Abstract:
A generator is provided that includes at least one conductor. The conductor is made up of aluminium. In a preferred embodiment, the generator includes at least one pole set representing one phase, each pole set having a plurality of poles, and the at least 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:
[0001]    This application claims priority of European Patent Office application No. 10163301.4 EP, filed May 19, 2010, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a generator and to a wind turbine. 
       BACKGROUND OF INVENTION 
       [0003]    In a direct drive permanent magnet generator windings are typically made of copper which is a relatively heavy metal and thereby results in a heavy generator which may create difficulties for transport and mantling of the generator. 
       SUMMARY OF INVENTION 
       [0004]    The power efficiency of a generator, especially of a large direct drive generator, can be increased by reducing the weight of the generator. Moreover, the installation of a generator in an electrical machine or power plant can be simplified if the weight of the generator is reduced. A reduced weight of the generator would also reduce the installation costs. 
         [0005]    Therefore, it is a first objective of the present invention to provide a generator, where the winding weight is reduced. It is a second objective of the present invention to provide an advantageous wind turbine. 
         [0006]    The above objectives are solved by the features of the independent claims. The depending claims define further developments of the invention. 
         [0007]    The inventive generator comprises at least one conductor. The conductor comprises aluminium. The at least one conductor can be only made of aluminium. 
         [0008]    The idea is to use aluminium windings instead of copper. The use aluminium instead of copper is advantageous, because aluminium has electrical conductivity of about 60% of the electrical conductivity of copper, but a mass density of only 30% of the mass density of copper. Therefore, it is advantageous to use more aluminium, e.g. 60% or more, volume-wise by increasing the slot area, compared with conventional copper windings. A similar or the same winding resistance can be obtained as for conventional copper windings. Furthermore, a significant reduction in the total winding weight by roughly 52% may be achieved: 
         [0000]    
       
         
           
             
               
                 m 
                 Al 
               
               = 
               
                 
                   
                     
                       
                         ρ 
                         Al 
                       
                       · 
                       
                         V 
                         Al 
                       
                       · 
                       
                         m 
                         cu 
                       
                     
                     
                       
                         ρ 
                         cu 
                       
                       · 
                       
                         V 
                         cu 
                       
                     
                   
                   ≈ 
                   
                     0.3 
                     * 
                     1.6 
                     * 
                     
                       m 
                       cu 
                     
                   
                 
                 = 
                 
                   0.48 
                    
                   
                     m 
                     cu 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where ρ is the mass density, V the volume and m is the weight. 
         [0009]    Considering the thermal properties of aluminium and copper, see table.1: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Mass, electrical and thermal properties of  
               
               
                 aluminium and copper, intended for comparison. 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Resistance 
               
               
                   
                   
                   
                   
                 Specific  
                 coefficient 
               
               
                   
                 Mass 
                 Electrical 
                 Thermal 
                 heat 
                 of 
               
               
                   
                 density  
                 resistivity 
                 Conductivity 
                 capacity 
                 temperature 
               
               
                   
                 ρ 
                 (Ω*m ) 
                 (W/m*K) 
                 (kJ/kg*K) 
                 (1/K) 
               
               
                   
               
             
          
           
               
                 aluminium 
                 2700 
                  2.82 × 10 −8   
                 205.0 
                 0.91 
                 0.0043 
               
               
                 copper 
                 8960 
                 1.722 × 10 −8   
                 385.0 
                 0.39 
                 0.0040 
               
               
                   
               
             
          
         
       
     
         [0010]    The temperature rise would be 11% lower in aluminium winding compared to copper winding which is an advantage: 
         [0000]    
       
         
           
             
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     T 
                     Al 
                   
                 
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     T 
                     Cu 
                   
                 
               
               = 
               
                 
                   
                     
                       Q 
                       Al 
                     
                     
                       
                         m 
                         Al 
                       
                       · 
                       
                         c 
                         Al 
                       
                     
                   
                   
                     
                       Q 
                       Cu 
                     
                     
                       
                         m 
                         Cu 
                       
                       · 
                       
                         c 
                         Cu 
                       
                     
                   
                 
                 = 
                 
                   
                     
                       
                         m 
                         Cu 
                       
                       · 
                       
                         c 
                         Cu 
                       
                     
                     
                       
                         m 
                         Al 
                       
                       · 
                       
                         c 
                         Al 
                       
                     
                   
                   ≈ 
                   0.89 
                 
               
             
             , 
           
         
       
     
         [0000]    where, c is the specific heat capacity and Q is the loss energy in the windings which is assumed to be the same for both types of winding. Regarding the resistance increase at higher temperatures, the aluminium resistance would be about 7.5% higher than copper for the same temperature rise, which is due to a larger temperature coefficient of aluminium. Hence, the aluminium winding resistance would not be larger than copper winding due to temperature rise but even might be less. 
         [0011]    In the present invention the relatively heavy copper as winding material in direct drive generators is at least partly replaced by aluminium, which has a much lower mass density. The right selection also stops compromising on the electrical and thermal conductivity of the winding and to some extent even improves it. Furthermore, this choice is more motivated by the need to have a more flexible and softer material that simplifies the winding manufacturing in direct drive generator technology. 
         [0012]    The inventive generator may comprise at least one pole set. One pole set may represent one phase. Each pole set can comprise a number of poles. At least one conductor can be turned about the poles of the particular pole set such that only half a single turn is associated to each pole. Preferably, a number of conductors which are connected in parallel are turned about the poles such that only half a single turn of each conductor is associated to each pole. 
         [0013]    Compared with a conventional coil composed of more than one series turn, the insulation between the conductors placed together in a slot of the inventive generator can significantly be reduced. This improves the slot fill factor resulting in higher torque or efficiency. Moreover, the inventive generator provides the possibility for a better cooling of, for example, a permanent magnet generator. The inventive generator may, for example, be used in direct drive wind turbine applications. 
         [0014]    In the inventive generator single turn wave winding replaces the conventional windings. The idea is that each phase in, for example, a three phase or multi phase generator has a single Go or Return path in each pole. In the frame work of the present invention a single Go or Return path is also designated as half a single turn. The Go and Return paths or half a single turns may form a wave configuration. For example, a single Go path may itself be composed of a number of parallel conductors. The parallel conductors return in the next pole and continue this way of distribution along the whole circumference of, for example, the stator of the generator. This gives the advantage of having less insulation in the slot. Thereby, a better cooling of the windings can be achieved and a higher slot fill factor can be realised. 
         [0015]    Advantageously, between 5 and 25, preferably between 10 and 20, conductors may be connected in parallel. Assuming that the same slot dimension as for conventional multi-turn windings is used for the wave winding, 10 to 20 parallel conductors or in that range will form the winding in order to reduce the proximity and skin effect losses. The optimal number of parallel conductors to give a low value of proximity and skin effect loss can be chosen analytically or can be obtained by a simulation or can be obtained experimentally. 
         [0016]    The conductors can be transposed from one pole to another pole. This improves the elimination of extra AC losses, for example losses due to the proximity and skin effect. The conductors can be partially or fully transposed in each or every second and winding. Advantageously, the conductors may be transposed at every neighbouring pole or at every second neighbouring pole of the particular pole set. Preferably, the number of poles in a pole set may be an integer multiple of the number of the conductors connected in parallel. To have completely balance out the extra AC loss a full transposition may be used, i.e. to transpose every parallel conductor at every pole while choosing the number of poles to be in integer multiple of the number of parallel conductors. Having a different number of poles than what is mentioned will still be an option, but with some extend higher relative AC loss due to proximity effect. 
         [0017]    The inventive generator may comprise an even number of poles per pole set. In a preferred embodiment of the invention such as for a generator for a direct drive wind turbine the number of poles is equal to or above 100. For example, the generator may comprise at least 1 pole set, preferably 3 pole sets. Furthermore, the generator may be a direct drive generator. Generally, the generator may comprise a stator and a rotor. The stator may comprise the at least one pole set. Alternatively or additionally the rotor may comprise the at least one pole set. 
         [0018]    In the proposed type of single turn winding, the number of poles may be equal to the sum of Go and Return paths of each phase winding. This means, that the number of poles may be equal to the sum of the half a single turns. 
         [0019]    In the present invention, the series turns made of copper in the slot are replaced effectively by aluminium windings. Preferably, additionally the series turns in the slot are replaced effectively by half a single turns or single wave winding which needs much less thinner insulation and increases the fill factor. The needed thinner insulation is caused by a less voltage difference between the series turns in one slot in the inventive generator. Taking the advantage of having less insulation for conductors and all the following improvement of the generator performance, some draw backs like high extra AC loss due to proximity and skin effect are reduced by transposing the conductors in an efficient way. 
         [0020]    The increased fill factor caused by using half a single turns or single wave winding instead of conventional series turns allows having a larger conductor area of aluminium without a need to change the overall design dimensions significantly. 
         [0021]    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 
         [0022]    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. 
           [0023]      FIG. 1  schematically shows a wind turbine. 
           [0024]      FIG. 2  schematically shows a comparative illustration of multi-turn and single turn wave windings for one phase and four poles. 
           [0025]      FIG. 3  schematically shows part of a single turn wave windings of the lower part of  FIG. 2  in a perspective view. 
           [0026]      FIG. 4  schematically shows the AC loss factor dependency for single turn winding of the number of parallel, fully transposed conductors. 
           [0027]      FIG. 5  schematically shows an arrangement of fully transposed  5  parallel conductors belonging to one phase. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0028]    An embodiment of the present invention will now be described with reference to  FIGS. 1 to 5 . 
         [0029]      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. 
         [0030]      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. 
         [0031]    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 . In the present invention the conductors  5  comprise aluminium. 
         [0032]    The lower part of  FIG. 2  schematically shows the inventive single turn wave windings for one phase of a preferable 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. 
         [0033]    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 aluminium or is made of aluminium. 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. 
         [0034]    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.    
         [0035]    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 . 
         [0036]    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 . 
         [0037]      FIG. 4  schematically shows the AC loss factor for single turn windings versus the number of parallel conductors which are always assumed to be fully transposed. The x-axis represents the number N of parallel and fully transposed conductors. The y-axis represents the AC loss factor L for a single turn winding in arbitrary units. The AC loss factor is caused by proximity and skin effect losses. The obtained curved  14  in  FIG. 4  shows a maximum AC loss factor for about two parallel conductors. With a further increasing number of parallel conductors the AC loss factor decreases nearly exponentially. For eight and more parallel conductors the AC loss factor L decreases only minimally. The curve  14  shows for ten and more parallel conductors a nearly straight line which is nearly parallel to the x-axis. This means, that the optimal number of parallel conductors to give a low value of proximity and skin effect loss is ten and more. 
         [0038]      FIG. 5  schematically shows an arrangement of fully transposed  5  parallel conductors belonging to one phase. In the shown arrangement the first pole  21  is followed by a second pole  22 , followed by a third pole  23 , followed by a fourth pole  24 , which is followed by a fifth pole  25  and so forth. Each of the poles  21 ,  22 ,  23 ,  24  and  25  comprises an upper side  15  and a bottom side  16 . The different conductors are designated by numbers 1 to 5. Each pole  21 ,  22 ,  23 ,  24  and  25  comprises five positions, a first position  31 , a second position  32 , a third position  33 , a fourth position  34  and a fifth position  35 , which follow each other from the upper side  15  to the bottom side  16 . 
         [0039]    In the first pole  21  the first conductor  1  is located at the first position  31 , the second conductor  2  is located at the second position  32 , the third conductor  3  is located at the third position  33 , the fourth conductor  4  is located at the forth position  34  and the fifth conductor  5  is located at the fifth position  35 . 
         [0040]    In the second pole  22  the next half a turn of the first conductor  1  changes to the second position  32 , the next half a turn of the second conductor  2  changes to the third position, the next half a turn of the third conductor  3  changes to the fourth position  34  and the next half a turn of the fourth conductor  4  changes to the fifth position  35 . The next half a turn of the fifth conductor  5  changes from the fifth position  35  in the first pole  21  to the first position  31  in the second pole  22 . This pattern is continued for the next poles as shown in  FIG. 5 . By arranging the conductors as shown in  FIG. 5  the 5 parallel conductors are completely transposed. 
         [0041]    In the present embodiment the generator comprises three phases, which means that it comprises three pole sets. Each pole set comprises ten poles. The pattern which is shown in  FIG. 5  is cyclically repeated for the other 5 poles which are not shown in  FIG. 5 . Generally, the generator comprises a rotor  26 , a stator  27  and an airgap  28  between the rotor  26  and the stator  27 . The stator  27  comprises the poles shown in  FIG. 5 . Alternatively, the rotor  26  may comprise the poles shown in  FIG. 5 . 
         [0042]    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. 
         [0043]    Based on the described transposed single turn wave winding configuration, it is theoretically clear that the parallel conductors in the slot may need no insulation or only some varnish as there is none or very small voltage difference between these parallel conductors. 
         [0044]    Furthermore, using aluminium conductors, which are softer and more flexible than conventional copper conductors, and additionally using the described single turn wave-like winding the manufacturing of the coils and the winding process becomes significantly easier and less costly than for conventional windings.