plates26,27and the ring-segment-shaped stator segments28as well as the junctions52,53between the ring-shaped rotor end plates18,19and the ring-segment-shaped rotor segments20are located substantially at a radius R1in relation to a centre axis A of the generator2which is smaller than the radius R2of the air gap34between stator elements33for the power generation arranged on the ring-segment-shaped stator segments28and rotor elements25for the power generation arranged on the ring-segment-shaped rotor segments20. Thus in case of the present embodiment of the invention the maximum diameters of the ring-shaped stator and rotor end plates are 2*R1. These diameters are significantly smaller than the diameter of the air gap34(2*R2).

A ring-segment-shaped rotor segment20comprises an exterior, ring-segment-shaped rotor supporting element54, a radially inwardly directed front ring-segment-shaped rotor connection element55arranged on the front side of the exterior, ring-segment-shaped rotor supporting element54and a radially inwardly directed rear ring-segment-shaped rotor connex connection ion element56arranged on the rear side of the exterior, ring-segment-shaped rotor supporting element54for establishing an inwardly open, substantially U-shaped ring-segment-shaped rotor segment20, wherein at least one rotor element25in form of at least one permanent magnet25is arranged on the outside of the exterior ring-segment-shaped rotor supporting element54. Thereby a ring-segment-shaped rotor segment20connects the front and the rear ring-shaped rotor end plates18,19with each other.

As shown inFIG. 2andFIG. 3a front ring-segment-shaped rotor connection element55comprises on its end a ring-segment-shaped flange21and a ring-segment-shaped supporting projection57. A rear ring-segment-shaped rotor connection element56comprises on its end a ring-segment-shaped flange22and a ring-segment-shaped supporting projection58. The front ring-shaped rotor end plate18has a ring-shaped flange23and the rear ring-shaped rotor end plate19has a ring-shaped flange24. In case of the present embodiment of the invention the flanges21and23as well as the flanges22and24are bolted together to build up the rotor16. In the described way all ring-segment-shaped rotor segments20are attached to the front and the rear ring-shaped end plates18,19. Thus the rotor16has substantially a hollow-cylindrical shape.

In a comparable way a ring-segment-shaped stator segment28comprises an exterior, ring-segment-shaped stator supporting element67, a radially inwardly directed front ring-segment-shaped stator connection element68arranged on the front side of the exterior, ring-segment-shaped stator supporting element67and a radially inwardly directed rear ring-segment-shaped stator connection element69arranged on the rear side of the exterior, ring-segment-shaped stator supporting element67for establishing an inwardly open, substantially U-shaped, ring-segment-shaped stator segment, wherein at least one stator element33in form of a winding form75with a winding76is arranged on the inside of the exterior ring-segment-shaped stator supporting element67. Thereby a ring-segment-shaped stator segment28connects the front and the rear ring-shaped stator end plates26,27with each other.

As shown inFIG. 2andFIG. 3a front ring-segment-shaped stator connection element68comprises on its end a ring-segment-shaped flange29and a ring-segment-shaped supporting projection60. A rear ring-segment-shaped stator connection element69comprises on its end a ring-segment-shaped flange30and a ring-segment-shaped supporting projection61. The front ring-shaped stator end plate26has a ring-shaped flange31and the rear ring-shaped stator end plate27has a ring-shaped flange32. In case of the present embodiment of the invention the flanges29and31as well as the flanges30and32are bolted together to build up the stator17. In the described way all ring-segment-shaped stator segments28are attached to the front and the rear ring-shaped stator end plates26,27. Thus also the stator17has substantially a hollow-cylindrical shape.

In case of the present embodiment of the invention each ring-segment-shaped rotor segment20is substantially arranged inside a ring-segment-shaped stator segment28. In particular during the transport, the assembly and the disassembly of the ring-segment-shaped stator and rotor segments20,28one ring-segment-shaped stator segment28and one ring-segment-shaped rotor segment20are able to build a stator/rotor segment unit. Thereby the ring-segment-shaped rotor segment20is moved into the ring-segment-shaped stator segment28until the ring-segment-shaped rotor segment20rests on the ring-segment-shaped stator segment28. More precisely the projections57,58,60,61engage, whereat the supporting projection60and the supporting projection57as well as the supporting projection61and the supporting projection58are supported against each other. In general the projections57,58,60,61engage if the air gap between the stator elements and the rotor elements for the power generation is reduced to a value a certain level below the nominal value of the air gap. The magnetic pull during the mounting of the stator/rotor segment unit is counteracted by suitable tools, e.g. a set of jacks. Once the projections57,58,60,61engage, the magnetic pull is taken by the projections and the suitable tools can be removed. In this position the magnetic circuits are as a rule closed and the stator/rotor segment unit no longer poses any risk in relation to the strong permanent magnets. The stator/rotor segment unit is able to be transported, assembled and disassembled with no special precautions.

On the site of erection of the wind turbine1first the supporting structures are assembled—the stationary shaft7, the main bearings11,12, the main shaft10, a third and a fourth bearing35,36, described later, and the ring-shaped rotor end plates18,19as well as the ring-shaped stator end plates26,27.

For the assembly of the generator2a stator/rotor segment unit is, as indicated above, arranged on the front and the rear ring-shaped end plates18,19,26,27. Thereby the flange29of a front ring-segment-shaped stator connection element68and the flange31of the front ring-shaped stator end plate26as well as the flange30of a rear ring-segment-shaped stator connection element69and the flange32of the rear ring-shaped stator end plate27are bolted together with schematically shown flange bolts62. This is done for all six stator/rotor segment units. Any necessary adjustment of the radial position of a ring-segment-shaped stator segment28is carried out with not explicitly shown shims in the junctions50,51between the flanges29,31as well as between the flanges30,32. Thereby the respective shims are inserted between the respective flanges and then the flange bolts62are inserted in respective bolt holes and tightened.

Afterwards the flanges21of the front ring-segment-shaped rotor connection elements55and the flange23of the front ring-shaped rotor end plate18and the flanges22of the rear ring-segment-shaped rotor connection elements56and the flange24of the rear ring-shaped rotor end plate19are bolted together with schematically shown flange bolts63. Thereby the ring-segment-shaped rotor segments20are as a rule pulled away from the resting position, in which the projections57,58,50,61engage. Any necessary adjustment of the radial position of a ring-segment-shaped rotor segment20is carried out with not explicitly shown shims in the junctions52,53between the flanges21,23as well as between the flanges22,24before the flange bolts63are inserted in respective bolt holes and finally tightened.

If a ring-segment-shaped rotor segment20or a ring-segment-shaped stator segment28needs e.g. replacement the described steps are carried out in reverse order and the replacement stator/rotor segment unit is assembled as described.

Based on trial rotations the radial positions of the ring-segment-shaped rotor segments20as well as the radial positions of the ring-segment-shaped stator segments28are able to be fine tuned with the shims in the junctions50-53. Thus the width of the air gap34between the electrical stator elements33of the stator17and the permanent magnets25of the rotor16is able to be adjusted to establish a preferably completely uniform and concentric air gap34.

In order that the rotor16can turn together with the main shaft10around the centre axis A of the main shaft10and relatively to the stator17the wind turbine1in particular the direct drive generator2comprise the already mentioned third or front generator bearing35and the already mentioned fourth or rear generator bearing36. The relative positions of the stator17and the rotor16are maintained by the third and the fourth bearing35,36.

The third bearing35is in case of the present embodiment of the invention attached to a flange37of the main shaft10. More precisely the inner bearing shell38of the third bearing35is firmly attached to the flange37of the main shaft10. The inner bearing shell38of the third bearing35is furthermore firmly attached to the front ring-shaped rotor end plate18, which supports the front part of the rotor16. The outer bearing shell39of the third bearing35is firmly connected to the front ring-shaped stator end plate26, which supports the front part of the stator17.

The rear part of the stator17is supported by the rear ring-shaped stator end plate27, which is firmly connected to the flange9of the stationary shaft7and thus to the retaining arrangement. In case of the present embodiment of the invention the inner bearing shell40of the fourth bearing36is firmly attached to the rear ring-shaped stator end plate27and the rear ring-shaped rotor end plate19supporting the rear part of the rotor16is firmly connected to the outer bearing shell41of the fourth bearing36.

Based on the described arrangement comprising the main shaft10, the first main bearing11, the second main bearing12, the rotor16, the stator17, the third bearing35and the fourth bearing36the main shaft10turns in operation of the wind turbine1together with the rotor16relatively to the stator17.

For avoiding situations in which the four bearing arrangement is statically undetermined in case of the present embodiment of the invention the front ring-shaped rotor end plate18firmly supported on the main shaft10and the rear ring-shaped stator end plate27firmly supported on the retaining arrangement comprise a certain and sufficient extent of flexibility in the directions of the centre axis A of the main shaft10. Thereby these end plates18,27act like membranes supporting the rotor16and the stator17substantially firmly in the radial direction so as to maintain the width of the air gap34, but flexing readily so as to allow e.g. a bending of the main shaft10with no major resistance. In particular the end plates18,27have such dimensions that they have a comparatively little bending stiffness. They simply flex passively when e.g. the main shaft10is shifted a bit by deflection. Thus when a bending of the main shaft10occurs to which the rotor16and the stator17are connected the front ring-shaped rotor end plate18and the rear ring-shaped stator end plate27bend in substantially a respective way in the directions of the centre axis A wherein the width of the air gap34is maintained substantially constant or within required tolerances.

As a consequence of the four bearing arrangement, in addition to the loads from the wind turbine rotor and the main shaft10the two main bearings11,12carry approximately half of the weight of the generator2, approximately the other half of the weight of the generator2is directly supported on the retaining arrangement. The third or front generator bearing35carries approximately half of the weight of the stator17, approximately the other half of the weight of the stator17is supported on the retaining arrangement. The fourth or rear generator bearing36carries approximately half of the weight of the rotor16, approximately the other half of the weight of the rotor16is supported on the main shaft10.

Based on the described design or structure of the wind turbine1in particular based on the described generator arrangement comprising the third and fourth bearing the rotor16and the stator17are supported on both sides, the front side and the rear side. This enables a more lightweight rotor and in particular a more lightweight stator construction with less dimensions of the stator structure in particular of the stator support structure like the end plates and so on to maintain in operation of the wind turbine1the width of the air gap34within the necessary tolerances along the directions of the centre axis A and around the perimeter.

Unlike to the afore described embodiment of the invention the front ring-shaped stator end plate26and the rear ring-shaped rotor end plate19are able to comprise the certain extent of flexibility in the directions of the centre axis A of the main shaft10, whilst the front ring-shaped rotor end plate18and the rear ring-shaped stator end plate27have not these flexibility. Also in this case the width of the air gap34is able to be held substantially constantly or at least within required tolerances.

The ring-shaped rotor end plate and the ring-shaped stator end plate which have the certain flexibility need not to have the flexibility in the whole end plates. Thus the ring-shaped end plates are able to have different areas. The respective ring-shaped rotor end plate may have e.g. a comparatively rigid area e.g. for the attachment of the third bearing and an area having the mentioned flexibility in the directions of the centre axis A. In the same way the respective ring-shaped stator end plate may have e.g. a comparatively rigid area e.g. for the attachment of the fourth bearing and an area having the mentioned flexibility in the directions of the centre axis A.

The front ring-shaped rotor end plate is able to be directly arranged on the main shaft. In this case the third bearing is able to be directly attached to the main shaft or to the front ring-shaped rotor end plate.

It is not necessary to attach the fourth bearing to the rear ring-shaped stator end plate. The fourth bearing is also able to be directly attached to the retaining arrangement e.g. the stationary shaft or the retaining frame or arm.

As a rule the ring-shaped end plates are made of an appropriate metal or metal alloy. The ring-shaped end plates do not need to have the same diameter. In fact the different ring-shaped end plates are able to have different diameters. In this case also the respective ring-segment-shaped segments have to be modified in relation to the radial extension of the radially inwardly directed ring-segment-shaped stator connection elements to build the stator or the rotor.

FIG. 4shows the view of the generator2of the wind turbine1in the direction of the arrows IV ofFIG. 1. InFIG. 4the third or generator bearing35, the front ring-shaped stator end plate26, the flanges29,31and the six ring-segment-shaped stator segments28building the stator ring71are cognizable. In case of the present embodiment of the invention the front ring-shaped stator end plate26comprises six man holes70providing access to the internals of the generator. In the same way the other ring-shaped end plates of the stator or the rotor are able to comprise man holes. Thereby the man holes are as a rule closed by means of a kind of door.

In case of the present embodiment of the invention in each ring-segment-shaped stator segment28a ring-segment-shaped rotor segment20is substantially centrically arranged. But it is also possible that two or more ring-segment-shaped rotor segments20are substantially arranged in a ring-segment-shaped stator segment28.

By the way the generator2is able to comprise less or more ring-segment-shaped stator segment28building the stator ring as well as less or more ring-segment-shaped rotor segment20building the rotor ring as described afore.

In case of the described embodiment of the invention the ring-segment-shaped stator segments28and the ring-shaped stator end plates26,27as well as the ring-segment-shaped rotor segments20and the ring-shaped rotor end plates18,19comprise axial extending flanges21-24,29-32for the mounting. Thereby the axial extending flanges21-24,29-32extend substantially in the directions of the centre axis A. But the mounting is also able to be done by radial extending flanges or other suitable means. Thereby the radial extending flanges extend substantially perpendicularly in relation to the centre axis A.

Unlike described before the direct drive generator is also able to be arranged on the downwind side of the tower.

By the way the wind turbines1comprise a housing H normally called the nacelle which contain the generator2and at least a part of the retaining arrangement.