Motor

Abstract

A motor 10 comprising a stator assembly 12 comprising a stator winding; a rotor 14; an impeller 16 connected to the rotor and configured to generate an air flow when the rotor rotates relative to the stator assembly. A guide structure adjacent to the winding is arranged to guide the air flow over the winding, where the guide structure is configured to locally accelerate the air flow in a region where the air flow passes over the winding. The guide structure may define an air flow passage comprising a constriction in the region where the air flow passes over the stator winding to locally accelerate the air flow. The constriction may comprise an intermediate portion of the air flow passage located between an upstream portion and a downstream portion of the air flow passage, and where a cross-sectional area of the air flow passage in a direction normal to the air flow may be reduced in the intermediate portion relative to the upstream portion and the downstream portion. The cross-sectional area of the air flow passage may reduce gradually from the upstream portion to the intermediate portion and may increase gradually from the intermediate portion to the downstream portion. The guide structure may comprise a turbulator arranged to generate turbulence in the region where the air flow passes over the stator winding.

Description

B ACKGROUND Brushless electric motors find utility in a wide range of devices and appliances. Such an electric motor typically includes a rotor and a stator with a set of windings which are sequentially energised to cause rotation of the rotor. SUMMARY According to a first aspect, there is provided a motor comprising: a stator assembly comprising a stator winding; a rotor; an impeller connected to the rotor and configured to generate an air flow when the rotor rotates relative to the stator assembly; and a guide structure adjacent to the stator winding and arranged to guide the air flow over the stator winding; wherein the guide structure is configured to locally accelerate the air flow in a region where the air flow passes over the stator winding. In this manner, the air flow generated by the impeller flows over the stator winding to cool the stator winding. As the guide structure locally accelerates the air flow in the region where the air flow passes over the stator winding, heat can be efficiently removed from the stator winding, thus promoting efficient cooling of the stator winding. In particular, increased air flow speed over the stator winding contributes to increased heat exchange between the stator winding and the air flow. Moreover, as the air flow is only accelerated locally, this results in any pressure drop in the air flow caused by the acceleration to be localised to the region where the air flow passes over the stator winding. This may minimise any pressure drop in the air flow exiting the motor caused by the air flow passing over the stator winding. Accordingly, the air flow efficiently cools the stator winding, whilst minimising an effect on pressure of the air flow output from the motor. The guide structure may comprise any suitable structure within the motor which guides the air flow from the impeller over the stator winding. The guide structure may, for example, define a channel or passageway through which the air flow passes. The guide structure is configured (i.e. shaped) such that the air flow is locally accelerated in the region where the air flow passes over the stator winding. Thus, the air flow in the region where the air flow passes over the stator winding may be faster than an upstream portion and a downstream portion of the air flow. Thus, as the air flow passes from the upstream portion into the region where the air flow passes over the stator winding, the air flow is locally accelerated, following which it is decelerated as it passes into the downstream portion, e.g. to return it to substantially the same speed as in the upstream portion. Various arrangements and shapes of guide structure can be used to achieve such a localised acceleration of the air flow, examples of which are provided below. The air flow may come into contact with the stator winding in the region where the air flow passes over the stator winding. For example, the stator winding may be directly exposed to the air flow. Alternatively, the stator winding may comprise (e.g. be covered by) a heat exchange component, such that the air flow comes into contact with the heat exchange component. In other words, the air flow can remove heat from the stator winding via the heat exchange component rather than by direct contact with the stator winding. The stator assembly may comprise a stator core (e.g. a permanent magnet) around which the stator winding is wound. The rotor is rotatably mounted relative to the stator assembly, and configured to rotate in response to a magnetic field generated by the stator assembly. The impeller is connected to the rotor so that the impeller rotates with the rotor. The impeller may be configured to generate an air flow along (e.g. parallel to) an axis of rotation of the rotor. The impeller may be located downstream of the stator assembly. In this manner, the air flow passes through the stator assembly before reaching the impeller. This may improve cooling efficiency of the stator winding, as this may result in cooler air being drawn through the guide structure prior to the air flow reaching the impeller. The motor is an electric motor. In particular, the motor may be a brushless permanent magnet motor. The guide structure may define an air flow passage comprising a constriction in the region where the air flow passes over the stator winding to locally accelerate the air flow. In this manner, the constriction can act to locally accelerate the air flow over the stator winding. The constriction in the air flow passage results in the air flow being temporarily accelerated as the air flow passes through (or around) the constriction, enhancing heat exchange with the stator winding. The constriction may comprise an intermediate portion of the air flow passage located between an upstream portion and a downstream portion of the air flow passage, and a cross-sectional area of the air flow passage in a direction normal to the air flow may be reduced in the intermediate portion relative to the upstream portion and the downstream portion. In other words, the cross-sectional area of the air flow passage is larger on either side of the constriction. In this manner, the air flow is accelerated as it passes from the upstream portion into the intermediate portion, and then decelerated as it passes from the intermediate portion into the downstream portion. This reduces a pressure drop of the air flow in the downstream portion compared to the intermediate portion, enabling a higher pressure air flow to be output from the motor. Such an increase in the cross-sectional area of the air flow passage after the intermediate portion may also reduce turbulence and promote laminar air flow in the downstream portion. Accordingly, a relatively high pressure laminar air flow may be output from the downstream portion of the air flow passage, which may be beneficial for many applications. The cross-sectional area of the air flow passage may reduce gradually from the upstream portion to the intermediate portion, and increase gradually from the intermediate portion to the downstream portion. In other words, there may be no sudden or sharp changes in the cross-sectional area of the air flow passage, but rather there are gradual (i.e. smooth) changes in cross-sectional area between the different portions of the air flow passage. Such gradual changes in the cross-sectional area of the air flow passage contribute to minimising turbulence at an interface between the upstream portion and the intermediate portion, and at an interface between the intermediate portion and the downstream portion. Accordingly, the gradual change in cross-sectional area of the air flow passage promotes smooth laminar flow along the air flow passage, thus reducing turbulence in the air flow output by the motor. Sidewalls of the air flow passage may be substantially smooth without sharp edges or corners. Similarly to the above, this may contribute to a laminar air flow being output by the motor. In some cases, the guide structure may comprise a turbulator arranged to generate turbulence in the region where the air flow passes over the stator winding. Increasing turbulence in the region where the air flow passes over the stator winding may increase heat exchange between the stator winding and the air flow, thus promoting efficient cooling of the stator winding. The turbulator may be arranged such that turbulence is substantially localised to the region where the air flow passes over the stator winding, with turbulence caused by the turbulator being resolved by the time the air flow exits the motor. For example, the turbulator may be provided in the upstream and / or intermediate portion of the air flow passage. In this manner, heat exchange with the stator winding can be enhanced, whilst minimising turbulence in the downstream portion. The turbulator may, for example, be an indentation and / or protrusion in a sidewall of the air flow passage. The guide structure may define an air flow passage comprising a Venturi passage configured to locally accelerate the air flow in the region where the air flow passes over the stator winding. The Venturi passage may provide a local narrowing of the air flow passage, which acts to locally accelerate the air flow. The Venturi passage may correspond to the intermediate portion of the air flow passage mentioned above. As discussed further below, the air flow passage(s) in the motor may comprise a first air flow passage which extends through a bobbin structure of the stator assembly, and / or the air flow passage may comprise a second air flow passage partially defined by an inner surface of a housing of the motor. Any features described above in relation to the guide structure and the air flow passage are equally applicable to each of the first air flow passage and second air flow passage, as detailed further below. The stator assembly may comprise a bobbin structure on which the winding is supported, and the guide structure may comprise a portion of the bobbin structure which defines a first air flow passage that extends through part of the bobbin structure. The first air flow passage may pass over the stator winding, and be configured to accelerate the air flow. In this manner, the bobbin structure may serve to both support the stator winding and to accelerate the air flow to cool the stator winding. Integrating the guide structure with the bobbin structure may therefore contribute to simplifying a construction of the motor and making the motor more compact. The bobbin structure may comprise a support around which the stator winding is wound. In some cases, the bobbin structure may comprise a stator core, such that the stator winding passes around (i.e. is wound around) the stator core. In other words, the stator core may be integrated with the bobbin structure. The guide structure may comprise a first support member and a second support member of the bobbin structure, the first support member and the second support member being arranged on either side of the winding, and wherein the first air flow passage extends through a gap between the first and second support members. In this manner, the first and second support members of the bobbin structure may define sidewalls of the first air flow passage. The gap between the first and second support members may thus act as a channel which guides the air flow over the stator winding and which locally accelerates the air flow. The bobbin structure may be arranged to support the stator winding such that the stator winding is spaced from the frame, such that the first air flow passage passes between the stator winding and the rotor (and / or the frame), with the first and second support members defining sidewalls of the first air flow passage. The first and second support members may connect the bobbin structure to the frame, e.g. the first and second support members may correspond to legs of the bobbin structure. The first and second support members may be removably connectable to the frame, or they may be formed continuously with the frame. In some cases, the stator core may include first and second side portions which extend along the first and second support members of the bobbin structure, respectively. In such a case, ends of the first and second side portions of the stator core may be connected or connectable to the frame, to secure the bobbin structure to the frame. In this manner, the bobbin structure may be connected to the frame via the first and second side portions of the stator core. The first and second side portions of the stator core may be integrated into the first and second support members of the bobbin structure. For example, the first and second support members may comprise a plastic material which is formed at least partially around (e.g. overmoulded at least partially around) the first and second side portions of the stator core, respectively. The frame provides a structure on which the bobbin structure is mounted. The bobbin structure and stator winding may provide a first stator sub-assembly which is mounted on the frame. In practice, there may be two or more stator sub-assemblies mounted on the frame, each stator sub-assembly having a respective stator winding supported on a respective bobbin structure. In each stator sub-assembly, a respective air flow passage which passes over the stator winding may be defined through bobbin structure to cool the stator winding. The air flow passage in each stator sub-assembly may be as described for the first air flow passage. Thus, the stator winding of each stator sub-assembly may be cooled by a locally accelerated air flow passing through its bobbin structure. A width of the gap may vary along a length of the first air flow passage to create a constriction in the first air flow passage. In this manner, the air flow is locally accelerated in the first air flow passage in the region where the air flow passes over the stator winding, to cool the stator winding. For instance, the gap between the first and second support members may become narrower along the first air flow passage to create the constriction. The constriction in the first air flow passage may provide effects corresponding to the constriction described above. Here, the width of the gap may refer to a width in a direction normal to the direction of air flow. The first and second support members may be shaped to provide the varying width of the gap along the length of the first air flow passage. The width of the gap may decrease from an upstream end of the bobbin structure towards an intermediate portion of the bobbin structure, and the width of the gap may increase from the intermediate portion towards a downstream end of the bobbin structure. In this manner, the gap is narrowest in the intermediate portion of the bobbin structure, so as to form the constriction in the first air flow passage. In other words, the first air flow passage through the bobbin structure may have an intermediate portion located between an upstream portion and a downstream portion of the first air flow passage, where a cross-sectional area of the first air flow passage in a direction normal to the air flow is reduced in the intermediate portion relative to the upstream portion and the downstream portion. In line with the discussion above, this reduces a pressure drop in the air flow output from the motor, and promotes laminar air flow exiting the motor. The width of the gap may vary gradually along the length of the first air flow passage. Edges of the first and second support members at an upstream end of the bobbin structure may be rounded. This avoids sharp edges around an entrance to the gap between the first and second support members, thus minimising turbulence in this region and promoting laminar air flow at the upstream end of the bobbin structure. Additionally or alternatively, edges of the first and second support members at a downstream end of the bobbin structure may be rounded. Similarly to the above, this avoids sharp edges for air flow exiting the gap between the first and second support members, thus minimising turbulence in this region and promoting laminar air flow exiting the motor. The motor may further comprise a housing disposed around the stator assembly, and the guide structure may comprise an inner surface of the housing that defines a sidewall of a second air flow passage that passes over part of the stator winding. In this manner, the housing is shaped to guide the air flow in a manner that locally accelerates the air flow over the stator winding to cool the stator winding. Integrating the guide structure with the housing may contribute to simplifying a construction of the motor and making the motor more compact. The housing may be arranged around the stator assembly to enclose all or part of the stator assembly. The inner surface of the housing corresponds to a surface on an inside of the housing which generally faces towards the stator assembly. The inner surface of the housing may include one or more guide features provided thereon, which are configured to guide the air flow over the stator winding and locally accelerate the air flow in the region where the air flow passes over the stator winding. The second air flow passage may extend in a gap between the inner surface of the housing and the stator winding. Advantageously, where the motor includes both the first air flow passage and the second air flow passage, different portions of the stator winding may be cooled by air flowing along the two air flow passages. In particular, the first air flow passage passes through part of the bobbin structure, and so may act to cool an inward-facing portion of the stator winding. On the other hand, the second air flow passage is defined between the housing and the stator winding, such that the second air flow passage may act to cool an outward facing portion of the stator winding. Thus, the combination of the first and second air flow passages may enable efficient cooling across a majority of the stator winding. A distance between the inner surface of the housing and the stator winding may vary along a length of the second air flow passage to create a constriction in the second air flow passage. Thus, in line with the discussion above, the constriction may locally accelerate air flowing along the second air flow passage in the region where air flows over the stator winding. As an example, the inner surface of the housing may comprise a portion that protrudes towards the stator winding, so as to reduce the distance between the inner surface of the housing and the stator winding and thereby create a constriction in the second air flow passage. The second air flow passage may include an upstream portion, a downstream portion, and an intermediate portion located between the upstream portion and the downstream portion, where the distance between the inner surface of the housing and the stator winding is smaller in the intermediate portion relative to the upstream portion and the downstream portion. In line with the discussion above, this enables the air flow to be locally accelerated in the intermediate portion, whilst reducing a pressure drop in the air flow output from the motor. The distance between the inner surface of the housing and the stator winding may vary gradually along the length of the second air flow passage, e.g. so that there are no sharp edges or comers at interfaces between the upstream portion and the intermediate portion, and between the intermediate portion and the downstream portion. According to a second aspect, there is provided a stator sub-assembly for a motor, the stator sub-assembly comprising: a stator winding; and a bobbin structure on which the stator winding is supported, wherein the bobbin structure comprises a first support member and a second support member arranged on either side of the winding; wherein an air flow passage is defined between the first and second support members, the air flow passage being arranged to guide an air flow over the stator winding; and wherein the air flow passage is configured to locally accelerate the air flow in a region where the air flow passes over the stator winding. The stator sub-assembly of the second aspect may form part of the motor of the first aspect described above. Accordingly, any features described in relation to the motor of the first aspect may be shared with the stator sub-assembly of the second aspect. According to a third aspect, there is provided an appliance comprising a motor according to the first aspect. For example, the appliance may comprise a vacuum cleaner, or a haircare appliance such as a hair dryer or hair styler. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic sectional view of a motor according to an embodiment; Figure 2 shows a schematic perspective view of a stator assembly of the motor of Figure 1; Figure 3 shows a schematic front view of a stator sub-assembly of the motor of Figure 1; Figure 4 shows a schematic cross-sectional view of a bobbin structure of the stator subassembly of Figure 3; Figure 5 shows a schematic perspective view of a variation of the stator sub-assembly of Figure 3; Figure 6 shows a schematic expanded sectional view of the motor of Figure 1; and Figure 7 shows a schematic diagram of an appliance according to an embodiment. DETAILED DESCRIPTION Fig. 1 shows a schematic sectional view of a motor 10. The motor 10 comprises a stator assembly 12, a rotor 14 which is rotatably mounted relative to the stator assembly 12, and an impeller 16 connected to the rotor 14. The sectional view of Fig. 1 corresponds to a section taken through the motor 10 along an axis of rotation of the rotor 14. A perspective view of stator assembly 12 and rotor 14 is shown in Fig. 2. In the example shown, the stator assembly 12 includes three stator sub-assemblies 18a, 18b, 18c which are mounted in a frame 20 of the stator assembly 12, such that they are regularly spaced about the axis of rotation of the rotor 14. Of course, other examples may include different numbers of stator sub-assemblies. The frame 20 provides a structure which holds the stator subassemblies 18a, 18b, 18c in place around the rotor 14. The frame 20 may be connected to an outer housing 62 of the motor 10, so as to secure the stator assembly 12 in the housing 62. Each stator sub-assembly 18a, 18b, 18c includes a respective stator winding, through which a current can be passed to generate a magnetic field. In use, current is passed through the stator winding of the stator sub-assemblies 18a, 18b, 18c, to generate a timevarying magnetic field which interacts with a permanent magnet 22 on the rotor 14, and which causes rotation of the rotor 14 and hence the impeller 16. The impeller 16 is configured to generate an air flow when it rotates, the air flow being generally along the axis of rotation of the rotor 14. For example, the impeller 16 may comprise a fan or the like. An individual stator sub-assembly 18 of the motor 10 is described in relation to Figs. 3-5, each of the stator sub-assemblies 18a, 18b, 18c having substantially the same structure. Fig. 3 shows a schematic front view of the stator sub-assembly 18. The stator sub-assembly 18 includes a bobbin structure 24 around which a stator winding 26 is wound. The stator winding 26 comprises an insulated wire, for example insulated copper wire. The bobbin structure 24 comprises a first support member 28 and a second support member 30, which are arranged on either side of the stator winding 26. The bobbin structure 24 further includes a bridging portion 31 (shown in Fig. 6) which extends between (i.e. connects) the first support member 28 and the second support member 30, with the stator winding 26 being wound around the bridging portion of the support structure 24. A stator core 33 is integrated into bobbin structure 24, such that the stator winding 26 is wound around at least part of the stator core 33. In particular, the stator core 33 extends within the bridging portion 31 of the bobbin structure 24, such that the stator winding 26 is wound around the stator core 33. Additionally, the stator core 33 includes first and second side portions 33a, 33b which extend along an outside of the first and second support members 28, 30, respectively. The bobbin structure 24 may be fitted to the stator core 33, e.g. via an overmoulding process. Ends of the first and second side portions 33a, 33b of the stator core 33 are connectable to the frame 20, to mount the bobbin structure 24 to the frame 20 of the stator assembly 12. In the example shown, the first and second side portions 33a, 33b include connecting portions 32, 34 (e.g. tooth tips) at their ends which are engageable with corresponding connection portions on the frame 20. In other examples, the bobbin structure 24 may be formed as part of the frame 20. The first and second support members 28, 30 act as support legs, which hold the stator winding 26 such that it is spaced away from the frame 20. The first and second support members 28, 30 further act as a guide structure for guiding the air flow generated by the impeller 16 over the stator winding 26, to cool the stator winding 26. In more detail, a first air flow passage 36 is defined between the stator winding 26 and the rotor 14 (e g. rotor magnet 22), in a gap between the first and second support members 28, 30. In this manner, the first and second support members 28, 30 define sidewalls of the air flow passage 36. As can be seen in Fig. 3, a surface of the stator winding 26 is exposed to the air flow passage 36, such that air flowing along the air flow passage 36 can come into contact with the stator winding 26 and remove heat from the stator winding 26. Thus, the bobbin structure 26 provides the dual functions of supporting the stator winding 26 and defining the first air flow passage 36 for cooling the stator winding 26. Furthermore, the first air flow passage 36 facilitates winding the stator winding 26 around the stator core. The first support member 28 and the second support member 30 are shaped so that the air flow generated by the impeller 16 is locally accelerated within the first air flow passage 36 where the air flow passes over the stator winding 26. Thus, the air flow generated by the impeller 16 is received in the first air flow passage 36, where it is locally accelerated. Fig. 4 shows a cross-sectional view through the bobbin structure 24 taken along a plane A-A indicated in Fig. 3. The arrows 38 in Fig. 4 indicate a direction of the air flow from the impeller 16 through the first air flow passage 36. As can be seen, the first and second support members 28, 30 are shaped so as to form a constriction in the first air flow passage 36 in the region where the air flow passes over the stator winding 26. The constriction is in the form of a narrowing of the air flow passage towards a mid-section of the first air flow passage 36, which causes the air flow to be accelerated. In more detail, the first air flow passage 36 includes an upstream portion 40 which acts an inlet of the first air flow passage 36, a downstream portion 42 which acts as an outlet of the first air flow passage 36, and an intermediate portion 44 located between the upstream and downstream portions 40, 42. A cross-sectional area of the first air flow passage 36 in a direction normal to the air flow is reduced in the intermediate portion 44 relative to the upstream portion 40 and the downstream portion 42. This is achieved by shaping the first and second support members 28, 30 such that a width of the gap between the first and second support members 28, 30 decreases from the upstream portion 40 to the intermediate portion 44, and then increases from the intermediate portion 44 to the downstream portion 42. Thus, in the direction of air flow, the cross-sectional area of the first air flow passage 36 reduces from the upstream portion 40 to the intermediate portion 44, and then increases again from the intermediate portion 44 to the downstream portion 42. Accordingly, the intermediate portion 44 with its reduced cross-sectional area (or width) acts as a constriction in the first air flow passage 36, which accelerates air flowing along the first air flow passage 36. In this manner, the first air flow passage 36 acts as a Venturi passage for the air flow. The relatively narrow intermediate portion 44 of the first air flow passage 36 relative to the upstream portion 40 results in a higher speed and lower pressure of the air flow in the intermediate portion 44 compared to the upstream portion 42. As the first air flow passage 36 widens again following the intermediate portion 44, the pressure of the air flow increases in the downstream portion 42 relative to the intermediate portion 44. Thus, the pressure drop associated with acceleration of the air flow can be localised to the intermediate portion 44, enabling a relatively high-pressure air flow to be output from the downstream portion 42 of the first air flow passage 36. As can be seen in Figs. 3 and 4, the cross-sectional area (or width) of the first air flow passage 36 reduces gradually (i.e. smoothly, progressively) from the upstream portion 40 to the intermediate portion 44. Likewise, the cross-sectional area (or width) of the first air flow passage 36 increases gradually (i.e. smoothly, progressively) from the intermediate portion 44 to the downstream portion 42. For example, the first and second support members 28, 30 can be shaped so that sidewalls of the first air flow passage 36 at an interface between the upstream portion 40 and the intermediate portion 44 are curved, so as to provide the gradual narrowing of the air flow passage 36. Similarly, the first and second support members 28, 30 can be shaped so that sidewalls of the first air flow passage 36 at an interface between the intermediate portion 44 and the downstream portion 42 are curved, so as to provide the gradual widening of the air flow passage 36. The sidewalls of the first air flow passage 36 defined by the first and second support members 28, 30 may be substantially smooth (e.g. without sharp corners or edges), so as to promote laminar flow and optimise heat exchange with the stator winding 26. Furthermore, upstream ends 46, 48 of the first and second support members 28, 30 can be rounded, so as to minimise turbulence of the air flow entering the air flow passage 36. Additionally or alternatively, downstream ends 50, 52 of the first and second support members 28, 30 can be rounded, so as to minimise turbulence of the air flow exiting the air flow passage 36. Fig. 5 shows a schematic perspective view of a modified version of the stator subassembly 18 described above. The stator sub-assemblies 18a, 18b, 18c may correspond to the stator sub-assembly shown in Fig. 5. In addition to all of the features of stator subassembly 18 described above in relation to Figs. 3 and 4, the stator sub-assembly 18 of Fig. 5 includes a pair of turbulators 54, 56, arranged in sidewalls of the first air flow passage 36. In more detail, the first turbulator 54 is arranged on an inner surface of the first support member 28 defining a first sidewall of the first air flow passage 36, and a second turbulator 56 is arranged on an inner surface of the second support member 30 defining a second, opposite sidewall of the first air flow passage 36. The turbulators 54, 56 are arranged at or near an interface between the upstream portion 40 and the intermediate portion 44 of the first air flow passage 36. The turbulators 54, 56 are arranged to generate turbulence in the air flow as it enters the intermediate portion 44 of the first air flow passage 36, which may enhance heat exchange between the stator winding 26 and the air flow. As the turbulators 54, 56 are located near the upstream portion 40 of the first air flow passage 36, turbulence generated by the turbulators 54, 56 may be reduced by the time the air flow reaches the downstream portion 42, such that a relatively laminar air flow may be output from the first air flow passage 36. The turbulators 54, 56 may take various forms. In the example shown, the turbulators 54, 56 are in the form of cavities (or indents) in the inner surfaces of the first and second support members 28, 30, respectively. In other examples, the turbulators 54, 56 may be implemented as protrusions which protrude from the inner surfaces of the first and second support members 28, 30, respectively. Fig. 6 shows an expanded sectional view of the motor 10, depicting a region around the stator sub-assembly 18a. The features described below in relation to Fig. 6 apply equally to the other stator sub-assemblies 18b and 18c. The sectional view of Fig. 6 corresponds to the same section through the motor 10 as Fig. 1. As shown in Fig. 6, the motor 10 may further comprise a second air flow passage 58, which is defined between an inner surface 60 of the housing 62 of the motor 10, and a surface of the stator winding 26. In other words, the second air flow passage 58 extends along a gap between the inner surface 60 of the housing 62 and the surface of the stator winding 26. The housing 60 is arranged to surround the stator assembly 12 and the impeller 16, such that the air flow generated by the impeller 16 is channelled within the housing 60. For illustration purposes, only a portion of the housing 60 is depicted in Figs. 1 and 6. The arrow 64 in Fig. 6 indicates a direction of air flow along the second air flow passage 58. As can be seen, the inner surface 60 of the housing 62 acts as a guide structure which guides the air flow over a surface of the stator winding 26. The inner surface 60 of the housing 62 includes a curved portion 66 which substantially follows a curvature of the stator winding 26, so as to guide the air flow in the second air flow passage 58 along the surface of the stator winding 26. A distance between the inner surface 60 and the stator winding 26 varies along a length of the second air flow passage 58, so as to create a constriction in the second air flow passage 58, in order to locally accelerate the air flow in the second air flow passage 58. In particular, the second air flow passage 58 includes an upstream portion, a downstream portion, and an intermediate portion located between the upstream and downstream portions, where the distance between the inner surface 60 and the stator winding 26 is smaller in the intermediate portion relative to the upstream and downstream portions. Thus, in the direction of air flow, the second air flow passage 58 gradually (progressively) narrows from the upstream portion to the intermediate portion, following which the second air flow passage 58 gradually widens from the intermediate portion to the downstream portion. In this manner, similarly to the first air flow passage 36 described above, the second air flow passage 58 acts as a Venturi passage which locally accelerates the air flow in the intermediate portion as it passes over the surface of the stator winding 26, to promote cooling of the stator winding 26. Accordingly, as with the first air flow passage 36 described above, the second air flow passage 58 serves to cool the stator winding 26 whilst reducing a pressure loss between the upstream and downstream portions of the second air flow passage 58, and allowing a relatively laminar flow to be output from the second air flow passage 58. Arrow 38 in Fig. 6 further indicates the direction of air flow along the first air flow passage 36. As can be seen the first air flow passage 36 and the second air flow passage 58 pass over different portions of the stator winding 26, so as to remove heat from different portions from the stator winding 26. In particular, air flowing along the first air flow passage 36 passes over a first, inner portion of the stator winding 26 which is located nearer the frame 20, whilst air flowing along the second air flow passage 58 passes over a second, outer portion of the stator winding 26 which is located nearer the housing 62. This arrangement promotes effective heat removal around the whole stator winding 26. In the motor 10, the impeller 16 is located downstream of the stator assembly 12. Thus, the air flow generated by the impeller 16 draws air through the stator assembly 12, in order to cool the stator assembly 12. In other examples, the impeller 16 may instead be located upstream of the stator assembly 12. When the motor 10 is activated by energising the stator windings 26 in the stator assembly 12, the rotor 14 and the impeller 16 rotate together, generating an air flow within the housing 62. In line with the above discussion, portions of the generated air flow pass through the first air flow passage 36 and the second air flow passage 58 associated with each respective stator sub-assembly 18a, 18b, 18c. Thus, each stator sub-assembly is cooled by air flowing along its corresponding first air flow passage 36 and second air flow passage 58. As the air flow generated by the impeller 16 is locally accelerated in the first and second air flow passages 36, 58, this promotes efficient cooling of the stator windings 26 in each stator sub-assembly, whilst reducing a pressure loss across the stator assembly 12, and allowing a relatively laminar air flow to be output from the stator assembly 12. For example, the arrangement of the first and second air flow passages 36, 58 described above may result in a relatively low pressure drop of the air flow across the stator assembly 12, whilst providing enhanced cooling the stator assembly 12. Additionally, the stator assembly 12 may be arranged such that only relatively small portions of the total air flow generated by the impeller 16 pass through the first and second air flow passages 36, 58 of the stator sub-assemblies 18a, 18b, 18c. For example, the stator assembly 12 may be arranged such that a majority of the air flow generated by the impeller passes through openings 68a, 68b, 68c between adjacent stator sub-assemblies (see Fig. 2). This provides effective cooling of the stator windings 26 whilst reducing the pressure drop across the stator assembly 12. Fig. 7 shows a schematic diagram of an appliance in the form of a vacuum cleaner 70. The vacuum cleaner 70 includes a housing 72 in which the motor 10 is mounted, the motor 10 being arranged to provide suction to a cleaner head 74, to draw in dust and particles into the vacuum cleaner 70 via the cleaner head 74. Of course, the motor 10 may be used in other types of appliances.

Claims

1. A motor compri sing:a stator assembly comprising a stator winding;a rotor;an impeller connected to the rotor and configured to generate an air flow when the rotor rotates relative to the stator assembly; anda guide structure adjacent to the stator winding and arranged to guide the air flow over the stator winding;wherein the guide structure is configured to locally accelerate the air flow in a region where the air flow passes over the stator winding.

2. A motor according to claim 1, wherein the guide structure defines an air flow passage comprising a constriction in the region where the air flow passes over the stator winding to locally accelerate the air flow.

3. A motor according to claim 2, wherein the constriction comprises an intermediate portion of the air flow passage located between an upstream portion and a downstream portion of the air flow passage, and wherein a cross-sectional area of the air flow passage in a direction normal to the air flow is reduced in the intermediate portion relative to the upstream portion and the downstream portion.

4. A motor according to claim 3, wherein the cross-sectional area of the air flow passage reduces gradually from the upstream portion to the intermediate portion, and increases gradually from the intermediate portion to the downstream portion.

5. A motor according to any preceding claim, wherein the guide structure comprises a turbulator arranged to generate turbulence in the region where the air flow passes over the stator winding.

6. A motor according to any preceding claim, wherein the guide structure defines an air flow passage comprising a Venturi passage configured to locally accelerate the air flow in the region where the air flow passes over the stator winding.

7. A motor according to any preceding claim, wherein the stator assembly comprises a bobbin structure on which the winding is supported, and wherein the guide structure comprises a portion of the bobbin structure which defines a first air flow passage that extends through part of the bobbin structure.

8. A motor according to claim 7, wherein the guide structure comprises a first support member and a second support member of the bobbin structure, the first support member and the second support member being arranged on either side of the winding, and wherein the first air flow passage extends through a gap between the first and second support members.

9. A motor according to claim 8, wherein a width of the gap varies along a length of the first air flow passage to create a constriction in the first air flow passage.

10. A motor according to claim 9, wherein the width of the gap decreases from an upstream end of the bobbin structure towards an intermediate portion of the bobbin structure, and increases from the intermediate portion towards a downstream end of the bobbin structure.

11. A motor according to any of claims 8 to 10, wherein edges of the first and second support members at an upstream end of the bobbin structure are rounded, and / or edges of the first and second support members at a downstream end of the bobbin structure are rounded.

12. A motor according to any preceding claim, further comprising a housing disposed around the stator assembly, wherein the guide structure comprises an inner surface of the housing that defines a sidewall of a second air flow passage that passes over part of the stator winding.

13. A motor according to claim 12, wherein a distance between the inner surface of the housing and the stator winding varies along a length of the second air flow passage to create a constriction in the second air flow passage.

14. A motor according to claim 13, wherein the second air flow passage includes an upstream portion, a downstream portion, and an intermediate portion located between the upstream portion and the downstream portion, and wherein the distance between the inner surface of the housing and the stator winding is smaller in the intermediate portion relative to the upstream portion and the downstream portion.

15. A stator sub-assembly for a motor, the stator sub-assembly comprising: a stator winding; anda bobbin structure on which the stator winding is supported, wherein the bobbin structure comprises a first support member and a second support member arranged on either side of the stator winding;wherein an air flow passage is defined between the first and second support members, and is arranged to guide an air flow over the stator winding; andwherein the air flow passage is configured to locally accelerate the air flow in a region where the air flow passes over the stator winding.

16. An appliance comprising a motor according to one of claims 1 to 14.

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