WO2024110654A1

WO2024110654A1 - A rotor

Info

Publication number: WO2024110654A1
Application number: PCT/EP2023/083052
Authority: WO
Inventors: Graeme HYSON; Simon ODLING; Jack HODGSON; Callum PICKEN
Original assignee: Yasa Limited
Priority date: 2022-11-25
Filing date: 2023-11-24
Publication date: 2024-05-30
Also published as: GB2624859A; GB202217713D0

Abstract

A disc shaped rotor that has an axis of rotation and also having a plurality of permanent magnets mounted to a first face of the rotor body circumferentially about the axis of rotation. The rotor has one or more structures arranged on a second face opposed the first face of the rotor body for reducing deflection of the rotor body when the rotor is rotated about the axis.

Description

A Rotor

FIELD OF THE INVENTION

The present invention relates to a rotor, in particular a rotor for an axial flux machine.

BACKGROUND OF THE INVENTION

A move to electric machines, away from internal combustion engines has meant an increase in research effort to maximise efficiencies, torque, speed and consequential power and power densities and to choose particular electric machine topologies to take best advantage of materials and production methods. For many years radial flux motors I generators dominated electric machines, despite the invention of a different, axial flux topology. Several reasons can be attributed to the slow rise of axial flux machines, not least difficulty in displacing incumbent technologies, but also not helped by challenges in efficient production techniques for mass production, of a challenging, yet ultimately superlative power dense topology, operating at speeds and torques that suit many industrial, and particularly automotive and similar traction and power generation applications. Advances continue to be made in axial flux topology, particularly improving power density and manufacturing techniques.

Power being a function of speed and torque, both parameters are sought to be maximised within materials and environmental constraints. Rotor speed, if increased, delivers increased power for a given torque, but whereas radial flux machine rotors are usually mass and stiffness balanced around a rotating axis, in contrast axial flux machine rotors usually have asymmetric distribution of both mass and stiffness along their rotation axis, though they are usually radially symmetric. Though this axial asymmetry is not a significant challenge for lower rotor speeds, asymmetry along the rotation axis becomes a source of movement in a spinning rotor due to a mismatch between bending moments caused by centripetal forces acting on a rotor’s asymmetric distribution of mass along the rotation axis and the component stiffnesses which resist them. Movement caused by bending moment forces on rotors with surface mount magnets is potentially a source of magnet cracking, adhesive failure and similar loss of integrity in rotors, leading to instability and possible failure. Of lesser import, but of equal concern is loss of machine efficiency, caused by axial movement of rotor with respect to the stator.

Whereas some attempts to improve mass and stiffness balance in axial flux machines, have sought to move surface mounted magnets closer to the rotation axis, witnessed by US2016329795A1 , thereby reducing bending moment, this approach is restrictive on an important degree of freedom in rotor design. In reviewing attempts to address these mass balance issues, it has been seen there is room for further improvement in rotor design to enable higher rotation speeds for axial flux machines.

SUMMARY OF INVENTION

The present invention provides a rotor in accordance with the independent claims appended hereto.

Further advantageous embodiments are also provided with reference to the dependent claims, also appended hereto.

We describe a rotor for an axial flux machine, comprising: a disc-shaped rotor body having an axis of rotation; a plurality of permanent magnets mounted to a first face of the rotor body circumferentially about the axis of rotation; one or more structures arranged on a second face opposed the first face of the rotor body for reducing deflection of the rotor body when the rotor is rotated about the axis.

Such an arrangement enables the deformation of the rotor under rotational forces to be reduced or minimised.

The one or more structures may comprise one or more slots in the second face of the rotor body, the one or more slots extending radially along at least a portion of the radial distance between the axis of rotation of the rotor and a circumferential outer edge of the rotor. The one or more slots may comprise a recess in the second face of the rotor body.

The slots reduce the hoop stiffness of the rotor body, thus reducing the amount of deflection of the rotor body under rotational forces. The one or more structures may comprise one or more counterweights mounted to the second face of the rotor body. The one or more counterweights may be mounted radially between the axis of rotation and a circumferential outer edge of the rotor.

When there are two or more counterweights, the two or more counterweights may be mounted on the second face of the rotor body at the same radial distance between the axis of rotation and the circumferential edge of the rotor. Alternatively, the two or more counterweights may be mounted on the second face of the rotor body at different radial distance between the axis of rotation and the circumferential edge of the rotor.

In any of the above, the rotor body may comprise a rotor back and a layer of metal laminate mounted to the rotor back, wherein the metal laminate forms the first surface and the magnets are mounted on or to the metal laminate. The rotor may comprise a filling compound provided in interlamination spaces of the metal laminate. The filling compound may comprise a resin.

The rotor may comprise an adhesive join between the metal laminate and the set of permanent magnets and the metal laminate.

The rotor may comprise a support member for receiving the plurality of permanent magnets, wherein the plurality of permanent magnets are mounted to and retained by the support member; and wherein the support member is secured to the first face of the rotor body. The support member may comprise an annular part from which a plurality of spokes project radially therefrom, each of the spaces between adjacent spokes defining a socket for receiving a respective permanent magnet. The annular part and the spokes may be formed of separate pieces.

Each of the spokes may be formed from a uni-directional carbon strip, and wherein the fibres of each of the carbon strips run perpendicular to the axial length of the respective spoke.

Each of the spokes may be formed from a material having isotropic properties in a plane that extends radially along the length of a respective spoke and a plane that extends perpendicular to the radial plane. Each of the spokes may be formed from a magnetic material. The magnetic material may have a magnetic field alignment that is orthogonal to a magnetic field alignment of one or more of the plurality of permanent magnets.

In any of the above, the rotor may comprise a retaining band extending around the permanent magnets, restricting or preventing radial outward movement of the permanent magnets, in use, and the retaining band being pre-stressed to apply an inward, radially directed load to the plurality of magnets. The retaining band may comprise a composite material. The retaining band may comprise windings of a reinforcing fibre material within a matrix of a suitable resin material. The fibre material may be substantially hoop wound.

We also describe an axial flux machine comprising: a stator comprising a plurality of stator bars disposed circumferentially at intervals around an axis, each of the stator bars having a set of windings wound therearound for generating a magnetic field generally parallel to the axis, the plurality of stator bars being arranged to provide a hollow region at the centre of the axis; and a rotor as described above, wherein the rotor is mounted for rotation about the axis of the stator, and the rotor being spaced apart from the stator along the axis to define a gap between the stator and rotor.

LIST OF FIGURES

The present invention will now be described, by way of example only, and with reference to the accompanying figures, in which:

Figures 1a to 1c show, respectively, a general configuration of a two-rotor axial flux machine, example topologies for axial flux permanent magnet machines, and a schematic side view of a yokeless and segmented armature (YASA) machine;

Figure 2 shows a perspective view of the YASA machine of Figure 1c;

Figure 3 shows a perspective exploded view of a stator and stator housing for a YASA machine; Figure 4 shows an example of a rotor;

Figure 5 shows a cut-through portion of an example rotor;

Figure 6 shows example stresses experiences by the rotor of Figure 5;

Figure 7 shows the supporting body in isolation;

Figure 8 illustrates the centrifugal forces acting on the magnets;

Figure 9 shows a side cut-through view of the rotor;

Figure 10 shows an example plot of a dishing rotor body;

Figures 11 and 12 show a first aspect for reducing deformation of the rotor;

Figures 13 and 14 show a second aspect for reducing deformation of the rotor;

Figure 15 and 16 show a different configuration of the aspect shown in figures 13 and 14;

Figure 17 shows an alternative arrangement of the magnets using a support member; and

Figure 18 shows a close up of an area of the alternative arrangement regarding the hoop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In brief, we describe a disc shaped rotor that has an axis of rotation and also having a plurality of permanent magnets mounted to a first face of the rotor body circumferentially about the axis of rotation. The rotor has one or more structures arranged on a second face opposed the first face of the rotor body for reducing deflection of the rotor body when the rotor is rotated about the axis. We will first discuss the background of the arrangement of axial flux machines, which is an example use of the rotor of the present invention. Referring first to Figures 1c, 2 and 3, which are taken from our PCT application WO2012/022974, Figure 1c shows a schematic illustration of a yokeless and segmented armature machine 10.

The machine 10 comprises a stator 12 and two rotors 14a, b. The stator 12 is a collection of separate stator bars 16 spaced circumferentially about a rotation axis 20 of the rotors 14a,b. Each bar 16 has its own axis (not shown) which is preferably, but not essentially, disposed parallel to the rotation axis 20. Each end of each stator bar is provided with a shoe 18a,b which serves a physical purpose of confining a coil stack 22, which stack 22 is preferably of square/rectangular section insulated wire 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.

The two rotors 14a,b carry permanent magnets 24a, b that face one another with the stator coil 22 between (when the stator bars are inclined - not as shown - the magnets are likewise). Two air gaps 26a, b are disposed between respective shoe and magnet pairs 18a/24a, 18b/24b. There are a 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 the coils do not all come into registration with the corresponding magnet pair at the same time and at the same rotational position of the rotor with respect to the stator. This serves to reduce cogging.

In a motor the coils 22 are energized 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 14a, b are generally connected together (for example by a shaft, not shown) and rotate together about the axis 20 relative to the stator 12. The magnetic circuit 30 is provided by two adjacent stator bars 16 and two magnet pairs 24a, b and a back iron 32a, b for each rotor links the flux between the back of each magnet 24a, b facing away from the respective coils 22. The stator coils 16 are enclosed within a housing that extends through the air gap 26a, b and which defines a chamber supplied with a cooling medium. Turning to Figure 3, a stator 12a is shown in which the stator coils are located between plastic material clam shells 42a, b. These clamshells have external cylindrical walls 44, internal cylindrical walls 46, and annular radially disposed walls 48. In the prior art example of Figure 3 the radial walls 48 include internal pockets 50 to receive the shoes 18a,b of the stator bars 16 and serve to locate the stator coil assemblies 16, 22, 18a,b when the two clam shell housings 42a, b of the stator 12a are assembled together. The stator housing 42a, b defines spaces 52 internally of the coils 22 and externally at 54 around the outside of the coils 22 and there are spaces 56 between the coils. The spaces 52,54,56 are interlinked defining a cooling chamber. Although not shown in Figure 3, when assembled, the stator housing 42a, b is provided with ports that allow cooling medium such as oil to be pumped into the spaces 52,54,56 to circulate around the coils and cool them.

Figure 4 shows an example of a rotor 14 and Figure 5 shows a cut-through portion of an example rotor 14. The rotor assembly consists of an array of permanent magnets 24 bonded to a supporting body 62. The supporting body may be a homogenous structure, typically manufactured from metallic materials. The magnet-body bond 66 may be a chemical adhesive such an epoxy.

The magnets 24 interact with electromagnetic fields generated by the motor stator to generate torque, which is then transmitted into the supporting structure 62 via the adhesive bond 66, then outwards from the motor via the body’s central hub 60.

The rotor assembly may also include a laminated back iron ring (LBIR) 64. The LBIR is a layer of metal laminate located between the supporting body 62 and the magnets 24 and is bonded to both. A filing compound (for example a resin) may be provided between the interlamination spaces of the metal laminate. The LBIR serves to direct magnetic flux and improve motor performance. The rotor 14 does not need to use a LBIR. Instead, the rotor 14 may comprise a supporting body 62 that received the magnets 24 directly. The presence, or absence, of the LBIR does not alter the fundamental principles described below.

A fundamental performance metric for motors is their maximum rotational speed. When used in electric road vehicle applications it is generally desirable for a motor to spin as fast as possible; high motor rotational speeds can provide efficiency and weight benefits in both the electric motor and the transmission system it is driving. One major constraint on motor speed is the maximum safe speed of the rotor due to mechanical limitations. If the rotor assembly is spun too fast permanent mechanical damage can occur.

One rotor failure mode is so-called ‘magnet ejection’. Magnet ejection occurs when one or more magnets 24 become detached from the support body 62 and hence escapes from the motor causing catastrophic damage. The primary cause of magnet ejection is over stressing of the magnet-body bond.

Figure 6 shows example stresses experiences by the rotor of Figure 5. The magnet-body bond may be overstressed by several different mechanisms, the two most significant of which are exceeding its shear strength 68 or its peel strength 70. Shear strength refers to the bond’s ability to resist in-plane shear stresses 68. Peel strength refers to the bond’s ability to resist normal stresses 70. In typical adhesives shear strength is significantly higher than peel strength, therefore it is desirable to minimise the latter and preferentially utilise the former. This target may be achieved by controlling stress and deformation within the rotor assembly in general and the bond area in particular.

Stress is generated in all parts of the rotor assembly 14 by its rotation. When the rotor assembly spins every part of it must be acted on by a centripetal force (acting towards the rotor central axis) for circular motion to be maintained. It is these centripetal forces which ensure that all parts of the rotor follow a circular path and the assembly maintains structural integrity.

It is also valid to describe this effect in terms of centrifugal forces (acting away from the rotor central axis). Centripetal forces act all on all parts of the rotor to maintain circular motion, Newton’s laws mean that these parts all generate centrifugal forces acting away from the rotor central axis due to their inertia. Therefore it can be said that rotation of the rotor assembly generates a set of ferees acting radially outwards from the rotor centre axis which are proportional to rotational speed squared. Any one element of the rotor assembly experiences a so-called ‘phantom force’ that acts to push it away from the rotor centre axis. The centrifugal forces have markedly different effects on each part of the rotor assembly due to their differing geometry and construction. Figure 7 shows the supporting body 62 in isolation. This component is a homogenous monolithic body. When it spins, centrifugal forces are generated within it as above, any one element of the rotor experiences a force which appears to act radially outwards 72. Since the supporting body is a continuous ring of material these radial forces can be reacted by circumferential stresses in the body material 74. High speed rotation of the supporting body generates high hoop stresses in it, but the body can maintain structural integrity so long as the tensile hoop strength of the body material is not exceeded.

Consider now the magnets in isolation, illustrated in Figure 8. As with the supporting body 62, rotation of the components means that any one element of the magnet array experiences a force appearing to act radially outwards 72. The magnet array 24 is not a continuous ring of material , so it cannot support any hoop stress. If the magnet array 24 were spun in isolation it is obvious that it would not maintain integrity and would instead rapidly disintegrate.

The magnet array 24 requires additional support in order to be able to rotate. In operation this is provided by the supporting body 62 by means of the magnet-body bond. However even when bonded to the supporting body the magnet array still cannot support hoop stresses. Therefore when spinning the rotor assembly it is desirable that the supporting body support all centrifugal forces generated by the magnet array, at high speed these forces may be very large.

Figure 9 shows a side cut-through view of the rotor 14. The magnet array 24 is bonded to only one side of the supporting body 62, this leads to an asymmetry of forces within the supporting body 62. Large centrifugal forces 72 from the magnets 24 are applied to one side of the supporting body62 with no counter-balancing force on the opposing side, where the magnet centrifugal forces are reacted by a load path 76 through the bonds (and LBIR if present), which leads to a large ‘dishing’ deformation of the body. This dishing is proportional the applied centrifugal forces, which are in turn proportional to rotor speed squared.

Figure 10 shows an example plot of a dishing rotor body. This dishing deformation creates undesirable peel stresses in the magnet-body bond 66, leading to magnet ejection. Reducing these peel stresses and instead utilizing the (superior) shear strength of the bond adhesive will increase maximum operational speed. This peel stress reduction can be achieved by reducing the dishing deformation of the rotor.

Deformation of this nature could be reduced by several mechanisms. The simplest would be to increase the overall stiffness of the supporting body, either by using a higher modulus material or by increasing thickness. This has a detrimental effect on assembly mass, inertia, size and cost and is not preferred.

Reducing magnet mass would decrease the scale of the unbalanced centrifugal forces and reduce dishing deflection. Lighter magnets could be realised with a higher performance magnetic material that facilitates smaller magnets, but this has a significant cost detriment. Reducing the physical size and hence mass of the magnets without changing material would decrease deformation, but at the cost of reduced motor output torque/power.

A more optimised way to reduce deformation is provided by the invention described herein. Deformation is reduced in one of two closely related ways. Both techniques rely on adding additional structures to the rear of the rotor support body 62 in order to reduce the amount of deformation experienced by the rotor 14 when rotating.

Figures 11 and 12 show a first aspect for reducing the deformation of the rotor when rotating. Figure 11 shows the rear of the support body 62. Figure 12 shows a cut-through view of the rotor 14. The magnets are not shown in this figure.

In this first aspect, the additional structures are one or more grooves, slots or recesses 78 cut or formed into the rear, or second face, of the rotor support body 62. The one or more slots extend radially along at least a portion of the radial distance between the axis of rotation of the rotor and a circumferential outer edge of the rotor. In the figures the slots are shown extending the full radial distance between the outer circumferential edge and the central hub 60. However, this need not be the case, and the radial length of the slots may be less than the full distance.

The undesirable dishing deformation is caused by the imbalance of centrifugal and hoop forces in the rotor assembly. The magnet array and the supporting body both generate large centrifugal forces, but in the body they are reacted by hoop stresses within the body material, whereas in the magnetic array they cannot be.

The first proposed solution for reducing deformation reduces the hoop stiffness of the supporting body. If the body’s hoop stiffness is reduced or removed the body can no longer react centrifugal forces with internal circumferential forces. This change means that the centrifugal forces generated by the magnet array and the supporting body are both reacted by radial forces within the rotor assembly, rather than by a combination of radial forces (in the array) and circumferential forces (in the body). This in turns means there are no longer asymmetrical centrifugal forces applied to the rotor assembly and the dishing deformation is much reduced or does not occur.

The reduction in hoop stiffness may be achieved as described above by modifying the supporting body to reduce hoop stiffness, for example by adding the radial grooves 78.

A second proposed technique to reduce deformation of the rotor is to include additional mass on the second, rear side of the supporting body 62 (the side opposing the magnet array).

Figures 13 and 14 show the second aspect for reducing the deformation of the rotor when rotating. Figure 13 shows the rear of the support body 62. Figure 14 shows a cut- through view of the rotor 14.

In this second aspect, the additional structures are one or more counterweights 80 mounted to the second face of the rotor body 62. The one or more counterweights 80 are mounted on the rear of the support body 62 radially between the axis of rotation of the rotor body and the circumferential outer edge of the rotor. They may be mounted on the second face of the rotor body at the same radial distance between the axis of rotation and the circumferential edge of the rotor, or they may be mounted on the second face of the rotor body at a different radial distance between the axis of rotation and the circumferential edge of the rotor.

In the extreme case, it can be seen that adding a second identical magnet array to the ‘reverse’ side of the supporting body 62 would act to perfectly counterbalance the effect of the primary magnet array by making the force distribution within the assembly symmetrical. However, such an array would be magnetically redundant and at considerable cost in terms of weight and thus power output of the machine.

One solution could be to modify the rotor geometry to include extra material similar to the magnet array on the rear of the support body. This material is most effective when it has discontinuous geometry, e.g. there are radial ‘breaks’ in the material, so that it cannot support hoop stresses. The geometry of this added material can be tuned to affect the dishing deformation to a greater or lesser extent.

Figures 15 and 16 show an alternative second aspect for reducing the deformation of the rotor when rotating. Figure 15 shows the rear of the support body 62. Figure 16 shows a cut-through view of the rotor 14. This solution again uses the counterweights 80 in order to reduce the deformation. In figures 15 and 16, the radial position of the counterweights 80 between the axis of rotation of the rotor and the circumferential edge of the rotor is different to that shown in figures 13 and 14. As with the second aspect shown in figures 13 and 14, the counterweights may be at the same radial distance as each other, or different.

It can be seen from the attached diagrams that adding additional structures (in the form of radial grooves into the rear face of the rotor support body and/or adding discontinuous material in the form of counterweights to the rear face of the support body) are solutions with a high degree of commonality. In practice a balance must be struck between adding counterbalancing mass (which increases overall rotor assembly mass) and reducing circumferential stiffness (which may have an effect on other rotor behaviours not related to maximum running speed).

Whilst we have described the magnets being mounted on or bonded to the rotor support body, alternative arrangements are envisaged wherein the permanent magnets are mounted to a support disc, and the support disc is mounted on or bonded to the rotor support body.

Referring to Figures 17 and 18, bonded, by an adhesive material, to the face of the rotor support body 62 is a support member 120. In a first aspect, the support member 120 is of a non-magnetic material, and is shaped to define a central annular part 120a from which a plurality of generally radially projecting spokes 120b extend. Adjacent ones of the spokes 120b define therebetween sockets or spaces within which respective permanent magnets 122 are located. The permanent magnets 122 and the support member 120 are bonded to the rotor support body 62 using an adhesive material. The adhesive material may be any type of adhesive material that is suitable for bonding the permanent magnets 122 and support member 120 to the rotor support body 62.

The adhesive material may, for example, be an adhesive such as an epoxy (or have similar characteristics or modulus to epoxy adhesives), which would provide a strong, rigid bond between the magnets 122 and support member 120 and the rotor support body 62. However, it is preferable that the adhesive material is one having a low or relatively low modulus and is of relatively high thermal conductivity. By way of example, the adhesive material may be of a silicone based form. Other examples of suitable materials include low modulus epoxy adhesives, polyurethane adhesives, acrylic adhesives and nitrile phenolic adhesives. It will be appreciated that this list is not exhaustive and that other materials could be used. In order to achieve the advantages of the invention, the strain to failure, or elongation, of the material should be sufficient to absorb all relative movements of the components that occur in normal use such that the stress in the adhesive does not carry significant radial loads. By way of example, the modulus should be less than 20MPa, and should preferably be in the region of 1MPa to 5MPa.

Surrounding the radially outer peripheries of the magnets 122 is a retaining band 124, the purpose of which is to apply a radially inwardly directed load to the magnets 122, urging them into the socket or spaces between the spokes 120b of the support member 120. The retaining band 124 is of a fibre reinforced composite material. By way of example, it may comprise windings of a suitable reinforcing fibre material such as carbon fibre, embedded within a suitable resin material matrix. The retaining band 124 is conveniently manufactured to be of a relaxed diameter smaller that the outer diameter of the part of the rotor defined by the permanent magnets 122, the retaining band 124 being resiliently stretched over the permanent magnets 122 during the assembly process to pre-stress the retaining band 124 in such a manner that the retaining band 124 applies the aforementioned radially directed loads to the magnets 122.

It will be appreciated that the retaining band 124 applies a relatively large radially inwardly directed load to the magnets 122, and one of the functions of the support member 120 is to react these loads. When bonded with a low modulus adhesive material, the support member 120 reacts substantially all of the radially inward loads applied to the magnets 122 that, without the support member 120, would result in failure of the adhesive joints. The shapes of the sides of the magnets 122 and adjacent parts of the support member 120 are such that a locking force arises between the magnets 122 and the support member 120 as a result of the hoop stress generated in the retaining band when fitted. The locking force arises from the radially inwardly directed loads urging the sides of the magnets 122 against the sides of each leg of the support member 120 as a result of the magnets 122 being wedge shaped. The magnets 122 thus become locked against the support member 120 with the legs thereof compressed between adjacent ones of the magnets 122, as illustrated diagrammatically in Figure 18. The modulus of this support member is important in making the rotor function and reacting the loads as the magnets are relatively stiff. By way of example, the support member 120 may be of a material with a high carbon fibre content. It should preferably have a modulus in excess of 20GPa.

The support member 120 may be of injection moulded form, or could alternatively be of press moulded form, machined or finished to substantially the shape illustrated, for example. However, in some aspects, some manufacturing techniques enable the support member 120 to be formed without the inner annular part 120a. Conveniently, the support member 120 and magnets 122 are preassembled to form a subassembly that is subsequently bonded, as a unit, to the rotor support body 62 by the adhesive 18b.

When the support member 120 is provided as separate pieces during the manufacturing process, this enables the different parts comprising the support member 120 to be made from different materials having different properties.

For example, the spokes 120b may comprise of a uni-directional (LID) carbon strip. For example, the spokes 120b may be cut from a pultruded profile, which is a continuous manufacturing process suitable for making predominantly LID strip materials in high volume with low labour content.

Using spokes 120b comprising UD carbon fibre pultrusion greatly increases the stiffness of the spokes, particularly across their width. For example, the planar modulus across their width may be in the region of 110-130Gpa, compared to their planar modulus along their length being in the region of 5Gpa.

Alternatively, the spokes 120b may comprise of an isotropic, or quasi-isotropic material. For example, a material having isotropic properties in the XY plane (where the XY plane is the face of the support member and magnets). Such materials couple comprise a Glass fibre SMC (Sheet Moulding Compound) may have an XY planar modulus of 10- 20GPa, a Carbon Fibre SMC would be higher, typically around 30-40GPa - this can be increased by selecting higher modulus grades of Carbon Fibre. In the Z plane, (through thickness) the modulus may be lower, for example in the region of 5-10GPa.

The spokes 120b may instead comprise a magnetic material. In such an arrangement, the magnetic field alignment of the spokes 120b is arranged to be orthogonal to the magnetic field alignment of the permanent magnets 122. For example, if the permanent magnets are arranged as N-S pairs, the spokes 120b may be arranged as having a E- W alignment. Such an arrangement may provide a Halbach array, which may improve the magnetic performance of the rotor due to augmentation of the magnetic field.

The inner ring 120a may be injection moulded. Its material properties are not as influential on the performance of the rotor as the magnet 122 and spoke 120b location.

As described before, the inner ring 120a may instead not be present, in which case the permanent magnets 122 are held by the spokes 120b in the moulding tool prior to a resin based filler being applied.

By producing the subassembly in this manner, it can be ensured that the magnets 122 are in intimate contact with the support member 120 and filler material, minimising or avoiding movement therebetween. Accordingly, upon subsequent fitting of the retaining band 124, no significant movement of the magnets 122 should occur, which would introduce stress or strain into the adhesive material.

Whilst the above-mentioned manufacturing techniques use a resin based filler material once the components (inner ring 120a, where applicable, spokes 120b and magnets 122) are located in a moulding tool, an alternative technique (a so-called co-moulding technique) involves using an adhesive between some or all of the components (inner ring 120a, where applicable, spokes 120b and magnets 122). Such a technique provides a much stronger joint strength between the respective components, particularly between the magnets 122 and the spokes 120b, compared to when a resin based filler is used. Using the co-moulding technique, the load path of from the magnets is directly into the material used to form the inner ring 120a, where applicable, spokes 120b and magnets 122 (for example a SMC material) and produces a sub-assembly of support member 120 and magnets 122 that has a high overall stiffness.

In use the rotation of the rotor about its axis results in the permanent magnets 122 being subject to significant centripetal loads, and the prestressed retaining band 124 serves to counter those loads, retaining the permanent magnets 122 in position. When the permanent magnets 122 are bonded to the rotor support body 62 using a low modulus adhesive material , the low modulus adhesive is able to flex, in use, accommodating relative movements whilst maintaining the integrity of the bonds between the permanent magnets and the rotor support body 62. The low modulus adhesive material can also accommodate movements arising from differential thermal expansion of parts of the rotor.

Although the rotor described hereinbefore is intended for use in a motor of the type in which a stator is located between a pair of single sided rotors, the invention is not restricted to such use and may be employed with others forms of motor and other forms of rotary electric machine, for example, the rotor may be used in a machine in which a single rotor is located between two stators. Furthermore, the machine may instead be a generator.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

CLAIMS:

1 . A rotor for an axial flux machine, comprising: a disc-shaped rotor body having an axis of rotation; a plurality of permanent magnets mounted to a first face of the rotor body circumferentially about the axis of rotation; one or more structures arranged on a second face opposed the first face of the rotor body for reducing deflection of the rotor body when the rotor is rotated about the axis.

2. A rotor according to claim 1 , wherein the one or more structures comprise one or more slots in the second face of the rotor body, the one or more slots extending radially along at least a portion of the radial distance between the axis of rotation of the rotor and a circumferential outer edge of the rotor.

3. A rotor according to claim 2, wherein the one or more slots comprise a recess in the second face of the rotor body.

4. A rotor according to any preceding claim, wherein the one or more structures comprise one or more counterweights mounted to the second face of the rotor body.

5. A rotor according to claim 4, wherein the one or more counterweights are mounted radially between the axis of rotation and a circumferential outer edge of the rotor.

6. A rotor according to claim 4 or 5, wherein, when there are two or more counterweights, the two or more counterweights are mounted on the second face of the rotor body at the same radial distance between the axis of rotation and the circumferential edge of the rotor.

7. A rotor according to claim 4 or 5, wherein, when there are two or more counterweights, the two or more counterweights are mounted on the second face of the rotor body at different radial distance between the axis of rotation and the circumferential edge of the rotor.

8. A rotor according to any preceding claim, wherein the rotor body comprises a rotor back and a layer of metal laminate mounted to the rotor back, wherein the metal laminate forms the first surface and the magnets are mounted on or to the metal laminate.

9. The rotor according to claim 8, comprising a filling compound provided in interlamination spaces of the metal laminate.

10. The rotor according to claim 9, wherein the filling compound comprises a resin.

11. The rotor according to any one of claims 8 to 10, comprising an adhesive join between the metal laminate and the set of permanent magnets and the metal laminate.

12. A rotor according to any preceding claim, comprising a support member for receiving the plurality of permanent magnets, wherein the plurality of permanent magnets are mounted to and retained by the support member; and wherein the support member is secured to the first face of the rotor body.

13. A rotor according to claim 12, wherein the support member comprises an annular part from which a plurality of spokes project radially therefrom, each of the spaces between adjacent spokes defining a socket for receiving a respective permanent magnet.

14. A rotor according to claim 13, wherein the annular part and the spokes are formed of separate pieces.

15. A rotor according to claims 14, wherein each of the spokes is formed from a unidirectional carbon strip, and wherein the fibres of each of the carbon strips run perpendicular to the axial length of the respective spoke.

16. A rotor according to claim 14 or 15, wherein each of the spokes is formed from a material having isotropic properties in a plane that extends radially along the length of a respective spoke and a plane that extends perpendicular to the radial plane.

17. A rotor according to claim 14, 15 or 16, wherein each of the spokes is formed from a magnetic material.

18. A rotor according to claim 17, wherein the magnetic material has a magnetic field alignment that is orthogonal to a magnetic field alignment of one or more of the plurality of permanent magnets.

19. A rotor according to any preceding claim, comprising a retaining band extending around the permanent magnets, restricting or preventing radial outward movement of the permanent magnets, in use, and the retaining band being pre-stressed to apply an inward, radially directed load to the plurality of magnets,

20. A rotor according to claim 19, wherein the retaining band is of a composite material.

21 . A rotor according to claim 20, wherein the retaining band comprises windings of a reinforcing fibre material within a matrix of a suitable resin material.

22. A rotor according to claim 21 , wherein the fibre material is substantially hoop wound.

23. An axial flux machine comprising: a stator comprising a plurality of stator bars disposed circumferentially at intervals around an axis, each of the stator bars having a set of windings wound therearound for generating a magnetic field generally parallel to the axis, the plurality of stator bars being arranged to provide a hollow region at the centre of the axis; and a rotor according to any one of claims 1 to 22, wherein the rotor is mounted for rotation about the axis of the stator, and the rotor being spaced apart from the stator along the axis to define a gap between the stator and rotor.