A stator assembly for an electric motor

Abstract

A stator assembly 10 for an electric motor includes a plurality of stator core segments 16. At least one of the stator core segments comprises a back, a tooth extending from the back, and a fin extend

Description

B ACKGROUND There is a general desire to improve electric machines, such as electric motors, in a number of ways. For example, improvements may be desired in terms of size, weight, power density, manufacturing cost, efficiency, reliability, and noise. SUMMARY According to a first aspect there is provided a stator assembly for an electric motor, the stator assembly comprising a plurality of stator core segments, wherein at least one of the stator core segments comprises a back, a tooth extending from the back, and a fin extending outwardly relative to at least one of the back and the tooth, wherein the fin comprises a groove formed therein, and the stator core segments are overmoulded by an overmoulding material such that the groove is exposed through the overmoulding material. By providing a segmented stator assembly, comprising a plurality of discrete stator core segments, winding of a coil of the stator assembly around the individual stator core segments may be more easily achieved in comparison to winding of a full annular stator core. This may allow for improved fill factor associated with the coil, which can enable increased power density to be achieved. When electric motors operate at higher powers, and higher speeds, higher stator losses can occur. Higher stator losses can result in increased temperatures of the stator assembly, and / or a rotor assembly of the electric motor, which can have negative implications for mechanical components of the electric motor, for example leading to component degradation and / or failure. By providing the fin comprising the groove, and the stator core segments being overmoulded such that the groove is exposed through the overmoulding material, the groove can be available to act as a heat sink, for example with the groove being exposed to airflow through an electric motor comprising the stator assembly in use. Use of a groove formed in the fin can provide an increased surface area for the fin, which may lead to an increased level of heat exchange between the stator core segment and an airflow flowing over the stator assembly in use. The stator core segments may be overmoulded by the overmoulding material such that the back of the at least one of the stator core segments is covered by the overmoulding material. The stator core segments may be overmoulded by the overmoulding material such that a portion of the tooth of the at least one of the stator core segments is covered by the overmoulding material. The stator core segments may be overmoulded by the overmoulding material such that a pole face of the tooth of the at least one of the stator core segments is exposed through the overmoulding material. The stator core segments may be arranged in an annular array. The fin may extend circumferentially outwardly relative to the back and the tooth. The tooth may project in a first direction away from the back, and the fin may extend in a second direction, different to the first direction, away from the back. The fin may comprise a radially outer surface, and the grove may be formed in the radially outer surface. The stator core segments may be overmoulded by the overmoulding material such that the radially outer surface of the fin of the at least one of the stator core segments is exposed through the overmoulding material. The pole face of the tooth may face generally radially inwardly. The groove may extend along a full length of the fin. This may provide improved heat exchange properties in use relative to an arrangement in which the groove extends only partially along the length of the fin. The fin may extend along a full length of the at least one of the stator core segments. The groove may have a width of at least 0.3mm. This may inhibit dust or debris entrained in airflow flowing over the stator assembly, when incorporated into an electric motor in use, from becoming stuck in the groove. The groove may have a width of no more than 1.5mm. The groove may have a width of around 0.3mm to 0.7mm, for example around 0.5mm. The groove may have a depth of at least 0.3mm. This may inhibit dust or debris entrained in airflow flowing over the stator assembly, when incorporated into an electric motor in use, from becoming stuck in the groove. The groove may have a depth of no more than 1mm. The groove may have a depth of around 0.3mm to 0.9mm, for example around 0.5mm. The groove may have a length of at least 0.16mm. The groove may have a length of no more than 10.6mm. The groove may have a length of around 5.3mm to 10.6mm, for example around 10.6mm. The groove may be disposed on a surface of the fin that faces in an opposite direction to a pole tip of the tooth. This may position the groove at a location which is more likely to be exposed to a primary airflow through an electric motor that incorporates the stator assembly in use. The groove may be formed on the radially outer surface of the fin. The pole tip may be formed at a radially inner end of the tooth. The surface of the fin, for example the radially outer surface of the fin, may be aligned, for example radially aligned, with a rear surface of the back of the at least one of the stator core segments. This may inhibit disruption of airflow over the stator assembly when the stator assembly is incorporated into an electric motor in use, for example in comparison to an arrangement where there is a step change between the back and the fin. The fin may comprise a plurality of grooves formed therein, and the grooves are positioned such that airflow through the electric motor in use flows over the plurality of grooves. Use of a plurality of grooves may increase a surface area of the fin, which may result in improved thermal transfer properties relative to arrangement with only one, or no, groove(s). The fin may comprise at least three grooves. The fin may comprise four grooves. The grooves may each extend along the full length of the fin. The grooves may be evenly spaced along the fin. This may provide for relatively even distribution of heat transfer along the fin. The grooves may extend across substantially an entire length of the fin. Adjacent ones of the grooves may be spaced by at least 0.35mm. This may provide mechanical integrity for wall sections between the grooves. The grooves may each have a same depth and / or width. This may provide for relatively even distribution of heat transfer between the grooves. The at least one of the stator core segments may comprise a further tooth, and a further fin extending outwardly relative to at least one of the back and the further tooth. The further fin may comprise a further groove formed therein, and the stator core segments may be overmoulded by the overmoulding material such that the further groove is exposed through the overmoulding material. Provision of a further fin and further groove in such a manner may provide for improved thermal transfer relative to an arrangement with only one, or no, fin and groove arrangement(s). The fin and the further fin may extend in opposite directions away from the back, for example with the fin and the further fin being located at opposite sides of the back. The fin and the further fin may have substantially the same form. The further fin may comprise a radially outer surface, and the further grove may be formed in the radially outer surface. The stator core segments may be overmoulded by the overmoulding material such that the radially outer surface of the further fin of the at least one of the stator core segments is exposed through the overmoulding material. The further groove may extend along a full length of the further fin. The further fin may extend along a full length of the at least one of the stator core segments. The further groove may have a width of at least 0.3mm. The further groove may have a width of no more than 1.5mm. The further groove may have a width of around 0.3mm to 0.7mm, for example around 0.5mm. The further groove may have a depth of at least 0.3mm. The further groove may have a depth of no more than 1mm. The further groove may have a depth of around 0.3mm to 0.9mm, for example around 0.5mm. The further groove may have a length of at least 0.16mm. The further groove may have a length of no more than 10.6mm. The further groove may have a length of around 5.3mm to 10.6mm, for example around 10.6mm. The further groove may be disposed on a surface of the further in that faces in an opposite direction to a pole tip of the further tooth. The further groove may be formed on the radially outer surface of the further fin. The pole tip may be formed at a radially inner end of the further tooth. The surface of the further fin, for example the radially outer surface of the further fin, may be aligned, for example radially aligned, with a rear surface of the back of the at least one of the stator core segments. The further fin may comprise a plurality of further grooves formed therein, and the stator core segments are overmoulded by the overmoulding material such that the plurality of further grooves is exposed through the overmoulding material. Use of a plurality of further grooves may increase a surface area of the further fin, which may result in improved thermal transfer properties relative to arrangement with only one, or no, further groove(s). The further fin may comprise at least three further grooves. The further fin may comprise four further grooves. The further grooves may each extend along the full length of the further fin. The further grooves may be evenly spaced along the further fin. The further grooves may extend across substantially an entire length of the further fin. Adjacent ones of the further grooves may be spaced by at least 0.35mm. The further grooves may each have a same depth and / or width. The at least one of the stator core segments may formed of a plurality of stamped laminations, each of the stamped laminations may comprise a sub-groove, and the subgrooves may collectively form the groove, wherein each sub-groove may be formed as part of a stamping process that forms the respective stamped lamination. Forming the subgrooves as part of the stamping process may reduce the need for further postmanufacturing processes, such as etching or laser-cutting, which may reduce complexity and / or cost of manufacture. Each of the plurality of stator core segments may comprise a back, a tooth extending from the back, and a fin extending outwardly relative to at least one of the back and the tooth, wherein the fin comprises a groove formed therein, and the stator core segments may be overmoulded by the overmoulding material such that the respective grooves are exposed through the overmoulding material. This may provide for improved thermal redistribution relative to an arrangement in which only one of the plurality of stator core segments has an exposed groove. Each of the plurality of stator core segments may comprise substantially the same form. Fins of adjacent ones of the stator core segments may be circumferentially spaced from one another. This may inhibit formation of magnetic circuits between adjacent stator core segments in use. Fins of adjacent ones of the stator core segments may be circumferentially spaced from one another by at least 3mm. The stator assembly may comprise a plurality of stator core sub-assemblies, each comprising a respective one of the plurality of stator core segments and a bobbin. The bobbins may each comprise respective connection formations for connecting to adjacent bobbins. The stator core segments may be overmoulded by the overmoulding material such that the overmoulding material defines the bobbins. Each of the stator core segments may be separately overmoulded by the overmoulding material. The plurality of stator core sub-assemblies may be overmoulded by the overmoulding material to form the stator assembly. The bobbins may be overmoulded onto the stator core segments, for example as part of a separate overmoulding process to that in which the plurality of stator core sub-assemblies are overmoulded by the overmoulding material. According to a second aspect there is provided a stator core segment for a stator assembly as claimed in any one of the preceding claims. According to a third aspect there is provided an electric motor comprising a stator assembly according to the first aspect, or a stator core segment according to the second aspect, a rotor assembly, and an impeller mounted to the rotor assembly, wherein the stator assembly is configured to drive rotation of the rotor assembly such that the impeller generates an airflow through the electric motor in use, and the groove is positioned such that the airflow flows over the groove. The rotor assembly may comprise a shaft having a rotational axis. The groove may face radially outwardly away from the rotational axis. The fin may extend circumferentially about the rotational axis. According to a fourth aspect there is provided an appliance comprising an electric motor according to the third aspect, a stator assembly according to the first aspect, or a stator core segment according to the first aspect. The appliance may be any one of a vacuum cleaner and a haircare appliance. Optional features of aspects may be equally applied to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa schematic cross-sectional view through a stator assembly; Figure 2 is a top plan schematic view of a stator core segment of the stator assembly of Figure 1; Figure 3 is a perspective schematic view of the stator core segment of Figure 2; Figure 4 is a schematic sectional view through an electric motor comprising the stator assembly of Figure 1; Figure 5 is a schematic perspective view of the electric motor of Figure 4; Figure 6 is a schematic view of a vacuum cleaner comprising the electric motor of Figure 4; and Figure 7 is a schematic view of a haircare appliance comprising the electric motor of Figure 4. DETAILED DESCRIPTION A stator assembly 10 is illustrated in Figure 1. The stator assembly 10 has three stator core sub-assemblies 14, each of which is identical in form. The stator core sub-assemblies 14 are arranged in a generally annular array to define a channel 12. Each stator core sub-assembly 14 has a stator core segment 16, a bobbin 18, and a winding 20. An individual stator core segment 16 is illustrated in Figures 2 and 3. The stator core segment 16 has a back 22, first 24 and second 26 arms, and first 28 and second 30 fins. The back 22 and the first 24 and second 26 arms give a generally c-shaped form, and the stator core segment 16 may be referred to as a c-core. The first 24 and second 26 arms each have a respective first portion 32,34 and a respective second portion 36,38. Each first portion 32,34 extends substantially orthogonally from the back 22, and each second portion 36,38 is angled at around 33 degrees relative to the respective first portion 32,34. The second portions 36,38 are angled inwardly toward one another, and collectively the back 22 and the first 24 and second 26 arms define a winding channel 40 within which the respective winding 24 is located. The second portions 36,38 have respective pole faces 42,44 disposed at ends of the respective second portions 36,38 distal from the back 22, with the pole faces 42,44 extending to either side of the respective second portions 36,38. The pole faces 42,44 are spaced apart from one another to define a slot gap 46, with the slot gap 46 defining a point of entry into the winding channel 40. The pole faces 42,44 are asymmetric to provide saliency, and are curved with each pole face 42,44 having a different center of curvature. The first fin 28 extends generally outwardly from the back 22 at a region where the first arm 24 extends from the back 22, such that the first fin 28 extends in a direction away from the first arm 24. The first fin 28 is generally curved in form, such that when the stator core segment 16 is in place in the stator assembly 10 the first fin 28 extends in a generally circumferential direction. The first fin 28 extends outwardly from the back 22 by a distance of around 5mm. An outer surface of the first fin 28, that faces in an opposite direction to the pole face 42 of the first arm 24, has four grooves 48 formed therein. The grooves 48 are evenly spaced along the length of the first fin 28, with a spacing of around 1mm between centrelines of adjacent grooves 48. Spacing of at least 0.35mm is envisaged, such that walls between adjacent grooves 48 provide sufficient mechanical integrity. Each groove 48 extends fully along a length of the first fin 28, with each groove 48 having a length of around 10.6mm. Each groove 48 has a width, measured in a direction along the length of the first fin 28, of around 0.3mm, and a depth, measured in a transverse direction across the first fin 28, of around 0.35mm. Widths of around 0.3mm to 0.7mm, and depths of around 0.3mm to 0.9mm, are also envisaged. Each groove 48 comprises generally curved edges, such that there are no sharp edges defined by the groove 48. The second fin 30 extends generally outwardly from the back 22 at a region where the second arm 26 extends from the back 22, such that the second fin 30 extends in a direction away from the second arm 26. The second fin 30 is generally curved in form, such that when the stator core segment 16 is in place in the stator assembly 10 the second fin 30 extends in a generally circumferential direction. The second fin 30 extends outwardly from the back 22 by a distance of around 5mm. An outer surface of the second fin 30, that faces in an opposite direction to the pole face 44 of the second arm 26, has four further grooves 50 formed therein. The further grooves 50 are evenly spaced along the length of the second fin 30, with a spacing of around 1mm between centrelines of adjacent further grooves 50. Spacing of at least 0.35mm is envisaged, such that walls between adjacent further grooves 50 provide sufficient mechanical integrity. Each further groove 50 extends fully along a length of the second fin 30, with each further groove 50 having a length of around 10.6mm. Each further groove 50 has a width, measured in a direction along the length of the second fin 30, of around 0.3mm, and a depth, measured in a transverse direction across the second fin 30, of around 0.35mm. Widths of around 0.3mm to 0.7mm, and depths of around 0.3mm to 0.9mm, are also envisaged. Each further groove 50 comprises generally curved edges, such that there are no sharp edges defined by the further groove 50. An inner surface of the second fin 30, that faces in the same general direction as the pole face 44 of the second arm 26, has a notch 52 formed therein. The notch 52 extends fully along a length of the second fin 30, and defines a welding point for the stator core segment 16. In particular, the stator core segment 16 is formed of a plurality of laminations, each having the form previously described, with the laminations welded together along the notch 52. The grooves 48, further grooves 50, and notch 52 are defined as part of the shape of each lamination during stamping of the lamination, and each lamination can be considered to comprise sub-grooves, sub-further grooves, and sub-notches, that collectively define the respective grooves 48, further grooves 50, and notch 52. Each bobbin 18 is formed from an overmoulding material that is overmoulded onto the respective stator core segment 16. Examples of suitable overmoulding materials include fibre reinforced plastic materials, such as PBT GF30. The bobbin 18 is overmoulded to the stator core segment 16 such that the bobbin 18 overlies inner and outer surfaces of the back 22, inner and outer surfaces of the first 24, and second 26 arms, and inner surfaces of the first 28 and second 30 fins. The bobbin 18 thereby lines the winding channel 40, and allows the winding 20 to be wound about the back 22 of the stator core segment 16. The bobbin 18 is overmoulded to the stator core segment 16 such that outer surfaces of the first 28 and second 30 fins, and hence the grooves 48 and the further grooves 50, are exposed through the overmoulding material. A first end 54 of the bobbin 18 has a connecting projection 56, and a second end 58 of the bobbin 18 has a connecting recess 60. The connecting projection 56 and the connecting recess 60 are correspondingly shaped, such that adjacent bobbins 18 within the stator assembly 10 are engaged with one another by virtue of the connecting projection 56 of one bobbin 18 being received within the connecting recess 60 of an adjacent bobbin 18. Given the form of the bobbins 18, fins 28,30 of adjacent ones of the stator core segments 16 are spaced from one another in the stator assembly 10, by around 5mm. When the stator core sub-assemblies 16 are connected by virtue of the bobbins 18, the outer surfaces of the first 28 and second 30 fins that have the grooves 48 and the further grooves 50 respectively formed therein are located at a same radial distance from a central axis of the channel, and generally at a same radial distance as the backs 22 of the stator core segments 16. The grooves 48 and the further grooves 50 can be considered to face generally radially outwardly, whilst the pole faces 42,44 of the first 24 and second 26 arms can be considered to face generally radially inwardly. An electric motor 100 incorporating the stator assembly 10 is illustrated schematically in Figures 4 and 5. The electric motor has a rotor assembly 102, an impeller 104, and a diffuser assembly 106. The rotor assembly 102 includes a shaft 108 and a permanent magnet 110 attached to the shaft 108. The rotor assembly 102 is positioned relative to the stator assembly 10 such that the shaft 108 and the permanent magnet 110 sit inside the channel 12 of the stator assembly 10, with the permanent magnet 110 generally axially aligned with the stator core segments 16 of the stator core sub-assemblies 14. The impeller 104 is mounted to the shaft 108 upstream of the stator assembly 10. The diffuser assembly 106 has an outer wall 112, and nine diffuser vanes 114 protruding inwardly from the outer wall 112. The outer wall 112 can define a housing of the electric motor 100. Although not shown in the figures, in some examples the diffuser assembly 106 also has an inner wall, with the diffuser vanes 114 extending between the inner wall and the outer wall 112. The inner wall can be such that the inner wall does not overlie the grooves 48 and the further grooves 50 of the stator core sub-assemblies 14. In some examples the diffuser assembly 106 can be overmoulded onto the stator assembly 10, whilst still leaving the grooves 48 and further grooves 50 exposed through any overmoulding material. The spacing between the diffuser vanes 114 defines an airflow path 116 through the electric motor 100, and the stator assembly 10 is positioned relative to the diffuser assembly 10 such that the grooves 48 and the further grooves 50 of each of the stator core segments 16 are exposed to the airflow path 116. In use, a voltage is applied to the windings 20 such that the stator assembly 10 generates a magnetic field. The magnetic field interacts with the permanent magnet 110 of the rotor assembly 102, and with appropriate control of the voltage applied to the windings 20 to vary the magnetic field produced by the stator assembly 10, the permanent magnet 110, and hence the shaft 108, is caused to rotate within the stator assembly 10. Rotation of the shaft 108 causes rotation of the impeller 104, and rotation of the impeller 104 generates an airflow along the airflow path 116, and through the diffuser assembly 106. As airflow flows along the airflow path 116, airflow passes over the grooves 48 and the further grooves 50 of the first 28 and second 30 fins of the stator segments of the stator assembly 10. During operation of the electric motor 100, stator losses can result in an increased temperature of the stator assembly 10, which can limit a speed at which the rotor assembly 102 is driven. In particular, when electric motors operate at higher powers, and higher speeds, higher stator losses can occur. Higher stator losses can result in increased temperatures of a stator assembly, and / or a rotor assembly of the electric motor, which can have negative implications for mechanical components of the electric motor, for example leading to component degradation and / or failure. The speed at which the rotor assembly can be driven is therefore often limited by the temperature increase of components due to stator losses. In the present case, the grooves 48 and further grooves 50 of the first 28 and second 30 fins increase a surface area of the stator core segments that is exposed to the airflow through the electric motor 100 in use. This may lead to an increased level of heat exchange between the stator core segments 16 and an airflow flowing over the stator assembly 10 in use, resulting in a reduction in temperature increase relative to an arrangement in which the grooves 48 and further grooves 50 are not provided. This can enable the rotor assembly 102 to rotate at higher speeds, which can in turn have benefits for an appliance in which the electric motor 100 is incorporated, A vacuum cleaner 200 comprising the electric motor 100 is illustrated schematically in Figure 6, whilst a haircare appliance 300 comprising the electric motor 100 is illustrated schematically in Figure 6. Whilst described above with particular forms, it will be appreciated that variations of the stator core segment 16, the stator assembly 10, and the electric motor 100, are also envisaged. For example, greater or fewer numbers of the grooves 48 and or the further grooves 50 are envisaged. Arrangements in which only one of the first fin 28 and the second fin 30 are provided are envisaged. Arrangements in which at least some of the grooves 48 and / or the further grooves 50 have differing dimensions are envisaged. Arrangements in which only some of the stator core segments 16 are provided with the first 28 and / or second 30 fin are envisaged. Arrangements in which the impeller 104 is located downstream of the stator assembly 10, with airflow passing over the stator assembly 10 before being worked on by the impeller 104 are envisaged. This list of variants is non-exhaustive, and appropriate variants will be apparent to a person skilled in the art. In each case, at least one stator core segment is provided having a fin with a groove formed therein, with the stator core segment overmoulded by an overmoulding material such that the groove is exposed through the overmoulding material.

Claims

1. A stator assembly for an electric motor, the stator assembly comprising a plurality of stator core segments, wherein at least one of the stator core segments comprises a back, a tooth extending from the back, and a fin extending outwardly relative to at least one of the back and the tooth, wherein the fin comprises a groove formed therein, and the stator core segments are overmoulded by an overmoulding material such that the groove is exposed through the overmoulding material.

2. An electric motor as claimed in Claim 1, wherein the groove extends along a full length of the fin.

3. A stator assembly as claimed Claim 1 or Claim 2, wherein the groove has a width of at least 0.3mm.

4. A stator assembly as claimed in any one of the preceding claims, wherein the groove has a depth of at least 0.3mm.

5. A stator assembly as claimed in any one of the preceding claims, wherein the groove is disposed on a surface of the fin that faces in an opposite direction to a pole tip of the tooth.

6. A stator assembly as claimed in any one of the preceding claims, wherein the fin comprises a plurality of grooves formed therein, and the grooves are positioned such that airflow through the electric motor in use flows over the plurality of grooves.

7. A stator assembly as claimed in Claim 6, wherein the grooves are evenly spaced along the fin.

8. A stator assembly as claimed in Claim 6 or Claim 7, wherein adjacent ones of the grooves are spaced by at least 0.35mm.

9. A stator assembly as claimed in any one of Claims 6 to 8, wherein the grooves each have a same depth and / or width.

10. A stator assembly as claimed in any one of the preceding claims, wherein the at least one of the stator core segments comprises a further tooth, and a further fin extending outwardly relative to at least one of the back and the further tooth, the further fin comprises a further groove formed therein, and the stator core segments are overmoulded by the overmoulding material such that the further groove is exposed through the overmoulding material.

11. A stator assembly as claimed in Claim 10, wherein the further fin comprises a plurality of further grooves formed therein, and the stator core segments are overmoulded by the overmoulding material such that the plurality of further grooves is exposed through the overmoulding material.

12. A stator assembly as claimed in any one of the preceding claims, wherein the at least one of the stator core segments is formed of a plurality of stamped laminations, each of the stamped laminations comprising a sub-groove, and the sub-grooves collectively form the groove, wherein each sub-groove is formed as part of a stamping process that forms the respective stamped lamination.

13. A stator assembly as claimed in any one of the preceding claims, wherein each of the plurality of stator core segments comprises a back, a tooth extending from the back, and a fin extending outwardly relative to at least one of the back and the tooth, wherein the fin comprises a groove formed therein, and the stator core segments are overmoulded by the overmoulding material such that the respective grooves are exposed through the overmoulding material.

14. A stator assembly as claimed in Claim 13, wherein fins of adjacent ones of the stator core segments are circumferentially spaced from one another.

15. A stator core segment for a stator assembly as claimed in any one of the preceding claims.

16. An electric motor comprising a stator assembly as claimed in any one of Claims 1 5 to 14, or a stator core segment as claimed in Claim 15, a rotor assembly, and an impeller mounted to the rotor assembly, wherein the stator assembly is configured to drive rotation of the rotor assembly such that the impeller generates an airflow through the electric motor in use, and the groove is positioned such that the airflow flows over the groove.10 17. An appliance comprises an electric motor as claimed in Claim 16, a stator assemblyas claimed in any one of Claims 1 to 14, or a stator core segment as claimed in Claim 15.

18. An appliance as claimed in Claim 17, wherein the appliance is any one of a vacuum cleaner and a haircare appliance.17

Images