Abstract:
An apparatus and method for holding permanent magnet components, particularly on a rotor of an electrical machine. A magnet carrier ( 10 ) comprises a channel ( 30 ) suitable for receiving an encircling magnet retainer ( 20 ) and a method of manufacturing a magnet assembly ( 100 ) comprises providing a magnet carrier ( 10 ) comprising a channel ( 30 ), mounting a magnet ( 60 ) on the magnet carrier ( 10 ), and encircling the magnet ( 60 ) and magnet carrier ( 10 ) with a magnet retainer ( 20 ), wherein the magnet retainer ( 20 ) passes through the channel ( 30 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an apparatus and method for magnet retention, in particular magnet retention for use with a permanent magnet rotor, such as for an electrical machine. 
       BACKGROUND 
       [0002]    Permanent magnet rotors have various applications in electrical machines, including use in a permanent magnet motor as shown in  FIG. 1 . Permanent magnet rotors, especially those with surface mounted magnets, often require additional mechanical means for retaining the magnets on the rotor hub. In particular, at high speeds, the magnets are subject to high centrifugal forces and it is not possible to rely on adhesive bonding of the magnet to the rotor hub surface alone. 
         [0003]    Relying on adhesive bonding on the contact surface between magnet and rotor back-iron also requires the magnet material to have sufficient strength under tension throughout its structure to prevent failure under centrifugal loads. The physical properties of sintered magnets typically used in high performance motors (e.g. neodymium iron boron NdFeB) can vary from batch to batch in production. Without resorting to X-ray inspection techniques, there is no way of identifying voids and potential crack propagation points in the material, and thus the quoted typical material properties cannot be relied upon. A suitable containment that acts to contain the magnets should they fracture in any way and prevents them from going in to tension (thus reducing the likelihood of fracture) is therefore often required. The containment must also prevent significant displacement of the magnets that would cause the rotor to become unbalanced. 
         [0004]    Retention of the magnet blocks can also be complicated by the requirement for segmented magnets. Typical magnet materials (e.g. sintered NdFeB) are also electrically conductive. Due to armature reaction and changes in the magnet working point as it rotates past the toothed structure of the machine stator, eddy currents are induced in the material leading to losses which reduce efficiency and lead to self-heating of the magnet material which can lead to a reduction in flux density and hence machine performance due to the temperature sensitive nature of the magnet material. Therefore the magnets are often laminated or segmented in order to impede the flow of eddy currents, usually with insulating bond planes between magnet segments. This can also reduce the mechanical strength of the magnet block. 
         [0005]    Typically, smaller rotors may have a composite sleeve placed over the magnets, as shown in  FIG. 2 , usually manufactured from a carbon fibre or glass fibre wound within an epoxy matrix. This may be “wet-wound” directly onto the magnet rotor and then cured. Alternatively, a tube may be pre-formed on a mandrel and fitted over the rotor. It is often desirable to achieve a pre-stress in the hoop direction of the fibres which minimises any movement of the magnets should they become dis-bonded. In the case of wet-wound rotors, this is achieved by winding the fibres under tension. For a pre-formed tube, the pre-stress may be achieved by using a thermal interference fit (e.g. rotor cooled before being dropped into a sleeve at room temperature). The outer surface of the overwrap sleeve may then be ground to achieve a high tolerance part compatible with the small airgap/clearances required in motors. 
         [0006]    Although larger diameter machines (used in marine propulsion or wind turbines for example) often run at low to medium speeds, the combination of large diameter and speed and/or use of relatively large magnetic blocks can lead to high centrifugal forces on the magnets. However, the method of overwrapping the complete rotor with a sleeve is problematic on large diameter rotors. The pre-tension requirements to overcome the strain in the carbon-fibre under loading and maintain negligible displacement in the magnets preclude its use as the overwrap thickness required would lead to poor electromagnetic performance due to the increased magnetic airgap. 
         [0007]    The present invention provides a method and apparatus for mounting magnets which has potential benefits in terms of ease of manufacture and mechanical strength while minimising the thickness of the retention system to ensure the magnetic airgap is minimised. It is particularly relevant to large diameter machines and high-speed applications where the centrifugal forces are significant and/or in applications with larger magnet poles. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention, there is provided a magnet carrier comprising a channel suitable for receiving an encircling magnet retainer. 
         [0009]    The magnet carrier may preferably be made of ferromagnetic material, most preferably steel. The floor of the channel may be chamfered and/or rounded and the channel may have a chamfered and/or rounded profile. The magnet carrier may comprise a plurality of channels suitable for receiving an encircling magnet retainer. The magnet carrier may further comprise mounting means suitable for mounting the magnet carrier to a rotor rim. The mounting means may comprise at least one tapped hole. 
         [0010]    There is further provided a magnet assembly comprising a magnet carrier comprising a channel suitable for receiving an encircling magnet retainer; a permanent magnet; and a magnet retainer which encircles the carrier and the permanent magnet, wherein the magnet retainer passes through the channel. 
         [0011]    The magnet retainer may comprise fibre filament. The fibre filament may be carbon fibre or glass fibre. The magnet retainer may further comprise an epoxy. The magnet retainer may be under tension. 
         [0012]    The magnet retainer may be thinner on a face of the magnet than it is in the channel. The magnet retainer may cover a greater surface area of a face of the magnet than it covers of the base of the magnet carrier. The magnet retainer may have a greater width on a face of the magnet than the width of the channel. 
         [0013]    The magnet may be a laminated magnet or a segmented magnet. The magnet may have at least one chamfered and/or rounded edge. 
         [0014]    There is further provided a permanent magnet rotor comprising a plurality of magnet assemblies comprising a magnet carrier comprising a channel suitable for receiving an encircling magnet retainer; a permanent magnet; and a magnet retainer which encircles the carrier and the permanent magnet, wherein the magnet retainer passes through the channel. 
         [0015]    The plurality of magnet assemblies may be coupled to a rotor rim. The rotor rim may be ferromagnetic. The channel may lie in a circumferential or an axial direction of the rotor. 
         [0016]    There is further provided a method of manufacturing a magnet assembly comprising providing a magnet carrier comprising a channel; mounting permanent magnet material on the magnet carrier; and encircling the permanent magnet material and magnet carrier with a magnet retainer, wherein the magnet retainer passes through the channel. 
         [0017]    The permanent magnet material may be unmagnetized when mounted to the magnet carrier. The permanent magnet material may be magnetized after it has been mounted on the magnet carrier. The permanent magnet material may be magnetized to create a permanent magnet after the permanent magnet material and magnet carrier have been encircled with the magnet retainer. 
         [0018]    The magnet retainer may comprise fibre filament. The magnet retainer may comprise carbon fibre. The magnet retainer may comprise glass fibre. The magnet retainer may further comprise an epoxy. The step of encircling the magnet and magnet carrier may comprise winding the fibre around the magnet and magnet carrier. The fibre may be wound under tension. 
         [0019]    The step of mounting permanent magnet material on the magnet carrier may include bonding the permanent magnet material to the magnet carrier using adhesive. The adhesive may be an epoxy. The adhesive may be an acrylic. 
         [0020]    There is further provided a method of manufacturing a permanent magnet rotor comprising manufacturing, in one of the ways discussed above, at least one magnet assembly; and mounting the at least one magnet assembly on a rotor rim. 
         [0021]    Mounting the at least one magnet assembly on a rotor rim may comprise loading the magnet assembly onto tooling or guideways; and guiding the magnet assembly onto the rotor rim using the tooling or guideways. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Embodiments of the present invention will now be described in detail by way of example with reference to the following figures in which: 
           [0023]      FIG. 1  shows a cross-sectional view of a permanent magnet motor; 
           [0024]      FIG. 2  shows a known permanent magnet rotor; 
           [0025]      FIG. 3  shows a permanent magnet rotor in accordance with an aspect of the present invention. 
           [0026]      FIG. 4  shows a permanent magnet rotor in accordance with an aspect of the present invention. 
           [0027]      FIG. 5  shows a permanent magnet rotor in accordance with an aspect of the present invention. 
           [0028]      FIG. 6  shows a perspective view of a magnet assembly in accordance with an aspect of the present invention; 
           [0029]      FIG. 7  shows an end view of the magnet assembly of  FIG. 6 ; 
           [0030]      FIG. 8  shows a side view of the magnet assembly of  FIG. 6 ; 
           [0031]      FIG. 9  shows a cross-sectional view of the magnet assembly of  FIG. 6 , taken along section A-A in  FIG. 8 ; 
           [0032]      FIG. 10  shows a perspective view of a magnet carrier in accordance with an aspect of the present invention; 
           [0033]      FIG. 11  shows a bottom view of the magnet carrier of  FIG. 10 ; 
           [0034]      FIG. 12  shows an end view of the magnet carrier of  FIG. 10 ; 
           [0035]      FIG. 13  shows a side view of the magnet carrier of  FIG. 10 ; 
           [0036]      FIG. 14  shows a cross-sectional view of the magnet carrier of  FIG. 10 , taken along section A-A in  FIG. 12 ; 
           [0037]      FIG. 15  shows a perspective view of a segmented permanent magnet. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 3  shows a section of a permanent magnet rotor  1  in accordance with an embodiment of the present invention. As may be seen in  FIG. 3 , magnet assemblies  100 , each comprising a magnet carrier  10 , a permanent magnet  60 , and a magnet retainer  20 , are mechanically fixed to a rotor rim  2 . The magnet assemblies  100  may be fixed to the rotor rim  2  by any suitable means; in the embodiment shown the magnet assemblies are fixed to the rotor rim  2  using bolts  4 . 
         [0039]    The magnets  60  in the magnet assemblies  100  may be alternately magnetized in “radial in” and “radial out” directions (which may be considered as alternate North and South magnets) as shown in  FIG. 4 . Alternatively, wider poles may be achieved by using two or more poles of the same polarity as shown in  FIG. 5 . 
         [0040]      FIGS. 6-9  show an embodiment of a magnet assembly  100 . A permanent magnet  60  is mounted to a magnet carrier  10  by means of a magnet retainer  20 . The magnet retainer  20  encircles the magnet  60  and magnet carrier  10 , passing through a plurality of channels  30  formed in the base of the magnet carrier  10 . In some embodiments, more than one permanent magnet  60  may be mounted on a single magnet carrier  10 . 
         [0041]    In a preferred embodiment, the magnet carrier  10  is formed of a ferromagnetic material, most preferably magnetic steel. Suitable steels may include Mild Steel (Grades such as 1020, 1020, EN1a), Silicon Steel (Carpenter Silicon-C), Ferritic and Martensitic Stainless Steel (i.e. Grades 410, 416, 446) or Cobalt Iron alloys. The magnet carrier  10  may serve as a mechanical interface to a rotor rim  2 , as shown in  FIG. 3 . The rotor rim  2  may be formed of a ferromagnetic material. Where the magnet carrier and rotor rim  2  are ferromagnetic, the magnet carrier  10  together with the rotor rim  2  may provide back iron for the electromagnetic circuit (e.g. return path for the magnetic flux between adjacent magnet poles). 
         [0042]    Preferably, the electromagnetic path should not be compromised in the rotor back iron. The magnet carrier  10  provides slots or channels  30  for the magnet retainer  20  to pass under the magnet assembly  100 . The intermediate projections or teeth  50  between the channels  30  allow the magnet flux to pass unimpeded (e.g. without creating a large airgap with high reluctance) from the magnet carrier  10  into the further ferromagnetic structure or rotor rim  2  which makes up the remainder of the rotor back iron. The teeth  50  of the magnet carrier  10  preferably provide sufficient cross-sectional area for the magnetic flux path. In a preferred embodiment, the magnet carrier is made of steel. Due to the higher saturation flux density of steel (typically 1.5-2.0T) the cross-sectional area of the steel within the flux path can be less than the cross-sectional area of the magnet  60  which typically would have a working flux density of 0.8-1.0T without there being any issues of magnetic saturation leading to high reluctance in the magnetic circuit and a reduction in magnetic performance. 
         [0043]    The magnet retainer  20  may preferably comprise a fibre filament wound in an epoxy matrix. Suitable fibre filaments include carbon fibre and glass fibre. Preferably, the fibre may be wound with a certain tension (preferably about 5-10 kg force, or about 49-98 N) applied to a tow to achieve a pre-stress in the finished cured part. The choice of fibre (e.g. carbon or glass) and epoxy matrix will depend on the required strength and also potentially any restrictions on cure cycle due to temperature limits of the magnet material. The fibres may be wet wound (i.e. dry fibres are run through a resin bath before being wound around the magnet and magnet carrier) or may be a pre-impregnated product or “pre-preg” which has an epoxy coating applied. Baking of the component in an oven or autoclave then cures the epoxy. A mould may be applied over the component during the curing process to ensure a final shape/surface finish of the epoxy. Preferably a tow may comprise between 3,000 and 15,000 carbon fibre filaments, most preferably 7,000. Preferably, the diameter of each carbon fibre may be between about 5 and 10 micrometres, most preferably 6 micrometres. 
         [0044]    The magnet retainer may preferably be added to the magnet and magnet carrier using a resin transfer mould process (RTM) in which the magnet and magnet carrier are dry wrapped with layers of carbon or glass fibre (which may be, for instance, in the form of matting, tape or strands) and then clamped and held in a mould cavity while low viscosity resin or epoxy is injected under pressure. The component is then cured with heat as described above to produce a highly integral part. 
         [0045]    The magnet retainer  20  may be subtended at an angle to cover a large area of the surface of the magnets  60  on the upper face  102 . Therefore, the width  24 A of the magnet retainer  20  on the upper face  102  may be larger than the width  24 B of the channel  30  on the magnet carrier  10 . In the case of the magnet retainer  20  comprising a fibre filament wound under tension, a force will be exerted by the magnet retainer  20  along the fibre direction. However, the maximum angle (θ) between an angled fibre and a zero-degree wrapped fibre (perpendicular to the axial direction) is preferably small. Therefore, the resolved force from the magnet retainer  20  along the axial direction, which is proportional to sin(θ), may be negligible. 
         [0046]    The aspect ratio of the cross section of the magnet retainer  10  may vary around the magnet assembly  100 . The magnet retainer  20  may form a thin layer (of thickness  22 A) on the upper face  102  of the magnet assembly  100  which may be adjacent to the electrical machine airgap (e.g. facing the stator in a conventional radial field permanent magnet machine) in use. The magnet retainer  20  may be comparatively thicker (thickness  22 B) in the channel  30 . The magnet retainer  20  may have a constant cross-sectional area. A thin magnet retainer  20  on the upper face  102  has the advantage of minimising the magnetic airgap in a machine employing the rotor. 
         [0047]      FIGS. 8-12  show the magnet carrier  10  in more detail. 
         [0048]    The magnet carrier  10  may also comprise a number of mechanical fixing points  12  to allow the carrier to be mounted onto the rotor structure as shown in  FIG. 1 . The mechanical fixing points  12  may be tapped holes. In the embodiment shown in  FIG. 1 , bolts  4  pass through clearance holes in the rotor rim  1 , and engage in the magnet carrier mechanical fixing points  12 . Additional fixing points  14  may also be included on other faces of the carrier that allow the magnet assembly to be securely held against magnetic forces as the magnets are mounted on to the rotor. Of course, the clearance and tapped holes may be located oppositely to the above, and alternative mechanisms for fixing may be employed instead. 
         [0049]    Preferably, the channels  30  have profiled edges  32 , which may be, for example, chamfered and/or rounded. Profiled edges  32  help avoid high stress concentrations in the magnet retainer  20 . Where the magnet retainer  20  comprises fibre filament, profiled edges also allow the fibre to pass around the structure without sharp corners causing breakage of the fibre tow as it is wound. Preferably the profiled edge has a radius that is larger than minimum bend radius of the fibre. It is also advantageous to have chamfers or radii on the leading edges  34  of the channels  30  to prevent the fibre tow snagging during winding and allowing it to pass into the channel  30  without breakages. 
         [0050]      FIG. 15  shows a permanent magnet  60  suitable for use with the magnet carrier  10 . The magnet  60  is segmented to avoid eddy currents, with bonded interfaces  62  between segments  64 . In an embodiment, the bonded interfaces  62  preferably comprise glass beads to prevent contact between segments  64 . Similarly to the magnet carrier channels  30  the magnet  60  may have profiled edges  66  to prevent stress concentrations and, in the case of a magnet retainer comprising fibre filament, reduce the bend radius of the fibre during winding. The removal of material in this region does not have significant impact on machine performance (e.g. airgap flux density and torque production) as the magnet material here usually contributes to inter-pole leakage (i.e. flux that leaks between magnet poles on the rotor without crossing the airgap and linking with the rest of the electrical machine). 
         [0051]    In the embodiment described above the magnet retainer  20  lies in the circumferential direction of the rotor. It is also possible that the magnet retainer  20  and hence channels  30  run axially along the magnet  60  and magnet carrier  10 , i.e. the magnet retainer may run parallel to the axis of rotation of the rotor. However, if the relevant edges of the magnet are profiled to remove sharp corners, this arrangement may increase the effective airgap on the main axis of the magnetic pole and can be detrimental to performance. In the circumferential case the profiled edge  66  occurs in the magnet inter-pole and loss of magnet material in this location has a significantly lower effect on performance. 
         [0052]    To manufacture a magnet assembly  100  such as described above, permanent magnet material is mounted on the magnet carrier  10  and the magnet retainer  20  is encircled around the magnet carrier  10  and permanent magnet material, passing through a channel  30  of the magnet carrier  10 . 
         [0053]    When the permanent magnet material is mounted on the magnet carrier  10 , it may be bonded onto the magnet carrier  10  using a suitable adhesive, such as an epoxy or acrylic. 
         [0054]    The adhesive may hold the permanent magnet material in place on the magnet carrier while the magnet retainer  20  is applied. The adhesive may also contribute to retaining permanent magnet  60  during use of magnet assembly  100  on a rotor rim  2 . 
         [0055]    The permanent magnet material may be mounted on the magnet carrier  10  in an unmagnetized state which simplifies the manufacturing process as high magnetic attractive forces between the permanent magnet material and magnet carrier  10  (which may squeeze out an adhesive) are not present and allows the optimum bondline to be achieved and handling of the permanent magnet material is far easier. The permanent magnet material may then be magnetized to create a permanent magnet  60  after the permanent magnet material has been mounted on the magnet carrier  10 , or after the magnet retainer  20  has been applied. Alternatively, the permanent magnet material may be already magnetized to form permanent magnet  60  before mounting on magnet carrier  10 . 
         [0056]    The permanent magnet  60  may comprise a segmented or laminated magnet. In some embodiments, more than one permanent magnet  60  may be mounted on a single magnet carrier  10 . 
         [0057]    Preferably, the magnet retainer  20  comprises a fibre filament. Typically the magnet retainer  20  would be formed with a single tow (or bunch of fibres) being wrapped around as the combined magnet  60  and magnet carrier  10  is rotated. The combined magnet  60  and magnet carrier  10 , and the fibre tow may also be moved laterally relative to each other in the direction of the axis of rotation to allow the fibre to traverse the length of the magnet  60  and magnet carrier  10 . Alternative traversing patterns can be employed such as winding a number of turns in each channel  30  before indexing to the next channel  30  or placing single or low number of turns in a channel  30  before moving to the next and using multiple traverses of the magnet  60  length. 
         [0058]    Preferably, the fibre is wound with a certain tension applied to the tow to achieve a pre-stress in the finished cured part. The choice of fibre (carbon or glass) and epoxy matrix will depend on the required strength and also potentially any restrictions on cure cycle due to temperature limits of the magnet material. 
         [0059]    To construct a permanent magnet rotor, a plurality of magnet assemblies  100  as described above may be mounted on a rotor rim  2 , as shown in  FIG. 3 . 
         [0060]    To facilitate the mounting of the magnet assemblies  100 , holes or fixings which aid the mechanical handling of the magnet assembly  100  may be included in the magnet carrier  10 . Sintered magnets are brittle and are difficult to machine and, therefore, fasteners or fixings cannot be directly applied to the magnet  60 . The ability to mechanically hold the magnet carrier  10  may be exploited to control the forces acting on the magnets  60  during the assembly of the rotor  1 . 
         [0061]    As a magnet  60  is loaded onto a rotor rim  2 , it is subject to a range of forces as it interacts with neighbouring magnets and a possibly ferromagnetic rotor rim  1 , which can make manual loading of magnets onto a rotor very challenging. However, the magnet assembly  100  may be loaded onto tooling/guideways and controlled forces applied to lower or guide the magnet assembly  100  onto the rotor rim  2 . Once in place, the magnet carrier is fixed to the rotor rim  1  (e.g. using bolts  4  through clearance holes in the rim  1  into mechanical fixing points  12  (e.g tapped holes) in the base of the magnet carrier  10 ). The tooling fixings may then be released and the rotor indexed round to the next position for the subsequent magnet assembly to be loaded. 
         [0062]    To complete assembly of the rotor  1 , magnet assemblies  100  may be mounted on the rotor rim  2  around the full circumference of the rotor  1 . The magnets  60  in the magnet assemblies  100  may be alternately magnetized in “radial in” and “radial out” directions (which may be considered as alternate North and South magnets) as shown in  FIG. 4 . Alternatively, wider poles may be achieved by using two or more poles of the same polarity as shown in  FIG. 5 . 
         [0063]    It will of course be understood that the present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention.