Abstract:
A generator for a wind power has teeth which are surrounded by windings and separated by slots, and permanent magnets that form the poles of the exciter field. The ratio of the number of slots to the product of the numbers of poles and winding phases is fractional and &gt;1. Alternatively or additionally, the front and/or rear edges of successive poles or groups of poles are oppositely inclined to the axis of rotation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a generator for a wind power installation. The generator has teeth which are surrounded by windings and separated by slots, and permanent magnets that form the poles of the exciter field. 
     2. Description of the Related Art 
     An important goal of the further development of wind power installations is reduction of the noise produced by their operation. A high noise level is produced above all when the tower or other parts of the plant are excited with their natural frequency. Therefore, to control noise, it is primarily necessary to prevent vibrational excitations. 
     One of the causes of noise can be the meshing of the teeth in a transmission transmitting the rotation of the rotor to a generator. But the generator is also a source of vibrational excitations. The principal cause of this is so-called slot cogging torques. A well-known means of suppressing slot cogging torques is oblique arrangement of the poles and/or teeth. 
     EP 0 995 257 B1 discloses a wind power installation, in which noise is reduced by using a synchronous generator, whose rotor carries n poles, which are arranged asymmetrically with respect to a plane that contains the axis of rotation of the rotor. Other measures for reducing local vibrational excitations in the generator consist in a pole geometry with the shape of an arrowhead and in pole pieces designed with a trapezoidal cross section. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide other possible means of reducing noise in wind power installations. 
     This object is met by a generator of the aforementioned type, in which the ratio of the number of slots to the product of the numbers of poles and winding phases is fractional and &gt;1 and/or the front and/or rear edges of successive poles or groups of poles are oppositely inclined to the axis of rotation. 
     In a generator of this type, it is advantageous if not all of the cogging torques hit a slot synchronously, since the slot cogging torque and thus vibrational excitations by the generator are largely suppressed in this way. Alternatively or additionally, vibrational excitations are suppressed during generator operation by the compensation of forces generated by the poles and acting on the teeth by successive poles or successive groups of poles generating forces opposed to each of the aforesaid forces. 
     The ratio described above is preferably between 1 and 1.5. In addition, in the preferred embodiment of the invention, the permanent magnets are joined with the rotor of the generator. The rotor is preferably an external rotor. Moreover, in the preferred embodiment of the invention, the rotor of the generator is provided for direct drive by the rotor of the wind power installation without a transmission connected between them. In this case, the measures that have been described are especially effective, because due to the high degree of integration of the generator in the tower of the wind power installation, no dampening coupling elements between the generator and the tower housing are possible. 
     The aforementioned groups of poles are preferably groups of two poles. 
     As a further measure for noise reduction, it can be provided that the poles are arranged asymmetrically with respect to a plane that contains the axis of rotation of the generator. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the Drawing: 
         FIGS. 1 to 3  are schematic partial views of different embodiments of generators of wind power installations according to the invention. 
         FIGS. 3 to 6  are various representations to illustrate forces arising on the stator of the generator of  FIG. 2 . 
         FIGS. 7 and 8  are partial views of additional embodiments of generators of wind power installations of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an unwound top view of a section of teeth  1 , which are distributed over the circumference of a generator stator and are separated from each other by a slot  2 . Each of the teeth is surrounded by a winding  3 .  FIG. 1  shows one such winding as an example. Corresponding to three winding phases that are present, the windings  3  of every third tooth  1  are joined with one another and connected in series. Permanent magnets lies opposite the end faces of the teeth  1  with radial separation to form the generator air gap. The permanent magnets  4  are connected with the generator rotor (not shown), and each of them forms a pole  5  of the exciter field. In the illustrated embodiment, both the end faces of the teeth  1  and the end faces of the poles  5  have a rectangular geometry. 
     As  FIG. 1  shows, for each pole  5  there are four slots. With three winding phases, the ratio of the number of slots to the product of the numbers of poles and winding phases is thus 1.33. This geometry of the generator significantly reduces cogging torques that generate noise. 
     The embodiment according to  FIG. 2  differs from the embodiment according to  FIG. 1  in that the poles  5   a  formed by permanent magnets  4   a  have the shape of a parallelogram instead of a rectangle. Parallel edges  6 ,  7  of the poles  5   a  are inclined to the direction of movement of the rotor, which is indicated by an arrow  8 , or to the axis of rotation. Compared to the embodiment illustrated in  FIG. 1 , this inclination results in further reduction of the cogging torque and thus even less noise generation of the wind power installation. 
     The embodiment illustrated in  FIG. 3 , which has permanent magnets  4   b  that form arrow-shaped poles  5   b , provides similar noise reduction relative to the embodiment according to  FIG. 1 . Here too, oblique edges of the poles  5   b  result in further “blurring” that counteracts the cogging torques. 
     Referring now to  FIGS. 4 to 6 , these Figures illustrate forces acting on the teeth  1  of the generator of  FIG. 2  during its operation. 
       FIGS. 4   a ,  5   a , and  6   a  each show a section through a plane that contains the axis of rotation  9  of the generator, and  FIGS. 4   b ,  5   b , and  6   b  show a radial top view of the generator. As  FIGS. 4   a ,  5   a , and  6   a  show, the stacks of sheets that form the teeth  1  are arranged on a foundation with a disk  10  that is perpendicular to the axis of rotation  9  and with support sheets  11  that are perpendicular to the disk. The sectional view of  FIGS. 4   a ,  5   a , and  6   a  also reveals a rotor ring  12  for mounting the permanent magnets  4   a.    
       FIG. 4  shows the case in which the center of the edge of a permanent magnet  4   a  that is the front edge  6  with respect to the direction of movement (arrow  8 ) is located above the center of a slot  2 . In this way, part of the permanent magnet  4   a  or of the pole  5   a  is already reaching the next tooth  1 ′, while another part still lies above the adjacent tooth  1 . Due to the different positions of the adjacent teeth  1 ,  1 ′, forces  13  that act radially outward are produced in the half of the tooth  1 ′ on side A of the generator, while forces  14  that are directed radially inward are produced in the half of the tooth  1  that lies on the other side B of the generator. In addition, tangential forces  15  that are directed opposite the direction of movement (arrow  8 ) are produced in the half of the tooth  1 ′ on side A, while tangential forces  16  that are directed in the direction of movement are produced in the half of the tooth  1  on side B. 
       FIG. 5  shows the case in which the center of an edge  6  of a permanent magnet  4   a  lies on the center of a front edge  17  of a tooth  1 . In this position, forces  18  that act radially inward are produced in an end region of the next tooth  1 ′ on side A of the generator. Forces  19  that act radially outward are produced in a central region of the tooth  1 , and forces  20  that are directed radially inward are produced in the end region of this tooth on side B of the generator. Tangential forces  21 - 23  are distributed in similar fashion. 
       FIG. 6  shows the case in which the center of the front edge  6  of a permanent magnet  4   a  lies above the center of a tooth  1 . Forces  24  that are directed radially inward are produced in the half of the tooth  1  on side A of the generator, while forces  25  that are directed radially outward are produced in the other half of the tooth on side B of the generator. Tangential forces are produced in the direction of movement in the half of the tooth  1  on side A, while tangential forces are produced in the other half in the direction opposite the direction of movement of the permanent magnet  4   a.    
     As a result of the force distributions described above, when the rotor rotates around the stator, tilting moments develop in the stator in both the radial and tangential directions. This results in periodic excitations to vibrations, which can be transferred from the stator to the tower housing and the tower and can be a further cause of noise in the wind power installation. 
     Therefore, further noise reduction relative to the embodiment of  FIG. 2  can be realized by the embodiments of a generator of a wind power installation that are illustrated in  FIGS. 7 and 8 . 
     According to  FIG. 7 , the respective edges  6   c ,  7   c  and  6   c ′,  7   ic ′ of successive permanent magnets  4   c ,  4   c ′ are oppositely inclined in relation to the direction of movement of the permanent magnets or in relation to the axis of rotation. The forces acting on the respective permanent magnets thus largely cancel each other. 
     An effect of this type is also achieved in the embodiment according to  FIG. 8 , in which two adjacent permanent magnets  4   d ,  4   d ′ that are parallel to each other are followed by a permanent magnet  4   d ″ that is inclined towards the preceding permanent magnet  4   d′.    
     While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.