Abstract:
In one aspect, a stator is provided having a first cylindrical surface with a plurality circumferentially spaced winding slots, each winding slot extending along an axial length of the first cylindrical surface and formed to embed a segment of a single stator winding, Each stator winding is preformed as a closed loop and two substantially parallel segments of each stator winding formed to be embedded in a first and a second winding slot of the stator, with the second winding slot being the third adjacent winding slot to the first winding slot. Each of the three stator windings has a first winding head segment leaving the winding slots in axial direction of the stator, the first winding head segment of a first winding of the three stator windings describing in space in parts a combination of a rotation about an axis and a translation along that axis.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 08016467.6 EP filed Sep. 18, 2008, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a group of three stator windings for a stator of an electric machine. The invention concerns also a stator arrangement comprising such stator windings, a generator—e.g. a direct drive generator for a wind turbine—comprising such a stator as well as a wind turbine comprising such a generator. 
       BACKGROUND OF INVENTION 
       [0003]    In large electrical machines or generators it is common to use a so called double-layer winding. This winding type has the benefit that all coils are identical and the coils are easy to manufacture. 
       SUMMARY OF INVENTION 
       [0004]    However, in electrical machines with a high number of poles and a small pole pitch the traditional double layer windings can be difficult to mount because an insertion of a last coil within dedicated slots require to temporarily lift a first coil out of the slots while completing the winding of the coil, so that the last coil can be inserted underneath the temporarily removed part of the first coil. 
         [0005]    Furthermore, for a segmented electrical machine double layer windings require coils that connect across the segment joint. 
         [0006]    It is possible to make single-layer windings that do not require the removal of the first inserted coils when inserting the last coils in a stator. It is also possible to make single-layer windings that do not need to cross segment joints. Consequently, single-layer windings can be advantageous for large electrical machines. 
         [0007]    Single-layer windings normally require both in-plane and out-of-plane winding overhangs. These winding overhangs are also called end windings or winding heads. 
         [0008]    It is possible to make a single layer winding that does not need to cross the segment joint of a segmented stator, but when using known single-layer technology this leads to large winding overhangs taking up a large space at the ends of the stator pack and consuming large amounts of copper for the winding. 
         [0009]    It is therefore an object of the present invention to provide a group of stator windings, a stator arrangement, a generator as well as a wind turbine in such a way, that the winding overhangs consume less copper. 
         [0010]    The object of the invention is inventively achieved by a group of three stator windings, a stator arrangement, by a generator, and by a wind turbine. 
         [0011]    Advantageous embodiments can be found in the dependent claims. 
         [0012]    According to the invention a group of three stator windings for a stator of an electric machine—e.g. a generator—is defined, whereas the stator having a first cylindrical surface with a plurality circumferentially spaced winding slots formed in it, each winding slot extending along an axial length of the first cylindrical surface and set up to embed a segment of a single stator winding. Each stator winding is pre-formed as a closed loop. Two substantially parallel segments of each stator winding are set up to be embedded in a first and a second winding slot of the stator, with the second winding slot being the third adjacent winding slot to the first winding slot. Each of the three stator windings has a first winding head segment leaving the winding slots in axial direction of the stator. According to the invention the first winding head segment of a first winding of the three stator windings describes in space in parts a combination of a rotation about an axis and a translation along that axis. 
         [0013]    More specific, the part of the first and/or second winding head segment, which is the combination of the rotation and the translation may be substantially screw axis symmetric. 
         [0014]    Additionally this axis may be substantially perpendicular to a plane of the winding slots. 
         [0015]    The invention is in particular advantageous because the overall length of all windings and therefore also the coils may be kept at a minimum length resulting in a reduction of material for the windings, especially copper. Besides, this also results in a reduction of weight, e.g. a couple of hundred kilograms in a large generator for example in a wind turbine. 
         [0016]    Additionally the invention allows reducing the axial extent of the winding heads so that the stator including the windings will have a smaller axial dimension. This may reduce the dimension of the housing of the electric machine and also the overall weight of the electric machine. 
         [0017]    Besides, the invention allows advantageously having all three coils with substantially the same length and therefore having all the same resistance. This is true for the three windings but particularly also for the three coils that each are comprised of a plurality of the same number of windings. This then also results in that the three coils pull the same current. 
         [0018]    Advantageously the three windings represent the three phases in an electrical machine with one pole per slot per phase and are arranged of equal angular pitch, and—in terms of the distance between the first and the second winding slot—with a pitch of three winding slot pitches. This means that between the first and the second winding slot of a first winding and between the second winding slot of a first winding to the first winding slot of another winding belonging to the same coil representing one phase, two winding slot gaps are existing that will be filled with two further windings belonging to two further coils for the other two phases. 
         [0019]    Besides, advantageously the invention concerns single-layer stator windings so that only one winding will be mounted in one winding slot. 
         [0020]    The stator windings may be pre-formed that way that the cross-section of a winding will be substantially rectangular. The cross section of the windings may be the same overall the whole loop of the winding. 
         [0021]    The winding heads may be arranged that the windings do not touch each other when mounted into adjacent winding slots. 
         [0022]    The above mentioned invention defines a first winding head extending in one axial direction of the stator. Obviously to close the loop also a second winding head is present extending in the other axial direction of the stator. In a preferred embodiment, the second winding head has the same dimension and structure as the first winding head, possibly being reflection symmetric and/or point symmetric to the first winding head. 
         [0023]    Besides, at least one of the three stator windings may itself be reflection symmetric with the axis of symmetry being parallel to the winding slots, including also the first and the second winding head. 
         [0024]    An advantageous shape for an example of the winding heads will be explained later with respect to a schematic drawing. 
         [0025]    The object of the present invention is also inventively achieved by a stator arrangement comprising the group of windings as described before, by a generator comprising such a stator arrangement and a wind turbine comprising such a generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention will in the following be explained in more detail with reference to the schematic drawings, wherein 
           [0027]      FIG. 1  shows three winding heads of three windings. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0028]      FIG. 1  shows a three-dimensional section of a stator S for a large conventional rotating electrical machine—especially a generator for a wind turbine. The complete stator S comprises a stator core SC and a large number of open-type radial winding slots WS 1 , WS 2 , . . . that are uniformly spaced around the circumference of the radially inner or outer surface of the stator core SC, depending whether the stator S is an inner or an outer stator of the electrical machine. Due to the large dimension of the stator S, the shown section of the stator S appears in  FIG. 1  as a cuboid with substantially parallel winding slots WS 1 , WS 2 , . . . , WSx (x being a integer value of 1 to 6). This is acceptable as an approximation but it need to be understood that the adjacent winding slots WS 1 , WS 2 , . . . are not completely parallel planes regarding their slots, because the slots all are directed radially to a centre axis of the electrical machine. As a further approximation—or in other situations this might be absolutely true—the slots are cuboid narrow channels with a rectangular cross-section. 
         [0029]    It has to be noted that in the example the winding slots WS 1 , WS 2 , . . . are arranged parallel to the longitudinal axis of the stator S, but it is also possible to have the winding slots formed at an angle to the longitudinal axis of the stator. 
         [0030]    The stator core SC may be laminated, i.e. formed from a plurality of axially stacked laminations. 
         [0031]      FIG. 1  shows furthermore three windings W 1 , W 2 , W 3  that are pre-formed and have a rectangular cross-section fitting right into the winding slot WS 1 , WS 2 , . . . grooves. 
         [0032]    In  FIG. 1  a single layer stator winding is shown, so that only a single segment of a single winding W 1 , W 2 , W 3  is placed into a single winding slot WS 1 , WS 2 , . . . —instead of placing a segment of a first winding and a segment of a second winding into single winding slot being substantially on top of each other in the slot, as it is done for double-layer winding. 
         [0033]    Furthermore the windings W 1 , W 2 , W 3  each have two segments that get inserted into two winding slots—e.g. winding W 1  will be inserted with one segment of the winding loop into the first winding slot WS 1  and with another segment of the winding loop into the second winding slot WS 2 . The windings W 1 , W 2 , W 3  are closed loops therefore requiring a section to close the loops between the substantially parallel sections of the windings inside the winding slots. This is done via winding heads WH 1 , WH 2 , WH 3  that extend outside of the stator S and close the loop at both axial ends of the stator S. 
         [0034]    The winding slots build pairs of slots to mount a single winding. In the FIGURE the winding slots WS 1  and WS 2  build a pair of slots for the winding W 1 , the winding slots WS 3  and WS 4  build a pair of slots for the winding W 2 , and the winding slots WS 5  and WS 6  build a pair of slots for the winding W 3 . 
         [0035]    To properly map the windings to the used terminology of the claims, the “first winding” of the claims corresponds to winding W 2 , the “second winding” to winding W 3 , and the “third winding” corresponds to winding W 1 . 
         [0036]    It is assumed that each of the windings is symmetric—e.g. axial symmetric to a radial plane in the centre of the stator S—, so in the following usually only one winding head of the windings is observed, but all that is said is also applicable to the winding heads at the other end of the stator S. 
         [0037]    Furthermore it is assumed that the length of all windings might not be identical but fairly similar in length. Besides a slight difference in length, the only difference between the first winding W 1 , the second winding W 2 , and the third winding W 3  is the three-dimensional shape of their winding heads WH 1 , WH 2 , WH 3 . 
         [0038]    Regarding the winding W 1  it is assumed that it is substantially formed that way that the winding head WH 1  stays in an area between the radial plane of the winding slot WS 1  and the radial plane of the winding slot WS 2 , so that the winding W 1  does not touch or interfere adjacent windings. The winding head WH 1  may extend in radial direction between these radial planes, but it is assumed only the space in one radial direction—radially inwards for an inner stator—should be used so to not interfere with the rotor, which is not shown in the FIGURE. Taking the circumferential surface of the stator core SC as the reference plane, for an inner stator the space from that reference plane facing the axis of the stator could be consumed for the winding head WH 1 . For an outer stator the space from that reference plane facing the radial outside of the electrical machine until the housing of the electrical machine could be consumed for the winding head WH 1 . 
         [0039]    The above said regarding winding W 1  and winding head WH 1  applies accordingly also for the winding W 2  and W 3  with their winding heads WH 2  and WH 3 . 
         [0040]    The windings W 1 , W 2 , and W 3  furthermore are preferably formed that way that they can easily be mounted by following a specific order of mounting the windings into the stator winding slots. Specifically the windings should not to be tangled up so that all three windings need to be mounted altogether in one step. Preferably the windings should be formed like this that as a first option all windings W 1  could be inserted overall the stator or overall one stator segment, followed by winding W 2  and finally by winding W 3 . As a second option the stator could be mounted with the windings W 1 , W 2 , W 3  winding by winding, putting in first a single W 1 , then single W 2 , followed by a single W 3 , and starting all over again with W 1 . 
         [0041]    In the following description, if the terms “underneath” or “above” or similar directional terms are used, this belongs to the orientation as it is shown in  FIG. 1 . “Above” or “upwards” means in the direction from where the windings get mounted into the stator S. For an inner stator this means the direction away from the centre axis of the stator. With “underneath” or “downwards” the opposite direction is meant, i.e. for an inner stator the direction to the centre axis of the generator. 
         [0042]    In  FIG. 1 , the winding head WH 3  extends in the plane of the two substantially parallel segments of the stator winding W 3 , which is the plane of the winding slots. The winding loop stays in one plane and is flat and not tilted out of the plane. In other words, once mounted, winding W 3  including its winding head WH 3  stays in the same plane as the plane of the winding slots WS 5  and WS 6  of the stator S. 
         [0043]    The length of the section of the winding head WH 3  that longitudinally extends the winding outside of the stator core SC is set up that way, that the winding length of winding W 3  has preferably substantially the same length as the length each of the other two windings W 1  and W 2 , as far as possible. 
         [0044]    Now regarding the winding W 1 , its winding head WH 1  comprises a section that is continuously tilted on a curvature, whereas the curvature is a 90 degree segment of a substantially circular arc. In the FIGURE, after the 90 degree turn has reached, the winding W 1  then further extends in a plane that is perpendicular to the plane of the substantially parallel winding segments or the plane of the winding slots WS 1  and WS 2 . 
         [0045]    The winding head WH 1  then proceeds underneath the other winding heads WH 2  and WH 3  with a substantially straight connection. 
         [0046]    Alternatively and not shown in the FIGURE the winding head WH 1  might be tilted at one pivotal point and then might extend in a plane different from the plane of the two substantially parallel segments of the stator winding W 1 . The pivotal point may be in the area of the winding head W 11  and close to the substantially parallel segments of the stator winding W 1 . Therefore the winding W 1  may be tilted close to the stator block. The winding head WH 1 , after the turn at the pivotal point then would extend underneath the other winding heads WH 2  and WH 3 . 
         [0047]    The winding W 1  might perform a “sharp turn” at the pivotal point but preferably might also perform a continuous change of direction over a specific section around the pivotal point. 
         [0048]    The change of direction at the pivotal point might be an angle between 0 and 90 degrees, measured from the plane of the two substantially parallel segments. 
         [0049]    Yet in another not shown alternative, the winding W 1  may differ from the previously said that the section that is continuously tilted on a curvature of the winding head WH 1  may have a curvature that is more than a 90 degree segment of a substantially circular arc. Specifically as an example, the angle, taken from the plane of the stator winding slots may be about 135 degrees. This allows a very compact arrangement. 
         [0050]    Not mentioned so far, the two windings W 1 , W 3  close their loops between the radial planes of their winding slots with a connection section perpendicular to these mentioned radial planes. The transition from the winding head sections located in the radial planes to/from the connection section is performed via a 90 degree turn with a fairly small radius. The connection itself is then substantially plain and straight. 
         [0051]    The winding W 3 , which is not tilted at all, can be viewed as a substantially rectangular belt. The winding W 1  can also be seen as such rectangular belt, only with the modifications that close to the corner the longer side of the rectangular is bent. 
         [0052]    Due to the fact that the cross-section of each winding W 1 , W 2 , W 3  is substantially rectangular, the connection section—i.e. the connection to overcome the distance between a pair of winding slots like WS 1  and WS 2 , parallel to the side surface of the cylindric stator core SC—of windings W 1  and W 3  itself is substantially cuboid and therefore located in a plane of the cuboid. Regarding winding W 3 , this plane of the connection section of winding head WH 3  is parallel to the side surface of the cylindric stator core SC. Regarding winding W 1 , the plane of the connection section of winding head WH 1  is substantially in a 90 degree angle in relation to the side surface of the cylindric stator core SC or, in other words, parallel to the plane of the stator winding slots. 
         [0053]    Proceeding to winding W 2 , this winding W 2  is mounted into the winding slots WS 3  and WS 4 . Displayed is a first winding head segment WH 2  of a the winding W 2 —which is specified as “first” winding in the claims—describing in space in parts a combination of a rotation about an axis and a translation along that axis. Substantially this segment part of the winding head WH 2  is screw axis symmetric, the axis being substantially perpendicular to a plane of the winding slots WS 3 , WS 4 . 
         [0054]    More specifically, winding head WH 2  describes for a part of the winding head WH 2  a three-dimensional form characterised in that the displayed segment of winding head WH 2  of the stator winding W 2  is—starting from the winding slot WS 4 —continuously tilted on a curvature downwards until the curvature is substantially a 90 degree segment of a substantially circular arc. Then the winding W 2  is continuously tilted on a curvature in the opposite direction in the same plane until the winding W 2  is directed again substantially in an axial direction of the stator S. At that point the winding head WH 2  is underneath the winding head WH 3 . Then the shape of the winding head WH 2  continues with a combination of the rotation in direction of winding slot WS 3  about an axis that is substantially perpendicular to the plane of the winding slots WS 3 , WS 4  and an upward translation along that axis. 
         [0055]    This combination of the rotation about the axis and the translation along that axis is continued for the winding head WH 2  until the winding cross-section faces the longitudinal extension of the winding slot WS 3  and being in the same plane as the winding slots WS 3 , WS 4 . Then, the winding head WH 2  is substantially above the winding head WH 1 , so that the winding head WH 2  can cross the winding head WH 1  without interference. Finally, the winding head WH 2  is continued above winding head WH 1  in the direction of the winding slot WS 3  in a straight fashion. 
         [0056]    All in all and by looking from the top at the winding W 2 , winding head WH 2  performs a half circle to close the winding W 2  between winding slots WS 3  and WS 4 . 
         [0057]    The just discussed embodiment has the advantage that the length of each winding can be reduced and that the length of the three windings is fairly the same. This allows a vast reduction of material for the windings and a major reduction of weight. 
         [0058]    It has to be understood that slight variations of the embodiment, especially regarding the perfectness of the circular arc, regarding the perfect planarity of the mentioned planes, or regarding specifically mentioned angles, are also covered by this invention.