GB2365931A

GB2365931A - Self-regulating wind turbine rotor

Info

Publication number: GB2365931A
Application number: GB0020147A
Authority: GB
Inventors: Michael Andrew Wastling; Robert James Cann; David Irving; John Charles Balson
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-08-17
Filing date: 2000-08-17
Publication date: 2002-02-27
Also published as: GB0020147D0

Abstract

A wind turbine rotor has coned blades 1 each mounted on a pitch axis offset from the centre of the hub 2 so that they tend to pitching to stall due to centrifugal forces. This tendency is reduced by the generator load. The change in pitch of the blades is linked by an annulus 6 connected to each blade by a pitch arm 5. A resilient member provides a spring bias towards a pre-set blade pitch position. The spring bias may be achieved by the deformation of a circular diaphragm 7 attached to the annulus 6 and fixed axially on the main shaft 4. The action of the wind on the surface of this diaphragm will maintain the blades in a stall position in high winds and the axial movement of the diaphragm provides a damping force to control rapid changes in blade pitch.

Description

<Desc/Clms Page number 1> Self- Regulating Wind Turbine Rotor Wind turbines are well known devices used to extract energy from the wind. Most commonly these are horizontal axis machines with one or more blades. A problem inherent with such machines is the need to regulate the power extracted from the wind to avoid excessive rotor speed or overload of the generator in high winds. Wind Turbines commonly employ a means of power regulation based on altering the pitch of the blades in order to reduce the power generated. This is commonly achieved using electrical or hydraulic actuators controlled by speed sensors to alter blade pitch.

Some designs make use of the torque reaction of the rotor hub relative to the main shaft to change blade pitch. This can be achieved by allowing relative rotation of the rotor hub on its shaft to alter blade pitch through mechanical linkages.

Other known means of achieving variable blade pitch make use of the forces generated by the rotational movement of the blade mass. Typically the radial centrifugal force due to the blades' rotational motion is used to operate a mechanism to alter blade pitch.

If the centre of mass of the blade is arranged so that it is offset from an axis about which the blade is allowed to rotate then the blade will experience a moment about this pitch axis which may be balanced against a biasing force to control blade pitch. One way to achieve the mass offset required to generate the pitching moment is to mount the blades at an angle (cone angle) to the plane of rotation and to offset the pitch axis of the blades (which lies in the plane of rotation) relative to the axis of rotation.

According to the present invention a wind turbine rotor is arranged so that its blades are mounted on a rotating hub which turns about a horizontal axis.

Each blade of the wind turbine is mounted on the rotating hub such that the blade's main axis is orientated at an angle relative to the plane of rotation of the hub about its axis. Each blade may rotate about its pitch axis which lies in the plane of rotation of the hub and is parallel to and displaced a distance from a radial line which lies in the plane of rotation of the hub and passes through the centre of rotation of the hub.

The resultant centrifugal force acting on the blade mass causes the blade to experience a pitch moment about its pitch axis.

Each blade is mounted in such a way that this pitch moment will cause it to rotate about its pitch axis and it is restrained in this rotation by means of a resilient member such that it encounters a resistance proportional to the degree of rotation in doing so.

The direction of rotation is so as to pitch the blade towards the direction of stall and therefore reduce the aerodynamic power produced by the turbine. This is the mechanism that is used to prevent the wind turbine from rotating too quickly.

The angular offset of the blade from its pitch axis means that the aerodynamic loading on the blades acting to turn the rotor also makes an additional contribution to the pitching moment. In this case the direction of rotation is to pitch the blades away from stall and therefore increase aerodynamic power. Provided the rotational inertia of the generator is

low compared to that of the blades and hub, this contribution to blade pitch moment can be significant only if the generator is providing a reaction torque. This is the mechanism that is used to ensure that the rotor can operate efficiently only when the generator is connected to a load. This provides additional protection against over speeding whenever there is a generator loss of load.

Preferably the pitch angle of all the blades should be identical. This may be achieved by means of rigid members one end of which are connected rigidly to the blade such that they rotate with changes in pitch of the blade. The other ends of these pitch arms are connected by means of a pinned joint to a rigid annulus which is allowed to rotate about the rotor axis. The in-plane component of the arc described by the end of each pitch arm results in rotation of the annulus thereby restraining each blade to identical pitch angle and change of pitch.

Additionally the axial component of the arc described by the end of each pitch arm displaces the annulus axially with change in blade pitch.

Preferably the annulus is restrained axially by means of a bias afforded by a resilient member arranged to offer a resistance proportional to the degree of rotation of the blade in pitch.

The axial movement of the annulus may be limited in one direction by means of fixed stops in order to give a set initial blade pitch angle. This also allows a pre-load to be applied to the resilient member so that the blade pitch moment must reach a set level before the blade begins to change pitch.

The resilient biasing member may comprise a circular diaphragm which can be elastically deformed in an out of plane direction such that axial movement away from its equilibrium position results in a resistance proportional to its deflection. The circular diaphragm is constrained axially and radially but is allowed to rotate about the rotor axis. A preload may be introduced in the diaphragm spring by restraining the blades from pitching past a certain point using fixed stops and by securing the centre portion of the diaphragm at a position axially removed from its undeformed position.

The rigid annulus may be connected to a surface so disposed with relation to the wind direction that due to its significant aerodynamic resistance to the wind it exerts an axial force on the annulus which increases with the wind speed.

The force exerted by this surface acts to move the rigid annulus and hence the ends of the pitch arms axially thereby causing the blades to pitch towards a stall position once the pre-load in the diaphragm is overcome by the combination of blade pitch moment due to centrifugal forces and the aerodynamic force on this surface. This will tend to maintain the blades in stall in extreme wind conditions and result in lower rotational speeds.

The axial movement of the surface which results from a change in pitch of the blades due to rotation about their pitch axes is also resisted by a force proportional to the speed of axial deflection due to the air resistance of the surface thereby providing a damping force to oppose changes in blade pitch. This damping force will increase in proportion to the rate of change of blade pitch thereby resisting rapid changes in blade pitch.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 Shows the geometry of the blade attachment and pitch axis.

Figure 2 Is a view on the main axis of the turbine rotor showing the arrangement of the blades with the pitch control components removed for clarity.

Figure 3 Is a side elevation of the turbine rotor showing the arrangement of one blade attached to the hub and with the pitch control components and other blades removed for clarity.

Figure 4 Is a view on the upwind root area of the blade showing the attachment of the outer blade pitch bearing and pitch arm and the blade end pitch pin.

Figure 5 Is an isometric view of the turbine rotor from a downwind direction showing the interconnection of the blades' pitch arms by the rigid annulus.

Figure 6 Is an isometric view of the turbine rotor from an upwind direction showing the rigid annulus located on the main shaft by the diaphragm spring member.

Figure 7 Is a side elevation of the turbine rotor showing the three blades connected to the rigid annulus and diaphragm spring damper.

Referring to Figure 1 the wind turbine is arranged so that its blades are mounted on a rotating hub which turns in direction R relative to a wind direction indicated by arrow U. Each blade of the wind turbine is mounted on the rotating hub such that the blade's main axis (B-B) is orientated at an angle (A1) relative to the plane of rotation of the hub about axis (Z-Z).

The blade may rotate about its pitch axis (X-X) which lies in the plane of rotation of the hub and is parallel to and displaced a distance (y) from a radial line (C-C) which lies in the plane of rotation of the hub and passes through the centre of rotation of the hub (O).

The resultant centrifugal force (Fc) acting on the blade mass (m) causes the blade to experience a pitch moment (M1) about its pitch axis (X-X).

The blade is mounted in such a way that pitch moment (M1) will cause it to rotate about its pitch axis (X-X) and it is restrained in this rotation by means of a resilient member (D) such that it encounters a resistance proportional to the degree of rotation in doing so. The direction of rotation is so as to pitch the blade towards the direction of stall and therefore reduce the aerodynamic power produced by the turbine. This is the mechanism that is used to prevent the wind turbine from rotating too quickly.

The angular offset of the blade from its pitch axis (A1) means that the aerodynamic loading on the blades acting to turn the rotor also makes an additional contribution to the pitching moment. In this case the direction of rotation is to pitch the blades away from stall and therefore increase aerodynamic power. Provided the rotational inertia of the generator is low compared to that of the blades and hub, this contribution to blade pitch moment can be significant only if the generator is providing a reaction torque. This is the mechanism that is used to ensure that the rotor can operate efficiently only when the

generator is connected to a load. This provides additional protection against over- speeding whenever there is a generator loss of load.

Figure 2 is a view in the direction of the main axis of the turbine and shows this principle applied to a turbine rotor in which three blades (1) are attached to a rotating hub (2) at an offset (y) from the centreline.

Figure 3 is a view on the side of the hub and shows how a typical blade (1) is mounted on a hub (2) with a cone angle (A1) between the blade centreline and the blade's pitch axis. The blade is located by a blade-end pin (10) and bearing assembly (11) and an outer blade attachment (8) and pin (9) with bearing assembly (12). This arrangement allows the blade to rotate about its pitch axis. Note that only one blade is shown and the pitch control mechanism has been removed for clarity.

Figure 4 shows the location of the blade outer attachment (8) & (9) and pitch arm (5) which are rigidly attached to the upwind surface of the blade (1). It also the blade end pin (10) which is coaxial with the pitch axis and the outer blade attachment pin (9).

A means of interconnecting the blades is shown in Figure 5. The three blades (1), (drawn with dotted lines for clarity), are connected via their respective pitch arms (5) to a rigid annulus (6). Rotation of the blades (1) about their pitch axes causes both rotation and axial deflection of the rigid annulus (6) and thus each blade (1) is restrained to rotate about its pitch axis by an equal amount.

Figure 6 shows the turbine rotor from the upwind direction with the blades (1) connected to the rigid annulus (6) by pitch arms (5). The rigid annulus (6) is attached to a diaphragm (7) which is located by its centre portion on the shaft (4), such that it is restrained in the radial and axial directions but free to rotate around the main axis of the rotor.

As the diaphragm (7) is restrained axially at its centre it is forced to deflect as the outer annular rim (6) moves axially and thereby behaves in such a way as to offer a biasing force to oppose this movement. The diaphragm (7) is positioned axially such that it acts as a biasing force to rotate the blades (1) about their pitch axes until they contact a fixed stop positioned to maintain them at a preselected pitch angle.

Figure 7 shows a side elevation of the turbine rotor with the diaphragm (7) deflected to give a pre-loaded spring bias to hold the blades (1) against the fixed stops until the pitch moment is sufficient to cause change in blade pitch.

As the rotational speed of the rotor increases the pitch moment experienced by the blades increases proportionally until at a chosen speed the pitch moment becomes greater than the biasing force provided by the diaphragm spring (7) and the blades will change pitch in a uniform manner. This change in pitch puts the blades into stall relative to the wind direction and reduces the power produced by the rotor.

When the rotation of the rotor is opposed by the presence of a load such as an electrical generator or similar there is an increased tendency for the blades to remain at their pre- selected pitch-to-run position as determined by the rigid stop. If this moment about the blade pitch axis due to the reaction torque of the load is lost, as occurs due to the loss of generator load, the tendency of the blade to rotate around its pitch axis towards a stall position and thus reduce power is increased.

The pitch control diaphragm (7) is also arranged such that it will be acted upon by the full force of the prevailing wind. Careful sizing of this surface allows a predictable additional axial force to be applied to the annular outer rim (6) which is connected to the pitch arms which control blade pitch. Thus in a strong wind the aerodynamic resistance of this surface will automatically move the blades to a stall position.

The axial movement of the surface in conjunction with the change in pitch of the blade allows the surface to contribute a damping force proportional to the rate of change of pitch angle. This damping force is derived from the displacement of air on either side of the moving surface and contributes significantly to reducing blade pitch instability.

Claims

CLAIMS 1 In a wind turbine with one or more blades which are arranged so as to have a tendency to pitch to stall under a combination of centrifugal forces and torque reaction due to a load, the blades are linked such that they rotate about their pitch axes uniformly relative to each other by means of members connected at one end to the blade such that they rotate with changes in pitch of the blade and to a rigid annulus at the other end which is constrained radially but is allowed to rotate about the rotor axis thus ensuring equal pitch change in all the blades. 2 A wind turbine as claimed in Claim 1 wherein the annulus linking the blades' pitch arms is constrained axially by a resilient member which provides a biasing force such that axial movement away from its equilibrium position results in a resistance due to its deflection which acts on the axial component of the arc described by the end of the pitch arm connected to the annulus in order to oppose rotation of the blade about its pitch axis. 3 A wind turbine as claimed in Claim 1 and Claim 2 wherein the use of fixed stops to set the initial pitch angle of the blades applies a pre-load to the resilient member to maintain the blades at nominal pitch angle until a selected pitch moment is achieved. 4 A wind turbine as claimed in Claim 1 or Claim 2 or Claim 3 wherein the annulus linking the blades' pitch arms is connected to a circular diaphragm which can be elastically deformed in an out of plane direction such that axial movement away from its equilibrium position results in a resistance proportional to its deflection which acts on the axial component of the arc described by the end of the pitch arm connected to the annulus in order to oppose rotation of the blade about its pitch axis. 5 A wind turbine as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 wherein the diaphragm spring as described in Claim 4 is subjected to a pre-load by restraining the blades from pitching past a certain point using fixed stops and by securing the centre portion of the diaphragm axially removed from its un-deformed position. 6 A wind turbine as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 wherein the annulus is connected to a surface so disposed with relation to the wind direction that due to its significant aerodynamic resistance it exerts an axial force on the annulus which varies with the wind speed. 7 A wind turbine as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 wherein the force exerted by the surface described in Claim 6 acts to move the ends of the pitch arms axially thereby causing the blades to pitch. 8 A wind turbine as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 wherein the axial movement of the surface described in Claim 6 and Claim 7 which results from a change in pitch of the blades due to rotation about their pitch axes is resisted by a force proportional to the speed of axial deflection due to the air resistance of the surface thereby providing a damping force to oppose rapid changes in blade pitch.