Electric motor

Abstract

An electric motor comprising a rotor having a rotational axis and a radial axis perpendicular to the rotational axis and a c-shaped stator core. The stator core comprises a back, a first arm and a second arm. Each of the first arm and the second arm extend from the back and comprising a pole face. The pole face of the first arm is spaced from the pole face of the second arm to define a slot opening. The stator core is mounted relative to the rotor such that an inner side of the back faces the rotor. The radial axis extends through the centre point of the slot opening and through the inner side. The inner side of the back is asymmetric with respect to the radial axis.

Description

[0001] 1 P004700-W001

[0002] ELECTRIC MOTOR

[0003] BACKGROUND

[0004] There is a general desire to improve electric motors, such as permanent magnet motors, in a number of ways. For example, improvements may be desired in terms of power density, size, weight, efficiency, performance and / or reliability.

[0005] SUMMARY

[0006] According to a first aspect, there is provided an electric motor comprising: a rotor having a rotational axis and a radial axis perpendicular to the rotational axis; and a c-shaped stator core comprising a back, a first arm and a second arm, each of the first arm and the second arm extending from the back and comprising a pole face, wherein: the pole face of the first arm is spaced from the pole face of the second arm to define a slot opening; the stator core is mounted relative to the rotor such that an inner side of the back faces the rotor; the radial axis extends through the centre point of the slot opening and through the inner side; and the inner side of the back is asymmetric with respect to the radial axis.

[0007] A smaller electric motor (which may be referred to interchangeably herein as a motor, for brevity) may be desirable. For example, a smaller motor may be more suited to compact and / or lightweight products, such as handheld electronic appliances. A motor with a stator core as described herein may have a diameter of the order of a few tens of millimetres, such as around 30 millimetres. This may be of the order of around 2 or 2.5 times smaller than existing motors. The c-shaped stator core has a generally c-shaped form, which may be a relatively compact arrangement for a stator core.

[0008] Smaller motors may have smaller stator cores, with relatively small slot openings. For example, the size of a slot opening may be restricted to limit magnet eddy loss so as to maintain a sufficiently low magnet temperature to reduce the risk of demagnetization. However, it can be challenging to wind wire round a stator core with a relatively small slot opening due to the difficulty in traversing the wire through the slot opening and into comers of an interior region defined by the stator core, especially if a relatively large 2 P004700-W001 diameter wire is used. It can thus be challenging to obtain a high fill factor for the winding of such a stator core.

[0009] The asymmetric inner side of the back in examples herein can create a larger line of sight into a comer of the stator core without increasing the slot opening. The line of sight is for example defined as the area between a straight line from one edge of the slot opening to one side of the comer and an opposite straight line from an opposite edge of the slot opening to an opposite side of the corner. The asymmetric inner side may result in a shift of the inner side relative to the radial axis so as to decrease the angle between each of these straight lines and the radial axis. This, in turn, increases the distance between these straight lines, thereby increasing the line of sight. The line of sight defines the diameter of wire that can be traversed through the slot opening into the comer. A larger line of sight therefore allows a wire with a larger diameter to pass through the slot opening and be wound around the back near the comer. In this way, the asymmetric inner side may facilitate winding of the back of the stator core to increase the fill factor that can be achieved, even with relatively large diameter wires. For example, the wire may be wound along a greater extent of a length of the back, such as into the comer to a greater extent, than would be achievable with a smaller line of sight. The increased fill factor can increase the power density of the motor compared to other approaches such as those involving reducing the amount of wire wound around the stator core, which can improve the motor performance.

[0010] The asymmetric inner side can enable the winding to be performed effectively without having to unduly increase the slot opening. Examples herein with the asymmetric inner side may therefore be more reliable than other approaches that increase the slot opening to a greater extent, as increasing the slot opening can result in increased magnet losses, which may increase the magnet temperature and increase the risk of demagnetization issues.

[0011] The asymmetric inner side may also or alternatively allow a relatively high fill factor to be obtained without unduly increasing the size of the stator core. The stator core may thus be relatively compact. A compact stator core may contribute to an improved performance of an appliance comprising the stator core. For example, a compact stator core such as this may allow a particular shape and / or size of an airflow area outside a motor comprising the 3 P004700-W001 stator core to be obtained in order to achieve a desired performance of an appliance comprising the motor, such as a vacuum cleaner.

[0012] The radial axis extends through the centre point of the slot opening, which for example corresponds to a point that is equidistant between opposing edges of pole portions of the first and second arms that comprise the pole faces. The radial axis may divide the slot opening substantially equally in two such that a distance, area and / or volume between an edge of the pole portion of the first arm and the radial axis is substantially equal to a distance, area and / or volume between an edge of the pole portion of the second arm and the radial axis. As used herein, the term “substantially equal” may be considered to refer to quantities that are equal to each other or equal to each other within acceptable measurement and / or manufacturing tolerances such as within 1%, 2% or 5%. Similarly, the term “substantially parallel” and “substantially perpendicular” may be considered to refer to elements that are, respectively, parallel or perpendicular to each other, or that are, respectively, parallel or perpendicular to each other within acceptable measurement and / or manufacturing tolerances such as within 1%, 2% or 5%.

[0013] A length of the first arm may be different from a length of the second arm, which may enable winding of the stator core with a relatively high fill factor. The lengths of the first and second arms may be taken along the first and second arms, respectively. Having first and second arms of different lengths may shift the first and second arms and inner side of the back in a direction perpendicular to the radial axis, for example without altering the position of the pole faces of the first and second arms. This can increase the line of sight for winding a wire around the back of the stator core while maintaining the positions of the pole faces so as to limit magnet losses.

[0014] A width of a first inner side portion of the inner side, from the first arm to the radial axis, may be different from a width of a second inner side portion of the inner side, from the second arm to the radial axis. The widths of the first and second inner side portions may be taken along the inner side. The inner side may be substantially perpendicular to the radial axis, in which case the widths of the first and second inner side portions may be taken substantially perpendicularly to the radial axis. Different widths of the first and second 4 P004700-W001 inner side portions may offset the inner side relative to the radial axis so that winding of the stator core can be performed with a relatively high fill factor, without unduly increasing the size of the stator core. The stator core may thus be relatively compact and exhibit acceptable power density and electromagnetic performance.

[0015] The inner side may be at a non-perpendicular angle relative to the radial axis. The inner side may be considered to have an angular shift with respect to a transverse axis perpendicular to the radial axis, so that the inner side is non-parallel relative to the transverse axis and hence non-perpendicular relative to the radial axis. This may shift a comer of the stator core facing the slot opening along the radial axis (compared to a perpendicular angle between the inner side and the radial axis) to increase the line of sight for filling the corner of the slot opening with wire during winding. For example, an angle between the inner side and the radial axis may differ from 90 degrees by more than 0 degrees and less than about 8 degrees (although in other examples, this angle may differ from 90 degrees by more than 8 degrees).

[0016] A first distance from the inner side to the pole face of the first arm, parallel to the radial axis, may differ from a second distance from the inner side to the pole face of the second arm, parallel to the radial axis. The difference between the first and second distances may result in a difference in an extent of a first line of sight from the slot opening into a first comer defined by the first arm and the inner side and a second line of sight from the slot opening into a second corner defined by the second arm and the inner side. For example, if the first distance is smaller than the second distance, the first line of sight may be smaller than the second line of sight. However, the asymmetric inner side may increase the first line of sight relative to an arrangement with a symmetric inner side. For example, the width of the first inner side portion may be smaller than the width of the second inner side portion to obtain a larger first line of sight. The second line of sight may be sufficiently large to enable winding of the wire into the second comer without the wire being obstructed by the first and second arms.

[0017] The first distance may be smaller than the second distance in the absence of the asymmetric inner side, such as with the inner side perpendicular to the radial axis. 5 P004700-W001

[0018] However, the asymmetry in the inner side may result in a larger first distance than the second distance, for example if the asymmetry in the inner side is such that the first arm extends at an acute angle relative to the inner side, with the inner side non-perpendicular relative to the radial axis, so that the first corner is located further from the pole face of the first arm than otherwise.

[0019] The first and second distances may differ by at least 0.3 millimetres. Although this difference between the first and second distances may increase the overall height of the stator core compared to examples in which these distances differ to a lesser extent (or in which these distances are the same), this may correspond to an increase in height of the stator core along one of the first and second arms and a reduction in height of the stator core along the other of the first and second arms. The reduction in height along the other of the first and second arms may compensate for the increase in height along the one of the first and second arms so that, overall, the performance of a magnet comprising the stator core may not be unduly affected.

[0020] A proximal portion of the first arm, proximal to the inner side, may extend from the inner side at a different angle than a proximal portion of the second arm, proximal to the inner side. This may increase the line of sight for the winding of the stator core so as to improve the fill factor.

[0021] Each of the first and second arms may comprise a first portion extending from the back and a second portion extending from, and obliquely angled relative to, the first portion. The first portions of the first and second arms may be substantially parallel to each other. The obliquely angled second portion may decrease a height of the stator core compared to an example in which the second portions are not obliquely angled relative to the first portion, thereby decreasing the overall size of the stator core. An oblique angle is for example an angle that is obtuse or acute, such that the oblique angle is an angle other than 90 degrees and other than 180 degrees.

[0022] An angle between the first and second portions of the first arm may be less than or equal to 0.9 times an angle between the first and second portions of the second arm. This may offset 6 P004700-W001 the inner side relative to the radial axis by a sufficient amount to allow a relatively high fill factor to be obtained within a relatively compact stator core.

[0023] A length of the second portion of the first arm may be less than or equal to 0.7 times a length of the second portion of the second arm. The lengths of the second portions of the first and second arms may be taken along the second portions of the first and second arms, respectively. These lengths of the second portions of the first and second arms may increase the line of sight for winding of the back of the stator core sufficiently to enable a relatively high fill factor to be achieved.

[0024] A length of the second portion of the first arm may be less than 0.6 times a length of the first portion of the first arm. Additionally or alternatively, the length of the second portion of the second arm may be between 0.7 times and 1.3 times the length of the first portion of the second arm. This may increase the line of sight for the winding, so as to increase the fill factor that is achievable. The lengths of the first portions of the first and second arms may be taken along the first portions of the first and second arms, respectively. Similarly, the lengths of the second portions of the first and second arms may be taken along the second portions of the first and second arms, respectively, as described above.

[0025] The inner side and the first portions of the first and second arms may define an interior region of the stator core. A cross-section of the interior region may be asymmetric with respect to the radial axis. The asymmetric cross-section may increase the cross-sectional area within the interior region that can be easily accessed during the winding, to enable a higher fill factor to be obtained.

[0026] The first arm may be formed of a portion extending substantially perpendicularly from the back and a pole face portion disposed at an end of the portion of the first arm and comprising the pole face of the first arm, and the second arm may comprise a first portion extending from the back and a second portion extending from, and obliquely angled relative to, the first portion of the second arm. The portion of the first arm may be substantially parallel to the first portion of the second arm. This arrangement may correspond to a maximum offset of the stator core, for example if the portion of the first 7 P004700-W001 arm and the first portion of the second arm each extend substantially perpendicularly from the inner side. This may maximize the line of sight for winding of the stator core, to maximize the fill factor that may be obtained.

[0027] At least one of the first arm and the second arm may extend at an acute angle relative to the inner side. The first arm may extend at the acute angle relative to the inner side and the second arm may extend at an obtuse angle relative to the inner side, or the second arm may extend at the acute angle relative to the inner side and the first arm may extend at the obtuse angle relative to the inner side. This arrangement may be relatively compact but provide a relatively large line of sight for the winding of the stator core. A respective portion of each of the first and second arms may be substantially parallel to each other to improve compactness.

[0028] The pole faces of the first arm and the second arm may be asymmetric. Asymmetric pole faces may facilitate the starting of a magnet comprising the stator core in a particular direction.

[0029] The electric motor may comprise a winding wound around the back of the stator core, wherein the back has an outer side opposite to the inner side, the winding comprises a layer having a first layer portion disposed around the inner side and a second layer portion disposed around the outer side, and a cross-section of the first layer portion is asymmetric with respect to the radial axis. The cross-section of the first layer portion may be taken in a plane parallel to the radial axis.

[0030] The asymmetric cross-section of the first layer portion may result from the asymmetric inner side. Providing the winding with an asymmetric first layer portion may enable a greater quantity of winding to be wound round the back, increasing the fill factor. This can in turn increase the power density of the magnet.

[0031] According to a second aspect, there is provided an electronic appliance comprising the electric motor of the first aspect. The electronic appliance may be a vacuum cleaner or an electronic haircare appliance. 8 P004700-W001

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure l is a cross-sectional view of part of an electric motor according to a first example;

[0034] Figure 2 is a cross-sectional view of part of an electric motor according to a second example;

[0035] Figure 3 is a cross-sectional view of part of an electric motor according to a third example;

[0036] Figure 4 is a perspective view of a stator assembly;

[0037] Figure 5 is a cross-sectional view through the stator assembly of Figure 4;

[0038] Figure 6 is a cross-sectional view of one of the sub-assemblies forming part of the stator assembly of Figures 4 and 5;

[0039] Figure 7 is a perspective view of a rotor assembly;

[0040] Figure 8 is a perspective view of an electric motor comprising the stator and rotor assemblies of Figures 4 and 5;

[0041] Figure 9 is a perspective cross-sectional view of the electric motor of Figure 8 taken in the plane indicated in Figure 8; and

[0042] Figure 10 is a schematic perspective view of a vacuum cleaner incorporating an electric motor.

[0043] DETAILED DESCRIPTION

[0044] A cross-sectional view of part of an electric motor 100 is illustrated in Figure 1. The electric motor 100 includes a stator core 101, which in Figure 1 is a c-shaped stator core, with a generally c-shaped form. The stator core 101 has a back 102, a first arm 104 and a second arm 106. The first and second arms 104, 106 extend from the back 102. In this example, the back 102 has a first arm connection portion 108, a second arm connection portion 110 and a central portion 112 between the first and second arm connection portions 108, 110. The first and second arms 104, 106 extend from the first and second arm connection portions 108, 110 of the back 102, respectively, in this example. The first and second arms 104, 106 each comprise a respective first portion 114, 116, a respective second portion 118, 120 and a respective pole portion 122, 124.

[0045] The first portions 114, 116 are proximal portions of the first and second arms 104, 106, which are proximal to the back 102. The first portions 114, 116 extend substantially 9 P004700-W001 parallel to each other and extend substantially perpendicularly from the back 102 in Figure 1 (although in other examples this need not be the case).

[0046] The second portions 118, 120 of the first and second arms 104, 106 are disposed between the first portions 114, 116 and the pole portions 122, 124 of the first and second arms 104, 106 respectively, and extend from the first portions 114, 116 to the pole portions 122, 124. The second portions 118, 120 are obliquely angled with respect to the first portions 114, 116 of the first and second arms 104, 106, respectively. An angle between the first and second portions 114, 118 of the first arm 104 is 0.9 times the angle between the first and second portions 116, 120 of the second arm 106, although in other examples the angle between the first and second portions 114, 118 of the first arm 104 may be less than 0.9 times the angle between the first and second portions 116, 120 of the second arm 106. A length of the second portion 118 of the first arm 104 is 0.7 times a length of the second portion 120 of the second arm 106, although in other examples, the length of the second portion 118 of the first arm 104 may be less than 0.7 times a length of the second portion 120 of the second arm 106. The length of the second portion 118 of the first arm 104 may be less than 0.6 times a length of the first portion 114 of the first arm 104. In some cases (as in the example of Figure 2, discussed below), the length of the second portion 118 of the first arm 104 is zero, i.e. the first arm lacks a second portion, and the pole portion of the first arm extends from the first portion of the first arm. The length of the second portion 120 of the second arm 106 may be between 0.7 times and 1.3 times the length of the first portion 116 of the second arm 106. The second portions 118, 120 of the first and second arms 104, 106 are angled towards each other, so as to form the general C-shape of the stator core 101.

[0047] The pole portions 122, 124 of the first and second arms 104, 106 are disposed at a distal end of the first and second arms 104, 106 respectively, and comprise pole faces 126, 128 of the first and second arms 104, 106, respectively. Whereas the first portions 114, 116 and the second portions 118, 120 each have a substantially constant width along their length (i.e. a substantially constant width in cross-section, as shown in Figure 1), the pole portions 122, 124 flare out, and thus generally increase in width in a direction along the pole portions 122, 124 away from the back 102 and towards the respective pole faces 126, 128. 10 P004700-W001

[0048] The pole portions 122, 124 thus extend to either side of the respective second portions 118, 120.

[0049] The pole portions 122, 124 are spaced from each other to define a slot opening 130 to allow a winding 132 to be wound around the back 102 of the stator core 101. An inner side 134 of the back 102 faces the slot opening 130 and defines, together with the first portions 114, 116 of the first and second arms 104, 106, an interior region 136 of the stator core 101.

[0050] The pole faces 126, 128 are asymmetric to provide saliency. In this example, the pole faces 126, 128 each have a different centre of curvature so that there is a different distance between each pole face 126, 128 and a centre point 138 of the slot opening 130. The pole face 126 of the first arm 114 is asymmetric with respect to the pole face 128 of the second arm 116 but each of the pole faces 126, 128 are also asymmetric about a central point along that pole face 126, 128.

[0051] The electric motor 100 of Figure 1 further includes a rotor 158 (a portion of which is shown in Figure 1), which comprises a permanent magnet in this example. The stator core 101 is mounted relative to the rotor 158 such that the inner side 134 of the back 102 faces the rotor 158. The rotor 158 is configured to rotate about a rotational axis 159 relative to the stator core 101, in use. The rotational axis 159 is into the page in the orientation illustrated in Figure 1. The rotor 158 has a radial axis 160, which lies in a plane perpendicular to the rotational axis 159. The radial axis 160 extends through the slot opening 130 and through the inner side 134. In Figure 1, the radial axis 160 is substantially parallel to the first portions 114, 116 of the first and second arms 104, 106, although this is merely an example. The radial axis 160 extends through the centre point 138 of the slot opening 130 and divides the slot opening 130 substantially equally in two. The centre point 138 is for example an imaginary point that is substantially equidistant from opposing edges of the pole portions 122, 124 of the first and second arms 104, 106. The slot opening 130 of Figure 1 thus comprises slot opening regions (which are for example three-dimensional regions of space) of substantially the same shape and size on opposite sides of the centre point 138. 11 P004700-W001

[0052] The inner side 134 of the back 102 is asymmetric with respect to the radial axis 160. In this example, the first portions 114, 116 and the second portions 118, 120 of the first and second arms 104, 106 are shifted relative to the pole portions 122, 124 so as to shift the inner side 134 relative to the radial axis 160. The first and second portions 114, 118 of the first arm 104 are shifted towards the radial axis 160 and the first and second portions 116, 120 of the second arm 106 are shifted away from the radial axis 160, in a direction perpendicular to the radial axis 160. This causes the inner side 134 to be offset with respect to the radial axis 160, so that a mid-point along a length of the inner side 134 (which is e.g. midway between the first arm connection portion 108 and the second arm connection portion 110 in the direction perpendicular to the radial axis 160) is misaligned with respect to the radial axis 160. Hence, due to the asymmetric inner side 134, a width 140 of a first inner side portion of the inner side 134, from the first arm 104 to the radial axis 160, is smaller than a width 142 of a second inner side portion of the inner side, from the second arm 106 to the radial axis 160. Each of these widths 140, 142 is taken along the length of the inner side 134 in Figure 1, which in this example is perpendicular to the radial axis 160.

[0053] The pole faces 126, 128 in this example remain in the same position relative to the rotor 158 as in a comparative example in which the inner side is symmetric with respect to the radial axis, so as to avoid unduly impacting an electromagnetic performance of a motor comprising the stator core 101. However, in other examples, the position of the pole faces 126, 128 may themselves be offset from their position in the comparative example with the symmetric inner side. The pole faces 126, 128 may be offset in this manner by rotating the pole portions 122, 124 and / or the pole faces 126, 128 relative to the rotor 158, for example to a different extent than in the comparative example.

[0054] A cross-section of the interior region 136 of the stator core 101, as shown in Figure 1, is also asymmetric with respect to the radial axis 160, with a first portion of the cross-section of the interior region 136 between the first portion 114 of the first arm 104 and the radial axis 160 having a smaller area than a second portion of the cross-section of the interior region 136 between the first portion 116 of the second arm 106 and the radial axis 160. 12 P004700-W001

[0055] The first portions 114, 116 of the first and second arms 104, 106 respectively meet the back 102 to form first and second corners 144, 146 within the interior region 136. The first portions 114, 116 of the first and second arms 104, 106 form one side of the first and second corners 144, 146, respectively, and the central portion 112 of the back 102 forms an opposite side of the first and second corners 144, 146.

[0056] The stator core 101 has first and second lines of sight 148, 150 into the first and second comers 144, 146 within the interior region 136, respectively. The first line of sight 148 corresponds to a region between substantially parallel lines from points on opposite sides of the first comer 144 towards points on opposite edges of the slot opening 130. The second line of sight 150 similarly corresponds to a region between substantially parallel lines from points on opposite sides of the second corner 144 towards points on the opposite edges of the slot opening 130. The first and second lines of sight 148, 150 constrain the diameter of the winding 132, which in this example is an electrically conductive wire, that can enter the slot opening 130 to be wound round the central portion 112 of the back 102, without impinging on the pole portions 122, 124.

[0057] The first and second lines of sight 148, 150 each have a width 152, 154 in a direction perpendicular to each of the parallel lines defining the respective lines of sight 148, 150. The asymmetry in the inner side 134 increases the width 152 of the first line of sight 150 relative to a stator core with a similar structure to the stator core 101 of Figure 1 but with a symmetric inner side 134 relative to the radial axis 160. The larger first line of sight 150 allows a winding 132 with a larger diameter to be wound round the central portion 112 of the back 102. For example, the winding 132 may have a diameter of around 1.07 millimetres, which may be larger than a winding that can be wound around a stator core with a symmetric inner side without impinging on the pole portions 122, 124 and with an adequate fill factor. Increasing the first line of sight 150 in this manner may also allow the winding 132 to be more easily wound into the first comer 144, to fill the first corner 144 to a greater extent than otherwise. The asymmetric inner side 134 can therefore enable a higher fill factor for the winding of the stator core 101 to be achieved. 13 P004700-W001

[0058] A length of the first arm 104 is smaller than a length of the second arm 106. The lengths of the first and second arms 104, 106 are taken along the first and second arms 104, 106, respectively, such as along longitudinal axes of the first and second arms 104, 106 from the pole faces 126, 128, along the first and second arms 104, 106 towards the first and second arm connection portions 108, 110 of the back 102, respectively. The decreased length of the first arm 104 relative to the second arm 106 aids in creating the asymmetric inner side 134 relative to the radial axis 160.

[0059] Figure 1 shows the stator core 101 with a winding 132 wound around the central portion 112 of the back 102. The central portion 112 has an outer side 156 opposite to the inner side 112 and the winding 132 comprises a layer 158 having a first layer portion disposed around the inner side 134 and a second layer portion disposed around the outer side 156. The first layer portion is the portion of the layer 158 that is located adjacent to the inner side 134, within the interior region 136 of the stator core 101, and the second layer portion is the portion of the layer 158 that is located adjacent to the outer side 156, outside the interior region 136 of the stator core 101. A cross-section of the first layer portion is asymmetric with respect to the radial axis 160, in this case due to the asymmetry of the inner side 134. A volume of the interior region 136 occupied by a part of the first layer portion between the first portion 114 of the first arm 104 and the radial axis 160 is smaller than a volume of the interior region 136 occupied by a further part of the first layer portion between the first portion 116 of the second arm 106 and the radial axis 160.

[0060] The winding 132 may be wound round the central portion 112 of the back 102 sequentially, by wrapping the winding 132 repeatedly along the central portion 112 of the back 102 in a direction along the length of the back 102 (such as from the first comer 144 to the second corner 146), to create the layer 158. A subsequent layer of the winding 132 may be formed in a similar manner by wrapping the winding 132 repeatedly around the central portion 112 of the back 102 (and around the previously-deposited layer 158) along the central portion 112 of the back 102, such as in the opposite direction to that for depositing the layer 158. This process may be performed repeatedly to create a plurality of layers of the winding 132. The stator core 101 may comprise a bobbin (not shown in Figure 1) located between the stator core 101 and the winding 132 to assist in the winding. 14 P004700-W001

[0061] For example, the bobbin may comprise grooves to receive the winding 132 and to arrange the winding 132 in a particular pattern as it is wound round the central portion 112. The grooves may form a winding guide to guide the winding 132 during winding of the stator core 101, which may be performed using spindle winding, for example.

[0062] Although not shown in Figure 1, it is to be appreciated that the outer side 156 of the central portion 112 (or an outer side of the back 102 in general) may also be asymmetric relative to the radial axis 160 to enable the stator core 101 to be correctly oriented during manufacture.

[0063] Figure 2 shows a further example of part of an electric motor 200 comprising a stator core 201. The stator core 201 of Figure 2 is similar to the stator core 101 of Figure 1 except that the first arm 204 of the stator core 201 of Figure 2 lacks a second portion. Features of Figure 2 that are similar to corresponding features of Figure 1 are labelled with the same reference numeral incremented by 100; corresponding descriptions are to be taken to apply. Certain features that are labelled in Figure 1 are omitted from Figure 2, for clarity.

[0064] In Figure 2, the first arm 204 is formed of a portion 214 that extends substantially perpendicularly from the back 202 and a pole face portion 222. In this case, the portion 214 extends from the first arm connection portion 208 of the back 202. The portion 214 of the first arm 204 in Figure 2 corresponds to the first portion 114 of the first arm 104 in Figure 1. The pole face portion 222 is at a distal end of the portion of 214 of the first arm 204, whereas the back 202 is at the proximal end of the portion 214 of the first arm 204. The pole face portion 222 comprises the pole face 226 of the first arm 204. The first arm 204 of Figure 2 thus lacks the second portion 118 of Figure 1. Instead, the portion 214 of the first arm 204 is directly connected at one end (the proximal end) to the back 202 and at the opposite end (the distal end) to the pole face portion 222 comprising the pole face 226.

[0065] The second arm 206 of Figure 2 is similar to the second arm 106 described with reference to Figure 1 and includes a first portion 216 extending from the back 202 and a second portion 220 extending from, and obliquely angled relative to, the first portion 216 of the second arm 206. 15 P004700-W001

[0066] Due to the different structure of the first and second arms 204, 206 in Figure 2, the length of the first arm 204, along the first arm 204, is less than the length of the second arm 206, along the second arm 206. This creates an asymmetric inner side 234 of the back 202 relative to the radial axis 260 of the rotor 258, which can increase the line of sight and improve the fill factor for the winding 232 as described with reference to Figure 1. The arrangement of Figure 2 may be taken to correspond to a maximum offset of the inner side 234 relative to the radial axis 260, in a plane comprising the radial axis 260 and in a direction perpendicular to the radial axis 260, for which a proximal portion of the first arm 204 (in this case, the portion 214) extends perpendicularly from the back 202.

[0067] Figure 3 shows a further example of part of an electric motor 300 comprising a stator core 301. The stator core 301 of Figure 3 is similar to the stator core 101 of Figure 1 except that the inner side 334 of the back 302 is at a non-perpendicular angle relative to the radial axis 360. Features of Figure 3 that are similar to corresponding features of Figure 1 are labelled with the same reference numeral incremented by 200; corresponding descriptions are to be taken to apply. Certain features that are labelled in Figure 1 are omitted from Figure 3, for clarity.

[0068] In Figure 3, the inner side 334 is angularly shifted with respect to the radial axis 360. Rather than the inner side 334 being positioned perpendicularly to the radial axis 360, the inner side 334 is instead at an oblique angle 362 relative to the radial axis 360. The oblique angle in examples such as Figure 3 may be any angle greater than zero and less than 90 degrees and may thus be an acute angle. In Figure 3, the angle 362 is around 6 degrees. Due to the oblique angle 362 between the inner side 334 and the radial axis 360, the inner side 334 is asymmetric with respect to the radial axis 360.

[0069] The angular displacement of the inner side 334 relative to the radial axis 360 results in a displacement of the first comer 344 away from the rotor 358 and the second corner 346 towards the rotor 358 compared to the stator cores 201, 101 of Figures 1 and 2. This arrangement increases the line of sight 348 into the first corner 344 compared to an arrangement with an inner side that is symmetric relative to the radial axis 360, without 16 P004700-W001 unduly affecting the line of sight 350 into the second corner 346. This can increase the fill factor during winding of the stator core 301, e.g. with a copper wire, for example using spindle winding. Figure 3 illustrates the motor 300 comprising a bobbin 364 disposed between the stator core 301 and the winding 332. The bobbin 364 overlies the inner and outer sides 334, 356 of the back 302, inner surfaces of the first portions 314, 316 of the first and second arms 304, 306, and inner surfaces of the second portions 318, 320 and part of the pole portions 322, 324 of the first and second arms 304, 306. The bobbin 364 thereby lines the interior region 358 of the stator core 301, and allows the winding 322 to be wound about the back 302 of the stator core 301.

[0070] There is a first distance from the inner side 334 to the pole face 326 of the first arm 304, parallel to the radial axis 360, which differs from a second distance from the inner side 334 to the pole face 328 of the second arm 306 due to the asymmetry in the inner side 334. The first and second distances may be between the inner side 334 and centre points of the pole faces 326, 328 of the first and second arms 304, 306 respectively or between the inner side 334 and points aligned with the centre points of the pole faces 326, 328 of the first and second arms 304, 306 respectively along an axis perpendicular to the radial axis 360 (for example if the inner side 334 does not overlap one or both of the centre points of the pole faces 326, 328).

[0071] The first distance is larger than the second distance in this example, to allow the winding to be wound more effectively into the first corner 344. The larger first distance increases the overall volume of the stator core 301. However, the smaller second distance may at least partially compensate for a reduction in performance of the motor 300 due to the larger first distance. For example, if the motor 300 is used to generate an airflow, the smaller second distance may compensate for a reduction in airflow area resulting from the larger first distance. This arrangement can thus enable the diffusion of air passing along the outer surface 356 of the stator core 301. The first distance may be larger than the second distance by at least 0.3 millimetres (mm). In the example of Figure 3, the first distance is around 0.7mm larger than the second distance. 17 P004700-W001

[0072] In Figure 3, the structure of the first and second arms 304, 306 is similar to the structure of the first and second arms 104, 106 of Figure 1. However, due to the obliquely angled inner side 334 of the back 302, a proximal portion of the first arm 304, proximal to the inner side 334 (which in this case is the first portion 314 of the first arm 304) extends from the inner side at a different angle than a proximal portion of the second arm 306 (which in this case is the first portion 316 of the second arm 306), proximal to the inner side 334. The first and second arms 304, 306 comprise second portions 318, 320 and pole portions 322, 324 which are similar to those described with reference to Figures 1 and 2. The first portion 314 of the first arm 304 is acutely angled relative to the inner side 334, whereas the first portion 316 of the second arm 306 is obtusely angled relative to the inner side 334. The first portions 314, 316 of the first and second arms 304, 306 are parallel to each other and parallel to the radial axis 360.

[0073] Various approaches may be used to identify if an inner side of a back of a stator core is asymmetric with a radial axis of a rotor as described herein. For example, measurements of various aspects of a motor may be obtained from a computed tomography (CT) scan of the motor. For example, it may be determined from a CT scan of the motor that the width of the first inner side portion is different from the width of the second inner side portion as described with reference to Figure 1 and / or that the first distance from the inner side to the pole face of the first arm differs from the second distance from the inner side to the pole face of the second arm, in a direction parallel to the radial axis, as described with reference to Figure 3.

[0074] A stator assembly 10 is shown in Figures 4 and 5. The stator assembly 10 comprises three stator core sub-assemblies 16 arranged annularly. The stator assembly 10 comprises a central aperture for receiving a rotor. The stator assembly 10 may further comprise a termination assembly (not shown) comprising terminals connected to the windings 5.

[0075] Referring now to Figure 6 (which shows just one of the stator core sub-assemblies), each of the stator core sub-assemblies 16 has the same structure and comprises a stator core 1, a bobbin 3 and a winding 5. 18 P004700-W001

[0076] The stator core 1 of each of the stator sub-assemblies 16 is similar to the stator cores 101, 201, 301 described with reference to Figures 1 to 3. However, the stator core 1 of each of the stator sub-assemblies 16 comprises first and second fins 7, 9, which extend generally outwardly from a back 2 of the stator core 1 at a region where first and second arms 4, 6 of the stator core 1 extend from the back 2. The first fin 7 therefore extends in a direction away from the first arm 4, and the second fin 9 extends is a direction away from the second arm 6. The stator core 1 is nevertheless a c-shaped stator core in that the back 2, the first arm 4 and the second arm 6 (i.e., those parts of the stator core 1 through which induced stator flux links with the rotor) form a c-shape. The first and second fins 7, 9 are angled relative to the back 2 such that they fins extends in a generally circumferential direction around the stator assembly 10. Outer surfaces of the first and second fins 7, 9 have a number of grooves 11, 13, which increases the surface area of the fins 7, 9. The fins 7, 9 act as heat sinks and dissipate heat generated by the stator assembly 10 to the surrounding environment.

[0077] Figures 5 and 6 also show a rotational axis 59 of a rotor received within the central aperture of the stator assembly 16 (which corresponds to the rotational axes 159, 259, 359 of Figures 1 to 3), as well as a radial axis 60 that extends through a centre point of the slot opening 30 of each of the stator cores 1 (corresponding to the radial axes 160, 260, 360 of Figures 1 to 3). As with the examples described above with reference to Figures 1 to 3, the inner side of the back 2 of each stator core 1 is asymmetric with respect to the radial axis 60 extending through the centre point of corresponding slot opening 30.

[0078] Figure 7 shows a rotor assembly 12 suitable for use with the stator assembly of Figure 4. The rotor assembly 12 may also be used as any of the rotors 158, 258, 358 shown in part in Figures 1 to 3. The rotor assembly 12 comprises a shaft 70, a magnet assembly 72, a bearing assembly 74 and an impeller 76. The magnet assembly 72 is mounted to the shaft 70 at a first end and comprises a permanent magnet 80 sandwiched between a pair of balance members 81, 82. The impeller 76 is mounted to the shaft 70 at a second end and comprises a centrifugal impeller. The bearing assembly 74 is mounted to the shaft 70 at a location between the magnet pack 72 and the impeller 76. The bearing assembly 74 comprises a pair of bearings 83, 84 that are spaced apart, and a spring 85 that applies a 19 P004700-W001 preload to each of the bearings 83, 84. In use, the rotor assembly 12 rotates about a rotational axis 59 (which corresponds to the rotational axes 159, 259, 359 of Figures 1 to 3 and is also shown in Figure 4), causing the impeller 76 to generate an airflow.

[0079] An electric motor 90 comprising the stator assembly 10 and the rotor assembly 12 of Figures 4 and 7 respectively is shown in Figures 8 and 9. The electric motor 90 comprises a housing 92, which defines a main body of the electric motor 90. The housing 92 is generally cylindrical in form and defines a channel within which the stator and rotor assemblies 10, 12 are received.

[0080] Any of the electric motors herein described, such as the electric motors 100, 200, 300 of Figures 1 to 3 or the electric motor 90 of Figures 8 and 9, may be used in various electronic appliances in which an airflow is to be generated, such as a vacuum cleaner or an electronic haircare appliance. As an example, a vacuum cleaner 500 comprising the electric motor 90 of Figure 8 is illustrated schematically in Figure 10.

[0081] In other examples, the rotor assembly of an electric motor according to examples herein need not comprise an impeller and may be used for other purposes than those involving the generation of an airflow. Electric motors comprising a rotor assembly according to these other examples may thus be comprised by electronic appliances which do not involve airflow generation.

[0082] In the examples described above, the rotor comprises a permanent magnet. However, the stator core and stator assembly may be used with other types of rotor, such as a ferromagnetic rotor or a rotor having a ferromagnet core with permanent magnets mounted to or embedded within the core. Accordingly, various types of electric motor may be used as the electric motor herein, such as a permanent-magnet motor, internal permanentmagnet motor, or switched reluctance motor.

Claims

20 P004700-W001CLAIMS1. An electric motor comprising: a rotor having a rotational axis and a radial axis perpendicular to the rotational axis; and a c-shaped stator core comprising a back, a first arm and a second arm, each of the first arm and the second arm extending from the back and comprising a pole face, wherein: the pole face of the first arm is spaced from the pole face of the second arm to define a slot opening; the stator core is mounted relative to the rotor such that an inner side of the back faces the rotor; the radial axis extends through a centre point of the slot opening and through the inner side of the back; and the inner side of the back is asymmetric with respect to the radial axis.

2. The electric motor of claim 1, wherein a length of the first arm is different from a length of the second arm.

3. The electric motor of claim 1 or claim 2, wherein a width of a first inner side portion of the inner side, from the first arm to the radial axis, is different from a width of a second inner side portion of the inner side, from the second arm to the radial axis.

4. The electric motor of any one of claims 1 to 3, wherein the inner side is at a nonperpendicular angle relative to the radial axis.

5. The electric motor of any one of claims 1 to 4, wherein a first distance from the inner side to the pole face of the first arm, parallel to the radial axis, differs from a second distance from the inner side to the pole face of the second arm, parallel to the radial axis.21 P004700-W0016. The electric motor of any one of claims 1 to 5, wherein a proximal portion of the first arm, proximal to the inner side, extends from the inner side at a different angle than a proximal portion of the second arm, proximal to the inner side.

7. The electric motor of any one of claims 1 to 6, wherein each of the first and second arms comprise a first portion extending from the back and a second portion extending from, and obliquely angled relative to, the first portion.

8. The electric motor of claim 7, wherein an angle between the first and second portions of the first arm is less than or equal to 0.9 times an angle between the first and second portions of the second arm.

9. The electric motor of claim 7 or claim 8, wherein a length of the second portion of the first arm is less than or equal to 0.7 times a length of the second portion of the second arm.

10. The electric motor of any one of claims 7 to 9, wherein a length of the second portion of the first arm is less than 0.6 times a length of the first portion of the first arm.

11. The electric motor of any one of claims 7 to 10, wherein a length of the second portion of the second arm is between 0.7 times and 1.3 times a length of the first portion of the second arm.

12. The electric motor of any one of claims 7 to 11, wherein the inner side and the first portions of the first and second arms define an interior region of the stator core, and a cross-section of the interior region is asymmetric with respect to the radial axis.

13. The electric motor of any one of claims 1 to 6, wherein the first arm is formed of a portion extending substantially perpendicularly from the back and a pole face portion disposed at a distal end of the portion of the first arm and comprising the pole face of the first arm, and the second arm comprises a first portion extending from the back and a22 P004700-W001 second portion extending from, and obliquely angled relative to, the first portion of the second arm.

14. The electric motor of any one of claims 1 to 13, wherein at least one of the first arm and the second arm extends at an acute angle relative to the inner side.

15. The electric motor of claim 14, wherein: the first arm extends at the acute angle relative to the inner side and the second arm extends at an obtuse angle relative to the inner side; or the second arm extends at the acute angle relative to the inner side and the first arm extends at the obtuse angle relative to the inner side.

16. The electric motor of any one of claims 1 to 15, wherein the pole faces of the first arm and the second arm are asymmetric.

17. The electric motor of any one of claims 1 to 16, comprising a winding wound around the back of the stator core, wherein the back has an outer side opposite to the inner side, the winding comprises a layer having a first layer portion disposed around the inner side and a second layer portion disposed around the outer side, and a cross-section of the first layer portion, is asymmetric with respect to the radial axis.

18. An electronic appliance comprising the electric motor of any one of claims 1 to 17.

19. The electronic appliance of claim 18, wherein the electronic appliance is a vacuum cleaner or an electronic haircare appliance.

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