Abstract:
Drive device for a wind mill comprising a large pulley disposed on a main shaft and at least one belt or chain adapted to transfer rotation from the pulley to a generator. The pulley is rotationally coupled to at least two secondary shafts, which are disposed parallel to the main shaft. One or more belts, which transfer the rotation, extend over the pulley and the secondary shafts. The secondary shafts are in turn rotationally coupled to at least one, preferably two, electric generators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of pending U.S. patent application Ser. No. 13/146,330 filed Jul. 26, 2011 which is a U.S. National Phase Application of PCT/EP2010/050987 filed Jan. 28, 2010. International application PCT/EP2010/050987 designates the United States and claims priority to Norway Patent No. 20090433 filed Jan. 28, 2009. Each of these applications is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a drive device for a wind turbine according to the preamble of the following claim  1 . 
       BACKGROUND 
       [0003]    In most of today&#39;s wind turbines the drive unit is placed in a nacelle, which also carries the wind turbine rotor. The nacelle is necessarily arranged on top of a high mast and rotates so that the rotor is always facing the wind. 
         [0004]    Such placement of the drive unit results in a great weight at the top of the mast and that access for maintenance is a challenge. Yet it is this position of the drive unit and the generators that is used extensively as the alternative is that the rotational torque must be transmitted via a shaft through the mast, something that leads to losses and requires that the mast can absorb the reaction torque from the rotating shaft. 
         [0005]    There is therefore an urgent need to simplify the drive unit and make it lighter. There is also a need to make access for maintenance easier. These are the main aims of the present invention, and these are achieved by the features that appear in the characterizing part of claim  1 . 
         [0006]    To use a belt to transmit the rotation from the rotor to a generator is known from, among others, WO 2008/028335 and JP 2005023893. However, the present invention aims to utilise the characteristics of the belt operation, or possibly the chain operation, better so that a more compact drive device can be achieved with additional operational benefits. 
         [0007]    Some of the benefits that can be achieved by the invention in relation to the belt operation, according to the known solutions are: 
         [0008]    One gets a doubling of the utilization of the capacity of the belts with respect to the known belt operation. This relates, in particular, to large (and therefore costly) belts and associated large belt wheels. 
         [0009]    The structure becomes more compact. 
         [0010]    The torque over the drive shaft is balanced. 
         [0011]    The generation of power can be divided onto two generators that can be open to a more reasonable generator control system. 
         [0012]    The time before one needs to replace belts can be made longer because one can drive the torque on a single generator when there is little wind. 
         [0013]    In comparison to traditional drive systems with cogged wheel transfer, or direct operation, one can achieve the following benefits: 
         [0014]    Reduction of shock loads (large momentum changes, vibration, etc. are dampened). 
         [0015]    Makes complex lubrication and cooling systems unnecessary. 
         [0016]    The system will be less prone to corrosion and have lower maintenance needs. This is particularly relevant for offshore wind turbines. The belt wheels can, for example, be given a zinc coating to reduce corrosion, something that is not possible with cogged wheel transmissions. 
         [0017]    A weight saving is achieved by integrating the drive shaft in the belt wheel and also by taking up the torque over a large radius instead of a small radius inside a gearbox. 
         [0018]    The number of parts that must be manufactured in the production of the drive system is significantly reduced. 
         [0019]    A large part of the maintenance can be done without the use of a large crane. The belts are most susceptible to wear, but even the big belts do not weigh more than 70-100 kg. The belt wheel will not be subjected to wear as the belt is soft. 
         [0020]    Scaling up to, for example, 5, 7 and 10 MW, will be possible without the mass of the drive system increasing exponentially. The mass of a 5 MW turbine with gearbox or direct drive will quickly become very heavy. 
         [0021]    The power from the rotor can be distributed to several standard generators, something which results in increased flexibility. 
         [0022]    A larger exchange can be brought about in one step than with cogged wheel transmission. This is because a large exchange in cogged wheel transmissions leads to high pressure forces on the cogs, something which results in much wear. With a belt operation, there will be a softer transmission and some slip may be acceptable at abrupt torque changes. 
         [0023]    A belt drive system also has advantages compared with direct operation. With direct operation a relatively low rotational speed is transmitted to the generator. This means that the generator must be large and heavy. At higher rotational speeds one can use smaller and lighter generators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention shall now be explained in more detail with reference to the accompanying drawings, where: 
           [0025]      FIG. 1  shows a known nacelle for a wind turbine with a rotor hub, a drive unit and a generator. 
           [0026]      FIG. 2  shows a nacelle similar to that shown in  FIG. 1 , but with a drive device according to the invention. 
           [0027]      FIG. 3  shows the drive device according to the invention in perspective. 
           [0028]      FIG. 4  shows the drive device according to the invention in perspective from the opposite side and 
           [0029]      FIG. 5  shows the drive device according to the invention in a split drawing. 
           [0030]      FIG. 6  shows a principle diagram of a drive device with three secondary shafts; and 
           [0031]      FIG. 7  shows an embodiment with a common generator for two secondary shafts. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Firstly, the known nacelle according to  FIG. 1  shall be briefly explained. It comprises a rotor hub  1  to which the wind turbine blades (not shown) are attached. The hub  1  is mounted in a main bearing  2  and is connected to a main shaft  3 . The main shaft is connected to a main gear  4 . The gear  4  is fitted with a brake  5 . The gear is connected to a generator  7  via a connection  6 . The nacelle is also fitted with a swivel bearing  8 , swivel gear  9  and swivel ring  10  for rotation of the nacelle in relation to a tower  11 , on which the nacelle is placed. 
         [0033]    The present invention aims to replace the following components in the known nacelle above: the main shaft  3 , the main gear  4 , the brake  5  and the connection  6 . 
         [0034]      FIG. 2  shows the nacelle in  FIG. 1  with the drive device according to the invention placed at an intended location and with the known nacelle as a background. 
         [0035]    However, before  FIG. 2  is explained, the drive device according to the invention shall be explained with reference to  FIGS. 3-5 , which show the drive device according to the invention separate from the nacelle  50 . 
         [0036]    The drive device is mounted in a frame  12 , which has a first opening  13  at the one end that faces away from the hub  1  and a second opening  14  at its other end facing towards the hub  1 . The opening  14  is circular and is set up to receive a rotation bearing  15 . The frame also includes a load-carrying wall  16  for a main shaft bearing  17 . The load-carrying wall  16  is equipped with openings  18  and  19 . The frame  12  is designed at the bottom to take up a swivel bearing for is rotation of the nacelle in relation to the tower. The frame also has an opening  20  in this area. 
         [0037]    A large belt disc wheel  21  is mounted between the swivel bearing  15  and main shaft bearing  17  with the help of a main shaft  22 . Two secondary shafts  23  and  24  are mounted in parallel with the large belt disc  21  and the main shaft  22 . Each of these is fitted with a small belt disc  25 ,  26  at the ends facing away from the hub  1 . A set of belts  27  extends around the large belt disc  21  and the secondary shafts  23 ,  24  to transmit the rotation torque from the large belt disc  21  to the secondary shafts  23 ,  24 . The secondary shafts  23 ,  24  are mounted between brackets  28 ,  29  at the side of the opening  14  and the load-bearing wall  16 . Two generators  30 ,  31  are attached to the frame  12  and have generator shafts  32 ,  33  that extend through the load-bearing wall  16 . These generator shafts  32 ,  33  are in rotational connection with each of the small belt discs  25 ,  26  via sets of belts  34 ,  35 . 
         [0038]    The large belt disc  21  is provided with spokes  36  so that four openings  37  are formed through the belt disc  21  itself. 
         [0039]    The above mentioned results in a compact unit that can form a load-bearing part of the nacelle  50 .  FIG. 2  shows how this unit will be placed in the nacelle  50 . The hub will be connected directly to the large belt disc  21  by means of a number of bolts that are screwed into the bolt holes  38  in the belt disc  21 . 
         [0040]    Thus the rotation of the hub will lead to a rotation of the large belt disc  21 . This will in turn be transmitted to the secondary shafts  23 ,  24  via the belts  27  and on to the generators  30 ,  31  via the small belt discs  25 ,  26  and the belts  34 ,  35 . Since the large belt disc  21  has a significantly larger diameter than the secondary shafts  23 ,  24 , even a small rotational speed of the hub will cause a large rotational speed of the secondary shafts  23 ,  24 . A very large exchange in one step is thereby achieved. 
         [0041]    The parallel and diametrically opposite secondary shafts  23 ,  24  provide a good balance the large belt disc  21 . The tightening of the belt can be adjusted by is moving the secondary shafts sideways, as is suggested by the oblong holes  39  in the brackets  28 ,  29  and the load-bearing wall  16 . The tightening of the belts  34 ,  35  can be carried out in a similar way, by displacing the generators  30 ,  31  sideways, as is indicated by the oblong holes  40  in the load-bearing wall  16 . 
         [0042]    If the wind is weaker than that required to provide half the power production, one of the generators can be disconnected. The possibility to disconnect half of the generator power means that one can achieve a wider regulating range for the wind turbine where there is little wind, than with the help of a single generator and conventional double-fed generator control. 
         [0043]    Although two secondary shafts and two generators are described in the above, there is nothing in the way of using several secondary shafts and generators. A principle diagram of a system with three secondary shafts  23   a ,  23   b ,  23   c , which by way of the one set of belts  27  is in rotational connection with a large belt disc  21 , is shown in  FIG. 6 . Instead of belts  27 ,  34 ,  35 , one or more chains that engage with the cogs of the large disc and the secondary shaft can also be used. 
         [0044]    Instead of each of the secondary shafts being in connection with their own generator, these can be rotationally connected with a common generator. An example of this is shown in  FIG. 7 , where the small belt discs  25 ,  26  are connected via a belt disc set  34 ,  35  to a common generator shaft  32   a  that goes into a common generator  30   a . Today, this is the most relevant embodiment. 
         [0045]    The way the frame  12  is designed and the way the drive unit is positioned in the frame, offer good access for inspection and maintenance. Personnel can come up in the frame  12  via the opening  20 , which is in connection with an opening at the top of the tower. From here, the personnel can get to the back of the nacelle via the opening  13  and to the front via the opening  18  or  19 . Moreover, it is possible to crawl through the large belt disc  21  itself via one of the openings  37  and from here into the hub. 
         [0046]    The drive device according to the invention will also be equipped with a brake is which will ensure that nothing rotates while there are people in the nacelle. This brake will most appropriately be arranged at the two small belt discs or on the generators and be active until the maintenance personnel have manually locked the large belt disc and the hub.