A rotor assembly

Abstract

A rotor assembly (10, Fig. 1) for a brushless permanent magnet motor is provided. The rotor assembly includes a shaft (12, Fig. 1), a bearing assembly (16, Fig. 1) mounted to the shaft; and a mounting

Description

Field of the Invention The present invention relates to a rotor assembly for a brushless permanent magnet motor, and to a brushless permanent magnet motor comprising such a rotor assembly. Background of the Invention There is a general desire to improve electric machines, such as brushless 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. Summary of the Invention According to a first aspect of the present invention there is provided a rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising: a shaft; a bearing assembly mounted to the shaft; and a mounting member for mounting the bearing assembly to a frame of the brushless permanent magnet motor, the mounting member comprising an outer portion attachable to the frame, an inner portion attached to the bearing assembly, and a plurality of arms extending from the outer portion to the inner portion, wherein the inner portion comprises a base, and a tab extending from the base along an outer surface of the bearing assembly. As the mounting member has an outer portion and an inner portion, with the plurality of arms extending from the outer portion to the inner portion, radial and axial flexibility may be provided for movement of the inner portion relative to the outer portion. This may enable the mounting member to apply a pre-load force to the bearing assembly, and may enable application of such a pre-load force with fewer component parts than, for example, an arrangement in which a spring is utilised in combination with an insert and an end cap to apply a pre-load force to the bearing assembly. The outer portion, the inner portion, and the plurality of arms may be integrally formed, for example such that the mounting member comprises a monolithic component. The mounting member may comprise an axial stiffness, for example a stiffness in a direction parallel to a longitudinal axis of the shaft, in the region of 1 N / mm to 100N / mm. The mounting member may comprise a radial stiffness, for example a stiffness in a direction orthogonal to the longitudinal axis of the shaft, in the region of 100N / mm to 1000N / mm. The inner portion of the mounting member may be attached to an outer surface of the bearing assembly, for example via an adhesive or via a press-fit. The bearing assembly may comprise a ball or roller bearing assembly, for example comprising an inner ring, an outer ring, and a plurality of ball bearings or rollers located between the inner ring and the outer ring. The inner portion of the mounting member may be attached to the outer ring of the ball or roller bearing assembly. The inner portion may be substantially annular about the bearing assembly. The outer portion may be substantially annular about the inner portion. The inner and outer portions may be substantially concentric. The mounting member may comprise a total axial thickness, for example a thickness in a direction parallel to a longitudinal axis of the shaft, in the region of 0.1mm to 4.0mm, for example around 0.5mm. The mounting member may comprise at least three arms extending from the outer portion to the inner portion. Provision of at least three arms may provide sufficient axial and / or radial flexibility whilst also inhibiting excessive rotation of the bearing assembly relative to the frame when mounted in a brushless permanent magnet motor. The mounting member may comprise no more than five arms extending from the outer portion to the inner portion. Provision of no more than five arms may provide sufficient axial and / or radial flexibility whilst also ensuring sufficient gaps between arms to allow airflow therethrough in use. At least one of the plurality of arms may extend from a first point on the outer member to a second point on the inner member, the second point circumferentially offset from the first point. The plurality of arms may have a combined cross-sectional area of no more than 50% of a total cross-sectional area of a region between the outer portion and the inner portion. This may provide sufficient axial and / or radial flexibility whilst also ensuring sufficient space between arms to allow airflow therethrough in use. The plurality of arms may define apertures therebetween. The plurality of arms may have a combined cross-sectional area of no more than 40%, no more than 30%, no more than 25%, or no more than 20%, of a total cross-sectional area of a region between the outer portion and the inner portion At least one of the plurality of arms may have a cross-sectional area that varies between the outer portion and the inner portion. This may enable the respective arm to provide desirable flexibility characteristics. The cross-sectional area may comprise a cross-sectional area viewed in a plane orthogonal to the shaft. The cross-sectional area may comprise a cross-sectional area viewed in a plane parallel to the shaft. At least one of the plurality of arms may have a cross-sectional area that varies between the outer portion and the inner portion in a plane orthogonal to the shaft and in a plane parallel to the shaft. At least one of the plurality of arms may be non-linear between the outer portion and the inner portion. Use of a non-linear arm may provide a greater number of design options for achieving desired flexibility and strength when compared to use of a linear arm. At least one of the plurality of arms may be curved between the outer portion and the inner portion. At least one of the plurality of arms may comprise a first portion extending in a first direction between the outer portion and the inner portion, and a second portion extending in a second direction different to the first direction between the outer portion and the inner portion. This may enable the arm to have a greater overall length than, for example, a strictly linear arm, which may provide improved flexibility and / or strength characteristics. The first portion may have a first length, and the second portion may have a second length different to the first length. This may enable an interface between the arm and the outer portion to be circumferentially displaced from an interface between the arm and the inner portion, which may enable increased relative rotation between the outer portion and the inner portion in comparison to an arrangement where the interfaces are circumferentially aligned. The first portion may have a first length, and the second portion may have a second length equal to the first length. This may enable an interface between the arm and the outer portion to be circumferentially aligned with an interface between the arm and the inner portion, which may enable increased relative rotation between the outer portion and the inner portion in comparison to an arrangement where the interfaces are circumferentially displaced. The plurality of arms may be evenly spaced about a periphery of at least one of the outer portion and the inner portion. This may provide even pre-loading about the periphery of the bearing assembly, and may enable relatively even axial and or radial motion of the bearing assembly when the rotor assembly rotates within a brushless permanent magnet motor in use. Each of the plurality of arms may extend from the outer portion at a respective outer interface, and, at at least one of the outer interfaces, at least one of the outer portion and the respective arm may comprise a region of reduced cross-sectional area. This may enable increased flexibility in comparison to an arrangement absent a region of reduced cross-sectional area. Each of the plurality of arms may extend from the inner portion at a respective inner interface, and, at at least one of the inner interfaces, at least one of the outer portion and the respective arm may comprise a region of reduced cross-sectional area. This may enable increased flexibility in comparison to an arrangement absent a region of reduced cross-sectional area. The outer member may comprise a cross-sectional thickness greater than a maximal cross-sectional thickness of any of the plurality of arms when viewed in a plane orthogonal to a longitudinal axis of the shaft. The outer member may comprise a region of reduced cross-sectional area at the outer interface between any of the plurality of arms and the outer member. The inner member may comprise a cross-sectional thickness greater than a maximal cross-sectional thickness of any of the plurality of arms when viewed in a plane orthogonal to a longitudinal axis of the shaft. The inner member may comprise a region of reduced cross-sectional area at the inner interface between any of the plurality of arms and the inner member. The inner portion comprises a base, and a tab extending from the base along an outer surface of the bearing assembly. The tab may provide a region of increased surface area for attaching the inner member to the bearing assembly. The base may be substantially annular in form. The tab may extend axially in a direction parallel to a longitudinal axis of the shaft. The inner portion may comprise a plurality of tabs extending from the base along the outer surface of the bearing assembly. The inner portion may comprise a base, and a projection extending from the base away from the bearing assembly. As the projection extends from the base away from the bearing assembly, the projection may be exposed to airflow, when the rotor assembly is mounted in a brushless permanent magnet motor in use, to a greater extent than if the projection were to extend along the outer surface of the bearing assembly. The projection may then act as a heat sink to remove heat from the bearing assembly in use, which may improve a lifetime of the bearing assembly when compared to an arrangement in which no heat sink is provided. The outer portion may comprise a plurality of first regions located at a first radial distance relative to the shaft, and a plurality of second regions located at a second radial distance relative to the shaft, the second distance greater than the first distance. This may enable formation of channels along an outer surface of the outer portion, which may facilitate airflow around the mounting member in use. Furthermore, the second regions may facilitate attachment of the mounting member to the frame, by providing discrete mounting points about a periphery of the outer portion of the mounting member. The second regions may be located intermediate adjacent first regions about a periphery of the outer portion. The mounting member may comprise a damping member for inhibiting transmission of vibrations from the inner portion to the outer portion. This may provide reduction in noise in use compared to an arrangement absent a damping member. The damping member may extend between the inner and outer portions. The damping member may be located between an arm of the plurality of arms and at least one of the inner and outer portions. The damping member may be formed of a resiliently deformable material, for example rubber or the like. The mounting member may be formed of a metallic material, for example stainless steel. Use of a metallic material may provide strength alongside the desired flexibility. The mounting member may be formed of a plastic material. Plastic material may provide strength alongside the desired flexibility. The outer portion may comprise an aperture for receiving a location member of a frame of a brushless permanent magnet motor. This may facilitate insertion of the mounting member into the frame of the brushless permanent magnet motor in use, and may, for example, enable the use of pins or heat stakes for securing the mounting member to the frame. The outer portion may comprise a plurality of apertures for receiving corresponding location members of a frame of a brushless permanent magnet motor, for example a plurality of apertures spaced about a periphery of the outer portion. The rotor assembly may comprise a further mounting member for mounting the bearing assembly to the frame of the brushless permanent magnet motor, the further mounting member may comprise a further outer portion attachable to the frame, a further inner portion attached to the bearing assembly, and a plurality of further arms extending from the further outer portion to the further inner portion. This may provide for application of an increased pre-load force relative to an arrangement with just one mounting member, whilst retaining flexibility in axial and / or radial motion of the bearing assembly. The further mounting member may have substantially the same form as the mounting member. The rotor assembly may comprise a spacer located between the mounting member and the further mounting member. The spacer may act to ensure appropriate spacing between the mounting member and the further mounting member in use. The spacer may act to provide damping for the mounting member and the further mounting member. The spacer may be formed of a different material to the mounting member and the further mounting member For example, the mounting member and the further mounting member may be formed of a metallic material, and the spacer may be formed of a plastic material. The bearing assembly may comprise a flange, and the inner portion may be in abutment with the flange. This may facilitate application of a pre-loading force to the bearing assembly by the mounting member. According to a second aspect of the present invention there is provided a brushless permanent magnet motor comprising: a frame; and a rotor assembly as claimed in any preceding claim, the outer portion attached to the frame. According to a third aspect of the present invention there is provided a vacuum cleaner comprising a brushless permanent magnet motor according to the second aspect of the present invention. According to a fourth aspect of the present invention there is provided a haircare appliance comprising a brushless permanent magnet motor according to the second aspect of the present invention. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. Brief Description of the Drawings Figure 1 is a schematic view of a first embodiment of a rotor assembly; Figure 2 is a schematic view of a first embodiment mounting member of the rotor assembly of Figure 1; Figure 3 is a schematic view illustrating the rotor assembly of Figure 1 mounted in a frame; Figure 4 is a schematic view illustrating a brushless permanent magnet motor comprising the rotor assembly of Figure 1; Figure 5 is a schematic view of a second embodiment of a rotor assembly; Figure 6 is a schematic view of a third embodiment of a rotor assembly; Figure 7 is a first schematic view of a second embodiment of a mounting member; Figure 8 is a second schematic view of the second embodiment of the mounting member; Figure 9A is a schematic view of a third embodiment of a mounting member; Figure 9B is a schematic view of a fourth embodiment of a mounting member; Figure 9C is a schematic view of a fifth embodiment of a mounting member; Figure 9D is a schematic view of a sixth embodiment of a mounting member; Figure 9E is a schematic view of a seventh embodiment of a mounting member; Figure 9F is a schematic view of an eighth embodiment of a mounting member; Figure 9G is a schematic view of a ninth embodiment of a mounting member; Figure 9H is a schematic view of a tenth embodiment of a mounting member; Figure 9I is a schematic view of an eleventh embodiment of a mounting member; 22 05 25 Figure 10 is a schematic view of a vacuum cleaner comprising a brushless permanent magnet motor; and Figure 11 is a schematic view of a haircare appliance comprising a brushless 5 permanent magnet motor. Detailed Description of the Invention A rotor assembly is illustrated schematically in Figure 1. The rotor assembly 10 10 comprises a shaft 12, first 16 and second 18 bearing assemblies, a balance ring 20, a permanent magnet 22, and a mounting member 24. The shaft 12 is elongate in form, having a first end 26 and a second end 28. The permanent magnet 22 is mounted generally centrally along the shaft 12. The first 15 16 and second 18 ball bearing assemblies are ball bearing assemblies, each having an inner ring, an outer ring, and a plurality of ball bearings located between the inner ring and the outer ring. The first bearing assembly 16 is mounted to the shaft 12 at the first end 26, with the balance ring 20 mounted to the shaft 12 between the first bearing assembly 16 and the permanent magnet 22. The 20 second bearing assembly 18 is mounted to the shaft 12 toward the second end 28 at an opposite side of the permanent magnet 22 to the first bearing assembly 16. A first embodiment of the mounting member 24 is shown in isolation in Figure 2. 25 The mounting member 24 comprises an inner portion 30 attached to the outer race of the first bearing assembly 16, an outer portion 32, and three arms 34,36,38 that extend between the inner portion 30 and the outer portion 32. The inner portion 30 is annular in form, and has a dimensions substantially corresponding to the outer race of the first bearing assembly 16. The inner 30 portion 30 can be press-fit or adhered to the outer race of the first bearing assembly 16. The outer portion 32 is substantially annular in form, and has a diameter greater than the diameter of the inner portion 30. The arms 34,36,38 extend between the inner 30 and outer 32 portions, and are evenly spaced about the circumference of the inner 30 and outer 32 portions. Each arm 34,36,38 has a first portion 40, a second portion 42, and a third portion 44. The first portions 40 of each arm extend from the inner portion 30 in a first non-radial direction. The second portions 42 of each arm 34,36,38 extend from the respective first portions 40 in a generally radial direction. The third portions 44 of each arm 34,36,38 extend from the respective second portions 42 in a second non-radial direction, opposite to the first non-radial direction, to the outer portion 32. The third portions 44 of each arm 34,36,38 are longer than the respective first portions 40 of the arms 34,36,38. The arms 34,36,38 of the mounting member 24 are shaped and dimensioned such that apertures 46 are defined through the mounting member 24. The arms 34,36,38 have a combined cross-sectional area that is no greater than 20% of the cross-sectional area of the region between the inner portion 30 and the outer portion 32. The inner portion 30, the outer portion 32, and the arms 34,36,38 are integrally formed, and have a thickness of around 0.5mm. Other thicknesses, for example in the region of 0.1mm to 4.0mm, are also envisaged. The mounting member 24 can, for example, be formed by laser etching or stamping a sheet of material, or may be formed via additive manufacturing or injection moulding. The mounting member 24 is formed of a metallic, thermally conductive, material, although use of a plastic material is also envisaged. The geometry and dimensions of the mounting member 24 are such that arms 34,36,38 enable relative movement between the inner portion 30 and the outer portion 32 in both an axial direction, along an axis parallel to a longitudinal axis of the shaft 12, and in a radial direction, generally orthogonal to the longitudinal axis of the shaft. The mounting member 24 has an axial stiffness, in the direction parallel to the longitudinal axis of the shaft 12, in the region of 1N / mm to 100N / mm. The mounting member 24 has a radial stiffness, in the direction orthogonal to the longitudinal axis of the shaft 12, in the region of 100N / mm to 1000N / mm. The rotor assembly 10 is illustrated in position relative to a frame 50 of a brushless permanent magnet motor 100 in Figure 3. The frame 50 comprises a channel 52 having a first region 54 of a first diameter, a second region 56 of a second diameter smaller than the first diameter, a third region 58 of a third diameter smaller than the second diameter, and a fourth region 60 of a fourth diameter smaller than the third diameter. Transitions between the different regions 54,56,58,60 of the channel 52 are step-wise transitions, such that planar surfaces are defined between different regions 54,56,58,60 of the channel 52 The rotor assembly 10 is positioned within the channel 52 such that the second bearing assembly 18, the permanent magnet 22, and the balance ring 20 are located within the third region 58 of the channel 52. The outer portion 32 of the mounting member 24 is affixed, via adhesive, to a planar surface of the frame 50 that defines a transition between the first region 54 of the channel 52 and the second region 56 of the channel 52. The first bearing assembly 16 bridges the first region 54 and the second region 56 of the channel 52. Given that the arms 34,36,38 of the mounting member 24 enable relative movement between the inner 30 and outer 32 portions of the mounting member 24, the mounting member 24 applies a pre-load force to the first bearing assembly 16, and may enable axial and / or radial motion of the first bearing assembly 16 relative to the frame 50 in use. The brushless permanent magnet motor 100 is illustrated more fully in Figure 4, and further comprises four stator cores assemblies 102. The stator core assemblies 102 are received in slots 104 formed in the region of the frame 50 that defines the third region 48 of the channel 52. In use, current driven into windings of the stator core assemblies 102 generates a magnetic field that interacts with the rotor assembly 10 to cause the rotor assembly 10 to spin within the frame 50. An impeller 106 is located at the second end 28 of the rotor assembly 10. An alternative rotor assembly 200 is illustrated schematically in Figure 5, where like reference numerals are used for sake of clarity. The rotor assembly 200 is substantially the same as the rotor assembly 10 of Figure 1, save for the form of the first bearing assembly 202. In the rotor assembly 200 of Figure 5, the outer race of the first bearing assembly 202 comprises an annular flange 204. The inner portion 30 of the mounting member 24 contacts the annular flange 204. The annular flange 204 can aid in application of a pre-load force to the first bearing assembly 202. A further alternative rotor assembly 300 is illustrated schematically in Figure 6 where like reference numerals are used for sake of clarity The rotor assembly of Figure 6 is substantially the same as the rotor assembly 10 of Figure 1, save for the presence of a further mounting member 302 and a damping member 304. The further mounting member 302 is substantially the same as the mounting member 24 of Figure 6, and is attached to the first bearing assembly in the same way as the mounting member 24. The further mounting member 302 is spaced apart from the mounting member 24, and the damping member 304 is located between the mounting member 24 and the further mounting member 302. The damping member 304 is formed of a plastic material, and can inhibit transmission of vibrations caused by axial and / or radial motion of the mounting member 24 in use. In other examples, the mounting member 24 can comprise additional or alternative forms of damping member, such as a damping member located on one or more of the arms 34,36,38, or between the arms 34,36,38 within the apertures 46. A second embodiment of the mounting member 400 is illustrated schematically in Figures 7 and 8. The second embodiment of the mounting member 400 comprises an inner portion 402, an outer portion 404, and three arms 406,408,410. The inner portion 402 comprises a main body 412, first tabs 414, and second tabs 416. The main body 412 is generally annular in form. The first tabs 414 are rectangular in form, evenly spaced about the main body 412, and extend orthogonally from the main body 412 in a first direction. The second tabs 416 are rectangular in form, evenly spaced about the main body 412, and extend orthogonally from the main body 412 in a second direction opposite to the first direction. The outer portion 404 comprises three arcuate sections 418 having a first diameter, and three protruding portions 420 having a second diameter greater than the first diameter. The protruding portions 420 are evenly spaced, with each arcuate section 418 located between two protruding portions 420. The protruding portions 420 are located on the outer portion 404 such that each first tab 414 on the inner portion 402 is intermediate adjacent ones of the protruding portions 420. The protruding portions 420 and the arcuate sections 418 collectively define channels between the protruding portions 420. Each arm 406,408,410 extends between the inner portion 402 and a respective protruding portion 420. Each arm 406,408,410 has two sections that extend in opposing directions, similar to the first portion 40 and the third portion 44 of the arms 34,36,38 of the first embodiment of the mounting member 24 of Figure 2. In use, the mounting member 400 is fixed to the outer race of a bearing assembly, such as the first bearing assembly 16. The first tabs 414 are fixed to the outer race of the first bearing assembly 16, whilst the second tabs 416 extend away from the first bearing assembly 16. The second tabs 416 can act as heat sinks, for example where the rotor assembly 10 comprises an impeller that draws airflow over the second tabs 416. The protruding portions 420 of the outer portion 404 are used to fix the mounting member 400 to a frame such as the frame 50. The arcuate sections 418 are then spaced from an internal wall of the frame 50, such that airflow passages within the channel 52 of the frame 50 are defined by the arcuate sections 418, the protruding portions 420, and the internal wall of the frame 50. Third through eleventh embodiments of the mounting member are shown in Figures 9A to 9I respectively. Each of the third through eleventh embodiments of the mounting member has the same general structure as the first embodiment 24 of the mounting member, with an inner portion 30, an outer portion 32, and arms 34,36,38 extending between the inner portion 30 and the outer portion 32. Like reference numerals are therefore used for sake of clarity. In the third embodiment 500 of the mounting member, as shown in Figure 9A, each arm 34,36,38 has a short section 502 extending in a first direction, and a long section 504 extending in a second direction opposite to the first direction. Relative dimensions of the short section 502 and the long section 504 can be varied to achieve desired axial and / or radial stiffness of the mounting member 500. The fourth embodiment 600 of the mounting member, as shown in Figure 9B, has v-shaped arms 602, with each section of the individual v-shaped arms 602 being of equal length. In the fifth 700 and sixth 800 embodiments of the mounting member, as shown in Figures 9C and 9D, the respective arms 702,802 are curved between the inner portion 30 and the outer portion 32. The seventh embodiment 900 of the mounting member, as shown in Figure 9E, also has curved arms 902, but each arm 902 has regions 904 of reduced cross-sectional area at interfaces between the arm 902 and the inner portion 30, and between the arm 902 and the outer portion 32. In the eighth embodiment 1000 of the mounting member, as shown in Figure 9F, the inner portion 30 and the outer portion 32 each have regions 1002 of reduced cross-sectional area located at their respective interfaces with the arms 1004. In the ninth embodiment 1100 of the mounting member, as shown in Figure 9G, each arm 1102 has regions 1104 of reduced cross-sectional area at interfaces between the arm 1102 and the inner portion 30, and between the arm 1102 and the outer portion 32. The inner portion 30 and the outer portion 32 also each have regions 1106 of reduced cross-sectional area located at their respective interfaces with the arms 1102. In the tenth embodiment 1200 of the mounting member, as shown in Figure 9H, there are five curved arms 1202 rather than three. In the eleventh embodiment 1300 of the mounting member, the outer portion 32 has apertures 1302 for receiving pins of a frame of a brushless permanent magnet motor. This may assist with locating the mounting member 1300 relative to the frame. Any of the mounting members discussed herein can be utilised in the brushless permanent magnet motor 100 in use. In each of the third through eleventh embodiments of the mounting member, the nature of the arms between the inner portion and the outer portion may enable a pre-load to be applied to a bearing assembly, whilst also enabling relative axial and / or radial movement of the bearing assembly in use. Characteristics of the inner portion, outer portion, and arms, can be chosen to obtain desired axial and / or radial stiffness characteristics. A vacuum cleaner 1400 comprising the brushless permanent magnet motor 100 is illustrated in Figure 10. 5 A haircare appliance 1500 comprising the brushless permanent magnet motor 100 is illustrated in Figure 11.

Claims

1. A rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising:a shafta bearing assembly mounted to the shaft; anda mounting member for mounting the bearing assembly to a frame of the brushless permanent magnet motor, the mounting member comprising an outer portion attachable to the frame, an inner portion attached to the bearing assembly, and a plurality of arms extending from the outer portion to the inner portion,wherein the inner portion comprises a base, and a tab extending from the base along an outer surface of the bearing assembly.

2. A rotor assembly as claimed in Claim 1, wherein the mounting member comprises at least three arms extending from the outer portion to the inner portion.

3. A rotor assembly as claimed in Claim 1 or Claim 2, wherein the plurality of arms have a combined cross-sectional area of no more than 50% of a total cross-sectional area of a region between the outer portion and the inner portion.

4. A rotor assembly as claimed in any preceding claim, wherein at least one of the plurality of arms has a cross-sectional area that varies between the outer portion and the inner portion.

5. A rotor assembly as claimed in any preceding claim, wherein at least one of the plurality of arms is non-linear between the outer portion and the inner portion.

6. A rotor assembly as claimed in any preceding claim, wherein at least one of the plurality of arms comprises a first portion extending in a first direction between the outer portion and the inner portion, and a second portion extending in a second direction different to the first direction between the outer portion and the inner portion.

7. A rotor assembly as claimed in Claim 6, wherein the first portion has a first length, and the second portion has a second length different to the first length.

8. A rotor assembly as claimed in Claim 6, wherein the first portion has a first length, and the second portion has a second length equal to the first length.

9. A rotor assembly as claimed in any preceding claim, wherein the plurality of arms are evenly spaced about a periphery of at least one of the outer portion and the inner portion.

10. A rotor assembly as claimed in any preceding claim, wherein each of the plurality of arms extends from the outer portion at a respective outer interface, and, at at least one of the outer interfaces, at least one of the outer portion and the respective arm comprises a region of reduced cross-sectional area.

11. A rotor assembly as claimed in any preceding claim, wherein the inner portion comprises a base, and a projection extending from the base away from the bearing assembly.

12. A rotor assembly as claimed in any preceding claim, wherein the outer portion comprises a plurality of first regions located at a first radial distance relative to the shaft, and a plurality of second regions located at a second radial distance relative to the shaft, the second distance greater than the first distance.

13. A rotor assembly as claimed in any preceding claim, wherein the mounting member comprises a damping member for inhibiting transmission of vibrations from the inner portion to the outer portion.

14. A rotor assembly as claimed in any preceding claim, wherein the outer portion comprises an aperture for receiving a location member of a frame of a brushless permanent magnet motor.

15. A rotor assembly as claimed in any preceding claim, wherein the rotor assembly comprises a further mounting member for mounting the bearing assembly to the frame of the brushless permanent magnet motor, the further mounting member comprising a further outer portion attachable to the frame, a further inner portion attached to the bearing assembly, and a plurality of further arms extending from the further outer portion to the further inner portion.

16. A rotor assembly as claimed in Claim 15, wherein the rotor assembly comprises a spacer located between the mounting member and the further mounting member.

17. A rotor assembly as claimed in any preceding claim, wherein the bearing assembly comprises a flange, and the inner portion is in abutment with the flange.

18. A brushless permanent magnet motor comprising:a frame; anda rotor assembly as claimed in any preceding claim., the outer portion attached to the frame19. A vacuum cleaner comprising a brushless permanent magnet motor as claimed in Claim 18.

20. A haircare appliance comprising a brushless permanent magnet motor as claimed in Claim 18.LO CXILO

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