Module for mounting stators in generators of wind turbines to reduce resonance effects and related method

Abstract

A generator (18) for a wind turbine (10) that reduces undesirable resonance effects during a startup operation of the wind turbine (10) is disclosed. The generator (18) includes a generator housing (44) defining a frame (36) and a stator (34) mounted within the frame (36) by a stator mounting module (70) connected to the stator (34) and connected to the frame (36). The stator mounting module (70) includes at least one stiffness-varying element (74) for providing the stator mounting module (70) with non-linear stiffness. Due to the non-linear stiffness characteristic, the stiffness of the stator mounting module (70) may be high in a region around the resonant frequency (ωr) of the stator (34) but decreases for frequencies away from the resonant frequency (ωr) of the stator (34). A method of assembling the generator (18) using the stator mounting module (70) is also disclosed.)

Description

[0001] MODULE FOR MOUNTING STATORS IN GENERATORS OF WIND TURBINES TO REDUCE RESONANCE EFFECTS AND RELATED METHOD

[0002] Technical Field

[0003] This invention relates generally to wind turbines, and more particularly to a stator mounting module for mounting a stator in a generator of a wind turbine that reduces resonance effects experienced by the stator during, for example, startup of the wind turbine. The invention also relates to a method of assembling the generator of the wind turbine using the stator mounting module so as to reduce resonance effects.

[0004]

[0005] Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine includes a tower and an energy generating unit positioned atop of the tower. The energy generating unit typically includes a nacelle to house mechanical and electrical components, such as a generator and a rotor operatively coupled to the components in the nacelle through a main shaft extending from the nacelle. The rotor, in turn, includes a central hub and a plurality of blades extending radially therefrom and configured to interact with the wind to cause rotation of the rotor. The rotor is supported on the main shaft, which is either directly or indirectly operatively coupled with the generator which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator. Wind turbines may be constructed onshore or offshore at locations with significant wind.

[0006] In many wind turbine designs, the main shaft is operatively coupled to one or more gear stages, which may be in the form of a gearbox, to produce a more suitable mechanical input to the generator. The gearbox relies on various gear arrangements to provide speed and torque conversions from the rotation of the rotor and main shaft to the rotation of a secondary drive shaft that operates as an input to the generator.

[0007] The generator typically includes a stator and a rotor concentrically disposed relative to each other within a generator housing. The stator is generally fixed to and stationary relative to a frame of the generator housing and includes a plurality of coils (fieldwindings), while the rotor includes a plurality of magnets (e.g., permanent magnets) and is configured to be rotatably driven relative to the stator by the output shaft from the gearbox. The magnets and coils are separated from each other across a radial air gap through which the magnetic field generated by the magnets must pass. In use, an electrical current is induced in the field windings of the stator by a fluctuating magnetic field produced by the rotation of permanent magnets located within the rotor according to the precepts of Faraday's Law. In this way, the stator and rotor of the generator cooperate to convert the mechanical energy received from the rotor into electrical energy so that the kinetic energy of the wind is harnessed for power generation.

[0008] In current generator designs, the stator is mounted within the generator housing using a generally flexible mounting arrangement, sometimes referred to as stator mounts. A flexible mounting arrangement is desired in order to reduce vibration transmission between the stator and the generator housing, which can aid in reducing the noise and tonal responses of the generator during operation of the wind turbine. In one design, the flexible mounting arrangement includes a flexible beam extending between two spaced apart frame members associated with the frame of the generator housing. The flexible beam is fixed to the stator along a central region of the flexible beam by stator mounting members. The generator may include multiple flexible mounting arrangements around the circumference of the stator for supporting the stator within the generator housing.

[0009] While mounting the stator within the generator housing using the flexible mounting arrangement as described above provides certain advantages, there may also be some drawbacks with such a design. In this regard, the magnetic forces imposed on the stator by the rotor operates as a time-varying forcing function dependent upon, for example, the rotational speed of the (generator’s) rotor. This forcing function induces a vibration in the stator. Like most vibrating systems, the vibrating stator mounted to the frame of the generator housing by the flexible mounting arrangement has a resonant frequency. The resonant frequency of the stator, however, is typically below the rotational speed of the rotor during normal operation of the wind turbine. Thus, the resonance behaviour of the stator is not generally problematic during periods when the wind turbine is normally producing electrical power. However, during other periods, such as startup of the wind turbine, for example, the frequency of the forcing function on the stator (e.g, corresponding to the rotational speed of the generator’s rotor) may have to pass throughthe resonant frequency in order to reach the normal operation of the wind turbine.

[0010] Furthermore, during the startup process of the wind turbine, the vibration of the stator may be susceptible to a magneto-elastic instability. The magneto-elastic instability is an interaction between the elastic nature of the coupling of the stator to the frame of the generator housing and the strength of the magnetic forces imposed on the stator from the rotor that causes the forcing function to increase (e.g., in amplitude). The increase in the forcing function, in turn, further increases the magnetic forces imposed on the stator by the rotor, which further increases the forcing function. Thus, the magnetoelastic instability may cause a self-sustaining runaway phenomena that causes the stator and rotor to eventually bridge the air gap such that the stator and rotor touch, possibly damaging the generator.

[0011] One approach to address the effects of resonance and the magneto-elastic instability on the stator is to make the mounting arrangement between the stator and the frame of the generator housing stiffer for all frequencies, which produces smaller amplitude vibrations for a given forcing function. This approach, however, moots the noise and tonal benefits provided by the flexible mounting arrangement. Moreover, the “stiff” mounting arrangement is subjected to higher stresses due to thermal expansion of the stator, increasing the likelihood of breakage of the mounting arrangement between the stator and the frame of the generator housing. In such a case, the wind turbine will have to be taken out of service and maintenance performed to replace the defective mounting arrangement, which is costly and time-consuming.

[0012] In view of the above drawbacks, manufacturers seek improved designs for connecting the stator to the frame of the generator housing that provides the benefits of a flexible mounting arrangement while also avoiding undesirable resonance effects, including those caused by the magneto-elastic instability during startup of the wind turbine.

[0013]

[0014] In one aspect of the invention, a generator for a wind turbine that provides low vibration and noise transmission and also reduces undesirable resonance effects during, for example, a startup operation of the wind turbine is disclosed. The generator includes a generator housing defining a frame, a stator mounted within the frame by at least onestator mounting module connected to the stator and connected to the frame, and a rotor. The stator is concentrically disposed about the rotor to define an air gap between the rotor and the stator. The air gap includes a nominal diameter DAGNom, where the at least one stator mounting module includes a pre-determined permitted maximum displacement Amax within the air gap. The generator includes a Generator Displacement Factor (GDF) of less than 0.001, which is defined as the ratio of the maximum allowable displacement Amax to the nominal air gap diameter DAGNom. To reduce resonance effects, the at least one stator mounting module includes at least one stiffness-varying element for providing the at least one stator mounting module with non-linear stiffness.

[0015] In one embodiment, the at least one stiffness-varying element may provide hardening stiffness to the at least one stator mounting module. In a configuration having hardening stiffness, the stiffness of the at least one stator mounting module increases with increasing amplitude of the displacement of the stator. Thus, in one embodiment, the at least one stiffness-varying element may be configured such that the stiffness of the at least one stator mounting module is a maximum near a resonant frequency associated with the stator and decreases at frequencies away from the resonant frequency. A stator mounting module displaying such stiffness characteristics may be generally flexible at stator frequencies away from the resonant frequency but have generally high stiffness at stator frequencies around the resonant frequency. Accordingly, the benefits of a generally flexible connection of the stator to the frame of the generator housing, such as low vibration and noise transmission from the generator during operation, may be retained at regions away from the resonant frequency of the stator while suppression of high amplitude vibrations and the magneto-elastic instability may be achieved around the resonant frequency of the stator.

[0016] In another embodiment, the permitted maximum displacement Amax of the stator and / or rotor over the air gap may be a fraction of the air gap for the generator. This ensures a factor of safety within the system, and Amax may be 10% of the air gap, for example. In one embodiment, a rate of change of keffwith displacement of the at least one stator mounting module (70) is constant to provide the stiffness-varying element with nonlinear stiffness. Alternatively, a rate of change of keffwith displacement of the at least one stator mounting module is non-constant to provide the stiffness-varying elementwith non-linear stiffness. For example, the rate of change of keffmay increase with increased displacement of the at least one stator mounting module. For instance, the effective stiffness keffmay change by at least 3% over the permitted maximum displacement Amax. As another example, the effective stiffness keffmay change by up to 5% over the permitted maximum displacement Amax. The change in the effective stiffness keffover the permitted maximum displacement Amax serves to avoid the resonance effects, and particularly the resonance effects during startup of the generator.

[0017] In one embodiment, the at least one stator mounting module may include a flexible beam having a first end connected to the frame and a second end connected to the frame and a stator mounting member connected to the flexible beam and connected to the stator. In one embodiment, the stator mounting member may be connected to the flexible beam at a location between the first end and the second end of the flexible beam, such as in a central region thereof. The at least one stator mounting module may further include an intermediate member connected to the frame at a location between the first end and the second end of the flexible beam and spaced from the flexible beam to define a region between the intermediate member and the flexible beam. In one embodiment, at least part of the stiffness-varying element may be disposed in the region between the intermediate member and the flexible beam and connected to both the intermediate member and the flexible beam to provide the non-linear stiffness.

[0018] In one embodiment, the at least one stiffness-varying element may include a spring device having a plurality of spring washers for varying the stiffness of the at least one stator mounting module. For example, in one embodiment, the spring device may include a spring snubber including a shank having a first end and a second end, a first head connected to the first end of the shank, and a second head connected to the second end of the shank. In this embodiment, the plurality of spring washers may be disposed between the first head and the second head of the spring snubber. More particularly, in one embodiment, one spring washer may be captured between the first head and the intermediate member, and another spring washer may be captured between the second head and the flexible beam. In one embodiment, one of the first head or the second head may be integrally formed with the shank (similar to a bolt head) and the other of the first head or the second head may be removably connected to theshank (e.g. similar to a nut). Additionally, in one embodiment, at least the second head may be formed with or permanently connected to the at least one stator mounting module. For example, at least the second head may be formed with or permanently connected to the flexible beam. In one embodiment, each of the plurality of spring washers may be formed as conical washers.

[0019] In one embodiment, the spring device may include a spring snubber assembly including a cylinder defining a chamber and a piston having a piston head positioned within the chamber and a piston shaft extending from the piston head and extending outside the chamber. A pair of mounting bolts may connect the spring snubber assembly to the intermediate member and the flexible beam. In this embodiment, the plurality of spring washers may be disposed in the chamber of the cylinder and on opposed sides of the piston head. More particularly, one spring washer may be captured between the piston head and an upper wall of the cylinder, and another spring washer may be captured between the piston head and a lower wall of the cylinder. In one embodiment, the spring snubber assembly may further include an adjustment mechanism for pre-tensioning the spring snubber assembly. In one embodiment, the adjustment mechanism may include a plurality of set screws for adjusting the pre-tension.

[0020] In one embodiment, the at least one stiffness-varying element may include a rubber damper having a rubber material for varying the stiffness of the at least one stator mounting module. For example, in one embodiment, the rubber damper may include an outer casing having a first end and a second end, a first plate adjacent the first end, a second plate adjacent the second end, and a rubber insert disposed between the first and second plates. In one embodiment, the rubber insert may be formed from a rubber material displaying non-linear stiffness. In one embodiment, a pair of mounting bolts may connect the rubber damper to the intermediate member and the flexible beam. Alternatively, the rubber damper may include opposed shafts extending from the first and second plates that are connected to the intermediate member and the flexible beam by mounting nuts.

[0021] In one embodiment, the at least one stiffness-varying element may include or further include a rubber bumper. Similar to the above, the rubber bumper may be formed from a rubber material displaying non-linear stiffness. In this embodiment, the rubber bumpermay be mounted to the stator mounting member of the at least one stator mounting module. For example, the rubber bumper may be mounted to an upper surface of the at least one stator mounting member. In this way, the rubber bumper is configured to contact and be squeezed between the intermediate member and the at least one stator mounting member as the stator vibrates. In addition, the rubber bumper may also be used in combination with the other forms of the stiffness-varying elements disclosed herein.

[0022] In one embodiment, the at least one stiffness-varying element may provide softening stiffness to the at least one stator mounting module. In a configuration having softening stiffness, the stiffness of the at least one stator mounting module may decrease with increasing amplitude of the displacement of the stator.

[0023] In one embodiment of this type, the at least one stiffness-varying element may include a buckling assembly having at least one flexible buckling element for varying the stiffness of the at least one stator mounting module. For example, in one embodiment, the buckling assembly may include a first head connected to a first end of the at least one flexible buckling element and a second head connected to a second end of the at least one flexible buckling element. More particularly, in one embodiment, the first head may confront and engage the intermediate member, and the second head may confront and engage the flexible beam.

[0024] In one embodiment, the stator may be mounted within the frame of the generator housing by a plurality of stator mounting modules. The plurality of stator mounting modules may be positioned, uniformly or non-uniform ly about the circumference of the stator.

[0025] In one embodiment, a wind turbine may include a tower, a nacelle disposed on the tower, and a generator according to the first aspect described above disposed in the nacelle.

[0026] In another aspect of the invention, a method of assembling a generator of a wind turbine that provides low vibration and noise transmission and also reduces undesirable resonance effects during, for example, a startup operation of the wind turbine is disclosed. The generator includes a generator housing defining a frame and a stator.The method includes providing a plurality of stator mounting modules and mounting the stator within the frame of the generator housing using the plurality of stator mounting modules. Each of the plurality of stator mounting modules includes at least one stiffnessvarying element for providing the at least one stator mounting module with non-linear stiffness.

[0027] Brief Description of the Drawings

[0028] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

[0029] Fig. 1 is a front perspective view of a wind turbine having a generator according to an embodiment of the invention.

[0030] Fig. 2 is a perspective, enlarged view of the nacelle and rotor hub of the wind turbine of Fig. 1.

[0031] Fig. 3 is a schematic cross-sectional perspective view of a vertical plane taken through a generator of a wind turbine.

[0032] Fig. 4 is a schematic perspective view of a generator housing and frame to which a stator of the generator is mounted.

[0033] Fig. 5 is a detailed view of part of Fig. 3 focused on a stator mounting bay of the generator with part of the housing removed.

[0034] Fig. 5A is a diagrammatic illustration showing an air gap between the stator and rotor of the generator.

[0035] Fig. 6 is a schematic view of a portion of a generator illustrating a stator mounting module having linear stiffness.

[0036] Fig. 7 is a schematic amplitude-frequency curve for a linear vibrating system having aresonant frequency.

[0037] Fig. 8 is a schematic amplitude-frequency curve for a non-linear vibrating system demonstrating hardening stiffening.

[0038] Fig. 9 is a schematic amplitude-frequency curve for a non-linear vibrating system demonstrating softening stiffening.

[0039] Fig. 10 is a schematic view of a portion of a generator illustrating a stator mounting module having non-linear stiffness provided by a spring snubber in accordance with an embodiment of the invention.

[0040] Fig. 11 is a schematic view of a portion of a generator illustrating an alternative stator mounting module having non-linear stiffness provided by a spring snubber in accordance with an embodiment of the invention.

[0041] Fig. 12 is a schematic view of a portion of a generator illustrating an alternative stator mounting module having non-linear stiffness provided by a spring snubber assembly in accordance with an embodiment of the invention.

[0042] Fig. 13 is a front detailed view of the spring snubber assembly shown in Fig. 12 for providing the non-linear stiffness of the stator mounting module.

[0043] Fig. 14 is a schematic view of a portion of a generator illustrating an alternative stator mounting module having non-linear stiffness provided by a rubber damper in accordance with an embodiment of the invention.

[0044] Fig. 15 is a schematic perspective view of the rubber damper shown in Fig. 14 for providing the non-linear stiffness of the stator mounting module.

[0045] Fig. 16 is a schematic perspective view of an alternative rubber damper for providing the non-linear stiffness of the stator mounting module.

[0046] Fig. 17 is a schematic view of a portion of a generator illustrating an alternative statormounting module having non-linear stiffness provided by rubber bumper in accordance with an embodiment of the invention.

[0047] Detailed Description

[0048] With reference to Figs. 1 and 2, a wind turbine 10 includes a tower 12, a nacelle 14 disposed at the apex of the tower 12, and a rotor 16 operatively coupled to a generator 18 via a gearbox 20 housed inside the nacelle 14. In addition to the generator 18 and gearbox 20, the nacelle 14 may house various components needed to convert wind energy into electrical energy and to operate and optimize the performance of the wind turbine 10. The tower 12 supports the load presented by the nacelle 14, rotor 16, and other wind turbine components housed inside the nacelle 14 and operates to elevate the nacelle 14 and rotor 16 to a height above ground level or sea level, as may be the case, at which air currents having lower turbulence and higher velocity are typically found.

[0049] The rotor 16 may include a central hub 22 and a plurality of blades 24 attached to the central hub 22 at locations distributed about the circumference of the central hub 22. In the representative embodiment, the rotor 16 includes three blades 24, however the number may vary. The blades 24, which project radially outward from the central hub 22, are configured to interact with passing air currents to produce rotational forces that cause the central hub 22 to spin about its longitudinal axis A1. The design, construction, and operation of the blades 24 are familiar to a person having ordinary skill in the art of wind turbine design and may include additional functional aspects to optimize performance. For example, pitch angle control of the blades 24 may be implemented by a pitch control mechanism (not shown) responsive to wind velocity to optimize power production in low wind conditions, and to feather the blades if wind velocity exceeds design limitations.

[0050] The rotor 16 may be coupled to the gearbox 20 directly or as shown, indirectly via a main shaft 26 extending between the hub 22 and the gearbox 20. The main shaft 26 rotates with the rotor 16 and is supported within the nacelle 14 by a main bearing support 28 which supports the weight of the rotor 16 and transfers the loads on the rotor 16 to the tower 12. The gearbox 20 transfers the rotation of the rotor 16 through a coupling to the generator 18. Wind exceeding a minimum level may activate the rotor 16, causingthe rotor 16 to rotate in a direction substantially perpendicular to the wind and applying torque to the input shaft of the generator 18. The electrical power produced by the generator 18 may be supplied to a power grid (not shown) or an energy storage system (not shown) for later release to the grid as understood by a person having ordinary skill in the art. In this way, the kinetic energy of the wind may be harnessed by the wind turbine 10 for power generation.

[0051] With reference firstly to the gearbox 20, a gearbox housing is generally cylindrical in form and is oriented such that its major rotational axis is horizontal, in the orientation of the drawings. The cylindrical configuration of the gearbox housing is due to the specific type of gearbox that is used in the illustrated embodiment, which is an epicyclic gearbox. As the skilled person would know, an epicyclic gearbox includes a series of planet gears that are arranged about a central sun gear, and which collectively are arranged within an encircling ring gear. The ratio of the number of teeth between the ring gear, the planet gear and the sun gears determine the gear ratio of the gearbox. For clarity, fine detail of the gearbox will not be described in further detail here as the gearbox is not the principal subject of the invention. Accordingly, it should be appreciated that other gearbox configurations could also be used, although it is currently envisaged that an epicyclic gearbox provides an elegant solution fit for the confines of a wind turbine nacelle.

[0052] Turning now to the generator 18, and as illustrated in Fig. 3, the output shaft of the gearbox 20 interfaces with a rotor 32 of the generator 18. As such, the major axis of the gearbox output shaft defines the rotational axis of the generator 18 which corresponds to the central axis of a stator 34 concentrically disposed about the rotor 32. The stator 34 therefore also defines the central axis of the generator 18.

[0053] The generator 18 in the illustrated embodiment is an IPM (interior permanent magnet) electric machine having an external stator 34 which surrounds the rotor 32. The stator 34 includes a stator core and a stator frame which surrounds and supports the stator core. The stator core includes a plurality of lamina stacked in an axial direction of the generator. Field windings of a conductive material such as copper are wound around the lamina of the stator core. An electrical current is induced in the field windings by a fluctuating magnetic field caused by the magnets attached to the rotor 32, which is rotated in use by the hub 22 of the wind turbine 10. Although the embodiment describedherein refers to an interior permanent magnet machine, it should be appreciated that in other embodiments the generator may include a rotor having an electro-magnet instead of permanent magnets.

[0054] The generator 18 includes a frame 36 within which the stator 34 is mounted by a stator mounting system as will be described in greater detail below. The frame 36 has a drive end face 38 located, in use, adjacent to the gearbox 20, and a non-drive end face 40 located opposite the drive end face 38 and separated therefrom in the direction of the central generator axis. A plurality of frame members 42 extend between the drive end face 38 and the non-drive end face 40 to provide structural rigidity to the frame 36 and mounting points for the various panels and systems of the generator 18.

[0055] The outer surface of the generator defines a generator housing 44 which includes the drive end face 38 and non-drive end face 40 of the frame 36. The outer surfaces of the housing 44 between the drive end face 38 and non-drive end face 40 include panels 46 which are removably attached to the frame 36. Referring to Fig. 4, the panels 46 each include an access opening which provides access to a stator mounting ‘bay’ 48. In this example, the stator mounting system of the generator 18 may include four stator mounting bays 48 which are equally spaced around the circumference of the stator 34. It will be understood that any suitable amount of stator mounting bays 48 may be used and it is not essential that four stator mounting bays 48 be provided.

[0056] The access openings are covered by an access panel 50 which is removably attached to the frame 36 by mechanical fasteners (not shown). In this embodiment, the access panels 50 (also herein referred to as the ‘access hatch cover’ or the ‘access hatch’) fit within the access openings such that the panel 46 of the generator housing 44 has a substantially planar finish. However, in an alternative embodiment, the access panels 50 may be attached to the outer surface of the panels 46. Alternatively, the access panels 50 may be connected to the frame 36 or panels 46 by hinges such that the panels 50 must be rotated about their hinges to gain access to the access openings.

[0057] Each stator mounting bay 48 may include two stator mounting modules 52. For clarity, only one stator mounting bay 52 is described in detail below. However, it will be understood that the configuration of each stator mounting bay 48 is substantially asdescribed below. It is not essential that each stator mounting bay 48 include two stator mounting modules 52. In alternative designs, for example, the stator mounting bays 48 may include only one stator mounting module 52, or more than two stator mounting modules 52. Similarly, it is not essential that each stator mounting bay 48 has the same number and / or configuration of stator mounting modules 52 as the other stator mounting bays 48 of the generator 18. For example, in the embodiment shown in Fig. 6, a first stator mounting module 52 may be located within the mounting bay 48 towards the nondrive end face 40 of the frame 36 and a second stator mounting module 52 may be located within the mounting bay 48 towards the drive end face 38 of the frame 36. The stator mounting modules 52 may be removably and adjustably attached to axially extending members 54 of the frame 36 which extend from the drive end face 38 to the non-drive end face 40. In one embodiment, the frame members 42 may operate as the axially extending members 54. The axially extending members 54 may be located substantially at each outer axially extending edge of the access openings such that the opening which provides access to the mounting bay 48 includes a reinforced opening in the generator housing 44. Each axially extending member 54 may include a plurality of equally spaced bolt holes for receiving a fixing bolt 56.

[0058] In one embodiment, the stator mounting modules 52 may each include a flexible beam 60 and a stator mounting member 62. The flexible beam 60 includes first and second ends which have bolt holes through which the fixing bolts 56 pass. The first and second ends of the flexible beam 60 may be removably and adjustably attached to the axially extending members 54 of the frame 36 by the fixing bolts 56 which pass through the bolt holes in the ends of the flexible beam 60 and engage with the bolt holes in the axially extending members 54. The axially extending members 54 each have an inwardly facing recessed portion facing towards the centre of the access opening. The first and second ends of the flexible beam 60 may be attached to the recessed portion of the axially extending members 54. The stator mounting member 62 may be removably attached to the stator 34 and to the flexible beam 60. In one embodiment, the stator mounting member 62 may be substantially cuboidal in form and include rows of mounting holes which pass through the stator mounting member 62 from a radially outer position to a radially inner position with respect to the central axis of the generator 18. Each of the rows of mounting holes in the stator mounting member 62 corresponds to rows of mounting holes located substantially in the centre of the flexible beam 60. The stator 34may include a mounting rail 64 along which a plurality of equally spaced bolt holes is provided.

[0059] Each stator mounting module 52 may be removably attached to the stator 34 by stator mounting bolts 66 which pass through the mounting holes in the flexible beam 60 and through the holes in the stator mounting member 62. The stator mounting bolts 66 attach to the bolt holes in the stator mounting rail 64. When the stator 34 is mounted in the frame 36 by the stator mounting system it is supported by the stator mounting modules 52 of the stator mounting bays 48 in a generally flexible manner. The flexible connection between the stator 34 and the frame 36 of the generator housing 44 is generally considered advantageous for reducing vibration transmission across that connection. The reduction in vibration transmission not only provides a structural benefit, e.g., reduced stress on the generator components and fixed connections, such as bolted and welded joints, but also provides a reduction in noise emanating from the generator 18 during its operation.

[0060] Fig. 5A is a diagrammatic illustration showing an air gap 31 of the generator 18, which is the space or radial clearance between the rotor 32 and the stator 34. As will be understood by a person skilled in the art, the air gap 31 provides radial clearance between the rotor 32 and stator 34 so that the rotor 32 may rotate freely within the stator 34. In that regard, the air gap 31 defines the maximum range of radial movement or displacement of the rotor 32 and / or stator 34 relative to the other before there is contact between the two components. Such radial movements of the rotor 32 and stator 34 may occur due to system vibrations, particularly during transient conditions like generator 18 startup or shut-down. As shown, the air gap 31 is generally defined as:

[0061] Air Gap (31) ^Stator ^Rotor

[0062] where Rstator is the inner radius of the stator 34 and Rpotor is the outer radius of the rotor 32. As shown, the air gap 31 forms a circumferential ring around the rotor 32 and can be characterized by DAGNom, which represents the nominal diameter of the air gap 31. The value of DAGNom varies depending on the size of the generator 18. For example, a smaller generator 18 may have DAGNom = 1,500 mm, while a larger generator 18 may have DAGNom = 3,000 mm.For typical wind turbine generators, the air gap 31 between the rotor 32 and the stator 34 ranges from approximately 0.5 mm to 10 mm, depending on the size and design of the generator 18. The maximum allowable displacement of the stator 34 or rotor 32 within this air gap 31 , referred to as the critical displacement threshold, denoted herein as Amax, is typically a fraction of the total air gap 31. In that regard, Amax ensures safe operation by preventing rotor 32 and stator 34 contact, which could lead to mechanical failure or electrical faults. The critical displacement threshold Amax is defined as a percentage of the total air gap 31 and is typically between 5% and 10% of the total air gap 31 , but in some circumstances may be less than 5% or greater than 10%, such as up to 20%, for example. To that end, Amax is specified by the generator manufacturer and varies by design. For example, a generator 18 with DAGNom = 3,000 mm and an air gap 31 of 10 mm may have A max — 0.6 mm, which corresponds to 6% of the air gap. A generator 18 with DAGNom = 1 ,500 mm and an air gap 31 of 5 mm may have Amax = 0.3 mm, also equating to 6% of the air gap.

[0063] To standardize the relationship between critical displacement limits and the size of the generator 18, the Generator Displacement Factor (GDF) is provided. The GDF is defined as the ratio of the maximum allowable displacement Amax to the nominal air gap diameter DAGNom, being:

[0064]

[0065] This dimensionless factor provides a benchmark for comparing different wind turbine generator 18 designs, ensuring that permissible displacement thresholds are proportional to the overall size of the generator 18 system. For wind turbines, the GDF is generally below 0.001, more commonly under 0.0005, and in many cases, it is even less than 0.0003. To that end, the generator 18 may have a GDF of about 0.0001.

[0066] Fig. 6 illustrates a schematic of a stator mounting module 52 according to existing designs for mounting a stator 34 to the frame 36 of the generator 18. In this regard, Fig.

[0067] 6 illustrates two spaced-apart axially extending members 54 that form part of the frame 36 of the generator 18. A flexible beam 60 of the stator mounting module 52 extendsbetween the two axially extending members 54 (e.g., in the circumferential direction) and is fixed to the two axially extending members 54 by fixing bolts 56. Along a central region of the flexible beam 60, the stator mounting member 62 is connected to the flexible beam 60 by stator mounting bolts 66 and is also connected to the stator 34, which is shown schematically in Fig. 6. When the rotor 32 of the generator 18 is rotated, electromagnetic forces from the rotating magnets carried by the rotor 32 induce a time varying, frequency dependent forcing function F on the stator mounting member 62 in a generally radial direction of the generator 18. This forcing function F, in turn, induces radial displacements of the flexible beam 60 of the stator mounting module 52, as illustrated by arrow A in Fig. 6. In other words, the stator mounting module 52 represents a system subject to vibrations. As such, the system is subject to the effects of resonance and includes a resonance frequency corat which the amplitude of the displacement of the flexible beam 60 (and thus the stator 34 carried thereby) is a maximum. That is, resonance occurs when the input frequency from the generator 18 rotating is close to or the same as the resonant frequency corof the stator mounting module 52, and in particular the flexible beam 60.

[0068] The vibrating system of the mounting module 52 illustrated in Fig. 6 generally displays linear stiffness, where the load and displacement of the flexible beam 60 have a linear relationship. Fig. 7 is an exemplary illustration of the amplitude-frequency curve Ciinear in an arrangement exhibiting linear stiffness, such as the mounting module 52 including the flexible beam 60. In such a system with linear stiffness, the relationship between load or force and displacement is proportional, meaning that as force is applied to the flexible beam 60, displacement increases in a predictable, linear manner. This behaviour follows Hooke’s Law, expressed as

[0069] F = kx

[0070] where F is the restoring force exerted by the flexible beam 60, k is the stiffness or spring constant of the flexible beam 60, and x is the displacement of the beam 60. For a fixed-end beam, k is determined by:

[0071]

[0072] where E is the Young’s modulus or modulus of elasticity of the flexible beam 60, w is the beam width, h is the beam thickness, and L is the beam length.

[0073] As show in Fig. 7, as the excitation frequency co increases from, for example, a stopped condition or very low frequency condition of the generator 18, the amplitude of the displacement x of the flexible beam 60 starts to increase. This behavior continues until the system reaches its resonant frequency corat which the amplitude of displacement x reaches a maximum. At this point, the system temporarily experiences maximum vibration amplitude, or resonance. This is a characteristic response of a simple harmonic oscillator when the driving frequency matches the natural resonance frequency of the system. To that end, while the flexible beam 60 exhibits linear stiffness, the amplitudefrequency relationship follows a non-linear resonant peak, as shown in Fig. 7. As the excitation frequency continues past the resonant frequency cor, the amplitude of displacement x of the flexible beam 60 starts to decrease.

[0074] As discussed above, the resonant frequency corof a generator 18 in accordance with that shown by the system illustrated in Fig. 6 is typically not near the normal operating state of the generator 18. However, the generator 18 may have to pass through the resonant range in order to reach the normal operating state of the generator 18. This is schematically illustrated, for example, by line P in Fig. 7. The region Rnthe right of line P represents the normal operating range of the generator 18 and the region Rsto the left of the line P represents a startup region of operation of the generator 18. As illustrated in Fig. 7, the resonant frequency coris removed from the line P but is generally in the startup region Rsof the generator 18. Thus, resonance can influence the generator 18 during startup of the generator 18 of the wind turbine 10 but generally does not have an effect on the generator 18 during normal operation.

[0075] As also discussed above, it has been discovered that the generator 18 in accordance with that shown by the system illustrated in Fig. 6 may be susceptible to a magnetoelastic instability. In this instability, the vibration of the stator 34 and the electromagnetic forces between the rotor 32 and the stator 34 align in such a manner to cause the forcing function F to grow, in some cases uncontrollably, which causes an increase in the amplitude of the displacement of the flexible beam 60, and the stator 34 which isconnected thereto. In some cases, the forcing function F and the amplitude of the displacement of the flexible beam 60 may remain in phase and continue to grow. This may cause movement of the stator 34 to exceed Amax, and in extreme circumstances breach the air gap 31 between the rotor 32 and stator 34 and potentially damage the generator 18. The magneto-elastic instability may be most pronounced around the resonant frequency corwhere the amplitude of the displacement of the stator 34 is at its maximum.

[0076] Existing stator 34 mounting systems, such as the stator mounting module 52 shown in Fig. 6, exhibit a linear stiffness, as described above. Although these mounting modules 52 and systems may not be perfectly linear in practical operation, any inherent nonlinearity present in existing stator mounting systems has been proven to be insufficient to avoid the effects of resonance. Consequently, conventional stator mounting systems remain susceptible to resonance during critical operational phases, particularly during startup conditions of the generator and wind turbine, as described earlier. Aspects of the present invention seek to retain the advantageous aspects of prior stator mounting modules, such as their reduction in vibration transmission to other parts of the generator and drivetrain as well as their reduction in noise transmission, while avoiding the potential undesirable resonance effects during startup of the generator 18 and wind turbine 10, including those posed by the magneto-elastic instability. Compared to existing linear systems in use with current generator 18 designs, the stator mounting modules described below exhibit non-linear behaviour with up to a 10% variation in spring constant k across the allowable displacement Amax. This degree of non-linearity within the generator 18 has been shown to effectively mitigate the resonance effects previously described, particularly during generator 18 startup. Stator mounting modules according to embodiments disclosed herein are particularly suitable for wind turbines having a GDF of below 0.001, and more particularly below 0.0005, and even more particularly below 0.0003. Embodiments of stator mounting modules described herein may be installed in a generator 18 having a GDF of about 0.0001.

[0077] In various embodiments, the above advantages may be achieved by designing the stator mounting modules to have non-linear stiffness, where the load and displacement of the system has a non-linear relationship. In such systems, the stiffness k is not constant but instead varies with the amplitude of displacement x, either increasing (hardening) ordecreasing (softening) as the displacement x of the system grows. This contrasts with linear stiffness, where the restoring force follows Hooke’s Law, F = kx, with a constant stiffness k. Instead, in a system exhibiting non-linear stiffness, the restoring force can be expressed as:

[0078] F = keffx

[0079] where keis the effective stiffness, which varies with deflection. That is, keis a function of the current deflection state of the system. Unlike in linear systems, where k remains fixed or constant, the effective stiffness keffin non-linear systems varies dynamically with displacement. This dependence may be generally expressed in terms of a non-linearity coefficient, expressed as:

[0080] ke^ = kQ+ axn

[0081] where kQis the initial stiffness, a is a non-linearity coefficient, and n determines the degree of stiffness variation with displacement. If a > 0, the system exhibits hardening stiffness, meaning resistance increases with displacement. If a < 0, the system exhibits softening stiffness, where stiffness decreases as displacement grows.

[0082] In another embodiment, as will be described below, the system may include springs in parallel or series that exhibit different spring or stiffness constants so as to provide the system with non-linear stiffness. The effective stiffness kefffor this type of non-linear system having two springs may be expressed by:

[0083] keff= k1+ k2

[0084] where ki is the spring constant for a first spring and k2 is the spring constant for the second spring. This formula can be extended to any number of different spring constants in the system, arranged either in parallel or series.

[0085] Fig. 8 illustrates a schematic amplitude-frequency curve Cni.h where the stiffness increases with increasing amplitude of the displacement, which is referred to ashardening stiffness. In one example, this phenomenon occurs when a > 0, as described above, meaning that as the oscillation amplitude grows, the restoring force F increases at a greater rate than in a linear system. As illustrated in this figure, the peak of the curve Cni.h is bent to the right as a result of the non-linearity in the system, meaning that at higher oscillation amplitudes, the system resonates at higher frequencies than it would in a linear stiffness configuration. That is, in a non-linear hardening system, the stiffness k increases with deflection.

[0086] For a non-linear hardening spring system, such as that shown in Fig. 8, where keffis the effective stiffness for the system, the restoring force in the system may be described as:

[0087] F = keffx)x

[0088] where the effective stiffness, fce^(x), increases with displacement x, leading to a progressively stiffer response as the rotor approaches the maximum permitted displacement x, being Amax. This relationship results in a force-displacement (F vs. x) curve that deviates from the linear behavior of a traditional linear spring, where stiffness remains constant. For a hardening spring system, the force-displacement curve exhibits an increasing slope as displacement grows, thereby providing greater resistance to deflection at higher amplitudes.

[0089] As an example of a non-linear hardening spring system in a generator 18 having an operational air gap limit of x=Amax and a GDF of about 0.0001 , the rate of change of keover Amax is non-constant. For example, at 10% displacement of Amax, the effective stiffness fce^(x) of the system may have only increased by 0.2%. However, by 30% of Amax, fce^(x) may have increased by 1%. By 50% of Amax, the stiffness increase may reach 1.5%, signifying that the system is now generating greater restoring force for each additional increment of displacement. This progressively steeper force-displacement curve effectively dampens larger vibrations, a defining characteristic of a non-linear hardening spring system. As displacement continues to grow, the increase in both keff^x) may become more pronounced. This trend may continue, with fce^(x) increasing by at least 3% as the displacement nears 100% of Amax. At this point, anyadditional displacement would require an increase in force, effectively preventing further motion and protecting the rotor from exceeding the permitted allowable deflection Amax.

[0090] As yet another example of a non-linear system in a generator 18 having an operational air gap limit of x=Amax and a GDF of about 0.0001 , the rate of change of

[0091]

[0092] over Amax may be constant. As an example of this type of non-linear system, the effective stiffness keff _x) may increase by .5% for every 10% increase in displacement up to 100% of Amax. In this example, the effective stiffness keffmay change by up to 5% over the permitted maximum displacement Amax. These exemplary non-linear systems serve to control displacement of the stator relative to the rotor, as well as avoid resonance within the generator system due to the non-linearity of the system. This behavior ensures that the rotor can respond to normal operational forces, such as during startup, without excessive rigidity while also preventing excessive movement that could lead to reaching the maximum permitted displacement between the rotor and stator.

[0093] Fig. 9 illustrates a schematic amplitude-frequency curve Cni.s for a system exhibiting softening stiffness (in contrast to the hardening system shown in Fig. 8), where the stiffness decreases with increasing displacement amplitude. In one example, this effect occurs when a < 0, meaning that as the amplitude increases, the system’s resistance to displacement reduces, leading to a downward shift in the resonant frequency. As illustrated in this figure, the peak of the curve Cni.s is bent to the left as a result of the non-linearity in the system, meaning that as the system oscillates at larger amplitudes, resonance occurs at lower frequencies. That is, in a non-linear softening system, the stiffness k decreases with deflection.

[0094] Fig. 10 is a schematic illustration of a stator mounting module 70 similar to that shown in Fig. 6 but having non-linear stiffness, and in particular non-linear hardening stiffness, in accordance with an embodiment of the invention. Fig. 10 illustrates the axially extending members 54, the flexible beam 60, and stator mounting member 62 similar to that shown in Fig. 6. In addition, however, Fig. 10 illustrates an intermediate axially extending member 72 extending between the drive end face 38 and the non-drive end face 40 to provide further structural rigidity to the frame 36 and to provide a location to stiffen the stator mounting module 70 in a non-linear manner. In one embodiment, theintermediate axially extending member 72 may be positioned between the two axially extending members 54. For example, in one embodiment, the intermediate axially extending member 72 may generally overlie the stator mounting member 62 (i.e., radially aligned). In an alternative embodiment, the axially extending member 72 may be (circumferentially) spaced from the stator mounting member 62 toward one side or the other.

[0095] In addition to the above, and in accordance with an aspect of the invention, the stator mounting module 70 may include at least one stiffness-varying element 74 connected at one end to the intermediate axially extending member 72 and connected at the other end to the flexible beam 60. The at least one stiffness-varying element 74 is configured to either increase or decrease the stiffness of the stator mounting module 70 as a function of rotor speed co of the rotor 32 of the generator 18. Fig. 10 illustrates an arrangement where the stiffness of the stator mounting module 70 is configured to increase with increasing amplitude of the displacement of the stator 34. In this regard, Fig. 10 illustrates the stator mounting module 70 having at least one spring snubber 76. In an exemplary embodiment, the stator mounting module 70 may include two spring snubbers 76. For example, the stator mounting module 70 may include a spring snubber 76 on opposed axial sides of the stator mounting member 62. However, a stator mounting module 70 may have more than two spring snubbers 76 and be located at different positions relative to the stator mounting member 62.

[0096] In one embodiment, the spring snubber 76 may include a shank 78 including an upper head 80 connected to the shank 78 at one end thereof and a lower head 82 connected to the shank 78 at the other end thereof. In this regard, the intermediate axially extending member 72 may include a bore 84 that aligns with a bore 86 in the flexible beam 60 and which receives the shank 78 therethrough. The upper head 80 may be connected to the shank 78 so as to confront an upper surface 88 of the intermediate axially extending member 72 and the lower head 82 may be connected to the shank 78 so as to confront a lower surface 90 of the flexible beam 60. In one embodiment, the upper and lower heads 80, 82 may take the form of nuts. The spring snubber 76 may further include a pair of spring washers in the form of, for example, conically shaped washers disposed about the shank 78. In one embodiment, the conical shaped washers may be configured as non-linear springs. As illustrated in Fig. 10, an upper spring washer 92 may bedisposed between the upper head 80 of the spring snubber 76 and the upper surface 88 of the intermediate axially extending member 72, and a lower spring washer 94 may be disposed between the lower surface 96 of the intermediate axially extending member 72 and the upper surface 98 of the flexible beam 60. The upper and lower spring washers 92, 94 effectively operate as springs with a spring constant

[0097]

[0098] that changes (i.e., increases) with the amplitude of the displacement of stator 34. To that end, the upper and lower spring washers 92, 94 may be a Belleville spring washer, which increase in stiffness or harden as they are compressed.

[0099] In this regard, as the flexible beam 60 is displaced in a direction toward the intermediate axially extending member 72, the lower spring washer 94 flexes away from the conical configuration and toward a flattened configuration (i.e., where the washer 94 is generally planar). If the amplitude of the displacement is large enough, the spring washer 94 will bottom out in its flattened configuration. When this occurs, the intermediate axially extending member 72, the lower spring washer 94, and the flexible beam 60 are in planar contact with one another and operate as an effective beam having a stiffness greater, and perhaps significantly greater than the stiffness of the flexible beam 60 in isolation. Thus, the stiffness of the system increases as the lower spring washer 94 flexes toward the flattened configuration and then the stiffness rapidly increases to a higher value when the amplitude of the displacement is such to cause the lower spring washer 94 to reach its flattened configuration. When the amplitude of the displacement of the flexible beam 60 decreases, such as after passing by the resonant frequency corof the system, the lower spring washer 94 will regain its conical configuration and the stiffness of the stator mounting module 70 will correspondingly decrease.

[0100] In somewhat of a similar manner, as the flexible beam 60 is displaced in a direction away from the intermediate axially extending member 72, the upper spring washer 92 flexes away from the conical configuration and toward a flattened configuration (i.e., where the washer 92 is generally planar). If the amplitude of the displacement is large enough, the spring washer 92 will bottom out in its flattened configuration. When this occurs, the upper head 80, the upper spring washer 92, and the intermediate axially extending member 72 are in planar contact with one another and operate as an effective beam having a stiffness greater, and perhaps significantly greater than the stiffness of the flexible beam 60 in isolation. Thus, the stiffness of the system increases as the upperspring washer 92 flexes toward the flattened configuration and then rapidly increases to a higher value when the amplitude of the displacement is such to cause the upper spring washer 92 to reach its flattened configuration. When the amplitude of the displacement of the flexible beam 60 decreases, such as after passing by the resonant frequency corof the system, the upper spring washer 92 will regain its conical configuration and the stiffness of stator mounting module 70 will correspondingly decrease.

[0101] Based on the above, it should be appreciated that, the spring snubber 76 has the general characteristics that when the amplitude of the deflections of the flexible beam 60 are relatively small, the stiffness of the stator mounting module 70 remains relatively low. Accordingly, the stator mounting module 70 offers low vibration and noise transmission from the stator 36 to the frame 36 and the generator housing 44. Thus, the stator mounting module 70 offers the beneficial attributes of current stator mounting modules, such as stator mounting modules 52 described above. However, when the amplitude of the deflections of the flexible beam 60 are relatively large, such as when around (e.g., + / - 10%, and preferably + / - 5%) the resonant frequency corof the system, the stiffness of the stator mounting module 70 increases which, in turn, changes the vibration characteristics of the system. More particularly, the increased stiffness of the stator mounting module 70 decreases the amplitude of the deflections that would otherwise occur for the same forcing function F. In this way, the increase in the stiffness of the stator mounting module 70 essentially cuts off the self-feeding aspect of the magnetoelastic instability and the system is able to stabilize in the resonance range. Once the system passes through the resonant frequency corof the stator 34, the amplitude of the deflections of the flexible beam 60 return to being relatively small. Thus, the stiffness of the stator mounting module 70 returns to being relatively lower and again offers low vibration and noise transmission from the stator 36 to the frame 36 and the generator housing 44. In total, the inclusion of the spring snubber 76 in the stator mounting module 70 offers the benefits of the prior arrangements as it pertains to lower transmitted vibrations and noise while also reducing the effects of resonance on the stator 36, even in the presence of the magneto-elastic instability.

[0102] In the embodiment shown in Fig. 10, the upper head 80 may be integral with the shank 78 (e.g., bolt head) and the lower head 82 may be removably connected to the shank 78 (e.g., nut). For example, the lower end of the shank 78 may be threaded and thelower head 82 may be internally threaded such that the lower head 82 may be connected to the shank 78 through a threaded connection. It should be recognized that other arrangements may also be possible. For example, in an alternative embodiment, the lower head 82 may be integral with the shank 78 (e.g., bolt head) and the upper head 80 may be removably connected to the shank 78 (e.g. nut). In this embodiment, the upper end of the shank 78 may be threaded and the upper head 80 may be internally threaded such that the upper head 80 may be connected to the shank 78 through a threaded connection.

[0103] A further alternative embodiment is illustrated in Fig. 11 , in which like reference numbers refer to like features shown in Fig. 10. This embodiment differs from that shown in Fig.

[0104] 10 by integrating the at least one spring snubber 76 into the flexible beam 60 of the stator mounting module 70. In this regard, at least a portion of the at least one spring snubber 76 may form part of or may be permanently connected to the flexible beam 60. For example, as illustrated in Fig. 11 , at least the lower head 82 of the at least one spring snubber 76 forms part of or is permanently connected to the flexible beam 60. In one embodiment, a threaded bore may be formed through the flexible beam 60 and the region around the threaded bore may be considered to constitute the lower head 82 of the at least one spring snubber 76. In another embodiment, the lower head 82 may be welded or otherwise permanently connected to the flexible beam 60. Furthermore, in one embodiment, the shank 78 of the at least one spring snubber 76 may be integrated with the lower head 82 such that the lower head 82 and shank 78 form part of or are permanently connected to the flexible beam 60. In an alternative embodiment, the lower end of the shank 78 may be threaded and is configured to make a threaded connection with the lower head 82. In this embodiment, only the lower head 82 is integrated with the flexible beam 60. The upper and lower spring washers 92, 94 may be assembled to the at least one spring snubber 76 during the assembly of the stator mounting module 70 to the frame 36 of the generator 18. The upper head 80, which may take the form of a threaded nut, may be connected to a threaded upper end of the shank 78 to complete the assembly of the at least one spring snubber 76.

[0105] Fig. 12 illustrates a further alternative embodiment in which like reference numbers refer to like features shown in Fig. 10. This embodiment differs from that shown in Fig. 10 in that the at least one stiffness-varying element 74 is configured as a spring snubberassembly 104. As best shown in Fig. 13, the spring snubber assembly 104 includes a cylinder 106, a piston 108, and the pair of spring washers 92, 94. The cylinder 106 includes a cylinder body 110 having at least one side wall 112, a lower wall 114 connected to a lower end of the at least one side wall 112, and an upper wall 116 connected to an upper end of the at least one side wall 112 that collective define an interior chamber 118. Additionally, the upper end of the at least one side wall 112 may include an outwardly directed peripheral flange 120, the purpose of which will be described below.

[0106] The piston 108 includes a piston head 122 and a piston shaft 124 extending therefrom. The piston head 122 is slidably positioned in the interior chamber 118 of the cylinder 106 and the piston shaft 124 extends from a lower surface of the piston head 122 and through an opening 126 in the lower wall 114. In one embodiment, the piston shaft 124 may be integrally formed with the piston head 122. In an alternative embodiment, the piston head 122 may include a threaded bore that receives a threaded upper end of the piston shaft 124 to form, for example, a threaded connection. A lower end of the piston shaft 124 may include a shaft head 128 that is below and outside of the interior chamber 118 of the cylinder 106.

[0107] In one embodiment, the cylinder 106 may be configured to be connected to the intermediate axially extending member 72 and the piston 108 may be configured to be connected to the flexible beam 60. In this regard, in one embodiment, the shaft head 130 of the piston 108 may be configured to be connected to the flexible beam 60 by a mounting bolt 128. For example, the shaft head 128 may include a threaded bore that is configured to receive the threaded end of the mounting bolt 130 to make a threaded connection. In one embodiment, a lower surface of the shaft head 128 may engage the upper surface 98 of the flexible beam 60 and the piston shaft may extend upwardly therefrom. In one embodiment, the cylinder 106 may be connected to the intermediate axially extending member 72 through an adjustment mechanism 132. While the above described the cylinder 106 being connected to the intermediate axially extending member 72 and the piston 108 connected to the flexible beam 60, in an alternative embodiment (not shown), the cylinder 106 being connected to the flexible beam 60 and the piston 108 connected to the intermediate axially extending member 72.In one embodiment, the adjustment mechanism 132 may include a plurality of set screws 134 that extend through respective bores 136 in the peripheral flange 120 and are configured to engage with the intermediate axially extending member 72. Each of the plurality of set screws 134 may include a screw head 138, a threaded shaft 140 and an adjustable stop 142 disposed on the threaded shaft 140. In one embodiment, for example, the adjustable stop 142 may take the form of a threaded nut. In one embodiment, the threaded shafts 140 of the set screws 134 are configured to be received in respective threaded bores (not shown) in the upper surface 88 of the intermediate axially extending member 72.

[0108] As illustrated in the figures, the spring washers 92, 94 may be disposed on opposed sides of the piston head 122 and within the interior chamber 118 of the cylinder 106. More particularly, the upper spring washer 92 may be positioned between an upper surface 144 of the piston head 122 and a lower surface 146 of the upper wall 116, and the lower spring washer 92 may be positioned between a lower surface 148 of the piston head 122 and an upper surface 150 of the lower wall 114. One of ordinary skill in the art will understand the operation of the spring snubber assembly 104 based on, for example, the operation of the spring snubber 76 described above. Accordingly, a further description of the operation of the spring snubber assembly 104 will be omitted for sake of brevity.

[0109] In one embodiment, the adjustment mechanism 132 may be used to pre-tension the spring snubber assembly 104. In this regard, the position of the cylinder 106 relative to the intermediate axially extending member 72 may be selectively varied to provide the pre-tension in the system. For example, to lower the position of the cylinder 106 relative to the intermediate axially extending member 72, the set screws 134 may be rotated in a first direction to move the set screws 134 toward the intermediate axially extending member 72. Similarly, to raise the position of the cylinder 106 relative to the intermediate axially extending member 72, the set screws 134 may be rotated in a second direction to move the set screws 134 away from the intermediate axially extending member 72. The movement of the cylinder 106 relative to the intermediate axially extending member 72 causes one of the spring washers 92, 94 to flex and become activated to provide the pre-tension.Fig. 14 illustrates a further alternative embodiment in which like reference numbers refer to like features shown in Fig. 10. This embodiment differs from that shown in Fig. 10 in that the at least one stiffness-varying element 74 may be configured as a rubber damper 154. As shown in Fig. 15, the rubber damper 154 includes an outer casing 156 defined by at least one side wall 158, a lower plate 160 adjacent a lower end of the outer casing 156 and within the at least one side wall 158, and an upper plate 162 adjacent an upper end of the outer casing 156 and within the at least one side wall 158 to define an interior chamber 164. The rubber damper 154 further includes a rubber insert 166 disposed within the interior chamber 164. In one embodiment, the rubber insert 166 substantially fills the interior chamber 164 and is in engagement with the lower and upper plates 160, 162. In one embodiment, the rubber insert 166 may be formed from a rubber material that provides non-linear stiffness to the stator mounting module 70, i.e. , the stiffness of the stator mounting module 70 increases with increasing amplitude of the deflections of the stator 34. By way of example, and without limitation, the rubber insert 166 may be formed from polyurethane, neoprene, silicone-based elastomers, and viscoelastic rubber compounds, for example.

[0110] In one embodiment, the lower and upper plates 160, 162 may include threaded bores 168 that are configured to receive mounting bolts 170 to connect the rubber damper 154 to the intermediate axially extending member 72 and the flexible beam 60. In this regard, the intermediate axially extending member 72 and the flexible beam 60 may include respective bores 84, 86 for receiving the mounting bolts 170. In one embodiment, the bore 84 in the intermediate axially extending member 72 may include a counter bore portion 170 that receives a portion of the outer casing 156 of the rubber damper 154. In an alternative embodiment, as illustrated in Fig. 16, the lower and upper plates 160, 162 of the rubber damper 154 may include threaded shafts 172 extending outwardly from their respective plates 160, 162 which are received in the bores 84, 86 in the intermediate axially extending member 72 and the flexible beam 60, respectively. The threaded shafts 172 are configured to receive mounting nuts 174 to connect the rubber damper 154 to the intermediate axially extending member 72 and the flexible beam 60.

[0111] Fig. 17 illustrates a further alternative embodiment in which like reference numbers refer to like features shown in Fig. 10. This embodiment differs from that shown in Fig. 10 in that the at least one stiffness-varying element 74 may be configured as a rubber bumper176. As shown in this figure, the rubber bumper 176 may be positioned between the lower surface 90 of the intermediate axially extending member 72 and an upper surface 178 of the stator mounting member 62. By way of example, in one embodiment, the rubber bumper 176 may be bonded or otherwise connected to the upper surface 178 of the stator mounting member 62. In one embodiment, the rubber bumper 176 may substantially fill the entire space between the intermediate axially extending member 72 and the stator mounting member 62. In an alternative embodiment, the rubber bumper 176 may only partially fill the space between the intermediate axially extending member 72 and the stator mounting member 62. Similar to the above, the rubber bumper 176 may be formed from a rubber material that provides non-linear stiffness to the stator mounting module 70, i.e. , the stiffness of the stator mounting module 70 increases with increasing amplitude of the deflections of the stator 34. By way of example, and without limitation, the rubber bumper 176 may be formed from polyurethane, neoprene, silicone-based elastomers, and viscoelastic rubber compounds, for example. It should be appreciated that in alternative embodiments, the rubber bumper 176 may be used in combination with the stiffness-varying elements 74 discussed above.

[0112] While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.

Claims

Claims1. A generator (18) for a wind turbine (10), comprising:a generator housing (44) defining a frame (36);a stator (34) mounted within the frame (36) by at least one stator mounting module (70) connected to the stator (34) and connected to the frame (36),a rotor (32), the stator (34) being concentrically disposed about the rotor (32) with an air gap being defined between the rotor (32) and the stator (34), the air gap having a nominal diameter DAGNom, wherein the at least one stator mounting module (70) includes a pre-determined permitted maximum displacement Amax within the air gap, wherein the generator includes a Generator Displacement Factor (GDF) of less than 0.001 , the GDF being the ratio of the maximum allowable displacement Amax to the nominal air gap diameter DAGNom, andwherein the at least one stator mounting module (70) includes at least one stiffness-varying element (74) for providing the at least one stator mounting module (70) with non-linear stiffness.

2. The generator (18) according to claim 1 , wherein the at least one stiffness-varying element (74) provides hardening stiffness to the at least one stator mounting module (70).

3. The generator (18) according to claim 1 or 2, wherein the at least one stiffnessvarying element (74) is configured such that the stiffness of the at least one stator mounting module (74) is a maximum near a resonant frequency (cor) associated with the stator (34) and decreases at frequencies (co) away from the resonant frequency (cor).

4. The generator (18) according to any of the preceding claims, wherein the nonlinear stiffness of the at least one stator mounting module (70) is characterized by an effective stiffness keffthat varies with displacement of the at least one stator mounting module (70).

5. The generator (18) according to claim 4, wherein Amax is a fraction of the air gap for the generator (18).

6. The generator (18) according to claim 4 or 5, wherein a rate of change of keffwith displacement of the at least one stator mounting module (70) is constant.

7. The generator (18) according to claim 4 or 5, wherein a rate of change of keffwith displacement of the at least one stator mounting module (70) is non-constant.

8. The generator (18) according to claim 7, wherein the rate of change of keffincreases with increased displacement of the at least one stator mounting module (70).

9. The generator (18) according to claim 7 or 8, wherein the effective stiffness keffchanges by at least 3% over the permitted maximum displacement Amax.

10. The generator (18) according to claim 7 or 8, wherein the effective stiffness keffchanges by up to 5% over the permitted maximum displacement Amax.

11. The generator (18) according to any of the preceding claims, wherein the at least one stator mounting module (70) comprises:a flexible beam (60) having a first end connected to the frame (36) and a second end connected to the frame (36);a stator mounting member (62) connected to the flexible beam (60) and connected to the stator (34), the stator mounting member (62) being connected to the flexible beam (60) at a location between the first end and the second end of the flexible beam (60); andan intermediate member (72) connected to the frame (36) at a location between the first end and the second end of the flexible beam (60) and spaced from the flexible beam (60) to define a region between the intermediate member (72) and the flexible beam (60),wherein the at least one stiffness-varying element (74) is at least partially disposed in the region between the intermediate member (72) and the beam (60).

12. The generator (18) according to any of the preceding claims, wherein the at least one stiffness-varying element (74) includes a spring device (76, 104) having a plurality of spring washers (92, 94) for varying the stiffness of the at least one stator mountingmodule (70).

13. The generator (18) according to claim 12, wherein the spring device (76, 104) includes a spring snubber (76) comprising:a shank (78) having a first end and a second end;a first head (80) connected to the first end of the shank (78); anda second head (82) connected to the second end of the shank (78), wherein the plurality of spring washers (92, 94) is disposed between the first head (80) and the second head (82).

14. The generator (18) according to claim 13, wherein one of the first head (80) or the second head (82) is integrally formed with the shank (78) and the other of the first head (80) or the second head (82) is removably connected to the shank (78).

15. The generator (18) according to claim 13, wherein at least the second head (82) is formed with or permanently connected to the at least one stator mounting module (70).

16. The generator (18) according to claim 12, wherein the spring device (76, 104) includes a spring snubber assembly (104) comprising:a cylinder (106) defining a chamber (118);a piston (108) having a piston head (122) positioned within the chamber (118) and a piston shaft (124) extending from the piston head (122) and extending outside the chamber (118),wherein the plurality of spring washers (92, 94) is disposed in the chamber (118) on opposed sides of the piston head (122).

17. The generator (18) according to claim 16, wherein the spring snubber assembly (104) further comprises an adjustment mechanism (132) for pre-tensioning the spring snubber assembly (104).

18. The generator (18) according to any of claims 1-11, wherein the at least one stiffness-varying element (74) includes a rubber damper (154) having a rubber material for varying the stiffness of the at least one stator mounting module (70).

19. The generator (18) according to claim 18, wherein the rubber damper (154) comprises:an outer casing (156) having a first end and a second end;a first plate (160) adjacent the first end;a second plate (162) adjacent the second end; anda rubber insert (166) disposed within the outer casing (156) and between the first and second plates (160, 162).

20. The generator (18) according to any of the preceding claims, wherein the at least one stiffness-varying element (74) includes or further includes a rubber bumper (176) for varying the stiffness of the at least one stator mounting module (70).

21. The generator (18) according to claim 20, when dependent from claim 11, wherein the rubber bumper (176) is mounted to the stator mounting member (62) of the at least one stator mounting module (70).

22. The generator (18) according to any of the preceding claims, wherein the stator (34) is mounted within the frame (36) by a plurality of stator mounting modules (70).

23. A wind turbine (10), comprising:a tower (12);a nacelle (14) disposed on the tower (12); anda generator (18) according to any of claims 1-22 disposed in the nacelle (14).

24. A method of assembling a generator (18) including a generator housing (44) defining a frame (36) and a stator (34), comprising:providing a plurality of stator mounting modules (70); andmounting the stator (34) within the frame (36) of the generator housing (44) using the plurality of stator mounting modules (70),wherein each of the plurality of stator mounting modules (70) includes at least one stiffness-varying element (74) for providing the at least one stator mounting module (70) with non-linear stiffness.

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