GB2525143A

GB2525143A - A generator

Info

Publication number: GB2525143A
Application number: GB1400141.6A
Authority: GB
Inventors: Giles Henry Rodway
Original assignee: Spinetic Energy Ltd
Current assignee: Spinetic Energy Ltd
Priority date: 2014-01-06
Filing date: 2014-01-06
Publication date: 2015-10-21
Anticipated expiration: 2034-01-06
Also published as: GB2525143B; GB201400141D0

Abstract

A generator comprising a stator 14, a rotor 2 configured to rotate relative to the stator, wherein the rotor comprises a plurality of radially extending vanes 12 which are arranged to direct air radially outwards as the rotor rotates, thereby cooling the generator. The vanes may be curved or straight along their radial length and the thickness and height may increase along their radial length. Preferably the rotor or stator is provided with vents towards the centre and preferably the rotor or stator comprises a plurality of coils 20, and the other of the stator or the rotor comprises a plurality of magnets 10a, 10b. Also, the coils may have an opening at their centre, the vanes may be offset or aligned with the magnets, the magnets may be Neodymium magnets and the generator may be a radial flux generator.

Description

A GENERATOR

The invention relates to a generator and particularly, but not exclusively, to a generator for a wind turbine.

Wind turbines are a common means of generating renewable energy. Wind turbines convert the mechanical energy of wind into electrical energy using a suitable generator.

The electrical energy can then either be stored locally or fed into an electrical grid.

Generators generally comprise a stator and a rotor which is configured to rotate relative to the stator. The stator is provided with a plurality of coils, whereas the rotor is provided with a plurality of magnets. The coils of the stator and the magnets of the rotor are in close proximity to one another such that the magnetic flux lines emanating from each of the magnets pass through the electrically conductive wires of the coils, thus generating a voltage within the wires.

Wind turbines are able to generate electricity at a wide range of wind speeds.

However, if the wind speed exceeds a maximum limit the wind turbine may be damaged. It is therefore necessary to implement control features to reduce or limit the speed of the turbine. Many different approaches may be taken to achieve this. For example, the angle of attack of the turbine blades may be increased so as to stall the blades or may be decreased such that the leading edges of the blades face directly into the wind. Mechanical or electrical braking methods are also widely used.

Electrical braking is particularly suitable for small-scale wind turbines. Typically, the coils are short-circuited which creates an electrical load and thus a braking torque, thereby reducing the rotational speed of the rotor. However, this increases the temperature of the stator coils as a result of Ohmic heating. The heat may be transferred from the coils of the stator to the rest of the generator, particularly the magnets of the rotor.

The magnets used within generators are often formed of Neodymium. Although such magnets are extremely strong, they are susceptible to heat and can lose their magnetism at as low as 80-100°C. Accordingly, if the temperature within the generator goes beyond this limit, the generator can become damaged and this can negatively affect the power output of the wind turbine.

It is therefore desired to provide a generator which overcomes this issue.

Therefore, in accordance with an aspect of the invention there is provided a generator comprising: a stator; a rotor configured to rotate relative to the stator; wherein the rotor comprises a plurality of radially extending vanes which are arranged to direct air radially outwards as the rotor rotates, thereby cooling the generator.

The vanes provide cooling air to the magnets and/or coils. This may be particularly beneficial where the coils are shorted to create a braking torque, thus increasing the temperature of the coils. Such heating can cause damage to the magnets. Further, by reducing the temperature of the coils, it may be possible to run the generator at higher speeds.

Each of the vanes may be curved along its radial length. The vanes are preferably back-curved but may also be forward-curved.

Each of the vanes may be straight along its radial length.

The thickness of each vane may increase along its radial length.

The axial height of each vane may increase along its radial length.

The stator may be provided with one or more vents. The vents may allow air to be drawn into the generator.

The vents may be positioned toward the centre of the stator. This may assist with the circulation of air through the generator.

One of the stator or the rotor may comprise a plurality of coils, and the other of the stator or the rotor may comprise a plurality of magnets.

The coils may each have an opening at their centre. This may allow air to pass therethrough, while also increasing the surface area of the coils exposed to the air.

The vanes may be offset from the magnets. This may cause air to be directed between the magnets thereby cooling the coils.

Each of the vanes may be aligned with one of the magnets. This may provide improved cooling of the magnets themselves.

The magnets may be Neodymium magnets. Such magnets are particularly susceptible to heat and thus the cooling provided by the vanes is particularly advantageous.

The generator may be a radial flux generator.

The magnets may be located on radially inner and outer arms which are arranged as a pair of concentric rings.

The inner and outer arms may be planar elements. This allows the magnets to also be planar which may reduce manufacturing costs.

Each of the vanes may be connected to one of the inner arms. The vanes may thus provide a structural reinforcement to the inner arms which prevents them from bending under centrifugal loading.

Each of the vanes may be connected to an upstream side of one of the inner arms.

The air flow may therefore be directed towards a tangential direction to improve circulation of the air around the magnets and coils.

The thickness of the vanes at their radially outer ends may correspond to that of the inner arms. Accordingly, the air flow may be channelled between the vanes.

The height of the vanes at their radially outer ends may correspond to that of the inner arms. Air is therefore directed over and around the arms.

The outer arms may form an annular ring. The annular ring may cause air to be directed across the diameter of the coils. The annular ring may also prevent the ingress of rain and debris into the generator.

The vanes and/or arms may be integrally formed with the rotor.

The coils may be mounted on an annular ring.

The generator may be an axial flux generator.

The axial height of the vanes may be equal to of less than the axial height of the magnets. Accordingly, the vanes sit below the level of the magnets and thus do not impede the proper functioning of the generator.

In accordance with another aspect of the invention, there is provided a wind turbine comprising a generator as described above.

The wind turbine may be a vertical axis wind turbine.

For a better understanding of the present disclosure, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-Figure 1 is a perspective view of a rotor of a radial flux generator according to an embodiment of the invention; Figure 2 is a perspective view of a stator of the radial flux generator; Figure 3 is an exploded view of the radial flux generator comprising the rotor and stator of Figures 1 and 2; Figure 4 is a side view of the assembled radial flux generator of Figure 3; Figure 5 is a perspective view of a rotor of a radial flux generator according to another embodiment of the invention; Figure 6 is a perspective view of a rotor of a radial flux generator according to another embodiment of the invention; Figure 7 is a perspective view of a rotor of a radial flux generator according to another embodiment of the invention; Figure 8 is a perspective view of an assembled radial flux generator comprising the rotor of Figure 7; Figure 9 is a perspective view of a rotor of an axial flux generator according to another embodiment of the invention; Figure 10 is a perspective view of a stator of the axial flux generator; and Figure 11 is a side view of the assembled axial flux generator comprising the rotor and stator of Figures 9 and 10.

Figure 1 shows a rotor 2 of a radial flux generator according to an embodiment of the invention.

The rotor 2 is formed by a disc 4 which is mounted for rotation about a central axis 6.

The disc 4 is provided with an opening 7 at its centre which is configured to receive an output shaft of a prime mover, such as a rotor of a wind turbine.

A plurality of pairs of arms 8 project perpendicularly from the surface of the disc 4.

Each pair of arms B comprises an inner arm Ba and an outer arm 8b which are aligned with, but spaced from one another along a radial direction. The pairs of arms 8 are spaced equally around the circumference of the disc 4. The arms 8 thus form a pair of concentric rings comprising an inner ring formed by the inner arms 8a and an outer ring formed by the outer arms 8b. However, for ease of manufacture, the inner and outer arms 8a, 8b may be formed by substantially planar elements which thus lie tangential to these concentric rings.

Each of the inner arms 8a is provided with an inner magnet ba which is affixed to an outer radial surface of the inner arm Ba. Similarly, each of the outer arms 8b is provided with an outer magnet lOb which is affixed to an inner radial surface of the outer arm 8b. The inner and outer magnets bOa, lOb are thus affixed to opposing radial surfaces of the inner and outer arms Ba, 8b such that they face one another.

The inner and outer magnets ba, lOb are, however, spaced from one another by a gap.

The inner and outer magnets ba, lOb are arranged so that opposite poles of the magnets ba, lOb oppose one another. Accordingly, magnetic flux passes radially between the inner and outer magnets ba, lOb.

A plurality of vanes 12 are provided on the surface of the disc 4. The vanes 12 project perpendicularly from the surface of the disc 4 and are oriented along the radial direction. The vanes 12 are located between the opening 7 and the inner arms 8a.

Specifically, the vanes 12 extend from a position which is adjacent to the opening 7 to a position which is adjacent to the inner arms 8a.

As shown in Figure 1, the vanes 12 are offset from the inner arms 8a such that each vane 12 lies between an adjacent pair of inner arms 8a. Over a radially outer portion of each vane 12, the axial extent of the vane 12 is substantially constant, whereas over a radially inner portion, the vane 12 tapers such that the axial extent reduces towards the centre of the disc 4.

The vanes 12 may act as stiffening ribs which strengthen the disc 4 of the rotor 2 and withstand axial deformation of the disc 4. This may allow the disc 4 to be made less substantial such that the introduction of the vanes 12 does not increase the weight of the rotor 2 and may even allow the weight of the rotor 2 to be reduced.

Figure 2 shows a stator 14 of the radial flux generator.

The stator 14 comprises a disc 16 and an annular ring 18 which is affixed to or integrally formed with the disc 16. The annular ring 18 extends perpendicularly from a surface of the disc 16. The annular ring 18 has a diameter which is greater than that of the concentric ring formed by the inner arms 8a and smaller than that of the concentric ring formed by the outer arms 8b. Consequently, the annular ring 18 of the stator 14 can be located in between the pairs of arms 8 of the rotor 2.

A plurality of coils 20 are spaced circumferentially around the annular ring 18. The spacing of the coils 20 about the annular ring 18 corresponds substantially to the spacing of the inner and outer magnets iDa, lOb of the rotor 2. The coils 20 are located within openings in the annular ring 18. The coils are arranged such that the axis of each coil 20 is oriented in a substantially radial direction. An opening 22 extends through the centre of each coil 20 and through the annular ring 18.

A plurality of vents 24 are provided through the disc 16 of the stator 14. As shown in Figure 2, the vents 24 are positioned toward the centre of the disc 16.

As shown in Figures 3 and 4, the rotor 2 is received over the stator 14 such that the annular ring 18 and its coils 20 are positioned in between the inner and outer arms 8a, 8b and their respective inner and outer magnets ba, lOb. The rotor 2 and stator 14 are arranged such that the centres of the inner magnets ba, the outer magnets lOb and the coils 20 lie in the same plane and thus are radially aligned with one another. In this position, the inner and outer arms 8a, Sb are spaced from the disc 16 of the stator 14, and the annular ring 18 of the stator 14 is spaced from the disc 4 of the rotor 2.

Accordingly, the rotor 2 is able to rotate freely with respect to the stator 14.

In use, the rotor 2 is rotated relative to the stator 14 by a prime mover, such as a rotor of a wind turbine. Accordingly, the coils 20 cut through magnetic flux passing between the inner and outer magnets ba, lOb. This induces a current within the coils 20 that can either be fed to the grid or stored locally.

As the rotor rotates, the vanes 12 of the rotor 2 force air radially outwards toward the inner and outer magnets ba, lOb and the coils 20. This draws air into the generator from the surrounding atmosphere via the vents 24 of the stator 14, and forces air from within the generator out to the surrounding atmosphere via the openings 22 in the coils 20. This circulating flow of air acts to cool the components of the generator, particularly the inner magnets iDa, the outer magnets lOb and the coils 20. This may be particularly beneficial during an electrical braking process where the coils 20 are short-circuited, which causes their temperature to increase as a result of Ohmic heating.

Figure 5 shows a rotor 102 of a radial flux generator according to another embodiment of the invention. The rotor 102 may be assembled with the stator 14, as described previously. As per the rotor 2, the rotor 102 comprises a disc 4 and a plurality of pairs of arms 8, comprising an inner arm 8a and an outer arm Sb, which project perpendicularly from the surface of the disc 4. A plurality of vanes 112 are also provided on the surface of the disc 4, however, the vanes 112 of the rotor 102 differ from the vanes 12 of the rotor 2.

B

Specifically, the vanes 112 are connected at their radially outer ends to the inner arms Ba. The vanes 112 are therefore aligned with the inner arms 8a, rather than being offset therefrom. However, it may also be possible for the vanes 112 to connect to adjacent pairs of inner arms Ba such that the vanes 112 are again offset from the inner arms Ba. This may be particularly beneficial where it is desired to cool the magnets iDa, lOb.

As shown in Figure 5, the vanes 112 increase in thickness from the radially inner end to the radially outer end, where they have a thickness which is approximately equal to the circumferential width of the inner arms Ba. As shown in Figure 5, the vanes 112 taper uniformly along their radial length, however, this need not be the case. In certain circumstances, it may be desirable for the vanes 112 to increase in thickness only toward their radially outer ends. This may create a more tangential circulation of air which may provide more effective cooling of the inner and outer magnets iDa, lOb.

The vanes 112 could also be simple planar elements (as with the vanes 12) with a uniform thickness along the radial length. Optionally, the air flow may be provided with a tangential component by curved elements located on the radially inner surface of the inner arms Ba.

As described previously with respect to the rotor 2, as the rotor 112 rotates, the vanes 112 force air radially outwards toward the inner and outer magnets ba, lOb and the coils 20, thus generating a circulating flow of air which acts to cool the components of the generator, particularly the inner magnets iDa, the outer magnets lOb and the coils 20.

As the rotor 102 rotates, a centrifugal force is exerted upon the inner arms Ba. The vanes 112 provide a structural reinforcement for the inner arms 8a, preventing them from bending outwards. The vanes 112 therefore ensure that the distance between the radially outer surfaces of the inner arms Ba and the radially inner surface of the annular ring 18 is uniform over all operating speeds. The reinforcement provided by the vanes 112 thus improves the efficiency of the generator and prevents contact between the rotor 102 and stator 14.

Figure 6 shows a rotor 202 of a radial flux generator according to another embodiment of the invention. The rotor 102 may be assembled with the stator 14, as described previously. As per the rotors 2, 102, the rotor 202 comprises a disc 4 and a plurality of pairs of arms 8, comprising an inner arm 8a and an outer arm 8b, which project perpendicularly from the surface of the disc 4. A plurality of vanes 212 are also provided on the surface of the disc 4, however, the vanes 212 of the rotor 202 differ from the vanes 12, 112 of the rotors 2, 102.

Like the vanes 112, the vanes 212 are connected at their radially outer ends to the inner arms Ba. However, the vanes 212 are curved along their radial length rather than being straight. Specifically, the vanes 212 are backward-curved and thus curve against the direction of rotation of the rotor 202 and connect to an upstream edge of the inner arms 8a. The vanes 212 may, however, be forward-curved and thus curve in the direction of rotation of the rotor 202.

In use, the vanes 212 and inner arms Ba thus direct air from a radial direction at the centre of the rotor 2 to towards a tangential direction at the inner arms Ba. This may be beneficial in circulating the flow of air around the inner and outer magnets ba, lOb.

As described previously, the vanes 212 connect to the inner arms Ba, thus providing a structural reinforcement to the inner arms Ba. The vanes 212 may, however, be discrete elements which are spaced from the inner arms Ba where such reinforcement is not required.

Figure 7 shows a rotor 302 of a radial flux generator according to another embodiment of the invention. The rotor 302 may be assembled with the stator 14, as described previously. As per the rotors 2, 102, 202, the rotor 302 comprises a disc 4 and a plurality of pairs of arms 8, comprising an inner arm 8a and an outer arm Bb, which project perpendicularly from the surface of the disc 4. A plurality of vanes 312 are also provided on the surface of the disc 4. Although the vanes 312 are shown in Figure 7 as being arranged in the same manner as the vanes 12, they may instead resemble the vanes 112, 212 of the rotors 102, 202 described previously.

In the rotor 302, adjacent outer arms Bb are connected via connecting pieces 326 which extend perpendicularly from the surface of the disc 4 to the same axial height as the outer arms Bb (although this need not be the case). The outer arms 8b and connecting pieces 326 thus form a continuous ring. The outer arms 8b and connecting pieces 326 may be integrally formed with one another, or the outer arms Sb may be embedded in a continuous ring.

Figure 8 shows an assembled generator which comprises the rotor 302 and the stator 24. VVlien assembled, the outer arms Sb and connecting pieces 326 are spaced from the disc 16 of the stator 14, and the annular ring 18 of the stator 14 is spaced from the disc 4 of the rotor 302. The rotor 302 is therefore able to rotate freely with respect to the stator 14. Moreover, the gap between the outer arms Sb and connecting pieces 326 and the disc 16 of the stator 14 forms a ventilation gap through which air exiting the generator can pass.

As the rotor rotates, the vanes 312 of the rotor 302 force air radially outwards toward the inner and outer magnets ba, lOb and the coils 20. The air passes over the inner magnets ba and through the openings 22 in the coils 20 or between the annular ring 18 of the stator 12 and the disc 4 of the rotor 302.

The continuous ring formed by the outer arms Sb and the connecting pieces 326 causes the air received through the openings 22 in the coils 20 or between the annular ring 18 of the stator and the disc 4 of the rotor 302 to flow axially across the diameter of the coils 20. This improves the cooling of the coils 20.

The connecting pieces 326 may also provide improved structural support for the outer arms 8b and thus avoid them bending under centrifugal loading. Further, the continuous ring formed by the outer arms Sb and the connecting pieces 326 prevents ingress of rain and debris into the generator. This may be particularly effective when the rotor 302 is arranged above the stator 14, as may be employed with a vertical-axis wind turbine. During rotation, the flow of air passing through the ventilation gap between the rotor 302 and stator 14 may also blow rain and debris away from the generator.

Figure 9 shows a first part of a rotor 402 of an axial flux generator according to another embodiment of the invention.

The first part of the rotor 402 is formed by a disc 404a which is mounted for rotation about a central axis 406. The disc 404a is provided with an opening 407 at its centre which is configured to receive an output shaft of a prime mover, such as a rotor of a wind turbine.

A plurality of magnets 410a are affixed to the surface of the disc 404a. The magnets 410a are positioned on the surface of the disc 404a in a circle, with the magnets 410a being spaced evenly from one another. As with the rotors described previously, a plurality of vanes 412 are provided on the surface of the disc 404a. The vanes 412 project perpendicularly from the surface of the disc 404 and extend in a radial direction.

The vanes 412 are located between the opening 407 and the magnets 410a.

Specifically, the vanes 412 extend from a position which is adjacent to the opening 407 to a position which is adjacent to the magnets 410a. As per the vanes 212 of the rotor 202, the vanes 412 are curved along their radial length. Specifically, the vanes 412 are backward-curved and thus curve against the direction of rotation of the rotor 402. In other embodiments, the vanes 412 may, however, be forward-curved or planar. The vanes 412 are preferably arranged so that they are directed towards the magnets 410a.

As will be described in detail below, as a result of the construction of the axial flux generator, the vanes 412 are low-profile in that they have a relatively small axial height and thus do not project a great distance from the surface of the disc 404a. The vanes 412 may have an axial height which substantially corresponds to that of the magnets 410a.

The vanes 412 may act as stiffening ribs which strengthen the disc 404a of the rotor 402 and withstand axial deformation of the disc 404a. This may allow the disc 404a to be made less substantial such that the introduction of the vanes 412 does not increase the weight of the rotor 402 and may even allow the weight of the rotor 402 to be reduced.

Figure 10 shows a stator 414 of the axial flux generator.

The stator 414 is formed by a disc 416. A plurality of coils 420 are embedded within the disc 416. The coils 420 are positioned in a circle, with the magnets 420 being spaced evenly from one another. The arrangement of the coils 420 corresponds substantially to that of the magnets 410a. The coils 420 are located within openings in the disc 416. The coils are arranged such that the axis of each coil 420 is oriented in a substantially axial direction. An opening 422 extends through the centre of each coil 420 and through the disc 416.

A plurality of vents 424 are provided through the disc 416 of the stator 414. As shown in Figure 2, the vents 424 are positioned toward the centre of the disc 416.

As shown in Figure 11, the rotor is formed by a pair of opposing first and second discs 404a, 404b which are connected to one another. The second disc 404b is provided with a plurality of magnets 410b, as described with respect to the first disc 404a. The first and second discs 404a, 404b are arranged such that the magnets 410a, 410b oppose one another. In particular, the magnets 410a of the first disc 404a and the magnets 410b of the second disc 404b are arranged so that opposite poles of the magnets 410a, 410b oppose one another. Accordingly, magnetic flux passes axially between the magnets 410a, 410b.

The magnets 410a of the first disc 404a and the magnets 410b of the second disc 404b are spaced from one another such that the stator 414 can be received therebetween, with the magnets 410a, 41Db and coils 420 all in axial alignment. It is desirable to minimise the distance between the magnets 410a, 410b and the coils 420 and the low-profile nature of the vanes 412 enables this.

In use, the rotor 402 is rotated relative to the stator 414 by a prime mover, such as a rotor of a wind turbine. Accordingly, the coils 420 cut through magnetic flux passing between the opposing magnets 410a, 410b. This induces a current within the coils 420 that can either be fed to the grid or stored locally.

As the rotor rotates, the vanes 412 of the rotor 402 force air radially outwards toward the magnets 410a, 410b and the coils 420. Air may pass axially across the stator 414 via the vents 424. The air flow acts to cool the components of the generator, particularly the magnets 410a, 41Db and the coils 420. This may be particularly beneficial during an electrical braking process where the coils 420 are short-circuited, which causes their temperature to increase as a result of Ohmic heating.

In the preceding embodiments, although the rotor has been described as having magnets and the stator has been described as having coils, this arrangement could of course be reversed, if necessary. It is, however, beneficial for the coils to remain stationary for ease of wiring. The magnets described previously may be Neodymium magnets or other types of magnets, which may include electromagnets.

In the preceding embodiments, the number of vanes corresponds to the number of magnets. However, it will be appreciated that this need not be the case and greater or fewer vanes may be provided.

Where it is described that the vanes extend in a radial direction, it will be appreciated that the vanes need only extend in a direction having a radial component. The vanes may therefore be curved, as described, or angled from a purely radial direction. The vanes are preferably integrally formed with the disc, but may also be discrete elements which are affixed to the disc. Similarly, where it is described that the vanes connect to the inner arms, it will be appreciated that the vanes and arms may be integrally formed with one another. Further, the arms may be formed by planar elements so as to allow planar magnets to be used, thus reducing cost.

Although the stator has been described as being provided with vents, it will be appreciated that the rotor may also comprise vents either in addition to or instead of those provided in the stator. Further, any number of vents may be used and arranged as desired over the surface of the rotor and/or stator. The vents may also be any appropriate shape. It is, however, beneficial for the vents to be located over only an inner portion of the generator to improve the circulation of air in the radial direction.

The invention is not limited to the embodiments described herein, and may be modified or adapted without departing from the scope of the present invention.

Claims

CLAIMS1. A generator comprising: a stator; a rotor configured to rotate relative to the stator; wherein the rotor comprises a plurality of radially extending vanes which are arranged to direct air radially outwards as the rotor rotates, thereby cooling the generator.
2. A generator as claimed in 1, wherein each of the vanes is curved along its radial length.
3. A generator as claimed in 1, wherein each of the vanes is straight along its radial length.
4. A generator as claimed in any preceding claim, wherein the thickness of each vane increases along its radial length.
5. A generator as claimed in any preceding claim, wherein the axial height of each vane increases along its radial length.
6. A generator as claimed in any preceding claim, wherein the rotor and/or stator is provided with one or more vents.
7. A generator as claimed in claim 6, wherein the vents are positioned toward the centre of the rotor and/or stator.
8. A generator as claimed in any preceding claim, wherein one of the stator or the rotor comprises a plurality of coils, and the other of the stator or the rotor comprises a plurality of magnets.
9. A generator as claimed in 8, wherein the coils each have an opening at their centre.
10. A generator as claimed in claim 9, wherein the vanes are offset from the magnets.
11. A generator as claimed in claim 9 or 10, wherein each of the vanes is aligned with one of the magnets.
12. A generator as claimed in claim 9 or 11, wherein the magnets are Neodymium magnets.
13. A generator as claimed in any of claims 9 to 12, wherein the generator is a radial flux generator.
14. A generator as claimed in claim 13, wherein the magnets are located on radially inner and outer arms which are arranged as a pair of concentric rings.
15. A generator as claimed in claim 14, wherein the inner and outer arms are planar elements.
16. A generator as claimed in claim 14 or 15, wherein each of the vanes is connected to one of the inner arms.
17. A generator as claimed in claim 16, wherein each of the vanes is connected to an upstream side of one of the inner arms.
18. A generator as claimed in any of claims 14 to 17, wherein the thickness of the vanes at their radially outer ends corresponds to that of the inner arms.
19. A generator as claimed in any of claims 14 to 18, wherein the height of the vanes at their radially outer ends corresponds to that of the inner arms.
20. A generator as claimed in any of claims 14 to 19, wherein the outer arms form an annular ring.
21. A generator as claimed in any of claims 14 to 20, wherein the coils are mounted on an annular ring.
22. A generator as claimed in any of claims 1 to 12, wherein the generator is an axial flux generator.
23. A generator as claimed in claim 22, wherein the axial height of the vanes is equal to of less than the axial height of the magnets.
24. A generator substantially as described herein with reference to and as shown in the accompanying drawings.
25. A wind turbine comprising a generator as claimed in any preceding claim.
26. A wind turbine as claimed in claim 25, wherein the wind turbine is a vertical axis wind turbine.