Motor assembly

Abstract

Disclosed is a shaft for an electric motor, the shaft formed from a hoop-wound unidirectional-fibre thermoplastic composite. Disclosed is a shroud for an impeller of an electric motor, the shroud formed from a hoop-wound unidirectional-fibre thermoplastic composite. Disclosed is a method comprising hoop-winding a unidirectional-fibre thermoplastic composite to form a shaft of an electric motor, and a method comprising hoop-winding a unidirectional-fibre thermoplastic composite to form a shroud for an impeller assembly of an electric motor. Also disclosed is a rotor assembly for an electric motor, and an electric motor.

Description

[0001] MOTOR ASSEMBLY

[0002] BACKGROUND

[0003] There is a general desire to improve electric machines, such as electric motors, in a number of ways. For example, improvements may be desired in terms of size, weight, power density, manufacturing cost, efficiency, reliability, and noise.

[0004] SUMMARY

[0005] A first aspect of the invention provides a shaft for an electric motor, the shaft formed from a hoop-wound unidirectional-fibre thermoplastic composite.

[0006] A shaft according to the first aspect has been found to exhibit increased specific stiffness and / or specific strength compared to a shaft for an electric motor formed from other materials, for example steel or injection moulded thermoplastics, or formed from a unidirectional-fibre thermoplastic composite that is not hoop-wound. Accordingly, a shaft according to the first aspect may be suitable for use at higher rotational speeds compared to a shaft for an electric motor formed from steel or injection moulded thermoplastics because the shaft can withstand higher stresses, which increase with the square of a rotational speed of the shaft, and because the shaft may exhibit less deflection under said stresses. A shaft according to the first aspect may have flexible modes at higher rotational speeds compared to a shaft for an electric motor formed from steel or injection moulded thermoplastics, and may thus be operable at higher rotational speeds without the need for soft mountings. A shaft according to the first aspect may be less energy intensive to manufacture and easier to recycle compared to shafts formed from thermosetting plastics. A shaft according to the first aspect may have a higher fracture toughness compared to a shaft formed from a thermosetting plastic.

[0007] A matrix of the hoop-wound unidirectional-fibre thermoplastic composite may be a material in the polyaryletherketone (PAEK) thermoplastic polymer family, for example poly etheretherketone (PEEK), poly etherketone (PEK), poly etherketoneketone (PEKK). Such materials exhibit high wear resistance compared to other materials suitable for use as a matrix in composite. Such high wear resistance may be desirable in the event that the shaft rubs against another surface in use.

[0008] Fibres of the unidirectional-fibre thermoplastic composite may have a modulus of at least lOOGPa. Moduli above lOOGPa may provide specific stiffness in the shaft to enable rotation of the shaft at speeds above 200,000 rpm, or above 500,000 rpm.

[0009] Fibres of the unidirectional-fibre thermoplastic composite may be continuous. A continuous fibre extends along a complete length of the composite. This may provide increased specific stiffness and strength in the shaft compared to a shaft formed from a hoop-wound thermoplastic composite with discontinuous fibres.

[0010] The unidirectional-fibre thermoplastic composite may have at least a 50%, or at least a 55%, or at least a 60%, volume fraction of fibres. Such volume fractions may provide a shaft with a higher stiffness than a shaft formed from a hoop-wound unidirectional-fibre thermoplastic composite having a lower volume fraction of fibres.

[0011] The unidirectional-fibre thermoplastic composite may be a tape. A shaft formed from a hoop-wound tape may have increased strength and stiffness compared to a shaft formed from another form of composite that is hoop-wound, for example a composite resin that is formed into a shaft by additive manufacturing the composite resin in hoops. Hoop-winding a tape may permit use of a composite with a higher volume fraction of fibres than hoopwinding a resin in an additive manufacturing process.

[0012] The tape may have a thickness in the region from 0.1mm to 0.3mm, for example 0.12mm to 0.25mm. The thickness is a distance between the major surfaces of the tape. This may provide sufficient strength and stiffness in the tape whilst permitting the tape to be hoopwound at a suitable radius to form a shaft for an electric motor.

[0013] The tape may have a density in the region from 1.4 to 2 g / cm3and a strength in the region of 2GPa. The tape may thus have a relatively high specific strength compared to other materials suitable for forming a shaft for an electric motor. The shaft may be entirely formed from the unidirectional-fibre thermoplastic composite. This may simplify manufacture of the shaft compared to a shaft comprising one or more additional materials.

[0014] The shaft may be formed from a plurality of hoop-wound unidirectional-fibre thermoplastic composite tapes. The plurality of tapes may each be helically hoop-wound, and may be arranged in a predefined pattern relative to one another. This may enable more rapid production of the shaft and may reduce manufacturing costs of the shaft.

[0015] The shaft may be for a rotor assembly of the electric motor.

[0016] The shaft may have a constant outer diameter. This may help to maintain a rotational balance of the shaft during rotation of the shaft.

[0017] The outer diameter may be no more than 30mm, or no more than 20mm, or no more than 12mm. This may help to minimise an overall size of an electric motor comprising the shaft, whilst providing stiffness in the shaft to enable the shaft to rotate at speeds above 200,000 rpm, or above 500,000 rpm.

[0018] The outer diameter may be no less than 8mm, or no less than 10mm. This may provide a stiffer shaft compared to a shaft with a smaller outer diameter.

[0019] The shaft may comprise a first portion having a first outer diameter, and a second portion having a second outer diameter different to the first outer diameter. Each of the first and second portions may be suitable for accommodating different components to the other portion.

[0020] At least a portion of the shaft may be hollow, which may not be possible for shafts of comparable size formed from more common materials, such as steel, used for shafts for rotor assemblies for electric motors because shafts formed from more common materials may lose structural integrity during use in an electric motor, for example may exhibit flexible modes at lower rotational speeds compared to solid shafts formed from such materials or not be able to sustain centrifugal body forces generated at high rotational speeds. Having a hollow portion may reduce a weight of the shaft compared to a solid shaft. This may also allow an additional component of an electric motor to be positioned within the shaft, which may help to reduce an overall size of an electric motor comprising the shaft.

[0021] The shaft may define a central bore. This may help to maintain a rotational balance of the shaft during rotation of the shaft, whilst allowing an additional component to be positioned within the central bore.

[0022] The central bore may extend along the shaft by at least a quarter, at least a third, or at least a half of the length of the shaft. The central bore may extend along a complete length of the shaft. A greater length of the central bore may help to reduce a weight of the shaft compared to a shaft with a central bore extending along the length of the shaft by a lesser amount.

[0023] The central bore may have a constant diameter. This may help to maintain a rotational balance of the shaft during rotation of the shaft.

[0024] An outer wall that defines the central bore may have a constant wall thickness. This may help to maintain a rotational balance of the shaft during rotation of the shaft, compared to a non-constant wall thickness. The wall thickness may be no more than 3mm, or no more than 2mm, or no more than 1mm, or no more than 0.5mm. Such wall thicknesses may provide stiffness in the portion of the shaft that defines the central bore to enable rotation of the shaft at speeds in excess of 200,000 rpm, whilst keeping a size and weight of the shaft less than with a greater wall thickness of the outer wall.

[0025] An entirety of the shaft may be hollow. This may simplify manufacture of the shaft compared to a non-hollow or a partially hollow shaft, since the shaft may be manufacturable only from the hoop-wound unidirectional-fibre thermoplastic composite without the need for any additional material to, at least partially, fill the shaft. This may further reduce a weight of the shaft compared to a shaft that is hollow along only a portion of the complete length of the shaft.

[0026] The shaft may have a constant inner diameter along the complete length of the shaft. This may help to maintain a rotational balance of the shaft during rotation of the shaft, compared to a non-constant inner diameter.

[0027] The unidirectional-fibre thermoplastic composite may form a wall having a constant wall thickness along the complete length of the shaft. This may help to maintain a rotational balance of the shaft during rotation of the shaft, compared to a non-constant wall thickness.

[0028] The wall may have a wall thickness of no more than 3mm, or no more than 2mm, or no more than 1mm, or no more than 0.5mm. Such wall thicknesses may provide sufficient stiffness in the portion of the shaft that defines the central bore to enable rotation of the shaft at speeds in excess of 200,000 rpm, whilst keeping a size and weight of the shaft less than with a greater wall thickness of the outer wall.

[0029] A second aspect of the invention provides a shroud for an impeller of an electric motor, wherein the shroud is formed from a hoop-wound unidirectional-fibre thermoplastic composite.

[0030] Such a shroud has been found to exhibit increased specific stiffness and / or specific strength compared to a shroud for an impeller that is formed other materials, for example from steel or injection moulded thermoplastics, or formed from a uni directional -fibre thermoplastic composite that is not hoop-wound. Accordingly, a shroud according to the second aspect may be suitable for use at higher rotational speeds compared to a shroud formed from steel or injection moulded thermoplastics because the shroud can withstand higher stresses, and because the shroud may exhibit less deflection under said stresses. A shroud according to the first aspect may be less energy intensive to manufacture and easier to recycle compared to shrouds formed from thermosetting plastics. A shroud according to the second aspect may have a higher fracture toughness compared to a shroud formed from a thermosetting plastic. A matrix of the hoop-wound unidirectional-fibre thermoplastic composite may be a material in the PAEK thermoplastic polymer family, for example PEEK, PEK, PEKK. This may provide an increase modulus of the composite compared to a composite having a matrix formed from another material. Such materials exhibit high wear resistance compared to other materials suitable for use as a matrix in composite. Such high wear resistance may be desirable in the event that the shroud rubs against another surface in use.

[0031] Fibres of the unidirectional-fibre thermoplastic composite may have a modulus of at least lOOGPa. Moduli above lOOGPa may provide stiffness in the shaft to enable rotation of the shaft at speeds above 200,000 rpm, or above 500,000 rpm.

[0032] Fibres of the unidirectional-fibre thermoplastic composite may be continuous. A continuous fibre extends along a complete length of the composite. This may provide increased specific stiffness in the shroud compared to a shroud formed from a hoop-wound unidirectional-fibre thermoplastic composite with discontinuous fibres.

[0033] The unidirectional-fibre thermoplastic composite may have at least a 50%, or at least a 55%, or at least a 60%, volume fraction of fibres. Such volume fractions may provide a shroud with a higher specific stiffness than a shroud formed from a hoop-wound unidirectional-fibre thermoplastic composite having a lower volume fraction of fibres.

[0034] The unidirectional-fibre thermoplastic composite may be a tape. A shroud formed from a hoop-wound tape may have increased strength and stiffness compared to a shroud formed from another form of composite that is hoop-wound, for example a composite resin that is formed into a shroud by additive manufacturing the composite resin in hoops, for example by fused deposition modelling (FDM). Hoop-winding a tape may permit use of a composite with a higher volume fraction of fibres than hoop-winding a resin in an additive manufacturing process.

[0035] The tape may have a thickness in the region from 0.1mm to 0.3mm, for example 0.12mm to 0.25mm. The thickness is a distance between the major surfaces of the tape. This may provide sufficient strength and stiffness in the tape whilst the tape to be hoop-wound at a suitable radius to form a shroud for an impeller for an electric motor.

[0036] The tape may have a density in the region from 1.4 to 2 g / cm3and a strength in the region of 2GPa. The tape may thus have a relatively high specific strength compared to other materials suitable for forming a shaft for an electric motor.

[0037] The shroud may be formed from a plurality of hoop-wound unidirectional-fibre thermoplastic composite tapes. The plurality of tapes may each be helically hoop-wound, and may be arranged in a predefined pattern relative to one another. This may enable more rapid production of the shroud and may reduce manufacturing costs of the shroud.

[0038] The shroud may have a tapered outer surface. A tapered outer surface may react both radial and linear forces induced during use of the shroud in an electric motor.

[0039] The shroud may form a frustum of a cone. This may simplify manufacture of the shroud compared to a shroud of another non-cylindrical shape.

[0040] A diameter of the tapered outer surface may change linearly along a length of the shroud. This may provide a generally smooth tapered outer surface which may increase performance of an impeller comprising the shroud by reducing losses caused by movement of the tapered outer surface.

[0041] The shroud may have a maximum outer diameter of no more than 60mm, or 50mm, or 40mm, or 30mm, or 20mm. This may help to keep minimise an overall size of an electric motor comprising the shroud, compared to greater outer diameters, whilst providing stiffness in the shroud to enable the shroud to rotate at speeds above 200,000 rpm, or above 500,000 rpm.

[0042] The unidirectional-fibre thermoplastic composite may form a wall having a constant wall thickness. The shroud wall may be entirely formed from the unidirectional-fibre thermoplastic composite. This may help to maintain a rotational balance of the shroud during rotation of the shroud, compared to a non-constant wall thickness and / or a wall formed of a plurality of materials.

[0043] The wall may have a wall thickness of no more than 3mm, or 2mm, or 1mm, or 0.5mm. Such wall thicknesses may provide sufficient stiffness and strength in the shroud to enable rotation of the shroud at speeds in excess of 200,000 rpm, whilst keeping a size and weight of the shroud less than a shroud with a greater wall thickness.

[0044] The shroud may comprise one or more features overmoulded onto the wall. Such features may be provided to improve performance of the shroud. For example, the one or more features may comprise a first feature overmoulded onto an inner surface of the shroud, the first feature configured to promote attachment of the shroud to an impeller. For example, the one or more features may comprise a second feature overmoulded onto an outer surface of the shroud, such as the tapered outer surface, the second feature configured to provide a surface profile or texture to promote function of the outer surface as an air bearing, for example to reduce windage of the shroud, during rotation of the shroud. For example, the one or more features may comprise a labyrinth seal overmoulded onto an outer surface of the shroud.

[0045] The one or more features may be configured to increase one or more of: an aerodynamic performance, an efficiency or a stall margin of the impeller to which the shroud is attached.

[0046] The one or more features may comprise a thermoplastic in the same family as the hoopwound unidirectional-fibre thermoplastic composite, such as the PAEK thermoplastic polymer family. This may promote adhesion between the unidirectional-fibre thermoplastic composite and the one or more features. The thermoplastic may be filled or unfilled.

[0047] A third aspect of the present invention provides an impeller assembly for an electric motor, the impeller assembly comprising: a hub and a plurality of blades, each of the plurality of blades comprising a tip; and a shroud according to the second aspect, the shroud fixed to the tips of the plurality of blades. Such an impeller assembly may exhibit reduced radial growth of the impeller assembly in use of the electric motor compared to an impeller assembly without a shroud or to an impeller assembly with a shroud that is not according to the second aspect. The shroud may help to reduce centrifugal load generated by the plurality of blades pulling on the hub in use of the electric motor due to the stiffness of the shroud being greater than that of the impeller and / or impeller blades.

[0048] The plurality of blades may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite of the shroud, such as the PAEK thermoplastic polymer family. This may promote adhesion between the shroud and the plurality of blades. The thermoplastic may be filled or unfilled.

[0049] The hub may be formed from the same material as the plurality of blades. This may simplify manufacture of the impeller assembly. The hub and the plurality of blades may be monolithic. This may provide a more robust impeller assembly compared to the hub and plurality of blades not being monolithic.

[0050] The plurality of blades may have a length extending along a longitudinal axis of the impeller assembly, and the shroud may have a length extending along the longitudinal axis of the impeller assembly that is equal to or greater than the length of the plurality of blades. This may increase performance of the impeller assembly in use, for example by reducing tip losses, compared to a shroud with a length that is less that the length of the plurality of blades.

[0051] The shroud may have a length extending along the longitudinal axis that is less than the length of the plurality of blades. This may reduce an overall weight of the impeller assembly. The shroud may be positioned towards, or at, an output end of the impeller assembly, as opposed to at an intake end of the impeller assembly. Efficiency losses at an output end of an impeller assembly are typically greater than at an intake end, so that providing a shroud at the output end may provide greater performance benefits than at the intake end. The shroud may be welded, for example ultrasonically welded, to the tips of one or more of the plurality of blades. This may provide more robust and consistent adhesion between the shroud and the one or more of the plurality of blades than other forms of adhesion. This may provide rapid, low-heat adhesion between the shroud and the one or more of the plurality of blades compared to other forms of adhesion. This may also reduce manufacturing cost compared to other forms of adhesion.

[0052] The shroud may be fixed to the tip of each of the plurality of blades along a complete length of the shroud. This may provide better adhesion between the shroud and the one or more of the plurality of blades compared to the shroud being fixed to the tip of each of the plurality of blades along only a portion of the complete length of the shroud.

[0053] A fourth aspect of the present invention provides a rotor assembly for an electric motor, the rotor assembly comprising a shaft according to the first aspect.

[0054] A rotor assembly according to the fourth aspect may exhibit improved performance compared to rotor assemblies that do not comprise a shaft according to the first aspect. For example, flexible rotor modes may occur at higher rotational speeds than rotor assemblies that do not comprise a shaft according to the first aspect. Accordingly, the rotor assembly may be operable at higher rotational speeds without the need for soft mountings. The rotor assembly may have no flexible rotor modes within an operational rotational speed range of the electric motor, for example no flexible rotor modes at rotational speeds up to at least 200,000 rpm, or at least 500,000 rpm, or at least 800,000 rpm. Balancing of the rotor assembly may be achieved more rapidly in production of the rotor assembly because the balancing can be performed at lower rotational speeds than balancing of a rotor assembly that has one or more flexible rotor modes within an operational rotational speed range of the rotor assembly.

[0055] The rotor assembly may comprise an impeller assembly fixed to the shaft. The impeller assembly may comprise a hub and a plurality of blades. The hub may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite of the shaft, such as the PAEK thermoplastic polymer family. This may enable more robust adhesion between the hub and the shaft compared to a hub formed of a different material. The thermoplastic may be filled or unfilled.

[0056] The plurality of blades may be formed from the same material as the hub. The plurality of blades and the hub may be monolithic. This may simplify manufacture of the impeller assembly and provide a more robust impeller assembly compared to the hub and plurality of blades not being monolithic.

[0057] The impeller assembly may be overmoulded onto the outer surface of the shaft. This may provide more robust adhesion between the hub and the shaft compared to an impeller assembly fixed to the shaft in a different way.

[0058] The impeller assembly may comprise a shroud, the shroud fixed to a tip of each of the plurality of blades. This may provide improved rotor assembly performance compared to an impeller assembly without the shroud, for example reduced tip losses and sensitivity to tip clearance.

[0059] The shroud may fully shroud the plurality of blades, which may provide a further reduction in tip losses and / or sensitivity to tip clearance than a shroud that partially shrouds the plurality of blades. The shroud may partially shroud the plurality of blades, which may reduce a weight of the impeller assembly compared to a shroud that fully shrouds the plurality of blades. For example, the shroud may be positioned towards, or at, an output end of the impeller assembly, as opposed to at an intake end of the impeller assembly. Efficiency losses at an output end of an impeller assembly are typically greater than at an intake end, so that providing a shroud at the output end may provide greater performance benefits than at the intake end.

[0060] The shroud may comprise a material comprising a thermoplastic in the same family as a material from which the plurality of blades is formed, such as the PAEK thermoplastic polymer family. This may provide improved adhesion between the shroud and the plurality of blades compared to a shroud formed of a material not comprising such a thermoplastic.

[0061] The shroud may be the shroud according to the second aspect. An impeller assembly comprising such a shroud may have increased specific stiffness, specific strength and / or fracture resistance compared to an impeller assembly that is not according to the second aspect.

[0062] The shroud may be formed from the same unidirectional-fibre thermoplastic composite as the shaft. This may simplify and / or quicken manufacture of the rotor assembly.

[0063] The shroud may be welded, for example ultrasonically welded, to the plurality of blades. This may provide more robust and consistent adhesion between the shroud and blades than other forms of adhesion. This may provide rapid, low-heat adhesion between the shroud and the blades compared to other forms of adhesion. This may also reduce manufacturing cost compared to other forms of adhesion.

[0064] The impeller assembly may be according to the third aspect.

[0065] The rotor assembly may comprise a magnet. The magnet may be disposed within the shaft. This may provide a more compact arrangement than a magnet positioned external to the shaft.

[0066] Placing the magnet within the hollow shaft may allow use of a solid magnet, as opposed to a hollow magnet that surrounds the shaft and thus is hollow. Use of a solid magnet may increase performance of the rotor assembly compared to a rotor assembly comprising a hollow magnet, for example by providing an improved flux-path across the magnet for a two-pole magnet and / or reducing rotational stress in the magnet during rotation of the magnet.

[0067] The magnet may be held within the shaft by interference fit between an outer surface of the magnet and an inner surface of the hollow portion. No adhesive may thus be required to retain the magnet within the shaft. This may also allow the magnet to be pressed out of the shaft at the end of the life of the rotor assembly, facilitating recycling of materials of the rotor assembly. The interference fit may reduce rotational stress in the magnet by constraining radial displacement of the magnet during rotation of the rotor assembly. The magnet may be in compression when the rotor assembly is not rotating, which may reduce maximum tensile stress during rotation of the rotor assembly for the same rotational speed, at the expense of increasing stress in the shaft compared to a non-interference fit.

[0068] The rotor assembly may comprise a balance ring. The balance ring may be disposed within the shaft. This may provide a more compact arrangement than a rotor assembly with a balance ring positioned external to the shaft. The balance ring may be retained within the shaft under interference or friction, which may negate a need for adhesive to retain the balance ring within the shaft.

[0069] The balance ring may have an outer diameter that is equal to an outer diameter of the magnet. This may allow both the balance ring and the magnet to be held within the shaft by interference fit, with the shaft having a constant inner diameter.

[0070] A fifth aspect of the present invention provides a rotor assembly for an electric motor, the rotor assembly comprising a shroud according to the second aspect.

[0071] A rotor assembly according to the fifth aspect may exhibit improved performance compared to rotor assemblies that do not comprise a shroud according to the second aspect. For example, the shroud may have a high specific stiffness and / or specific strength compared to a shroud formed from a material and manufacturing method more typically used to form a shroud for an impeller assembly.

[0072] The rotor assembly may comprise a hub and a plurality of blades extending radially outward from the hub, the shroud attached to tips of the plurality of blades. Providing such a rotor assembly may increase performance of the rotor assembly in use, for example by reducing tip losses at the tips of the plurality of blades. The plurality of blades may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite of the shroud, such as the PAEK thermoplastic polymer family. This may promote adhesion between the shroud and the plurality of blades. The thermoplastic may be filled or unfilled.

[0073] The hub may be formed from the same material as the plurality of blades. This may simplify manufacture of the rotor assembly. The hub and the plurality of blades may be monolithic. This may provide a more robust rotor assembly compared to the hub and plurality of blades not being monolithic.

[0074] The plurality of blades may have a length extending along a longitudinal axis of the rotor assembly, and the shroud may have a length extending along the longitudinal axis of the rotor assembly that is equal to or greater than the length of the plurality of blades. This may increase performance of the rotor assembly in use, for example by reducing tip losses, compared to a shroud with a length that is less than the length of the plurality of blades.

[0075] The shroud may have a length extending along the longitudinal axis that is less than the length of the plurality of blades. This may reduce an overall weight of the rotor assembly. The shroud may be positioned towards, or at, a trailing edge of each of the plurality of blades, as opposed to a leading edge of each of the plurality of blades. Efficiency losses at a trailing edge of the plurality of blades are typically greater than at a leading edge, so that providing a shroud at the trailing edge may provide greater performance benefits than at the leading edge.

[0076] The shroud may be welded, for example ultrasonically welded, to the tip of one or more of the plurality of blades. This may provide more robust and consistent adhesion between the shroud and the one or more of the plurality of blades than other forms of adhesion. This may provide rapid, low-heat adhesion between the shroud and the one or more of the plurality of blades compared to other forms of adhesion. This may also reduce manufacturing cost compared to other forms of adhesion. The shroud may be fixed to the tip of each of the plurality of blades along a complete length of the shroud. This may provide better adhesion between the shroud and the one or more of the plurality of blades compared to the shroud being fixed to the tip of each of the plurality of blades along only a portion of the complete length of the shroud.

[0077] The rotor assembly may comprise a shaft. The shaft may be fixed to the hub. The hub and plurality of blades may be overmoulded on to the shaft. This may provide more robust adhesion between the hub and the shaft compared to the hub and plurality of blades being fixed to the shaft in a different way.

[0078] The shaft may comprise a thermoplastic in the same family as a material from which the hub is formed, such as the PAEK thermoplastic polymer family. This may provide improved adhesion between the shaft and the hub compared to a shaft formed of a material not comprising such a thermoplastic.

[0079] The shaft may be according to the first aspect. A rotor assembly comprising such a shaft may have increased specific stiffness and specific strength compared to a shaft that is not according to the fist aspect, for example a solid steel shaft.

[0080] The shaft may be formed from the same material as the shroud. This may simplify and / or quicken manufacture of the rotor assembly.

[0081] The rotor assembly may comprise a magnet. The magnet may be disposed within the shaft. This may provide a more compact arrangement than a magnet positioned external to the shaft.

[0082] Placing the magnet within the hollow portion of the shaft may allow use of a solid magnet, as opposed to a hollow magnet that surrounds the shaft and thus is hollow. Use of a solid magnet may increase performance of the rotor assembly compared to a rotor assembly comprising a hollow magnet, for example by providing an improved flux-path across the magnet for a two-pole magnet and / or reducing rotational stress in the magnet during rotation of the magnet. The magnet may be held within the shaft by interference fit between an outer surface of the magnet and an inner surface of the hollow portion. No adhesive may thus be required to retain the magnet within the shaft. This may also allow the magnet to be pressed out of the shaft at the end of the lift of the rotor assembly, facilitating recycling of materials of the rotor assembly. The interference fit may reduce rotational stress in the magnet by constraining radial displacement of the magnet during rotation of the rotor assembly. The magnet may be in compression when the rotor assembly is not rotating, which may reduce maximum tensile stress during rotation of the rotor assembly for the same rotational speed, at the expense of increasing stress in the shaft compared to a non-interference fit.

[0083] The rotor assembly may comprise a balance ring. The balance ring may be disposed within the shaft. This may provide a more compact arrangement than a rotor assembly with a balance ring positioned external to the shaft. The balance ring may be retained within the shaft under friction or interference, which may negate a need for adhesive to retain the balance ring within the shaft.

[0084] The balance ring may have an outer diameter that is equal to an outer diameter of the magnet. This may allow both the balance ring and the magnet to be held within the shaft by interference fit, with the shaft having a constant inner diameter.

[0085] A sixth aspect of the present invention provides a rotor assembly for an electric motor, the rotor assembly comprising the impeller assembly according to the third aspect.

[0086] A rotor assembly according to the sixth aspect may exhibit improved performance compared to rotor assemblies that do not comprise an impeller assembly according to the third aspect. For example, the shroud may have a high specific stiffness compared to a shroud formed from a material more typically used to form a shroud for an impeller assembly, which may reduce stress in the hub and / or the plurality of blades in use, for example by helping to constrain radial deflection. The rotor assembly may comprise a shaft. The shaft may be fixed to the hub. The impeller assembly may be overmoulded on to the shaft. This may provide more robust adhesion between the hub and the shaft compared to an impeller assembly fixed to the shaft in a different way.

[0087] The shaft may comprise a material comprising a thermoplastic in the same family as a material from which the hub is formed, such as the PAEK thermoplastic polymer family. This may provide improved adhesion between the shaft and the hub compared to a shaft formed of a material not comprising such a thermoplastic.

[0088] The shaft may be according to the first aspect. A rotor assembly comprising such a shaft may have increased specific stiffness and specific strength compared to a shaft that is not according to the fist aspect.

[0089] The shaft may be formed from the same material as the shroud. This may simplify and / or quicken manufacture of the rotor assembly.

[0090] The rotor assembly may comprise a magnet. The magnet may be disposed within the shaft. This may provide a more compact arrangement than a magnet positioned external to the shaft.

[0091] The magnet may be within a hollow portion of the shaft, which may allow use of a solid magnet, as opposed to a hollow magnet that surrounds the shaft and thus is hollow. Use of a solid magnet may increase performance of the rotor assembly compared to a rotor assembly comprising a hollow magnet, for example by providing an improved flux-path across the magnet for a two-pole magnet and / or reducing rotational stress in the magnet during rotation of the magnet.

[0092] The magnet may be held within the shaft by interference fit between an outer surface of the magnet and an inner surface of the hollow portion. No adhesive may thus be required to retain the magnet within the shaft. This may also allow the magnet to be pressed out of the shaft at the end of the lift of the rotor assembly, facilitating recycling of materials of the rotor assembly. The interference fit may reduce rotational stress in the magnet by constraining radial displacement of the magnet during rotation of the rotor assembly. The magnet may be in compression when the rotor assembly is not rotating, which may reduce maximum tensile stress during rotation of the rotor assembly for the same rotational speed, at the expense of increasing stress in the shaft compared to a non-interference fit.

[0093] The rotor assembly may comprise a balance ring. The balance ring may be disposed within the shaft. This may provide a more compact arrangement than a rotor assembly with a balance ring positioned external to the shaft. The balance ring may be retained within the shaft under friction, which may negate a need for adhesive to retain the balance ring within the shaft.

[0094] The balance ring may have an outer diameter that is equal to an outer diameter of the magnet. This may allow both the balance ring and the magnet to be held within the shaft by interference fit, with the shaft having a constant inner diameter.

[0095] A seventh aspect of the present invention provides a method comprising hoop-winding a unidirectional-fibre thermoplastic composite to form a shaft of an electric motor.

[0096] Such a method may be less energy intensive than a method of forming a shaft for an electric motor from thermosetting plastics. A shaft formed using the method may have a higher fracture toughness compared to a shaft formed from a thermosetting plastic. A shaft formed using the method may have improved specific strength and specific stiffness compared to a shaft formed from injection moulded thermoplastics. A shaft formed using the method may be lighter than a shaft formed from steel, for example. A shaft formed using the method may have higher wear resistance compared to a shaft formed from other materials, such that the shaft may be suitable for defining a surface of an air bearing.

[0097] The unidirectional-fibre thermoplastic composite may be a tape. Hoop-winding a tape may provide a shaft with a higher strength and stiffness compared to hoop-winding unidirectional-fibre thermoplastic composite that is in a different form. For example, a higher volume fraction of fibres may be provided in a tape compared to a composite resin suitable for use in additive manufacturing, such as fused deposition modelling, for example.

[0098] The hoop-winding may comprise melting the tape on to a mandril. This may provide consistent and reliable shaft dimensions and adhesion between adjacent layers of the tape. The method may comprise removing the shaft from the mandril.

[0099] The unidirectional-fibre thermoplastic composite may comprise a plurality of tapes. The hoop-winding may comprise simultaneously hoop-winding the plurality of tapes. This may enable more rapid production of the shaft and may reduce manufacturing costs of the shaft.

[0100] The method may comprise hoop-winding the unidirectional-fibre thermoplastic composite to form a hollow structure, and cutting the hollow structure in a radial direction to form the shaft. This may provide a relatively rapid and repeatable way to form the shaft.

[0101] The hoop-winding may confer desirable properties to the shaft for use in an electric motor, for example as a shaft of a rotor assembly of the electric motor. Such desirable properties may comprise increased specific strength and specific stiffness compared to a solid or hollow metallic or thermoplastic injection moulded shaft, and a rotor assembly having no flexible rotor modes below a particular rotational speed, for example below 200,000rpm, 500,000rpm or 850,000 rpm.

[0102] A matrix of the hoop-wound unidirectional-fibre thermoplastic composite may be a material in the PAEK thermoplastic polymer family, for example PEEK, PEK, PEKK. Such materials exhibit high wear resistance compared to other materials suitable for use as a matrix in composite. Such high wear resistance may be desirable in implementations in which the shaft rubs against another surface in use.

[0103] Fibres of the unidirectional-fibre thermoplastic composite may have a modulus of at least lOOGPa. Moduli above lOOGPa may provide sufficient stiffness in the shaft to enable rotation of the shaft at speeds above 200,000 rpm, or above 500,000 rpm. Fibres of the unidirectional-fibre thermoplastic composite may be continuous. A continuous fibre extends along a complete length of the composite, such that the continuous fibre is helically wound around and along the shaft. This may provide increased specific stiffness in the shaft compared to a shaft formed from a hoop-wound unidirectional-fibre thermoplastic composite without continuous fibres.

[0104] The unidirectional-fibre thermoplastic composite may have at least a 50%, or at least a 55%, or at least a 60%, volume fraction of fibres. Such volume fractions may provide a shaft with a higher stiffness and strength than a shaft formed from a hoop-wound unidirectional-fibre thermoplastic composite having a lower volume fraction of fibres.

[0105] The, or each, tape may have a thickness in the region from 0.1mm to 0.3mm, for example 0.12mm to 0.25mm. The thickness is a distance between the major surfaces of the tape. This may provide sufficient strength and stiffness in the tape whilst permitting a small enough bend radius.

[0106] The, or each, tape may have a density in the region from 1.4 to 2 g / cm3and a strength in the region of 2GPa. The tape may thus have a relatively high specific strength compared to other materials suitable for forming a shaft for an electric motor.

[0107] The shaft may be according to the first aspect of the invention.

[0108] The method may comprise fixing an impeller assembly to an outer surface of the shaft to form a rotor assembly. The fixing may comprise overmoulding the impeller assembly to the outer surface of the shaft. This may allow the impeller assembly to be attached to the shaft without adhesive.

[0109] The impeller assembly may comprise a hub, a plurality of blades extending outwardly from the hub, and a shroud surrounding the hub and plurality of blades. Alternatively, the shroud may be omitted. The method may comprise fixing or overmoulding the hub and blades to the outer surface of the shaft, and welding, for example ultrasonically welding, the shroud to the blades.

[0110] The hub may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite, such as the PAEK thermoplastic polymer family. The overmoulding may thus provide better adhesion between the hub and the shaft than if the hub is formed from another material. The thermoplastic may be filled or unfilled.

[0111] The impeller assembly may be according to the third aspect. The shroud may be according to the second aspect.

[0112] The shaft may be at least partially hollow along a length of the shaft, and the method may comprise placing a magnet within the shaft. The placing may comprise pressing the magnet into the shaft in an axial direction of the shaft. This may provide a rapid way to form a shaft comprising a magnet therein.

[0113] The method may comprise placing the magnet within the shaft with an interference fit. This may allow the magnet to be placed within the shaft without adhesive. The interference fit may reduce rotational stress in the magnet by constraining radial displacement of the magnet during rotation of the rotor assembly. The magnet may be in compression when the rotor assembly is not rotating, which may reduce maximum tensile stress during rotation of the rotor assembly for the same rotational speed, at the expense of increasing stress in the shaft compared to a non-interference fit.

[0114] The hoop-winding may comprise hoop-winding the unidirectional-fibre thermoplastic composite around a magnet. This may provide a rapid way to form a shaft comprising a magnet therein.

[0115] The magnet may be a solid magnet. When used within a rotor assembly for an electric motor, this shaft with the solid magnet may provide better performance compared to a hollow magnet, for example by providing an improved flux-path across the magnet for a two-pole magnet and / or reducing rotational stress in the magnet during rotation of the magnet.

[0116] The method may comprise pressing a balancing ring into a hollow portion of the shaft. The method may comprise modifying the balancing ring to provide a balancing correction. The modifying may comprise, with the balancing ring disposed in the hollow portion of the shaft, machining the balancing ring in an axial direction of the shaft. This may provide a rapid, accurate and relatively simple way to provide a balancing correction.

[0117] An eighth aspect of the present invention provides a method comprising hoop-winding a unidirectional-fibre thermo-plastic composite to form a shroud for an impeller assembly of an electric motor.

[0118] Such a method may be less energy intensive than a method of forming a shroud for an impeller of an electric motor from thermosetting plastics or from a metallic material. A shroud formed using the method may have a higher fracture toughness compared to a shroud formed from a thermosetting plastic. A shroud formed using the method may have improved specific strength and specific stiffness compared to a shroud formed from injection moulded thermoplastics, or from a shroud that is not formed by hoop-winding such a composite. A shroud formed using the method may be lighter than a shroud formed from steel, for example. A shroud formed using the method may have higher wear resistance compared to a shroud formed from other materials, such that the shroud may be suitable for defining a surface of an air bearing.

[0119] The unidirectional-fibre thermoplastic composite may be a tape. Hoop-winding a tape may provide a shroud with a higher strength and stiffness compared to hoop-winding unidirectional-fibre thermoplastic composite that is in a different form. For example, a higher volume fraction of fibres may be provided in a tape compared to a composite resin suitable for use in additive manufacturing. The hoop-winding may comprise melting the tape on to a mandril. This may provide consistent and reliable shroud dimensions and adhesion between adjacent layers of the tape. The method may comprise removing the shroud from the mandril.

[0120] The unidirectional-fibre thermoplastic composite may comprise a plurality of tapes. The hoop-winding may comprise simultaneously hoop-winding the plurality of tapes. This may enable more rapid production of the shroud and may reduce manufacturing costs of the shroud.

[0121] The method may comprise hoop-winding the unidirectional-fibre thermoplastic composite to form a hollow structure, and cutting the hollow structure in a radial direction to form the shaft. This may provide a relatively rapid and repeatable way to form the shaft.

[0122] The hoop-winding may confer desirable properties to an impeller assembly comprising the shroud for use in an electric motor. Such desirable properties may comprise increased specific strength and specific stiffness compared to a shroud formed by another method.

[0123] A matrix of the hoop-wound unidirectional-fibre thermoplastic composite may be a material in the PAEK thermoplastic polymer family, for example PEEK, PEK, PEKK. Such materials exhibit high wear resistance compared to other materials suitable for use as a matrix in composite. Such high wear resistance may be desirable in implementations in which the shroud rubs against another surface in use.

[0124] Fibres of the unidirectional-fibre thermoplastic composite may have a modulus of at least lOOGPa. Moduli above lOOGPa may provide sufficient stiffness in the shroud to enable rotation of the shroud at speeds above 200,000 rpm, or above 500,000 rpm.

[0125] Fibres of the unidirectional-fibre thermoplastic composite may be continuous. A continuous fibre extends along a complete length of the composite, such that the continuous fibre is helically wound around and along the shroud. This may provide increased stiffness in the shroud compared to a shroud formed from a hoop-wound unidirectional-fibre thermoplastic composite without continuous fibres. The unidirectional-fibre thermoplastic composite may have at least a 50%, or at least a 55%, or at least a 60%, volume fraction of fibres. Such volume fractions may provide a shroud with a higher specific stiffness than a shroud formed from a hoop-wound unidirectional-fibre thermoplastic composite having a lower volume fraction of fibres.

[0126] The, or each, tape may have a thickness in the region from 0.1mm to 0.3mm, for example 0.12mm to 0.25mm. The thickness is a distance between the major surfaces of the tape. This may provide sufficient strength and stiffness in the tape whilst permitting a small enough bend radius.

[0127] The, or each, tape may have a density in the region from 1.4 to 2 g / cm3and a strength in the region of 2GPa. The tape may thus have a relatively high specific strength compared to other materials suitable for forming a shaft for an electric motor.

[0128] The method may comprise hoop-winding the unidirectional-fibre thermo-plastic composite with a non-constant diameter, for example with a linearly changing diameter. The resulting shroud may thus be tapered, or frustoconical, which, in use, may allow the shroud to react a thrust load generated during use of an electric motor comprising an impeller assembly comprising the shroud.

[0129] The method may comprise overmoulding one or more features onto a surface of the shroud, the one or more features configured to promote performance of the shroud in use in an electric motor. The one or more features may comprise a labyrinth seal. The one or more features may comprise a profile and / or a pattern on an exterior surface of the shroud configured to promote the use of the exterior surface as a surface of an air bearing, to reduce noise generated by the shroud in use and / or to reduce windage of the shroud in use. The one or more features may be configured to increase one or more of aerodynamic performance, aerodynamic efficiency and stall margin of the shroud or an impeller assembly comprising the shroud.

[0130] The shroud may be according to the second aspect. The method may comprise welding, for example ultrasonically welding, an inner circumferential surface of the shroud to tips of blades of an impeller to form an impeller assembly. Welding, particularly ultrasonic welding, may permit robust, consistent, non- invasive and rapid adhesion between the shroud and the tips of the blades which may be particularly suitable in a high-volume production line. Ultrasonic welding may also reduce manufacturing cost compared to other forms of adhesion.

[0131] The blades may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite of the shroud, such as the PAEK thermoplastic polymer family. This may provide improved adhesion between the blades and the shroud compared to blades formed from a different material. The thermoplastic may be filled or unfilled.

[0132] The method may comprise overmoulding one or more features onto an interior surface of the shroud, the one or more features configured to promote welding, for example ultrasonic welding, of the shroud to the blades. The one or more features may comprise a thermoplastic in the same family as the unidirectional-fibre thermoplastic composite of the shroud, such as the PAEK thermoplastic polymer family. The one or more features may have a lower volume fraction of fibres than the hoop-wound unidirectional-fibre thermoplastic composite. This may provide additional material that can more readily melt during the ultrasonic welding to form a stronger bond between the shroud and the impeller. The thermoplastic may be filled or unfilled.

[0133] The method according to the eighth aspect may comprise the method according to the seventh aspect.

[0134] A ninth aspect of the present invention provides a rotor assembly for an electric motor, the rotor assembly comprising a shaft that is at least partially hollow, and a magnet within a hollow portion of the shaft.

[0135] This may provide a more compact arrangement that an electric motor comprising a magnet that is external to the hollow shaft. Placing the magnet within the hollow shaft may allow use of a solid magnet, as opposed to a hollow magnet that surrounds the shaft and thus is hollow. Use of a solid magnet may increase performance of the rotor assembly compared to a rotor assembly comprising a hollow magnet, for example by providing an improved flux-path across the magnet for a two-pole magnet and / or reducing rotational stress in the magnet during rotation of the magnet.

[0136] The magnet may be held within the shaft by interference fit. No adhesive may thus be required to retain the magnet within the shaft. The interference fit may reduce rotational stress in the magnet by constraining radial displacement of the magnet during rotation of the rotor assembly. The magnet may be in compression when the rotor assembly is not rotating, which may reduce maximum tensile stress during rotation of the rotor assembly for the same rotational speed, at the expense of increasing stress in the shaft compared to a non-interference fit.

[0137] The shaft may be completely hollow. This may reduce the amount of material required to manufacture the shaft. This may also reduce a weight of the shaft.

[0138] The rotor assembly may comprise one or more balancing rings within the hollow shaft. This may provide a more compact arrangement that a rotor assembly with balancing rings external to the hollow shaft.

[0139] The one or more balancing rings may be held within the shaft by interference fit. No adhesive may thus be required to retain the one or more balancing rings within the shaft.

[0140] The shaft may be the shaft according to the first aspect of the invention.

[0141] The rotor assembly may be the motor assembly according to the fourth aspect, the fifth aspect or the sixth aspect.

[0142] A tenth aspect of the present invention provides an electric motor comprising an impeller and a shroud located about the impeller, wherein an external surface of the shroud defines an air bearing surface. An electric motor comprising such an air bearing surface may be capable of performing at higher rotational speeds than electric motors with rolling element bearings.

[0143] The external surface may be formed from a unidirectional-fibre thermoplastic composite. Such composites have been found to have higher wear resistance than other materials more typically used for shrouded impellers, and thus be particularly suitable for use as air bearing surfaces, particularly for aerodynamic bearings in which mechanical contact occurs at zero rotational speed and / or when the motor starts and stops rotating.

[0144] The unidirectional-fibre thermoplastic composite may be a hoop-wound tape having any of the features described herein with reference to the shroud of the second aspect. This may provide a shroud with higher specific stiffness and specific strength than other configurations of the unidirectional-fibre thermoplastic composite. Such a shroud may help to reduce radial growth of the impeller, and better bear centrifugal load of blades of the impeller pulling on a hub of the impeller, compared to a shroud of different composition.

[0145] The external surface may be tapered along a length of the impeller. The external surface may thus be capable of reacting both radial and longitudinal loads in use as a surface of an air bearing.

[0146] The external surface may comprise one or more features that reduce windage of the impeller. The one or more features may be overmoulded on the external surface.

[0147] The shroud may be a shroud according to the second aspect. Together the shroud and impeller may define an impeller assembly. The impeller assembly may be according to the third aspect.

[0148] The electric motor may comprise a housing surrounding the shroud. The housing may form part of a stator of the electric motor. A surface of the housing adjacent to the external surface may form a surface of the air bearing opposing the external surface. The surface of the housing may be formed from a unidirectional-fibre thermoplastic composite, which may be the same as the unidirectional-fibre thermoplastic composite from which the external surface is formed. Alternatively, the surface of the housing may be formed from a metal or other kind of material with a low coefficient of friction with the external surface of the shroud.

[0149] The electric motor may comprise one or more of a shaft according to the first aspect, a shroud for an impeller according to the second aspect, an impeller assembly according to the third aspect, a rotor assembly according to the fourth aspect, a rotor assembly according to the fifth aspect, a rotor assembly according to the sixth aspect, and a rotor assembly according to the ninth aspect.

[0150] The electric motor may provide any advantages of the relevant aspects described herein.

[0151] An eleventh aspect of the present invention provides an electric motor, the electric motor comprising one or more of a shaft according to the first aspect, a shroud for an impeller according to the second aspect, an impeller assembly according to the third aspect, a rotor assembly according to the fourth aspect, a rotor assembly according to the fifth aspect, a rotor assembly according to the sixth aspect, and a rotor assembly according to the ninth aspect.

[0152] The electric motor may provide any advantages of the relevant aspects described herein.

[0153] For an electric motor according to the eleventh aspect comprising a shaft and / or a shroud formed from hoop-wound unidirectional-fibre thermoplastic composite, such as a shaft according to the first aspect or a shroud according to the second aspect, an inner or outer surface of the shaft and / or an outer surface of the shroud may define an air bearing surface. The hoop-wound unidirectional-fibre thermoplastic composite may provide high wear resistance compared to other materials typically used to form a shaft and / or a shroud for an electric motor, and thus may withstand, in use, rubbing and / or tribological wear against an adjacent surface of the electric motor that forms an opposing surface of the air bearing during start up and stop of the electric motor. A twelfth aspect of the present invention provides an electric motor with a first critical speed above 850k rpm or 700krpm of 600krpm of 500krpm or 400krpm.

[0154] Such a motor may enable higher rotational speeds of an electric motor to be employed than a motor with a first critical speed below 850k rpm. Such a motor may be suitable for use in appliances that require high electric motor speeds, for example a motor to generate an airflow in a vacuum cleaner.

[0155] The electric motor may comprise one or more of: a shaft according to the first aspect, a shroud for an impeller according to the second aspect, an impeller assembly according to the third aspect, a rotor assembly according to the fourth aspect, a rotor assembly according to the fifth aspect, a rotor assembly according to the sixth aspect, and a rotor assembly according to the ninth aspect.

[0156] The electric motor may be manufactured by a method comprising the method according to the seventh aspect and / or the method according to the eighth aspect.

[0157] The electric motor may provide any advantages of the relevant aspects described herein.

[0158] As applied to any of the aspects described herein, the term ‘hoop-wound’ is intended to describe continuous circumferential winding of the unidirectional-fibre thermoplastic composite, for example about an axis. The hoop-wound unidirectional-fibre thermoplastic composite may have a pitch of zero degrees, or may have an axial component such that the pitch of greater than zero degrees and the unidirectional-fibre thermoplastic composite is therefore helically hoop-wound, unidirectional-fibre thermoplastic composite.

[0159] BRIEF DESCRIPTION OF THE DRAWINGS

[0160] Figure 1 shows a schematic sectional view of an electric motor according to an example;

[0161] Figure 2 shows an exploded view of a rotor assembly of the electric motor of Figure 1; Figure 3 shows a shroud of the rotor assembly of Figure 2;

[0162] Figure 4 shows a first example method;

[0163] Figure 5 shows a second example method; and

[0164] Figure 6 shows a third example method.

[0165] DETAILED DESCRIPTION

[0166] An electric motor 1 is schematically illustrated in cross-section in Figure 1. The electric motor 1 comprises a motor housing 2, a stator assembly 4 and a rotor assembly 10. The motor housing 2 surrounds the stator assembly 4 and the rotor assembly 10. The motor 1 is a mixed flow compressor, but in other examples may be an axial compressor.

[0167] The rotor assembly 10 is shown in exploded view in Figure 2, and comprises a shaft 12, an impeller assembly 14 fixed to the shaft 12, a magnet 16 disposed within the shaft 12, and two balancing rings 18 disposed within the shaft 12 adjacent to, and on opposing sides of, the magnet 16.

[0168] The shaft 12 comprises a shaft wall 20, which is formed from hoop-wound unidirectional- fibre thermoplastic composite, is tubular, hollow, and has a length of around 50 mm. The shaft wall 20 has a constant thickness of around 0.5 mm along a complete length of the shaft, with an inner diameter of around 10 mm and an outer diameter of around 11 mm. The shaft has a first end 21 and a second end 22 opposite to, and downstream from, the first end 21.

[0169] A bearing portion 24, denoted by the dashed lines in Figures 1 and 2, of an outer surface 26 of the shaft wall 20 is immediately adjacent to a first bearing surface 6 of the motor housing 2. The first bearing surface 6 surrounds the bearing portion 24 of the outer surface 26 and has an inner diameter of around 12 mm. In this example, the bearing portion 24 of the outer surface 26 is towards the second end 22 of the shaft 12 and downstream of the impeller assembly 14, but in other examples is towards the first end 21 of the shaft and upstream of the impeller assembly 14.

[0170] The impeller assembly 14 comprises a hub 26, a plurality of blades 28 extending radially outward from the hub 26, and a shroud 30 surrounding the hub 26 and the plurality of blades 28. The hub 26 has an inner diameter that is substantially equal to the outer diameter of the shaft 12, and is overmoulded on to the shaft 12 at the first end 21. The shroud 30 is ultrasonically welded to a tip 32 of each blade of the plurality of blades 28 along a complete length of the blades 28.

[0171] The impeller assembly 14 has a first impeller end 31 towards the first end 21 of the shaft 12, and extends along the shaft 12 to a second impeller end 32. The shroud 30 extends from the first impeller end 31 to the second impeller end 32, and tapers in diameter between the first impeller end 31 and the second impeller end 32. The shroud wall 33 has a constant thickness of around 1 mm. At the first impeller end 31, the shroud wall 33 has an outer diameter of around 30 mm and at the second impeller end 32, the shroud wall 33 has an outer diameter of around 35 mm. The diameter of the shroud wall 33 increases linearly between the first impeller end 30 and the second impeller end 32. In other examples, the shroud wall 33 extends only partially between the first impeller end 31 and the second impeller end 32. In other examples the shroud wall 33 extends beyond the first impeller end 31 and second impeller end 32.

[0172] An outer surface 34 of the shroud wall 33 is immediately adjacent to a second bearing surface 8 of the motor housing 2. The second bearing surface 8 surrounds the outer surface 34 and has a diameter that is less than 2 mm greater than an adjacent part of the outer surface 34.

[0173] The shroud wall 33 is formed from the same unidirectional-fibre thermoplastic composite as the shaft wall 20 of the shaft 12, which is hoop-wound to form the shroud wall 33.

[0174] The shroud 30 comprises a thermoplastic layer 39 overmoulded onto the outer surface 34 of the shroud wall 33, the thermoplastic layer 39 comprising a chevron-like pattern 40, shown in more detail in Figure 3. The chevron-like pattern 40 comprises a plurality of chevrons that each protrude from a remainer of the thermoplastic layer 39 by around 0.2 mm. Each chevron of the chevron-like pattern 40 is equal in size and shape to each other chevron in the chevron-like pattern 40 in this example. Respective apexes 41 of the chevrons each point in the same circumferential direction around the shroud 30. It will be appreciated that different patterns may be employed in other examples. For example, the thermoplastic layer 39 may comprise a plurality shapes, such as chevrons, recessed into the thermoplastic layer 39.

[0175] In this example, the thermoplastic layer 39 has a constant thickness across the outer surface 34 of the shroud wall 33, and has a lower volume fraction of fibres than the unidirectional- fibre thermoplastic composite of the shroud wall 33. In other examples, the thermoplastic layer 39 varies, optionally linearly, in thickness from the first impeller end 31 to the second impeller end 32.

[0176] The shroud 30 comprises a layer 42 of thermoplastic overmoulded onto an entirety of the inner surface 44 of the shroud wall 33. The layer 42 has a constant thickness and has a lower volume fraction of fibres than the unidirectional-fibre thermoplastic composite of the shroud wall 33. In other examples, the shroud 30 comprises features 42 of thermoplastic overmoulded onto portions of the inner surface 44 of the shroud wall 33. In other examples, the layer 42 varies, optionally linearly, in thickness from the first impeller end 31 to the second impeller end 32.

[0177] The magnet 16 is a sintered magnet in the form of a solid cylinder having an outer diameter of around 10 mm and a length of around 9 mm. In other examples the magnet 16 may be a bonded or other form of magnet. The magnet 16 is disposed within the shaft 12 between the impeller assembly 14 and the bearing portion 24, and is held within the shaft 12 by an interference fit with an inner surface 36 of the shaft wall 20.

[0178] The balancing rings 18 are solid cylinders of brass having an outer diameter of around 10mm and a length of around 5mm. The balancing rings 18 each have a milled portion 19 extending partially therethrough, if required to be machined during a balancing procedure of the rotor. The balancing rings 18 are held within the shaft 12 by an interference fit with the inner surface 36 of the shaft wall 20.

[0179] The stator assembly 4 comprises four coils 38 disposed around the shaft 12 in such a way that the coils 38 overlap the magnet 16 in a direction along the length of the shaft 12. A different configuration of coils 38 may be employed in other examples. The coils 38 are electrically connected to an electrical power source, for example a battery assembly and / or a mains power socket.

[0180] In use of the electric motor, electrical power is provided to the coils 38 in sequence to generate magnetic fields that interact with the magnet 16 and cause the rotor assembly 10 to spin about a longitudinal axis 100 of the shaft 12. This causes the impeller assembly 14 to generate an airflow in a direction, denoted by arrow A, from the first end 21 towards the second end 22. The rotation of the rotor and / or the airflow energises a first air bearing between the bearing portion 24 and the first bearing surface 6 of the motor housing 2, and a second air bearing between the outer surface 34 of the shroud wall 33 and the second bearing surface 8 of the motor housing 2. The taper of the shroud 30 allows the second air bearing to react both radial and thrust loads generated by the impeller assembly 14 in use.

[0181] In this example, the electric motor 1 is outlet cooled; the impeller assembly 14 is upstream of the stator assembly 4. In examples in which the impeller assembly is towards the second end 22 of the shaft 12, the electric motor 1 is inlet cooled; the impeller assembly 14 is downstream of the stator assembly 4.

[0182] The shaft 12 is formed by a method 400 as shown in Figure 4, and comprises hoopwinding 402 the unidirectional-fibre thermoplastic composite, in this example in the form of a tape. The tape has continuous fibres that each extend along a complete length of the tape. The tape has a modulus of around 120 GPa, a volume fraction of fibres of around 60%, a thickness of around 0.15 mm, a width of around 5 mm, a density of around 1.6 g / cm3, and a strength of around 2 GPa. It will be appreciated that, in other examples, continuous-fibre thermoplastic composite of different construction may be employed. In this example, the hoop-winding 402 comprises hoop-winding the tape onto a mandril 403 having a diameter of 10 mm. In this example, the mandril is heated to partially melt a matrix of the composite to adhere adjacent turns of the composite to one another. In some examples, the tape is heated prior to being wound onto the mandril.

[0183] The hoop-winding 402 comprises winding 404 the tape around the magnet 16. The method 400 then comprises removing 406 the hoop-wound tape with the magnet 16 within from the mandril, and cutting 408 the hoop-wound tape to a length of 50 mm to form the shaft wall 20 with the magnet 16 disposed within.

[0184] The shroud 30 is formed by a method 500 as shown in Figure 5, which comprises forming the shroud 30 by hoop-winding 504 the unidirectional-fibre thermoplastic composite, in this example with the same tape as used to form the shaft 12. In this example, the hoopwinding 504 comprises hoop-winding the tape onto a mandril 505 to form the shroud wall 33. In this example, the mandril is heated to partially melt a matrix of the composite to adhere adjacent turns of the composite to one another. In some examples, the tape is heated prior to being wound onto the mandril. The method 500 then comprises removing 506 the shroud wall 33 from the mandril.

[0185] The method 500 comprises overmoulding 508 the thermoplastic layer 39 having the chevron-like pattern 40 onto the outer surface 34 of the shroud wall 33 and overmoulding 510 the layer 42 onto the inner surface 44 of the shroud wall 33, to form the shroud 30.

[0186] The rotor assembly 10 is formed by the method 600 shown in Figure 6. The method comprises forming the shaft 12, in this example by the method 400 described above with reference to Figure 4. The method 600 further comprises forming the hub and the blades 28 of the impeller assembly 14 by overmoulding 602 the hub 26 and the blades 28 on to the shaft 12 at the first end 21 of the shaft 12. In this example, the hub 26 and the plurality of blades 28 are injection moulded 602 as a single part, but it will be appreciated that other moulding operations may be employed in other examples.

[0187] The method 600 comprises forming the shroud 30, in this example by the method 500 described above with reference to Figure 5, and fixing 604 the shroud 30 to the tip 32 of each blade of the plurality of blades 28 to form the impeller assembly 14. In this example, the fixing 604 comprises ultrasonically welding 605 the shroud 30 to the tip 32 of each blade of the plurality of blades 28 along a complete length of the tip 32. During the ultrasonic welding, the layer 42 is at least partially melted and bonded to the tip 32 of each blade of the plurality of blades 28.

[0188] The method 600 further comprises pressing 606 the balancing rings 18 into the shaft 12 in a direction along the longitudinal axis 100 with an interference fit. The balancing rings are pressed, in this example, to a position immediately adjacent to the magnet 16. The method 600 comprises milling 608 the balancing ring 18 in-situ, within the shaft 12, to balance the rotor assembly 10.

[0189] The hub 26, plurality of blades 28, the chevron-like pattern 40 and the layer 42 are each formed from a material comprising a thermoplastic in the PEAK thermoplastic family. This aids adhesion of the chevron -like pattern 40 and the layer 42 with the shroud wall 33, the hub 26 with the shaft 12, and the plurality of blades 28 with the shroud 30. The layer 42 has a lower volume fraction of fibres than the shroud wall 33. In other examples, one or more of the hub 26, plurality of blades 28 the chevron -like pattern 40 and the layer 42 are not formed from a material comprising a thermoplastic in the same thermoplastic family as the shaft wall 20 and / or the shroud wall 33.

[0190] In other examples, one or more of the blocks of Figures 4, 5 and 6 shown with dashed lines may be omitted. For example, the tape may not be wound onto a mandril, the magnet may be pressed into the shaft after the hoop-winding, no pattern or layer may be added to the shroud wall or may be added in a different way, and / or the shroud may be fixed to the blades in a different manner to ultrasonic welding.

[0191] There is a general desire to produce electric motors with increased power density. One of the main design levers to help achieve this is to increase a rotational speed of the rotor assembly. However, as the rotational speed of the rotor assembly increases, significant mechanical, thermal and aeroacoustic challenges present themselves. Such challenges may include maintaining the robustness / integrity of components of the electric motor, keeping acoustic output of the electric motor below a threshold, and controlling the dynamic behaviour of the rotor assembly. Maintaining air flow speeds below the speed of sound is another consideration.

[0192] Increasing rotational speed of the rotor may result in a desire to reduce the diameter of components of the rotor. However, this may result in lower stiffness of the rotor, which may in turn have an adverse effect on rotor dynamics of the rotor, particularly as rotational forces increase with the square of the rotor speed. For example, a flexible rotor mode of a rotor with a lower stiffness may fall within running speeds of the electric motor (or have insufficient safety margin to such running speeds) and thus necessitate provision of soft mountings for the rotor to maintain rigid rotor behaviour at higher speeds, increasing the cost and complexity of employing the electric motor. The electric motor 1, and in particular the rotor assembly 10 help to overcome such challenges.

[0193] It will be appreciated that other examples are envisaged and fall within the scope of the claims, as described briefly, and by way of example only, below.

[0194] In other examples the shroud 30 is omitted from the rotor assembly 10. In other examples, the impeller is an axial flow impeller and the shroud 30 cylindrical rather than tapered.

[0195] In other examples one or both of the shaft 12 and the shroud 30 are not formed from hoopwound unidirectional-fibre thermoplastic composite, and / or are not formed from the same hoop-wound unidirectional-fibre thermoplastic composite.

[0196] In other examples the shaft 12 is only partially hollow and / or has a non-constant outer diameter.

[0197] In other examples the hub 26 is fixed to the shaft 12 and the shroud 30 is fixed to the tips 32 of the plurality of blades 28 in a different way, for example by non-ultrasonic welding and / or with adhesive. In other examples the magnet is hollow. In other examples the magnet is positioned around the shaft 12, rather than within the shaft 12.

[0198] In other examples one or more rolling element bearings are provided instead of the first and / or second air bearings.

[0199] In other examples one or both of the balancing rings are omitted. In other examples balancing operations (addition or subtraction of material) are performed on portions of the impeller and / or shroud and / or shaft.

[0200] In other examples, the impeller is moulded and then ultrasonically welded to the shaft.

[0201] In other examples, the balance rings and / or the magnet are ultrasonically inserted into the hollow shaft.

[0202] In other examples, at least a portion of an inner surface of the shaft or shroud defines an air bearing surface, or an interface to a rolling element bearing.

[0203] In other examples, the layer 42 may comprise discrete regions of thermoplastic composite, rather than covering an entirety of the inner surface 44 of the shroud wall 33.

[0204] In other examples the method may comprise hoop-winding a unidirectional-fibre thermoplastic composite in a different way, for using additive manufacturing to form the shaft and / or the shroud from a continuous-fibre thermoplastic composite resin.

[0205] Such variations are provided by way of example only. Others are envisaged without departing from the scope of the claims.

Claims

CLAIMS1. A shaft for an electric motor, the shaft formed from a hoop-wound unidirectional- fibre thermoplastic composite.

2. The shaft according to claim 1, wherein at least a portion of the shaft is hollow.

3. The shaft according to claim 2, wherein an entirety of the shaft is hollow.

4. The shaft according to any one of the preceding claims, having an outer diameter of no more than 30mm.

5. The shaft according to any one of the preceding claims, having an outer diameter of no less than 8mm.

6. A shroud for an impeller of an electric motor, wherein the shroud is formed from a hoop-wound unidirectional-fibre thermoplastic composite.

7. The shroud according to claim 6, comprising a tapered outer surface.

8. The shroud according to claim 6 or claim 7, wherein the uni directional -fibre thermoplastic composite forms a wall having a wall thickness of no more than 3mm.

9. The shaft according to any one of claims 1 to 5, or the shroud according to any one of claims 6 to 8, wherein fibres of the unidirectional-fibre thermoplastic composite have a modulus of at least lOOGPa.

10. The shaft according to any one of claims 1 to 5 or 9, or the shroud according to any one of claims 6 to 8, wherein fibres of the unidirectional-fibre thermoplastic composite are continuous.

11. The shaft according to any one of claims 1 to 5, 9 or 10, or the shroud according to any one of claims 6 to 10, wherein the unidirectional-fibre thermoplastic composite has at least a 50% volume fraction of fibres.

12. The shaft according to any one of claims 1 to 5 or 9 to 11, or the shroud according to any one of claims 6 to 11, wherein the unidirectional-fibre thermoplastic composite is a tape.

13. A rotor assembly for an electric motor, the rotor assembly comprising the shaft according to any one of claims 1 to 5 or 9 to 12.

14. A rotor assembly for an electric motor, the rotor assembly comprising a shroud according to any one of claims 6 to 12.

15. A method comprising hoop-winding a uni directional -fibre thermoplastic composite to form a shaft of an electric motor.

16. The method according to claim 15, wherein the hoop-winding comprises hoopwinding the unidirectional-fibre thermoplastic composite around a magnet.

17. The method according to claim 15 or claim 16, wherein the unidirectional-fibre thermoplastic composite is a tape.

18. The method according to any one of claims 15 to 17, comprising overmoulding an impeller onto the shaft.

19. The method according to claim 18, wherein a matrix of the unidirectional-fibre thermoplastic composite of the shaft comprises a thermoplastic in the polyaryletherketone (PAEK) thermoplastic family, and the impeller is formed from a thermoplastic in the PAEK thermoplastic family.

20. A method comprising hoop-winding a unidirectional-fibre thermoplastic composite to form a shroud for an impeller assembly of an electric motor.

21. The method according to claim 20, wherein the unidirectional-fibre thermoplastic composite is a tape.

22. The method according to claim 20 or claim 21, comprising welding the shroud to tips of blades of an impeller.

23. The method according to claim 22, wherein a matrix of the unidirectional-fibre thermoplastic composite of the shroud comprises a thermoplastic in the polyaryletherketone (PAEK) thermoplastic family, and the blades of the impeller are formed from a thermoplastic in the PAEK thermoplastic family.

24. The method according to any one of claims 20 to 23, comprising overmoulding one or more features onto a surface of the shroud.

25. An electric motor comprising one or more of a shaft according to any one of claims 1 to 5 or 9 to 12; a shroud according to any one of claims 6 to 12; or a rotor assembly according to claim 13.

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