WO2009105837A1

WO2009105837A1 - An electric generator

Info

Publication number: WO2009105837A1
Application number: PCT/AU2009/000252
Authority: WO
Inventors: Joseph Bertony
Original assignee: Windworks Engineering Limited
Priority date: 2008-02-28
Filing date: 2009-02-27
Publication date: 2009-09-03

Abstract

An electric generator (40) is disclosed. The generator has a first (41) and a second (42) component having a common axis (3) and rotatable relative to each other around the common xis (3), the components being arranged for the generation of electricity on their relative rotation. There is a first set of poles (50) connected to the first component (41), the poles being spaced apart around the common axis (3). There is a second (45A) and a third set (45B) of poles connected to the second component (42), the poles of each respective second (45A) and third (45B) set being spaced apart around the common axis (3), the second set (45A) and the third set (45B) of poles being located with respect to each other so as to reduce cogging.

Description

AN ELECTRIC GENERATOR

Field of the Invention

The present invention relates generally to electric generators and, particularly but not exclusively, to electric generators suitable for coupling to vertical wind turbines, such as direct coupling.

Background of the Invention

Electric generators are typically operated by prime movers such as steam engines, steam turbines, water powered turbines and similar machines all of which have an appreciable starting torque. The prime mover usually has no difficulty in getting a rotor of the electric generator to commence rotation. However, for some prime movers which have a low starting torque, for example wind turbines, rotation at low wind speed often does not commence even if the wind velocity is sufficient for maintaining rotation of the rotor.

The phenomenon that the starting torque required to commence rotation is greater than the running torque to maintain rotation is in large part due to a phenomenon known as "cogging" .

As each magnetic pole on a rotor approaches a pole piece of a stator, it is attracted towards the pole piece. When the two are aligned, the reluctance of the magnetic flux paths is at a minimum and the generator settles in a minimum energy state. Similarly, as the magnetic pole moves away from alignment with the pole piece, then the rotor is retarded because the reluctance of the magnetic path is being increased. Often as the rotor comes to a standstill, the rotor will be seen to make a final movement in the forward direction, and rotate slightly in the reverse direction before then coming to a stop. This happens because the magnetic pole on the rotor passes a pole piece on the stator and is attracted rearwardly with a force that is sufficient to overcome the remaining momentum in the rotor. Consequently, the rotor does not progress onto the next alignment of magnetic pole and pole piece, but instead reverses back to the previous alignment thereby ensuring a minimum energy position at which the rotor stops .

One way of overcoming this cogging phenomenon is to have a number of magnetic poles that is not an integer multiple of the number of pole pieces. For example, if there are 64 magnetic poles and 64 pole pieces, then the minimum energy arrangement is when each pole piece is aligned with a magnetic pole. However, if there are 64 magnetic poles, and say, 63 pole pieces, then it is not geometrically possible for all the pole pieces to be simultaneously aligned with a magnetic pole. This has the consequence that it is easier to start initial rotor rotation but the electrical operation of the machine at normal operating speeds is reduced because the number of pole pieces is not at an optimum (that is 64 in this example) .

Summary of the Invention

According to one aspect of the invention, there is provided an electric generator comprising: a first and a second component rotatable relative to each other around a common axis, the first and second components having elements, the elements including magnetic poles and at least one conductor and being arranged for generation of an electrical current when the first and second components rotate relative to each other about the common axis, the first component including a first set of the elements that are spaced apart form each other and positioned around the common axis, the second component including a second set of the elements that are spaced apart from each other and positioned around the common axis, the second component further comprising a third set of the elements that are spaced apart from each other and positioned around the common axis, wherein the elements of the second set and the elements of the third set have relative positions that are selected so that cogging with the elements of the first set is reduced.

As cogging is reduced, wind turbines having such an electric generator have the advantage that rotation of a rotor may commence at a lower wind speed. This has the desirable consequence that the electric generators operate more often and overall more electricity is produced.

The at least one conductor may be a coil. The coil may be one of a plurality of coils .

In one embodiment the elements of the second set are spaced apart from the elements of the third set in a direction along the common axis.

At least some, in a specific embodiment all, of the elements of the first set may each have substantially the same spacing from the common axis . The elements of the first set may be positioned in a first plane and may form a first ring of elements.

Further, at least some, in a specific embodiment all, of the elements of the second set may each have substantially the same spacing from the common axis. The elements of the second set may be positioned in a second plane and may form a second ring of elements .

In addition, at least some, in a specific embodiment all, of the elements of the third set may each have substantially the same spacing from the common axis. The elements of the third set may be positioned in a third plane and form a third ring of elements .

In one specific embodiment all elements of the second ring and all elements of the third ring have substantially the same spacing from the common axis. One of the second and third rings may be positioned over the other one of the second and third rings .

At least, in a specific embodiment all, of the elements of the second set may have angular positions relative to the common axis that are different to those of all elements of the third set. The elements of the second set may be rotated relative to the elements of the third set by an angle that corresponds to less than a spacing between adjacent elements of the first set of elements. The second set of elements may comprise the same number of elements as the third set of elements and each element of the second set may be rotated by the same angle relative to a corresponding element of the third set.

In an embodiment each element of the first set comprises a pole of a respective magnet, such as a permanent magnet or an electromagnet. The second and third set of elements may each comprise a plurality of interconnected pole piece pairs, each pole piece pair having a coil positioned between them. The interconnected pole piece pairs may be magnetically permeable.

The generator may be arranged so that rotation of the first component relative to the second component results in the first set of elements sweeping past the second and third sets of elements generating an electric current in the at least one conductor.

The first component may be a rotor that is at least partially surrounded by the second component, which may be a stator. The second and third sets of elements may at least partially surround the first set of elements.

The elements of the first set may be regularly spaced apart around the common axis . The elements of the second set of elements may also be regularly spaced apart around the common axis. Further, the elements of the third set of elements may be regularly spaced apart around the common axis . The elements of the first set and the elements of the second set may have the same spacing between them.

The generator may be arranged for changing an electrical coupling of the elements of the second set of the elements. For example, the second and third sets of elements may each comprise a number of coils. The generator may be arranged so that at least some of the coils may be included in or excluded from an electrical circuit through which in use generated electricity is directed, so that a variable output of the electric generator may be controlled. Further, the second and third sets of elements may be arranged so that at least some of the coils, may be coupled in parallel or in series, which also influences an output of the generator over a wide range of parameters .

The second component may comprise magnetically permeable annuli, one associated with the second set of elements and another one associated wit the third set of elements . Each annulus may have a plurality of inward protrusions each constituting one of the poles. The inward protrusions may be integral with their respective annulus.

In an embodiment each element of the first set comprises a magnetic pole piece and adjacent magnetic pole piece of the first set have opposite polarity. — o —

In one embodiment the number of elements of the first set is greater than the number of elements of either one of the second and third sets . A ratio of the number of elements of the first component, typically including magnetic poles, to either one of the second or third sets, typically including pole pieces and coils, may be nil (n: integer grater than 1), such as 6:1. The electric generator in accordance with embodiments of the present invention is arranged for direct coupling of the generator to relatively large wind turbines operating at low or very low rotations per minute (RPM) .

The electric generator may be arranged to be coupled to a low torque prime mover. The prime mover may be a wind turbine. The common axis may be a vertical axis and the wind turbine may be a vertical wind turbine. The first component may be driven by the wind turbine via at least two elastically extendable links .

In a second aspect of the present invention there is provided a wind turbine comprising the generator in accordance with the first aspect of the present invention.

Brief Description of the Figures

In order to achieve a better understanding of the nature of the present invention, embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a plan view of an embodiment of an electrical generator;

Fig. 2 is a vertical cross -sectional view taken along the line II - II of Fig. 1;

Fig. 3 is a schematic perspective view of the stator of the generator of Figs. 1 and 2;

Fig. 4 is a plan view of half of the generator of Fig. 1 showing the arrangement of magnetic pole pieces; Fig. 5 is a vertical cross-section of the half stator of Fig. 4;

Fig. 6 is a plan view of an "upper deck" of the half stator of Fig. 5; and Fig. 7 is a plan view of a "lower deck" of the stator of Fig. 5.

Detailed Description of Embodiments of the Invention

A plan view of an embodiment of a generator is shown in

Fig. 1, generally indicated by the numeral 40. As seen in Figs. 1 and 2, this embodiment of a wind turbine has a vertical rotatable mast 3 defining a central axis which has its base supported by a bearing 30 which is in turn mounted on a concrete foundation 36. In some embodiments, the mast may have an orientation other than vertical, such as horizontal. Also located on the concrete foundation 36 is the generator 40 having a first component, in this embodiment a rotor 41, and a second component, in this embodiment a stator 42.

In this embodiment, the rating of the generator 40 is in the range of IkW to 10,00OkW however it will be appreciated that a generator having a rating outside of this range may also be used. As seen in Fig. 1 a pair of flexible connecting links 44, which may be manufactured from an elastomer as such as polyurethane, interconnects the mast 3 and the rotor 41. The flexibility and elastic nature of the connecting links 44 means that any one or more of vibration, non-uniform rotation, or small eccentricity caused by the wind turbine operation and reflected in the rotation of the mast 3, may be not transmitted to the rotor 41, or may be transmitted at a reduced magnitude .

It will be appreciated that, at least in this embodiment, the rotor 41 and the stator 42 are each an annulus, and as schematically illustrated in Fig. 3, each is constructed in two 180 degree portions or halves.

Turning now to Figs. 4 and 5, it will be seen that the stator 42 has an upper deck 42A and a lower deck 42B spaced apart along the axis, each of which are essentially identical and each of which has a magnetically permeable yoke 45A, 45B respectively. Each yoke 45 is generally annular with inwardly protruding legs or poles 46 between each pair of which is wound a coil 47. The coils 47 can be connected in series or in parallel, in single phase or multiple phases as required. Further, some of the coils may be switched in and out of the circuitry so that a variable output of the generator may be controlled. In this embodiment the connection may be switched electronically and optionally automatically. The yokes 45A, 45B are mounted on an aluminium body 48 and the coils 47 are protected by a housing 49.

The rotor 41 is provided with a multiplicity of magnets 50 each of which is mounted radially in front of an annular magnetically permeable ring 51 housed in an aluminium body 58. In this embodiment, the magnets are permanent but may in some embodiments are electromagnets. As seen in Fig. 4 the permanent magnets 50 are arranged so as to present alternating north and south poles to the poles 46. It will be seen that each pair of adjacent poles 46 spans six permanent magnets 50 so as to be aligned with magnetic poles of opposite polarity. Thus the ratio of magnetic poles to pole pieces is 6:1 for each deck of the stator 42.

Turning now to Figs. 6 (upper deck) and 7 (lower deck) , it will be seen that the upper deck formed by yoke 45A is movable circumferentially relative to the lower deck formed by yoke 45B. In particular, the two decks are positioned so that their final installation position differs by a fraction of the spacing between the adjacent magnets 50, such as by more than 10%, more than 30%, more than 50%, more than 70% or more than 90% of the spacing. In this way, the attraction between the pole pieces 46 of the upper deck and the rotor 41 can be, say, clockwise whilst the attraction between the pole pieces 46 of the lower deck and the rotor 41 is anticlockwise so that the two attractions are effectively balanced or cancelled.

It will be appreciated that some embodiments may have a very low starting torque since the cogging phenomenon is substantially reduced. To commence the rotor rotation, in some embodiments, it may be only a matter of overcoming the static coefficient of friction in the bearing 30 which is quite low and may be substantially less than conventional cogging forces. As a result some embodiments of a wind turbine may be able to commence rotation at much lower wind speeds and thereby rotate for longer periods of each day, especially where winds are light or fluky for much of the time. Thus the total energy produced by such a generator may be substantially increased.

The foregoing describes only one embodiment of the present invention and modifications can be made thereto without departing from the spirit and scope of the present invention. For example, the permanent magnets 50 can be placed on the stator 42 and the yoke 45 and pole pieces 46 can be placed on the rotor 41, however, the arrangement as illustrated above provides for a brushless electrical connection to the coils 47. The generator may take the form of an alternator with electromagnets replacing the permanent magnets .

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

1. An electric generator comprising: a first and. a second component rotatable relative to each other around a common axis, the first and second components having elements, the elements including magnetic poles and at least one conductor and being arranged for generation of an electrical current when the first and second components rotate relative to each other about the common axis, the first component including a first set of the elements that are spaced apart form each other and positioned around the common axis, the second component including a second set of the elements that are spaced apart from each other and positioned around the common axis, the second component further comprising a third set of the elements that are spaced apart from each other and positioned around the common axis, wherein the elements of the second set and the elements of the third set have relative positions that are selected so that cogging with the elements of the first set is reduced.

2. The electric generator of claim 1 wherein at least some of the elements of the second set have angular positions relative to the common axis that are different to those of all elements of the third set.

3. The electric generator of claim 1 or 2 wherein the elements of the second set are rotated relative to the third set of elements by an angle that corresponds to less than a spacing between adjacent elements of the first set of elements .

4. The electric generator of any one of the preceding claims wherein the elements of the second set comprise the same number of elements as the third set and each element of the second set is rotated by the same angle relative to a corresponding element of the third set.

5. The electric generator of any one of the preceding claims wherein the elements of the first set have substantially the same spacing from the common axis and form a first ring of elements.

6. The electric generator of any one of the preceding claims wherein the at least one conductor is a coil .

7. The electric generator of any one of the preceding claims wherein the elements of the second set are spaced apart from the elements of the third set in a direction along the common axis and form second and third rings of elements, each ring having elements that have substantially the same spacing from the common axis.

8. The electric generator of claim 7 wherein one of the second and third rings is positioned over the other one of the first and second rings.

9. The electric generator of any one of the preceding claims wherein each element of the first set comprises a pole of a respective magnet and wherein the second and third set of elements each comprise a plurality of interconnected pole piece pairs, each pole piece pair having a coil positioned between them.

10. The electric generator of any one of the preceding claims wherein the generator is arranged so that rotation of the first component relative to the second component results in the first set of elements sweeping past the second and third sets of elements generating an electric current in at least one conductor.

11. The electric generator of any one of the preceding claims wherein the first component is a rotor that is at least partially surrounded by the second component, which is a stator.

12. The electric generator of any one of the preceding claims wherein the second and third sets of elements is at least partially surround the first set of elements.

13. The electric generator of any one of the preceding claims wherein the elements of the first set are regularly spaced apart around the common axis and wherein the elements of the second set of elements are regularly spaced apart around the common axis and wherein the elements of the third set of elements are regularly spaced apart around the common axis .

14. The electric generator of claim 13 wherein the elements of the first set and the elements of the second set have the same spacing between them.

15. The electric generator of any one of the preceding claims wherein the second and third sets of elements each comprise a number of coils.

16. The electric generator of claim 15 wherein the generator is arranged for changing an electrical coupling of the elements of the second set with the elements of the third set.

17. The electric generator of claim 16 wherein the generator is arranged so that at least some of the coils may be included in or excluded from an electrical circuit through which in use generated electricity is directed, so that a variable output of the electric generator may be controlled.

18. The electric generator of claim 16 or 17 wherein the second and third sets of elements are arranged so that at least some of the coils may be coupled in parallel or in series .

19. The electric generator of any one of the preceding claims wherein the second component comprises magnetically permeable annuli, one associated with the second set of elements and another one associated wit the third set of elements .

20. The electric generator of any one of the preceding claims wherein the number of elements on the first set is greater than the number of elements of either one of the second and third sets .

21. The electric generator of any one of the preceding claims wherein the first set comprises a magnetic pole piece and adjacent magnetic pole piece of the first set have opposite polarity.

22. The electric generator of any one of the preceding claims wherein the electric generator is arranged to be coupled to a vertical axis wind turbine.

23. The electric generator of claim 22 wherein the first component is in use driven by the wind turbine via at least two elastically extendable links .

24. A vertical axis wind turbine comprising the electric generator in accordance with anyone of the preceding claims.