Abstract:
An electric machine ( 10;100 ) comprises a rotor ( 14   a ), preferably having permanent magnets ( 24   a,b ), although other field generation means are available, and a stator ( 12 ). The stator has coils ( 22 ) wound on stator bars ( 16 ) for interaction with the magnetic field of the rotor across an air gap ( 26   a,b ) defined between them. The rotor ( 14 ) comprises a housing ( 54 ) of a chamber ( 70 ) containing refrigerant ( 82 ). The rotor housing ( 54 ) has heat dissipating fins ( 96 ) accessible by the open environment whereby air movement relative to the housing caused at least by rotation of the rotor absorbs heat from the fins. The machine may bean axial flux machine, the coils being wound on bars that are disposed circumferentially spaced around a fixed axle of the machine forming a rotational axis ( 80 ) of the rotor. The machine may be a wheel motor for a vehicle, wherein the wheel is mounted directly on the rotor housing ( 14 ).

Description:
[0001]    This invention relates to an electric machine comprising a stator and a rotor journalled for rotation around the stator. The stator is provided with coils and the rotor is provided with means to generate a rotor magnetic field to cooperate with the coils across an air gap between the rotor and stator. The machine will usually be a motor but it may be a generator and is in many embodiments an axial flux machine. In particular it relates to a yokeless and segmented armature machine (hereinafter termed a “Y machine”), particularly to a permanent magnet machine. 
       BACKGROUND 
       [0002]    Woolmer and McCulloch [1] describe the topology of a Y machine, discussing its advantages of reduced iron in the stator enabling an improvement in torque density. It comprises a series of coils wound around bars spaced circumferentially around the stator, ideally axially disposed, (ie parallel the rotation axis of the rotor). The rotor has two stages comprising discs provided with permanent magnets that face either end of each coil of the stator. The magnetic path at any stage of operation is: through a first coil into a first magnet on a first stage of the rotor; across a back iron of the rotor to an adjacent second magnet on the first stage; through a second coil of the stator adjacent the first coil; into a first magnet on the second stage of the rotor aligned with the second magnet on the first stage; across the back iron of the second stage to a second magnet on the second stage and aligned with the first magnet on the first stage; and completing the circuit through the first coil. 
         [0003]    One difficulty with electric machines generally is to provide adequate cooling. This is a particular problem with a Y machine having a high torque density that significant heat is generated in the coils at high torques and is often a limiting factor in the torques that can be employed, at least for extended periods of time. Also, the coils are isolated from one another and therefore cooling only one region of the motor is insufficient as there is low conduction of heat between coils. 
         [0004]    WO-A-2006/066740 discloses a Y machine comprising a housing having a cylindrical sleeve mounting stator coils internally, the sleeve being hollow whereby cooling medium is circulated. However, the coils are embedded in a thermally conducting material to carry heat to stator housing. A rotor is rotatably journalled in the housing. The stator bars appear to be laminated, as they are in GB-A-2379093 that also discloses a Y machine, as does WO-A-03/094327. No cooling arrangements are mentioned. 
         [0005]    U.S. Pat. No. 6,720,688 discloses a Y machine in which the rotor acts as a vane pump to circulate fluid within a chamber defined by a stator housing through which a rotor shaft, supported on bearings in the housing and carrying the rotor, extends. The fluid cools stator coils. 
         [0006]    Of course, cooling problems are not limited to Y motors. 
         [0007]    The idea of evaporative cooling has been employed in, for example, SU-955379, where a hollow rotor shaft appears to extend into an external rotating housing so that refrigerant evaporating in the shaft cools the rotor and vapour condenses in the external housing releasing its heat before returning as a liquid. U.S. Pat. No. 5,394,040 discloses a similar arrangement. SE-A-7411152 likewise appears to disclose evaporative cooling of a motor. These devices are passive, where the cooling circuit is self-driven, but more active arrangements are disclosed. U.S. Pat. No. 3,217,193 sprays liquid refrigerant on the hot parts of a motor or generator. US-A-2007/0199339 discloses a complete refrigerant circuit with valves ensuring correct directional flow between passages through the stator and an external heat exchanger. This development is of particular interest to the present invention which finds a primary application in wheel motors for vehicles. 
         [0008]    Indeed, it is an object of the present invention to provide an electric machine with effective, but nevertheless passive, evaporative cooling arrangements. It is a particular object to minimize the additional elements and equipment required for cooling, whereby the cost in terms of power lost through the cooling arrangements can be minimized. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    In accordance with the present invention there is provided an electric machine comprising a rotor having a rotor field and a stator having coils for magnetic interaction with the rotor field across an air gap defined between the rotor and stator, wherein the stator is a fixed component with respect to a mounting for the machine and the rotor rotates around the stator externally thereof forming a rotating housing defining a sealed chamber between the rotor and stator incorporating cooling medium to cool the coils, and wherein the cooling medium has a boiling point less than a design temperature of operation of the stator and more than design temperature of operation of the rotor housing, and wherein the rotor housing has a heat dissipating external surface accessible by a coolant. Preferably, the coolant is ambient air of the open environment. 
         [0010]    The chamber in use comprises a volume that is preferably filled to less than about 25% by volume with said cooling medium when liquid. The volume ratio of liquid to vapour changes as temperature rises and pressure increases. The fill volume of less than 25% refers to ambient conditions. Preferably the remaining volume is filled only with the cooling medium in vapour form. The pressure in the chamber is preferably ambient when the machine is not operating. Pressure accommodating means they may be provided. 
         [0011]    Preferably, the electric machine is an axial flux machine, the coils being wound on bars that are disposed circumferentially spaced around a fixed axle of the machine forming a rotational axis of the rotor. Preferably, the bars are parallel to the rotational axis, the rotor comprising two stages each having permanent magnets interacting with each end of the bars. 
         [0012]    Preferably, the stator is mounted on a stub axle of said mounting and on which stub axle said rotor is rotationally journalled. 
         [0013]    Preferably, the rotor stages each comprise an annular dish, whose outer rims are connected together by a sleeve and which mount said permanent magnets, an inner rim of one stage being journalled on said stub axle, and the other stage being closed by a plate. 
         [0014]    Preferably, said plate is also journalled on an end of said stub axle extending through said stator. 
         [0015]    Preferably, said sleeve has fins to shed heat externally. The fins may be radially directed or axially directed. The sleeve may be coned so as to ensure that some area of it is available for contact by cooling medium vapour. The chamber is preferably sealed on said stub axle by a lip seal. However, any suitable alternative may be employed. 
         [0016]    Preferably, diffusive material is disposed around and between said coils whereby liquid cooling medium is captured and transported to the coils and vapour from the liquid cooling medium evaporating from the coils escapes. The diffuse material acts as a wick, similar to its application in a heat pipe. 
         [0017]    Preferably, paddles are disposed internally of the rotor housing to wash liquid cooling medium around the stator. 
         [0018]    Preferably, scrapers are disposed on the stator to scoop onto the stator liquid cooling medium held against the inside of the rotor housing by centripetal acceleration. 
         [0019]    In one embodiment, said machine is a motor. Indeed, it may be a wheel motor of a vehicle. 
         [0020]    Thus, in one aspect, the invention provides a vehicle having a stub axle suspended from vehicle and on which a motor as defined above is mounted. Preferably, a wheel of the vehicle is mounted on said rotor housing. Preferably, regenerative braking is employed to retard the vehicle. In any event, preferably, a brake disc is mounted on said rotor housing. Said disc may be mounted on said one stage of the rotor. The wheel preferably has apertures to allow access to air passing the vehicle to flow through the wheel and act as said coolant to cool the rotor housing. Preferably, said fins are shaped to draw air through said apertures. Said shape may be the fins having a helical form on said housing. 
         [0021]    In another embodiment, said machine is a generator, means being provided to rotate the rotor about said stator. Said means may comprise a belt and pulley. While the invention can perfectly feasibly operate as a generator, in that event, if the generator is stationary and accessible by users, it may be necessary, or at least desirable, to guard the rotating rotor to prevent accidental contact with users. The guard needs to have access to coolant flow (preferably airflow) in order to enable the rotor to shed heat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic side view of a yokeless and segmented armature machine to which the present invention primarily (but not exclusively) relates; 
           [0024]      FIG. 2  is a perspective view of the arrangements of  FIG. 1 ; 
           [0025]      FIG. 3   a  and  b  are respectively a side section of a motor in accordance with the present invention and a section on the line B-B in  FIG. 3   a;    
           [0026]      FIG. 4   a  and  b  are respectively a perspective view of the stator of the motor of  FIG. 3  and a side view of a detail of  FIG. 4   a;    
           [0027]      FIG. 5  is a similar view to  FIG. 3   a , but where the bearing arrangement is modified; 
           [0028]      FIG. 6  is a schematic diagram of a generator arrangement in accordance with the invention; 
           [0029]      FIG. 7  is another schematic variation of the arrangements of  FIGS. 4 and 5 ; and 
           [0030]      FIG. 8  is a side section through a motor and wheel similar to that shown in  FIG. 3   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0031]    A yokeless and segmented armature machine  10  is illustrated schematically in  FIG. 1 . The machine  10  comprises a stator  12  and two rotors  14   a,b . The stator  12  is a collection of separate stator bars  16  spaced circumferentially about a rotation axis  20  of the rotors  14   a,b . Each bar  16  has its own axis  16   a  which is disposed parallel to the rotation axis  20 . However, that is not absolutely essential. In an axial flux machine, the axis  16   a  is indeed parallel the rotation axis  20 . However, it can be disposed at any angle thereto, even radially with respect to the rotation axis  20 . The following discussion is in respect of an axial flux machine, but this should not be understood to be limiting in any sense and, where the context permits, the invention equally applies to other inclinations of the stator bars  16 . 
         [0032]    Each end of each stator bar is provided with a shoe  18   a,b  which serves a physical purpose of confining a coil stack  22 , which stack  22  is preferably of square section insulated wire (or possibly rectangular section) so that a high fill factor can be achieved. The coils  22  are connected to an electrical circuit (not shown) that (in the case of a motor) energizes the coils so that the poles of the resultant magnetic fields generated by the current flowing in the coils is opposite in adjacent stator coils  22 . 
         [0033]    The two rotors  14   a,b  carry permanent magnets  24   a,b  that face one another with the stator coil  22  between. Indeed, in the axial flux machine, the rotors and their magnets are radially disposed, but when the stator bars are inclined, then they are likewise. Two air gaps  26   a,b  are disposed between respective shoe and magnet pairs  18   a / 24   a ,  18   b / 24   b . There are an even number of coils and magnets spaced around the axis of rotation  20  and, preferably, there are a different number of coils and magnets so that each coil does not come into registration with a corresponding magnet pair all at the same time and at the same rotational position of the rotor with respect to the stator. This serves to reduce cogging. 
         [0034]    In a motor (with which the present invention is primarily concerned) the above-mentioned electric circuit is arranged to energize the coils  22  so that their polarity alternates serving to cause coils at different times to align with different magnet pairs, resulting in torque being applied between the rotor and the stator. The rotors  14   a,b  are generally connected together (for example by a shaft, not shown, although see below) and rotate together about the axis  20  relative to the stator  12 , which is generally fixed (for example in a housing, not shown, although, again, see below). One advantage provided by the arrangement is illustrated in  FIG. 1  in that the magnetic circuit  30  is provided by two adjacent stator bars  16  and two magnet pairs  24   a,b . Thus, no yolk is required for the stator  12 , although a back iron  32   a,b  is required for each rotor linking the flux between the back of each magnet  24   a,b  facing away from the respective coils  22 . 
         [0035]    Thus, in the case of a motor, by appropriate energization of the coils  22 , the rotor  14  can be urged to rotate about the axis  20 . Of course, in the situation of a generator, rotation of the rotor  14   a,b  induces currents in the stator coils  12  according to the changing magnetic flux induced in the stator bars  16  as the rotors  14   a,b  rotate. 
         [0036]    However, in either case heat is generated in the coils  22  and the efficiency of the machine is reduced, and its capacity limited, if this heat is not removed. Accordingly, the present invention suggests enclosing the stator coils  16  within a housing formed by the rotor and which is supplied with a refrigerant cooling medium. 
         [0037]    Turning to  FIG. 3   a , a motor  100  according to the present invention comprises a stator  12  mounted on a stub axle  50  of a vehicle (not shown). The stator comprises coils  22  mounted on bars  16  supported between stator plates  52  fixed to the stub axle  50 . A rotor  14  comprises a housing  54  formed of  2  annular plates  56 , 58  connected at their outer rims by a sleeve  60 . Annular plate  58  is rotationally journalled through bearings  62  on the stub axle  60  which passes through an aperture  64  in the annular plate  58 . Annular plate  56  is filled with a centre plate  66  that is also journalled on end  50   a  of the stub axle through bearings  68 . Rotor housing  14 , formed by the annular plates  56 , 58 , sleeve  60  and end plate  66 , encloses a chamber  70  that is sealed by rotary lip seals  72  disposed between the annular plate  58  and the stub axle  50 . 
         [0038]    Rotor plates  56 , 58  are made from any convenient material and are preferably non-ferromagnetic. In that event, annular back irons  32   a,b  are provided which are of magnetically linking material and on which permanent magnets  24   a,b  are disposed, aligned with the stator bars  16 , and defining the air gaps  26   a,b  between them. 
         [0039]    A wheel  90  of the vehicle is mounted by any convenient means (not shown) on the rotor housing  14 . Also, inboard of the housing plate  58 , a brake disc (not shown) may be mounted on the rotor housing  54  for interaction with a caliper (not shown) mounted on the stub axle  50 . 
         [0040]    Power electronics  92  for operating the motor  100  may be mounted on a front flange  50   b  of the stub axle  50 . They may be supplied from the vehicle with cabling through channels (not shown) through the stub axle  50 . The power electronics  92 , control energization of the cols  22  to cause magnetic interaction with the magnets  24   a,b , as described above with reference to  FIGS. 1 and 2 , and in order to drive the rotor around the stub axle  50 , around an axis of rotation  80 . 
         [0041]    Heat generated by the coils  22  must be dissipated; otherwise the torque capacity of the motor will be limited. For this purpose, the chamber  70  incorporates liquid refrigerant coolant  82  which, when the vehicle is stationary, collects by gravity at the lowest point of the chamber  70 . However, when the rotor begins to rotate, paddles  84  disposed periodically around the inside  83  of the sleeve  60  scoop the liquid coolant  82  and splash the coils  16  around the entire periphery of the stub axle  50 . As the speed of rotation of the rotor  14  increases, centripetal acceleration may retain the fluid against the internal wall of the rotor  14 . Accordingly, at the top of the stator  12 , and potentially elsewhere around its periphery, are disposed scoops  86  that catch the liquid and splash it into contact with the coils  22 . 
         [0042]    The paddles  84  may be omitted, on the ground that, until speed builds up, cooling of the coils will not generally be necessary. Once speed has built up, however, centripetal acceleration and friction between the liquid and inner surface  83  of the sleeve will retain the liquid against the surface, so that it rotates also around the stub axle  50  with the rotor  14 . In this event, the scoops  86  may be arranged closer to the surface  83  of the sleeve  60 . Indeed, with reference to  FIG. 3   b , in the direction of rotation (eg normal forwards rotation for a vehicle in the direction of the Arrow C), the scoops  86   a,b,c  may be progressively closer to the surface. Thus, if there are three of them as shown, at ninety degree locations, paddle  86   a  will scrape off a first depth of the rotating liquid  82  (shown collected at the bottom), scoop  86   b  a second depth, closer to the surface  83 , and scoop  86   c  scraping off most of what remains. 
         [0043]    However, interspersed between and around the coils  22  is diffuse material  94  (see  FIGS. 4   a ,b) that absorbs the liquid coolant and wets the coils  22 . Indeed, the material may be a wicking material, akin to cotton wool, such as rock wool, for example. As heat begins to be generated by the coils  22 , this is transferred to the liquid refrigerant. This is arranged to have a boiling point below the desired working temperature of the coils  22 . Accordingly, when that temperature is reached, the liquid boils and evaporates so that it is driven away from the coils  22  taking heat with it, further liquid coolant being drawn in by the wicking material  94 . The vapor then filling the chamber  70  contacts the surfaces of the housing  14  which, at least around its periphery, is provided with fins  96 . At ambient temperature, the coolant is arranged to condense and give up its heat to the material of the rotor housing  14  and return to liquid form where the cycle can be repeated. 
         [0044]    As the temperature of the motor builds, the vapour pressure in the chamber  70  builds up but it cannot escape if the seal provided by the lip seal  72  is gas tight. While it does not escape and pressure builds as more heat is generated by the coils  22 , the boiling temperature of the liquid increases so that the temperature of the rotor housing also increases, shedding heat more quickly. Consequently, although the temperature of the coils also rises, the system is self regulating in shedding heat more quickly as it gets hotter. However, by virtue of the evaporative heat transfer, the temperature gradient between the coils  22  and housing  54  is much less while still transporting heat. 
         [0045]    An advantage of the present invention is that the rotor  96 , in rotating as it is driven, is exposed to a cooling airflow not only from its own progress, but also from the progress of the vehicle (when, as shown in  FIG. 3   a , the motor is mounted in a vehicle). In the case of a wheel motor as also shown in  FIG. 3   a , the wheels  90  may have apertures at  98  whereby airflow a can wash over the rotor housing  14  from outside the vehicle. Indeed, both the wheel  90  and the fins  96  may be arranged to scoop air and draw it over the housing  14 . For example, the wheel  90  may be provided with scalloped apertures  98 , so that air is directed towards the rotor housing  14 . Alternatively, or in addition, the fins  96  may be arranged as a helix screw so that air is pumped, for example, in the direction of the arrow BB in  FIG. 3   a.    
         [0046]    The bearings  62 , 68  are shown in contact with the chamber  70 . If this is the case, it is important that the coolant  82  act as a lubricant for the bearings. Alternatively, it would be possible to isolate the bearings  68  from the chamber  70 . For example, seal  72  could be inboard (with respect to the chamber  70 , of the bearing  64 . Another lip seal would be required for bearing  68  to isolate that from the chamber  70 . It is also important that the coolant  82  not affect the electronics  92 . 
         [0047]    Turning to  FIG. 5 , an alternative embodiment of a motor  100 ′ is illustrated. Here, a stub shaft  50 ′ of a vehicle (not shown) has a hollow end  50   a ′ in the bore  102  of which is received the shaft  104  of rotor plate  66 ′. The shaft  104  is supported on bearings  62 ′, 68 ′ which are spaced apart by a spacer  106  and retained by circlips  108 , 110 . Circlip  110  may be replaced by a nut serving to preload the bearings  62 ′, 68 ′. A seal  112  isolates the bearings  62 ′, 68 ′ from chamber  70 ′ containing liquid refrigerant coolant  82 . 
         [0048]    The rotor  14 ′ has a plate  66 ′ that is connected to rotor housing plate  56 ,′ which is itself connected to corresponding rotor housing plate  58 ′ via a coned sleeve element  60 . The taper of the sleeve  60  ensures that the liquid coolant  82  collects in a most radially distant (from the rotation axis  80 ) corner  70   a  of the chamber so that more of the internal surface  60   a  of the sleeve  60  is exposed to direct contact by vapour in the chamber  70 , rather than being insulated therefrom by liquid coolant  82 . 
         [0049]    Fins  96  are disposed, as in the embodiment described above, on the sleeve  60 . In addition, however, fins  96   a,b  (being axially directed and circumferentially disposed) are provided on the annular plates  56 ′, 58 ′. Otherwise, the arrangement is much as described above, with the wheel  90  being bolted by nuts  118  on studs  120  received in the plate  66 ′. A brake disc  122  is mounted on the inside annular plate  58 ′ and a seal  72 ′ isolates the chamber  70 ′ from the external environment. A further bearing  62   a  may support the brake disc  122 . 
         [0050]    In  FIG. 6 , a generator  100 ″ has essentially the internal structure of the motor illustrated in  FIG. 5  or  6 . Here a stub axle  50 ″ is mounted on the ground and the rotor housing  14 ″ is driven by a pulley  150  and belt  152 . The rotor has fins  96  and the entire rotating assembly is protected by an apertured guard  154 . 
         [0051]    In  FIG. 7 , rotor housing  14   x  has a vehicle tyre  160  mounted directly on flanges  162  of the housing. In this case, the rotor may be in two parts, a first hub part  5660 , bolted onto the vehicle hub  66   x  by bolts  118 , and a second flange part  58   x , secured by bolts  162  to the hub part  5660 , and sealed thereto by means not shown. As in the arrangement of  FIG. 5 , the hub  66   x  is journalled for rotation in the stub axle  50   x , suspended from the vehicle (not shown). Fins  96   x  are on the sides of the rotor  14   x . The rotor  14   x  may be recessed at  166  between tyre beads  168  to pool liquid coolant  82  and permit the edges/corners  70   b,c  of the chamber  70   x  to be free of liquid giving direct access of vapour to the cooler surfaces of the rotor  14   x.    
         [0052]    In  FIG. 8 , a motor  100  comprises a hub  50 ″ for fixing to vehicle suspension (not shown). The hub carries a bearing  62 ″ that rotatably mounts a rotor flange  56 ″. The flange  56 ″ mounts both a rotor housing member  54 ″ and a brake disc  57 ″, by means of studs and nuts  59 ″. The rotor housing member  54 ″ is finned at  96 ″ to shed heat, as described above with reference to  FIG. 3   a.    
         [0053]    The hub  50 ″ also mounts a stator disc  52 ″ on which a number of stator bars  16  are fixed, around which are wound coils  22 ″. The disc  52 ″ mounts power electronics circuit board  92 ″. To a face edge of cupped rotor housing member  54 ″ is secured by fasteners  53 ″ apertured rotor plate  58 ″. Its aperture  61 ″ is closed and sealed by a cover plate  66 ″, although access can be gained to the electronics board  92 ″ by removing fasteners  63 ″. The rotor components  54 ″, 58 ″ mount permanent magnets  24 ″ a,b , disposed adjacent the stator coils  22 . 
         [0054]    A seal  72 ″ closes the chamber  70 ″ defined between rotor  14 ″ and hub  50 ″ and surrounding the stator  12 ″. Hub  50 ″ has a central bore  51 ″ that is open, but it is plugged (by a plug not shown) to complete the seal and close chamber  70 ″, although cabling (not shown) from the vehicle for powering and controlling motor  100 ″ will pass through the plug to connect with electronics board  92 ″. 
         [0055]    Rotor plate  58 ″ mounts a wheel  90 ″ that has an internal flange  92 ″ for this purpose, through studs and nuts  94 ″. Otherwise, the arrangement is as described above and refrigerant in the chamber  70 ″ cools both the coils  22 ″ and the electronics board  92 ″. The fins  96 ″ may be spirally arranged on the face of the rotor housing member  54 ″ to promote circulation as the rotor rotates about the hub  50 ″. 
         [0056]    Suitable coolants are known to those skilled in the art (such as water, methyl acetate, flouro benzene, 2-heptene) and may be arranged so that the coils have a working temperature in the range ambient-150° C., with a coolant boiling point in the range 50-80° C. between 0.1 bar and 5 bar of pressure, depending on the application. In this scenario, the temperature of the housing  54  will be in the range ambient-80° C. 
         [0057]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0058]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0059]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
         [0060]    References 
         [0061]    [1] T J Woolmer and M D McCulloch “Analysis of the Yokeless and Segmented Armature Machine”, International Electric Machines and Drives Conference (IEMDC), 3-5 May 2007