Abstract:
The invention relates to a generator, to a magnetic flux conducting unit for a generator, and to a power generation machine comprising such a generator. In an embodiment of the invention, a generator is disclosed which comprises at least one coil assembly and at least one magnetic flux conducting unit. The magnetic flux conducting unit comprises at least one magnet, a pair of opposed magnetic flux conducting elements defining a space therebetween for receiving the coil assembly, and at least one connection portion extending between the opposed magnetic flux conducting elements. The at least is arranged relative to the opposed magnetic flux conducting elements such that the magnetic attraction forces between the elements are redacted through and balances with the connection portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority to the benefit of International Patent Application PCT/GB2007/000883 with an International Filing Date on Mar. 14, 2007 with subsequent publication as International Publication Number WO 2007/104976 on Sep. 20, 2007. PCT/GB2007/000883, in turn, claims priority to Great Britain Patent Application No. 0605298.9, filed Mar. 16, 2006. The disclosures of each of the aforementioned patent documents are incorporated herein by reference in their entirety. 
     
    
       [0002]    Not Applicable 
       BACKGROUND 
       [0003]    The present invention relates to a generator, to a magnetic flux conducting unit for a generator, and to a power generation machine comprising such a generator. In particular, but not exclusively the present invention relates to a direct drive generator and to a magnetic flux conducting unit for a direct drive generator. 
         [0004]    In the field of electricity generation, it is well known to provide a generator coupled to a fluid driven turbine such as those found in oil, gas, coal and nuclear power stations. Conventional generators comprise a rotor having an iron core with a number of current-carrying coils wound on the core, and an iron-cored stator carrying a winding. A magnetic field is generated by passing a current along the rotor coils such that, on rotation of the rotor, a current is induced in the coils of the stator winding. The rotors of generators found in power stations are coupled to the turbine by a drive shaft which rotates at a high rotational velocity, of the order of several thousand rpm, and with a relatively low drive torque. Conventional power generators, manufactured with this in mind, have therefore been designed for high velocity, low torque operation. 
         [0005]    In recent years, significant research has been conducted worldwide into sustainable electricity generation methods, including wind, wave and tidal power generation. 
         [0006]    Existing wind machines comprise a prime mover in the form of a large diameter rotor. The rotor has a number of rotor blades, mounted on a rotor shaft, which is coupled to a power generator. The turbine rotor typically rotates at relatively low rotational speeds and at a high output torque, for example, 20 rpm for a 2MW machine, with an output torque of around 955 kNm. It will therefore be understood that turbines of this type operate at relatively low rotational speeds, but with a relatively high torque output. In order to successfully generate power in such low speed, high torque machines, conventional power generators (designed for high speed, low torque operation) require to be connected to the turbine rotor through a gearbox. The gearbox increases the rotational speed and decreases the torque of the output from the turbine rotor which is input to the generator. 
         [0007]    Use of gearboxes of this type is generally undesired as there are a number of significant disadvantages. In particular, the gearboxes are relatively large and heavy, greatly increasing the weight of the unit provided in the nascelle at the top of the wind turbine tower. Additionally, provision of a gearbox between the output shaft of the turbine rotor and the input shaft of the generator reduces the efficiency of the machine. 
         [0008]    Furthermore, these gearboxes have been found to be surprisingly unreliable under typical wind turbine operating conditions. The main cause for this is the constant variation in the operating speed and torque transmitted through the gearbox, due to fluctuations in wind velocity. 
         [0009]    Similar problems have been experienced in power generation systems using wave and tidal forces, where the prime movers of the systems operate at even lower rotational or cycling velocities, and hence at still higher torques or thrust forces. 
         [0010]    To address these problems, different types of power generators have been developed which are designed for low speed, high torque operation, for direct connection to, for example, the rotor of a wind machine. 
         [0011]    Examples of these types of generator include conventional permanent magnet generators, and high force density machines such as the Transverse Flux Machine (TFM) and the Vernier Hybrid Machine (VHM) of Newage AVK SEG, which have been proposed for direct drive systems. A particular application of these generators which has been identified is in wave power machines. A linear VHM machine includes opposed magnetic flux conducting cores which are generally C-shaped in cross-section, with a number of successively oppositely polarized pairs of magnets arrayed on arms of the cores, on either side of an air gap between the two opposed cores. A translator with upper and lower castellated surfaces is arranged in the air gap and is coupled to the prime mover of the power generating machine. In use, the translator reciprocates back and forth within the air gap and as the castellated portions of the translator successively align with oppositely polarized pairs of magnets, the magnetic flux flow between the two cores switches, the frequency of this switching depending upon the velocity of reciprocation of the translator. Coils are arranged on arms of the cores and thus power is generated as the magnetic field switches. 
         [0012]    Machines of this type, those with an iron core on the stationary and moving members, suffer from significant disadvantages, particularly in that extremely large magnetic attraction forces exist between the two cores. This requires a very large and heavy support structure for the cores, in order to maintain the air gap, with a resultant effect upon the size and weight of the generator. Additionally, manufacture and assembly of the generator, due to these large magnetic attraction forces, is extremely difficult. 
         [0013]    In an effort to address problems such as those associated with the aforementioned iron cored machines, a low force density generator, disclosed in International Patent Application Number PCT/GB02/02288, has been proposed. The generator disclosed in PCT/GB02/02288 is designed for use with a wind turbine, and is therefore a rotary generator. In the disclosed generator, the iron on the stator of the generator has been removed, and coils on the stator are supported by a non-magnetic material. In this machine, flux coming out of the moving iron surface of an iron core on a rotor of the machine has no iron surface to flow into, hence the magnetic flux effectively sees an infinitely large magnetic air gap. The flux density is therefore relatively low, and the efficiency and effectiveness of the machine is significantly reduced when compared with other generators. Accordingly, significantly more magnetic material is required on the rotor in order to achieve anything like a suitable operating efficiency. As a result, the physical diameter of the machines is required to be greatly increased. For example, for a 5MW air-cored machine, it is estimated that the machine would be 26 meters in diameter, being of the order of two to three times the diameter of an equivalent iron-cored generator. 
         [0014]    In an alternative type of rotary generator, two iron discs are located in opposition with an air gap therebetween, and with an air cored winding sandwiched between the two moving discs. Magnets are provided on the iron discs, with successive pairs of magnets (in a circumferential direction) being of opposite polarity. When the discs rotate, the stationary windings successively experience a switching magnetic flux flow, thereby generating electricity. 
         [0015]    However, machines of this type have extremely large magnetic attraction forces between the two discs, presenting problems of requiring a large and heavy support structure of the type described above. This presents a particularly difficult problem during manufacture of these relatively large machines, as it is extremely difficult to maintain the small air gap required (in order to maximize flux density) whilst keeping the iron discs apart. 
         [0016]    It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the forgoing disadvantages. 
       SUMMARY 
       [0017]    According to first aspect of the present invention, there is provided a generator comprising at least one coil assembly and at least one magnetic flux conducting unit, the at least one magnetic flux conducting unit comprising: at least one magnet; a pair of opposed magnetic flux conducting elements defining a space therebetween for receiving a coil assembly; and at least one connecting portion extending between the opposed magnetic flux conducting elements; wherein the at least one magnet is arranged relative to the opposed magnetic flux conducting elements such that magnetic attraction forces between the elements are reacted through and balanced within the connecting portion. 
         [0018]    According to a second aspect of the present invention, there is provided a magnetic flux conducting unit for a generator, the magnetic flux conducting unit comprising: at least one magnet; a pair of opposed magnetic flux conducting elements defining a space therebetween for receiving a coil assembly of a generator; and at least one connecting portion extending between the opposed magnetic flux conducting elements; wherein the at least one magnet is arranged relative to the opposed magnetic flux conducting elements such that magnetic attraction forces between the elements are reacted through and balanced within the connecting portion. 
         [0019]    By reacting the magnetic attraction forces which exist between the flux conducting elements through the connection portion, and by balancing the forces within the connecting portion, it is not necessary to provide a large, heavy support structure in order to maintain the air gap between the flux conducting elements. It will be understood that references herein to the magnetic attraction forces between the flux conducting elements being reacted through and balanced within the connecting portion are to mechanical loading on the flux conducting unit as a result of these attraction forces being transmitted to the connecting portion from the flux conducting elements, and to the flux conducting unit being arranged such that the mechanical forces in the flux conducting elements act against each other and therefore balance or cancel out. This greatly reduces the weight of the generator; eases manufacture; reduces manufacturing time; and consequently reduces cost when compared to known generators. 
         [0020]    It will be understood that the connecting portion, in extending between the opposed magnetic flux conducting elements, thereby defines a maximum extent of the space or air gap between the elements. 
         [0021]    In preferred embodiments, the generator is a direct drive generator and is adapted to be coupled directly to a prime mover of a power generation machine. The generator may therefore be adapted to be coupled to a drive member (such as an output shaft or rotor) of a wind power generation machine; a tidal power generation machine; or a wave power generation machine; or a free piston Stirling engine in a combined heat and power plant, for example. It will be understood that a direct drive generator is one where the generator is driven directly from or by the drive member of a power generation machine. 
         [0022]    Alternatively, the generator may be an indirect or non-direct generator, for indirect or non-direct drive applications; some wind turbine applications involve a single stage gearbox to step the speed from, say, 10 rpm to 100 rpm. The generator may therefore be utilized in such circumstances, which is still considered to be a low speed application. Furthermore, the generator could be used in both motoring and generating applications at any speed. 
         [0023]    Preferably, the at least one magnet is arranged relative to the opposed magnetic flux conducting elements such that a magnetic flux flow path within the magnetic flux conducting unit extends through the connecting portion. Thus the connecting portion may be magnetic flux conducting and may be located within or may define part of a flux flow path in the unit. 
         [0024]    The magnetic flux conducting unit may be generally C-shaped in cross-section, the connecting portion forming a base or central member and the elements coupled in a cantilever arrangement relative to the base. The space or air gap may be defined between the two opposed elements with the coil assembly received within the air gap. Alternatively, the magnetic flux conducting unit may be generally I shaped in cross-section, the connecting portion forming a base or central member and the elements coupled to the central member so as to form two cantilevered sections either side of the central member, with two spaces or air gaps between the flux conducting elements on either side of the connecting portion. There may be two coil assemblies, one received in each air gap. In each above case, the at least one magnet may be arranged such that magnetic attraction forces between the flux conducting elements generate mechanical loads within the elements, these mechanical loads transmitted to the connecting portion(s) and reacted against each other. Thus the mechanical loads are contained within the unit. Where the elements are cantilevered, or include sections that are cantilevered, relative to the connecting portion(s), turning moments may be generated about the connection portion. However, the at least one magnet may be arranged such that the turning moments of each element are equal and opposite and centered about a neutral axis of the connecting portion, to balance out the loads. 
         [0025]    In a further alternative, the magnetic flux conducting unit may be generally rectangular or square in cross-section, with two connection portions extending between the opposed magnetic flux conducting elements and with a space or air gap defined between the two connecting portions, and a coil assembly located within the air gap. The mechanical loads in the elements may be transmitted to both connection portions, and the at least one magnet may be arranged so that turning moments around central axes of the connection portions are balanced out, as described above. 
         [0026]    In embodiments of the invention, the magnet is arranged within the space or air gap defined between the elements. The unit may comprise a magnet coupled to each element, the magnets arranged with opposite poles facing one another and with a coil assembly disposed between the opposing surfaces of the magnets. The unit may comprise a C-core, the connecting portion forming a base or side of the C-core, and the flux conducting elements forming opposing arms of the C-core. The unit may comprise two such C-cores provided back to back, which may share a common connecting portion. It will therefore be understood that such a unit may be generally I-shaped and may therefore form an I-core. The generator may therefore comprise two coil assemblies, the coil assemblies disposed either side of the connecting portion, and two pairs of magnets coupled to the elements either side of the connecting portions. 
         [0027]    In alternative embodiments of the invention, the at least one magnet may define or form the connecting portion of the flux conducting unit. The at least one magnet may therefore serve to define the air gap between the flux conducting elements. The unit may be generally C-shaped, the magnet forming a base or central member, and the flux conducting elements forming opposing arms. The unit may comprise two such assemblies provided back to back, which may therefore share a common magnet, and it will be understood that such a unit may be generally I-shaped. The generator may comprise two coil assemblies, one disposed in each of the spaces or air gaps defined either side of the magnet. 
         [0028]    In further alternative embodiments of the invention, the unit may comprise two magnets extending between the flux conducting elements, each magnet defining a connecting portion, and a space or air gap defined between the magnets for receiving a coil assembly. Alternatively, the unit may comprise a one-piece body defining the flux conducting elements, and thus the flux conducting units may form one continuous section with an optionally rectangular portion removed from the centre. Magnets may be positioned on opposite faces and a winding sandwiched between the two in the remaining airspace. The unit may be generally rectangular or square in cross-section. 
         [0029]    Preferably, the generator comprises a plurality of magnetic flux conducting units, and the direction of flow of magnetic flux within each unit and through the respective at least one space or air gap may be opposite to that in the or each adjacent flux conducting unit. In this fashion, relative movement between the flux conducting units progressively switches a direction of flux flow through the coil assembly, generating a current within the coil assembly. 
         [0030]    The generator may be a rotary generator and may comprise a rotor and a stator, the rotor adapted to be coupled to a drive member of a prime mover of a power generation machine and thereby adapted for rotation relative to the stator. The at least one coil assembly may be provided on one of the rotor and the stator, and the at least one magnetic flux conducting unit may be provided on the other one of the rotor and the stator. Where the generator comprises a plurality of units, the units may be arranged circumferentially around the rotor or stator and may be arranged such that a main axis or plane of the flux conducting elements of the units are parallel to an axis of a shaft of the rotor, or perpendicular to the rotor axis. 
         [0031]    Alternatively, the generator may be a linear generator and may comprise a translator and a stator, the translator adapted to be coupled to a drive member of a prime mover of a power generation machine. The coil assembly may be provided on one of the translator and the stator and the at least one flux conducting unit on the other one of the translator and the stator. The generator may comprise a plurality of flux conducting units, the units extending along a plane parallel to a plane of the translator. The generator may comprise a plurality of translators and corresponding stators, each translator coupled to a common drive member of a prime mover. 
         [0032]    Where the generator comprises a plurality of units, adjacent units may be separated by an air gap or by spacers which are non-magnetically conductive/magnetically insulating, or of negligible magnetic conductivity compared to the magnetic flux conducting units. Alternatively the adjacent units could be butted up against one another so that they are not separated by an airgap/spacer. 
         [0033]    The at least one magnet may be a permanent magnet and may be magnetized following location within the unit. This may facilitate assembly of the unit by ensuring that the magnet is placed in a desired location prior to magnetization and thus before any magnetic attraction forces are generated. To facilitate construction of the unit, a clip or clamp may be provided for locating the at least one magnet in the unit prior to magnetization. Alternatively, the at least one magnet may be magnetized prior to location within the unit. 
         [0034]    The at least one coil assembly may comprise a plurality of current conducting coils and may be of copper or other suitable material. The magnetic flux conducting elements may be of iron, a ferrous alloy such as steel or the like. The at least one connecting portion may similarly be of iron or of a ferrous alloy. 
         [0035]    According to third aspect of the present invention, there is provided a generator comprising at least one coil assembly and at least one magnetic flux conducting unit, the at least one magnetic flux conducting unit comprising: a pair of opposed magnetic flux conducting elements defining a space therebetween for receiving a coil assembly; and at least one magnet extending between the opposed magnetic flux conducting elements, the at least one magnet arranged relative to the opposed magnetic flux conducting elements such that magnetic attraction forces between the elements are reacted through and balanced within the at least one magnet. 
         [0036]    According to a fourth aspect of the present invention, there is provided a magnetic flux conducting unit for a generator, the magnetic flux conducting unit comprising: a pair of opposed magnetic flux conducting elements defining a space therebetween for receiving a coil assembly of the generator; and at least one magnet extending between the opposed magnetic flux conducting elements, the at least one magnet arranged relative to the opposed magnetic flux conducting elements such that magnetic attraction forces between the elements are reacted through and balanced within the at least one magnet. 
         [0037]    According to a fifth aspect of the present invention, there is provided a power generation machine comprising the generator of the first or third aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0039]      FIG. 1  is a schematic view of a magnetic flux conducting unit for a generator, in accordance with an embodiment of the present invention; 
           [0040]      FIG. 2  is a schematic side view of part of a generator incorporating the magnetic flux conducting unit of  FIG. 1 , in accordance with an embodiment of the present invention; 
           [0041]      FIG. 2A  is a schematic side view of a power generation machine incorporating the generator shown in  FIG. 2 ; 
           [0042]      FIG. 3  is a schematic side view of part of a generator incorporating the magnetic flux conducting unit of  FIG. 1 , in accordance with an alternative embodiment of the present invention; 
           [0043]      FIG. 4  is a schematic front view of part of a generator incorporating the magnetic flux conducting unit of  FIG. 1 , in accordance with a further alternative embodiment of the present invention; 
           [0044]      FIG. 5  is an end view of the generator of  FIG. 4 ; 
           [0045]      FIG. 6  is a schematic view of a magnetic flux conducting unit for a generator, in accordance with a further alternative embodiment of the present invention; 
           [0046]      FIG. 7  is a view of the unit shown in  FIG. 6 , illustrating a bearing assembly by which a coil assembly is mounted to the unit; 
           [0047]      FIG. 8  is a perspective view of a generator incorporating magnetic flux conducting units similar to the unit shown in  FIGS. 6 and 7 , in accordance with a preferred embodiment of the present invention; 
           [0048]      FIG. 9  is a view of part of the generator shown in  FIG. 8 ; 
           [0049]      FIGS. 10 and 11  are views of the magnetic flux conducting units and of a coil assembly, respectively, forming parts of the generator shown in  FIG. 9 ; 
           [0050]      FIG. 12  is a schematic end view of a generator in accordance with a still further alternative embodiment of the present invention; 
           [0051]      FIG. 13  is an end view of part of a generator, incorporating a magnetic flux conducting unit, in accordance with a still further alternative embodiment of the present invention; 
           [0052]      FIG. 14  is a perspective view of the magnetic flux conducting unit shown in  FIG. 13 ; 
           [0053]      FIG. 15  is a perspective, partially cut-away view of a generator, incorporating a magnetic flux conducting unit, in accordance with a still further alternative embodiment of the present invention; 
           [0054]      FIG. 16  is a longitudinal sectional view of the generator shown in  FIG. 15 ; 
           [0055]      FIG. 17  is a view of the flux conducting unit of  FIG. 15 , cut-away as shown in  FIG. 15 ; 
           [0056]      FIG. 18  is a view of a power generation machine incorporating the generator of  FIG. 15 ; and 
           [0057]      FIG. 19  is a schematic illustration of a power generation machine incorporating the generator of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0058]    Turning firstly to  FIG. 1 , there is shown a schematic view of a magnetic flux conducting unit for a generator, in accordance with an embodiment of the present invention, the unit indicated generally by reference numeral  10 . Part of a generator  12  incorporating the magnetic flux conducting unit  10  shown in  FIG. 1  is illustrated in the schematic side view of  FIG. 2 . As shown in  FIGS. 1 and 2 , the magnetic flux conducting unit  10  comprises a pair of magnets  14 ,  16 , a pair of opposed magnetic flux conducting elements in the form of arms  18  and  20  and a connecting portion  22  extending between the arms  18  and  20 . The magnets  14  and  16  are arranged such that a magnetic flux flow path  24  (indicated in broken outline) extends in an anti-clockwise direction, when viewing  FIG. 1 . It will be understood that, in order to achieve to this, the poles of the magnets  14  and  16  may be oriented S-N/S-N when viewing  FIG. 1  from top to bottom. The arms  18  and  20 , together with the connecting portion  22 , form a generally C-shaped core and are of a magnetic flux conducting material such as iron, or an iron alloy such as steel. 
         [0059]    A space or air gap  26  is defined between the opposed arms  18 ,  20 , and the magnets  14  and  16  are located within the air gap, the magnet  14  magnetically coupled to the arm  18  and the magnet  16  to the arm  20 . If desired, the magnets  14 ,  16  may be magnetized in situ, and may be held in position by a clip, clamp or support prior to magnetization. The coil assembly  28  of the generator  12  is located in the air gap  26  between the opposed magnets  14 ,  16  and, as will be understood by persons skilled in the art, the coil assembly  28  comprises a number of coils of current conducting material such as copper. 
         [0060]    The generator  12  shown in  FIG. 2  is of a rotary type, and comprises a number of the units  10  arranged around a circumference of a rotor disc, wheel or the like  30  which is mounted on a rotor shaft  32 . The rotor shaft  32  is coupled to a prime mover of a power generation machine.  FIG. 2A  illustrates a power generation machine in the form of a wind machine  33  having a prime mover in the form of a rotor  35 , the rotor coupled in a direct drive to the generator  12  by a shaft  37 . The generator  12  is thus a direct drive generator, being directly driven by the output shaft  37  of the rotor  35 . 
         [0061]    The generator  12  also includes a stator  34  having a stator disc, frame or the like  36  to which the coil assembly  28  is mounted. The stator  34  is mounted on the shaft  32  by a bearing  39 . As shown in  FIG. 2 , the units  10  are arranged around a periphery of the rotor disc  30 , respective planes of the unit arms  18  and  20  being disposed perpendicular to the rotor shaft axis  40 . 
         [0062]    In use of the generator  12 , magnetic attraction forces exist between the pairs of opposed arms  18 ,  20  of each magnetic flux conducting unit  10 . These magnetic attraction forces seek to close the air gap  26  and thereby impart mechanical loads on the arms  18 ,  20 . By virtue of the connection with the portion  22 , and the fact that the magnets  14 ,  16  are arranged such that the magnetic flux flow path  24  extends from the magnet  16 , through the arm  20 , through the connecting portion  22 , through the arm  18  and to the magnet  14 , these mechanical loads are transmitted to the connecting portion  22 . In effect, the arms  18  and  20  are cantilevered relative to the connection portion  22 . The attraction force between the arms  18 ,  20  imparts a mechanical load on the arm  18 . This generates a turning moment about a central or neutral axis  42  of the C-core in an anti-clockwise direction when viewing  FIG. 1 . In contrast, mechanical loading on the arm  20  generates a turning moment about the neutral axis  42  in a clockwise direction. These turning moments effectively balance and cancel out with a result that the magnetic attraction forces between the arms  18  and  20  are reacted through and balanced within the connecting portion  22 . By this arrangement, it is not necessary to provide a large, heavy support structure in order to maintain the air gap between the arms  18  and  20  and thus the overall size and weight of the generator  12 , when compared to existing, direct drive generators, is greatly reduced. This is achieved whilst maintaining a high flux density and a small air gap between the arms  18  and  20 , thus ensuring efficient operation of the generator  12 . 
         [0063]    The generator  12  operates to generate electricity as follows. As noted above, the various units  10  are arranged around the periphery  38  of the rotor disc  30 . The magnetic flux flow path in the units adjacent to the unit  10  are in opposite directions. Thus the flux flow path in the units adjacent to the unit  10  shown in  FIG. 2  flow in clockwise directions. This is achieved by reversing polarity of the magnets  14 ,  16  on the units adjacent to the unit  10  shown in the figure. 
         [0064]    Accordingly, in use and when the rotor disc  30  is driven by the wind machine rotor shaft  37 , the coil assembly  28  is exposed to successively changing magnetic flux flow, this generating a current in the coils of the coil assembly. It will be understood that the units  10  may be provided on the stator  34  and the coil assembly  28  on the rotor  29 , if desired. 
         [0065]    Turning now to  FIG. 3 , there is shown a schematic side view of part of a generator incorporating the unit  10  shown in  FIG. 1 , in accordance with an alternative embodiment of the present invention, the generator indicated generally by reference numeral  112 . Like components of the generator  112  with the generator  12  of  FIG. 2  share the same reference numerals, incremented by  100 . 
         [0066]    The generator  112  includes two rotor discs  130   a  and  130   b , each carrying an array of flux conducting units  10  spaced around a circumference of each of the discs  130   a ,  130   b  adjacent to the respective edges  138   a ,  138   b . The respective arms  18 ,  20  of the units  10  on each rotor  130   a ,  130   b  are arranged such that the planes of the arms are parallel to the rotor shaft axis  140 . A stator  134  of the generator  112  carries two sets of coil assemblies  128   a ,  128   b  for each of the units  10  on the rotors  130   a ,  130   b . The generator  112  thus permits two rotors  130   a ,  130   b  to be driven from a common rotor shaft  132 , providing improved efficiency without a significant increase in dimensions. 
         [0067]    Turning now to  FIGS. 4 and 5 , there are shown schematic side and end views of part of a generator incorporating the magnetic flux conducting unit  10  of  FIG. 1 , in accordance with a further alternative embodiment of the present invention, the generator indicated generally by reference numeral  212 . Like components of the generator  212  with the generator  12  of  FIG. 2  share the same reference numerals, incremented by  200 . 
         [0068]    The generator  212  is of a linear type, suitable for use with a linear power generation machine such as a wave power generation machine or a free-piston Stirling engine in a domestic CHP unit (not shown). In the illustrated embodiment, a number of flux conducting units  10   c ,  10   d ,  10   e  and  10   f  are shown and are mounted on a translator  44  which is directly coupled to the prime mover of the machine. A coil assembly  228  is arranged in the air gaps  26   c  to  26   f  of the respective units  10 . As shown in the figure, the direction of flux flow in the adjacent units  10   c  to  10   f  are in opposite directions, the arrow tail indicating flow into the paper and the arrow head indicating flow out of the paper in the respective flow paths  24   c  to  24   f . Accordingly, on translation of the units  10   c  to  10   f  back and forth in the direction of the arrows X-X′, the coil assembly  228  experiences successively changing magnetic flux flow, generating a current in the coils. As shown in  FIG. 5 , a bearing assembly  46  is provided between a support  48  on which the coil assembly  228  is mounted, to facilitate relative movement between the units  10  and the coil assembly  228 . 
         [0069]    Turning now to  FIG. 6 , there is shown a schematic view of a magnetic flux conducting unit for a generator, in accordance with an alternative embodiment of the present invention, the unit indicated generally by reference numeral  310 . Like components of the unit  310  with the unit  10  of  FIG. 1  share the same reference numerals, incremented by  300 . 
         [0070]    As shown in the figure, the unit  310  includes a pair of opposed magnetic flux conducting elements in the form of arms  318 ,  320  which are coupled together by two magnets  314 ,  316 , each of which defines a connecting portion of the unit  310 . A space or air gap  326  is defined between surfaces  50  and  52  of the respective arms  318 ,  320  in which a coil assembly  328  of a generator is received. 
         [0071]    The unit  310  is generally rectangular in cross-section and the coil assembly  328  is provided centrally within the structure. As shown in  FIG. 7 , bearing assemblies  346  mount the coil assembly  328  within the air gap  326 , and facilitate relative movement between the unit  310  and the coil assembly  328 . 
         [0072]    The magnets  314 ,  316  are arranged relative to the arms  318 ,  320  such that two magnetic flux flow paths  324   a  and  324   b  are generated, in two loops extending from the magnets  314 / 316 , into the arm  318 , through the air gap  326 , into the arm  320  and back to the respective magnet  314 / 316 . These magnetic flux flow paths  324   a ,  324   b  extend in clockwise and anti-clockwise directions, respectively, when viewing  FIG. 6 . 
         [0073]    By this arrangement of the magnets  314 ,  316 , magnetic attraction forces between the arms  318 ,  320  are balanced on both sides of the unit  310  within the magnets  314  and  316 , about respective neutral axes  342   a  and  342   b . Accordingly, in a similar fashion to the unit  10  of  FIG. 1 , magnetic attraction forces between the arms  318  and  320  are reacted through and balanced within the connecting portions (magnets  314  and  316 ). This avoids a requirement to provide a large and heavy support structure. Additionally, provision of the two magnets  314 ,  316  provides a higher magnetic flux density in the air gap  326  in comparison to that present in the air gap  26  of the unit  10  shown in  FIG. 1 , providing improved efficiency of a generator incorporating the unit  310 . 
         [0074]    A generator incorporating magnetic flux conducting of like design to the units  310  is shown in the perspective view of  FIG. 8 , and is indicated generally by reference numeral  412 . Like components of the generator  412  with the generator  12  shown in  FIG. 2  share the same reference numerals, incremented by  400 . The generator  412  comprises three arrays of flux conducting units and corresponding coil assemblies  54 ,  54 ′ and  54 ″. One of the arrays,  54 ′, is shown in  FIG. 9  separately from a remainder of the generator  412 , for ease of reference. It will be understood that the generator  412  is of a linear type, similar to that described above with reference to  FIG. 4 . 
         [0075]    The array  54  comprises a number of flux conducting units  310   g  to  310   j  and, as with the generator  212  of  FIG. 4 , the magnetic flux flow paths in adjacent units flow in opposite directions. The units  310   g  to  310   j  are each separated by non-magnetically conductive spacers  56 , and are coupled to the translator of a reciprocating prime mover or reciprocating mechanical load (not shown), such as may be found in a wave power machine. The coil assembly  428  includes a number of coil sections  58 , and the units  310   g  to  310   j  and coil assembly  428  are shown separately in  FIGS. 10 and 11 , respectively. 
         [0076]    Each of the coil assemblies  428 ,  428 ′ and  428 ″ ( FIG. 8 ) are mounted to a stationary frame  60 , and each coil assembly  428 ,  428 ′ and  428 ″ is a three-phase winding comprising three layers of coil sections or windings  58 . The uppermost coil assembly  428  is shown in  FIGS. 9 to 11 , and comprises three layers of windings  58   a ,  58   b  and  58   c , each layer representing one phase. It will be understood that the arrays  428 ′ and  428 ″ are of similar structure. The units  310  of each array  54 ,  54 ′ and  54 ″ are mounted one on top of another on a moveable base  62 . In use, the units  310  of the arrays  54 ,  54 ′ and  54 ″ are reciprocated back and forth in the direction of the arrows Y-Y′, as shown in  FIG. 8 . This reciprocating movement and the variation in flux flow in adjacent units  310  of each array  54  ensures that each coil section  58  of the respective coil assembly  428  experiences progressively changing magnetic flux flow directions, generating current. 
         [0077]    By providing a generator  412  with arrays  54 ,  54 ′ and  54 ″ in this fashion, a common drive source may be utilized whilst optimizing size and weight of the generator  412  and providing improved efficiencies. 
         [0078]    Turning now to  FIG. 12 , there is shown a schematic end view of part of a generator in accordance with a further alternative embodiment of the present invention, the generator indicated by reference numeral  512 . 
         [0079]    The generator  512  is a linear generator similar to the generator  212  of  FIG. 4  and the generator  412  of  FIG. 8 . However, the generator  512  includes a number of magnetic flux conducting units  510 , like components of the unit  510  with the unit  10  of  FIG. 1 , and of the generator  512  with the generator  12  of  FIG. 2 , sharing the same reference numerals incremented by  500 . 
         [0080]    The unit  510  essentially comprises two of the units  10  of  FIG. 1  provided back-to-back, and with a single magnet  514  extending between flux conducting arms  518 ,  520  of the unit  510 . Two magnetic flux flow paths  524   a  and  524   b  are generated within the unit  510 , extending from the magnet  514  into the arm  518 , across air gaps  526   a / 526   b , into the arm  524  and back to the magnet  514 . The magnet of an adjacent unit is of opposite polarity, such that the flow paths in the adjacent unit extend in opposite directions. 
         [0081]    The units  510  are each coupled to a translator of a power generation machine such as a wave power machine (not shown), and are reciprocated in the same fashion as the generators  212 ,  412 . Coil assemblies  528   a ,  528   b  are provided in the air gaps  526   a ,  526   b  and are mounted on the stationary frame  560  by bearing assemblies  546   a ,  546   b . The generator  512  operates in a similar fashion to the generator  412  to generate electricity. 
         [0082]    Turning now to  FIG. 13 , there is shown an end view of part of a generator, incorporating a magnetic flux conducting unit, in accordance with a still further alternative embodiment of the present invention, the generator indicated generally by reference numeral  612  and the flux conducting unit by reference numeral  610 . Like components of the generator  612  with the generator  12  of  FIG. 2 , and of the unit  610  with the unit  10  of  FIG. 1 , share the same reference numerals, incremented by  600 . However, only the substantive differences will be described herein in detail. 
         [0083]    The generator  612  is in fact of similar construction to the generator  412  of  FIG. 8 , and is thus a linear generator comprising a number of arrays of flux conducting units and coil assemblies, one of which is shown and given the reference numeral  654 . The array  654  includes a number of flux conducting units  610  arranged in abutment, one of which is shown in the Figure. The units  610  each comprise a pair of spaced opposed arms  618 ,  620  carrying respective magnets  614  and  616 . The arms  618  and  620  are generally C-shaped in cross section and include lips or end sections  64  and  66 , respectively, which together define connecting portions  622 . The array  654  includes a coil assembly  628  which is located in an air gap  626  defined between the magnets  614  and  616 , and which includes a number of separate windings or coil sections  658   a ,  658   b  and  658   c . Bearings  68  are mounted between shoulders  70  and  72  of the arms  618  and  620 , and could be a low friction material such as PTFE, hydrostatic bearing, magnetic bearing or more conventional roller bearing. In this example a slider bearing is shown. As best shown in  FIG. 14 , which is a perspective view of the unit  610 , the bearings  68  include channels  74 . The coil assembly  628  includes mountings  76  which are shaped to engage within the bearing channels  74 , and which permit sliding movement of the units  610  relative to the coil assembly  628 . Accordingly, in a similar fashion to the generator  412  shown in  FIG. 8 , the units  610  of the array  654  are reciprocated back and forth relative to the coil assembly  628 , generating an alternating current. 
         [0084]    In use, two flux flow paths  624   a  and  624   b  are generated in the unit  610 , and the attraction forces between the arms  618  and  620  are balanced within the connecting portions  622  by abutment between the lips  64  and  66 . It will be understood, however, that the bearings  68  also resist the attraction forces between the arms  618  and  620  and thus may be considered to form part of the connecting portions  622 . Additionally, it will be appreciated that attraction forces between the lips  64  and  66  hold the arms  618  and  620  together. 
         [0085]    The units  610  are arranged in the generator  612  in a similar fashion to the units  310  shown in  FIG. 8 , save that spacers are omitted so that each unit  610  is located in abutment with the adjacent unit or units. This is because the inventors have discovered that flow of flux in an axial direction along the arrays of units  610  is beneficial and improves efficiency of the generator  612  in use. For the arrangement of the unit  610  shown in  FIG. 13 , with flux flow directions shown by the arrows in the flow paths  624   a  and  624   b , flux flow also occurs from the arm  620  of one unit  610  in the direction Y′ in  FIG. 14  (as indicated by the arrow tail going into the paper in  FIG. 13 ); into an arm  620  on an adjacent unit  610  (not shown); up through magnets  616  and  614  of the adjacent unit; into the arm  618  of the adjacent unit; and back into the arm  618  of the unit  610  shown in  FIG. 13  (as indicated by the arrow head coming out of the paper). 
         [0086]    Turning now to  FIG. 15 , there is shown a perspective, partially cut-away view of a generator, incorporating a magnetic flux conducting unit, in accordance with a still further alternative embodiment of the present invention. The generator is indicated generally by reference numeral  712  and is a rotary generator incorporating a number of circumferentially arranged units  710 . Like components of the unit  710  with the unit  10  of  FIG. 1 , and of the generator  712  with the generator  12  of  FIG. 2 , share the same reference numerals incremented by  700 . The generator  712  is generally of similar construction to the generator  112  shown in  FIG. 3 , save that it includes only a single circumferential array of flux conducting units  710  and coil assemblies  728 . 
         [0087]    The generator  712  is shown in more detail in the longitudinal sectional view of  FIG. 16 , which is drawn to a smaller scale, and in  FIG. 17 , which is a view of the flux conducting units  710 , cut-away as shown in  FIG. 15  and also drawn to a smaller scale. 
         [0088]    The generator  712  includes a rotor  729  having a rotor shaft  732  carrying a rotor disc  730 . The circumferentially arranged flux conducting units  710  are each mounted to the rotor disc  730  around a circumferential edge of the disc, and are of similar structure to the units  10  shown in  FIG. 3 , save that arms  718  and  720  are of greater length. As with the units  620  of the generator  612  of  FIG. 13 , the units  710  are butted-up to one another and thus provided without spacers, improving efficiency. A coil assembly  728 , which comprises a number of coil sections  758 , is mounted on a stator plate  736 , such that coil sections  658  extend into the respective air gaps  726  of the units  710 . The coil sections  658  are mounted in the air gaps  726  using suitable bearings  746 . The generator  712  is provided as part of a wind machine  733 , which is shown in  FIG. 18 . 
         [0089]    In use, the stator  734  carrying the coil assembly  728  is mounted in a nascelle  78  of the wind machine  733 , whilst the rotor shaft  732  is coupled to a prime mover in the form of a turbine blade assembly  80 . In this fashion, rotation of the blade assembly  80  transmits a drive force to the rotor shaft  732  and thus to the rotor disc  730 . 
         [0090]    This rotates the flux conducting units  710 , generating an alternating current in the fashion described above. 
         [0091]    Turning finally to  FIG. 19 , there is shown a schematic illustration of a power generation machine in the form of a wave device  633  incorporating the generator  612  shown in  FIG. 13 . The generator  612  is illustrated schematically in the Figure. The wave device  633  includes a buoy  82 , which is shown floating on a sea surface  84 , however, the inherent buoyancy of the buoy  82  relative to the weight of the remaining components of the device  633  may be such that the buoy is submerged below the surface  84 . 
         [0092]    The buoy  82  is coupled to a series of arrays of flux conducting units by a coupling assembly  86 , however, only a single such unit  654  is shown in the Figure. A stator  634  of the wave device  633  is provided on a seabed  88 , and the coil assembly is mounted on a base  90  of the stator  634 . 
         [0093]    In use, the buoy  82  moves up and down under applied wave loading, moving the array  654  up and down and thus translating the flux conducting units  610  relative to the coil assembly  628 , thereby generating an alternating current. End stops  92  and  94  define the maximum permissible extent of movement of the array  654  relative to the coil assembly  628 . 
         [0094]    Various modifications may be made to the foregoing without departing from the spirit and scope of the present invention. 
         [0095]    For example, it will be understood that the generator may be used with or provided in a wide range of different types of machines, provided as a rotary or liner generator as required or desired, and may be used with or provided in, inter alia, wave, wind, tidal and marine current power generation machines.