Abstract:
An electrical machine comprises a longitudinally movable device and an intermediate rotor. A first arrangement of magnets is disposed on a first surface of the longitudinally movable device, and a second arrangement of magnets is disposed on a first surface of the intermediate rotor. The longitudinally movable device is movable in a first direction. The intermediate rotor&#39;s first surface is held adjacent the first surface of the longitudinally movable device such that it cannot move in the first direction. The intermediate rotor is, however, rotatable about an axis and the arrangement of the first and second arrangements of magnets translates movement of the longitudinally movable device into rotation of the intermediate rotor about the axis.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/062,127, filed Mar. 28, 2011, and entitled “Electrical Machine,” which is herein incorporated by reference. U.S. application Ser. No. 13/062,127 is a national stage application of PCT/GB09/51121, filed Sep. 3, 2009, which claims the benefit of GB 0816248.9, filed Sep. 5, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an electrical machine, and in particular to a machine that can be used to generate electrical current efficiently from a slowly moving body. 
       BACKGROUND 
       [0003]    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 3000 rpm achieved in a conventional power station. 
         [0004]    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. 
       SUMMARY  
       [0005]    According to a first aspect of the present invention, an electrical machine comprises a longitudinally movable device and an intermediate rotor. A first arrangement of magnets is disposed on a first surface of the longitudinally movable device, and a second arrangement of magnets is disposed on a first surface of the intermediate rotor. The longitudinally movable device is movable in a first direction. The intermediate rotor&#39;s first surface is held adjacent the first surface of the longitudinally movable device such that it cannot move in the first direction. The intermediate rotor is, however, rotatable about an axis and the arrangement of the first and second arrangements of magnets translates movement of the longitudinally movable device into rotation of the intermediate rotor about the axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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:— 
           [0007]      FIG. 1  is a schematic diagram, illustrating a part of a machine in accordance with the present invention. 
           [0008]      FIG. 2  shows a part of the machine of  FIG. 1 , to a larger scale. 
           [0009]      FIG. 3  is a cross-sectional view through the part shown in  FIG. 2 . 
           [0010]      FIG. 4  shows a first arrangement of magnets on the surfaces of the first and second rotors in the machine of  FIG. 1 . 
           [0011]      FIG. 5  shows a second alternative arrangement of magnets on the surfaces of the first and second rotors in the machine of  FIG. 1 . 
           [0012]      FIG. 6  shows a third alternative arrangement of magnets on the surfaces of the first and second rotors in the machine of  FIG. 1 . 
           [0013]      FIG. 7  shows another aspect of the arrangement of magnets on the surfaces of the first and second rotors in the machine of  FIG. 1 . 
           [0014]      FIG. 8  shows an alternative concave section cylinder form of the second rotor. 
           [0015]      FIG. 9  shows a second alternative arrangement of the first and second rotors. 
           [0016]      FIG. 10  shows an alternative convex section cylinder barrel form of the second rotor. 
           [0017]      FIG. 11  shows a third alternative arrangement of the first and second rotors. 
           [0018]      FIG. 12  shows a fourth alternative arrangement of the first and second rotors. 
           [0019]      FIG. 13  illustrates a further machine in accordance with the invention, having a wheeled support for the second rotors. 
           [0020]      FIG. 14  shows a first arrangement of a linear generator in accordance with the present invention. 
           [0021]      FIG. 15  shows a second arrangement of a linear generator. 
           [0022]      FIG. 16  shows a cross section of a linear generator. 
           [0023]      FIG. 17  shows a cross section of an alternative linear generator. 
           [0024]      FIG. 18  shows a wave energy converter incorporating a generator in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 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. 
         [0026]    The machine  8  of  FIG. 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 20 rpm for the case of a typical 1.5 MW wind turbine. In addition, although  FIG. 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. 
         [0027]    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. 
         [0028]    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 . 
         [0029]    Visible through the gap  16  in  FIG. 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 . 
         [0030]    Although  FIG. 1  shows only one cylindrical second rotor  18 , many such second rotors are in fact located within the first rotor. 
         [0031]      FIG. 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  FIG. 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. 
         [0032]    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. 
         [0033]      FIG. 3  is a cross-sectional view through the first rotor  10 , second rotor  18 , and stator  24 . 
         [0034]    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 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. 
         [0035]    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. 
         [0036]      FIG. 4  shows a first possible arrangement of magnets on the surfaces  26 ,  28  of the first and second rotors. It will be apparent that the arrangements are the same, but are displaced from each other. In addition, it will be noted 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 a first magnet  34 , made from permanent magnet material magnetized in a first direction, then a piece of iron  36 , then a second magnet  38 , made from permanent magnet material magnetized in a second direction opposite to the first direction, then a second piece of iron  40 , then a third magnet  42 , made from permanent magnet material magnetized in the first direction. 
         [0037]    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. 
         [0038]    In this case, the arrangement of 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  FIG. 4 . 
         [0039]      FIG. 5  shows a second possible arrangement of magnets on the surfaces  26 ,  28  of the first and second rotors. Again, it will be apparent that the arrangements are the same, but are displaced from each other, and it will be noted that the arrangements are shown here schematically as if the two surfaces are planar, rather than circular. 
         [0040]    In  FIG. 5 , the illustrated section of the surface  26  has a first magnet  54 , made from permanent magnet material magnetized in a first direction, then a second magnet  56 , made from permanent magnet material magnetized in a second direction opposite to the first direction, then a third magnet  58 , made from permanent magnet material magnetized in the first direction, then a fourth magnet  60 , made from permanent magnet material magnetized in the second direction, and so on. A piece of ferromagnetic material, for example iron,  62  is connected to one end of each of these magnets  54 ,  56 ,  58 ,  60 . 
         [0041]    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 . 
         [0042]    In this case, the arrangement of magnets on the surfaces  26 ,  28  has a pitch p equal to the width of two of the magnets as shown in  FIG. 5 . 
         [0043]      FIG. 6  shows a third possible arrangement of magnets on the surfaces  26 ,  28  of the first and second rotors. Again, it will be apparent that the arrangements are the same, but are displaced from each other, and it will be noted that the arrangements are shown here schematically as if the two surfaces are planar, rather than circular. 
         [0044]    In  FIG. 6 , the illustrated section of the surface  26  has permanent magnet material  82  magnetized in such a way as to produce a succession of North and South poles at the surface  26  as shown and very little magnetic field on the opposite surface  83 , in an arrangement known as a Halbach array to a person skilled in the art. 
         [0045]    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 , again forming a Halbach array. 
         [0046]    Again, the arrangement of 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  FIG. 6 . 
         [0047]    Whether the magnets are as shown in  FIG. 4 , or as shown in  FIG. 5 , or as shown in  FIG. 6 , they produce a degree of coupling between the first rotor  10  and the second rotor  18 . It is also possible to use an arrangement of magnets which is based on a mixture of the schemes outlined in  FIGS. 4 ,  5  and  6 . For instance a machine could be designed based on the magnets at surface  28  of  FIG. 6  co-operating with the magnets shown at surface  26  of  FIG. 5 . 
         [0048]    It is also possible to produce the magnetic field at surfaces  26  or  28  by using conventional electrical machine windings. 
         [0049]      FIG. 7  shows in more detail the arrangements of the magnets on the surfaces  26 ,  28 . Specifically, the 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 perpendicular axis of rotation. It is impossible to provide identical helices on surfaces  26  and  28  for the case of the torus and cylinder but this is not necessary. 
         [0050]    From a stationary position, in which the arrangements of magnets have settled into positions in which the attraction between the magnets of opposite poles and the repulsion between the magnets of the same polarity is maximized, rotation of the first rotor  10  about its axis of rotation causes rotation of the second rotor  18  about its axis of rotation (since it is unable to move with the first rotor about the axis of rotation of the first rotor) in order to maintain a position in which this attraction is maximized. In addition, the fact that the second rotor has a rotational radius that is much smaller than the rotational radius of the first rotor causes a gearing effect. 
         [0051]    If the first rotor moves a peripheral distance equal to the pitch p of the magnetic helix, for example as shown in  FIG. 4 ,  5  or  6 , the second rotor rotates a full 360 degrees. For example, if the first rotor  10  has an outside diameter of 5 m and the second rotor  18  has an outside diameter of around 0.5 m, 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. 
         [0052]    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. 
         [0053]    Although one basic structure has been illustrated, it will be appreciated that other structures are possible. 
         [0054]      FIG. 8  shows an alternative form of the first and second rotors. As discussed above with reference to  FIG. 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 . Stated alternatively, the first rotor  10  is in the form of a circumferentially-sliced torus, such that a cross section of the torus forms an arc and an annular gap  16  across a radially outer portion of the torus. In the embodiment shown in  FIG. 8 , the second rotor  18   a  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. 8 , as that conforms more closely to the surface of the inside of the first rotor  10 . 
         [0055]      FIG. 9  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  FIG. 10 , as in this case that shape conforms more closely to the surface of the inside of the first rotor  10 . 
         [0056]      FIG. 11  shows a further alternative arrangement, in which the first rotor  120  is formed in the shape of an incomplete torus having two side pieces  122 ,  124 , by omitting the part of the torus&#39;s 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. Stated alternatively, the first rotor  120  is in the form of a doubly-sliced torus, the torus being sliced along opposite faces of a disk centered in the torus&#39;s midplane to form the two concentric side pieces  122 ,  124 , which are positioned on first and second portions, respectively, of the torus&#39;s circular cross section. The second rotor  126  is held between these two parts  122 ,  124 . 
         [0057]      FIG. 12  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. Stated alternatively, the first rotor  130  is in the form of a doubly-sliced torus, the torus being sliced along opposite sides of a rectangular shape centered in the torus&#39;s circular cross section and perpendicular to the midplane of the torus, thereby forming annular gaps between radially inner and outer surfaces thereof. The second rotor  136  is held between these two parts  132 ,  134 . 
         [0058]    In order to illustrate the advantages of the invention, an outline design of a 6.5 MW wind turbine generator is provided, based on the arrangement of first and second rotors  110 ,  118  as shown in  FIG. 9 . In this example, the first rotor  110  has an outside diameter of 5 m, and a rotational speed of 16 rpm (revolutions per minute). There are sixteen second rotors  118 , each having an outside diameter of 0.5 m and a length of 0.4 m, and having a rotational speed of 2800 rpm. The active parts of this device have a total mass of 9 T (tonnes). This can be compared with the estimated total mass of the active parts of a conventional direct drive permanent magnet 6.5 MW wind turbine rotating at 16 rpm, which is around 42 T. It also compares favourably with that of an existing experimental 5 MW wind turbine (built by Repower), which has an asynchronous doubly fed generator, operating at a speed of 670-1170 rpm driven by a mechanical gearbox, in which the gearbox has a mass of 63 T and the generator has a mass of 17 T. 
         [0059]    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. 
         [0060]      FIG. 13  shows a machine of this type. The first and second rotors  110 ,  118  are of the type shown in  FIG. 10 , 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 . 
         [0061]    The required clearance between the first and second rotors  110 ,  118  is maintained by a structure in which rails  124 ,  126  are provided on the outer surface of the first rotor  110 . In this case, the rails  124 ,  126  each have a rectangular profile. 
         [0062]    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  110 . 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  FIG. 13 ) 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. 
         [0063]    The invention has been described so far with reference to a machine in which the initial motion is rotational. However, a similar structure is possible where the initial motion provided by the primary energy source is linear, rather than rotational. For example, some sources of renewable energy give rise to a reciprocating linear motion, such as that found in many wave energy converters. If the first rotor shown in  FIG. 1  above is replaced by a straight tube, which is driven by this reciprocating linear motion, then this movement can be converted into rotation, and hence used to generate electrical power. 
         [0064]    A machine, suitable for use as a generator in this situation, is shown in  FIG. 14 . 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 magnets  194 ,  196 . 
         [0065]    As a result of the interaction between the two helical arrangements of 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. 
         [0066]    A rotor (not shown, but well understood by the person skilled in the art) can than be mounted on the cylinder  180  so as to cooperate with a stationary stator to generate electrical power. 
         [0067]      FIG. 15  shows an alternative arrangement, which is identical to that shown in  FIG. 14 , 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  FIG. 15 ) can be mounted on the tube  184  so as to cooperate with a stationary stator to generate electrical power. 
         [0068]      FIG. 16  is a cross section through the machine of  FIG. 15 , 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. 
         [0069]    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  FIGS. 15 and 16 , 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. 
         [0070]    As described above, the embodiments shown in  FIGS. 14 and 15  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. 
         [0071]      FIG. 17  shows a modification of the arrangement shown in  FIG. 16 , which is arranged to produce a more continuous output power. 
         [0072]    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 magnets (not shown in  FIG. 17 ). 
         [0073]    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. 
         [0074]    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. 
         [0075]    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. 
         [0076]    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. 
         [0077]    Also, the stators  201 ,  301  can be arranged so that the electrical output is in a convenient form. 
         [0078]    The machine shown in  FIG. 17  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  FIG. 17 , 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. 
         [0079]      FIG. 18  shows a further modification of the machine, allowing the smoothing of the output power, even in circumstances where the input energy, in the form of the reciprocating motion, is not constant. For example, if a machine in accordance with the invention were to be used as part of a sea wave energy converter, it would be preferable if the electrical output from the device were reasonably smooth, despite the fact that typically the pattern of sea waves is not regular. In the embodiment of the invention shown in  FIG. 18 , means are provided to store energy in the converter in order to smooth out variations in input power. 
         [0080]    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 magnets (not shown in  FIG. 17 ). 
         [0081]    The arrangement is described here with reference to a situation in which reciprocating linear motion of the cylinder  180  is converted to rotation of the tube  184 , as described above with reference to  FIG. 15 , although it will be appreciated that similar arrangements can be provided in the other embodiments of the invention described above. 
         [0082]    The rotating tube  184  can be mechanically coupled, for example via a shaft or mechanism  403  to a hydraulic pump  401 . The hydraulic pump  401  then drives a hydraulic motor  404  which will in turn drive an electrical generator  405 . In this case, the fluid flow path between the hydraulic pump  401  and the hydraulic motor  404  is provided with at least one hydraulic accumulator  406 . Energy storage is thus provided by the hydraulic accumulator  406  so that, even though the varying supply of energy to the cylinder  180  means that the tube  184  will not be rotating at a constant speed, the fluctuations will be smoothed by the effect of the hydraulic accumulator, so that the output of the electrical generator will be more nearly constant. 
         [0083]    As mentioned above, similar arrangements can be provided in the cases of the embodiments of the invention. For example, the tube  184  can be held against rotation, and the cylinder  180  can thus be caused to rotate. In this case, the smoothing effect can be achieved by coupling the pump  401  to the cylinder  180 . 
         [0084]    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. 
         [0085]    Although the magnetic gearing is thus described in the context of electrical machines, the same magnetic gearing mechanisms can be used in other situations, for example where the gearing mechanism is used to change the speed of some other type of machine. For example, in the arrangement shown in  FIG. 1 , the second rotors could incorporate hydraulic motors or pumps or compressors, and may not have any electrical context.