RELATED APPLICATIONS 
     The present invention is a U.S. National Stage under 35 USC 371 patent application, claiming priority to Serial No. PCT/EP2013/072558, filed on 29 Oct. 2013; which claims priority from European Application No. 12190733.1, filed 31 Oct. 2012, the entirety of both of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to an electrical machine. 
     BACKGROUND 
     In many types of electrical machine, electric currents in a stator create a travelling magnetic field that interacts with a set of electric currents, a set of permanent magnets or a set of ferromagnetic features on the moving part of the machine, i.e. a rotor in the case of a rotary machine, or a translator in the case of a linear machine. The usual method of creating the travelling field is to use three component windings housed together in a uniform array of slots in a laminated iron stator and fed with three alternating currents from a three-phase electrical power supply. Each component winding has a cyclic distribution of coils and the three component windings are placed with their magnetic axes spaced at intervals of ⅓ of a wavelength along the stator. The alternating currents fed to the windings have relative phase differences of 120 degrees. As a result, the combined magnetic flux approximates a sinewave of constant amplitude travelling at a speed equal to one wavelength of the cyclic distribution during one cycle of the alternating current. Other polyphase windings are possible but are seldom used because the prevalent form of power supply is the three-phase type. 
     The most common type of electrical drive, the three-phase induction motor of the type used in many industrial applications, employs this method to produce a rotating field. Referring to  FIG. 1( a ) , a rotor  12  carries a set of conducting bars (not shown) in which electric currents are induced and which interact with the rotating field to produce torque on an output shaft. 
     In domestic and light industrial applications, where a three-phase electrical supply is not available, an alternative arrangement is used whereby two component windings are supplied with alternating currents differing in phase. The first current is provided directly by an available single-phase supply and the second is obtained from the same supply usually via a capacitor that introduces a phase shift. The phase shift is of the correct degree only under one load condition so under most conditions the operation of such motors is not ideal. Such machines are referred to as single-phase motors because they operate from a single-phase supply but the windings are more accurately described as two-phase windings. If a balanced two-phase supply providing two currents displaced in phase by 90 degrees were available, electrical machines of this type with component windings displaced by ¼ of the wavelength would create a rotating field of constant amplitude and could be just as effective as standard three-phase machines. 
     In recent years, linear electric motors have attracted interest for several applications including guided ground transport and for electromagnetic launch systems on aircraft carriers. Also linear generators have been used in certain wave-power devices relying on reciprocating movement. A linear electrical machine can be considered as a standard rotary machine that has been cut and un-rolled, as shown in  FIG. 1( a )  . . . ( c ). When this is done, the magnetic force of attraction indicated by the force lines  16  between stator  10  and translator  14  is no longer balanced by equal and opposite forces as in the case of the stator  10  and rotor  12 . 
     A common approach to overcome this problem is to use two stators  10 ′,  10 ″ placed on opposite sides of a single translator  14  as shown in  FIG. 2 . In most cases magnetic flux  18  passes from the first stator  10 ′, through the translator  14 , through the second stator  10 ″, and through the translator for a second time to complete its circuit. 
     The two-stator linear machine could be rolled up around the original axis (shown as A in  FIG. 1( b ) ) to form a rotary machine with two co-axial stators enclosing a hollow cylindrical rotor and with magnetic flux passing radially from the inner stator, through the inner gap, the cylindrical rotor and the outer gap to the outer stator. Machines of this configuration have applications in servo control systems where a drive of very low inertia is required. 
     Alternatively, the linear machine may be rolled up around an axis orthogonal to the original axis (shown as B in  FIG. 2 ) to create a rotary machine where the flux passes axially across two planar airgaps rather than radially through cylindrical airgaps. Axial-flux rotating machines of this type have been used particularly as permanent-magnet generators for renewable energy applications, notably small wind turbines. In such cases each of the two stators normally carries a three-phase winding comprising three component windings as described above. 
     It is an object of the present invention to provide an improved electrical machine. 
     SUMMARY 
     According to the present invention there is provided an electrical machine as claimed in claim  1 . 
     In embodiments of the invention, a two-stator machine has a two-phase winding in which one component winding is housed in each stator and the two component windings are aligned with their magnetic axes displaced by approximately 90 electrical degrees. 
     The displacement between the magnetic axes of the two windings is ideally 90° and the windings are ideally supplied with alternating currents that differ in phase by 90°. The invention applies to motors or generators in linear or either radial- or axial-flux rotary form. Each stator carries a single winding and this removes the restrictions caused by each winding having to share a common array of slots. As a result:
     1. The winding distribution can be arranged to approximate closely to a sinusoidal pattern because the slots no longer need to be uniformly distributed and can be distributed according to the requirements of just the one winding housed in them.   2. The winding layout is simplified because the winding does not need to overlap other windings and so it may be a straightforward set of planar coils in the case of a linear machine or an axial-flux rotary machine   3. The insulation is simpler because the winding needs only to be isolated from ground and not from other windings.   4. As a consequence of 2 and 3 it becomes practicable to form each winding from a continuous length of insulated cable. For undersea applications, this is especially beneficial as it simplifies the waterproofing the stator electrics as all winding connections can be readily made within a single sealed unit.   

     The movable member, whether a rotor or translator, may comprise a conducting body so that the machine operates as an induction motor or generator. Such an induction generator is tolerant to hypocycloidal motion produced by movement of a shaftless rotor within a turbine of the type shown in  FIGS. 3-5 . The rotor or translator may also include ferromagnetic parts to assist the passage of magnetic flux between the two stator structures. 
     Alternatively, the rotor or translator may comprise an array of permanent magnets or electromagnets so that the machine operates as a synchronous machine. With appropriate control of the electrical feed, such a machine would form a so-called brushless-dc machine. 
     A further alternative is to use a rotor or translator that includes an array of ferromagnetic features that interact with the magnetic field of the stator to create torque in the case of a rotary machine or thrust in the case of a linear machine. With appropriate control of the electrical feed, such a machine could form a synchronous reluctance machine, a switched-reluctance machine or a stepper motor. 
     The two windings may be fed from a two-phase electrical source or, in the case of a generator, they may feed into a two-phase load. A two-phase source may be provided by:
     1. a special-purpose two-phase alternator;   2. a power-electronic converter with two-phase output, such as may be arranged using two H-bridges using IGBTs and connected to a common dc link;   3. a three-phase source with a set of transformers connected to convert between three-phase and two-phase systems, such as a Scott-T connected transformer; or   4. a single-phase source with a phase-shifting impedance as used in a single-phase motor.   

     A two-phase load for a generator may employ one of the above arrangements operating with reversed power flow or it may comprise simply resistive load banks. In the case of a long linear machine or a large-diameter rotary machine, it is preferred to divide the stators into a number of manageable sections. For a rotary machine, the sections may be a set of arcs disposed around the machine or alternatively if a large number of sections is used, then in the case of an axial-flux machine it may be convenient and more cost effective to make the sections straight and disposed tangentially. If the sections are spaced apart, then the emfs in their windings differ in phase. If the distance between sections is ⅓ of a wavelength then the phase difference is 120 degrees and if the stator has been divided by a multiple of three, then a three-phase supply can be used to supply the sections of one stator. If the distance is ⅙ of a wavelength then a three-phase source may be used with the windings of alternate blocks reversed. In either case, a second three-phase supply with its three outputs in phase quadrature with those of the first three phase supply can be used to supply the sections of the second stator. If the windings of the sections of the first stator are connected in star, then the required second three-phase supply can be provided by connecting the windings of the second stator in delta and to the same three-phase source. In this way a standard three-phase source can be used. It is noted that the voltage applied to the windings connected in delta is higher than the voltage applied to the windings connected in star and the current is less. The windings of the two stators are therefore preferably arranged to operate at different voltage and current by specifying proportionately different numbers of turns of conductor of different cross section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIGS. 1( a ) to ( c )  show schematically the components of a rotary and a linear electrical machine; 
         FIG. 2  shows a two-stator linear machine; 
         FIG. 3  shows in elevation a housing for an open centered hydro-turbine including an electrical generator according to an embodiment of the present invention; 
         FIG. 4  shows a perspective view of the housing of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view through the line V-V and the plane V-V indicated in  FIGS. 3 and 4 ; 
         FIG. 6  shows an exemplary layout of windings on a stator for the turbine of  FIGS. 3-5 ; 
         FIGS. 7( a ) and ( b )  show in detail the layout of windings for a pole of the stator of  FIG. 6 ; 
         FIG. 8  shows a cross-section of an exemplary laminate for accommodating the windings of  FIGS. 6 and 7 ; 
         FIG. 9  shows a cross-section of an alternative laminate for accommodating windings for a stator; 
         FIG. 10  shows an electrical connection for the windings of the two stators arranged to operate with a standard three-phase electrical system; 
         FIG. 11  shows a two-bridge converter for use with the system of  FIG. 10 ; and 
         FIG. 12  shows a converter providing a two-phase output for use in alternative implementations of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One particular application for the present invention is an axial-flux, low-speed, direct-coupled, rotary generator for use in an open-centre, shaftless, tidal-current turbine. Referring now to  FIGS. 3 and 4 , there is shown such a turbine  30  including an electrical motor/generator according to an embodiment of the present invention. The turbine comprises a runner, comprising a set of blades  32  that are fixed between an inner ring  34  and an outer ring  36 , housed in a shroud  38 . Typically, the turbine housing also includes mounts which allow the turbine to be secured to a subsea structure such as a gravity base—although this detail is not shown in the present case. 
     Turning now to  FIG. 5  which shows a portion of the turbine in cross-section. The outer ring  36  runs within a channel defined by a front annular support  40 , an outer cylindrical support  42  and a rear annular support  44 —front and rear being defined relative to the movement of fluid through the turbine indicated by the arrow F. Nonetheless, it will be appreciated that the turbine can be bidirectional and so responsive to fluid flow in either direction. Each support  40 ,  42 ,  44  is separated from the ring  36  by a respective bearing  40 ′,  42 ′,  44 ′. In the embodiment, none of these components need be magnetic and can be designed purely from a mechanical transmission efficiency point of view. 
     In this embodiment, a motor/generator assembly is disposed axially away from the turbine assembly. The motor/generator assembly again comprises a front annular support  46 , a cylindrical outer support  48  and a rear annular support  50 . The front annular support  46  can be fixed to the rear annular support  44  or indeed these can be produced as an integral component. A first circular stator comprises a set of coils  52  wound around a circular laminate  53  as described in more detail below and the laminate  53  is fixed to the front support  46 . A second circular stator comprises a set of coils  54  wound around a circular laminate  55  again as described in more detail below and the laminate  55  is fixed to the rear support  50 . An annular rotor  56  is disposed between the two stators  52 ,  54  and this motor/generator rotor  56  is directly coupled to the outer ring  36  of the runner via a transmission ring  60 . Thus, when the motor/generator  52 ,  54 ,  56  is driven to start the turbine, rotation is transmitted to the runner via the ring  60 , and once running, the runner drives the motor/generator via the ring  60 . It will nonetheless be appreciated that in the right tidal conditions, tidal flow could be used to start the turbine without the need to kick-start the turbine electrically. However, the possibility of kick-starting the turbine means that it can operate across a greater range of conditions including low flow conditions than if this option were not available. 
       FIG. 6  shows the construction of the stators  52 ,  54  of  FIG. 5  in more detail. In some applications, the stator could have a mean diameter of 9 m and in this case a pole number of 48, i.e. the winding  53 ,  55  on each stator  52 ,  54  is a single-phase 48-pole arrangement. While it will be appreciated that the second stator  54  is identical to and facing the first  52 , in a 48 pole arrangement it is circumferentially displaced from the first by an angle of 3.75° which is half of a pole pitch. 
     In one embodiment, the winding of each stator is formed from 12 lengths of insulated cable laid into 864 slots.  FIG. 7 a    illustrates the first stage  60 ( 1 ) of the cable layout for one pole  60 , where the number of slots per pole has been reduced from 18 to 10 in order to simplify the illustration. Cable laying progresses in a helical manner from the outside to the inside to form a single-layer, 9-turn, planar coil. Cable laying then proceeds as shown dotted in  FIG. 7( a )  and a second layer  60 ( 2 ) is formed in the same slots in the pattern shown in  FIG. 7( b ) , which finishes at the outside. On completion of the second layer, cable laying may proceed to form two further layers in the same slots. Alternatively, cable laying may proceed to form two layers on an adjacent pole. In the present example, each of the 12 lengths of cable is used to form a total of 16 pairs of layers and these may be arranged as 2 layers on each of 16 poles, 4 layers on each of 8 poles or 8 layers on each of 4 poles. It will be seen that the two ends of each length of cable are situated on the outside of the windings and so it is convenient to provide sufficient spare length to reach an electrical equipment enclosure (not shown) where the cable ends may be passed through watertight glands. This arrangement is favourable for subsea use as it avoids any cable joints or splices that are expensive to form and are vulnerable points where electrical faults could occur. The 12 lengths of cable may be connected in parallel to a common single-phase ac source or they may be isolated and connected to separate independent single-phase sources so that in the event of a cable failure or failure of a source, the system can continue to operate. 
     Each of the stator cores  53 ,  55  is ideally a continuous laminated ring that could be formed from a continuous spiral of electrical steel sheet. However a continuous ring of the required dimensions could be unwieldy and in some applications, it is preferred to divide the core into a number of arcs or short straight lengths that assemble to form a polygon. In the present example, the preferred arrangement has 12 arcs or straight lengths and the cable winding is formed as four poles of eight layers on each. The winding may therefore be formed before the core sections are assembled to form the complete stator ring (or polygon). 
     In one embodiment, slots  80  are distributed as shown in  FIG. 8  with positions selected to give progressively higher concentrations of turns further from the pole axes, giving a good approximation to a sinusoidal current distribution.  FIG. 8  shows sections from the two stators facing each other and displaced by a half pole pitch. 
     In alternative arrangements, the slots can be arranged in a uniform array of slots but with each slot containing different numbers of turns. This can be achieved by omitting some of the inner loops of cable around each pole for the first few layers. In this case, the slot depth may differ according to the number of turns allocated or the slots may be all of the same depth and those containing fewer turns may contain inert packing. The illustration in  FIG. 9  shows a lamination with deep slots  90  furthest from the pole axis which could contain 8 layers of cable. The first two layers of the winding for each pole would have 2 loops. Closer to the pole axis, shallower slots allow the next two layers to have 3 loops and the last four layers contain 4 loops. 
     A combination of the approach shown in  FIGS. 8 and 9  for achieving a sinusoidal distribution could also be adopted. The benefit of achieving a close approximation to a sine wave is the reduction in losses associated with space harmonic components of the current distribution. Harmonic components give rise to flux components that rotate at speeds different from the speed of the fundamental and induce unwanted currents within the rotor leading to additional losses. 
       FIG. 10  is an electrical diagram showing an exemplary electrical system for the generator. In one implementation, three windings a, b and c are connected in star to the source/grid  100  and are housed with one stator; whereas a second set of windings A, B and C are connected in delta to the source/grid and housed with the other stator. Each stator structure is split into sections with the sections arranged so that the travelling magnetic field within the machine induces single phase alternating emf in the sections displaced in phase by 120 electrical degrees. Thus, each winding a, b, c, A, B, C is connected to a respective section of each stator. The delta-connected windings A, B, C should have approximately √3 times as many turns as those in star connection because they operate with the same magnetic flux but at √3 times higher voltage. However they carry only 1/√3 as much current to deliver the same power and so their windings may be of correspondingly smaller cross section. 
     Exemplary power conversion circuitry  100  comprising a voltage-source inverter is described in PCT/EP2012/065701.  FIG. 11  in particular shows a two-bridge converter where both a first stage converter  110  and a second stage converter  120  comprise a 3-phase voltage source inverter connected via a DC link  140 . The phase ports of the first stage  110  are connected to the source/grid, whereas the phase ports P 1 , P 2 , P 3  of the second stage  120  are connected to the stator blocks as shown in  FIG. 10 . 
     Referring now to  FIG. 12 , in other implementations, each stator&#39;s single phase winding can be connected to a power converter  110 ,  140 ,  125  comprising a respective H bridge that converts a dc source  140  to a single-phase ac source, phase A and phase B, the dc ports of the two H bridges  125  being connected to a common dc source  140  and their ac ports being arranged to provide voltages phase A and phase B, that differ in phase. 
     The rotor  56  is an annular sheet or plate of electrical conductor that rotates in the gap between the two stators as illustrated in  FIG. 5 . The annulus may be fabricated from several arcs or trapezoidal sheets. It is best to avoid using a number of sheets equal to the number of stator sections so that any torque pulsations created by the joints tend to mutually cancel.
An electrical machine comprising a stator and a member arranged to move relative to said stator. The stator comprises a first stator structure incorporating a first plurality of windings disposed along the direction of relative motion and displaced from the movable member in a direction perpendicular to the direction of motion, and a second stator structure incorporating a second plurality of windings displaced in a second direction from the movable member, so that the movable member lies between the two stator structures. The movable member is arranged to electrically couple with the windings. The first plurality of windings includes a single phase winding connected to a first single-phase ac supply, and the second plurality of windings includes a single phase winding of the same pole pitch being displaced in the direction of relative motion from first plurality of windings and connected to a second single-phase ac supply whose phase is displaced from that of the first supply.