EP2792058A2

EP2792058A2 - Electrical machine

Info

Publication number: EP2792058A2
Application number: EP12806642.0A
Authority: EP
Inventors: David Rodger; Hong Cheng Lai
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-12-16
Filing date: 2012-12-14
Publication date: 2014-10-22
Also published as: KR20140103169A; GB2497591A; CN104054255A; GB201121714D0; IN2014MN01440A; WO2013088166A2; WO2013088166A3

Abstract

An electrical machine, comprises a first rotor, rotatable about, or movable along, a first axis, and having a first arrangement of ferromagnetic salient poles on a first surface thereof; and a second rotor, held with a first surface thereof adjacent the first surface of the first rotor and such that it is rotatable about a second axis, and having a second arrangement of magnets on the first surface thereof. The first and second arrangements of ferromagnetic salient poles and magnets are such that rotation of the first rotor about the first axis, or movement of the first rotor along the first axis, causes rotation of the second rotor about the second axis.

Description

ELECTRICAL MACHINE

This invention relates to an electrical machine, and in particular to a machine that can be used to generate electrical current efficiently from a slow moving body.

Electrical machines in the form of generators are very well known, in which a primary source of energy is used to rotate a body, and this rotor cooperates with a stator to produce an electric current. However, where the primary source of energy is one of the common sources of renewable energy, such as wind, tide, or wave, the rotor typically moves rather slowly, at least compared with the 3000rpm achieved in a conventional power station.

The effect of this relatively slow movement is that the generator must be relatively large, which in turn means that the cost and mass of the generator is high. If conventional mechanical gearing is used to convert the slow rotation into a faster rotation of a rotor in a generator, then the gearing is a source of losses due to friction, and also reduces the reliability.

A recent patent application (EP-A- 2335344) describes machines which have an integrated magnetic gearing system which converts the slow rotation of a prime mover into faster rotation of a rotor in a generator. Double-sided arrays of magnets are employed to produce a very torque dense magnetic gearing system which results in a smaller machine. However, in some cases a high torque density is not necessary. The present invention uses one array of magnets co-operating with an array of salient ferromagnetic poles to produce a gearing effect. In some applications, this could be less expensive and more robust than the previous double-sided magnet system.

According to a first aspect of the present invention, there is provided an electrical machine, comprising:

a first rotor, rotatable about, or movable along, a first axis, and having a first arrangement of ferromagnetic salient poles on a first surface thereof;

a second rotor, held with a first surface thereof adjacent the first surface of the first rotor and such that it is rotatable about a second axis, and having a second

arrangement of magnets on the first surface thereof; wherein the first and second arrangements of ferromagnetic salient poles and magnets are such that rotation of the first rotor about the first axis, or movement of the first rotor along the first axis, causes rotation of the second rotor about the second axis. For a better understanding of the present invention, and to show how it can be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a schematic diagram, illustrating a part of a machine in accordance with the present invention.

Figure 2 shows a part of the machine of Figure 1 , to a larger scale.

Figure 3 is a cross-sectional view through the part shown in Figure 2.

Figure 4 shows a first arrangement of ferromagnetic salient poles and magnets on the surfaces of the first and second rotors in the machine of Figure 1.

Figure 5 shows a second alternative arrangement of ferromagnetic salient poles and magnets on the surfaces of the first and second rotors in the machine of Figure 1.

Figure 6 shows a third alternative arrangement of ferromagnetic salient poles and magnets on the surfaces of the first and second rotors in the machine of Figure 1. Figure 7 shows another aspect of the arrangement of ferromagnetic salient poles on the surfaces of the first or second rotors in the machine of Figure 1.

Figure 8 shows another aspect of the arrangement of magnets on the surfaces of the first or second rotors in the machine of Figure 1.

Figure 9 shows an alternative concave section cylinder form of the second rotor. Figure 10 shows a second alternative arrangement of the first and second rotors. Figure 1 1 shows an alternative convex section cylinder barrel form of the second rotor. Figure 12 shows a third alternative arrangement of the first and second rotors.

Figure 13 shows a fourth alternative arrangement of the first and second rotors. Figure 14 illustrates a further machine in accordance with the invention, having a wheeled support for the second rotors.

Figure 15 shows a first arrangement of a linear generator in accordance with the present invention.

Figure 16 shows a second arrangement of a linear generator. Figure 17 shows a cross section of a linear generator. Figure 18 shows a cross section of an alternative linear generator.

Figure 1 shows the general structure of an electrical machine 8 in accordance with the present invention. The electrical machine is described herein in the form of a generator, in which a rotation of a body is used to generate electrical power. However, it will be appreciated by the person skilled in the art that the same principle can be used to construct a motor, in which electrical power is applied, and used to cause a body to rotate. The machine 8 of Figure 1 has a first rotor 10, which is connected to an axle 12 by a support structure in the form of spokes 14. Rotation of the axle 12 then causes the rotor 10 to rotate about the axis defined by the axle. The rotation of the axle 12 can be driven by a power source such as a wind turbine, a tidal current machine, or a wave energy converter, and although it can of course be driven by any power source, the machine of the present invention is particularly suitable for situations where the driving rotation is at a relatively low speed, for example at about 20rpm for the case of a typical 1.5MW wind turbine. In addition, although Figure 1 shows the rotor 10 being driven through the axle 12, it can be driven directly by a body that is being caused to rotate by the external power source. For example, it may be mounted directly onto the hub of a wind turbine. The rotor 10 is generally toroidal. That is, it has an annular shape, which can be generated by rotating a circle about an axis that lies in the plane of the circle but outside the circle. This axis is then the axis about which the rotor is caused to rotate. However, the surface of the rotor is not a complete torus. Specifically, the part of the circular cross-section that lies furthest away from the axis of rotation is omitted, leaving an annular gap 16.

Visible through the gap 16 in Figure 1 is a cylindrical second rotor 18, which has an outer circular cross-section that is slightly smaller than the inner circular cross-section of the rotor 10.

Although Figure 1 shows only one cylindrical second rotor 18, many such second rotors are in fact located within the first rotor.

Figure 2 shows in more detail the part of the machine 8 in the region of the second rotor 18. Specifically, the second rotor 18 (and each of the other second rotors, not shown in Figure 1 or 2) is mounted on a support structure 20, which makes it unable to move in the direction of rotation of the first rotor 10, but allows it to rotate about an axis 22 of its own circular cross-section.

Located within the second rotor 18 is a stator 24. As is well known, the second rotor 18 and the stator 24 can be designed such that rotation of the second rotor 18 about its axis 22 causes an electrical current to be generated in the stator 24, which can be supplied through output electrical circuitry (not shown) to electrical power supply lines, electrical power storage devices, etc.

Figure 3 is a cross-sectional view through the first rotor 10, second rotor 18, and stator 24.

As mentioned above, the first rotor 10 is rotatable about an axis that lies in the plane of this cross-section. Meanwhile, the second rotor 18 is prevented from rotating about the axis of rotation of the first rotor, but is able to rotate about the axis 22. Provided on a first, inner, surface 26 of the first rotor 10, and on a first, outer, surface 28 of the second rotor 18 are arrangements of ferromagnetic salient poles and magnets that have the effect that, as the first rotor 10 is caused to rotate about its axis of rotation, the second rotor 18 is forced to rotate about the axis 22. This will be described in more detail below.

In addition, provided on a second, inner, surface 30 of the second rotor 18 and on a first, outer, surface 32 of the stator 24 are the arrangements that are required such that rotation of the second rotor 18 about its axis 22 causes an electrical current to be generated in coils of wire mounted on the stator 24. Suitable forms of these arrangements will be well known to the person skilled in the art, and will not be described further herein.

Figure 4 shows a first possible arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 of the first and second rotors. It will be apparent that the arrangements are shown here schematically as if the two surfaces are planar, rather than circular. The illustrated section of the surface 26 has ferromagnetic salient poles 36, 40 as shown. Between the poles are non ferromagnetic slots 34, 38, 42.

The illustrated section of the surface 28 has a first magnet 44, made from permanent magnet material magnetized in the second direction, then a piece of iron 46, then a second magnet 48, made from permanent magnet material magnetized in the first direction, then a second piece of iron 50, then a third magnet 52, made from

permanent magnet material magnetized in the second direction.

In this case, the arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 has a pitch p equal to the width of two of the magnets plus two of the pieces of iron, as shown in Figure 4.

Figure 5 shows a second possible arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 of the first and second rotors. Again, it will be apparent that the arrangements are shown here schematically as if the two surfaces are planar, rather than circular.

In Figure 5, the illustrated section of the surface 26 has ferromagnetic salient poles as shown at 54 and 58. Between the poles are non ferromagnetic slots 56 and 60.

The illustrated section of the surface 28 has a first magnet 64, made from permanent magnet material magnetized in the second direction, then a second magnet 66, made from permanent magnet material magnetized in the first direction, then a third magnet 68, made from permanent magnet material magnetized in the second direction, then a fourth magnet 70, made from permanent magnet material magnetized in the first direction, and so on. A piece of ferromagnetic material, for example iron, 72 is connected to one end of each of these magnets 64, 66, 68, 70.

In this case, the arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 has a pitch p equal to the width of two of the magnets as shown in Figure 5.

Figure 6 shows a third possible arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 of the first and second rotors. Again, it will be noted that the arrangements are shown here schematically as if the two surfaces are planar, rather than circular.

In Figure 6, the illustrated section of the surface 26 has ferromagnetic salient poles as shown at 82 and 84. Between the poles are non ferromagnetic slots 83 and 85.

The illustrated section of surface 28 has permanent magnet material 92 magnetized in such a way as to produce a succession of magnetic North and South poles at the surface 28 as shown and very little magnetic field on the surface 93, forming a structure known to a person skilled in the art as a Halbach array.

Again, the arrangement of ferromagnetic salient poles and magnets on the surfaces 26, 28 has a pitch p equal to the distance between two successive North poles, or between two successive South poles, as shown in Figure 6.

Whether the ferromagnetic salient poles and magnets are as shown in Figure 4, or as shown in Figure 5, or as shown in Figure 6, they produce a degree of coupling between the first rotor 10 and the second rotor 18.

In any event, while there is described here an embodiment in which the ferromagnetic salient poles are on the surface 26, while the magnets are on the surface 28, the opposite arrangement would also be possible, with the ferromagnetic salient poles on the surface 28 and the magnets on the surface 26. However, because the magnets are a more expensive component than the

ferromagnetic salient poles, there would be an advantage in an arrangement in which the magnets extend over a smaller area than the salient poles. Thus, for example, in the type of generator shown in Figure 1 , in which, in the case of a tidal generator, the diameter of the first rotor 10 might be of the order of 5m, there might be, say, six second rotors 18, which together extend around 25-50% of the circumference of the first rotor. In such a machine, there would be an advantage in providing the magnets on the surfaces of the second rotor and the salient poles on the s u rf ace of th e f i rst roto r.

It is also possible to produce the magnetic field at surfaces 26 or 28 by using conventional electrical machine windings instead of magnets. The coupling with the ferromagnetic salient poles may be enhanced by using a conventional electrical machine winding round each ferromagnetic salient pole.

Figures 7 and 8 show in more detail the arrangements of the ferromagnetic salient poles and magnets on the surfaces 26 or 28. Specifically, the ferromagnetic salient poles and magnets are arranged in helical patterns. These helical patterns have the effect that rotation of the first rotor 10 about its axis of rotation causes rotation of the second rotor 18 about its axis of rotation. For the case in which both the first and second rotors are cylinders, identical helices on both surfaces 26 and 28 would be advantageous. It is impossible to provide identical helices on surfaces 26 and 28 for the case where one of the rotors is a torus, but this is not necessary.

If the first rotor moves a peripheral distance equal to the pitch p of the magnetic helix, for example as shown in Figure 4, 5 or 6, the second rotor rotates a full 360 degrees. For example, if the first rotor 10 has an outside diameter of 5m and the second rotor 18 has an outside diameter of around 0.5m, a gear ratio of around 150: 1 (that is, the second rotor rotates 150 times for each rotation of the first rotor) may be

advantageous. The gear ratio can be altered by changing the diameter of the first rotor and/or of the second rotor, by changing the pitch p of the magnets, or by using more starts on the helical thread patterns. There is thus provided an electrical machine that can convert relatively slow rotation efficiently into a faster rotation that can be used more conveniently for generating electrical power. Although one basic structure has been illustrated, it will be appreciated that other structures are possible.

Figure 9 shows an alternative form of the first and second rotors. As discussed above with reference to Figure 1 , the first rotor 10 is in the form of a torus, from which the part of the circular cross section that lies furthest from the axis of rotation is omitted, leaving an annular gap 16. In the embodiment shown in Figure 9, the second rotor 18a is not in the form of a right circular cylinder, but rather is a cylindrical object formed by rotating a curved line about the axis 22. In particular, it may be advantageous to arrange for a concave surface, as illustrated in Fig 9, as that conforms more closely to the surface of the inside of the first rotor 10.

Figure 10 shows a further alternative form of the first and second rotors, in which the first rotor 110 forms an incomplete torus in which the part of the circular cross section that lies nearest to the axis of rotation is omitted, leaving an annular gap 116, with the second rotor 118 being visible through this gap. In this case the second rotor might advantageously be formed by rotating a curved line about the axis 22 so as to form a barrel shaped body with a convex surface as illustrated in more detail in Figure 11 , as in this case that shape conforms more closely to the surface of the inside of the first rotor 10.

Figure 12 shows a further alternative arrangement, in which the first rotor 120 is formed in the shape of an incomplete torus having two parts 122, 124, by omitting the part of the circular cross section that lies nearest to the axis of rotation of the first rotor and also the part of the circular cross section that lies furthest from the axis of rotation. The second rotor 126 is held between these two parts 122, 124.

Figure 13 shows a further alternative arrangement, in which the first rotor 130 is formed in the shape of an incomplete torus having two parts 132, 134, by retaining only the part 132 of the circular cross section that lies nearest to the axis of rotation and the part 134 of the circular cross section that lies furthest from the axis of rotation, while omitting two annular side pieces. The second rotor 136 is held between these two parts 132, 134.

In most rotating or linear electrical machines, it is important to maintain a small mechanical clearance between moving parts. If this is to be done in the case of a large electrical machine, it often means that the mass of supporting structure, used to impart rigidity, but not electromagnetically active, is increased. The mass problem can be alleviated in the case of the present invention by allowing the structure to be relatively light and flexible, while maintaining the necessary clearances by using wheels to support the second rotors, running on tracks which are attached to the first rotor.

Figure 14 shows a machine of this type. The first and second rotors 1 10, 118 are of the type shown in Figure 11 , in which the first rotor 110 forms an incomplete torus in which the part of the circular cross section that lies nearest to the axis of rotation is omitted, and the second rotor 118 is barrel-shaped. The second rotor 118 is mounted on a support structure 120, which allows it to rotate about an axis 122.

The required clearance between the first and second rotors 1 10, 118 is maintained by a structure in which rails 124, 126 are provided on the outer surface of the first rotor 1 10. In this case, the rails 124, 126 each have a rectangular profile.

Connected to the axle 122 above the second rotor 118 is a mechanism 127 comprising a first rod 128, which is at 90° to the axle 122, and is connected to a second rod 130 at an angle of about 90°. Connected to this second rod 130 are three wheels 132, 134, 136. The first wheel 132 is located so that it can run along a surface 138 of the rail 126 that is perpendicular to the outer surface of the first rotor 1 10. The second wheel 134 is located so that it can run along a surface 140 of the rail 126 that is parallel to the outer surface of the first rotor 110. The third wheel 136 is located so that it can run along a surface (not visible in Figure 14) of the rail 126 that is perpendicular to the outer surface of the first rotor 110 and opposite the surface 138. A similar mechanism 142 is connected between the axle 122 above the second rotor 118 and the rail 124. Further similar mechanisms 144, 146 are connected between the axle 122 below the rotor 118 and the rails 126, 124 respectively. If the first rotor shown in Figure 1 above is replaced by a straight tube, which is driven by a prime mover which supplies reciprocating linear motion, then this movement can be converted into rotation, and hence used to generate electrical power. A machine, suitable for use as a generator in this situation, is shown in Figure 15. A first tube 184 is connected to a primary source of energy, such that it is driven along its axis in a reciprocating linear motion, as shown by the arrows A. Provided on the inner surface 186 of the tube 184 is a helical arrangement of magnets 188, 190. The tube 184 is mounted around a second smaller cylinder 180. Provided on the outer surface 192 of the tube 180 is a helical arrangement of ferromagnetic salient poles 194 and non ferromagnetic slots 196.

As a result of the interaction between the two helical arrangements of ferromagnetic salient poles and magnets, similar to that described above, the reciprocating linear motion of the tube 184 is converted into reciprocating rotation in the smaller cylinder 180 as shown by the arrows B. The helical arrangements of the ferromagnetic salient poles and magnets on the respective surfaces can be as shown in Figures 7 and 8. Identical helices on both surfaces would be advantageous. While there is described here an embodiment in which the ferromagnetic salient poles are on the surface 192, while the magnets are on the surface 186, the opposite arrangement would also be possible, with the ferromagnetic salient poles on the surface 186 and the magnets on the surface 192. However, because the magnets are a more expensive component than the

ferromagnetic salient poles, there would be an advantage in an arrangement in which the magnets extend over a smaller area than the salient poles.

Thus, for example, in the type of generator shown in Figure 15, when in use as a wave energy converter, one of the rotor and the linear component will probably have a length at least equal to the stroke of the device, which might be in the region of 2-4m. Thus, for example, the rotor component 180 will probably have a length of 2-4m, while the linear component 184 might have a length of less than 1 m, and thus perhaps extends along 25-50% of the length of the rotor. In such a machine, there would be an advantage in providing the magnets on the surfaces of the linear component and the salient poles on the surface of the rotor. However, there might also be an advantage in the surface carrying the magnets being longer than the surface carrying the salient poles.

A rotor (not shown, but well understood by the person skilled in the art) can then be mounted inside or on the cylinder 180 so as to cooperate with a stationary stator to generate electrical power.

Figure 16 shows an alternative arrangement, which is identical to that shown in Figure 15, except that the cylinder 180 is driven along its axis in a reciprocating linear motion by a primary source of energy, as shown by the arrows C, and this movement is converted into reciprocating rotation in the tube 184, as shown by the arrows D. A rotor (not shown in Figure 16) can be mounted on the tube 184 so as to cooperate with a stationary stator to generate electrical power. With the type of generator shown in Figure 16 in use as a wave energy converter, again, one of the rotor and the linear component will probably have a length at least equal to the stroke of the device, which might be in the region of 2-4m. Thus, for example, the 180 will probably have a length of 2-4m, while the rotor 184 might have a length of less than 1 m, and thus perhaps extends along 25-50% of the length of the linear component. In such a machine, there would be an advantage in providing the magnets on the surfaces of the rotor and the salient poles on the surface of the linear component. However, there might also be an advantage in the surface carrying the magnets being longer than the surface carrying the salient poles. Figure 17 is a cross section through the machine of Figure 16, also showing the arrangement for generating electrical power. Specifically, a rotor part 198 of a generator is mounted on the outside of the tube 184, and this is located within the stator part 200 of the generator. Thus, as the cylinder 180 reciprocates as shown by the arrows C, the cylinder 184 will rotate, with changes in the rotational direction, and electrical power can be generated.

All of the embodiments so far have referred to electrical machines in the form of generators, where movement is converted to output electrical power. The same structures, with appropriate changes to the electrical connections as will be apparent to the person skilled in the art, can also be used as electric motors. Thus, for example, in the case of the structure shown in Figures 16 and 17, a linear motor may also be realised, if electrical power is provided to the stator 200, causing the rotor 198 to rotate, and hence causing the cylinder 180 to move along its axis.

As described above, the embodiments shown in Figures 15 and 16 are intended for use in situations where the primary energy source is a reciprocating motion, and will usually produce a reciprocating motion on the output side. If continuous rotation in one direction of the rotor 198 is required, however, this is also possible.

Figure 18 shows a modification of the arrangement shown in Figure 17, which is arranged to produce a more continuous output power.

In this arrangement, as before, a first tube 184 is mounted around a second smaller cylinder 180. Provided on the inner surface 186 of the tube 184, and on the outer surface 192 of the tube 180, are helical arrangements of ferromagnetic salient poles and magnets (not shown in Figure 18).

In this case, there are two rotors 202, 302 mounted on the outside of the tube 184, but they are not directly driven by the tube 184. Rather, two sprag clutches 204, 304 are connected to the tube 184, and drive the rotors 202, 302. The two rotors 202, 302 then co-operate with stators 201 , 301 respectively, to produce electrical power as described above. The sprag clutches (or any other similar device, which could be mechanical, hydraulic, electromechanical and so on) have the property that they produce a positive drive to a load in one direction, but will allow the load to overrun if the rotational speed of the load is greater than the input rotational speed. These clutch arrangements will be well known to the person skilled in the art, and will not be described further herein.

When the machine is being driven by a reciprocating motion of the cylinder 180, the magnetic gearing between the cylinder 180 and tube 184 will cause the tube 184 to rotate, alternating between opposite first and second rotational directions as the cylinder 180 reciprocates.

While the tube 184 is rotating in the first direction, it can drive the rotor 202 through the sprag clutch 204, which allows drive in the first direction and allows the rotor 202 to overrun in the second direction. While the tube 184 is rotating in the second direction, it can drive the rotor 302 through the sprag clutch 304, which allows drive in the second direction and allows the rotor 302 to overrun in the first direction. In this way, the rotors 202 and 302 can act as flywheels to store energy while the cylinder 180 is stationary, so being able to deliver more constant electrical power. Also, the stators 201 , 301 can be arranged so that the electrical output is in a convenient form.

The machine shown in Figure 18 may be modified for the case in which the

reciprocating energy source consists of a power stroke in a first direction and a weaker return stroke in a second direction opposite to the first direction. This situation could occur for instance where a buoy floating in the sea pulls a chain attached to the tube 180 providing the power stroke and a spring provides the return stroke. In the machine of Figure 18, the stator 301 , rotor 302 and sprag clutch 304 could be omitted. The sprag clutch 204 then drives the rotor 202 round on the power stroke and allows the rotor 202 to overrun on the return stroke.

There are thus described various electrical machines, in the form of generators and electric motors, in which an input motion of a first component is converted to an output motion of a second component, with the first and second components being coupled together by means of a magnetic gearing.

Claims

1. An electrical machine, comprising:

a first rotor, rotatable about a first axis, and having a first surface;

a second rotor, held with a first surface thereof adjacent the first surface of the first rotor and such that it is rotatable about a second axis;

wherein one of the first surfaces of the first rotor and the second rotor carries an arrangement of ferromagnetic salient poles and the other has an arrangement of magnets,

wherein the arrangements of ferromagnetic salient poles and magnets are such that rotation of the first rotor about the first axis causes rotation of the second rotor about the second axis.

2. An electrical machine as claimed in claim 1 , wherein:

the first rotor is in the form of an at least partial hollow torus or cylinder, with the first surface thereof being an internal surface;

the second rotor is in the form of a cylinder, located within the hollow torus or cylinder, with the first surface thereof being an external surface.

3. An electrical machine as claimed in claim 2, wherein the first rotor is in the form of a torus extending fully around the first axis, but extending only partially around the second axis.

4. An electrical machine as claimed in claim 3, wherein the first rotor is in the form of a torus having an annular gap at a radially outer surface thereof.

5. An electrical machine as claimed in claim 3, wherein the first rotor is in the form of a torus having an annular gap at a radially inner surface thereof.

6. An electrical machine as claimed in claim 3, wherein the first rotor is in the form of a torus having annular gaps at radially inner and outer surfaces thereof.

7. An electrical machine as claimed in claim 3, wherein the first rotor is in the form of a torus having annular gaps between radially inner and outer surfaces thereof.

8. An electrical machine as claimed in claim 3, wherein the first rotor is in the form of a cylinder.

9. An electrical machine as claimed in one of claims 2 to 8, comprising a plurality of said second rotors, held spaced apart within the hollow torus or cylinder at

predetermined positions around or along the first axis.

10. An electrical machine as claimed in any preceding claim, wherein:

the arrangement of ferromagnetic salient poles on the first surface of the first or second rotor comprises a helical arrangement; and

the arrangement of magnets on the first surface of the second or first rotor comprises a corresponding helical arrangement.

1 1. An electrical machine as claimed in any preceding claim, wherein the

arrangement of magnets extends adjacent to only a part of the respective first surface carrying the arrangement of ferromagnetic salient poles.

12. An electrical machine as claimed in claim 11 , wherein the part is less than 50% of the respective first surface.

13. An electrical machine as claimed in any preceding claim, in the form of a generator, and further comprising at least one stator, wherein the or each second rotor and an associated stator are positioned relative to each other such that rotation of the second rotor about the second axis causes an electrical current to be generated.

14. An electrical machine as claimed in claim 13, wherein the or each stator is located inside the associated second rotor.

15. An electrical machine as claimed in any of claims 1-12, in the form of a motor, comprising means for applying electrical energy to cause motion of one of said first and second rotors.

16. An electrical machine as claimed in any of claims 1-15, wherein the first rotor carries ferromagnetic salient poles and the second rotor carries magnets.

17. An electrical machine as claimed in any of the claims 1-15, wherein the first rotor carries magnets and the second rotor carries ferromagnetic salient poles.

18. An electrical machine, comprising:

a linear component, movable along a first axis, and having a first surface;

a rotor, held with a first surface thereof adjacent the first surface of the linear component and such that it is rotatable about a second axis;

wherein one of the first surfaces of the linear component and the rotor carries an arrangement of ferromagnetic salient poles and the other has an arrangement of magnets,

wherein the arrangements of ferromagnetic salient poles and magnets are such that motion of the linear component along the first axis causes rotation of the rotor about the second axis.

19. An electrical machine as claimed in claim 18, wherein:

the linear component is in the form of a cylinder, with the first surface thereof being an external surface.

20. An electrical machine as claimed in claim 18 or 19, wherein:

the rotor is in the form of a hollow cylinder, located around the linear component, with the first surface thereof being an internal surface.

21. An electrical machine as claimed in claim 20, wherein the second axis is coincident with the first axis.

22. An electrical machine as claimed in one of claims 18 to 21 , comprising a plurality of said rotors, held spaced apart along the linear component at predetermined positions around or along the first axis.

23. An electrical machine as claimed in claim 18, wherein:

the rotor is in the form of a cylinder, with the first surface thereof being an external surface.

24. An electrical machine as claimed in claim 18 or 23, wherein:

the linear component is in the form of a hollow cylinder, located around the rotor, with the first surface thereof being an internal surface.

25. An electrical machine as claimed in claim 24, wherein the second axis is coincident with the first axis.

26. An electrical machine as claimed in any of claims 18-25, wherein:

the arrangement of ferromagnetic salient poles on the first surface of the linear component or the rotor comprises a helical arrangement; and

the arrangement of magnets on the first surface of the rotor or the linear component comprises a corresponding helical arrangement.

27. An electrical machine as claimed in any of claims 18-26, wherein the

28. An electrical machine as claimed in claim 27, wherein the part is less than 50% of the respective first surface.

29. An electrical machine as claimed in any of claims 18-26, wherein the

arrangement of ferromagnetic salient poles extends adjacent to only a part of the respective first surface carrying the arrangement of magnets.

30. An electrical machine as claimed in claim 29, wherein the part is less than 50% of the respective first surface.

31. An electrical machine as claimed in any of claims 18-30, in the form of a generator, and further comprising at least one stator, wherein the or each rotor and an associated stator are positioned relative to each other such that rotation of the rotor about the second axis causes an electrical current to be generated.

32. An electrical machine as claimed in any of claims 18-31 , in the form of a motor, comprising means for applying electrical energy to cause motion of one of said linear component and said rotor.

33. An electrical machine as claimed in any of claims 18-32, wherein the linear component carries ferromagnetic salient poles and the rotor carries magnets. An electrical machine as claimed in any of claims 18-32, wherein the linear ponent carries magnets and the rotor carries ferromagnetic salient poles.