Abstract:
The invention relates to a wind turbine rotor comprising at least one blade and at least on pitch mechanism comprising a ring shaped motor for controlling the blade. The wind turbine rotor is characterized in that, the ring shaped motor controls the blade through gearing means of a planetary type. The invention further relates to a rotation controlling mechanism comprising, at least one ring shaped motor for controlling the rotation of at least one first part in relation to at least one second part. The rotation controlling mechanism is characterized in that, the ring shaped motor controls the relative rotation through gearing means of a planetary type. Even further the invention relates to a method for controlling at least one blade of a wind turbine rotor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of pending International patent application PCT/DK2007/000084, filed Feb. 21, 2007, which designates the United States and claims priority from Denmark patent application no. PA 2006 00291, filed Feb. 28, 2006, the content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a wind turbine rotor comprising at least one blade, and at least one pitch mechanism comprising a ring shaped motor for controlling rotation of at least one blade, and a method for controlling at least one blade of a wind turbine rotor. 
     BACKGROUND OF THE INVENTION 
     A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on  FIG. 1 . 
     The pitching of wind turbine blades is commonly done with a hydraulic system based on electrical powered oil pump, proportional valve and hydraulic cylinder acting on the blade. A much more direct way is to let an electrical motor act directly on the blade. Pitch systems has been made with electromechanical activation. One known way is to make the blade rotation with a geared motor, which rotate the blade by an open gear. Another known way is to have a linear activator (spindle and motor) to replace the function of the hydraulic cylinder. Both solutions have the inhered disadvantage, which is the motion of moving contact in gears or in treats. This means risk of wear, which can limit the life of the elements. 
     A contact free pitch motor could be advantageous. Such can be imagined. Think of an electrical motor, where the case is flanged to the hub and the blade is mounted on the rotor. If the motor is dominantly disk-shaped-like and merged with a blade bearing, say the motor has almost the same diameter as the blade bearing, a motorized slewing unit is made. 
     A motor for such design will have a large diameter and will naturally be without centre. The motor will look like a slewing ring, where the one ring is the rotor and the other is the stator. An example of this is disclosed in the international patent application WO 2005/019642 A1, where the rotor of a direct drive motor is attached more or less directly to a wind turbine blade and the stator is connected to the hub. 
     In this design, the direct drive motor has to handle the full blade torque moment. This demands a motor with very high torque and very low speed, which results in a large and expensive motor. 
     WO 2005/019642 A1 further disclose that the direct drive motor could act on the blade through a bull gear. But this solution has the implications, that to reduce the torque the bull gear ring—on which the pinion acts—has to be as large as possible, which results in limited space for a direct drive motor concentric to the pinion. 
     Say the bull gear is dentations of the inner ring of the blade bearing and almost in plane with the inner of the blade and hub. Then the motor cannot be much larger than the pinion. In such case the use of a direct motor makes no sense. Decreasing the bull gear diameter makes space for a larger direct motor, but reduces the gear ratio; hence the direct motor must have a larger torque capacity. The solution with a pinion and bull gear is contradictive to their statement about the benefit of the direct motor. 
     The use of a pinion also requires separate bearings for both the pinion and the direct motor. Most disadvantageously is the problem of having one large pinion in mesh with the bull gear. The tooth, which is in mesh at the dominant tip angle will be loaded frequently, which will cause wear and fatigue considerations. 
     An object of the invention is therefore to provide for an advantageous technique for controlling the blade of a wind turbine rotor, which do not include the above mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     The invention provides for a wind turbine rotor comprising at least one blade and at least one pitch mechanism comprising a ring shaped motor for controlling the blade. The wind turbine rotor is characterized in that, the ring shaped motor controls the blade through gearing means of a planetary type. 
     Using a ring shaped motor is advantageous, in that a ring shaped motor has a free centre, i.e. the motor is formed as an annular ring providing free access through its centre to the inside of the blade. 
     Furthermore it is advantageous to gear the motor torque, in that it hereby is possible to reduce the motor torque needed to control the blade, and using a planetary gear for this purpose is advantageous, in that planetary gears are the most compact embodiment of a gearbox, and in applications, such as pitch mechanisms for wind turbine blades—where minimum size and weight are important—the use of planetary gears are very advantageous. 
     Even further, the combination of a ring shaped motor and a planetary gear for controlling the blade is advantageous in that, such a combination is very suitable for active controlling of the blade with small service cost. The wear parts could basically be reduced brake pads, brake beam and backup batteries. All item to replace is of sizes, which can be changed without large cost to manipulate them. 
     In an aspect of the invention, said ring shaped motor controls said blades pitch angle in relation to a hub of said rotor. 
     Hereby, an advantageous embodiment of the invention is achieved. 
     In an aspect of the invention, an annulus gear of said gearing means is rigidly directly or indirectly mounted on a hub of said rotor. 
     Connecting the annulus gear of the planetary gear rigidly to the hub is advantageous, in that the two parts hereby can increase each others rigidity. 
     In an aspect of the invention, said gearing means comprise a plurality of planet gear wheels, such as more than three planet gear wheels. 
     The more planet gears the planetary gear comprises, the more evenly the torque from the ring shaped motor is distributed to the blade. If e.g. the entire torque where to be transferred through a single gear—like in traditional pitch gearmotor and pinion gear systems—the entire torque is in principle transferred through a single point. This would require gears of a very large module and the entire system (blade, hub, gears etc.) would have to be very rigid for not to be distorted, deformed or damaged, when transferring this huge load substantially through a single point, which again would lead to a disadvantageous weight increase in the hub and a significant increase in the manufacturing costs. 
     Making the planetary gear comprise a plurality of planet gears are therefore advantageous, in that the torque hereby is transferred more evenly and gentle to the blade. 
     Furthermore, by using a plurality of planet gears, the total play between the planets and the annulus gear and sun gear, is reduced, which then reduces the possibility of backlash in the gear, in that inaccuracies will level each other out. And when using a plurality of planet gears, the torque that the individual planet has to transfer, is reduced, which means that the module of the planets can be reduced. The lower the module, the more refined and precise the gear parts are made and this fact will also contribute to reducing the play and thereby reducing or eliminating backlash through the gear. 
     In an aspect of the invention, said plurality of planet gear wheels is directly or indirectly mounted on said at least one blade. 
     Making the blade directly or indirectly act as planet carrier in the planetary gear is advantageous, in that a simple and advantageous design of the planetary gear hereby is achieved. 
     In an aspect of the invention, said plurality of planet gear wheels is flexible mounted. 
     If the planetary gear comprises more than three planet gears the design becomes statically indeterminate, and if the planetary gear comprises e.g. twelve planet gears it becomes almost impossible to ensure that that all the planets mesh equally with both sun and annulus gear at all times. 
     The planetary gear parts and the parts, to which they are attached, would have to be extremely rigid and they would have to be manufactured with a very high degree of accuracy to ensure that e.g. all twelve planet gears mesh perfectly with both sun gear and annulus gear at all times. Such a system would be both heavy and expensive. 
     By mounting the planet gear shafts e.g. by means of flexible bearings, these bearings could compensate for any inherited alignment or angle inaccuracies or any distortion of the part during transferring of great torques. This will severely reduce both the weight and the manufacturing costs of the gear parts and the related rotor parts. 
     Furthermore, a flexible suspension of the planets e.g. by mounting the planet gear shafts in flexible bearings will reduce the transferring of gear vibrations to other parts of the rotor, hereby reducing the noise emission from the wind turbine. 
     In an aspect of the invention, at least one of said plurality of planet gear wheels comprises at least two different gear stages for altering the gear ration of said gearing means of a planetary type. 
     A planetary gear with a large sun gear and a large annulus gear diameter do only have a gear ratio just above 1:2. To enhance the gear ratio and still maintain the benefit of the planetary gear, planet gears with at least two gear stages for different gear diameters can advantageously be used. The planet gear stage with the larger diameter can e.g. mesh with the sun gear and the planet gear stage with the smaller diameter can e.g. mesh with the annulus gear. Hereby, the gear ratio of the planetary gear can be altered and increased to e.g. 1:4 or 1:5. This is advantageous, in that the motor torque needed to control the blade can be reduced accordingly, hereby reducing the size, weight and cost of the ring shaped motor. 
     In an aspect of the invention, a rotor part of said ring shaped motor comprises a sun gear of said gearing means. 
     Hereby is achieved an advantageous embodiment of the invention. 
     In an aspect of the invention, said sun gear of said gearing means are formed integrally in said rotor part of said ring shaped motor. 
     Ring shaped motors for this purpose will most likely have to be made specifically for the given task. Providing the rotor part of the ring shaped motor with a toothed rim is therefore advantageous, in that weight, assembly time and manufacturing time can be reduced. 
     In an aspect of the invention, said ring shaped motor comprise at least one motor bearing substantially fixating the gab between a rotor part and a stator part of said ring shaped motor. 
     The forces between the rotor part and stator part of the ring shaped motor are large and the gab between the parts is small (typically 0.5 mm). If the rotor part is not governed precisely, the rotor can easily collide with the stator part. The larger the diameter is, the more likely this is to happen. The hub also distorts under loads to a magnitude, where it is unsound to have a stiff rotor part spanning over the diameter of the hub. It is therefore advantageous to make the ring shaped motor comprise at least one motor bearing for ensuring the gab between the rotor and the stator. 
     In an aspect of the invention, said gearing means of a planetary type reduces the rotation speed of said blade in relation to a rotor part of said ring shaped motor. 
     By reducing the speed of the blade pitch, the blade becomes easier to control, and when making the gear reduce the rotational speed it also increases the torque accordingly, hereby amplifying this advantage. 
     The invention further relates to a rotation controlling mechanism comprising, at least one ring shaped motor for controlling the rotation of at least one first part in relation to at least one second part. The rotation controlling mechanism is characterized in that, the ring shaped motor controls the relative rotation through gearing means of a planetary type. 
     Using the combination of a ring shaped motor (commonly known as a direct drive or torque motor) in combination with at planetary gear for controlling the relative rotation between two parts is advantageous, in that it provides for at compact, light and relatively inexpensive way of controlling such a rotation. 
     Even further the invention relates to a method for controlling at least one blade of a wind turbine rotor, said method comprising the steps of
         establishing a torque by means of a ring shaped motor   increasing said torque by means of gearing means of a planetary type   making said increased torque control said blade.       

     By increasing the torque provided by the motor through a planetary gear, it is possible to reduce the size, weight and cost of the ring shaped motor. 
     Furthermore, a planetary gear is a very compact, light and efficient gear type, which is advantageous to use in systems where these criteria&#39;s are of great importance, such as in pitch mechanisms for wind turbine blades. 
     In an aspect of the invention, said ring shaped motor controls said blades pitch angle in relation to a hub of said rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in the following with reference to the figures in which 
         FIG. 1  illustrates a large modern wind turbine known in the art, as seen from the front, 
         FIG. 2  illustrates a cross section of a wind turbine blade connected to a hub through an embodiment of a pitch bearing known in the art, as seen from the side, 
         FIG. 3  illustrates a cross-section of a ring shaped motor controlling the pitch of a blade through a planetary gear, as seen from the side, 
         FIG. 4  illustrates a cross-section of a ring shaped motor controlling the pitch of a blade through a planetary gear, as seen from the top, 
         FIG. 5  illustrates a planetary gear comprising two-stage planets, as seen from the side, 
         FIG. 6  illustrates a three-ring pitch bearing, a ring shaped motor and a planetary gear, as seen from the side, 
         FIG. 7  illustrates a three-ring pitch bearing, a ring shaped motor and a planetary gear comprising two stage planets, as seen from the side, and 
         FIG. 8  illustrates an embodiment of how the planet gears could be mounted, as seen from the side. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a modern wind turbine  1 , comprising a tower  2  and a wind turbine nacelle  3  positioned on top of the tower  2 . The wind turbine rotor  4 , comprising three wind turbine blades  5 , is connected to the nacelle  3  through the low speed shaft  6  which extends out of the nacelle  3  front. 
       FIG. 2  illustrates a cross section of a wind turbine blade  5  connected to a hub  6  through an embodiment of a pitch bearing  7 . In this embodiment the pitch bearing  8  comprise an inner ring  8  connected directly to the root of the blade  5  and an outer ring  9  connected directly to the hub  6 . The pitch bearing  7  is in this case a single row ball bearing, but it could also be a double, triple or four rowed bearing, and the elements enabling free rotation relatively between the rings  8 ,  9  are in this embodiment balls, but it could also be rollers, needles or other. 
       FIG. 3  illustrates a cross-section of a ring shaped motor controlling the pitch of a blade through a planetary gear, as seen from the side. 
     In this embodiment of the invention a stator part  11  of a ring shaped motor  10  is connected to the hub  6 . A rotor part  12  is positioned inside the stator part  11 , and an upper part of the rotor part  12  is provided with teeth  13  meshing with a number of planet gears  14 , making the rotor part  12  act as a sun gear  16  in a planetary gear  17 . The opposite sides of the planet gears  14  mesh with teeth  15  formed integrally in the upper rim of the inside of the blade hole in the hub  6 , making the hub  6  act like an annulus gear  22  of a planetary gear  17 . The planet gears  14  are—by means of planet gear shafts  18 —indirectly and rigidly connected to the blade  5  through the inner ring  8  of the pitch bearing  7 , making the blade  5  act as a planet carrier  19  of a planetary gear  17 . 
     In this embodiment of the invention the ring shaped motor  10  is what normally is called a Direct Drive DC torque motor. A direct drive motor is a servo actuator, which is made to be directly attached to the load it drives. It has a permanent magnet field and a wound armature, which act together to convert electrical power to torque. This torque can then be utilized in positioning or speed control systems. In general, torque motors are designed for high torque at low speeds systems. Direct drive torque motors are particularly suited for servo system applications where it is desirable to minimize size, weight, power and response time, and to maximise rate and position accuracies. 
     Various principles of electrical motors can be used. Direct current, eddy current, synchronous, asynchronous or step motors are all options. Direct current motors are not particular suitable, as the brushes do not work well in small reversing movements. Eddy current motors are not particular suitable for low speed and high torque. Asynchronous motors are not effective at low speed. Remaining is synchronous or step motors. Both motor have the characteristics, that the rotor rigid follows the motion of the electrical field in the stator. If the field is reversed, the rotor moves one pole displacement. The rotor will typically have permanent magnets  20 , but can also be with electromagnets. Electromagnets will have to be powered and when placed on the rotor part  12  of the ring shaped motor  10 , the electrical connection will have to follow the motion of the rotor  12 . For the pitch bearing motor  10 , where the rotation is limited to 90 degrees, the electrical connection can be made by flexible cables. Permanent magnets  20  are expensive and difficulty to handle. For these reasons electrical magnets are of interest. Almost all ring shaped motors  10  on the market have permanent magnets  20  in order to allow the rotor  12  to rotate freely. Wound rotors  12  are as such not common. The fact that the motor  10  must be able to pitch without grid power, makes the electric connection to the rotor  12  a risk for failure and the industrial standard is already with permanent magnets  20 , which makes the permanent magnet  20  on the rotor part  12  a preferred choice. 
     For the wound armature  21  of the stator part  11  and/or if the rotor part is provided with electromagnets cooling might be considered. This can e.g. be done by air cooling, cooling pipes circulating some sort of coolant, cooling fins or other. 
     When a motor is geared, the torque goes down with the gear ratio and the speed goes up accordingly. In this embodiment of the invention a planetary gear  17  is therefore implemented between the ring shaped motor  10  and the blade  5 . 
     The ring shaped motor  10  does not necessarily in itself secure, that the blade  5  stays in the achieved position. It could therefore be advantageous to combine the pitch motor  10  with a brake unit (not shown). 
     Say the brake in this brake unit is a friction brake (not shown), which has spring loaded brake force and electrical relieved brake force, the pitch bearing  7  can be held in desired position with or without the system is energized (fail safe system). This brake system will see large number of load cycles and reversed load directions, and must therefore be designed to endure this, e.g. by making the brake pad play free with the hub  5  or other. 
     Furthermore, the pitching system can be provided with mechanical pitch locks in case of malfunctioning brakes and for parking locking. 
     To know, what the tip angle is at all time, a sensor like an encoder or electrical lineal can be implemented in the system. The feed back from the sensor tells a computer where the blade  5  is and the information can be used to control the force and the motion of the electrical field in the pitch motors  10 . 
     A pitch movement sequenced can comprise one or more of the following steps:
         Turbine controller sense power increase above set level.   The ring shaped motor  10  is told to pitch to higher tip angle (towards 90 deg.)   The rotor magnets (if electrical) and the brake are powered. Brake force is relieved.   The stator  11  is applied a forward moving electro magnetically field in the wound armature  21 .   The magnetic flux between the magnets  20  in the rotor  12  and the stator  11  creates a tangential force between the parts  11 ,  12 , which pulls the rotor  12  to rotate (pitch torque).   The phase angle between rotor  12  and stator  11  poles is measured and controls the current regulator to set the right strength of the magnet flux.   The speed of the moving field is depending on the deviation from the actual tip angle (rotor position) and the desired position.   When the desired position has been achieved and no new position is expected, the system power down and the brake spring hold the blade  5  in position. When actively pitching, where there is hardly any rest, the rotor magnets (if electrical and the brake is powered continuously.   Reversing pitch directions are made by reversing the motion direction of the electromagnetic field in the stator  11 .       

     This system is free from backlash problems due to play in mechanical transmissions and that the forces are transmitted directly to the blade with the reaction acting on the rim of the hub. 
     The forces between the rotor part  12  and stator part  11  of the ring shaped motor  10  are large and the gab  23  between the parts  11 ,  12  is small (typically 0.5 mm). If the rotor part  12  is not governed precisely, the rotor  12  can easily collide with the stator part  11 . The larger diameter the more likely this is to happen. The hub  6  also distorts under loads to a magnitude, where it is unsound to have a stiff rotor part  12  spanning over the diameter of the hub  6 . As so the best motor is a thin section motor  10  with its own bearings  24  between stator  11  and rotor  12 , but using the already existing pitch bearing  7  to maintain the gab  23  between the rotor and stator part  11 ,  12  of the ring shaped motor  10  is also within the scope of the invention. 
     The weight of the pitch system with ring shaped motor  10  in the hub  6  is approximately 300 kg pr. Blade  5  plus 100 kg for the motor  10  controllers. Say the pitch system in the hub  6  has a weight of 1.0 ton. On top of this a rotating transformer has to be applied in the nacelle  3 . 
     The weight of a hydraulic pitch system known in the art is 2 tons, of which halve is from components located in the hub  6 . 
     An electric pitch system for a traditional three blade  5  wind turbine  1  requires the following components: 
     Three ring shaped pitch motors  10   
     Three planetary gear units  17   
     Three brake calibers/brake discs 
     Three motor  10  control units 
     Three battery back up units 
     One rotating electrical power connector 
     One rotating communication connector 
     One hub  6  controller cabinet 
     One set of cables 
     Three position encoders 
     Pitch motor  10  requirements:
         The ring shaped motor  10  is placed in the hub  6  under or in close proximity of the pitch bearing  7 .   The ring shaped motor  10  acts on the blade  5  through a planetary gear  17  with a reduction of between 1:1 and 1:50, preferably between 1:1.5 and 1:20 and most preferred between 1:2 and 1:10, such as 1:4 or 1:5   The maximum blade  5  speed is 12 deg/sec   The maximum blade  5  torque on a traditional 2-3 MW three blade wind turbine is in the range of between 60-100 kNm       

       FIG. 4  illustrates a cross-section of a ring shaped motor  10  controlling the pitch of a blade  5  through a planetary gear  17 , as seen from the top. 
     In this embodiment of the invention a toothed  13  rotor part  12  of a ring shaped motor  10  functions as a sun gear  16  in the planetary gear  17 . The teeth of the rotor part  12  mesh with fifteen planet gears  15  distributed evenly around the sun gear  16 . In another embodiment the planetary gear  17  could be provided with between 2 and 50, preferably between 3 and 30 and most preferred between 4 and 25 planet gears  15 , such as 8, 12, 16 or 24. 
     Through their planet gear shafts  18 , the planet gears  15  are directly mounted on the blade  5  (not shown in  FIG. 4 ) and the planets  14  further mesh with a toothed part  15  of the hub  6 . In another embodiment the annulus gear  22  could also be an independent internally toothed gear ring rigidly attached to the hub  6 , and likewise, the sun gear  16  could be an independent externally toothed gear ring rigidly attached to the rotor part  12  of the ring shaped motor  10 . 
     In a preferred embodiment of the invention the ring shaped motor  10  is placed concentric with the pitch bearing  7  and the root of the blade  5 , but in another embodiment of the invention the ring shaped motor  10  could be positioned eccentric to either the pitch bearing  7 , the root of the blade  5  or both, e.g. to efficiently use the limited space in the hub  6 . 
     The described system could also be used in other connections where rotating of large diameter loads is needed. This could e.g. be as a yaw mechanism in a wind turbine  1 , the rotating mechanism for a construction crane, the pitch mechanism for the blades of a ships propeller or other places where relatively high torque has to be transferred over a relatively large diameter. 
       FIG. 5  illustrates a planetary gear  17  comprising two-stage planets  14 , as seen from the side. 
     A planetary gear  17  with large sun gear  16  and annulus gear  22  diameter do only have a reduction ratio just above 2. To enhance the gear ratio and still maintain the benefit of the planetary gear  17 , planet gears  14  with two different gear diameter D 1 , D 2  can be used. The larger diameter D 1  of the planet gears  14  mesh with the sun gear  16  and the smaller diameter D 2  of the planets  14  mesh with the annulus gear  22 . 
     It can be advantageous to make the ring shaped motor  10  at the largest diameter possible. The motor torque is in square of the diameter and proportional with the length. Price is proportional to length and diameter. The large diameter is the preferred feature of this type of motor. 
     Best is a ring shaped motor  10  with a diameter substantially equal to the inner rim of the hub  6  or the pitch bearing  7 . This motor  10  must be geared to reduce the torque and hereof the cross section of the motor  10 . A planetary gear  17  with large annulus  22  and sun gear  16  diameter can have many planets  14 . Say the planets  14  are small and many e.g. 12 planets or more, the load on each is small and the gear module of the sun  16 , annulus  22  and planet gears  14  can be small. The classical planetary gear  17  with small planets  14  has a gear ratio just over 2. To enhance the gear ratio and still maintain the benefit of the planetary gear  17 , planet gears  14  with two different gear diameter D 1 , D 2  incorporated in the planets  14 , can stages the gear ratio of the planetary gear  17  to four or more. 
     This design reduces the motor torque to 20% or 25% of the blade torque. This does also reduce the cost of the ring shaped motor  10  with a factor possibly in the magnitude of three to four. The cost of the planetary gear  17  is marginal in relation to the cost of the ring shaped motor, which is one of the parameters that justifies the use of a planetary gear in combination with the ring shaped motor for pitching wind turbine blades. Furthermore it is possible to use an open gear solution. 
       FIG. 6  illustrates a part of a cross section of a pitch bearing  7  comprising two columns of each two rows of bearing balls. Between the outer bearing ring  9  and the centre bearing ring  25  is positioned two rows of balls on the same diameter. Between the centre bearing ring  25  and the inner bearing ring  8  is positioned two other rows of bearing balls on another common diameter. The inner and outer bearing rings  8 ,  9  are rigidly connected to the hub  6 , and the centre ring  25  is rigidly connected to the root of the blade  5 . 
     In this embodiment of the invention the planet gears  14  are not connected to the pitch bearing  7 , but are instead via a blade flange  26  connected indirectly to the blade  5 . An internally toothed ring gear—acting as annulus gear  22  in the planetary gear  17 —is connected to the inside surface of the inner bearing ring  8 . 
     In another embodiment of the invention the planets  14  could—by means of their planet gear shafts  18 —be connected directly to the blade  5 , or the planets  14  could be rigidly connected to the blade  5  in a more indirect way e.g. as shown in  FIG. 6  or through a plate connected to the blade  5  and covering the entire hole in the root of the blade  5  (which in this case would acts as the planet carrier  19  of the planetary gear  17 ), through fixtures or fittings connected to the blade  5  or the bearing ring or rings  8 ,  9 ,  25  of the pitch bearing  7  which is/are connected to the blade  5 . Hence, the planets  14  can be connected to the blade in many ways e.g. directly, where the blade serve as the planet carrier in the planetary gear  17 , or more indirectly where something else in between the planets  14  and the blade acts as planet carrier  19  or the connection can be done in another way as long as it ensures a substantially rigid connection between the planet gears  14  and the blade  5 . 
       FIG. 7  illustrates a three-ring pitch bearing  7 , a ring shaped motor  10  and a planetary gear  17  comprising two stage planets  14 , as seen from the side. 
       FIG. 8  illustrates an embodiment of how the planet gears  14  could be mounted, as seen from the side. 
     In this embodiment of the invention the planet gears  14  are mounted by means of flexible bearings  27 . In this embodiment these flexible bearings  27  does not enable rotation of the shaft  18  but does only serve at flexible means for allowing a small displacement of the planet gears  14  angle and/or rotation axis. The rotation of the planet gears  14  is in this embodiment enabled by a planet bearing  28  placed between the planet gears  14  and the shaft  18 . These planet bearings  28  could in an preferred embodiment be plain bearings but in another embodiment they could also be ball bearings, needle bearing, roller bearings or any combination hereof. 
     In another embodiment of the invention the flexible bearings  27  suspending the planet shafts  18  could be combined with spherical bearings in the planet gear  14  for ensuring that the planets  14  at all times are placed correctly and in the right angle. The inaccuracy compensation could also be done by mounting the shafts  18  by means of spherical bearings, by incorporating a flexible bearing  27  in the planets  14  or both. 
     Providing the planet gears with a small degree of flexibility can also be done in a number of other ways within the scope of the invention. The planet gears  14 , the annulus gear  22  and sun gear  16 , the planet carrier  19  and/or the planet gear shafts  18  could be made in a slightly flexible material, the sun gear  16  and/or and or the annulus gear  22  could be divided into an number of toothed segments, which all where individually and flexibly mounted or the system could in another way be provided with means compensating for any inaccuracy of the parts  5 ,  6 ,  13 ,  14 ,  15 ,  16 ,  18 ,  19 ,  22 , any inaccuracy between the parts  5 ,  6 ,  13 ,  14 ,  15 ,  16 ,  18 ,  19 ,  22  or any slight distortion of the parts  5 ,  6 ,  13 ,  14 ,  15 ,  16 ,  18 ,  19 ,  22 . 
     The invention has been exemplified above with reference to specific examples of ring shaped motors  10 , planetary gears  17  and use of such. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims. 
     LIST 
     
         
         
           
               1 . Wind turbine 
               2 . Tower 
               3 . Nacelle 
               4 . Rotor 
               5 . Blade 
               6 . Hub 
               7 . Pitch bearing 
               8 . Inner bearing ring 
               9 . Outer bearing ring 
               10 . Ring shaped motor 
               11 . Stator part of ring shaped motor 
               12 . Rotor part of ring shaped motor 
               13 . Rotor teeth 
               14 . Planet gear 
               15 . Hub teeth 
               16 . Sun gear 
               17 . Planetary gear 
               18 . Planet gear shaft 
               19 . Planet carrier 
               20 . Permanent magnet 
               21 . Wound armature 
               22 . Annulus gear 
               23 . Gab 
               24 . Motor bearing 
               25 . Centre bearing ring 
               26 . Blade flange 
               27 . Flexible bearing 
               28 . Planet bearing 
             D 1 . First planet gear diameter 
             D 2 . Second planet gear diameter