Abstract:
A deflection resistant wind turbine generator having a stator arranged about an axis and a rotor operably mounted with respect to the stator to generate electricity. The rotor is rotatably communicating with wind turbine blades rotating substantially about the axis and the rotor and the stator are configured to maintain an airgap therebetween. The stator and the rotor have selectively engageable surfaces that maintain a substantially stable airgap and permit rotation of the rotor during engagement. The engageable surfaces engage when the rotor deflects to a predetermined amount of deflection.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to electric generators, and more particularly, to methods and systems for controlling an airgap between a rotor and a stator in a wind-powered turbine generator. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recently, wind turbines have received increased attention as an environmentally safe and a relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient. 
         [0003]    Generally, a wind turbine includes a plurality of blades coupled to a hub forming a turbine rotor. Utility grade wind turbines (i.e. wind turbines designed to provide electrical power to a utility grid) can have large turbine rotors (e.g., seventy or more meters in diameter). Blades on these turbine rotors transform wind energy into a rotational torque or force that drives a rotor of one or more generators. The turbine rotor is supported by the tower through a set of internal bearings that include a fixed portion coupled to a rotatable portion. The set of internal bearings is subject to a plurality of loads including the weight of the turbine rotor, a moment load of the turbine rotor that is cantilevered from the set of internal bearings, symmetric and asymmetric wind loads loads, such as, horizontal and shears, yaw misalignment, and natural turbulence. 
         [0004]    In a direct drive wind turbine generator, the generator rotor is directly coupled to the turbine rotor. The generator rotor and stator are separated by an airgap. During operation, a magnetic field generated by permanent magnets or an excited wound field mounted on the generator rotor passes through the airgap between the rotor and the stator. The passage of the magnetic field through the airgap is at least partially dependent on the uniformity of the airgap. Asymmetric and/or transient loads on the generator may be introduced through the turbine rotor from the blades. Such loads are transmitted from the turbine rotor to the wind turbine base through the set of internal bearings and may tend to deflect structural components of the generator rotor and stator in the load path such that the airgap distance is reduced and/or made non-uniform. One proposed solution includes fabricating wind turbine components from stiffer and/or stronger materials capable of withstanding the loads on the rotor. However, the size and/or weight drawbacks of stiffer and/or stronger materials and/or components make their use prohibitive. Additionally, the substantial structure needed to control the airgap would use up valuable hub-access space needed to install and service systems such as pitch-control and other devices. 
         [0005]    Thus, what is needed is a method and system to provide a wind turbine generator having an arrangement of a rotor and a stator that provides airgap stability. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the disclosure includes a wind turbine generator having a stator arranged about an axis and a rotor operably mounted with respect to the stator to generate electricity. The rotor is rotatably communicating with wind turbine blades rotating substantially about the axis and the rotor and the stator are configured to maintain an airgap therebetween. The stator and rotor have selectively engageable surfaces that maintain a substantially stable airgap and permit rotation of the rotor during engagement. The engageable surfaces engage when the rotor deflects to a predetermined amount of deflection. 
         [0007]    Another aspect of the disclosure includes a method for maintaining a stable airgap in a wind turbine generator. The method includes providing a wind turbine generator having a stator arranged about an axis and a rotor operably mounted with respect to the stator to generate electricity. The rotor is rotatably communicating with wind turbine blades rotating substantially about the axis. The rotor and the stator are configured to maintain an airgap therebetween. The method further includes engaging the engageable surfaces to maintain a substantially stable airgap and permit rotation of the rotor when the rotor deflects to a predetermined amount of deflection. 
         [0008]    One advantage of the present deflection resistant system and method is that the components do not significantly add to the weight or space within the wind turbine and provide deflection resistance without loss of operational or power capacity. 
         [0009]    Another advantage is that the components do not increase rotational resistance when little or no deflection of the rotor is present. 
         [0010]    Another advantage is that the stator mechanical stiffness may be improved without increasing sectional moments of inertia. This improved stiffness reduces the relative deflections between the rotor and the stator induced by bending loads. 
         [0011]    Another advantage is that the structural mass may be significantly reduced leading to a cost reduction in the generator frame. 
         [0012]    Another advantage includes electromagnet airgap variation reduction by selectively coupling the stator and the rotor frames so that the stator and the rotor frames deflect at the same time as opposed to independently. This advantage exists because gravity loads may dominate the deflections on the stator whereas wind loads that oppose gravity loads may dominate the deflection on the rotor. This concurrent deflection ensures that the rotor and the stator respond to the wind load such that the generator (including the rotor and the stator frames) bulk deformation is in the same direction. 
         [0013]    Another advantage is that the disclosure beneficially provides an additional control variable that can be used to shift the natural frequencies of the generator frames by adjusting stiffness. 
         [0014]    Another advantage is that the selective coupling serves as an alternate load path between the rotor and the stator. 
         [0015]    Another advantage is that the system of the present disclosure may be retrofitted to direct drive wind turbines in the field. 
         [0016]    Another advantage is that the system of the present disclosure permits the use of lighter generators. Among other benefits, lighter generators permit use of smaller capacity cranes to be used during wind turbine assembly in the field. 
         [0017]    Further aspects of the method and system are disclosed herein. The features as discussed above, as well as other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a side view of a wind turbine according to an embodiment of the present disclosure. 
           [0019]      FIG. 2  shows a schematic view according to an embodiment of the present disclosure. 
           [0020]      FIG. 3  shows an enlarged view of a portion of a wind turbine generator according to an embodiment of the present disclosure. 
           [0021]      FIG. 4  shows an enlarged view of a portion of the wind turbine generator according to another embodiment of the present disclosure. 
           [0022]      FIG. 5  shows a cutaway perspective view of a direct drive generator according to another embodiment of the present disclosure. 
           [0023]      FIG. 6  shows an enlarged view of a portion of a wind turbine generator according to an embodiment of the present disclosure. 
       
    
    
       [0024]    Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. 
         [0026]    As shown in  FIG. 1 , a wind turbine  100  generally comprises a nacelle  102  housing a generator (not shown in  FIG. 1 ). The nacelle  102  is a housing mounted atop a tower  104 , only a portion of which is shown in  FIG. 1 . The height of the tower  104  is selected based upon factors and conditions known in the art, and may extend to heights up to 60 meters or more. The wind turbine  100  may be installed on any terrain providing access to areas having desirable wind conditions. The terrain may vary greatly and may include, but is not limited to, mountainous terrain or off-shore locations. The wind turbine  100  also comprises one or more rotor blades  108  attached to a rotating hub  110 . Although the wind turbine  100  illustrated in  FIG. 1  includes three of the one or more rotor blades  108 , there are no specific limits on the number of the one or more rotor blades  108  required by the present invention. 
         [0027]      FIG. 2  shows a direct drive generator  200  according to an embodiment of the present disclosure. The direct drive generator  200  includes a rotor  201  mounted to a set of bearings  204  wherein the set of bearings  204  are attached to a shaft  202 . The rotor  201  is also in rotational communication to the rotating hub  110 , about which the one or more rotor blades  108  rotate. The rotor  201  is further disposed within a stator  203  with an airgap  207  disposed therebetween. The rotor  201  and the stator  203  include a permanent magnet, electromagnetic windings, combinations thereof, or other magnetic devices arranged to provide a magnetic flux across the airgap  207  sufficient to generate electricity. For example, in one method of operation, a magnetic field generated by permanent magnets and/or electromagnets mounted on the rotor  201  passes through the airgap  207  between the rotor  201  to the stator  203 . The present disclosure may also include other arrangements of the rotor  201  and the stator  203  and include one or more of the airgaps  207  for inductive generation of electricity. 
         [0028]    The passage of the magnetic field through the airgap  207  requires at least some uniformity of the airgap  207 . Excessive closure of the airgap  207  and/or non-uniform distances across the airgap  207  decrease power production and may result in operational problems. Furthermore, complete closure of the airgap  207  whereby the rotor physically contacts the stator while rotating can cause significant physical damage and potential catastrophic failure. As shown in  FIG. 2 , the rotor  201  rotates about a center axis  209 . The set of bearings  204  provide support and facilitate rotation of the rotor  201  about the center axis  209 . During operation of the wind turbine  100 , increases in wind speed (e.g., wind gusts) creating non-uniform forces on the one or more rotor blades  108  and/or the rotating hub  110  may result in deflection of the rotor  201  from the center axis  209 , causing non-uniformity in the airgap  207 . In addition, other causes, such as gravity sag and electromagnetic (EM) attraction may also contribute to non-uniformity in the airgap  207 . Other sources of deflection are ambient and operational temperature swings. These swings can also impact the airgap  207 . In addition, an excitation frequency, which is near the natural frequency of the wind turbine  100 , may impact the airgap  207 . “Deflection”, “deflecting”, and grammatical variations thereof, as used herein include translational or rotational motion of structural components due to loading applied to the wind turbine  100 , particularly variations and deviations from the center axis  209 . These sources of deflection can also cause deflection of the stator relative to the center axis  209 . Acceptable deflection tolerance of the airgap  207  for example, may include a closing of less than 20%. 
         [0029]    The direct drive generator  200  further includes a stator contact surface  211  and a rotor contact surface  213  arranged at an end of a contact arm  215 . The stator contact surface  211  and the rotor contact surface  213  are arranged such that the rotor  201  is permitted to rotate within the stator  203 . In one embodiment, the stator contact surface  211  and the rotor contact surface  213  are selectively engageable. That is, the stator contact surface  211  and the rotor contact surface  213  engage or are otherwise in contact during deflection of the rotor  201 . The engagement of the stator contact surface  211  and the rotor contact surface  213  is preferably a low friction contact to permit continued rotation of the rotor  201 , such as by sliding contact or a rolling contact. The engageable surfaces may also be configured so that engagement always occurs, which may reduce dynamic shock in a engageable surface that only contacts only under certain loads. An engageable surface that is always preloaded (i.e., in constant contact) may also allow for easier control of the structural natural frequencies by adjusting stiffness to prevent vibration of the structure near an excitation frequency. 
         [0030]    As shown in  FIG. 3 , the stator contact surface  211  and the rotor contact surface  213  are disposed at an end of the contact arm  215 , which is affixed to and/or is a portion of the stator  203 . The contact arm  215  is preferably arranged and disposed with sufficient strength to withstand forces associated with deflection of the rotor  201 . The contact arm  215  sufficiently reacts to the force provided by the rotor  201  to maintain substantial uniformity of the airgap  207 . In addition, the contact arm  215  may be configured to permit deflection or elastic deformation in order to react to force provided by deflecting the rotors  201 . The stator contact surface  211  and rotor contact surface  213  are preferably configured to provide an alternate load path for the rotor  201  to transfer the load to the stator  203 . In addition, the stator contact surface  211  and rotor contact surface  213  interface provides a means to better couple the rotor/stator deflections such that they move in the same general direction. The stator contact surface  211  includes a set of internal bearings  301  attached to a rod or a post fixed to the stator  203 . Optionally, a channel  303  for the airgap  207  can be configured to engage a contact roller or the low friction bar  401  (not shown in  FIG. 3 ) under sufficiently high loads. The contact rollers are comprised of the set of wheels or rollers  301  having internal bearings wherein the internal bearings are comprised of individual bearings any bearing type including, roller bearings, ball bearings or any other bearing structure that permits rolling contact. In one embodiment the set of wheels or rollers  301  having internal bearings are disposed circumferentially about the stator  203  to provide support along the periphery of the rotor  201  during deflection. While the embodiment shown in  FIG. 3  includes the set of wheels or rollers  301  having internal bearings affixed to the stator  203  and the channel  303  on the rotor  201 , the set of wheels or rollers  301  having internal bearings may be positioned on the rotor  201  and the channel  303  may be positioned on the stator  203 . 
         [0031]    As shown in  FIG. 4 , the stator contact surface  211  and the rotor contact surface  213  are disposed at an end of the contact arm  215 , which is affixed to and/or is a portion of the stator  203 , substantially as shown in  FIG. 3 . The stator contact surface  211  includes a low friction bar  401 . The rotor contact surface  213  includes the channel  303  configured to receive the low friction bar  401 . The low friction bar  401  includes a stator contact surface  211 , which is slidable and which provides sliding contact and engagement with the channel  303  upon the rotor  201  being deflected. The low friction bar  401  may be a low friction material or coating or may include lubricant or lubricant systems to maintain a low coefficient of friction. In one embodiment, the low friction bar  401  would be continuous around the circumference of the machine to maximize surface area and to minimize non-uniform wear patterns. In yet another embodiment, plurality of the low friction bars  401  are disposed circumferentially about the stator  203  to provide support along the periphery of the rotor  201  during deflection. While the embodiment shown in  FIG. 4  includes the low friction bar  401  affixed to the contact arm  215  of the stator  203  and the channel  303  on the rotor  201 , the low friction bar  401  may be positioned on the rotor  201  and the channel  303  may be positioned on the stator  203 .  FIG. 4  depicts a simplified embodiment not including the rod or post or internal bearing. 
         [0032]    While the above embodiments show the rotor contact surface  213  as including the channel  303 , the rotor contact surface is not so limited any may include planar surfaces or surfaces having geometries other than channel geometries. In addition, while the above has been shown with respect to the contact arm  215  and the stator contact surface  211  and the rotor contact surface  213  at an end of the rotor  201  and the stator  203 , the contact arm  215  and the stator contact surface  211  and the rotor contact surface  213  may be positioned in any location that is capable of receiving the rotor  201 , as deflected, maintaining the airgap  207  as substantially uniform. The contact arm  215  can be in several configurations. While the above have been shown with respect to contact arms  215  being a portion of the stator  203 , the contact arms  215  may be extensions or protrusions from the stator  203  extending to a stator contact surface  211  for selective engagement with the stator contact surface  213 . 
         [0033]      FIG. 5  shows a cutaway perspective view of a direct drive generator  200  according to an embodiment of the disclosure. As shown in  FIG. 5 , the rotor  201  and the stator  203  can be oriented with the stator  203  being inside of the rotor  201 . The rotor contact surface  213  and the stator contact surface  211  can be on the opposite side of the rotor. The stator contact surface  211  includes a low friction bar. The rotor contact surface  213  includes the channel  303  configured to receive the low friction bar  401 . The low friction bar  401  includes a stator contact surface  211 , which is slidable and which provides sliding contact and engagement with the channel  303  upon the rotor  201  being deflected. The low friction bar  401  may be a low friction material or coating or may include lubricant or lubricant systems to maintain a low coefficient of friction. 
         [0034]      FIG. 6  shows an enlarged view of a portion of a wind turbine generator according to an embodiment of the present disclosure. As shown in  FIG. 6 , multiple airgaps  207  may be present wherein the stator contact surface  211  and the rotor contact surface  213  are disposed at an end of the contact arm  215 , which is affixed to and/or is a portion of the stator  203 . The stator contact surface  211  includes a set of wheels or rollers  301  having internal bearings attached to a rod or a post fixed to the stator  203 . The contact rollers  301  are comprised of the set of internal bearings wherein the internal bearings are comprised of individual bearings any bearing type including, roller bearings, ball bearings or any other bearing structure that permits rolling contact. The multiple airgaps  207  are oriented such that they are between the stators  203  and the rotors  201 . 
         [0035]    An exemplary embodiment of a wind turbine generator system is described above in detail. The generator components illustrated are not limited to the specific embodiments described herein, but rather, components of each may be utilized independently and separately from other components described herein. 
         [0036]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.