A rotor assembly

Abstract

A rotor assembly 100 for an electric motor rotatable about an axis A including a rotor 110 having a magnet assembly 114 including a first side 114a having a first surface 114a’, and a second side 114b having a second surface 114b’ which faces in the opposite direction to the first surface, where the first surface is closer to the axis than the second surface, and a support assembly 118 for supporting the magnet assembly, where the second surface includes a formation 116 in which the support assembly is received. The support assembly may cover a portion of the second surface and at least another portion of the second surface is not covered by the support assembly. The support assembly may include a first surface which faces away from the axis, and the first surface of the support assembly and the second surface of the magnet assembly may be aligned with each other. The formation may be a recess and may have a first side that extends in a direction perpendicular to the axis, and a second side that extends in a direction parallel to the axis. The formation may be shaped with an arcuate portion, rectangular portion, kite-shaped and / or Y-shaped portion. The rotor assembly may include first 110 and second 130 rotors arranged around a stator 200.

Description

FIELD The present invention relates to electric motors. More particularly, the present invention relates to a rotor assembly for electric motors. BACKGROUND An electric motor includes a rotor assembly and stator. The rotor assembly includes one or more magnets, and the stator includes electric conductors arranged around the stator, often in the form of windings. When electric current is passed through the electric conductors, the magnetic field from the magnets interacts with the electric current to exert a force on the rotor assembly which causes the rotor assembly to rotate. Figure 1 is a schematic cross-section view showing a portion of a prior art rotor assembly 10 and stator 11 with the cross section taken through a plane which is perpendicular to the plane in which the rotor assembly 10 and stator 11 lie. The rotor assembly 10 includes annular shaped inner and outer rotors 12, 14 which rotate about a common axis R. Annular shaped stator 11 is positioned between the inner and outer rotors 12, 14. The outer rotor 14 includes an annular shaped magnet 18 which is held in place by support assembly 20, e.g., a retaining ring or sleeve, provided on the surface of the outer rotor 14 which faces towards the stator 11, typically through an interference fit. The inner rotor 12 similarly includes an annular shaped magnet 22 which is held in place by a support assembly 24, e.g., a retaining ring or sleeve, provided on the surface of the inner rotor 12 which faces towards the stator 11. The support assemblies 20, 24 provide a compressive force on the magnets 18, 22, to secure the magnets 18, 22 in place and / or provide mechanical support to inhibit damage to the magnets 18, 22 under the centrifugal forces acting on the magnets 18, 22 during rotation of the rotor assembly 10. The distance between each respective surface of the magnets 18, 22 which faces towards the stator 11 is known in the art as the air gap a. The size of the air gap a determines the motor diameter, power efficiency, and the amount of current required to produce a given power output. Smaller air gaps provide for better performance against these parameters compared to larger air gaps. There is a need for an alternative and / or improved rotor assembly. BRIEF DESCRIPTION OF THE INVENTION According to an aspect of the present invention we provide a rotor assembly for an electric motor rotatable about an axis A during use including: a rotor having a magnet assembly including a first side having a first surface, and a second side having a second surface which faces in the opposite direction to the first surface, wherein the first surface is closer to axis A than the second surface; and a support assembly for supporting the magnet assembly, wherein the second surface includes a formation in which the support assembly is received. Optionally or preferably the support assembly covers a portion of the second surface and at least another portion of the second surface is not covered by the support assembly. Optionally or preferably the support assembly includes a first surface which faces away from the axis A during use, and wherein the first surface of the support assembly and the second surface of the magnet assembly are aligned with each other. Optionally or preferably the formation is a recess. Optionally or preferably the formation has a first side that extends in a direction generally perpendicular to the axis A, and a second side that extends in a direction generally parallel to axis A. Optionally or preferably the formation includes an arcuate portion which connects the first side to the second side. Optionally or preferably the formation is rectangular-shaped in cross-section. Optionally or preferably the formation is kite-shaped in cross-section. Optionally or preferably the formation is Y-shaped in cross-section. Optionally or preferably the magnet assembly includes third side and fourth sides which connect respective ends of the first and second sides respectively. Optionally or preferably the formation is positioned at or near one of the third and fourth sides, or the formation is positioned generally centrally of the first side. Optionally or preferably the rotor assembly includes a plurality of said formations. Optionally or preferably the plurality of formations includes formations positioned at or near both of the third and fourth sides. Optionally or preferably the support assembly includes a plurality of support members which are received in the plurality of said formations. Optionally or preferably the magnet assembly includes a plurality of magnets connected together and optionally or preferably the formation or the plurality of said formations have respective portions thereof defined by portions of the plurality of magnets. A rotor assembly according to any preceding aspect including first and second said rotors, wherein one or both of the first and second rotors has a said magnet assembly including a first side having a first surface, and a second side having a second surface which faces in the opposite direction to the first surface, wherein the first surface is closer to axis A than the second surface; and includes first and / or second said support assemblies for supporting respective ones of the said magnet assemblies, and wherein the second surface of one or both of the said magnet assemblies includes a said formation in which the first and / or second said support assemblies are received. Optionally or preferably the first rotor extends around the second rotor and the stator is positioned in the space between them. Optionally or preferably the first and second rotors are configured to produce radially extending flux lines with respect to the axis A. Optionally or preferably the first and second rotors are configured to produce axially extending flux lines with respect to the axis A. Optionally or preferably the support assembly has a connecting section which covers a portion of the second surface, and optionally or preferably wherein the connecting section has a reduced thickness compared to the thickness of the portion of the support assembly which is received in the formation. According to another aspect of the present invention we provide an electric motor including a stator and a rotor assembly according to any preceding aspect. Optionally or preferably the at least another portion is within an active length of the stator. Optionally or preferably the portion of the second surface covered by the connecting section is within an active length of the stator. Optionally or preferably the rotor assembly and stator are configured as a Halbach array. A rotor assembly, or electric motor according to any preceding aspect, optionally or preferably the magnet assembly or magnet assemblies and support assembly or support assemblies are generally annular-shaped, and optionally or preferably the stator is generally annular-shaped. BRIEF DESCRIPTION OF THE FIGURES In order that the present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic cross-section view of a part of a prior art rotor assembly and stator; FIGURE 2a is a schematic cross-section view of a rotor assembly and stator according to an aspect of the present disclosure; FIGURE 2b is a schematic cross-section view of a rotor assembly and stator according to another aspect of the present disclosure; FIGURE 2c is a schematic cross-section view of a rotor assembly and stator according to another aspect of the present disclosure; FIGURE 3 is a schematic cross-section view of a rotor assembly and aspect of the present disclosure; FIGURE 4 is a schematic cross-section view of a rotor assembly and aspect of the present disclosure; FIGURE 5 is a schematic cross-section view of a rotor assembly and aspect of the present disclosure; FIGURE 6 is a schematic cross-section view of a rotor assembly and stator according to another stator according to another stator according to another stator according to another aspect of the present disclosure; FIGURE 7 is a perspective view of a rotor assembly according to another aspect of the present disclosure; FIGURE 8 is a perspective view showing a portion of the rotor assembly shown in figure 7 with certain components not shown; FIGURE 9 is a cross-section view showing a portion of the rotor assembly shown in figure 7; and FIGURE 10 is a schematic cross-section view of a rotor assembly and stator according to another aspect of the present disclosure. DETAILED DESCRIPTION OF THE DISCLOSURE Referring to the figures 2 to 10, these show various rotor assembly arrangements for an electric motor which embody one or more aspects of the present disclosure. The present disclosure relates to a rotor assembly which is rotatable about an axis during use. The rotor assembly includes a magnet assembly including a first side having a first surface, and a second side having a second surface which faces in the opposite direction to the first surface. The first surface is closer to the axis than the second surface. The rotor assembly includes a support assembly for supporting the magnet assembly. The second surface includes a formation in which the support assembly is received. By way of illustration, reference is made to figure 2a which is a cross-section view showing a portion of an example of a rotor assembly 100 and stator 200 embodying the present disclosure. The crosssection is taken through a plane perpendicular to the plane on which the rotor assembly 100 and stator 200 lie. Rotor assembly 100 is rotatable about an axis A. The rotor assembly 100 is generally annular shaped in this example. The rotor assembly 100 may include an inner rotor 110 and an outer rotor 130. The inner rotor and outer rotor 110, 130 are annular shaped with the outer rotor 130 having a larger diameter than the inner rotor 110. The inner rotor 110 may be positioned within a central space defined by the outer rotor 130. A radial distance D separates the inner rotor 110 from the outer rotor 130. Stator 200 may be generally annular shaped and may be positioned in the space between the inner rotor 110 and the outer rotor 130. The inner rotor 110 may include a member 112 positioned on an inwardly facing side of the rotor, e.g. the side which faces towards axis A, on which the various components of inner rotor 110 are supported. The member 112 may be in the form of a shaft, hub or laminate structure which is generally annular shaped and on which the components of inner rotor 110 are supported. The inner rotor 110 may include a magnet assembly 114. The magnet assembly 114 may be generally annular shaped. The magnet assembly 114 may be positioned adjacent an outer surface of the member 112 with respect to axis A and connected to the member 112 about the outer surface. The magnet assembly 114 includes a first side 114a having a first surface 114a’ and a second side 114b having a second surface 114b’ which faces in the opposite direction to the first surface 114a’. The first surface 114a’ is closer to axis A than the second surface 114b’. The magnet assembly 114 includes third side and fourth sides 114c, 114d which connect respective ends of the first and second sides 114a, 114b respectively. In examples, the magnet assembly 114 may be in the form of a singular permanent magnet. In other examples, the magnet assembly 114 may include a plurality of magnets, e.g., permanent magnets, connected together. The second surface 114b’ may include a plurality of formations 116. In this example, the plurality of formations 116 include first and second formations 116a, 116b. In this example, the first formation 116a is positioned at the third side 114c and the second formation 116b is positioned at the fourth side 114d. The formations 116a, 116b may take various forms. The formations 116a, 116b may each include a first side 116a’, 116b’ which extends generally perpendicularly to the axis A, and a second side 116a”, 116b” which extends in a direction generally parallel to axis A. The formations 116a, 116b may be defined by portions of the second side 114b and second surface 114b’. In this example, the formations 116a, 116b are recesses having respective first and second sides 116a’, 116b’ and 116a”, 116b”. In this example, the first sides 116a’, 116a” are perpendicular to axis A but need not be perpendicular in other examples. The second sides 116b’, 116b” are parallel to axis A in this example but need not be perpendicular in other examples. In this example, the formations 116a, 116b may be positioned adjacent or at the third and fourth sides 114c, 114d respectively. The rotor assembly 100 includes a support assembly 118 for supporting the magnet assembly 114. In this example, the support assembly 118 covers a portion of the second surface 114b’ and at least another portion of the second surface 114b’ is not covered by the support assembly 118. The portion of the second surface 114b’ not covered by the support assembly 118 is a portion which is within the active length of the stator 200. In other examples, this may not be the case and the support assembly 118 may cover all or substantially all of the second surface 114b’. The support assembly 118 may include a plurality of support members which are received in the plurality of formations 116. In this example, the plurality of support members include support members 118a, 118b which are received in first and second formations 116a, 116b respectively. The support members 118a, 118b are identically shaped in this example and are annular shaped. In this example, the spaces defined by formations 116a, 116b that receive the support members 118a, 118b have shapes which correspond to the shape of the support members 118a, 118b. In this example, the formations 116a, 116b are rectangular shaped in side cross-section. In other words, the recesses defined by the formations 116a, 116b is shaped to fit the shape of the support members 118a, 118b. The magnetic assembly 114 is shaped to include the formations 116a, 116b. These formations 116a, 116b may be formed by subtractive processes applied to the magnet or magnets after manufacturing by removing material to create the formations 116a, 116b, or alternatively, during manufacture of the magnet or magnets which form the magnetic assembly 114, e.g. directly during manufacture using additive manufacturing techniques. In this way the magnet or magnets forming the magnetic assembly 114 are shaped specifically for receiving the support members 118a, 118b so that the mechanical support members retain the magnet assembly 114. In this example, the magnetic assembly 114 is top-hat shaped. The support assembly 118 may be made from different materials. For example, the support assembly 118 may be made from a composite material, e.g. carbon, a metal, or fibres overwrapped onto the magnet assembly 114. Any material of sufficient strength may be used to form the support assembly 118. In examples, the material may also have low electrical conductivity as well. In examples, the support assembly 118 may be an assembly of rigid components, e.g. ceramic, polymer or metals which have low electrical and magnetic conductivity assembled together. The support assembly 118 is mechanically retained by the formations 116 and / or may be attached to the magnet assembly 114 by an interference fit in examples. In this example, the support assembly 118 is arranged so that it is aligned with the second surface 114b’ of the magnet assembly 114. In other words, the support assembly 118 does not sit proud or extend past the magnet assembly 114 into the space which exists between the stator 200 and the rest of the inner rotor 110. In particular, the support assembly 118 may include one or more surfaces which face away from axis A during use and these surfaces may be aligned with the second surface 114b’ of the magnet assembly. For example, the outwardly facing surfaces of the first and second formations 116a, 116b are aligned with the second surface 114b’ of the magnet assembly 114 and lie in the same plane when viewed in side cross-section. The support assembly 118 effectively extends into predefined or preformed formation(s) in the magnet assembly 114. The outer rotor 130 may include a member 132 positioned on an outwardly facing side of the rotor, e.g. the side which faces away from axis A, on which the various components of outer rotor 130 are supported. The member 132 may be similar to member 112 and share one or more features in common with it, including its shape and the materials from which it is made. The outer rotor 130 may include a magnet assembly 134. The magnet assembly 134 may be generally annular shaped. The magnet assembly 134 may be positioned adjacent an inner surface of the member 132 with respect to axis A and may be connected to the member 132 about the inner surface. In examples, the magnet assembly 134 may be in the form of a singular permanent magnet. In other examples, the magnet assembly 134 may include a plurality of magnets, e.g., permanent magnet connected together. In this example, the magnet assembly 134 does not have any formations like 116a or 116b of the magnet assembly 114, nor is there any support assembly, e.g. similar to support assembly 118, provided to support the magnet assembly 134. In other examples, a support assembly in the form of a sleeve similar to that found in the prior art, e.g., support assembly 20 shown in figure 1, may be provided on the magnet assembly 134. Alternatively, in other examples, the magnet assembly 134 may include one or more formations similar to formations 116 in its outer surface, i.e. the surface furthest from axis A for receiving a support assembly therein, e.g. a support assembly similar to the support assembly 118. The inner and outer rotors 110, 130 are configured to produce radially extending flux lines with respect to the axis A. An advantage of the inner rotor 110 is that it is configured to minimise the air gap or distance between the second surface 114b’ of the magnet assembly 114 and the stator 200 whilst having a support assembly 118 which retains the magnet assembly 114 in place and / or provides structural integrity to inhibit deformation / structural failure of the magnet assembly 114. This advantage is achieved because the support assembly 118 is received within the formations 116a, 116b which allows the magnet assembly 114 to be positioned closer to the stator 200 compared to the prior art rotors for which the known support assembly such as the retaining ring or sleeve shown in figure 1 cause the magnet assembly to be positioned relatively further away from the stator. In this example, the remainder of the second surface 114b’ is not covered by the support assembly 118 at all and so this part of the second surface 114b’ is even closer to the stator 200 resulting in a relatively minimised air gap of ai. The portions of the second surface 114b’ that define the formations 116a, 116b have a relatively larger air gap of a2. However, these form a smaller proportion of the second surface 114b’ compared to the rest of the second surface 114b’ and so, overall, the air gap is advantageously reduced compared to prior art designs. In examples, the support assembly 118 may entirely cover the second surface, e.g. a portion may extend over the part of the second surface 114b’ which extends between the formations 116a, 116b. An example having such a configuration is shown in figure 2c. The rotor assembly 100 and stator 200 are the same as that shown in figure 2a other than the support assembly 118 being a single component having members 118a, 118b which are received in formations 116a, 116b, and the assembly 118 having a connecting section between them which extends overs the part of the second surface 114b’ which extends between the formations 116a, 116b, i.e. with the active length of the stator 200. The connecting section has a reduced thickness or depth compared to the members 118a, 118b and so the air gap ai across this section is still relatively reduced. The configuration may advantageously provide greater mechanical retention for applications which involve higher speeds whilst still minimising the air gap in comparison to the prior art. This is because the formations 116a, 116b and the support members 118a, 118b providing the greater mechanical retention, whilst the thickness of the support assembly 118 which extends over this part may be greatly reduced, i.e. reducing the air gap ai, compared to prior art arrangements such as that shown in figure 1. This means the air gap is still advantageously relatively reduced compared to such prior art arrangements. By having a relatively reduced air gap, electric motors may be designed to be smaller, have higher power efficiency, and / or reduce the amount of current required to produce a given power output. In examples, electric motors may be produced that require fewer electrical components and / or reduced magnetic materials due to the increased efficiencies obtained. In examples, the rotor assembly and stator 200 may form part of an electric motor for which the magnet assemblies and stator are configured as a Halbach array (known in the art) to provide further advantages. In the example shown in figure 2a, the formations 116a, 116b are positioned relative to the stator 200 so that the stator 200 overlies the formations 116a, 116b. The formations 116, 116b are positioned within the active length of the stator 200. The stator 200 also overlies the support assembly 118, e.g. the support members 118a, 118a, in this example. The support assembly 118 is positioned with the active length of the stator 200. In the example shown in figure 2b, which is otherwise the same as the example of figure 2a, except that the rotor assembly 100 and stator 200 are configured so that the formations 116a, 116b are outside of the active length of the stator 200. The stator 200 does not overlie the formations 116a, 116b or the support assembly 118. This may be advantageous in that the support assembly 118 may be made from a non-magnetic material and so performance may be improved by having them outside of the active length of the stator 200. In other examples, the formations 116a, 116b and / or support assembly 118 may only extend partially inside the active length. The example shown in figure 2 is one arrangement of a rotor assembly 100 and stator 200 according to the present disclosure. In examples, one or more of the formations 116a, 116b may be configured to reduce stress on the magnet assembly 114 during operation. Referring to figure 3, this shows formations 116a, 116b having arcuate portions 117a’ and 117b’ which have this effect. Each arcuate portion 117a’, 117b’ connects the respective first and second sides 116a’, 116a” and 116b’, 116b”. In examples, one or both of the first and second magnet assemblies 114, 134 may include a single formation 116 which receives a single member of a support assembly 118 rather than a plurality of formations 116. For example, referring to figure 4, this shows an inner rotor 110 which is similar to the inner rotor 112 shown in figure 1 other than the magnet assembly 114 having a single formation 116 having a similar form and configuration to formations 116a, 116b. The formation 116 is positioned generally centrally between the third and fourth sides 114c, 114d of the magnet assembly 114 but may be positioned at a different point between these sides in other examples. The formation 116 receives a single support member 118’ which is similar to the support members 116a, 116b. In examples, the formation or formations may have other shapes which are not rectangular. The respective first and second sides of each formation may be inclined relative to one another and form an acute angle where they meet. For example, referring to figure 5, this shows rotor 110 which is similar to the rotor 110 shown in figure 1 other than the shape of the formations 116a, 116b being kite-shaped in side cross-section rather than rectangular shaped. The support members 118a, 118b are also kite-shaped in cross-section to match the shapes of the formations 116a, 116b in crosssection. Referring to figure 6 this shows a rotor assembly 1100 and stator 1200 sharing features in common with the rotor assembly 100 and stator 200 other than the configuration of the inner rotor 1110. Common features are denoted by the same reference numerals with the addition of 1000. The inner rotor 1110 includes a magnet assembly 1114 including a plurality of identical magnets 1114’ each connected to respective members 1112. By having the magnet assembly 1114 formed from a plurality of smaller magnets reduces losses caused by eddy currents or other frequency related losses. In addition, using a plurality of smaller magnets rather than a single larger magnet reduces the amount of stress or deflection of the individual magnets in comparison. Each magnet 1114’ is generally similar in shape and configuration as the single magnet which forms the magnet assembly shown in figure 2, i.e. top-hat shaped, but is smaller in cross-section by comparison. The magnets 1114’ and members 1112 are connected together, e.g. by adhesive or mechanical means. In examples, there may be a singular member 1112 which is connected to the magnets 1114’. The magnet assembly 1114 may include a plurality of formations 1116a, b, c, d which are generally annular shaped and have rectangular shaped recesses in cross-section provided on the second surface 1114b’ of the magnet assembly 1114. This surface 1114b’ is formed by the respective surfaces of each magnet 1114’ connected together. The formations 1116 may have respective portions which are defined by portions of the plurality of magnets. In this example, some of the formations 1116b, c are defined by adjacent magnets 1114’ connected together whilst the formations 1116a, d at respective third and fourth sides 1114c, 1114d of the magnet assembly 1114 are half the size of the other formations 1116b, c because they are each only defined by a single magnet. Figures 7 to 9 show another example of a rotor assembly 2000 according to the present disclosure. The rotor assembly 2000 includes a single rotor 2110. The rotor 2110 and is similar to the inner rotor 110 of the example shown in figure 4 with common features denoted by the same reference numeral with the addition of 1000. The rotor 2110 differs from inner rotor 110 in that the magnet assembly 2114 is formed by a plurality of individual, smaller magnets 2114’ connected together. In this example, the magnet assembly includes magnets 2114’ positioned side by side as pairs and the pairs are connected together to extend circumferentially to form an annular shape. The magnet assembly 2114’ includes a formation 2116 positioned generally centrally between the third and fourth sides and which extends annularly around the magnet assembly. The formation 2116 is Y-shaped in cross-section and is formed by respective portions of the magnets 2114. Respective half-sections of the formation 2116 are formed by pairs of magnets 2114’ as viewed in cross-section. Each magnet 2114’ has an identical shape in this example. The support assembly 2118 is formed of a singular support member which is annular and has a cross-section which corresponds to the cross-section of the formation 2116. The support assembly 2118 is thus Y-shaped in cross-section. The rotor 21110 may be assembled by first positioning the support assembly 2118 and then placing the respective magnets 2114’ around the support assembly 2118 so that the support assembly 2118 is received in the formation 2116. The magnets 2114’ may be connected to the support assembly 2118 by an interference fit and / or the use of adhesive or other means. For example, the support assembly 2118 may be provided as a composite fibre material which is overwrapped to the magnets 2114’ and which material is cured into place on the magnets 2114’. Another example of a rotor assembly 3100 and stator 3200 according to the present disclosure is shown in figure 10. This arrangement may be used to form an axial flux motor. Figure 10 is a crosssection view through a plane which is perpendicular to the plane in which the rotor assembly and stator lie. The rotor assembly 3100 and stator 3200 share common features with the rotor assembly 100 and stator 200 that are denoted by the same reference numeral with the addition of 300. Only a portion of the rotor assembly 3100 and stator 3200 is shown in figure 10. The rotor assembly 3100 and stator 3200 are arranged differently from the examples shown in the preceding figures. The rotor assembly 3100 includes a first rotor 3110 and a second rotor 3130 which are spaced apart along an axis which is parallel to axis A, with axis A being the axis about which the rotors 3110, 3130 rotate. The stator 3200 is positioned in the space defined between the axially facing end faces of the rotors 3110, 3130. The rotors 3110, 3130 and stator 3200 are annular shaped. The stator 3200 primarily extends radially with respect to axis A rather than axially such as the stators shown in the figures 2 to 6 which primarily extend axially with respect to axis A. The rotor assembly 3100 and stator 3200 are arranged so that the magnetic flux lines produced by the rotor assembly 3100 extend generally axially, i.e. parallel to axis A. The rotor assembly 3100 is generally annular shaped in this example. The first rotor 3110 and outer rotor 3130 each have the same diameter / size in this example. An axial distance D separates the first rotor 3110 from the outer rotor 3130. The first and second rotors 3110, 3130 have a similar structure to the inner rotor 110 in that they each include a member 3112, 3132, a formation 3116a, 3116b, a support member 3118a, 3118b, and a magnet assembly 3114, 3134 having similar annular shapes. The properties and functionality of corresponding components from the previously described examples may apply in the same or a similar way to the components of the present example. The main difference is that the formations 3116a, 3116b are formed at respective only one end of the magnet assemblies 3114, 3134 rather than at both ends thereof compared to the rotor 110 and they are positioned adjacent the members 3112, 3132 because the stator 3200 is positioned between the axially facing faces, i.e. the surfaces of the sides 3114d, 3134c of the magnet assemblies 3114, 3134. The support members 3118a, 3118b perform the same function as the support members of the previously described examples, in that they protect the magnet assemblies 3114, 3134 from damage occurring due to the centrifugal forces experienced by them during operation whilst reducing the respective air gaps ai, a2 between them and the stator 3200. In the described examples within the scope of the present disclosure, the rotor assembly may only have a single rotor, e.g. a single rotor with the stator positioned inside the space defined by the rotor, single rotor with the stator adjacent the rotor, or a single rotor with the rotor being positioned inside the space defined by the stator with no outer rotor. Although certain examples have been described in relation to a given topology or arrangement of a motor having a rotor assembly and stator as described, it should be noted that other types of motor topologies and motor arrangements may utilise a rotor assembly based on one or more aspects described in the present disclosure without departing from its scope. For example, raxial flux motors, or transverse flux machines may include a rotor assembly having a support structure which supports a magnet assembly of the rotor assembly and which support structure has one or more of the features described in the present disclosure as will be understood by the skilled reader. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The invention may also broadly consist in the parts, elements, steps, examples and / or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and / or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein. Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure. Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

1. A rotor assembly for an electric motor rotatable about an axis A during use including: a rotor having a magnet assembly including a first side having a first surface, and a second side having a second surface which faces in the opposite direction to the first surface, wherein the first surface is closer to axis A than the second surface; anda support assembly for supporting the magnet assembly, wherein the second surface includes a formation in which the support assembly is received.

2. A rotor assembly according to claim 1 wherein the support assembly covers a portion of the second surface and at least another portion of the second surface is not covered by the support assembly.

3. A rotor assembly according to claim 1 or 2 wherein the support assembly includes a first surface which faces away from the axis A during use, and wherein the first surface of the support assembly and the second surface of the magnet assembly are aligned with each other.

4. A rotor assembly according to claim 1, 2 or 3 wherein the formation is a recess.

5. A rotor assembly according to any preceding claim wherein the formation has a first side thatextends in a direction generally perpendicular to the axis A, and a second side that extends in a direction generally parallel to axis A.

6. A rotor assembly according to claim 5 wherein the formation includes an arcuate portion which connects the first side to the second side.

7. A rotor assembly according to any preceding claim wherein the formation is rectangularshaped in cross-section.

8. A rotor assembly according to any preceding claim wherein the formation is kite-shaped in cross-section.

9. A rotor assembly according to any preceding claim wherein the formation is Y-shaped in cross-section.

10. A rotor assembly according to any preceding claim wherein the magnet assembly includes third side and fourth sides which connect respective ends of the first and second sides respectively.

11. A rotor assembly according to claim 10 wherein the formation is positioned at or near one of the third and fourth sides, or the formation is positioned generally centrally of the first side.

12. A rotor assembly according to any preceding claim including a plurality of said formations.

13. A rotor assembly according to claim 12 when dependent on claim 10 wherein the plurality offormations includes formations positioned at or near both of the third and fourth sides.

14. A rotor assembly according to claim 12 or 13 wherein the support assembly includes a plurality of support members which are received in the plurality of said formations.

15. A rotor assembly according to any preceding claim wherein the magnet assembly includes a plurality of magnets connected together and optionally or preferably the formation or the plurality of said formations have respective portions thereof defined by portions of the plurality of magnets.

16. A rotor assembly according to any preceding claim including first and second said rotors, wherein one or both of the first and second rotors has a said magnet assembly including a first side having a first surface, and a second side having a second surface which faces in the opposite direction to the first surface, wherein the first surface is closer to axis A than the second surface; andincludes first and / or second said support assemblies for supporting respective ones of the said magnet assemblies, andwherein the second surface of one or both of the said magnet assemblies includes a said formation in which the first and / or second said support assemblies are received.

17. A rotor assembly according to claim 16 wherein the first rotor extends around the second rotor and the stator is positioned in the space between them.

18. A rotor assembly according to claim 16 or 17 wherein the first and second rotors are configured to produce radially extending flux lines with respect to the axis A.

19. A rotor assembly according to claim 16 or 17 wherein the first and second rotors are configured to produce axially extending flux lines with respect to the axis A.

20. A rotor assembly according to any preceding claim wherein the support assembly has a connecting section which covers a portion of the second surface, and optionally or preferably wherein the connecting section has a reduced thickness compared to the thickness of the portion of the support assembly which is received in the formation.

21. An electric motor including a stator and a rotor assembly according to any preceding claim.

22. An electric motor according to claim 21 when directly or indirectly dependent on claim 2wherein the at least another portion is within an active length of the stator.

23. An electric motor according to claim 21 when directly or indirectly dependent on claim 20 wherein the portion of the second surface covered by the connecting section is within an active length of the stator.

24. An electric motor according to any one of claims to 21 to 23 wherein the rotor assembly and stator are configured as a Halbach array.

25. A rotor assembly, or electric motor according to any preceding claim wherein the magnet assembly or magnet assemblies and support assembly or support assemblies are generally annular-shaped, and optionally or preferably the stator is generally annular-shaped.

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