Patent Application
20130300124
The present disclosure generally relates to wind turbines and, more particularly, relates to electrical generators of direct drive wind turbines.
A direct drive wind turbine includes a set of two or three large rotor blades mounted to a hub. The rotor blades and the hub together are referred to as the rotor. The rotor blades aerodynamically interact with wind and create lift, which is then translated into a driving torque by the rotor. The rotor of the direct drive wind turbine is directly coupled to a generator, typically to a rotor of the generator, without any speed increasing gearbox therebetween. By virtue of connecting the rotor of the direct drive wind turbine directly to the generator, this topology eliminates the complexity and expense of the gearbox that is typically found in a conventional wind turbine.
Given that the direct drive generator is directly coupled to the rotor of the wind turbine, the generator turns at the same low speed as the rotor. By turning at the low rotor speeds of the wind turbine, the generator exacts a number of trade-offs. For instance, a direct drive generator is typically constructed so that it is very large in diameter. The large diameter in part results in the need to increase the surface speed of the rotor relative to the stator to minimize the amount of active material in the generator. However, with such a large generator, and with the rotor of the generator directly coupled to the rotor of the wind turbine, large deflections can occur between the rotor and the stator of the generator. Specifically, transient dynamic effects and large aerodynamic moments can cause deflections of the wind turbine's rotor. These deflections can be transferred to the generator because of its direct coupling with the wind turbine rotor, causing the generator rotor and stator to deflect relative to one another. Such generator deflections are particularly relevant to the large generators of a modern multi-megawatt direct drive wind turbine.
To counteract the deflections in a direct drive wind turbine generator, the air gap between the rotor and stator must be designed large enough so that the largest expected deflections do not result in contact between the rotor and stator during operation. Any contact between the rotor and the stator can cause catastrophic failure of generator components and possibly the wind turbine. Generally speaking, as the diameter and/or stack length of the generator increases, the propensity of the generator to encounter large deflections increases as well; and so the air gap between the rotor and the stator must be made larger to accommodate those deflections. Larger air gaps reduce the performance of the generator in a known manner. Alternatively, the air gap can be minimized by increasing the stiffness of the generator's internal structure and supporting structure to minimize the relative deflections between the rotor and stator. However, this increased stiffness is generally accomplished by increased size, weight, complexity, and cost of the structures, especially in a direct drive generator which generally has a large diameter.
Accordingly, it would be beneficial if an effective mechanism were developed that could account for the deflections within the rotor and the stator, while minimizing the required width of the air gap.
In accordance with at least some embodiments of the present disclosure, a wind turbine is disclosed. The wind turbine may include a wind turbine rotor comprising a hub and a plurality of blades radially extending from the hub. The wind turbine may further include an electric generator operatively driven by the wind turbine rotor. The electric generator may comprise a generator stator, a generator rotor spaced apart from the generator stator, and a main shaft defining an axis of rotation. The main shaft may be at least indirectly connected to the generator rotor for rotation. The electric generator may further comprise a bearing assembly supporting the main shaft and defining a center of deflection on the axis of rotation. The center of deflection may lie in a substantial center of a stack length of the generator. The electric generator may also comprise a profiled air gap defined between the generator stator and the generator rotor with reference to the center of deflection. The air gap may have a maximum width at axial ends of the stack length and a minimum width at the substantial center of the stack length. The maximum width of the air gap may correspond to regions of maximum deflection and the minimum width of the air gap may correspond to regions of minimum deflection of at least one of the generator stator and the generator rotor.
In accordance with some other aspects of the present disclosure, a wind turbine is disclosed. The wind turbine may include a wind turbine rotor directly driving a generator. The wind turbine rotor may comprise a hub and a plurality of blades radially extending from the hub. The generator may comprise a generator stator, a generator rotor spaced apart from the generator stator, and a main shaft defining an axis of rotation. The main shaft may be at least indirectly connected to the generator rotor for rotation. The generator may further comprise a bearing assembly supporting the main shaft, configured to resist deflections of the main shaft, and defining a center of deflection on the axis of rotation, the center of deflection lying substantially longitudinally in-line with the bearing assembly along a stack length of the generator. The generator may further comprise a profiled air gap defined between the generator stator and the generator rotor. The air gap may have a maximum width in regions farthest away from the center of deflection and a minimum width in regions substantially longitudinally in-line with the center of deflection.
In accordance with yet other aspects of the present disclosure, a direct drive fluid-flow turbine is disclosed. The direct drive fluid-flow wind turbine may include a wind turbine rotor having a plurality of rotor blades that interact with the fluid in motion to produce a torque about the wind turbine rotor, the wind turbine rotor supported for rotation on a bearing assembly that minimizes deflections of the wind turbine rotor. The direct drive fluid-flow wind turbine may also include a generator having a generator stator and a generator rotor, the generator rotor being driven by the wind turbine rotor to rotate at the same rotational speed therewith, the generator stator and the generator rotor having an air gap therebetween of sufficient width to avoid mutual contact due to deflections of the generator stator or the generator rotor relative to the other, the generator stator and the generator rotor being positioned radially around the bearing assembly and the air gap width being profiled by convexly curving one of the surface of the generator rotor or the surface of the generator stator, with a greater width of the air gap occurring at each axial end of the air gap and the minimum width of the air gap occurring approximately in the axial middle of the air gap.
For a more complete understanding of the disclosed methods and apparatuses, reference should be made to the embodiments illustrated in greater detail on the accompanying drawings, wherein:
While the following detailed description has been given and will be provided with respect to certain specific embodiments, it is to be understood that the scope of the disclosure should not be limited to such embodiments, but that the same are provided simply for enablement and best mode purposes. The breadth and spirit of the present disclosure is broader than the embodiments specifically disclosed and encompassed within the claims eventually appended hereto.
Referring to
In addition to the components of the wind turbine 2 described above, the up tower section 4 of the wind turbine may include several auxiliary components, such as, a yaw system on which the up tower section 4 may be positioned to pivot and orient the wind turbine in a direction of the wind current or another preferred wind direction, a pitch control system (not visible) situated within the hub 12 for controlling the pitch (e.g., angle of the blades with respect to the wind direction) of the blades 10, and the like. Several other auxiliary components, such as various cooling units, back-up power units, etc., that may be present within the wind turbine 2 are contemplated and considered within the scope of the present disclosure.
Turning now to
It will be understood although only one of the bearing assembly 24 has been shown in
The generator 14 may also include a stator assembly 32 connected at least indirectly to the bearing assembly 24 and to the base 31, and a rotor assembly 34 mounted at least indirectly to the main shaft 28, in general alignment and spaced away from the stator assembly. In at least some embodiments and, as shown, the rotor assembly 34 may surround the stator assembly 32 in an “inside-out” configuration, while in some other embodiments the rotor assembly may be mounted within the stator assembly for rotation. The stator assembly 32 may include a stator frame 36 having a stator rim 38 on which a plurality of stator laminations 39 and stator windings 40 may be mounted. The rotor assembly 34 on the other hand may include a rotor frame 41 having a plurality of magnets (e.g., permanent magnets) 42 mounted circumferentially thereon and facing the stator windings 40. In at least some embodiments, the diameter of the rotor assembly 34 may be at least twice that of the stack length 20.
An air gap 44 is defined between the stator assembly 32 and the rotor assembly 34. Specifically, the air gap 44 may be an annular air gap that may be defined between the magnets 42 of the rotor assembly 34 and the stator windings 40 of the stator assembly 32. The air gap 44 is a profiled air gap designed to minimize the air space between the stator assembly 32 and the rotor assembly 34, while accounting for any relative deflections that may occur between those components, thereby avoiding contact therebetween. While the bearing assembly 24 may help in minimizing deflections between the stator assembly 32 and the rotor assembly 34, the bearing assembly cannot completely arrest the deflections because it is not infinitely stiff. The air gap 44 is therefore designed to provide sufficient clearance between the rotor assembly 34 and the stator assembly 32 along with the bearing assembly 24 to prevent any contact during operation.
Various profiles of the air gap 44 are described in greater detail below with respect to
Referring now to
Furthermore, all of the
With respect to the center of deflection 46, it may be a point that lies on the axis of the rotation 30 about which the rotor assembly 34 may rotate and with respect to which the deflections within the rotor and/or the stator assemblies may be determined. The center of deflection 46 may lie anywhere on the axis of the rotation 30 depending upon several factors, such as, the type of the bearings within the bearing assembly 24, the number of bearings within the bearing assembly and the location of those bearings with respect to the stack length 20. For example, in at least some embodiments, having a single bearing within the bearing assembly 24, the center of deflection 46 may lie on the axis of rotation 30 longitudinally substantially in-line with the bearing. In at least some other embodiments having two or more bearings within the bearing assembly 24, the center of deflection 46 may lie on the axis of rotation 30 at a longitudinally offset distance from one or more of the bearings. Generally speaking, intersection of the apices of the rollers constituting the bearings within the bearing assembly 24 with the axis of rotation 30 may define the point of the center of deflection 46 in embodiments having more than a single bearing within the bearing assembly 24.
Further, although typically the center of deflection 46 may be located as described above, in at least some embodiments and depending upon the type of bearing, the center of deflection need not always lie in-line with the bearing in a single bearing configuration, while the center of deflection may lie longitudinally in-line with one of the bearings in a multiple bearing configuration. Specifically, the type (e.g., internal geometery) of the bearings within the bearing assembly 24 may influence the location of the center of deflection 46 for both single and multiple bearing configurations. For example, one or more of the bearings within the bearing assembly 24 may be constructed to have a single race with a single row of rollers or, alternatively the bearing(s) may have a single race with multiple rows of rollers. In yet other embodiments, one of more of the bearings may have different prescribed cone angles as well. Such various configurations of the bearings may vary the intersection of the roller apices of the bearings with the axis of rotation 30, thereby varying the center of deflection 46. Furthermore, in some embodiments, there may be more than one center of deflection 46 as well.
Referring now specifically to
Furthermore, the air gap 44 may be profiled by contouring surfaces 52 and 54 of the stator assembly 32 and/or the rotor assembly 34, respectively. As shown in
In contrast to the configuration of the generator 14 of
Although not shown, it will be understood that similar to contouring the surface 52 of the stator assembly 32, in at least some embodiments, the surface 54 of the rotor assembly 34 may be contoured to gradually increase the width 56 of the air gap 44 from the end 62 to the end 64, when the single bearing 48 and the center of deflection 46 are both located toward the end 62. In at least some other embodiments, the single bearing 48 and the center of deflection 46 may be located toward the end 64 instead of the end 62 of the axial ends 50. In those cases, the deflections may be the greatest at the end 62 and the air gap 44 may be profiled such that the width 56 of the air gap is maximum at the end 62 and minimum at the end 64. Again, the air gap 44 may be profiled by contouring (e.g., tapering) the surfaces 52 or 54 of the stator assembly 32 or the rotor assembly 34, respectively. In yet other embodiments, the single bearing 48 and, therefore the center of deflection 46 may be located in any position in between the axial ends 50 and the center (or substantial center) of the stack length 20 with the greatest deflections being in regions farthest away from the center of deflection. The air gap 44 in such instances may be profiled to accommodate the regions of the maximum deflections by contouring the surfaces 52 or 54 such that the width 56 is maximum in the regions of greatest deflections.
While the regions of greatest deflections in the stator assembly 32 and/or the rotor assembly 34 are generally the regions that are farthest away from the center of deflection 46, the actual regions and the amount of deflections may be modeled by known engineering tools and techniques, the description of which is not necessary herein and will be understood by those of ordinary skill in this art, and the pattern and extent of those maximum deflected regions may determine how the air gap 44 may be profiled. For example, a maximum deflection profile may be determined by measuring the deflections of the rotor for all load conditions throughout the entire load range and superimposing these deflections upon one another. By selecting only the maximum deflection at any given position along the stack length, the worst case deflected rotor volume may be established. A specific contour can subsequently be determined therefrom to provide a minimum permissible air gap.
Depending upon several factors, such as, the loads that are applied from the rotor 8, the hub 12 and the support structure 18 onto the generator 14, the type and arrangement of the bearing assembly 24, the location of the center of deflection 46, the offset distance between the bearing and the center of deflection, etc., the regions of maximum deflection may vary. It will also be understood that for a given generator 14, there may be several regions of maximum deflections and several regions of minimum deflections. In such instances, the air gap 44 may be profiled to have the maximum width 56 in all the regions of maximum deflections and a minimum width in all the regions of minimum deflections.
Turning now to
In at least some other embodiments, the first bearing 66 and the second bearing 68 may be located in a center portion of the stack length 20, or in any position in between the center and the axial ends 50. The location of the center of deflection 46 in these instances may vary as well. In all of such cases, the regions for maximum deflections may be modeled by engineering tools and the air gap 44 may be profiled to have a maximum clearance in the regions of maximum deflections.
With reference now to
Notwithstanding the exemplary profiles of the air gap 44 and the generator configurations described above, several variations to the above are contemplated and considered within the scope of the present disclosure. For example, while the air gap 44 has been shown with the rotor assembly 34 surrounding the stator assembly 32, in at least some embodiments, the air gap may be similarly profiled as described above with the stator assembly surrounding the rotor assembly. Furthermore, although in
Thus, the present disclosure sets forth a direct drive wind turbine having a generator coupled directly to the rotor of the wind turbine. A radial air gap exists between the rotor and stator of the generator. The air gap may be profiled to permit deflection of the rotor and stator relative to one another and prevent mechanical contact or interference between the stator and rotor. Specifically, the air gap may be profiled to have a maximum width in regions of greatest deflections, and a minimum width in regions of minimum deflection. Such a profiled air gap minimizes the radial distance between the rotor and stator to increase the operating efficiency of the generator. Profiling the air gap may also have the benefit of minimizing the changes in voltage or current that may occur during periods of high deflection by maintaining a relatively constant average air gap throughout the range of possible deflections between the stator and rotor.
It will be understood that while the generator and the air gap within the generator have been discussed in relation with a wind turbine, the disclosure may be equally applicable to applications other than wind turbines as well using synchronous or other types of generators and electric motors that may be prone to component deflections. Furthermore, while the disclosure above has been described with respect to a direct drive wind turbine, the disclosure may be applicable to other types of direct drive fluid flow turbines such as a tidal or ocean current powered turbine. It will also be understood that the disclosure may be applicable to non-direct drive generators for wind turbines, such as, including but not limited to, geared generators and medium speed generators, without departing from the scope of this disclosure.
While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.
