Stator assembly

Abstract

A stator assembly for a radial flux motor comprises a stator core and a coil 32 comprising a plurality of turns about the core. Each turn has a rectangular cross-section, a minor edge thereof forming

Description

Technical Field The present invention relates to a stator assembly for a radial flux motor, and a radial 5 flux motor comprising the stator assembly. Background There is a general desire to improve electric machines, such as brushless motors, in a number of ways. For example, improvements may be desired in terms of size, weight, 10 power density, manufacturing cost, efficiency, reliability, and noise. Summary According to a first aspect of the present invention, there is provided a stator assembly for a radial flux motor, the stator assembly comprising: a stator core; and a coil 15 comprising a plurality of turns about the stator core, wherein each of the plurality of turns has a rectangular cross-section, and wherein a minor edge of the rectangular crosssection forms part of an inner periphery of the coil, wherein the inner periphery faces the stator core. 20 The plurality of turns about the stator core each having the rectangular cross-section may allow for a high fill factor. A high fill factor increases a maximum torque output compared with a lower fill factor. Further, the rectangular cross-section may reduce resistance to a current in the coil and thereby DC losses compared to a circular crosssection). The rectangular cross-section may be a constant size along an entire length of 25 the coil. This uniformity may further contribute to the high fill factor and reduction in DC losses. The inner periphery of the coil may consistently face towards the stator core along an entire length of each turn, and thereby the coil. This means that the inner periphery of 30 the coil is positioned opposite to the stator core. In some examples the inner periphery may directly face the stator core. That is the inner periphery may be parallel to a surface of the stator core. An arrangement of the coil so that the minor edge of the rectangular 27 03 25 cross-section forms part of the inner circumference means that each turn is arranged with a smaller surface being the closest to the surface of the stator core, and a larger surface facing an adjacent turn. This can reduce eddy currents within the coil which may in turn reduce AC losses in use. As such, the stator assembly may be more efficient 5 than stator assemblies of the prior art. The arrangement may also minimise a required space of the coil on the stator core while still providing a high-power output for a motor comprising the stator assembly. This may allow for the stator assembly to be used in a small but power dense motor. This motor can therefore be used for a plurality of devices, including small devices such as hair care appliances, without adding more 10 weight and / or taking up more space than conventional motors. The coil may partially enclose the stator core such that a single turn of the coil overlies both a radially inner and a radially outer surface of the stator core. In some examples, the stator core is annular, and the radially inner and outer surfaces are concentric. 15 Arranging the plurality of turns such that each single turn overlies both the radially inner and outer surfaces of the stator core may provide an increased surface area of the coil, compared to a stator core having only a radially outer surface about which turns can be wound. This may increase the power output of a motor using the stator assembly by providing an increase in torque output, compared to a motor having a turn wound 20 about a surface. In some examples, an aspect ratio of the rectangular cross-section is from 4.50:1 to 4.8:1. Having an aspect ratio in this range may allow for a wire (which is wound into the plurality of turns making up the coil) to be sufficiently bendable to avoid damaging 25 a layer of insultation which surrounds said wire. To give this aspect ratio, the rectangular cross-section may have a minor dimension of 0.2 mm and a major dimension of 0.925 mm, for example. The dimensions of the rectangular cross-section being 0.2mm by 0.925mm may provide for a smaller motor with a higher power output than some examples in the prior art. 30 The coil may comprise a single wire wound in the plurality of turns. The single wire wound into turns can utilise a high voltage with a low current. As such, the stator 27 03 25 assembly may produce a higher-power output from a high voltage power source, compared with a stator assembly comprising a coil comprising multiple different wires. The coil may comprise a first wire and a second wire different to the first wire, where 5 the first and second wires define the plurality of turns. The first wire may be wound in a first turn, and the second wire may be wound in a second turn adjacent to the first turn. Multiple different wires wound into turns can utilise a low voltage with a high current. As such, the stator assembly may produce a higher-power output from a high current power source (like a battery), compared with a stator assembly comprising a 10 coil comprising a single wire. Winding two different wires (or a single wire) alternately may reduce the overall diameter of the stator assembly, compared with having one wire radially outward of another, because a radially outer wire will have a larger bend radius than that of a 15 radially inner wire. Further, for adjacently wound wires, the first and second wire may both have the same bend radius about the stator core. This may allow for a greater volume of wire to fit into the same radial space compared to radially wound wires. Further, this then may then reduce eddy currents when an alternating current is driven through the wire(s). This may be particularly beneficial for small motors; for example, 20 motors for fitting into handheld appliances. In some examples, the first wire and second wire are wound in a repeating alternating pattern of turns. The first and second wire may be air wound in alternating turns of the coil prior to assembly around the stator core. 25 Alternate turns of wire may also help to balance alternating current within the wires of the stator assembly. This may reduce hot spotting within the wires and may be particularly useful in radial flux motors 30 The first and second wire may be electrically connected in parallel with one another. Using two different wires electrically connected in parallel means that a higher current can be passed through the coil, than the same arrangement with a single wire. This may 27 03 25 be advantageous when a motor using the stator assembly has a power source which is a battery. As a higher power output from the motor may be achievable compared to only one wire when the voltage is lower 5 The stator assembly may comprise a plurality of sub-assemblies, each sub-assembly comprising a respective coil comprising a plurality of turns about a stator core segment. Each of the plurality of turns may have a rectangular cross-section, and a minor edge of the rectangular cross-section may form part of an inner periphery of the coil, wherein the inner periphery faces the stator core. 10 Splitting the stator assembly into a number of sub-assemblies may be desirable to facilitate manufacturing of the stator assembly. For example, an increase in number of segments that form an annular stator core may enable the stator core segments to be more linear than if a lower number of segments were utilised to form the annular stator 15 core. This may facilitate formation of laminations that form the stator core segments, for example by enabling an increased number of laminations to be formed from a single sheet of material, reducing material wastage and cost. Furthermore, slotting an air-wound coil over the stator core may be achievable when 20 that stator core is in segments, compared with a single annular stator core. This may be easier to manufacture, compared with winding coils onto a stator core or segments thereof. In some examples, the stator assembly comprises exactly three sub-assemblies. 25 Splitting the stator assembly into sub-assemblies may increase a reluctance of the stator assembly in view of the air gaps between stator core segments, thereby providing resistance to magnetic flux attempting to pass between the stator core segments. Furthermore, for each incremental integer increase in the number of segments of the stator core, it may become more challenging to reliably and securely connect the stator 30 core segments together, which can result in variations in global form of the stator assembly. Such variations in form may introduce saliency, which may inhibit production of desirable magnetic torque in a motor. 27 03 25 In some examples, the stator assembly is a three-phase stator assembly. Three phase stator assemblies may generate more torque than a single-phase stator assembly when driven at the same voltage. 5 Each stator core segment may span an arc length of 120 degrees. Having three stator core segments with the same arc length may mean the three sub-assemblies have substantially the same form. This may reduce cost and complexity of manufacture of the stator assembly compared to, for example, a stator assembly where different forms 10 of stator core assembly are used. The curvature of the sub-assemblies may give the stator assembly an annular shape. An annular stator assembly may allow for a more efficient use of available space in an appliance than, for example, a rectangular stator assembly. This may be particularly 15 advantageous when the stator assembly will be used in a pipe, for example. In some examples, each coil comprises at least twenty turns. Using at least twenty turns for each coil may allow the stator assembly to produce a high-power output in a motor with the available space of the stator core segment, compared with stator assemblies 20 having less turns. In examples where the coil comprises the first and second wires which are alternately wound each coil, comprises at least ten turns of each wire. Each of the sub-assemblies may comprise a further coil, such that the stator assembly comprises six coils in total. The use of six coils may provide relatively little radial 25 loading on a rotor assembly associated with the stator assembly in use compared to, for example, an arrangement in which three coils are utilised. The further coil may be identical to the coil or a mirror image of the coil. The coil and further coil may be substantially evenly spaced about the stator core segment. 30 Each of the sub-assemblies may comprise a bobbin, each bobbin comprising a first and a second connection formation connected to respective second and first connection formation of an adjacent bobbin. 27 03 25 The bobbin may act to insulate the stator core segment from the coil, reducing undesirable currents or losses. Provision of connection formations on each bobbin, rather than on the stator core segments, may reduce reluctance and / or saliency which 5 could otherwise occur due to, for example, variations in shape of the stator core segments due to tolerances of manufacture. Each bobbin may comprise a slot for an external apparatus to manipulate the assembly during manufacture. The placement slot on the bobbin can be used during 10 manufacturing to make manipulation of the stator assembly (by the external apparatus), easier. The placement slot may be no more than 1.5mm in width thus making manufacture of the stator assembly easier without a significant impact on available space for coil(s) slotted on to the bobbin. The placement slot may space apart the coil and further coil described above. This positioning may maximise available space for 15 these coils (to maximise a number of turns). The stator assembly may have an outer diameter of no more than 25mm. In some examples, the stator assembly may have an outer diameter of no more than 20mm. This outer diameter may be smaller than stator assemblies of the prior art. Minimising a size 20 of the stator assembly may allow for a radial flux motor which uses this assembly to be reduced in size compared to motors using larger stator assemblies. Particularly in appliances and components which are small and handheld (i.e., a hair care appliance) a small motor improves the end user’s comfort and ease of use. 25 According to a second aspect of the present invention, there is provided a radial flux motor comprising the stator assembly as described above. Any of the above-described features may be included in the stator assembly alone or in combination. The radial flux motor may comprise any of the above-described features of the stator 30 assembly alone or in combination. The radial flux motor may further comprise a rotor assembly disposed within the stator assembly. 27 03 25 According to a third aspect of the present invention, there is provided a haircare appliance comprising a radial flux motor discussed above. According to a fourth aspect of the present invention, there is provided a vacuum 5 cleaner comprising a radial flux motor discussed above. According to a fifth aspect of the present invention, there is provided a stator assembly comprising: a stator core; and a coil comprising a first wire and a second wire different to the first wire, wherein the first and second wires are each wound in plurality of turns 10 interleaved about the stator core; wherein each of the first and second wires each have a rectangular cross-section, and wherein a minor edge of each rectangular cross-section forms part of an inner periphery of the coil, the inner periphery facing the stator core. The stator assembly may further include any of the above-described features, alone or in combination. 15 Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. 20 Brief Description of the Drawings Figure 1 is a perspective view of a stator assembly; Figure 2 is an exploded view of the stator assembly of Figure 1; Figure 3 is a perspective view of a stator core assembly of the stator assembly of Figure 1; 25 Figure 4 is an exploded view of the stator core assembly of Figure 3; Figure 5 is a perspective view of an example coil of the present invention, and Figure 6a is an enlarged view of a portion of the coil of Figure 5; Figure 6b is a cross-sectional view of Figure 6a, with the coil arranged on a stator core; 30 Figure 7a is a cross-sectional view of a portion of an alternative coil to Figure 5; 27 03 25 Figure 7b is a perspective view of a stator core sub-assembly comprising the alternative coil of Figure 7a; Figure 8 is a schematic illustration of a brushless permanent magnet motor comprising the stator assembly of Figure 1; 5 Figure 9 is a schematic illustration of a vacuum cleaner comprising the brushless permanent magnet motor of Figure 8; and Figure 10 is a schematic illustration of a haircare appliance comprising the brushless permanent magnet motor of Figure 8. 10 Detailed Description A stator assembly 10 is illustrated schematically in Figures 1 and 2, and comprises first 12, second 14 and third 16 stator core sub-assemblies, and a busbar assembly 18. The first stator core assembly is shown in Figures 3 and 4. Each of the first 12, second 14, 15 and third 16 stator core sub-assemblies has substantially the same form, and so the second 14 and third 16 stator core assemblies will not be described in detail here for the sake of brevity. The first stator core sub-assembly 12 comprises a stator core segment 22, a bobbin 24, 20 and first 32 and second 34 coils. The stator core segment 22 is formed of a stack of steel laminations (not shown), and is generally arcuate in form, with a height greater than its length and width. The stator core segment 22 spans an arc length of substantially 120 degrees. Circumferential end 25 faces of the stator core segment 22 are generally planar in form. When connected together the generally circumferential faces of the stator core segments 22 are substantially in contact with one another, such that a generally annular stator core is formed by the stator core segments 22. 30 The bobbin 24 is formed of a plastics material, and is overmoulded onto the stator core segment 22. The bobbin 24 comprises a first and second body portion 35, 36, which 27 03 25 follow the arcuate form of the stator core segment 22 that they overmould, and a slot 38. The slot 38 is generally rectangular in cross-section and at least partly separates the first 5 and second body portions 35, 36, providing a line-of-sight to a radially outer face of the stator core segment 22. The bobbin 24 comprises a frame 39 around the periphery of the slot 28, which protrudes outwardly from the first and second body portions 35,36. The slot 38 enables an appropriate magnet to hold the stator core segment 22 in place during assembly of the first stator core assembly 12 10 The first 32 and second 34 coils are formed from turns 505 of copper wire. Figure 5 shows the first coil 32 in isolation, with Figure 6a illustrating an enlarged portion of that coil 32. The first 32 and second 34 coils are each formed from a single piece of copper wire with an insulative coating and, prior to assembly, air wound using a 15 continuous winding process. The second coil 34 is a mirror image of the first coil 32. So, for brevity, only the first coil 32 will be described in detail. The first coil 32 comprises twenty substantially uniform turns 505. Each turn 505 comprises a rectangular cross-section having two minor edges 506, 506’ and two major 20 edges 507, 507’. The minor edges 506, 506’ are 0.2 mm in length, and the major edges 507, 507’ are 0.93 mm in length, giving the cross-section an aspect ratio of 4.65:1. The aspect ratio is generally consistent across the wire, as the cross-section of the wire remains generally the same. Although bent portions (described below) may have small variations in aspect ratio due to the bending of the wire. 25 Each turn 505 comprises a first bent portion 509, at a first end 508 (corresponding to the first end of the stator assembly 48) and a second bent portion 509’ at second end 510, with two straight extensions 511, 511’ between the bent portions 509, 509’.The bent portions 509, 509’ each curve about a bend axis A, B with a total curvature of 180 30 degrees, such that the curvature of the bent portion 509 results in the straight extensions 511, 511’ extending in parallel extension directions. 27 03 25 The second bent portion 509’ at the second end 510 of a first turn 505 is also the second bent portion 509’ of an adjacent turn 505, such that two adjacent turns share a common second bent portion 509’, such that the turns 505 wind around the stator core 22 when the first coil 32 is located about the stator core 22. These second bent portions 509’ are 5 in line with one another, as are the first bent portions 509. There may be some spacing between adjacent turns 505, or adjacent turns 505 may be in partial or complete contact. The inner surfaces of each turn 505, when arranged together, collectively form an inner periphery 514 of the first coil 32, which slots over the stator core 22. 10 Figure 6b illustrates an exploded cross-sectional view of four example turns 505 arranged around the stator core 22 (with the overmoulded bobbin 24). Each turn 505 is orientated on the stator core 22 such that the minor edge 506 of the cross-section faces the stator core 22 and is substantially parallel to the surface of the stator core 22. The bend axis A, B of the bent portions 509, 509’ is parallel to the minor edge 506. This 15 means that the collective inner periphery 514 of the turns 505 that faces the stator core 22 corresponds to the minor edges 506, 506’ of the cross-section. Or put another way, the turns are bent with one of the thinner surfaces facing the stator core. A final turn 505a,b at each end of the first coil 32 comprises a termination connection 20 512a,b adjacent to the first bent portion 509 at the first end 508. The termination 512a,b is an exposed portion of the copper wire, with no insulative coating. The termination connections 512a,b extend away from the first coil 32 in a direction which is generally perpendicular to the bend axis 516 of the turn 505. 25 When assembled on the bobbin 24 the termination connections 512a,b protrude away from the bobbin at the first end 48 of the stator assembly. The busbar assembly 18 is located at the first end 48 of the stator assembly 10 and comprises a plastic carrier 52 and three electrically conductive busbars 54. The electrically conductive busbars 54 are welded to the coils 32, 34 at the first end 48 of the stator assembly 10. Each busbar 54 30 is thereby electrically connected to two coils of the stator assembly 10. The configuration of the busbar assembly is not pertinent to the invention and so shall not be included in any detail for the sake of brevity. 27 03 25 Figure 7a illustrates a sectional view of an alternative coil embodiment arranged on the stator core 22, with Figure 7b illustrating a perspective view of the stator core subassembly 12 with this coil alternative as a first and second coil 710, 712 (which are 5 again mirror images of one another). This embodiment is arranged substantially the same as the above coil in Figures 5 to 6b, except each coil 710, 712 comprises a first wire 700 interleaved with a second wire 702. Each of these wires comprises ten turns 704, 706 (giving the coil a total of twenty turns). These turns 704, 706 are substantially identical to the above described turns 505. 10 In this embodiment, the turns 704, 706 are interleaved such that both major edges 707, 707’ of a cross-section of a turn 704 of the first wire 700 are each adjacent to a major edge 707’ of a cross-section of a turn 706 of the second wire 702. That is, adjacent turns 704, 706 alternate between the first and second wires 700, 702. 15 Each turn 704, 706 has a bent portion coupling to the next turn of that wire. When the wires and their respective turns are viewed individually, adjacent turns have a common bent portion as described in the above embodiment. 20 A final turn of each wire 700, 702 comprises a termination connection 708, 709 at each end of the first coil. As there are two wires in the first coil, the first coil has two termination connections at each end. The busbar assembly 18 is arranged in this embodiment to electrically connect to the termination connections of each wire in parallel, such that a current flowing through the first wire 700 flows in an opposite 25 direction to a current flowing in the second wire 702. Collectively, the first 12, second 14 and third 16 stator core sub-assemblies, when connected together, define an annulus having a central bore 46 for receiving a rotor assembly, with the stator assembly 10 having a diameter of no more than 20mm. The 30 stator core sub-assemblies 12, 14 and 16 each comprise one of the above-described coil embodiments. The coil embodiments are the same for each stator core sub-assembly. 27 03 25 In use, the stator assembly 10 is paired with a rotor assembly 800 to form a radial flux motor 802, as illustrated schematically in Figure 8. The rotor assembly 800 comprises a shaft 804 and a permanent magnet 806 mounted to the shaft 804. When the coils 32,34 are driven with an appropriate voltage, here of up to around 400V, the stator assembly 5 10 generates a magnetic field that interacts with the permanent magnet 806 to rotate the rotor assembly 800. A vacuum cleaner 900 comprising the radial flux motor 802 is illustrated schematically in Figure 9. 10 A haircare appliance 1000 comprising the radial flux motor 802 is illustrated schematically in Figure 10. The above embodiments are to be understood as illustrative examples of the invention. 15 Further embodiments of the invention are envisaged. Further arrangements of the coils on the stator core are envisaged. For example, the first and second coils may be identical, rather than a mirror image of one another; and or the coils may have differently positioned terminations. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other 20 features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. It should be understood that slight variations, such as minor non-uniformity of the wire are considered encompassed by the invention. Furthermore, equivalents and modifications not described above may also be employed without departing from the 25 scope of the invention, which is defined in the accompanying claims. 27 03 25

Claims

1. A stator assembly for a radial flux motor, the stator assembly comprising: a stator core; and5 a coil comprising a plurality of turns about the stator core,wherein each of the plurality of turns has a rectangular cross-section, andwherein a minor edge of the rectangular cross-section forms part of an inner periphery of the coil, wherein the inner periphery faces the stator core, 10 wherein an aspect ratio of the rectangular cross-section is from 4.50:1 to 4.8:1.

2. The stator assembly of claim 1 wherein the coil comprises a single wire wound in the plurality of turns.15 3. The stator assembly of claim 1 or 2, wherein the coil comprises a first wire anda second wire different to the first wire, and the first and second wires define the plurality of turns,wherein the first wire is wound in a first turn, the second wire is wound in a second turn adjacent to the first turn.

204. The stator assembly of claim 3, wherein the first and second wire are electrically connected in parallel with one another.

5. The stator assembly of any preceding claim, wherein the stator assembly 25 comprises a plurality of sub-assemblies, each sub-assembly comprising arespective coil comprising a plurality of turns about a stator core segment, wherein each of the plurality of turns has a rectangular cross-section, and wherein a minor edge of the rectangular cross-section forms part of an inner periphery of the coil, wherein the inner periphery faces the stator core.

6. The stator assembly of claim 5, wherein the stator assembly comprises exactly three sub-assemblies.27 03 257. The stator assembly of claim 6, wherein each stator core segment spans an arc length of 120 degrees.5 8. The stator assembly of any of claims 5 to 7, wherein each coil comprises at leasttwenty turns.

9. The stator assembly of any of claims 5 to 8, wherein each of the sub-assemblies comprises a further coil, such that the stator assembly comprises six coils in 10 total.

10. The stator assembly of claim 8, wherein each of the sub-assemblies comprises a bobbin, each bobbin comprising a first and a second connection formation connected to respective second and first connection formation of an adjacent 15 bobbin.

11. The stator assembly of claim 10, wherein each bobbin comprises a slot for an external apparatus to manipulate the assembly during manufacture.

12. The stator assembly of any preceding claim, wherein the stator assembly has an 20 outer diameter of no more than 25mm.

13. A radial flux motor comprising the stator assembly as claimed in any preceding claim.25 14. A haircare appliance comprising the radial flux motor as claimed in claim 13.

15. A vacuum cleaner comprising the radial flux motor as claimed in claim 13.

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