Abstract:
A generator is provided that includes at least one pole set representing one phase. Each pole set includes a plurality of poles. Only one conductor is turned about the poles of a particular pole set such that only half a single turn is associated to each pole of the particular pole set.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European Patent Office application No. 10163306.3 EP filed May 19, 2010, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The present invention relates to a generator and to a wind turbine. 
     BACKGROUND OF INVENTION 
     Based on today&#39;s technology in direct drive generators, each coil is composed of more than one series turn while a chosen number of coils may also be connected in series. The two aforementioned selectable numbers, i.e. the number of series in turn and the number of coils connected in series, may be chosen to obtain the terminal voltage of the generator which itself may have already been chosen based on power electronics/grid requirements. In the described conventional type of winding, the series turns in each slot need to be electrically insulated from each other due to the voltage difference between the turns in series. Furthermore, the whole conductors in the slot need to be insulated from the neighbouring lamination via so called slot insulation due to the voltage difference between the turns and the lamination which is grounded through the coupling with shaft and wind tower. 
     The aforementioned type of insulation used in the slot of the machines with conventional windings, results in different disadvantages: Poor heat transfer coefficient of the insulation makes it very difficult for the main source of heat in the generator, i.e. the windings to get cooled down through the neighbouring laminations. Using the required insulation in the slots decreases the slot space for the active material, which is usually copper, and thereby the so called fill factor. This in turn reduces the out put torque for the same current density or decreases the efficiency for the same torque. A typical value of fill factor for conventional type of winding is in the range of 70-80%. 
     SUMMARY OF INVENTION 
     It is a first objective of the present invention to provide a generator with an increased slot fill factor and with decreased manufacturing costs. It is a second objective of the present invention to provide an advantageous wind turbine. 
     The above objectives are solved by the features of the independent claims. The depending claims define further developments of the present invention. 
     The inventive generator comprises at least one pole set representing one phase. Each pole set comprises a number of poles. Only one conductor is turned about the poles of a particular pole set such that only half a single turn is associated to each pole. The only one conductor, which may preferably a single turn solid or compact winding, can replace the conventional windings giving the advantage of having less insulation in the slot. This reduces the insulation between the conductor and a neighboring lamination of the pole, so called slot insulation. Moreover, the conventional turn-turn insulation between the conductors within a slot becomes unnecessary. Furthermore, a better cooling of the windings and a higher slot fill factor can be achieved. Another advantage is that the manufacturing of the coils and the winding process becomes significantly easier and less costly than for multi-turn conventional windings. Furthermore, using only one solid or compact large conductor with the proposed suitable slot shape results in low skin effect loss. 
     The inventive generator may be a three phase or a multi-phase generator. In the inventive generator each phase may have a single Go or Return path, also designated as half a single turn, in each pole. The single Go or Return paths or half a single turns are arranged in a wave-like configuration. For example, the half a single turn itself can be a solid or compact conductor. It may return in the next pole and may continue this way of wave-like distribution along the whole circumference of the stator or rotor. 
     The inventive generator may comprise a stator comprising the at least one pole set. Additionally or alternatively, the generator may comprise a rotor comprising the at least one pole set. Preferably, the generator may comprise a stator, a rotor and a rotation axis. The stator may be located radially inside of the rotor (inner stator machine). 
     Advantageously the conductor comprises a number of, preferably solid or compact, conductor elements. The conductor elements can be conductively connected to each other. The conductor elements can be connected to each other by welding or by a mechanical fixation, for example. 
     Generally, the generator can comprise at least one slot between the poles. Preferably, the conductor elements may have the shape of straight bars or arcs. For example, a first straight bar can be placed in a slot and can be connected to a second straight bar in another slot by means of an arc. In this way the conductor elements can be connected to each other forming a wave-like configuration. 
     Generally, the conductor may at least partly be located in at least one slot between the poles. The generator may comprise a rotation axis. The slot may be closed or partly closed at a radial position related to the rotation axis. Advantageously, the slot may have a rectangular or trapezoidal or triangular shape in a plane perpendicular to the rotation axis. Preferably, the slot may have an incremental opening in radial direction related to the rotation axis. 
     Moreover, the slot can comprise at least one bridge which partially or fully closes the slot at a radial position. The bridge can be integral part of an element forming the slot, for example of a lamination of the pole. Alternatively, the bridge can be a separate element. In this case, the bridge can mechanically be fixed to the element forming the slot, for example to the pole lamination. For instance, the slot may comprise the side face. The element or lamination forming the slot can comprise a cavity in the side face of the slot. The cavity can be used for mechanical fixing the bridge. The mentioned bridges can be simply built by re-designing the tooth shape of the poles, especially of the lamination. 
     The bridge may comprise soft magnetic composite material or ferromagnetic material. For example, the bridge may be made of soft magnetic composite material or may be made of ferromagnetic material, via punching the lamination with the appropriate shape. 
     The use of separate conductor elements, as previously described, allows it to use a modular winding structure. This modular winding structure can be used for closed and semi-closed slots. This reduces the skin effect. Furthermore, it significantly simplifies the winding manufacturing, especially compared with conventional winding. For example, the conductor elements, preferably the straight bars, can be inserted into the slot, which may be a closed or semi-closed slot. 
     The use of trapezoidal or triangular slots with incremental opening, especially towards an air gap in inner stator machines, reduces the skin effect loss which emerges in the rectangular conductors with large radial height or deep slots. 
     The shape of the conductor can be optimized regarding the reduction of skin effect losses. In the same way, the shape of the slot and the shape of neighboring lamination teeth of the pole can be optimized. Moreover, having only one solid large conductor pole instead of having parallel conductors the insulation between series turns in conventional windings does not exist and completely disappear. In this aspect, the present invention is more efficient than having parallel conductors where still some thin varnish may be needed around the parallel conductors. Furthermore, the slot insulation can be optimized in thickness. 
     A further possibility to reduce the skin effect in the single turn solid conductor is to reduce the radial height or depth of the conductor. For example, the at least one slot may have an average width w and a radial depth h related to the rotation axis of the generator. The average width w can be of a value of at least 90% of the value of the depth h. Preferably the average width w can be comparable or larger than the depth h. Keeping the area of the conductor fixed, the slot width then need to become larger. As a result, the number of slots versus the poles will be reduced. Therefore, a concentrated winding, for example in form of only one single turn solid or compact conductor, as previously described, can be used. Such a concentrated winding has a low slot to pole number ratio. 
     In combination with the previously mentioned optimised conductor shape, especially if an inner stator is used, the slots can be practically be made in a trapezoidal shape. In this case, the teeth or poles can be kept rectangular. The combination of these two parameters using shallow and trapezoidal slots reduces the skin effect effectively. Moreover, to improve the magnetic circuit and/or to reduce the skin effect even further closed or semi-closed slots or bridges in the middle of the slots may be used. 
     FE 2D analysis has shown that each of these suggested shapes will reduce the skin effect loss significantly. Furthermore, a combination of these suggested cases reduces the skin effect even further. 
     One draw back of using the previously described bridge is that the slot leakage will be increased. This is to some extent an advantage since it reduces the short circuit current and this way adds to reliability of the generator, for example the direct drive generator. However, possibly more significantly the output torque of the generator will fall due to flux leakage. Therefore, the decision on thickness of the bridges is a trade off between having lower skin effect on the one hand and not losing too much torque on the other hand. 
     Generally, the generator may comprise at least 3 pole sets. The generator may be a direct drive generator. 
     In the present invention, the conventional insulation between the conductors placed together in a slot and also the slot insulation are significantly reduced. This improves the slot fill factor and enhances the cooling of the generator, for example of a permanent magnet generator. Such a permanent magnet generator can, for example, be used in direct drive wind turbine applications. Furthermore, simple and modular windings are defined and provided by the present invention. This significantly simplifies the manufacturing and the winding process, thereby lowering the associated costs. 
     The requirement of having turn-turn insulation was removed by using single turn solid coil winding. Furthermore, the slot insulation (between the single conductor and the neighbouring teeth) is minimized by choosing its thickness optimally for each slot, realizing that each slot has a different/unique voltage difference with its neighbouring teeth. This optimal selection is not possible for conventional multi-turn windings as more than one voltage difference exists in each slot; thereby the selection is based on the worst case. 
     Taking the advantage of having less winding and slot insulation and all the following improvement of the generator performance, a drawback of having high extra AC loss due to skin effect are effectively reduced by the described conductor/slot shapes. 
     The inventive wind turbine comprises an inventive generator, as previously described. The inventive wind turbine has the same advantages as the inventive generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, properties and advantages of the present invention will become clear from the following description of an embodiment in conjunction with the accompanying drawings. All mentioned features and properties are advantageous alone or in any combination with each other. 
         FIG. 1  schematically shows a wind turbine. 
         FIG. 2  schematically shows a comparative illustration of multi-turn and single turn wave windings for one phase and four poles. 
         FIG. 3  schematically shows part of a single turn wave windings of the lower part of  FIG. 2  in a perspective view. 
         FIG. 4  schematically shows a single turn wave winding arrangement of conductor elements in a perspective view. 
         FIG. 5  schematically shows a single turn wave winding arrangement of conductor elements in a sectional view. 
         FIG. 6  schematically shows a conductor element in form of an arc in a perspective view. 
         FIG. 7  schematically shows a rectangular slot in a sectional view. 
         FIG. 8  schematically shows a rectangular closed slot in a sectional view. 
         FIG. 9  schematically shows a trapezoidal and a triangular slot in a sectional view. 
         FIG. 10  schematically shows a rectangular semi-closed slot in a sectional view. 
         FIG. 11  schematically shows a variant of a rectangular semi-closed slot in a sectional view. 
         FIG. 12  schematically shows a rectangular closed slot in a sectional view. 
         FIG. 13  schematically shows part of a generator in a sectional view. 
         FIG. 14  schematically shows a variant of a trapezoidal slot in a sectional view. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     An embodiment of the present invention will now be described with reference to  FIGS. 1 to 14 . 
       FIG. 1  schematically shows a wind turbine  71 . The wind turbine  71  comprises a tower  72 , a nacelle  73  and a hub  74 . The nacelle  73  is located on top of the tower  72 . The hub  74  comprises a number of wind turbine blades  75 . The hub  74  is mounted to the nacelle  73 . Moreover, the hub  74  is pivot-mounted such that it is able to rotate about a rotation axis  79 . A generator  76  is located inside the nacelle  73 . The wind turbine  71  is a direct drive wind turbine. 
       FIG. 2  schematically shows a comparative illustration of multi-turn and single turn wave windings for one phase and four poles. The upper part of  FIG. 2  shows the distributed winding with slots per pole and phase equal to 1 for a 3-phase machine, phases A, B and C. A, B and C correspond to Go direction of the phases and A′, B′ and C′ correspond to Return direction, i.e. opposite direction, of the phases. 
     In the middle part of  FIG. 2  two poles  4  representing the first phase are shown. Each of the poles  4  comprises a number of conductor windings  5  with multiple-turns per pole  4 . The strokes  6  indicate the more than one series turns. The conductors  5  are connected in series. This is indicated by the dashed line  7 . Due to the series turns each of the poles  4  or coils comprises a number of Go paths  17  and a number of Return paths  18 . 
     The lower part of  FIG. 2  schematically shows the inventive single turn wave windings for one phase of an inventive generator. The pole set belonging to the first phase A comprises a number of poles  4 , from which four poles  4   a ,  4   b ,  4   c  and  4   d  are shown. Generally, the poles  4  may comprise a lamination. 
     Each pole  4  comprises a right side  10 , a left side  11 , a front side  12  and a back side  13 . A conductor  8  is wave-like turned about the poles  4 . The conductor  8  comprises a first half turn  8   a , a second half turn  8   b , a third half turn  8   c  and a fourth half turn  8   d . The first half turn  8   a  represents a Return path A′, the second half turn  8   b  represents a Go path A, the third half turn  8   c  represents a Return path A′ and the fourth half turn  8   d  represents a Go path A. 
     The first half turn  8   a  proceeds along the right side  10  of the first pole  4   a  and proceeds further along the back side  13  of the first pole  4   a . Then it proceeds further along the left side  11  of the first pole  4   a  and at the same time along the right side  11  of the second pole  4   b . This means, that the conductor passes a slot between the first pole  4   a  and the second pole  4   b . Then the conductor  8  further proceeds along the front side  11  of the second pole  4   b , then along the left side  11  of the second pole  4   b  and at the same time along the right side  10  of the third pole  4   c . The conductor  8  further proceeds along the back side  13  of the third pole  4   c  and along the left side of the third pole  4   c  and at the same time along the right side  10  of the fourth pole  4   d.    
     In this wave-like configuration the first half a turn  8   a  is associated to the first pole  4   a , the second half a turn  8   b  is associated to the second pole  4   b , the third half a turn  8   c  is associated to the third pole  4   c  and the fourth half a turn  8   d  is associated to the fourth pole  4   d .  FIG. 3  schematically shows part of the single turn wave windings of the lower part of  FIG. 2  in a perspective view. The poles  4  are separated from each other by slots  19 . 
     A number of conductors  8  are connected in parallel and are turned about the poles in such a way that only half a single turn of each conductor is associated to each pole, as shown in the lower part in  FIG. 2  and in  FIG. 3 . The optimal number of parallel conductors to give a low value of proximity and skin effect loss can be chosen analytically or experimentally or by simulation. An example is shown in  FIG. 4 . 
     Generally, the generator  76  can comprise an inner stator, which means that the stator is located radially inside of the rotor of the generator related to the rotation axis  79  of the rotor. Alternatively, the generator can comprise an outer stator, which means that the stator is located radially outside of the rotor of the generator related to the rotation axis  79  of the rotor. In both cases the rotor and/or the stator can comprise the described single turn wave winding. 
       FIG. 4  schematically shows a single turn wave winding arrangement of conductor elements in a perspective view. The conductor  8  comprises a number of conductor elements  20 ,  21 . The solid or compact conductor elements  20 ,  21  are connected to each other such that they form a single turn wave winding as shown in the lower part of  FIG. 2  and in  FIG. 3 . 
     In  FIG. 4  the conductor  8  comprises a number of conductor elements  20 , which have the form of straight bars, and a number of conductor elements  21 , which have the form of an arc. The conductor elements  20  in form of a straight bar have a first end  23  and a second end  24 . The conductor elements  21  in form of an arc have a first end  25  and a second end  26 . A first conductor element in form of a straight bar  20   a  is connected to a first conductor element in form of an arc  21   a  such that the second end  24  of the first conductor element  20   a  in form of a straight bar is connected to the first end  25  of the first conductor element  21   a  in form of an arc. The second end  26  of the first conductor element in form of an arc  21   a  is connected to the first end  23  of a second conductor element  20   b  in form of a straight bar. The second end  24  of the second conductor element  20   b  in form of a straight bar is connected to the first end  25  of a second conductor element  21   b  in form of an arc. The second end  26  of the second conductor element  21   b  in form of an arc is connected to the first end  23  of a third conductor element  20   c  in form of straight bar. By connecting a number of conductor elements  20 ,  21  in the described way a single turn wave winding as shown in  FIG. 3  is obtained. 
       FIG. 5  schematically shows part of the single turn wave winding arrangement of conductor elements, which is shown in  FIG. 4 , in a sectional view. The axial direction is designated as z-axis and is indicated by means of an arrow.  FIG. 7  schematically shows the second conductor element  21   b  in form of an arc in a perspective view along z-direction. The current direction in the conductor element  21   b  is indicated by means of arrows. 
       FIGS. 7 to 12  schematically show different slot forms in sectional views.  FIG. 7  schematically shows a rectangular slot  19  in a sectional view. The slot  19  is formed by a first pole  4   a  and a second pole  4   b . The poles  4  comprise iron. They further may comprise a lamination. The radial direction is indicated by an arrow  28 . The slot  19  comprises an opening in radial direction  28 . 
       FIG. 8  schematically shows a rectangular closed slot in a sectional view. Again, the slot  19  is formed by a first pole  40   a  and a second pole  40   b . The poles  40  have the same properties as the previously described poles  4 . At the position of the opening of the slot  19  in radial direction  28  the slot  19  in  FIG. 8  is closed by means of a bridge  27 . The bridge  27  connects the first pole  40   a  with a second pole  40   b.    
     Generally, all bridges, which are shown in the  FIGS. 8 ,  10  to  12  and  14 , can be integral part of the adjacent poles or can be separate elements. In the last case they can mechanically be fixed to the poles. Moreover, all shown bridges can comprise or can be made of soft magnetic composite material or ferromagnetic material, for example iron. 
       FIG. 9  schematically shows a trapezoidal slot  19  in a sectional view. The slot  19  is formed by a first pole  41   a  and a second pole  41   b . The poles  41  can have the same properties as the previously described poles  4 . In  FIG. 9  the slot  19  has an increasing width w. The width w increases in radial direction  28 . In a plane perpendicular to the rotation axis, which is identical with the shown sectional view, the slot  19  has a trapezoidal shape. The dashed line  43  in  FIG. 9  indicates a further variant, wherein the slot  19  has a triangular shape with an increasing width w in radial direction  28 . 
       FIG. 10  schematically shows a semi-closed rectangular slot in a sectional view. The slot  19  is formed by a first pole  42   a  and a second pole  42   b , which have the same properties as the previously described poles  4 . The opening of the slot in radial direction  28  is partly closed by means of a bridge  29 . The bridge  29  comprises a first portion  29   a  and a second portion  29   b . The first portion  29   a  can be part of the first pole  42   a  or it can be a separate element which is connected to the first pole  42   a . The second portion of the bridge  29   b  can be part of the second pole  42   b  or it can be a separate element which is connected to the second pole  42   b . Between the first portion  29   a  and the second portion  29   b  of the bridge an opening  33  of the slot  19  in radial direction  28  is formed. 
       FIG. 11  schematically shows a variant of a rectangular semi-closed slot  19  in a sectional view. The slot  19  in  FIG. 11  is formed by a first pole  44   a  and a second pole  44   b , which have the same properties as the previously described poles  4 . The slot  19  comprises inner side faces  32 . The height h of the slot  19  is indicated by an arrow. The slot  19  is semi-closed by means of a bridge  30 . The bridge  30  comprises a first portion  30   a  and a second portion  30   b . The bridge  30  is located at about half of the height or depth h of the slot  19 . The first portion  30   a  of the bridge is part of the first pole  44   a  or is connected to the first pole  44   a  at the side face  32  of the slot  19 . The second portion  30   b  of the bridge is part of the second pole  44   b  or is connected to the second pole  44   b  at the side face  32   b  of the slot  19 . The bridge  30  divides the slot  19  into an outer slot part  19   a  and an inner slot part  19   b . The outer slot part  19   a  is located radially outside of the inner slot part  19   b . The outer slot part  19   a  and the inner slot part  19   b  are connected to each other by means of an opening  33  between the first bridge portion  30   a  and the second bridge portion  30   b.    
       FIG. 12  schematically shows a rectangular closed slot in a sectional view. The slot  19  of  FIG. 13  is faulted by a first pole  45   a  and a second pole  45   b , which has the same properties as the previously described poles  4 . The poles  45   a  and  45   b  are connected to each other by means of a bridge  31 . The bridge  31  is located at about half of the height h or depth h of the slot  19 . The slot  19  comprises a first side face  32   a  and a second side face  32   b . The bridge  31  connects the first side face  32   a  with the second side face  32   b  of the slot  19 . The bridge  31  divides the slot  19  into a radially outer slot part  19   a  and a radially inner slot part  19   b.    
     The slots  19  of  FIGS. 7 ,  8 ,  10  to  12  have a rectangular shape in a plane perpendicular to the rotation axis, which is identical with the shown sectional views. 
       FIG. 13  schematically shows a generator  76  in a sectional view. The generator  76  comprises a rotation axis  79 , a stator  78  and a rotor  77 . In  FIG. 13  the rotor  77  is located radially outside of the stator  78 . This means, that the generator  76  of  FIG. 13  is an inner stator generator. Close to the rotation axis  79  a shaft  9  is located. The stator  78  is connected to the shaft  9 . The stator  78  comprises a number of poles  4  which are arranged about circumference of the stator  78 . Between the poles  4  slots  19 , as previously described, are formed. In the sectional view shown in  FIG. 13  the poles  46  have a rectangular shape and the slots  19  have a nearly trapezoidal shape. 
     The stator  78  and the poles  4  may comprise an iron lamination. The rotor  77  comprises a number of permanent magnets  80 . The permanent magnets  80  are arranged about the whole circumference of the rotor  77 . 
       FIG. 14  schematically shows enlarged view of part of the stator  78 .  FIG. 14  schematically shows a slot  19 , which is formed by a first pole  46   a  and a second pole  46   b , which have the same properties as the previously described poles  4 . The slot  19  has a trapezoidal shape in a plane perpendicular to the rotation axis  79 . The slot  19  has an increasing width w in radial direction  28 . Advantageously, the slot  19  has an average width w which is larger than the depth or height h of the slot  19  in radial direction  28 . 
     The slot  19  can be partly closed by a bridge  34 . The bridge  34  can be located at about half of the height or depth h of the slot  19 . The bridge  34  can connect the first pole  46   a  with the second pole  46   b . Alternatively or additionally, the slot  19  can be semi-closed by means of a bridge  35 , which comprises a first part  35   a  and a second part  35   b . The first bridge part  35   a  can be part of or can be connected to the first pole  46   a  and the second bridge part  35   b  can be part of or can be connected to the second pole  46   b . Between the bridge parts  35   a  and  35   b  an opening  33  can be present. The bridge  35  is preferably located at the maximum width w of the slot  19 .