A motor

Abstract

A motor 10 comprising a rotor assembly 14, a stator assembly 12, and a frame 16within which the rotor assembly and the stator assembly are located. The frame includes a cavity in which at least one of

Description

BACKGROUND There is a general desire to improve electric machines, such as motors. For example, improvements may be desired in terms of cooling motors. SUMMARY A first aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet for allowing a flow of air into the cavity, and a frame outlet for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, and a ratio of a maximal length of the protrusion to a maximal width of the protrusion is at least 2. In use, at least a portion of airflow through the motor, for example along an airflow path between a housing of the motor and the frame, may flow into the frame inlet, through the cavity and out the frame outlet. This may help to convectively cool the rotor assembly and / or the stator assembly as the flow of air may come into close contact with the rotor assembly and / or the stator assembly. As the frame inlet protrudes into the airflow path, by virtue of being defined in the protrusion, this may help to increase the volume of air flowing into the frame inlet, for example relative to an arrangement in which the frame inlet is formed parallel to a bulk direction of airflow through the motor, which may help to further improve cooling of the stator assembly and / or the rotor assembly. Furthermore, as the protrusion, and hence the frame inlet, protrudes into the airflow path, a relatively higher pressure may be provided at the frame inlet, which may increase a percentage of flow flowing through the frame inlet, and hence through the cavity, in use. The frame inlet may be located at an upstream end of the cavity. The frame outlet may be located at a downstream end of the cavity. A ratio of the maximal length of the protrusion to the maximal width of the protrusion, also referred to as an aspect ratio of the protrusion, being at least 2 may reduce turbulence in comparison to an arrangement having an aspect ratio of less than 2. The ratio of the maximal length of the protrusion to the maximal width of the protrusion may be no greater than 4. Aspect ratios of greater than 4 may provide diminishing returns in reducing turbulence, whilst taking up an increasing amount of space along the length of the motor. The ratio of the maximal length of the protrusion to the maximal width of the protrusion may be from 3 to 4. This has been found to be a good compromise between the above mentioned competing factors. The ratio of the maximal length of the protrusion to the maximal width of the protrusion may be around 3.2. The maximal length of the protrusion may be from 10mm to 15mm, for example around 13.2mm. The maximal width of the protrusion may be from 2mm to 6mm, for example around 4.12mm. The maximal length of the protrusion may be from 20% to 200% of a length of stator core of the stator assembly. The maximal width of the protrusion may be from 5% to 50% of a maximal diameter of an impeller of the rotor assembly. The rotor assembly may comprise a permanent magnet, and at least a portion of the permanent magnet may be located within the cavity. The stator assembly may comprise a stator core, and at least a portion of the stator core may be located within the cavity. The protrusion may be generally tear-drop shaped when viewed along an axis orthogonal to a central longitudinal axis of the motor. The rotor assembly may comprise a shaft, and the shaft may be coaxial with the central longitudinal axis of the motor. The protrusion may comprise an upstream end, and the upstream end may increase in width toward the maximal width of the protrusion. The protrusion may comprise a downstream end, and the protrusion may decrease in width between the maximal width of the protrusion and the downstream end. When viewed in cross-section, a surface of the protrusion in which the frame inlet is defined may be angled at from 5 degrees to 40 degrees relative to the central longitudinal axis of the motor. Such an angle, also referred to as an angle of attack, may encourage airflow to attach to the protrusion, thereby reducing a risk of turbulent flow, in comparison to arrangements with a larger angle of attack. The cross-section may comprise a crosssection taken along the central longitudinal axis of the motor, for example with the angle measured in a plane in which the central longitudinal axis of the motor, and a radial axis of the motor, extend. The surface of the protrusion may be angled at from 15 degrees to 35 degrees relative to the central longitudinal axis of the motor. This may provide a good compromise between the protrusion, and hence the frame inlet, protruding into the airflow path, and encouraging less turbulent airflow through the motor in use. The surface of the protrusion may be angled at around 30 degrees relative to the central longitudinal axis of the motor. The frame inlet may comprise an inlet aperture formed in the protrusion, and the inlet aperture may have a maximal width that is from 40% to 90% of the maximal width of the protrusion. If the inlet aperture is too large or too small, then an efficiency of cooling provided by the scoop may be reduced, and / or an efficiency of the motor as a whole may be reduced. For example, an increase in size in inlet aperture may create excess turbulence whilst leading to poor structural rigidity. The maximal width may be measured across a periphery of an open end of the frame inlet, for example across the surface of the protrusion. The inlet aperture may have a maximal width that is from 50% to 80% of the maximal width of the protrusion. Such a maximal width may be a good compromise between efficiency of cooling provided by the airflow between the frame inlet and the frame inlet, and the overall efficiency of the motor. The inlet aperture may have a maximal width that is around 61% of the maximal width of the protrusion. When viewed in a cross-sectional plane comprising the central longitudinal axis of the motor, a portion of a surface of the protrusion adjacent a radially outer periphery of the frame inlet may be curved, and a radius of curvature of the surface is at least 20% of the maximal length of the protrusion. Such a radius of curvature may encourage airflow to attach to the portion of the surface of the protrusion, and may inhibit turbulent airflow, when compared to a smaller radius of curvature. The portion of the surface of the protrusion may, when viewed in the cross-section, curve toward a maximal height of the protrusion, for example a maximal height of the protrusion relative to the central longitudinal axis of the motor, before a remainder of the surface of the protrusion decreases in height in a direction toward the downstream end of the protrusion. The radius of curvature may be measured at the maximal height of the protrusion, and may, for example, comprise a distance between the maximal height of the protrusion and a wall that defines the frame inlet. The radius of curvature of the portion of the surface may be from 20% to 40% of the maximal length of the protrusion. The radius of curvature of the portion of the surface may be around 25% of the maximal length of the protrusion. When viewed in a plan view, a portion of a surface of the protrusion may curve between the frame inlet and the maximal width of the protrusion, and a radius of curvature of the portion of the surface may be at least 30% of the maximal length of the protrusion. Such a radius of curvature may encourage airflow to attach to the portion of the surface of the protrusion, and may inhibit turbulent airflow, when compared to a smaller radius of curvature. The radius of curvature may be measured along an axis parallel to a radial axis of the motor, and tangential to the maximal height of the protrusion. The plan view may be in a radial direction, for example along a radial axis of the motor. The radius of curvature of the portion of the surface may be at least 50% of the maximal length of the protrusion. The radius of curvature of the portion of the surface may be no more than 80% of the maximal length of the protrusion. The radius of curvature of the portion of the surface may be around 52% of the maximal length of the protrusion. The portion of the surface of the protrusion that curves between the frame inlet and the maximal width of the protrusion in a plan view may be the same portion of the protrusion that curves when viewed in a cross-sectional plane comprising the central longitudinal axis of the motor. An angle between a central longitudinal axis of the protrusion, and a central longitudinal axis of the motor, when viewed in a plan view, is from between -5 degrees to 5 degrees. This may provide greater efficiency of cooling provided by the scoop, and / or a greater efficiency of the motor as a whole, when compared with angles of greater magnitude. The plan view may be in a radial direction, for example along a radial axis of the motor. The central longitudinal axis of the protrusion may bisect the protrusion. The central longitudinal axis of the protrusion may extend through a centre of a most downstream point of the protrusion and through a centre of a most upstream point of the protrusion, when viewed in plan view. The angle between the central longitudinal axis of the protrusion, and the central longitudinal axis of the motor, may be from -3 degrees to 3 degrees. The angle between the central longitudinal axis of the protrusion, and the central longitudinal axis of the motor, may be around 0 degrees. A maximal height of the protrusion may be from 30% to 90%, or from 50% or 80%, of a maximal diameter of the cavity. This may provide a reasonable comprise between impacting on airflow through the airflow path and promoting airflow through the frame inlet. The maximal height of the protrusion may be from 3mm to 6mm. The maximal height of the protrusion may be from 4mm to 5mm. The frame may comprise a plurality of frame inlets located at the upstream end of the cavity, and a plurality of respective frame outlets located at the downstream end of the cavity. Each frame inlet may be defined in a respective protrusion that protrudes outwardly from the frame into the airflow path through the motor. Each protrusion may have substantially the same form. The protrusions may be substantially evenly circumferentially spaced about the periphery of the frame. A second aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet located at an upstream end of the cavity for allowing a flow of air into the cavity, and a frame outlet located at a downstream end of the cavity for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, wherein, when viewed in cross-section along a central longitudinal axis of the motor, a surface of the protrusion in which the frame inlet is defined is angled at from 5 degrees to 40 degrees relative to the central longitudinal axis of the motor. A third aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet located at an upstream end of the cavity for allowing a flow of air into the cavity, and a frame outlet located at a downstream end of the cavity for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, wherein the frame inlet comprises an inlet aperture formed in the protrusion, and the inlet aperture has a maximal width that is from 40% to 90% of a maximal width of the protrusion. A fourth aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet located at an upstream end of the cavity for allowing a flow of air into the cavity, and a frame outlet located at a downstream end of the cavity for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, wherein when viewed in a cross-sectional plane comprising a central longitudinal axis of the motor, a portion of a surface of the protrusion adjacent a radially outer periphery of the frame inlet is curved, and a radius of curvature of the portion of the surface is at least 20% of a maximal length of the protrusion. A fifth aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet located at an upstream end of the cavity for allowing a flow of air into the cavity, and a frame outlet located at a downstream end of the cavity for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, wherein, when viewed in a plan view, a portion of a surface of the protrusion curves between the frame inlet and a maximal width of the protrusion, and a radius of curvature of the portion of the surface is at least 50% of a maximal length of the protrusion. A sixth aspect provides a motor comprising: a rotor assembly; a stator assembly; and a frame within which the rotor assembly and the stator assembly are located, wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet located at an upstream end of the cavity for allowing a flow of air into the cavity, and a frame outlet located at a downstream end of the cavity for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, wherein an angle between a central longitudinal axis of the protrusion, and a central longitudinal axis of the motor, when viewed in a plan view, is from between -5 degrees to 5 degrees. A seventh aspect provides a vacuum cleaner comprising a motor according to the first aspect. Optional features of aspects may be equally applied to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a perspective view of a motor; Figure 2 shows a perspective view of a stator assembly of the motor; Figure 3 shows a perspective view of a rotor assembly of the motor; Figure 4 shows a cross-sectional view through the motor with the rotor assembly removed; Figure 5 shows a first cross-sectional view of the motor; Figures 6 to 8 show a plan view of the motor; Figures 9 to 11 show a second cross-sectional view of the motor; Figures 12 and 13 show combined plan and cross-sectional views of portions of the motor; Figure 14 shows apian view of the motor; Figures 15 and 16 show two orthogonal cross-sections through the motor; Figure 17 schematically illustrates a vacuum cleaner comprising the motor; and Figure 18 schematically illustrates an alternative form of outlet aperture. DETAILED DESCRIPTION A motor 10 is illustrated in Figures 1 to 5. The motor 10 comprises a stator assembly 12, a rotor assembly 14, and a frame 16. The stator assembly 12 is illustrated in isolation in Figure 2, and comprises four stator core sub-assemblies 20 and a termination assembly 22. The stator core sub-assembly 20 comprises a stator core 23 (shown in Figure 16), a bobbin 24, and a winding 26. The stator core has a generally C-shaped form and may be referred to as a c-core. The bobbin 24 is over moulded to the stator core 23, and comprises first 28 and second 30 connection portions. The first 28 and second 30 connection portions are complementarity shaped, such that adjacent bobbins 24 in the stator assembly 12 can be connected to one another by axially sliding the relevant connection portions 28, 30 together. The winding 26 is wound about the bobbin 24. The termination assembly 22 comprises a first, upper, terminal 32, a second, lower, terminal 34, a first termination tab 36, and a second termination tab 38. Each of the first 32 and second 34 terminals is generally annular in form, with the first terminal 32 overlying the second terminal 34. The windings 26 of the stator core sub-assemblies 20 are connected to the first 32 and second 34 terminals. The first 36 and second 38 termination tabs project upwards from the termination assembly 22 and are each connected to one of the first terminal 32 and the second terminal 34. The termination tabs 36,38 are used to provide power to the terminals 32, 34. The rotor assembly 14 is shown in isolation in Figure 3. The rotor assembly 14 comprises a shaft 40, a permanent magnet 42, first 44 and second 46 bearings, first 48, second 50 and third 52 balancing rings, and an impeller 54. The shaft 40 is elongate in form, having an upstream end 56 and a downstream end 58, with upstream and downstream referring generally to a direction of airflow over the motor 10 in use. The permanent magnet 42 is mounted generally centrally along the shaft 40. The first balancing ring 48 is mounted to the shaft 40 at the upstream end 56, with the first bearing 44 mounted to the shaft 40 adjacent to the first balancing ring 48. The second balancing ring 50 is mounted to the shaft 40 between the first bearing 44 and the permanent magnet 42. The impeller 54 is mounted to the downstream end 58 of the shaft 40. The second bearing 46 is mounted to the shaft 40 adjacent to the impeller 54, with the third balancing ring 52 mounted to the shaft 40 between the second bearing 46 and the permanent magnet 42. The rotor assembly 14 comprises a pre-load spring 60 for applying a pre-load to the first bearing 44, and a seal in the form of an o-ring 62 located about the first bearing 44. The frame 16 can be seen in Figures 1, 4 and 5, and comprises a main body 64 and an end cap 68. Details of the end cap 68 are not pertinent to the present application, and so will not be described here for sake of brevity. The main body 64 is generally cylindrical in form with four projections 65 and comprises a thermoplastic material. The main body 64 defines first 70 and second 72 bearing seats for the respective first 44 and second 46 bearings. The main body 64 also defines a cavity 74 within which the rotor assembly 14 is located and into which the stator assembly 12 extends. Each of the projections 65 overlies a stator core sub-assembly 16. The main body 64 of the frame 16 comprises a plurality of inlet apertures 78, and a plurality of outlet apertures 80. The inlet apertures 78 define a frame inlet, and the outlet apertures 80 define a frame outlet. The plurality of inlet apertures 78 are located in a region below the first bearing seat 70, and are spaced about the periphery of the main body 64. Each inlet aperture 78 is defined in a respective protrusion 82 formed on the main body 64 of the frame 16, and extends through the main body 64 of the frame 16 to the cavity 74. Each protrusion 82 is located between two adjacent projections 65 about the circumference of the main body 64 of the frame 16. Each protrusion 82 has a same form, and a single protrusion 82 is shown in plan view in Figures 6 to 8, and in a cross-sectional view in Figures 9 to 11. The protrusion 82 is generally teardrop shaped in form in the plan view, having an upstream end 84 and a downstream end 86. The protrusion 82 has a maximal length ML of around 13.2mm, in a direction measured parallel to a central longitudinal axis CL of the motor 10. The protrusion 82 increases in width away from the upstream end 84 before reaching a maximal width MW in a direction measured orthogonal to the central longitudinal axis CL of the motor 10. The maximal width MW is around 4.12mm. An aspect ratio of the protrusion 82 is thereby around 3.2 Aspect ratios of from 2 to 4 are also envisaged. The portion of the protrusion 82 between the upstream end 84 and the maximal width MW is curved in the plan view, with the surface having a radius of curvature RC1 in the plan view that is around 52% of the maximal length ML of the protrusion 82. Percentages of from 30% to 80% are also envisaged. The radius of curvature RC1 here is around 8.29mm. The protrusion 82 decreases in width from the maximal width MW toward the downstream end 86. The protrusion 82 is arranged on the main body 64 of the frame 16 such that a central longitudinal axis SA of the protrusion 82 bisects the upstream end 84 and the downstream end 86, and such that the central longitudinal axis SA of the protrusion 82 is parallel to the central longitudinal axis CL of the motor 10. That is, there is an angle of 0 degrees between the central longitudinal axis SA of the protrusion 82 and the central longitudinal axis CL of the motor 10. Angles of from -5 degrees to 5 degrees are also envisaged. When viewed in the cross-sectional view, a portion of a surface 88 of the protrusion 82 immediately downstream of the inlet aperture 78 is angled at an attack angle a of around 30 degrees relative to a central longitudinal axis CL of the motor 10. Angles from 5 degrees to 40 degrees are also envisaged. The portion of the surface 88 curves away from the inlet aperture toward a region of maximal height MH of the protrusion from the main body 64 of the frame 16. A radius of curvature RC2 of the portion of the surface 88 in the cross-sectional view is around 25% of the maximal length ML of the protrusion 82. Percentages of from 20% to 40% are also envisaged. The radius of curvature RC2here is around 8.29mmmm. The maximal height of the protrusion 82 is around 4.69mm, although maximal heights of from 3mm to 6mm are also envisaged. Examples where the maximal height of the protrusion is from 30% to 90%, or from 50% or 80%, of a maximal diameter of the cavity are also envisaged. The inlet aperture 78 has a maximal width, measured in the cross-sectional view, of around 61% of the maximal width MW of the protrusion. Percentages of from 40% to 90% are also envisaged. The plurality of outlet apertures 80 are located in a region of the second bearing seat 72, and are spaced about the periphery of the main body 64. Each outlet aperture 80 is aligned with a corresponding inlet aperture 78 in a direction parallel to the central longitudinal axis CL of the motor 10. Each outlet aperture 80 extends through the main body 64 of the frame 16 to the cavity 74, from an inner surface 90 of the frame 16 to an outer surface 92 of the frame 16, and is defined by an internal wall 94. A total minimal cross-sectional area of the outlet apertures 80 is greater than or equal to the minimal free cross-sectional area of the cavity 74. Each outlet aperture 80 has a same form, and a single outlet aperture 80 is shown in plan views and in cross-sectional views in Figures 12 to 14. In the plan view of Figure 14, the outlet aperture 80 generally has the shape of a quadrant, with a periphery of the outlet aperture 80 having a first linear portion 96, a second linear portion 98, and a curved portion 100. The curved portion 100 has a radius of curvature RC3 that is around 58.1% of a maximal width WM of the outlet aperture 80. Percentages of from 20% to 90% are also envisaged. The radius of curvature RC3 of the curved portion 100 is around 5.47mm, and the maximal width WM of the outlet aperture 80 is from 50% to 90% of the height of the cavity. The internal wall 94 of the outlet aperture 80 is shaped to bias airflow exiting the outlet aperture 80 to one side of the outlet aperture 80, as will be described hereinafter. The internal wall 94 of the outlet aperture has an upstream portion 102, a downstream portion 104, and first 106 and second 108 side portions extending between the upstream portion 102 and the downstream portion 104. The cross-sectional view of Figure 12 is a cross-sectional view taken in a plane orthogonal to the central longitudinal axis CL of the motor 10. In the cross-sectional view of Figure 12, only a portion of the frame 16 corresponding to the outlet aperture 80 is shown, and the downstream portion 104 of the internal wall 94 of the outlet aperture 80 is omitted, for the sake of clarity. As can be seen in Figure 12, the first side portion 106 of the internal wall 94 extends between the inner surface 90 and the outer surface 92 of the frame 16 such that the first side portion 106 of the internal wall is generally orthogonal to tangent lines to the inner surface 90 and the outer surface 92 at the points at which the internal wall 94 meets the inner surface 90 and the outer surface 92 of the frame 16. In contrast, the second side portion 108 of the internal wall 94 is angled to bias airflow toward one side of the outlet aperture 80. In particular, the second side portion 108 of the internal wall 94 meets the inner surface 90 of the frame at a point, but instead of extending orthogonally relative to a tangent line T to the point, the second side portion 108 of the internal wall 94 defines an angle of around 32 degrees relative to an axis A that extends orthogonally relative to the tangent line T. Angles of from 3 degrees to 60 degrees are also envisaged. A width of the outlet aperture 80 thereby increases from the inner surface 90 of the frame 16 to the outer surface 90 of the frame 16. It can also be seen in Figure 12 that a transition from the second side portion 108 of the internal wall 94 to the outer surface 92 of the frame 16 is curved. The transition has a radius of curvature of around 18.3% of the maximal width WM of the outlet aperture 80. Percentages of from 5% to 50% are also envisaged. The radius of curvature of the transition is around 3.18mm. The cross-sectional view of Figure 13 is a cross-sectional view taken along the central longitudinal axis CL of the motor 10, with the central longitudinal axis CL in the plane of the cross-section. In the cross-sectional view of Figure 13, only a portion of the frame 16 corresponding to the outlet aperture 80 is shown, and the first 106 and second 108 side portions of the internal wall 94 omitted, for the sake of clarity. As can be seen in Figure 13, the upstream portion 102 of the internal wall 94 extends between the inner surface 90 of the frame 16 and the outer surface 92 of the frame 16 in a direction generally orthogonal to the central longitudinal axis CL of the motor 10. In contrast, the downstream portion 104 of the internal wall 94 extends between the inner surface 90 of the frame 16 and the outer surface 92 of the frame 16 in a direction obliquely angled relative to the central longitudinal axis CL of the motor 10. A width of the outlet aperture 80 thereby increases from the inner surface 90 of the frame 16 to the outer surface 90 of the frame 16. A transition from the downstream portion 146 of the internal wall 94 to the outer surface 92 of the frame 16 is curved. The transition has a radius of curvature of around 18.3% of the maximal width WM of the outlet aperture 80, and is the same as the radius of curvature of the transition from the second side portion 108 of the internal wall 94 to the outer surface 92 of the frame. Percentages of from 5% to 50% are also envisaged. The radius of curvature of the transition between the downstream portion 104 of the internal wall 94 and the outer surface 92 of the frame 16 is around 3.18mm. Cross-sectional views through the motor 10 are shown in Figures 15 and 16. As can be seen, the rotor assembly 14 sits within the frame 16, with the first bearing 44 located at the first bearing seat 70, the second bearing 46 located at the second bearing seat 72, and the permanent magnet 42 aligned with the stator cores of the stator assembly 12. The impeller 54 is thereby located on a downstream side of the stator assembly 12, and the first bearing 44 is located on an upstream side of the stator assembly 12. The permanent magnet 42 is located within the cavity 74 defined by the frame 16. In use, operation of the motor 10 causes the impeller 54 to rotate, which in turn generates an airflow across the motor 10. Airflow flows across the motor 10 in a direction toward the impeller 54, and the impeller can be thought of as being located at a downstream end of the motor 10. References to upstream and downstream herein will be construed accordingly, with downstream being located closer to the impeller in a direction parallel to the central longitudinal axis CL of the motor 10. As airflow flows across the motor 10, a portion of the airflow is directed into the inlet apertures 78, whilst the remainder of the airflow flows across the outer surface 92 of the frame 16. The portion of the airflow that is directed into the inlet apertures 78 can be thought of as a secondary airflow, whilst the remainder of the airflow flows that across the outer surface 92 of the frame 16 can be thought of as a primary airflow. As the inlet apertures 78 protrude into the airflow path of the primary airflow, by virtue of being defined in the protrusions 82, this may help to increase the volume of air flowing into the inlet apertures 78, which may help to improve cooling of the stator assembly 12 and / or the rotor assembly 14. For example, as the protrusions 82, and hence the inlet apertures 78, protrude into the airflow path of the primary airflow, a relatively higher pressure may be provided at the inlet apertures 78, which may increase a percentage of flow flowing through the inlet apertures 78, and hence through the cavity 74, in use. The particular geometries and relative dimensions of the protrusions 82 discussed herein have been found to reduce an impact of diverting the secondary airflow through the inlet apertures 78 on the primary airflow. For example, the particular geometries and relative dimensions of the protrusions 82 may reduce turbulence and / or encourage the primary airflow to attach to the frame 16, in comparison with other geometries and relative dimensions. The secondary airflow flows through the cavity 74, and passes the permanent magnet 42 of the rotor assembly 14. As the secondary airflow passes the permanent magnet 42, heat from the permanent magnet 42 is transferred to the secondary airflow to cool the permanent magnet 42. As the secondary airflow passes through the cavity 74, heat from other components within the motor 10 (such as the first 44 and second 46 bearings) is transferred to the secondary airflow to cool such components. The secondary airflow through the cavity 74 also cools components of the stator assembly 12 (such as the stator core 23) from within the cavity 74 (in addition to the cooling that occurs from the flow of air passing over the frame 16. The heated flow of air is then transported out of the cavity via the outlet apertures 80. Given the shape of the internal wall 94 of each outlet aperture 80 described above, airflow exiting the outlet apertures 80 is provided with both an axial component in a downstream axial direction, parallel to the central longitudinal axis CL of the motor 10, and a circumferential component in a direction around the central longitudinal axis CL of the motor 10. This may modify a course of the primary airflow, and may be considered to be pre-swirling of the airflow before it reaches the impeller 54. The particular geometries and relative dimensions of the outlet apertures 80 discussed herein have been found to reduce a negative impact of reintroduction of the secondary airflow to the primary airflow, whilst also pre-swirling the airflow before the impeller 54. For example, less turbulence may be created by the rejoining of the secondary airflow to the primary airflow. This may provide for improved efficiency of the motor 10. A vacuum cleaner 200 comprising the motor 10 is illustrated schematically in Figure 17. A second example form of an outlet aperture 300 that may replace the outlet aperture 80 of Figures 12 to 14, is illustrated schematically in Figure 18. The outlet aperture 300 of Figure 18 is partly defined by a main body 302 of a frame, and partly defined by a protrusion 306 that curves outwardly from the main body 302 of the frame. An internal wall 308 of the protrusion 306 is curved such that airflow exiting the outlet aperture 300 has a circumferential component, in manner similar to that described above for the outlet 5 apertures 80 of the frame 16 of Figures 12 to 14. Whilst particular examples have been described, it should be understood that these are illustrative examples only and that various modifications may be made without departing from the scope of the invention as defined by the claims. 10

Claims

1. A motor comprising:a rotor assembly;a stator assembly; anda frame within which the rotor assembly and the stator assembly are located,wherein the frame comprises a cavity in which at least one of the rotor assembly and the stator assembly is at least partly located, a frame inlet for allowing a flow of air into the cavity, and a frame outlet for allowing the flow of air out of the cavity, the frame inlet is defined in a protrusion that protrudes outwardly from the frame into an airflow path through the motor, and a ratio of a maximal length of the protrusion to a maximal width of the protrusion is at least 2.

2. A motor as claimed in Claim 1, wherein the ratio of the maximal length of the protrusion to the maximal width of the protrusion is no greater than 4.

3. A motor as claimed in Claim 1 or Claim 2, wherein the ratio of the maximal length of the protrusion to the maximal width of the protrusion is from 3 to 4.

4. A motor as claimed in any one of the preceding claims, wherein, when viewed in cross-section along a central longitudinal axis of the motor, a surface of the protrusion in which the frame inlet is defined is angled at from 5 degrees to 40 degrees relative to the central longitudinal axis of the motor.

5. A motor as claimed in Claim 4, wherein the surface of the protrusion is angled at from 15 degrees to 35 degrees relative to the central longitudinal axis of the motor.

6. A motor as claimed in Claim 4 or Claim 5, wherein the surface of the protrusion is angled at around 30 degrees relative to the central longitudinal axis of the motor.

7. A motor as claimed in any one of the preceding claims, wherein the frame inlet comprises an inlet aperture formed in the protrusion, and the inlet aperture has a maximal width that is from 40% to 90% of the maximal width of the protrusion.

8. A motor as claimed in Claim 7, wherein the inlet aperture has a maximal width that is from 50% to 80% of the maximal width of the protrusion.

9. A motor as claimed in Claim 7 or Claim 8, wherein the inlet aperture has a maximal width that is around 61% of the maximal width of the protrusion.

10. A motor as claimed in any one of the preceding claims, wherein when viewed in a cross-sectional plane comprising a central longitudinal axis of the motor, a portion of a surface of the protrusion adjacent a radially outer periphery of the frame inlet is curved, and a radius of curvature of the portion of the surface is at least 20% of the maximal length of the protrusion.

11. A motor as claimed in Claim 10, wherein the radius of curvature of the portion of the surface is from 20% to 40% of the maximal length of the protrusion.

12. A motor as claimed in Claim 10 or Claim 11, wherein the radius of curvature of the portion of the surface is around 25% of the maximal length of the protrusion.

13. A motor as claimed in any one of the preceding claims, wherein, when viewed in a plan view, a portion of a surface of the protrusion curves between the frame inlet and the maximal width of the protrusion, and a radius of curvature of the portion of the surface is at least 50% of the maximal length of the protrusion.

14. A motor as claimed in Claim 13, wherein the radius of curvature of the portion of the surface is no more than 30% of the maximal length of the protrusion.

15. A motor as claimed in Claim 13 or Claim 14, wherein the radius of curvature of the portion of the surface is around 52% of the maximal length of the protrusion.

16. A motor as claimed in any one of the preceding claims, wherein an angle between a central longitudinal axis of the protrusion, and a central longitudinal axis of the motor, when viewed in a plan view, is from between -5 degrees to 5 degrees.

17. A motor as claimed in Claim 16, wherein the angle between the central longitudinal axis of the protrusion, and the central longitudinal axis of the motor, is from -3 degrees to 3 degrees.

18. A motor as claimed in Claim 16 or Claim 17, wherein the angle between the central longitudinal axis of the protrusion, and the central longitudinal axis of the motor, is around 0 degrees.

19. A motor as claimed in any one of the preceding claims, wherein a maximal height of the protrusion is from 30% to 90%, or from 50% or 80%, of a maximal diameter of the cavity.

20. A vacuum cleaner comprising a motor as claimed in any one of the preceding claims.

Images