A stator assembly

Abstract

A stator core assembly (200) suitable for an electric motor (100, figure 2), has a first stator tooth (208) formed of non-grain-oriented electrical steel (NGOES); a second stator tooth (216) formed of

Description

Field of the Invention The present invention relates to a stator assembly for an electric motor. Background of the Invention 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 of the Invention According to a first aspect of the present invention there is provided a stator core assembly for an electric motor, the stator core assembly comprising: a first stator tooth formed of a first material; a second stator tooth formed of a second material different to the first material; and a coil located about the second stator tooth; wherein the first material is non-grain-oriented electrical steel, and the second material is grain-oriented electrical steel. Using grain-oriented electrical steel for the second stator tooth, about which the coil is located, may provide for improved motor efficiency for an electric motor comprising the stator core assembly in comparison to an arrangement where non-grain-oriented electrical steel is utilised for a stator tooth about which a coil is located. Motor efficiency as referred to herein can be a ratio of motor input power to motor shaft output power for an electric motor. Use of grain-oriented electrical steel for the second stator tooth, about which the coil is located, may enable the second stator tooth to be thinner in comparison to an arrangement where non-grain-oriented electrical steel is utilised for a stator tooth about which a coil is located. For example, as grain-oriented steel can handle a higher flux density before magnetic saturation, a narrower second tooth can be used relative to an arrangement where non-grain-oriented steel is used. Such a reduction in width of the second stator tooth may provide for increased area in which to locate the coil relative to a stator core assembly with a same structure but a greater width of the second stator tooth. Such an increased area may enable a greater volume of wire to be utilised, which may lead to an increase in motor efficiency. The first stator tooth may comprise an unwound stator tooth, for example a stator tooth that does not have a corresponding coil located about the stator tooth. As such, the first stator tooth may not see as much magnetic flux as the second stator tooth, and use of grain-oriented steel for the first stator tooth may be unnecessary. The first stator tooth comprises a first main body and first tooth tips extending outwardly from the first main body, the second stator tooth comprises a second main body and second tooth tips extending outwardly for the second main body, the first main body has a first cross-sectional width, and the second main body has a second cross-sectional width substantially the same as the first cross-sectional width. Use of grain-oriented steel for the second stator tooth may enable a width of the second stator tooth to be reduced relative to an arrangement in which non-grain-oriented steel is used for the first stator tooth. This may enable a width of the second stator tooth to be similar to a width of the first stator tooth, which may provide for an increased slot size. This may enable a greater volume of wire to be utilised, which may lead to an increase in motor efficiency. The grain-oriented steel of the second stator tooth may be oriented along a length of the stator tooth, for example in a direction from a radially inner end of the second stator tooth to a radially outer end of the second stator tooth. The stator core assembly may comprise a yoke formed of the first material. As the yoke is formed from the first material, whereas the second stator tooth is formed from the second material, the second stator tooth may be formed separately from the yoke. This may enable the coil to be located about the second stator tooth, for example wound about the second stator tooth, at a location remote from the yoke, before the second stator tooth is attached to the yoke. This may provide increased flexibility in choice of winding methods for the coil, and may enable use of winding methods that achieve a relatively high winding fill factor. The yoke may be substantially annular in form. The yoke may comprise a substantially uninterrupted annulus. The yoke and the first stator tooth may be integrally formed, for example integrally formed from the first material. This may reduce a number of components compared to an arrangement where the yoke and the first stator tooth are formed as separate components. The second stator tooth may be fixed to the yoke by an adhesive. This may provide a relatively simple mechanism for attaching the second stator tooth to the yoke. The second stator tooth may comprise a projection, the yoke may comprise a recess, and the projection may be received within the recess. This may facilitate location of the second stator tooth relative to the yoke. Engagement of the projection within walls defining the recess may inhibit removal of the second stator tooth from the yoke in a radial direction. The first stator tooth may comprise a channel, and the second stator tooth may comprise a rib received within the channel. This may facilitate location of the second stator tooth relative to the first stator tooth. A radially inner end of the first stator tooth may comprise the channel, and a radially inner end of the second stator tooth may comprise the rib. The stator core assembly may comprise a plurality of first stator teeth formed of the first material, a plurality of second stator teeth formed of the second material, and a plurality of coils, each coil located about a respective one of the plurality of second stator teeth, and wherein the plurality of first stator teeth and the plurality of second stator teeth may be arranged in an alternating pattern, such that each second stator tooth is located intermediate two adjacent first stator teeth. In such a manner each wound stator tooth may be located between adjacent ones of the first stator teeth, with the coil located in slots either side of the second stator tooth and each slot at least partially defined by a first stator tooth and a second stator tooth. Use of grain-oriented steel for the second stator teeth may then allow for the width of the second stator teeth to be lesser than would be require for nongrain oriented teeth to achieve the same flux, which may allow for increased space in which to locate a winding in the manner previously described. The stator core assembly may comprise a same number of first stator teeth and second stator teeth. The stator core assembly may comprise n second stator teeth, and n may be an integer multiple of three. The stator core assembly may comprise a three-phase stator core assembly. The stator core assembly may comprise exactly six first stator teeth and exactly six second stator teeth. According to a second aspect of the present invention there is provided an electric motor comprising a stator core assembly according to the first aspect of the present invention. The electric motor may comprise an outer diameter of no more than 75mm, no more than 55mm, no more than 45mm, or no more than 35mm. According to a third aspect of the present invention there is provided a vacuum cleaner comprising an electric motor according to the second aspect of the present invention. According to a fourth aspect of the present invention there is provided a cleanerhead for a vacuum cleaner, the cleaner head comprising an electric motor according to the second aspect of the present invention. The cleaner head may comprise a housing, and a roller rotatable relative to the housing, and the electric motor may be configured to cause rotation of the roller relative to the housing. Optional features of aspects of the present invention may be equally applied to other aspects of the present invention, where appropriate. Brief Description of the Drawings Figure 1 is a schematic view of a first embodiment of a stator assembly; Figure 2 is a schematic view an electric motor comprising the stator assembly of Figure 1; Figure 3 is a schematic view of a second embodiment of a stator assembly; Figure 4 is a schematic view of a vacuum cleaner comprising the electric motor of Figure 2; and Figure 5 is a schematic view of the cleanerhead of the vacuum cleaner of Figure 4. Detailed Description of the Invention A first embodiment of a stator assembly 10 is illustrated in Figure 1. The stator assembly 10 comprises a first stator core component 12 and six winding subassemblies 14. It will be appreciated that more or less sub-assemblies can be utilised depending on a desired number of teeth of the stator assembly. The first stator core component 12 comprises a yoke 16, and six first stator teeth 18. The yoke 16 is annular in form, and is formed of a stack of laminations (not shown) of non-grain-oriented electrical steel. Each first stator tooth 18 is substantially similar in form, and is integrally formed with the yoke 16, with each lamination having a yoke portion and six first stator tooth portions. The first stator teeth 18 are thereby integrally formed with the yoke 16, and are formed of the same non-grain-oriented electrical steel. The first stator teeth 18 extend radially outwardly from the yoke 16, and are evenly spaced about an outer circumference of the yoke 16. The first stator teeth 18 each comprise a main body 20, and tooth tips 22. The main body 20 has a generally rectangular cross-sectional shape, and extends substantially radially from the yoke 16, and comprises undercuts 24 located either side of the main body 20 at respective interfaces with the yoke 16. The tooth tips 22 extend circumferentially outwardly from a radially outer end of the main body 20. Each winding sub-assembly 14 comprises a second stator core component in the form of a second stator tooth 26, and a coil 28. It will be appreciated that each winding sub-assembly 14 has substantially the same form, and so only one winding sub-assembly is described here for sake of brevity. It will further be appreciated that each winding sub-assembly 14 may comprise further components, such as a bobbin, that are not illustrated here for sake of clarity. The second stator tooth 26 is a separate component to the first stator core component 12, and is formed of a stack of laminations (not shown) of grain-oriented electrical steel. The second stator tooth 26 comprises a main body 30, a guide portion 32, and tooth tips 34. The main body 30 is substantially rectangular in form, and has a width substantially the same as a width of the main body 20 of a corresponding first stator tooth 18. The main body 30 is formed such that a 0° direction of the grain-oriented electrical steel is aligned with a major axis M of the main body 30. The guide portion 32 is located at a first end of the main body 30, and comprises first 38 and second 40 protrusions that extend circumferentially outwardly from the main body 30. The first 38 and second 40 protrusions are shaped and dimensioned to be received within respective undercuts 24 of adjacent ones of the first stator teeth 18. The tooth tips 34 are located at a second end of the main body 30, and extend circumferentially outwardly from the second end of the main body 30. The first 38 and second 40 protrusions extend outwardly from the main body 30 by a greater extent than the tooth tips 34. The coil 28 comprises a copper wire of circular cross-sectional shape wound about the main body 30 of the second stator tooth 26 in a plurality of turns. Wires of other cross-sectional shape are also envisaged. To assemble the first embodiment of the stator assembly 10, the coil 28 is wound about the second stator tooth 26 at a location remote from the first stator core component 12. The winding sub-assembly 14 is positioned relative to the first stator core component 12 by axially sliding the winding sub-assembly 14 such that the first 38 and second 40 protrusions are received within respective undercuts 24 of adjacent ones of the first stator teeth 18. Adhesive may be applied prior to and / or after positioning of the winding sub-assembly 14, to at least one of the first stator core component 12 and the winding sub-assembly 14, to hold the winding sub-assembly 14 in place. The above is repeated for each winding sub-assembly 14, until all six winding sub-assemblies 14 are in place. It will be appreciated that each winding subassembly 14 may be wound before any winding sub-assembly is affixed to the first stator core component 12. An electric motor 100 comprising the stator assembly 10 is illustrated in Figure 2. The electric motor 100 comprises an outer rotor 102 comprising fourteen permanent magnets 104. The coils 28 are connected in a three-phase fashion, details of which are not pertinent, and so will not be described for sake of brevity. In use, a voltage is applied to the coils 28 such that magnetic fields are generated. Such magnetic fields interact with the permanent magnets 104 to rotate the outer rotor 102 relative to the stator assembly 10. By segmenting the second stator teeth 26 from the yoke 16, a greater range of winding methods may be achievable in comparison to an arrangement where second stator teeth are integrally formed with a yoke. This may enable use of winding methods that can achieve a greater fill factor for a given slot area. By winding fill factor is meant a ratio of a cross-sectional area of the conductive material of the coil to a cross-sectional area of open space within a slot in which the coil is located. An increased winding fill factor may result in a reduction in copper losses for the electric motor 100 comprising the stator core assembly 10, hence leading to increased motor efficiency. As the first 18 and second 26 stator teeth alternate about the circumference of the yoke 16, and only the second stator teeth 26 are wound, an increased slot volume available for copper may be achieved relative to an arrangement in which both the first 18 and second 26 stator teeth are wound. For example, in an arrangement in which both the first 18 and second 26 stator teeth are wound, two coils would be located in the same slot, and hence an air gap would be required between the two coils. There is no such need for an air gap where only the second stator teeth 26 are wound, as only a single coil is located within a slot. However, only winding the second teeth 26 would typically require each second stator tooth 26 to have a relatively large width to ensure appropriate magnetic performance. To mitigate for this, the second stator teeth 26 are formed of grain-oriented steel. Use of grain-oriented electrical steel for the second stator teeth 26 also enables the second stator teeth 26 to be thinner in comparison to an arrangement where non-grain-oriented electrical steel is utilised for a stator tooth about which a coil is located, as grain-oriented steel can handle a higher flux density before magnetic saturation. Such a reduction in width of the second stator teeth 26 provides for increased area in which to locate the coils 28, with a greater coil volume leading to increased motor efficiency for a motor comprising the stator assembly 10. In the manner described above, the stator assembly 10 may have an optimised slot volume available to accommodate a coil therein, as well as accommodating the coil with a relatively high fill factor for the given slot volume. A second embodiment of a stator assembly 200 is illustrated in Figure 3. The second embodiment of the stator assembly 200 comprises a first stator core component 202 and six winding sub-assemblies 204. The first stator core component 202 comprises a yoke 206, and six first stator teeth 208. The yoke 206 is annular in form, and is formed of a stack of laminations (not shown) of non-grain-oriented electrical steel. The yoke 206 comprises six receiving channels 210, with each receiving channel 210 located centrally between two adjacent first stator teeth 208. The six receiving channels 210 extend axially along the yoke 206, and are shaped and dimensioned to receive a corresponding protrusion 232 of a second stator tooth 216 of a respective winding sub-assembly 204. Each first stator tooth 208 is substantially similar in form, and is integrally formed with the yoke 206, with each lamination having a yoke portion and six first stator tooth portions. The first stator teeth 208 are thereby integrally formed with the yoke 206, and are formed of the same non-grain-oriented electrical steel. The first stator teeth 208 extend radially outwardly from the yoke 206, and are evenly spaced about an outer circumference of the yoke 206. The first stator teeth 208 each comprise a main body 212, and tooth tips 214. The main body 212 has a generally rectangular cross-sectional shape, and extends substantially radially from the yoke 206. The tooth tips 214 extend circumferentially outwardly from a radially outer end of the main body 20. Each winding sub-assembly 14 comprises a second stator core component in the form of a second stator tooth 216, and a coil 218. It will be appreciated that each winding sub-assembly 204 has substantially the same form, and so only one winding sub-assembly is described here for sake of brevity. It will further be appreciated that each winding sub-assembly 204 may comprise further components, such as a bobbin, that are not illustrated here for sake of clarity. The second stator tooth 216 is a separate component to the first stator core component 202, and is formed of a stack of laminations (not shown) of grain-oriented electrical steel. The second stator tooth 216 comprises a main body 220, a guide portion 222, and tooth tips 224. The main body 220 is substantially rectangular in form, and has a width substantially the same as a width of the main body 212 of a corresponding first stator tooth 208. The main body 220 is formed such that a 0° direction of the grain-oriented electrical steel is aligned with a major axis M of the main body 220. The guide portion 222 is located at a first end of the main body 220, and comprises first 228, second 230, and third 232 protrusions that extend outwardly from the main body 220. The first 228 and second 230 protrusions extend circumferentially outwardly from the main body 220. The third protrusion 232 extends radially from the main body 220, and is shaped and dimensioned to be received within a corresponding receiving channel 210 of the yoke 206 such that radial separation of the yoke 206 and the second stator tooth 216 is inhibited. The tooth tips 224 are located at a second end of the main body 220, and extend circumferentially outwardly from the second end of the main body 220. The coil 218 comprises a copper wire of circular cross-sectional shape wound about the main body 220 of the second stator tooth 216 in a plurality of turns. Wire of other cross-sectional shapes is also envisaged. The second embodiment of the stator assembly 200 differs from the first embodiment of the stator assembly 10 in the form of attachment between its first stator core component 202 and its winding sub-assemblies 204, with the combination of a third protrusion 232 and a respective receiving channel 210 acting to inhibit radial separation of the stator core component 202 and the winding sub-assembly. Otherwise, the second embodiment of the stator assembly 200 is substantially similar to the first embodiment of the stator assembly 10, particularly with regard to use of a combination of grain-oriented and non-grain-oriented electrical steel for its stator teeth, and the associated technical effects described above. It will be appreciated that the second embodiment of the stator assembly 200 may be utilised in an electric motor similar to the electric motor 100 of Figure 2. A vacuum cleaner 300 comprising the electric motor 100 is illustrated 5 schematically in Figure 4. The vacuum cleaner 300 comprises a main unit 302, a wand 304, and a cleanerhead 306. The cleanerhead 306 is illustrated schematically in Figure 5, and comprises a housing 308, a brushbar 310, and the electric motor 100. The electric motor 100 10 is located within the brushbar 310, and is used to drive motion of the brushbar 310 within the housing 308 in use. Further details of the cleanerhead 306 are not pertinent, and so will not be described here for sake of brevity. Whilst particular examples and embodiments have thus far been described, it 15 should be understood that these are illustrative only and that various modifications may be made without departing from the scope of the invention as defined by the claims.

Claims

1. A stator core assembly for an electric motor, the stator core assembly comprising:a first stator tooth formed of a first material;a second stator tooth formed of a second material different to the first material; anda coil located about the second stator tooth;wherein the first material is non-grain-oriented electrical steel, and the second material is grain-oriented electrical steel, andwherein the first stator tooth comprises a first main body and first tooth tips extending outwardly from the first main body, the second stator tooth comprises a second main body and second tooth tips extending outwardly for the second main body, the first main body has a first cross-sectional width, and the second main body has a second cross-sectional width substantially the same as the first cross-sectional width2. A stator core assembly as claimed in Claim 1, wherein the stator core assembly comprises a yoke formed of the first material.

3. A stator core assembly as claimed in Claim 2, wherein the yoke and the first stator tooth are integrally formed.

4. A stator core assembly as claimed in Claim 2 or Claim 3, wherein the second stator tooth is fixed to the yoke by an adhesive.

5. A stator core assembly as claimed in any of Claims 2 to 4, wherein the second stator tooth comprises a projection, the yoke comprises a recess, and the projection is received within the recess.

6. A stator core assembly as claimed in any preceding claim, wherein the first stator tooth comprises a channel, and the second stator tooth comprises a rib received within the channel.

7. A stator core assembly as claimed in any preceding claim, wherein the stator core assembly comprises a plurality of first stator teeth formed of the first material, a plurality of second stator teeth formed of the second material, and a plurality ofcoils, each coil located about a respective one of the plurality of second stator teeth, and wherein the plurality of first stator teeth and the plurality of second stator teeth are arranged in an alternating pattern, such that each second stator tooth is located intermediate two adjacent first stator teeth.

8. A stator core assembly as claimed in Claim 7, wherein the stator core assembly comprises a same number of first stator teeth and second stator teeth.

9. A stator core assembly as claimed in Claim 7 or Claim 8, wherein the stator core assembly comprises n second stator teeth, and n is an integer multiple of three.

10. A stator core assembly as claimed in any of Claims 7 to 9, wherein the stator core assembly comprises exactly six first stator teeth and exactly six second stator teeth.

11. An electric motor comprising a stator core assembly as claimed in any preceding claim.

12. A vacuum cleaner comprising an electric motor as claimed in Claim 11.

13. A cleanerhead for a vacuum cleaner, the cleaner head comprising an electric motor according to Claim 11.

14. A cleanerhead as claimed in Claim 13, wherein the cleaner head comprises a housing, and a roller rotatable relative to the housing, and the electric motor is configured to cause rotation of the roller relative to the housing.

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