Rotor Assembly

Abstract

A rotor assembly 10 for a brushless permanent magnet motor, the rotor assembly comprising a shaft 32; a permanent magnet 23 mounted to the shaft; and a rotor assembly component 20 mounted to the shaft

Description

Field of the Invention The present invention relates to a rotor assembly for a brushless permanent magnet motor, and 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 permanent magnet mounted to the shaft; and a rotor assembly component mounted to the shaft and in contact with the permanent magnet at an interface between the rotor assembly component and the permanent magnet; wherein at least one of the rotor assembly component and the permanent magnet comprises a through-hole located at the interface. By placing the rotor assembly component in contact with the permanent magnet, ease of manufacturing of the rotor assembly may be increased compared to, for example, an arrangement where no rotor assembly components are in contact with the permanent magnet. For example, contact of the permanent magnet with the rotor assembly component may act to properly axially locate the permanent magnet relative to the shaft during an assembly process, and may mitigate the need for appropriate tools to hold the permanent magnet in its desired axial location during assembly. By providing a through-hole at the interface between the rotor assembly and the permanent magnet, ease of assembly of the rotor assembly may be increased and / or cost of assembly of the rotor assembly may be reduced, compared to, for example, an arrangement where no such through-hole is present. For example, it may be desirable to bond the permanent magnet to the shaft using an adhesive. Such a through-hole may enable excess adhesive to leave a channel between the permanent magnet and the shaft, whereas if no such through-hole were present then no such leakage path for adhesive would be present. Furthermore, it may be desirable for an inner diameter of the permanent magnet to be as concentric with the shaft as possible. Where adhesive is applied between the permanent magnet and the shaft, such adhesive may be largely hidden by the permanent magnet. This may inhibit the use of certain types of curing process to cure the adhesive, for example ultra-violet (UV) curing processes, as there is no way to guarantee that all of the adhesive will be reached by UV light, and hence fully cure. It may therefore be necessary to use alternative processes for curing the adhesive, such as, for example, heat curing processes. However, heat curing processes may take significantly longer than UV curing processes. This may result in a window in which the permanent magnet is not securely held in position relative to the shaft, and in which misalignment of the permanent magnet relative to the shaft may occur. Correct alignment of the permanent magnet and the shaft may be important for reliable operation of a brushless permanent magnet motor comprising the rotor assembly. By providing the through-hole at the interface between the rotor assembly component and the permanent magnet, a line-of-sight path may be provided such that a relatively quick-curing UV adhesive can be used to tack the permanent magnet relative to the shaft whilst a longer heat curing process is used to fully secure the permanent magnet to the shaft. It will be appreciated that the through-hole may be filled with adhesive in the assembled rotor assembly, but that such a through-hole may provide line-of-sight to the shaft when adhesive is not present. The through-hole may comprise a cut-out in an end of the at least one of the rotor assembly component and the permanent magnet, for example such that the at least one of the rotor assembly component and the permanent magnet comprises an end that is castellated in form. At least one of the rotor assembly component and the permanent magnet may comprise a plurality of through-holes located at the interface. By providing a plurality of through-holes, increased adhesive run-off channels and / or increased line of sight at different locations about a periphery of the interface may be provided in comparison to an arrangement with a single through-hole. The plurality of through-holes may be evenly spaced about the at least one of the rotor assembly component and the permanent magnet. This may provide increased security of positioning of the permanent magnet to the shaft during an assembly process relative to an arrangement with an uneven spacing of through-holes. The at least one of the rotor assembly component and the permanent magnet may comprise at least three through-holes located at the interface. Providing at least three through-holes may provide a relatively good compromise between structural integrity and providing leakage paths / line of sight. The at least one of the rotor assembly component and the permanent magnet may comprise exactly three through-holes located at the interface The rotor assembly component may comprise the through-hole. This may provide increased structural integrity and ease of manufacture in comparison to an arrangement where the permanent magnet comprises the through-hole, and such increased structural integrity may enable the permanent magnet to withstand higher rotational speeds and / or higher temperatures in use, with a reduced risk of failure. Furthermore, this may provide an increased volume of magnetic material for a given packaging volume in comparison to an arrangement where the permanent magnet comprises the through-hole. The shaft may comprise a first portion to which rotor assembly component is mounted, the first portion having a first diameter, a second portion to which the permanent magnet is mounted, the second portion having a second diameter less than the first diameter, and a transition region intermediate the first and second portions, the transition region having third diameter less than the first and second diameters, and the through-hole may overlap the transition region. Such a transition region may facilitate manufacture of the rotor assembly, for example by providing ease of access to a channel between the permanent magnet and the second portion of the shaft during assembly, such that adhesive can be injected into the channel. The through-hole may overlap the transition region and the first portion. The rotor assembly component may comprise a bore, the bore comprising a first region having a first diameter, and a second region having a second diameter less than the first diameter, and the first region may be located at the interface. Providing a region of increased diameter at the interface may define a region in which adhesive used to bond the permanent magnet to the shaft can collect. Providing a region in which adhesive may collect may facilitate bonding of the adhesive to the rotor assembly component, which may inhibit adhesive from detaching from the rotor assembly component when the rotor assembly spins in use. The rotor assembly component may comprise one or more of a balance ring, an impeller, and a bearing assembly. The rotor assembly component may be a balance ring. Balance rings may typically be provided such that material can be removed from the balance ring to obtain desired rotor dynamics. This may lend itself to provision of a through-hole without impacting the primary functionality of the balance ring. The rotor assembly may comprise a bearing assembly mounted to the shaft, and the balance ring may be located intermediate the bearing assembly and the permanent magnet. This may space the bearing assembly, which may typically comprise ferromagnetic components, from the permanent magnet, whilst also facilitating desirable rotor dynamics performance. The rotor assembly may comprise a further bearing assembly mounted to the shaft on an opposite side of the permanent magnet to the bearing assembly. Provision of bearing assemblies either side of the shaft may provide for improved performance relative to, for example, a corresponding arrangement having only one bearing assembly, or two bearing assemblies located on the same side of the permanent magnet. The rotor assembly may comprise a further balance ring mounted to the shaft between the further bearing assembly and the permanent magnet. Provision of a further balance ring may provide increased flexibility in rotor dynamics performance in comparison to an arrangement having only one balance ring. The further balance ring may comprise a smaller mass then the balance ring. Provision of such an asymmetric balance ring arrangement may provide increased flexibility in motor construction in comparison to an arrangement having only one balance ring, or two identical balance rings. The rotor assembly may comprise an impeller mounted to the shaft, and the balance ring may be located intermediate the impeller and the permanent magnet. This may provide for improved rotor dynamics compared to, for example, an arrangement where the permanent magnet is located intermediate the impeller and the balance ring. According to a second aspect of the present invention there is provided a brushless permanent magnet motor comprising a rotor assembly according to the first aspect of the present invention. The brushless permanent magnet motor may comprise a stator assembly comprising one or more coils which, when energised, generate a magnetic field that interacts with the permanent magnet to rotate the shaft relative to the stator assembly. 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. According to a fifth aspect of the present invention there is provided a method of manufacturing a rotor assembly for a brushless permanent magnet motor, the method comprising providing a shaft, providing a rotor assembly component and a permanent magnet, at least one of the rotor assembly component and the permanent magnet comprising a through-hole, mounting the rotor assembly component to the shaft, and placing the permanent magnet relative to the shaft such that the permanent magnet is in contact with the rotor assembly component at an interface, and the through-hole is located at the interface. The method may comprise providing an adhesive between the permanent magnet and the shaft such that the adhesive is visible through the through-hole, and curing the adhesive using a light curing process to tack the permanent magnet to the shaft. The method may comprise further curing the adhesive using a heat curing process. 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 cross-sectional view of a rotor assembly; Figure 2 is a schematic cross-sectional view of a shaft of the rotor assembly of Figure 1; Figure 3 is a schematic perspective view of a first balance ring of the rotor assembly of Figure 1; Figure 4 is a flow diagram illustrating assembly steps of the rotor assembly of Figure 1; Figure 5 is a schematic illustration of a brushless permanent magnet motor comprising the rotor assembly of Figure 1; Figure 6 is a schematic illustration of a vacuum cleaner comprising the brushless permanent magnet motor of Figure 5; Figure 7 is a schematic illustration of a haircare appliance comprising the brushless permanent magnet motor of Figure 1; Figure 8 is a schematic illustration of an alternative shaft and balance ring for the rotor assembly of Figure 1; and Figure 9 is a schematic illustration of an alternative shaft and impeller for the rotor assembly of Figure 1. Detailed Description of the Invention A rotor assembly 10 is illustrated schematically in Figure 1. The rotor assembly 10 comprises a shaft 12, an impeller 14, first 16 and second 18 bearing assemblies, first 20 and second 22 balance rings, and a permanent magnet 23. The shaft 12 is illustrated in isolation in the schematic cross-section of Figure 2. The shaft 12 comprises a first portion 24, a second portion 26 adjacent to the first portion 24, a first transition region 25 between the first 24 and second 26 portions, a third portion 28 adjacent to the second portion 26, and a second transition region 27 between the second 26 and third 28 portions. In such a manner the second portion 26 is considered to be intermediate the first 24 and third 28 portions. The first portion 24 defines a first end 30 of the shaft 12, and the third portion 28 defines a second end 32 of the shaft 12 opposite to the first end 30. The shaft is a monolithic stainless steel component, such that the first 24, second 26 and third 28 portions are integrally formed. The shaft 12 has a relative magnetic permeability of around 20. In some alternative examples, the first 25 and / or second 27 transition regions may be omitted. The first portion 24 has a first shaft diameter A in the region of 5.0mm, and a length in the region of 25mm. The first shaft diameter A defines a maximal diameter of the shaft 12. Ends of the first portion 24 are tapered inwardly slightly from the first shaft diameter A. The second portion 26 has a second shaft diameter B in the region of 4.5mm, and a length in the region of 15mm. An end of the second portion 26 closest to the third portion 28 is tapered inwardly slightly from the second shaft diameter B. The third portion 28 has a third shaft diameter C in the region of 3.0mm, and a length in the region of 10mm. An end of the third portion 28 distal from the second portion 26 is tapered slightly inwardly from the third shaft diameter C. The first transition region 25 has a fourth shaft diameter D which is smaller than the first A and second B shaft diameters, but greater than the third shaft diameter C. The second transition region 27 has a fifth shaft diameter E which is smaller than the third shaft diameter C. Thus the first portion 24 has a first shaft diameter A greater than the second shaft diameter B of the second portion 26, and greater than the third shaft diameter C of the third portion 28. In particular, the first shaft diameter A is around 11% greater than the second shaft diameter B, and around 66% greater than the third shaft diameter C. The second shaft diameter B is around 50% greater than the third shaft diameter C. The first portion 24 has a length around 66% greater than the length of the second portion 26, and a length around 150% greater than the length of the third portion 28. The second portion 26 has a length around 50% greater than the length of the third portion 28. The first 24 and second 28 portions of the shaft 12 are precision ground, with a surface roughness in the region of 0.1 to 0.3 Ra. The second portion 26 of the shaft 12 is precision ground to a surface roughness greater than that of the first 24 and second 28 portions of the shaft 12, with a surface roughness typically in the region of 0.3 to 0.5 Ra. The impeller 14 is a mixed flow impeller and is press-fit to the first portion 24 such that the impeller 14 is located at the first end 30 of the shaft 12. Axial and / or radial flow impellers are also envisaged. The impeller 14 is injection moulded using a PEEK material. The first bearing assembly 16 comprises a ball bearing assembly, and is press-fit to the first portion 24 of the shaft 12 such that the first bearing assembly 16 lies partly within a hollow interior of the impeller 14. The first bearing assembly 14 thereby has an inner diameter substantially corresponding to the first shaft diameter A. The first bearing assembly 16 has an outer diameter greater than outer diameters of each of the second bearing assembly 18, the first 20 and second 22 balance rings, and the permanent magnet 23. The second bearing assembly 18 comprises a ball bearing assembly, and is press-fit to the third portion 28 of the shaft 12 such that the second bearing assembly 18 is located at the second end 32 of the shaft 12. The second bearing assembly 18 thereby has an inner diameter substantially corresponding to the third shaft diameter C. The second bearing assembly 18 has an outer diameter substantially corresponding to outer diameters of the second balance ring 22 and the permanent magnet 23, but smaller than an outer diameter of the first balance ring 20. The first 16 and second 18 bearing assemblies are located at points on the respective first 24 and third 28 portions of the shaft 12 such that the stride between the first 16 and second 18 bearing assemblies is around 30mm. The first balance ring 20 is shown in isolation in Figure 3. The first balance ring 20 has a base portion 36 and an upstanding wall 38. The base portion 36 is substantially annular and solid in form, with a central bore 40. The central bore 40 has a diameter substantially corresponding to the first shaft diameter A of the first portion 24 of the shaft 12, such that the first balance ring 20 is press-fit to the first portion 24 of the shaft 12 when assembled. The upstanding wall 38 is integrally formed with the base portion 36 from a plastics material such that the first balance ring 20 is a monolithic component. The upstanding wall 38 projects from the base portion 36 annularly about the central bore 40, and has three through-holes 42 evenly spaced about a periphery of the upstanding wall 38, which may also be referred to as cut-outs, such that the upstanding wall 38 has a generally castellated form. When mounted to the shaft 12, the through-holes 42 span the first portion 24 and the transition region 25 of the shaft 12. The second balance ring 22 is substantially annular and solid in form, and is formed of a plastics material. The second balance ring 22 is press-fit to the third portion 28 of the shaft 12, with the second bearing assembly 18 located closer to the second end 32 of the shaft 12 than the second balance ring 22. The second balance ring 22 has a smaller mass than the first balance ring 20. The permanent magnet 23 is a two-pole sintered magnet, and is mounted to the second portion 26 of the shaft 12 via an adhesive. When mounted to the second portion 26 of the shaft 12, the permanent magnet 23 is in contact with the upstanding wall 38 of the first balance ring 20 at an interface 44. The interface 44 is illustrated by a dashed line in Figure 1. To assemble the rotor assembly 10, the impeller 14 is initially press-fit onto the first portion 24 of the shaft 12. The first bearing assembly 16 is then also press-fit onto the first portion 24 of the shaft 12, with the first bearing assembly 16 inserted from the second end 32 of the shaft 12. The first balance ring 20 is then then also press-fit onto the first portion 24 of the shaft 12, with the first bearing assembly 16 inserted from the second end 32 of the shaft 12. Subsequently, the permanent magnet 23 is bonded to the second portion 26 of the shaft 12 via an adhesive. It may be desirable for an inner diameter of the permanent magnet 23 to be as concentric with the shaft 12 as possible. Where adhesive is applied between the permanent magnet 23 and the shaft 12, such adhesive may be largely hidden by the permanent magnet 23. This may inhibit the use of certain types of curing process to cure the adhesive, for example ultra-violet (UV) curing processes, as there is no way to guarantee that all of the adhesive will be reached by UV light, and hence fully cure. It may therefore be necessary to use alternative processes for curing the adhesive, such as, for example, heat curing processes. However, heat curing processes may take significantly longer than UV curing processes. This may result in a window in which the permanent magnet 23 is not securely held in position relative to the shaft 12, and in which misalignment of the permanent magnet relative to the shaft may occur. Correct alignment of the permanent magnet 23 and the shaft 12 may be important for reliable operation of a brushless permanent magnet motor comprising the rotor assembly 10. The form of the first balance ring 20, and the contact of the first balance ring 20 and the permanent magnet 23 at the interface 44, may facilitate proper alignment of the permanent magnet 23 and the shaft 12 during manufacture of the rotor assembly 10. In particular, the permanent magnet 23 can be slid along the shaft 12 from the second end 32 of the shaft 12 until an end of the permanent magnet 12 contacts the upstanding wall 38 of the first balance ring 20. By placing the first balance ring 20 in contact with the permanent magnet 23 proper axial location of the permanent magnet 23 relative to the shaft 12 during an assembly process, may be achieved. Furthermore, as the through-holes 42 are located at the interface 44 between the rotor assembly component and the permanent magnet, a line-of-sight path may be provided such that a relatively quick-curing UV curing process can be used to tack the permanent magnet 23 relative to the shaft 12 whilst a longer heat curing process is subsequently used to fully cure the adhesive to secure the permanent magnet 23 to the shaft 12. It will be appreciated that the through-holes 42 may be at least partially filled with adhesive, such as tacking adhesive, in the final assembled rotor assembly 10. A method 100 in accordance with the above is illustrated in the flow diagram of Figure 4. The method 100 comprises providing 102 a shaft, and providing 104 a rotor assembly component and a permanent magnet, at least one of the rotor assembly component and the permanent magnet comprising a through-hole. The method comprises mounting 106 the rotor assembly component to the shaft; and placing 108 the permanent magnet relative to the shaft such that the permanent magnet is in contact with the rotor assembly component at an interface, and the through-hole is located at the interface. It will be appreciated that the method 100 is described here more generally in relation to a rotor assembly component, and that other components, such as the first bearing assembly 16 or the impeller 14, or indeed the permanent magnet 23 itself, may be shaped to provide the through-holes, and that other components such as the first bearing assembly 16 or the impeller 14 may be placed in contact with the permanent magnet 23 to ensure proper axial alignment. A further benefit associated with the rotor assembly 10 arises from the form of the shaft 12, and in particular the first portion 24 having the first shaft diameter A greater than the second shaft diameter B of the second portion 26, and greater than the third shaft diameter C of the third portion 28, and the second shaft diameter B being greater than the third shaft diameter C. In particular, this may effectively decouple an inner diameter of the permanent magnet 23 from the inner diameters of the first 16 and second 18 bearing assemblies, as well as decoupling the inner diameters of the first 16 and second 18 bearing assemblies from one another, thereby enabling greater flexibility compared to, for example, a shaft having a constant diameter along its length, which would require the permanent magnet and the first and second bearing assemblies to have a substantially similar inner diameter. In other words, in the rotor assembly 10, an inner diameter of the permanent magnet 23 may be specified independently of the inner diameters of the first 16 and second 18 bearing assemblies, and the inner diameters of the bearing assemblies 16,18 may be specified independently of one another. Although specific dimensions of the first 24, second 26, and third 28 portions of the shaft 12 have been illustrated in the specific example above, alternative shaft dimensions and ratios are also envisaged. For example, the first shaft diameter A may be in the region of 1 % to 20% greater than the second shaft diameter B, may be in the region of 35% to 100% greater than the third shaft diameter C, and may be in the region of 3.0 to 7.0mm. The second shaft diameter B may be in the region of 10% to 75% greater than the third shaft diameter C, and may be in the region of 2.5mm to 6.5mm. The third shaft diameter C may be in the region of 1.0mm to 5.0mm. In use, the rotor assembly 10 is paired with a stator assembly 200 to form a brushless permanent magnet motor 202, as illustrated schematically in Figure 5. The stator assembly 200 comprises three coils 204, and, when the three coils 204 are driven with an appropriate voltage, the stator assembly 200 generates a magnetic field that interacts with the permanent magnet 23 to rotate the rotor assembly 10. A vacuum cleaner 300 comprising the brushless permanent magnet motor 202 is illustrated schematically in Figure 6. A haircare appliance 400 comprising the brushless permanent magnet motor 202 is illustrated schematically in Figure 7. An alternative shaft 500 and first balance ring 502 are shown schematically in Figure 8, where like reference numerals are used for sake of clarity. The shaft 500 of Figure 8 differs from the shaft 12 of Figures 1 and 2 in that the shaft 500 of Figure 8 omits the first transition region 25. The first balance ring 502 of Figure 8 has a bore with first 504 and second 506 regions of differing diameters. The first region 504 has a diameter larger than the diameter of the second region 506, and defines a region in which the adhesive used to bond the permanent magnet 23 to the shaft 500 can collect. Providing a region in which adhesive may collect may facilitate bonding of the adhesive to the first balance ring 502, which may inhibit adhesive from detaching from the first balance ring 502 when the rotor assembly spins in use. A further alternative shaft 600 and an impeller 14 are shown schematically in Figure 9, where like reference numerals are used for sake of clarity. The shaft 600 of Figure 9 differs from the shaft 12 of Figures 1 and 2 in that the shaft 600 of Figure 9 has a fourth portion 602 having a sixth shaft diameter F that is less than the first shaft diameter A. The impeller 14 is mounted to the fourth portion 602. This may allow for greater flexibility for aerodynamic design of the impeller 14 compared to, for example, the embodiment of Figures 1 and 2. It will be appreciated that other features of the rotor assembly 10 of Figures 1 and 2 may be utilised alongside the embodiments of Figures 8 and 9. Whilst particular examples and embodiments have thus far been described, it should be understood that these are illustrative only and that various modifications may be made without departing from the scope of the invention as defined by the claims. Clauses 1. A rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising: a shaft; a permanent magnet mounted to the shaft; and a rotor assembly component mounted to the shaft and in contact with the permanent magnet at an interface between the rotor assembly component and the permanent magnet; wherein at least one of the rotor assembly component and the permanent magnet comprises a through-hole located at the interface. 2. The rotor assembly according to clause 1, wherein the at least one of the rotor assembly component and the permanent magnet comprises a plurality of through-holes located at the interface. 3. The rotor assembly according to clause 2, wherein the plurality of through-holes are evenly spaced about the at least one of the rotor assembly component and the permanent magnet. 4. The rotor assembly according to any preceding clause, wherein the at least one of the rotor assembly component and the permanent magnet comprises at least three through-holes located at the interface. 5. The rotor assembly according to any preceding clause, wherein the rotor assembly component comprises the through-hole. 6. The rotor assembly according to any preceding clause, wherein the shaft comprises a first portion to which rotor assembly component is mounted, the first portion having a first diameter, a second portion to which the permanent magnet is mounted, the second portion having a second diameter less than the first diameter, and a transition region intermediate the first and second portions, the transition region having third diameter less than the first and second diameters, and wherein the through-hole overlaps the transition region. 7. The rotor assembly according to any preceding clause, wherein the rotor assembly component comprises a bore, the bore comprising a first region having a first diameter, and a second region having a second diameter less than the first diameter, and the first region is located at the interface. 8. The rotor assembly according to any preceding clause, wherein the rotor assembly component comprises one or more of a balance ring, an impeller, and a bearing assembly. 9. The rotor assembly according to any preceding clause, wherein the rotor assembly component is a balance ring. 10. The rotor assembly according to clause 9, wherein the rotor assembly comprises a bearing assembly mounted to the shaft, and the balance ring is located intermediate the bearing assembly and the permanent magnet. 11. The rotor assembly according to clause 10, wherein the rotor assembly comprises a further bearing assembly mounted to the shaft on an opposite side of the permanent magnet to the bearing assembly. 12. The rotor assembly according to clause 11, wherein the rotor assembly comprises a further balance ring mounted to the shaft between the further bearing assembly and the permanent magnet. 13. The rotor assembly according to clause 12, wherein the further balance ring comprises a smaller mass then the balance ring. 14. The rotor assembly according to any one of clauses 9 to 13, wherein the rotor assembly comprises an impeller mounted to the shaft, and the balance ring is located intermediate the impeller and the permanent magnet. 15. A brushless permanent magnet motor comprising the rotor assembly according to any preceding clause. 16. A vacuum cleaner comprising the brushless permanent magnet motor according to clause 15. 17. A haircare appliance comprising the brushless permanent magnet motor according to clause 15. 18. A method of manufacturing a rotor assembly for a brushless permanent magnet motor, the method comprising: providing a shaft; providing a rotor assembly component and a permanent magnet, at least one of the rotor assembly component and the permanent magnet comprising a through-hole; mounting the rotor assembly component to the shaft; and placing the permanent magnet relative to the shaft such that the permanent magnet is in contact with the rotor assembly component at an interface, and the through-hole is located at the interface. 19. The method according to clause 18, wherein the method comprises providing an adhesive between the permanent magnet and the shaft such that the adhesive is visible through the through-hole, and curing the adhesive using a light curing process to tack the permanent magnet to the shaft. 20. The method according to clause 19, wherein the method comprises providing further curing the adhesive using a heat curing process. 5 Claims 1. A rotor assembly for a brushless permanent magnet motor, the rotor assembly comprising: a shaft; a permanent magnet mounted to the shaft; and a rotor assembly component mounted to the shaft and in contact with the permanent magnet at an interface between the rotor assembly component and the permanent magnet; wherein: at least one of the rotor assembly component and the permanent magnet comprises a through-hole located at the interface; and the through-hole is configured to provide a line-of-sight to the shaft. 2. The rotor assembly according to claim 1, wherein the through-hole is configured to provide the line-of-sight to the shaft when the through-hole is not filled with adhesive. 3. The rotor assembly according to claim 2, wherein the through-hole is filled with an adhesive. 4. The rotor assembly according to claim 3, wherein the adhesive is a UV-curing adhesive. 5. The rotor assembly according to claim 3 or 4, wherein the adhesive is also applied between the permanent magnet and the shaft. 6. The rotor assembly according to any preceding claim, wherein the through-hole comprises a cut-out in an end of the at least one of the rotor assembly component and the permanent magnet. 7. A rotor assembly as claimed in any preceding claim, wherein the at least one of the rotor assembly component and the permanent magnet comprises a plurality of through-holes located at the interface. 8. A rotor assembly as claimed in Claim 7, wherein the plurality of through-holes are evenly spaced about the at least one of the rotor assembly component and the permanent magnet. 9. A rotor assembly as claimed in any preceding claim, wherein the at least one of the rotor assembly component and the permanent magnet comprises at least three through-holes located at the interface. 10. A rotor assembly as claimed in any preceding claim, wherein the rotor assembly component comprises the through-hole. 11. A rotor assembly as claimed in any preceding claim, wherein the shaft comprises a first portion to which rotor assembly component is mounted, the first portion having a first diameter, a second portion to which the permanent magnet is mounted, the second portion having a second diameter less than the first diameter, and a transition region intermediate the first and second portions, the transition region having third diameter less than the first and second diameters, and wherein the through-hole overlaps the transition region. 12. A rotor assembly as claimed in any preceding claim, wherein the rotor assembly component comprises a bore, the bore comprising a first region having a first diameter, and a second region having a second diameter less than the first diameter, and the first region is located at the interface. 13. A rotor assembly as claimed in any preceding claim, wherein the rotor assembly component comprises one or more of a balance ring, an impeller, and a bearing assembly. 14. A rotor assembly as claimed in any preceding claim wherein the rotor assembly component is a balance ring. 15. A rotor assembly as claimed in Claim 14, wherein the rotor assembly comprises a bearing assembly mounted to the shaft, and the balance ring is located intermediate the bearing assembly and the permanent magnet. 16. A rotor assembly as claimed in Claim 15, wherein the rotor assembly comprises a further bearing assembly mounted to the shaft on an opposite side of the permanent magnet to the bearing assembly. 17. A rotor assembly as claimed in Claim 16, wherein the rotor assembly comprises a further balance ring mounted to the shaft between the further bearing assembly and the permanent magnet. 18. A rotor assembly as claimed in Claim 17, wherein the further balance ring comprises a smaller mass then the balance ring. 19. A rotor assembly as claimed in any of Claims 14 to 18, wherein the rotor assembly comprises an impeller mounted to the shaft, and the balance ring is located intermediate the impeller and the permanent magnet. 20. A brushless permanent magnet motor comprising a rotor assembly as claimed in any preceding claim. 21. A vacuum cleaner comprising a brushless permanent magnet motor as claimed in Claim 20. 22. A haircare appliance comprising a brushless permanent magnet motor as claimed in Claim 20. 23. A method of manufacturing a rotor assembly for a brushless permanent magnet motor, the method comprising: providing a shaft; providing a rotor assembly component and a permanent magnet, at least one of the rotor assembly component and the permanent magnet comprising a through-hole; mounting the rotor assembly component to the shaft; and placing the permanent magnet relative to the shaft such that the permanent magnet is in contact with the rotor assembly component at an interface, and the through-hole is located at the interface; wherein the through hole is configured to provide a line-of-sight to the shaft. 24. A method as claimed in Claim 23, wherein the method comprises providing an adhesive between the permanent magnet and the shaft such that the adhesive is visible through the through-hole, and curing the adhesive using a light curing process to tack the permanent magnet to the shaft. 25. A method as claimed in Claim 24, wherein the method comprises further curing the adhesive using a heat curing process.

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