Rotor, electric machine, and methods of manufacturing a rotor

Abstract

Various embodiments provide a rotor. The rotor comprises: a rotor core comprising a lateral outer surface; a plurality of magnet elements disposed around the lateral outer surface of the rotor core, each magnet element comprising an outer surface facing away from the rotor core; and a magnet retaining assembly comprising a plurality of magnet retaining elements, each magnet retaining element adjacent one or more magnet elements. Each magnet retaining element comprises a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of the one or more magnet elements such as to impart a centripetal force on the one or more magnet elements during rotation of the rotor to prevent said one or more magnet elements separating from the rotor core. At least a portion of the outer surface of the one or more magnet elements is exposed outside of the magnet retaining assembly. Some other embodiments provide an electric machine, and a method of manufacturing a rotor.

Description

[0001] Rotor, Electric Machine, and

[0002] Methods of Manufacturing a Rotor

[0003] Field of the Invention

[0004] The present invention relates to a rotor, an electric machine comprising the rotor, and a method of manufacturing the rotor.

[0005] Background

[0006] In a permanent magnet rotor for a motor or generator, a plurality of magnets is disposed around the rotational axis of the rotor. In a surface permanent magnet (SPM) rotor, permanent magnets are positioned on the outside of a rotor core and accordingly the magnets need to be secured to the rotor core such that when the rotor rotates, a centripetal force is exerted on the magnets such that they rotate with the rotor core as opposed to becoming detached from the rotor core. In conventional SPM rotors, this securing of the magnets to the rotor core is achieved using retaining means such as an outer cover or sleeve encircling an outer surface of the magnets and compressing them inwards towards the rotor core; this outer cover / sleeve is thus positioned between the outer surface of the magnets and the stator such that the maximum radial dimension of the rotor is increased. The magnets may also be bonded to the surface of the rotor core using an adhesive or the like, but typically an outer cover / sleeve is still required due to the bond between the magnet and rotor core having insufficient tensile strength to withstand the centripetal force that must be exerted on the magnets to prevent them detaching from the rotor core.

[0007] GB2468718A proposes a structure for an SPM rotor wherein the rotor core is laminated from a plurality of sheets, the sheets comprising deformable clamping elements that allow the magnets to be secured to the rotor core with sufficient strength that an outer cover / sleeve can be omitted and at least a portion of an outer surface of each magnet is exposed on an outer surface of the rotor - a sleeveless rotor. This structure may allow the gap between the rotor magnets and the stator to be reduced compared to rotors that comprise an outer cover / sleeve.

[0008] However, further improvements in the performance of such sleeveless rotors are desired.

[0009] Summary of the Invention

[0010] In the rotor of GB2468718A, the rotor magnets are secured to the rotor core by the deformation of the clamping elements that are formed as part of the rotor core sheets upon insertion of magnets on both sides of the clamping elements. Accordingly, in the assembled rotor, said clamping elements are interposed between adjacent magnets on the outer surface of the rotor core, are formed from the same material as (that is, are integral with) the rotor core (e.g. steel), and the material of the clamping elements has a stress being exerted on it by the adjacent magnets even when the rotor is stationary. The rotors and electric machines of the present disclosure have been developed in order to provide improved performance and efficiency over the rotor of GB2468718A and electric machines incorporating such a rotor.

[0011] In a first aspect there is provided a rotor for an electric machine, the rotor comprising: a rotor core comprising a lateral outer surface; a plurality of magnet elements disposed around the lateral outer surface of the rotor core, each magnet element comprising an outer surface facing away from the rotor core; and a magnet retaining assembly comprising a plurality of magnet retaining elements, each magnet retaining element adjacent one or more magnet elements; wherein: each magnet retaining element comprises a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of the one or more magnet elements such as to impart a centripetal force on the one or more magnet elements during rotation of the rotor to prevent said one or more magnet elements separating from the rotor core; and at least a portion of the outer surface of the one or more magnet elements is exposed outside of the magnet retaining assembly. The rotor may be a sleeveless rotor.

[0012] In this way, the rotor according to the first aspect comprises a plurality of magnet elements on the outer surface of the rotor core that are securely fastened thereto by the magnet retaining assembly (e.g. the plurality of magnet retaining elements), and the magnet retaining assembly does not cover at least a portion of the outer surface of the one or more magnet elements. In this way, said at least a portion of the outer surface of each of the plurality of magnet elements may be exposed on a lateral outer surface of the rotor. The rotor may be for an electric machine (e.g. a motor or generator). The outer surface of the magnet elements being exposed on a lateral outer surface of the rotor is advantageous in that when the rotor is included within an electric machine comprising a stator, the gap between the rotor magnet elements and the stator can be made smaller than in conventional electric machines, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor, thereby increasing the motor efficiency. The rotor may have a rotational axis. The magnet retaining assembly retains the magnet elements against the rotor core by the retention portion of each magnet retaining element that laterally overlaps the outer surface of a magnet element exerting a force on the outer surface of said magnet element during rotation of the rotor, and that force will have a component in the radially inward direction (i.e. towards the rotational axis of the rotor: a centripetal force) due to the contact being on the outer surface of the magnet element.

[0013] Each magnet retaining element may be interposed between two adjacent magnet elements. For example, when each magnet retaining element is interposed between two adjacent magnet elements, each magnet retaining element may comprise a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of each of the adjacent magnet elements such as to impart a centripetal force on each of the adjacent magnet elements during rotation of the rotor to prevent said magnet elements separating from the rotor core; and at least a portion of the outer surface of each of the adjacent magnet elements is exposed outside of the magnet retaining assembly. Alternatively, each magnet retaining element may be interposed between an adjacent magnet and another adjacent magnet retaining element (i.e. the sequence of elements around the rotor may be: first magnet retaining element

[0014] - first magnet element - second magnet retaining element - third magnet retaining element - second magnet element - fourth magnet retaining element - fifth magnet retaining element - etc.)

[0015] Separation of the magnet element from the rotor core may be understood to mean detachment, unseating, and / or dismounting of the magnet element from the rotor core.

[0016] The rotor core may comprise two base surfaces and the lateral surface may extend between the two base surfaces. The base surfaces may lie substantially perpendicular to the rotational axis of the rotor and accordingly the lateral surface may extend substantially parallel to the rotational axis of the rotor. The lateral surface may also be understood to be the circumferential outer surface of the rotor. ‘Circumferential’, as used herein, may be understood to refer to a surface extending around the rotational axis of the rotor and generally parallel to the rotational axis of the rotor, rather than denoting a cylindrical surface. That is, the lateral / circumferential surface may face radially outwards from the axis of rotation of the rotor. ‘Radial’, as used herein, may be understood to refer to a direction extending from the rotational axis of the rotor and perpendicular to the rotational axis of the rotor, rather than requiring that the outer surface of the component being referred to (e.g. the rotor core, magnet element, magnet retaining element, rotor, etc.) is cylindrical or spherical. ‘Lateral direction’, as used herein, may be understood to refer to a direction parallel to a lateral surface and extending substantially perpendicularly to the rotational axis of the rotor (i.e. a direction encircling the rotational axis of the rotor and lying in a plane substantially perpendicular to the rotational axis of the rotor).

[0017] The rotor core and / or rotor may be rotationally symmetrical (e.g. may have an order of rotational symmetry of two or more). The rotational axis of the rotor may be coincident with the axis of rotational symmetry of the rotor and / or rotor core.

[0018] The rotor may further comprise a shaft attached to the rotor core. The rotor shaft may be formed separately from the rotor core. The rotor core may comprise a rotor shaft hole for receipt of the rotor shaft. The rotor shaft hole may comprise a keyway that is configured to interlock with a key provided on an outer surface of the rotor shaft. In this way, relative rotation between the rotor core and rotor shaft is prevented when the key and keyway are interlocked. The rotor core and / or rotor may be rotationally symmetrical apart from for the rotor shaft hole and / or a rotor shaft keyway within the rotor shaft hole.

[0019] A surface being ‘exposed’, as used herein, may be taken to mean that there is no element or structure overlying the surface in a radially outward direction of the rotor; for example, a portion of the outer surface of the one or more magnet elements (or of each of the adjacent magnet elements) being exposed outside of the magnet retaining assembly may be understood to mean that there is at least a portion of the outer surface of each of the plurality of magnet elements that does not have the magnet retaining assembly overlying it in the radially outward direction. That is, the magnet retaining assembly may not cover the whole of the outer surface of a magnet element. In an embodiment, at least a portion of the outer surface of each magnet element defines a radially outmost lateral surface of the rotor. In another embodiment, the outermost circumference of the rotor is defined by the magnet elements and the magnet retaining elements, for example, in an alternating sequence of: an outer surface of a first magnet element, an outer surface of a first magnet retaining element, an outer surface of a second magnet element, an outer surface of a second magnet retaining element, etc.

[0020] Each magnet element may comprise, or consist of, a magnet e.g. a permanent magnet. In an embodiment, each magnet element comprises a permanent magnet and a magnet housing containing (e.g. partially or completely enveloping) the permanent magnet. The magnet elements may be elongate and may have a longitudinal axis substantially parallel to the rotational axis of the rotor. Each magnet element may comprise an inner surface facing towards the rotor core. Each of the plurality of magnet elements may be in contact with the lateral outer surface of the rotor core, for example, the inner surface may be in contact (e.g. in direct contact) with the lateral outer surface of the rotor core.

[0021] The fastening portion may be integrally formed with the retention portion. By the fastening portion of each magnet retaining elements being interlocked with the rotor core, it can be understood that each magnet retaining element is separate from the rotor core, that is, the plurality of magnet retaining elements are not integrally formed with the rotor core. For example, the plurality of magnet retaining elements may be formed from a different material to the rotor core.

[0022] The rotor and / or the magnet retaining assembly may not comprise an outer cover encircling the plurality of magnet elements for retention of the magnet elements on the rotor. In an embodiment, the magnet retaining assembly comprises only the plurality of magnet retaining elements, that is, the only mechanism provided in the rotor for ensuring that the magnet elements are retained on the rotor core in use may be the plurality of magnet retaining elements. Preferably, the rotor may not comprise any outer cover encircling the plurality of magnet elements for the purposes of magnet element retention or otherwise.

[0023] Advantageously, by omitting an outer cover encircling the plurality of magnet elements, the rotor diameter can be reduced and the gap between the rotor magnet elements and the stator of a motor or generator that the rotor is included within can be made smaller, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor, thereby increasing the motor efficiency. Moreover, omitting such an outer cover can reduce the rotor inertia, thereby improving motor acceleration and deceleration.

[0024] Each magnet retaining element may be formed of a polymer or a fibre-reinforced polymer. Forming the magnet retaining elements from polymers or fibre-reinforced polymers is advantageous for several reasons. Firstly, fibre-reinforced polymers provide high specific strength compared to, for example, metals and metal alloys, thereby allowing the rotor inertia to be reduced compared to if the magnet retaining elements were formed from metals or metal alloys. Secondly, when positioned between adjacent magnet elements, polymers or fibre-reinforced polymers can reduce eddy currents between adjacent rotor magnets due to their high electrical resistivity compared to, for example, metals and metal alloys. Thirdly, polymers and fibre-reinforced polymers are highly temperature resistant and can have their thermal expansion coefficient tailored to match that of the rotor core and magnets by manipulating their composition and structure. The fibre-reinforced polymer may comprise a polymer matrix reinforced with fibres. The fibres may comprise glass fibres or carbon fibres. The polymer matrix may be a thermoplastic. The polymer matrix may comprise polyamide, for example nylon 6 (polycaprolactam) or nylon 66 (Poly(hexamethylene adipamide)). The fibre-reinforced polymer may be suitable for injection moulding.

[0025] The fibre-reinforced polymer have a melting temperature (for example, measured according to ISO 11357-1 / -3) greater than or equal to 180°C, greater than or equal to 200°C, greater than or equal to 220°C, greater than or equal to 240°C, greater than or equal to 260°C, greater than or equal to 280°C, or greater than or equal to 300°C. The fibre-reinforced polymer have a melting temperature (for example, measured according to ISO 11357-1 / -3) less than or equal to 200°C, less than or equal to 220°C, less than or equal to 240°C, less than or equal to 260°C, less than or equal to 280°C, or less than or equal to 300°C.

[0026] The fibre-reinforced polymer may have a tensile strength (for example, measured according to ISO 527- 2 / 1 A and, optionally, with a specimen prepared according to ISO 3167 A) greater than or equal to 180 MPa, greater than or equal to 190 MPa, greater than or equal to 200 MPa, greater than or equal to 210 MPa, greater than or equal to 220 MPa, or greater than or equal to 230 MPa. The fibre-reinforced polymer may have a tensile strength (for example, measured according to ISO 527-2 / 1 A and, optionally, with a specimen prepared according to ISO 3167 A) less than or equal to 180 MPa, less than or equal to 190 MPa, less than or equal to 200 MPa, less than or equal to 210 MPa, less than or equal to 220 MPa, or less than or equal to 230 MPa.

[0027] The fibre-reinforced polymer may have an electrical resistivity (for example, measured according to IEC 62631-3-1) greater than 1 x 1010Ohm meters, greater than or equal to 1 x 1011Ohm meters, greater than or equal to 1 x 1012Ohm meters, greater than or equal to 1 x 1013Ohm meters, greater than or equal to 1 x 1014Ohm meters, greater than or equal to 1 x 1016Ohm meters, or greater than or equal to 1 x 1018Ohm meters. The fibre-reinforced polymer may have an electrical resistivity (for example, measured according to IEC 62631-3-1) less than or equal to 1 x 1010Ohm meters, less than or equal to 1 x 1011Ohm meters, less than or equal to 1 x 1012Ohm meters, less than or equal to 1 x 1013Ohm meters, less than or equal to 1 x 1014Ohm meters, less than or equal to 1 x 1016Ohm meters, or less than or equal to 1 x 1018Ohm meters.

[0028] More generally, each magnet retaining element may be formed of a non-ferrous material, preferably a non-metallic material.

[0029] Each magnet element may comprise an inner surface facing towards the rotor core and the outer surface of the magnet element may comprise a pair of taper surfaces extending away from the inner surface (i.e. extending away from the rotor core) and tapering inward from the inner surface towards each other.

[0030] Each magnet retaining element may laterally (e.g. circumferentially) overlap at least a portion of a taper surface of each magnet element it is adjacent. In this way, the retention portion of the magnet retaining element can have the form of a wedge (e.g. a trapezoidal prizm with the longitudinal axis of the prizm substantially parallel to the rotational axis of the rotor) that increases in lateral extent with increasing distance from the rotor core, such that the wedge overlies the taper surfaces to retain the magnet without needing to extend over the outermost portion of the outer surface of the magnet. Where the magnet retaining element has the form of a wedge, a taper angle of each magnet retaining element may be selected in order to achieve the desired torque ripple performance from the motor. The wedge may have two elongate faces: an inner face facing towards the rotor core and an outer face facing away from the rotor core; the fastening portion may extend away from the retention portion from the inner face of the wedge. The outer face and inner face may be parallel.

[0031] The outer surface of each magnet element may further comprise a lateral (e.g. circumferential) face extending between the pair of taper surfaces.

[0032] Each magnet retaining element may further comprise a lip, the lip laterally overlapping a portion of the lateral face of the one or more magnet elements. Advantageously, the lip extending over the lateral face of the magnet element may increase a maximum rotational speed of the rotor that the magnet retaining elements can withstand compared to if the magnet retention elements only laterally overlapped the taper surfaces of the one or more magnet elements. This is because the force exerted on the magnet retaining element is distributed over a larger contact area with the magnet element. Each magnet retaining element may comprise two lips, and each lip may laterally overlap a portion of the lateral face of a respective adjacent magnet element.

[0033] One or more of the inner surface, the lateral face and the taper surfaces of a magnet element may lie in a plane parallel to the rotor’s rotational axis. In this way, each magnet element may have a substantially trapezoidal cross section in a plane perpendicular to the rotational axis of the rotor. By way of example, each magnet element may have a substantially isosceles trapezoidal cross section in a plane perpendicular to the rotational axis of the rotor. Said magnet element may thus have the shape of a trapezoidal prism.

[0034] The lateral face of the magnet element may be convex in the lateral direction, and the geometry of the adjacent magnet retaining element (e.g. the lateral extent and radial thickness of the lip of a magnet retaining element) may be such that the maximum radial distance to the outer surface of the magnet element is greater than the maximum radial distance to the adjacent magnet retaining element (e.g. to the lip of the magnet retaining element).

[0035] Each magnet retaining element may be interlocked with the rotor core via a tongue and groove joint. Advantageously, the magnet retaining elements and rotor core being interlocked in this manner allows the magnet retaining elements to withstand the forces exerted on them by the magnet elements during rotation of the rotor without the magnet retaining elements detaching from the rotor core, thereby retaining the magnet elements in position on the lateral outer surface of the rotor core.

[0036] The tongue and groove joint may extend along a longitudinal axis substantially parallel to the rotational axis of the rotor. The tongue and groove joint may extend along the entire axial extent of the rotor core. The fastening portion of each magnet retaining element may comprise a tongue; and the rotor core may comprise a plurality of grooves, each groove in receipt of the tongue of a respective magnet retaining element.

[0037] Alternatively, the rotor core may comprise a plurality of tongues, and the fastening portion of each magnet retaining element may comprise a groove in receipt of a respective tongue of the rotor core.

[0038] Where the rotor core is formed of a higher density material than the magnet retaining elements, the magnet retaining element comprising the tongue and the rotor core comprising the groove advantageously provides the rotor with lower inertia compared to the alternative arrangement of the rotor core comprising the tongues and the magnet retaining elements comprising the grooves. Moreover, compared to the alternative arrangement, the magnet retaining elements comprising the tongues and the rotor core comprising the grooves can reduce eddy currents between the adjacent magnets where the electrical resistivity of the rotor core is lower than that of the magnet retaining elements.

[0039] Each groove may extend along a longitudinal axis substantially parallel to the rotational axis of the rotor and each tongue may extend along a longitudinal axis substantially parallel to the rotational axis of the rotor, such that each tongue may be received in a respective groove with the longitudinal axes of the tongue and groove substantially parallel to each other and substantially parallel to the rotational axis of the rotor, e.g. the longitudinal axis of the tongue may be substantially coaxial with the longitudinal axis of the groove.

[0040] The cross-section of a tongue in a plane perpendicular to the rotational axis of the rotor may be substantially congruent to (or may have a shape which cooperates with a shape of) the cross-section of a groove the tongue is received in said plane.

[0041] Each groove may comprise a slot extending into the rotor core and terminating in a socket, the socket having a greater lateral extent than the slot; each tongue may comprises a stem terminating in a head, the head having a greater lateral extent than the stem and the slot; and each tongue may be received in the respective groove such that said slot is in receipt of said stem and said socket is in receipt of said head. In this way, the fastening portion of the magnet retaining element cannot be detached from the rotor core by a force applied to the magnet retaining element in a radial direction of the rotor because the head is unable to pass through the slot in the radial direction.

[0042] The head may have a cross-section in a plane perpendicular to the rotational axis of the rotor that is circular, ovular, square, rectangular, triangular, pentagonal, hexagonal, or octagonal, or irregular.

[0043] The stem may subtend an angle of less than or equal to 3 degrees in the lateral direction of the rotor and about the axis of rotation of the rotor. The stem may subtend an angle of less than or equal to 2.5 degrees, less than or equal to 2.2 degrees, less than or equal to 2.1 degrees, less than or equal to 2.0 degrees, less than or equal to 1 .9 degrees, or less than or equal to 1 .8 degrees in the lateral direction of the rotor. The stem may subtend an angle of greater than or equal to 1 .0 degrees in the lateral direction of the rotor. The stem may subtend an angle of greater than or equal to 1 .5 degrees, greater than or equal to 1 .6 degrees, greater than or equal to 1 .7 degrees, greater than or equal to 1 .8 degrees, greater than or equal to 1 .9 degrees, or greater than or equal to 2.0 degrees in the lateral direction of the rotor.

[0044] The head may subtend an angle of less than or equal to 6.0 degrees in the lateral direction of the rotor and about the axis of rotation of the rotor. The stem may subtend an angle of less than or equal to 5.0 degrees, less than or equal to 4.5 degrees, less than or equal to 4.4 degrees, less than or equal to 4.3 degrees, less than or equal to 4.2 degrees, or less than or equal to 4.1 degrees in the lateral direction of the rotor. The stem may subtend an angle of greater than or equal to 2.5 degrees in the lateral direction of the rotor. The stem may subtend an angle of greater than or equal to 3.0 degrees, greater than or equal to 3.5 degrees, greater than or equal to 3.6 degrees, greater than or equal to 3.7 degrees, greater than or equal to 3.8 degrees, greater than or equal to 3.9 degrees, greater than or equal to 4.0 degrees, greater than or equal to 4.1 degrees, greater than or equal to 4.2 degrees, or greater than or equal to 4.5 degrees in the lateral direction of the rotor.

[0045] The angle subtended by the head in the lateral direction of the rotor may be approximately 2 degrees greater than the angle subtended by the stem in the lateral direction of the rotor.

[0046] The lateral outer surface of the rotor core may comprise a plurality of locating elements; and optionally each magnet element may be interposed between two adjacent locating elements; said locating elements constraining the lateral position of said magnet element on the lateral outer surface of the rotor core. That is, each pair of adjacent locating elements and the portion of the lateral outer surface of the rotor core therebetween may define a magnet slot, and one of the plurality of magnets elements may be received in each magnet slot. Advantageously, the locating elements can assist in positioning the magnet elements in the correct position on the lateral outer surface of the rotor core prior to providing the magnet retention elements to fix the position of the magnet elements.

[0047] At least a portion of each locating element may laterally overlap a portion of the outer surface of the respective adjacent magnet element. In this way, the locating elements can assist the magnet retaining elements in retaining the magnet elements against the rotor core, because the portions of the locating elements laterally overlapping the outer surface of the magnet elements will exert a force on the outer surface of the magnet elements during rotation of the rotor, that force having a component in the radial direction (i.e. a component that is a centripetal force). Alternatively, each locating element may taper inwardly as said locating element extends radially outwards from the rotational axis of the rotor. That is, the lateral extent of each locating element may reduce with increasing radial distance from the rotational axis of the rotor. In this way, a locating element may not laterally overlap a portion of the outer surface of the adjacent magnet element. By the locating element not laterally overlapping a portion of the outer surface of the adjacent magnet, a reduction in the eddy currents between adjacent magnets elements can be realised by the space between adjacent magnets elements being occupied by magnet retaining elements that are formed of a material having a high electrical resistivity rather than the rotor core that is typically formed of a material having a low electrical resistivity, e.g. electrical steel.

[0048] Each magnet retaining element may be laterally (e.g. circumferentially) coincident with a respective locating element. For example, each tongue and groove joint may be laterally coincident with a respective locating element, e.g. the groove in the rotor core may be provided through the locating element. Each locating element may form a pair of lip structures either side of the groove (i.e. the two lip structures may be disposed on opposite sides of the groove in the lateral direction).

[0049] Each magnet retaining element may be injection moulded directly into a cavity bounded by the one or more magnet elements (or the adjacent magnet elements) and the rotor core. In this way, a rotor can be provided wherein there is no air gap between the magnet retaining element and the surfaces of the rotor core and magnet elements that the magnet retaining element abuts, thus improving the stiffness of the rotor by reducing the amount by which the magnet elements can move / vibrate relative to the rotor core and magnet retaining elements. The cavity may be further defined by an outer mould element that constrains the outer face and the lips (where present) of the magnet retaining element during injection moulding, but that is removed after solidification of the magnet retaining element (i.e. the outer mould element is not a component of the rotor).

[0050] The rotor may be configured to rotate at greater than or equal to 12000 revolutions per minute (RPM), greater than or equal to 13000 RPM, greater than or equal to 13000 RPM, greater than or equal to 14000 RPM, greater than or equal to 15000 RPM, greater than or equal to 16000 RPM, greater than or equal to 17000 RPM, or greater than or equal to 18000 RPM. The rotor may be configured to rotate at less than or equal to 12000 revolutions per minute (RPM), less than or equal to 13000 RPM, less than or equal to 13000 RPM, less than or equal to 14000 RPM, less than or equal to 15000 RPM, less than or equal to 16000 RPM, less than or equal to 17000 RPM, or less than or equal to 18000 RPM.

[0051] The rotor core may be formed of electrical steel.

[0052] The rotor core may comprise a plurality of inertia holes extending therethrough.

[0053] The rotor core may comprise a plurality of sheets laminated together. The sheets may lie substantially perpendicular to the rotational axis of the rotor.

[0054] In a second aspect there is provided an electric machine comprising: a stator; and a rotor according to any preceding claim, the rotor rotatably received within the stator.

[0055] By incorporating the rotor according to the first aspect into an electric machine, the gap between the rotor magnet elements (and thus the rotor magnets) and the stator (e.g. the stator magnets) can be made smaller than in conventional electric machines, thereby increasing magnetic flux density. The electric machine may be a motor (e.g. a permanent magnet motor). In the case of a motor, using the rotor according to the first aspect increases the torque density and the torque applied to the rotor, thereby increasing the motor efficiency. Alternatively, the electric machine may be a generator.

[0056] The electric machine may have no intervening elements between the stator and the outer surface of the magnet elements at the maximum outer diameter of the rotor as set by the outer surface of the magnet elements.

[0057] The electric machine may further comprise a housing holding the stator and rotatably supporting the rotor. Where the rotor comprises a rotor shaft, the housing may rotatably support the rotor by the rotor shaft extending between two or more bearings received in the housing. The rotor shaft may have a first end portion extending from a first base surface of the rotor core and a second end portion extending from a second base surface of the rotor core, the first and second end portions extending through first and second bearings such that the rotor shaft is supported on both sides of the rotor core.

[0058] The construction of the rotor according to the first aspect facilitates the gap between the outer surfaces of the plurality of magnets elements and the stator being made smaller than in conventional motors, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor, thereby increasing the motor efficiency.

[0059] In a third aspect there is provided a method of manufacturing a rotor, the method comprising the steps of: providing a rotor core comprising a lateral outer surface; disposing a plurality of magnet elements around the lateral outer surface of the rotor core, each magnet element comprising an outer surface facing away from the rotor core when disposed around the lateral outer surface of the rotor core; and providing a magnet retaining assembly comprising a plurality of magnet retaining elements such that each magnet retaining element is adjacent one or more magnet elements; wherein: each magnet retaining element comprises a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of the one or more magnet elements such as to impart a centripetal force on the one or more magnet elements during rotation of the rotor to prevent said one or more magnet elements separating from the rotor core; and at least a portion of the outer surface of the one or more magnet elements is exposed outside of the magnet retaining assembly.

[0060] In this way, the rotor provided by the method according to the third aspect comprises a plurality of magnet elements on a lateral outer surface of the rotor core that are securely fastened thereto by the magnet retaining assembly (e.g. the plurality of magnet retaining elements), and the magnet retaining assembly does not cover at least a portion of the outer surface of each of the plurality of magnet elements such that said portion of the outer surface of each of the plurality of magnet elements is exposed on a lateral outer surface of the rotor. The outer surface of the magnet elements being exposed on a lateral outer surface of the rotor is advantageous in that when the rotor is included within an electric machine comprising a stator, the gap between the rotor magnet elements and the stator can be made smaller than in conventional electric machines, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor, thereby increasing the motor efficiency. The magnet retaining assembly retains the magnet elements against the lateral outer surface of the rotor core by the retention portion of each magnet retaining element that laterally overlaps the outer surface of a magnet exerting a force on the outer surface of said magnet element during rotation of the rotor, that force having a component in the radial direction due to the contact being on the outer surface of the magnet element (i.e. a component of the force is a centripetal force).

[0061] As in the first aspect, in the third aspect, each magnet retaining element may be interposed between two adjacent magnet elements. The step of providing the magnet retaining assembly may comprise injection moulding each magnet retaining element into a cavity bounded by the one or more magnet elements (or the adjacent magnet elements) and the rotor core. In this way, a rotor can be provided wherein there is no air gap between the magnet retaining element and the surfaces of the rotor core and magnet elements that the magnet retaining element abuts, thus improving the stiffness of the rotor by reducing the amount by which the magnet elements can move / vibrate relative to the rotor core and magnet retaining elements. The cavity may be further defined by an outer mould element that constrains the outer face and the lips (where present) of the magnet retaining element during injection moulding, but that is removed after solidification of the magnet retaining element (i.e. the outer mould element is not a component of the rotor).

[0062] The step of providing the magnet retaining assembly may comprise: forming a plurality of magnet retaining elements; and subsequently interlocking each magnet retaining element with the rotor core. In this way, a method of manufacturing the rotor according to the first aspect can be provided wherein the magnet elements do not undergo heating during the step of providing the plurality of magnet retaining elements, thereby reducing the likelihood of demagnetisation of the magnets.

[0063] Each magnet retaining element may be interlocked with the rotor core by a tongue and groove joint, and the step of interlocking each magnet retaining element with the rotor core may comprise sliding the magnet retaining element along an axis substantially parallel to the rotational axis of the rotor core such as to interlock the tongue and groove.

[0064] In a fourth aspect there is provided a method of manufacturing an electric machine, the method comprising the steps of: manufacturing a rotor using the method of the third aspect; providing a stator; and rotatably receiving the rotor within the stator such that the rotor is received in the stator with at least a portion of the outer surface of each of the plurality of magnet elements is exposed outside of the magnet retaining assembly.

[0065] In a fifth aspect there is provided an electric vehicle comprising an electric machine according to the second aspect, the electric machine being a motor.

[0066] In a sixth aspect there is provided a servo (e.g. a servomotor) comprising an electric machine according to the second aspect, the electric machine being a motor.

[0067] In a seventh aspect there is provided a robot comprising an electric machine according to the second aspect, the electric machine being a motor.

[0068] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0069] Summary of the Figures

[0070] The principles of the present disclosure will now be discussed with reference to the accompanying figures in which:

[0071] Figure 1 is a perspective view of a first rotor; Figure 2 is a cross-section through the first rotor along cutting line A - A’ in Figure 1 ;

[0072] Figure 3A is a cross-section through a magnet retaining element of the first rotor in a plane perpendicular to a rotational axis of the rotor;

[0073] Figure 3B is a detail view of a cross-section through a rotor core of the first rotor in a plane perpendicular to a rotational axis of the rotor;

[0074] Figure 4 is a detail view of a cross-section through the rotor core and a magnet element of the first rotor;

[0075] Figure 5A is a detail view of a cross-section through a second rotor in a plane perpendicular to a rotational axis of the rotor;

[0076] Figure 5B is a detail view of a cross-section through a third rotor in a plane perpendicular to a rotational axis of the rotor;

[0077] Figure 5C is a detail view of a cross-section through a fourth rotor in a plane perpendicular to a rotational axis of the rotor;

[0078] Figure 6 is a perspective view of a modification of the rotor core of the first rotor;

[0079] Figure 7A is a perspective view of the first rotor further comprising a rotor shaft;

[0080] Figure 7B is a perspective view of a motor incorporating the first rotor;

[0081] Figures 8A - 8C are illustrations of a rotor according to the present disclosure after different steps in its manufacture; and

[0082] Figure 8D is flow chart for a method of manufacturing the first, second, or third rotor.

[0083] Detailed Description of the Invention

[0084] Aspects of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0085] Figure 1 is a perspective view of a first rotor 100. The rotor 100 comprises a rotor core 110 about which ten permanent magnet elements 120 are disposed. The rotor core 110 comprises a lateral outer surface 111 that extends between a first base surface 114 and a second base surface (not visible in Figure 1) that lie substantially perpendicular to a rotational axis 101 of the rotor. At the centre of the rotor core 110 is provided a rotor shaft hole 112 that is configured for receipt of a rotor shaft (not shown in Figure 1). The rotor shaft hole 112 comprise a keyway 113 that is configured to interlock with a key provided on an outer surface of the rotor shaft such that relative rotation between the rotor core 110 and rotor shaft is prevented when the key and keyway 113 are interlocked (i.e. when the rotor core 110 is mounted onto the rotor shaft). The rotor core 110 further comprises a plurality of inertia holes 115 extending therethrough that reduce the inertia of the rotor core 110, thereby allowing for improved rotor acceleration and deceleration when the rotor 100 is included in an electric machine. The magnet elements 120 are disposed around the lateral outer surface 111 of the rotor core 110 in an array. Each magnet element 120 comprises an outer surface 121 facing away from the rotor core 110 and an inner surface that abuts the lateral surface 111 of the rotor core 110. The magnet elements 120 are elongate, having a longitudinal axis that extends substantially parallel to the rotational axis 101 of the rotor 100; the geometry of the magnet elements 120 is discussed further in relation to Figure 4 and Figures 5A - 5C. Each magnet element 120 may consist of a magnet (e.g. a permanent magnet) or may comprise a magnet, e.g. comprising a magnet and a magnet housing (e.g. a coating over the magnet).

[0086] As the rotor 100 rotates about its rotational axis 101 (as illustrated by the dashes arrow about the rotational axis 101), a centripetal force must be exerted on the magnet elements 120 in order to prevent the magnet elements 120 from separating / unseating / dismounting from the rotor core 110. The greater the rotational speed of the rotor 100, the greater the required centripetal force. Accordingly, the rotor 100 further comprises a magnet retaining assembly configured to exert such a centripetal force on the magnet elements 120. The magnet retaining assembly comprises a plurality of magnet retaining elements 130 that are not integrally formed with the rotor core 110. Each magnet retaining element 130 is interposed between two adjacent magnet elements 120 and each magnet element 120 is interposed between two adjacent magnet retaining elements 130. In order to exert the required centripetal force on the magnet elements 120 that it is interposed between, each magnet retaining element 130 comprises a fastening portion that is interlocked with the rotor core 110 and a retention portion that laterally overlaps a portion of the outer surface of each adjacent magnet element 120; the fastening portion, retention portion, and the lateral overlap of the magnet retention element 130 are discussed further in relation to Figures 3A - 3B and 5A - 5C. The magnet retention elements 130 are configured such that they only laterally overlap a portion of the outer surface 121 of each of the adjacent magnet elements 120, and in this way another portion of the outer surface 121 of each of the magnet elements 120 is exposed outside of the magnet retaining assembly and thus, since the rotor 100 does not comprise an outer cover encircling the plurality of magnet elements 120, a portion of the outer surface 121 of each magnet element 120 is exposed on a lateral outer surface of the rotor 100. The outer surface 121 of the magnet elements 120 being exposed on the lateral outer surface of the rotor 110 is advantageous in that when the rotor 110 is included within an electric machine comprising a stator, the gap between the rotor magnet elements 120 and the stator can be made smaller than in conventional electric machines, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor 100, thereby increasing the motor efficiency.

[0087] The magnet retention elements 130 are formed of a fibre-reinforced polymer (e.g. glass fibres embedded in a nylon 6 matrix or carbon fibres embedded in a nylon 66 matrix) such that the magnet retention elements 130 have sufficient tensile strength to exert the necessary centripetal force on the magnet elements 120 and are lightweight compared to, for example, if the magnet retention elements 130 were formed of steel, or other metal alloys or metals, thereby reducing rotor inertia.

[0088] Figure 2 is a cross-section through the first rotor along cutting line A - A’ in Figure 1 . Figure 2 more clearly illustrates how the magnet retention elements 130 act to retain the magnet elements 120 by having a substantially wedge-shaped cross-section to the retention portion in the plane of Figure 2. The wedge shape is such that the retention portion increases in its lateral extent with increasing distance from the rotor core 110 (i.e. with increasing radial distance from the rotational axis of the rotor). The magnet retention elements 130 also act to retain the magnet elements 120 by having lips that extend even further over the outer surface 121 of the magnet elements 120 at the outermost position of the magnet retention elements 130 from the rotor core 110. The fastening portion of the magnet retention elements 130 interlock with the rotor core 110 such that the magnet retention elements 130 are unable to more in the radial direction relative to the rotor core 110. Accordingly, movement of the magnet elements 120 in a radially outward direction is prevented by the magnet retention elements 130.

[0089] Figure 3A is a cross-section through a magnet retaining element 130 of the first rotor in a plane perpendicular to a rotational axis of the rotor, and Figure 3B is a detail view of a cross-section through a rotor core 110 of the first rotor in a plane perpendicular to a rotational axis of the rotor. In Figures 3A and 3B the magnet retaining element 130 and rotor core 110 are shown separately for clarity but are in fact interlocked in the assembled rotor.

[0090] As discussed above in relation to Figure 1 , the magnet retaining element 130 comprises a fastening portion 132 configured to interlock with the rotor core 110 and a retention portion 131 configured to laterally overlap a portion of the outer surface of each magnet element that it is adjacent.

[0091] In the first rotor, the magnet retaining element 130 and rotor core 110 interlock by way of a tongue and groove joint, which allows the magnet retaining element 130 to withstand the forces exerted on it by the magnet elements during rotation of the rotor without detaching from the rotor core 110, thereby retaining the magnet elements in position on the lateral outer surface 111 of the rotor core 110. Specifically, the rotor core 110 comprises a groove in its lateral outer surface 111 and the fastening portion 132 of the magnet retaining element 130 comprises a tongue, with the cross-section of the tongue in the plane of Figure 3A being substantially congruent to the cross-section of the groove in said plane. Each groove in the rotor core 110 extends along a longitudinal axis that is substantially parallel to the rotational axis of the rotor and each tongue similarly extends along a longitudinal axis that is substantially parallel to the rotational axis of the rotor. The congruence of their cross-sections and their extension along parallel axes allows each tongue to be received in a respective groove with their longitudinal axes coincident and optionally substantially parallel to the rotational axis of the rotor.

[0092] In order to be able to exert a centripetal force on the magnet elements, the tongue and groove joint between the rotor core 110 and magnet retaining element 130 is such that the magnet retaining element 130 cannot be unlocked from the rotor core 110 by movement in a radial direction of the rotor. Specifically, the groove comprises a slot 114 extending into the rotor core 110 from the lateral outer surface 111 thereof and the slot 114 terminates in a socket 113, with the socket 113 having a greater lateral extent that the slot 114. Similarly, each tongue comprises a stem 134 terminating in a head 133, with the head 134 having a greater lateral extent than the stem 134 and the slot 114. Consequently, the fastening portion 132 of the magnet retaining element 130 cannot be detached from the rotor core by a force applied to the magnet retaining element in a radial direction of the rotor because the head 133 is unable to pass through the slot 114 in a radial direction. In the case of Figures 3A and 3B, the head 133 and socket 113 have a circular cross-section in a plane perpendicular to the rotational axis of the rotor, but other shapes can be used. Nevertheless, where the magnet retaining element 130 is formed and subsequently interlocked with the rotor core 110 by sliding the tongue of the magnet retaining element into the groove along the longitudinal axis of the groove, it is advantageous for the head 133 and socket 113 to have a circular cross section as this reduces the interfacial surface area between the head 133 and socket 113 and thus makes the sliding of the tongue into the groove easier than with a head 133 and socket 113 having a differently-shaped cross section.

[0093] In order for each magnet retaining element 130 to laterally overlap at least a portion of the outer surface of each magnet element it is adjacent, the retention portion 131 of the magnet retaining element 130 has a wedge-shaped (e.g. a trapezoidal prism) cross-section in the plane of Figure 3A. The wedge-shaped cross section increases in lateral extent with increasing distance from the rotor core 110 (i.e. the width of the retention portion 131 in the lateral direction increases with increasing distance from the rotor core 110 in the radial direction). The retention portion 131 thus has an inner face facing towards the rotor core 110 and an outer face facing away from the rotor core 110. The fastening portion 133 thus extends away from the retention portion 131 from the inner face of the retention portion 131 . In the case of the magnet retaining element 130 in Figure 3A, the inner and outer surfaces are substantially parallel. The magnet retaining element 130 further comprises a pair of lips 135 that form part of the retention portion 131 , the lips 135 extending in the lateral direction from the radially outermost portion of the magnet retaining element 130 to provide further lateral overlap over the outer surface of the magnet elements. The lips 135 and the retention of the magnet elements by the retention portion are discussed further in relation to Figures 5A - 5C.

[0094] Figure 4 is a detail view of a cross-section through the rotor core 110 and a magnet element 120 of the first rotor.

[0095] As discussed above with reference to Figure 1 , each magnet element 120 is disposed on the lateral outer surface 111 of the rotor core 110, with an inner surface 122 that abuts the lateral outer surface 111 of the rotor. The outer surface 121 of the magnet element 120 comprises a pair of taper surfaces 124 extending away from the inner surface 122 (i.e. extending away from the rotor core 110) and a lateral face 123 that extends between the pair of taper surfaces 124. The taper surfaces 124 taper inward towards each other from the inner surface 122 towards the outer surface 121 , such that the lateral extent of the magnet 120 reduces with increasing distance from the rotor core 110 (i.e. with increasing radial distance from the rotational axis of the rotor). Consequently, the magnet element 120 has a substantially trapezoidal cross section in the plane shown in Figure 4, albeit with the lateral face 123 being convex.

[0096] As shown in detail in Figure 4, the rotor core 110 further comprises a plurality of locating elements 112 that are used in positioning the magnet elements 120 in the correct position on the lateral outer surface 111 of the rotor core 110 prior to providing the magnet retention elements to fix the position of the magnet elements 120. As illustrated in Figure 4, the magnet element 120 is interposed between two adjacent locating elements 112. The locating elements 112 extend radially outward from the remainder of the rotor core 110 such that a pair of adjacent locating elements 112 and the lateral outer surface of the rotor core effectively provide a magnet slot in which the lateral position of the magnet element 120 is constrained on the lateral outer surface 111 of the rotor core 110.

[0097] In Figure 4, the locating elements 112 are laterally (e.g. circumferentially) coincident with the groove of the tongue and groove joint that facilitates the interlocking of the magnet retaining elements with the rotor core 110. As such, the grooves are provided through the locating element 1 12 and the locating element 112 takes the form of two lips either side of the groove.

[0098] In the case of the locating elements 1 12 shown in Figure 4, a portion of each locating element laterally overlaps a portion of an adjacent chamfer surface 124 of the magnet 120 when the magnet is received in the magnet slot. Accordingly, the portions of the two locating element 112 laterally overlapping the respective chamfer surface 124 of the magnet prevent the magnet 120 from moving radially outward from the rotor core 110. In this way, during rotation of the rotor the locating elements 112 are thus able to assist the magnet retaining elements in retaining the magnet elements 120 against the rotor core 110 by exerting a force on the chamfer surfaces 124 of the magnet element 120 that has a component in the radial direction (i.e. a centripetal force). However, the overlap of the locating elements 112 with the chamfer surfaces 124 is not sufficient alone to retain the magnet elements 120 in position, hence the use of the magnet retaining elements that laterally overlap a greater portion of the outer surface 121 of the magnet elements 120.

[0099] An alternative form of the locating elements 112 is discussed below with reference to Figures 5B and 5C.

[0100] Figure 5A is a detail view of a cross-section through a second rotor in a plane perpendicular to a rotational axis of the rotor. The rotor core 110 and magnet elements 120 are the same as those in the first rotor discussed above with reference to Figures 1 - 4 and accordingly the forgoing description of these components in the first rotor is applicable to the second rotor. However, the second rotor differs from the first rotor in relation to the magnet retaining elements 130 of the second rotor illustrated in Figure 5A (which can be compared to the magnet retaining elements of the first rotor illustrated in Figure 3A).

[0101] The magnet retaining element 130 in Figure 3A is configured to be formed and subsequently interlocked with the rotor core 110. In particular, the inner face of the retention portion 131 is substantially planar and does not conform to the curved outer surface 111 of the rotor core 110 at the magnet locating element. The reason for this is two-fold: having the inner face of the retention portion not exactly conform to the lateral outer surface 11 1 of the rotor it abuts makes interlocking the magnet retaining element 130 and rotor core 110 easier when the interlocking is effected by sliding the tongue of the pre-formed magnet retaining element 130 into the groove of the rotor core because the interfacial area between the magnet retaining element 130 and rotor core 110 is reduced; and the manufacture of the magnet locating element 130 is simpler because the geometry of the magnet retaining element 130 is simpler. However, a byproduct of this arrangement is that there will be an air gap between the magnet retaining element 130 and the surfaces of the rotor core 110 and magnet element 120 that it abuts once the magnet retaining element 130 is interlocked with the rotor core 110.

[0102] In contrast, the magnet retaining element 130 in Figure 5A is formed by injection moulding material directly into a cavity bounded by the adjacent magnet elements 120 and the rotor core 110. The cavity is further defined by an outer mould element (not shown) that constrains the outer face and the lips 135 of the magnet retaining element 130. Consequently, the second rotor is provided such that there is no air gap between the magnet retaining element 130 and the outer surfaces of the magnet elements 120 and rotor core 110 that it abuts (e.g. there are no air gaps adjacent the locating element 112). Thus, the stiffness of the rotor can be improved compared to the first rotor by reducing the amount by which the magnet elements 120 can move / vibrate relative to the rotor core 110 and magnet retaining elements 130.

[0103] In order to facilitate injection moulding of the magnet retaining elements 130 and so that the magnet retaining elements 130 to provide the necessary tensile strength to retain the magnet elements 120, the magnet retention elements 130 are formed of a fibre-reinforced polymer such as glass-fibre-reinforced nylon 6 or carbon-fibre-reinforced nylon 66.

[0104] However, such fibre-reinforced polymers typically have melt temperatures for injection moulding between 200°C and 350°C. Consequently, injecting molten fibre-reinforced polymer into the cavity bounded by the adjacent magnet elements 120 and the rotor core 110 may cause the magnet elements 120 to be heated to above their typical operating temperature. Heating of magnets can lead to demagnetisation (the extent of demagnetisation depending on the temperature and exposure time), which would then reduce the efficiency of the rotor when included in an electric machine (e.g. a motor); consequently, care is required to control the temperature reached by the magnet elements 120 to avoid or limit heat-induced demagnetisation.

[0105] Like the magnet retaining element 130 of Figure 3A, the magnet retaining element 130 in Figure 5A comprises lips 135 that extend laterally over a portion of the lateral face 123 of the adjacent magnet elements 120. Because of this lateral overlap with a portion of the outer surfaces 121 of the magnet elements 120, the lips 135 also contribute to the retention of the magnet elements 120 by exerting a centripetal force on the magnet elements 120 during rotation of the rotor. This is in addition to the centripetal force provided by the wedge-shaped section of the retention portion 131 that laterally overlaps with the taper surfaces 124 of the magnet elements 120.

[0106] It can also be appreciated from Figure 5A in combination with Figure 2 that the geometries of the magnet retaining element 130 and magnet element 120, and in particular the lateral extent and radial thickness of the lips 135 and the convex lateral face of the magnet element 120, are such that the maximum radial distance to the outer surface 121 of the magnet element (i.e. the outermost point on the lateral face 123) is greater than the maximum radial distance to the outer face of the retention portion 131. Accordingly, the presence of the lips 135 on the magnet retaining element 120 is not a constraint on how small a gap between the outer surface of the magnet elements 120 and a stator in which the rotor is received can be made. Figure 5B is a detail view of a cross-section through a third rotor in a plane perpendicular to a rotational axis of the rotor.

[0107] The magnet elements 120 are the same as those in the first rotor discussed above with reference to Figures 1 - 4 and accordingly forgoing description of the magnet elements in the first rotor is applicable to the second rotor. However, the third rotor differs from the first rotor in relation to the magnet retaining elements 130 of the third rotor illustrated in Figure 5A (which can be compared to the magnet retaining elements 130 of the first rotor illustrated in Figure 3A and the magnet retaining elements of the second rotor illustrated in Figure 5A) and the magnet locating elements 112 of the third rotor illustrated in Figure 5B (which can be compared to the locating elements 130 of the first rotor illustrated in Figure 4).

[0108] The magnet retaining element 130 in Figure 5B firstly differs from that in Figure 5A in that the magnet retaining element 130 in Figure 5B has not been injection moulded directly into a cavity bounded by the adjacent magnet elements 120 and the rotor core 110. This is illustrated by the fact that there are air gaps at the point at which the magnet retaining element 130, magnet element 120 and rotor core 1 10 meet (e.g. there are air gaps adjacent the locating element 112). Instead, the magnet retaining element 130 has been separately formed, for example, by injection moulding fibre-reinforced polymer into a separate mould, and subsequently interlocked with the rotor core 110 via the tongue and groove joint illustrated in Figure 5B, for example, by the tongue of the magnet retaining element 130 being slid longitudinally into the groove provided in the rotor core 110.

[0109] In order to make sliding the magnet retaining element 130 into the groove in the rotor core 1 10 easier, the width of the stem 134 of the tongue and the corresponding width of the slot of the groove in the rotor core 110 is made wider than in the magnet retaining element 130 in Figure 5A such that the stiffness of the tongue is increased.

[0110] The magnet retaining element 130 in Figure 5B further differs from that in Figure 5B in that the magnet retaining element 130 in Figure 5B does not comprise lips extending laterally over the lateral face 123 of the magnet 120. Consequently, the only section of the outer surface 121 of the magnet elements 120 that the magnet retaining element 130 laterally overlaps is the taper edges 124. Moreover, the entire lateral face 123 of each magnet element 120 is at a greater radial distance from the rotational axis of the rotor than any point on the outer face of the retaining portion 131 .

[0111] Additionally, the locating element 112 of the rotor core in Figure 5B differs from those in the first rotor described with reference to Figure 4. Specifically, there is no portion of the locating element 112 that laterally overlaps a portion of an adjacent chamfer surface 124 of the magnet 120 when the magnet is received in the magnet slot. Rather, the locating element 112 tapers inwardly as said locating element 112 extends radially outwards. That is, the lateral extent of each locating element 112 reduces with increasing radial distance from the rotational axis of the rotor. By the locating element 112 not laterally overlapping a portion of the outer surface of the adjacent magnet elements 120, a reduction in the eddy currents between adjacent magnets elements 120 can be realised by the space between adjacent magnets 120 elements being occupied by magnet retaining elements 130 that can be formed of a material having a high electrical resistivity rather than the rotor core 110, which is typically formed of a material having a high electrical resistivity, e.g. electrical steel.

[0112] Figure 5C is a detail view of a cross-section through a fourth rotor in a plane perpendicular to a rotational axis of the rotor.

[0113] The geometry of the interface between the magnet retaining element 130, magnet elements 120 and the locating element 1 12 of the rotor core 110 in the fourth rotor of Figure 5C is similar to that of the third rotor illustrated in Figure 5B. Accordingly, only the differences between the third and fourth rotor are discussed below and for similar features between the rotors, reference can be made back to the description of Figure 5B.

[0114] Firstly, as can be appreciated by the absence of air gap between the magnet retaining element 130 and the taper surfaces 124 of the magnet elements 120 and the lateral outer surface 11 1 of the rotor core 110 that it abuts, the magnet retaining element 130 has been injection moulded directly into a cavity bounded by the rotor core 110 and the adjacent magnet elements 120. In contrast, the magnet retaining element in the third rotor was formed separately from the rotor core and magnets and subsequently interlocked with the rotor core. Because the magnet retaining element 130 has been injection moulded directly into position within the rotor, the stem 134 of the tongue and the corresponding width of the slot of the groove in the rotor core 110 is made narrower than in the magnet retaining element 130 in Figure 5B. Making the stem 134 narrower also allows the head 133 of the tongue to be made narrower, since the head 133 needs to have a greater lateral extent than the stem 134. Thus, the mass of material being injected into the cavity bounded by the magnet elements 120 can be reduced, thereby reducing the amount of thermal energy introduced into the rotor by the injection moulding process.

[0115] Secondly, the locating elements 112 in the fourth rotor have a subtly different shape to those in the third rotor shown in Figure 5B. Comparing Figures 5B and 5C, the locating elements 112 of the fourth rotor core do not extend as far radially from the rotational axis of the rotor core as the locating elements of the third rotor core. This firstly means that in the fourth rotor core 110 a greater proportion of the volume between the two adjacent magnet elements 120 is occupied by the magnet retaining element 130 rather than the rotor core 110; where the magnet retaining element 130 is formed of a material with a higher electrical resistivity than the rotor core material (e.g. where the magnet retaining element 130 is formed of a fibre-reinforced polymer and the rotor core 110 is formed of electrical steel) more of said volume being filled with the higher-resistivity material can reduce eddy currents between adjacent rotor magnets. Moreover, where the magnet retaining element 130 is formed from a lower density material than the rotor core (e.g. where the magnet retaining element 130 is formed of a fibre-reinforced polymer and the rotor core 110 is formed of electrical steel), the geometry in Figure 5C can reduce the inertia of the rotor compared to that of Figure 5B, thereby improving rotor acceleration and deceleration when the rotor is included in an electric machine.

[0116] Figure 6 is a perspective view of a modification of the rotor core of the first rotor in which the rotor core 110 is formed from a plurality of laminated sheets. The sheets are stacked together such that the sheets lie substantially perpendicular to the rotational axis of the rotor. The sheets have substantially the same shape as each other such that the cross section of the rotor core is uniform along the rotational axis of the rotor. Laminating the rotor core in this manner is advantageous both from the perspective of making machining the rotor core shape easier and reducing eddy currents in the rotor core when the resulting rotor is in use within an electric machine.

[0117] Figure 7A is a perspective view of the first rotor 100, wherein the first rotor 100 further comprises a rotor shaft 140. The rotor shaft 140 is received in the rotor shaft hole in the rotor core (see Figure 1) and a key (not shown in Figure 7A) on the rotor shaft interlocks with the keyway 113 in the rotor shaft hole (see Figure 1) such that relative rotation between the rotor core 110 and rotor shaft 140 is prevented when the key and keyway 113 are interlocked. The rotor shaft 140 comprises a first end portion 141 extending from the first base surface 114 of the rotor core and a second end portion 142 extending from a second base surface (not visible in Figure 7A) of the rotor core 110.

[0118] Figure 7B is a perspective view of a motor 1 incorporating the first rotor 100. The motor further comprises a stator 200 encircling the rotor and a housing 300 for the stator 200 and rotor 100. In Figure 7B, a front face of the housing 300 is cut away to illustrate the arrangement of the rotor 100 and stator 200 therein. The rotor 100 comprises the rotor shaft 140 as in Figure 7A that extends out from the rotor core and stator 200. The stator 200 comprises a central bore that is substantially cylindrical, and the rotational axis of the rotor 100 is coincident with the central longitudinal axis of the bore. As can be appreciated from Figure 7B, there is no outer cover / sleeve encircling the plurality of magnet elements 120 and accordingly there is only an air gap between the outer surface of the magnet elements 120 and an inner surface of the stator 200 (i.e. a surface of the bore). This arrangement facilitates the gap between the outer surfaces of the plurality of magnets elements 120 and the stator 200 being made smaller than in conventional electrical machines, thereby increasing magnetic flux density. In the case of a motor, this increases the torque density and the torque applied to the rotor 100, thereby increasing the motor efficiency.

[0119] The motor further comprises the housing 300 that holds the stator and rotatably supports the rotor 100 within the stator 200 by the rotor shaft extending between two bearings received in the housing. A first bearing (not shown) is provided on one side of the rotor core to support the first end portion of the rotor shaft and a second bearing (not shown) is provide on the opposite side of the rotor core to support the second end portion of the rotor shaft (i.e. the rotor shaft is supported on both sides of the rotor core).

[0120] Figures 8A - 8C are illustrations of a rotor according to the present disclosure after different steps in its manufacture and Figure 8D is a flow chart for a method of manufacturing the rotor, steps S100, S200 and S300 in Figure 8D corresponding to the steps shown in Figures 8A, 8B and 8C, respectively. The method commences at step S100, in which a rotor core 100 is provided. As shown in Figure 8A, the rotor core comprises a lateral outer surface 111. Subsequently, at step S200, a plurality of magnet elements 120 are disposed around the lateral outer surface 111 of the rotor core 110. As shown in Figure 8B, each magnet element 120 comprises an outer surface 121 facing away from the rotor core 110 when disposed around the lateral outer surface 111 of the rotor core 110. Then, at step S300, a magnet retaining assembly is provided. As shown in Figure 8C, the magnet retaining assembly comprises a plurality of magnet retaining elements 130 such that each magnet retaining element 130 is interposed between two adjacent magnet elements 120. As discussed above in relation to Figures 1 and 3A, each magnet retaining element 130 comprises a fastening portion 132 interlocked with the rotor core and a retention portion 131 laterally overlapping a portion of the outer surface 121 of each of the adjacent magnet elements 120. At least a portion of the outer surface 121 of each of the adjacent magnet elements 120 is exposed outside of the magnet retaining assembly.

[0121] As discussed above with reference to Figures 5A - 5C, the step of providing the magnet retaining assembly may comprise injection moulding each magnet retaining element 130 into a cavity bounded by the adjacent magnet elements 120 and the rotor core 110. In this way, a rotor 100 can be provided wherein there is no air gap between the magnet retaining element 130 and the surfaces of the rotor core 110 and magnet elements 120 that the magnet retaining element 130 abuts, thus improving the stiffness of the rotor 100 by reducing the amount by which the magnet elements 120 can move / vibrate relative to the rotor core 110 and magnet retaining elements 130.

[0122] Alternatively, the step of providing the magnet retaining assembly may comprise: forming a plurality of magnet retaining elements 130; and subsequently interlocking each magnet retaining element 130 with the rotor core 110. In this way, the rotor manufacturing method can avoid heating the magnet elements 120 during the step of providing the plurality of magnet retaining elements 130, thereby reducing the likelihood of demagnetisation of the magnets.

[0123] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0124] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0125] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0126] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0127] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0128] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

Claims:1 . A rotor, the rotor comprising: a rotor core comprising a lateral outer surface; a plurality of magnet elements disposed around the lateral outer surface of the rotor core, each magnet element comprising an outer surface facing away from the rotor core; and a magnet retaining assembly comprising a plurality of magnet retaining elements, each magnet retaining element adjacent one or more magnet elements; wherein: each magnet retaining element comprises a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of the one or more magnet elements such as to impart a centripetal force on the one or more magnet elements during rotation of the rotor to prevent said one or more magnet elements separating from the rotor core; and at least a portion of the outer surface of the one or more magnet elements is exposed outside of the magnet retaining assembly.

2. The rotor according to claim 1 , wherein the rotor does not comprise an outer cover encircling the plurality of magnet elements for retention of the magnet elements on the rotor.

3. The rotor according to any preceding claim, wherein each magnet retaining element is formed of a fibre-reinforced polymer.

4. The rotor according to any preceding claim, wherein each magnet element comprises: an inner surface facing towards the rotor core; and the outer surface of the magnet element comprises a pair of taper surfaces extending away from the inner surface and tapering inward from the inner surface to towards each other.

5. The rotor according to claim 4, wherein each magnet retaining element laterally overlaps at least a portion of a taper surface of the one or more magnet elements.

6. The rotor according to claim 4 or 5, wherein: the outer surface of each magnet element further comprises a lateral face extending between the pair of taper surfaces; and each magnet retaining element further comprises a lip, the lip laterally overlapping a portion of the lateral face of an adjacent magnet element.

7. The rotor according to any preceding claim, wherein each magnet retaining element is interlocked with the rotor core via a tongue and groove joint.

8. The rotor according to claim 7, wherein: the fastening portion of each magnet retaining element comprises a tongue; andthe rotor core comprises a plurality of grooves, each groove in receipt of the tongue of a respective magnet retaining element.

9. The rotor according to claim 8, wherein: each groove comprises a slot extending into the rotor core and terminating in a socket, the socket having a greater lateral extent than the slot; each tongue comprises a stem terminating in a head, the head having a greater lateral extent than the stem and the slot; and each tongue is received in the respective groove such that said slot is in receipt of said stem and said socket is in receipt of said head.

10. The rotor according to any preceding claim wherein: the lateral outer surface of the rotor core comprises a plurality of locating elements; and each magnet element is interposed between two adjacent locating elements, said locating elements constraining the lateral position of said magnet element on the lateral outer surface of the rotor core.

11. The rotor according to claim 10, wherein at least a portion of each locating element laterally overlaps a portion of the outer surface of the respective adjacent magnet element.

12. The rotor according to claim 10, wherein each locating element tapers inwardly as said locating element extends radially outwards.

13. The rotor according to any of claims 10 to 12, wherein each magnet retaining element is laterally coincident with a respective locating element.

14. The rotor according to any preceding claim, wherein each magnet retaining element is injection moulded directly into a cavity bounded by the one or more magnet elements and the rotor core.

15. An electric machine comprising: a stator; and a rotor according to any preceding claim, the rotor rotatably received within the stator.

16. The electric machine according to claim 15, the electric machine further comprising a housing holding the stator and rotatably supporting the rotor.

17. A method of manufacturing a rotor, the method comprising the steps of: providing a rotor core comprising a lateral outer surface; disposing a plurality of magnet elements around the lateral outer surface of the rotor core, each magnet element comprising an outer surface facing away from the rotor core when disposed around the lateral outer surface of the rotor core; andproviding a magnet retaining assembly comprising a plurality of magnet retaining elements such that each magnet retaining element is adjacent one or more magnet elements; wherein: each magnet retaining element comprises a fastening portion interlocked with the rotor core and a retention portion laterally overlapping a portion of the outer surface of the one or more magnet elements such as to impart a centripetal force on the one or more magnet elements during rotation of the rotor to prevent said one or more magnet elements separating from the rotor core; and at least a portion of the outer surface of the one or more magnet elements is exposed outside of the magnet retaining assembly.

18. The method according to claim 17, wherein the step of providing the magnet retaining assembly comprises injection moulding each magnet retaining element into a cavity bounded by the one or more magnet elements and the rotor core.

19. The method according to claim 17, wherein the step of providing the magnet retaining assembly comprises: forming a plurality of magnet retaining elements; and subsequently interlocking each magnet retaining element with the rotor core.

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