Stator core

Abstract

A stator core 1 for a stator (3, fig 3) of an electric machine (5, fig 3) preferably in a vehicle (V, fig 1). The stator core 1 has a central stator axis (X1, fig 2); and a plurality of winding slots

Description

TECHNICAL FIELD The present disclosure relates to a stator core. The stator core is suitable for use in a stator of an electric machine. Aspects of the invention relate to a stator core, a stator, an electric machine and a vehicle. BACKGROUND It is known to use one or more electric machine in a vehicle. Such electric machines may operate as motors or as generators. Electric machines may operate as traction motors for propelling a vehicle such as an automobile, van, truck, motorcycle, boat, or aeroplane. Electric machines may be used in place of, or in addition to, an internal combustion engine. Such electric machines comprise a stator and a rotor, separated by an air gap, for example as part of a permanent magnet synchronous motor. The stator is a stationary element of the electric machine which may comprise a plurality of slots within which electrical stator windings are located. The rotor is a rotating element of the electric machine allowing a transfer of electrical energy input into the motor to a mechanical output, such as the rotation of a driveshaft of the vehicle. The vehicle may, for example, comprise a battery electric vehicle (BEV), a plug-in hybrid electric vehicle (PHEV) or a hybrid electric vehicle (HEV) where the electric machine is a traction motor for the vehicle. It is desirable to have the lightest possible traction motor with optimised energy conversion from an electrical energy input to a mechanical energy output whilst maintaining the integrity of the traction motor. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a stator core, a stator, an electric machine and a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided a stator core according to claim 1. According to an aspect of the present invention there is provided a stator core for a stator of an electric machine, the stator core comprising: a central stator axis; a plurality of winding slots in the stator core; and at least one first cooling duct in the stator core, the at least one first cooling duct extending substantially parallel to the central stator axis and being radially separated from the winding slots by a region of the stator core; wherein the or each first cooling duct has a first profile in transverse section comprising a first edge disposed in a radially innermost position and extending substantially perpendicular to a radial axis of the stator core. The first edge provides an increased surface area in the first cooling duct which faces inwardly. In use, this arrangement may promote heat transfer from the stator core into the or each first cooling duct. This 1 may promote heat rejection from the stator core into the or each first cooling duct. In use, a fluid flows through the first cooling duct to transfer heat from the stator core. The fluid may, for example, be a liquid coolant. At least in certain embodiments, heat generated by the stator windings may be transferred to the liquid coolant. The or each first cooling duct may be formed integrally in the stator core. The winding slots may be formed integrally in the stator core. The first cooling ducts may each be associated with one of the winding slots. The or each first cooling duct may be at least substantially aligned with the associated winding slot. The or each first cooling duct may be disposed radially outboard of an associated one of the winding slots. The first edge of each first cooling duct may be disposed adjacent to one of the winding slots. The winding slots may comprise a slot base which is disposed in a radially outermost position. The slot base may extend substantially perpendicular to a radial axis of the stator core. The first edge of the first cooling duct may extend at least substantially parallel to the slot base of the associated winding. The first profile may comprise or consist of a first polygon. The first polygon may be composed of a plurality of edges (sides) and a plurality of corners. The first edge may be one of the plurality of edges. The edges (sides) of the polygon may be rectilinear. Alternatively, the edges (sides) of the polygon may be curved, for example concave or convex. The edges of the polygon may be arcuate. At least one of the corners of the first polygon may be rounded. The at least one corner may comprise a radius. This arrangement may help to distribute mechanical loads in the stator core. At least one of the edges of the first polygon may be rectilinear. In certain embodiments, each edge of the first polygon may be rectilinear. Alternatively, at least one of the edges of the first polygon may be curved. The at least one of the edges of the first polygon may be concave or convex, for example. In certain embodiments, each edge of the first polygon may be curved. In a variant, one or more of the edges of the first polygon may be rectilinear and one or more edges of the first polygon may be curved. The first polygon may comprise a triangle. The triangle may have one or more rounded corners. The triangle may be an equilateral triangle or an isosceles triangle, for example. The or each first cooling duct may comprise at least one internal fin. The at least one internal fin may project into the first cooling duct. At least in certain embodiments, this may promote heat transfer from the stator core to a fluid in the first cooling duct. The or each cooling duct may comprise at least one of the following: at least one internal fin disposed at one or more corners of the cooling duct; and at least one internal fin disposed on one or more edges of the cooling duct. The at least one internal fin may be disposed at a mid-point of the edge of the cooling duct The or each first cooling duct may comprise a first line of (reflection) symmetry. The first line of symmetry may be at least substantially aligned with a radial axis of the stator core. The radial axis may extend substantially 2 perpendicular to the central stator axis. The radial axis may be a central slot axis of one of the winding slots formed in the stator core. In this arrangement, the first line of symmetry of the first cooling duct may be aligned with the central slot axis. The first polygon may comprise or consist of one of the following: a rhombus; a kite; a trapezoid, for example an isosceles trapezoid; a rectangle; a square; a pentagon; and a hexagon. The first polygon may be a regular polygon. The first polygon may be symmetrical about a radial axis of the stator core. The stator core may comprise at least one second cooling duct. The at least one second cooling duct may extend substantially parallel to the central stator axis. The or each second cooling duct may be formed integrally in the stator core. The or each second cooling duct may be radially separated from the winding slots by a region of the stator core. The or each second cooling duct may have a second profile in transverse section. The second profile may comprise a first edge disposed in a radially outermost position. The first edge may extend substantially perpendicular to a radial axis of the stator core. The stator core may comprise a plurality of stator teeth. The stator teeth may be formed integrally in the stator core. The stator teeth may be disposed between the stator slots. The second cooling ducts may each be associated with one of the stator teeth. The or each second cooling duct may be at least substantially aligned with an associated one of the stator teeth. The or each second cooling duct may be disposed radially outboard of the associated stator tooth. The or each second cooling duct may comprise a second line of (reflection) symmetry. The second line of symmetry may be at least substantially aligned with a radial axis of the stator core. The radial axis may extend substantially perpendicular to the central stator axis. The radial axis may be a central tooth axis of a stator tooth formed in the stator core. In this arrangement, the second line of symmetry of the second cooling duct may be aligned with the central tooth axis. At least in certain embodiments, the first line of symmetry of the or each first cooling duct may be at least substantially aligned with a central slot axis; and the second line of symmetry of the or each second cooling duct may be at least substantially aligned with a central tooth axis. In a variant, the first cooling ducts may each be associated with one of the stator teeth. The or each first cooling duct may be at least substantially aligned with the associated stator tooth. The or each first cooling duct may be disposed radially outboard of an associated one of the stator teeth. The first line of symmetry of the or each first cooling duct may be at least substantially aligned with a central tooth axis. Alternatively, or in addition, the second cooling ducts may each be associated with one of the winding slots. The or each second cooling duct may be at least substantially aligned with the associated winding slot. The or each second cooling duct may be disposed radially outboard of an associated one of the winding slots. The second line of symmetry of the or each second cooling duct may be at least substantially aligned with a central slot axis. The first polygon may comprise a triangle. The triangle may have one or more rounded corners. The triangle may be an equilateral triangle or an isosceles triangle, for example. The first and second cooling ducts may be offset from each other in a radial direction and / or a circumferential direction in the stator core. The first and second cooling ducts may at least partially overlap each other in a radial direction and / or at least partially overlap each other in a circumferential direction. The first and second cooling ducts may at least partially interlock with each other, i.e. the first and second cooling ducts may be tessellated. The first and second cooling ducts may comprise respective first and second triangles. Alternatively, the first and second cooling ducts may comprise first and second rhombuses. The first and second cooling ducts may comprise first and second isosceles trapezoids. The orientation of the first and second isosceles trapezoids may be reversed to form an interlocking arrangement of the first and second cooling ducts. The first and second cooling ducts may have edges disposed adjacent to each other. The adjacent edges may be oriented at least substantially parallel to each other. The adjacent edges of the first and second cooling ducts may be oriented at an acute (non-zero) angle to a radial axis of the stator core. A bridge may be formed in the stator core between adjacent pairs of the first and second cooling ducts, The or each bridge may have a central axis oriented at an acute angle to a radial axis of the stator axis. According to a further aspect of the present invention there is provided a lamination for forming the stator core described herein. The stator core may be formed from a plurality of the laminations. The laminations may be arranged in a stack. According to a further aspect of the present invention there is provided a stator comprising a stator core as described herein. The stator is an assembly comprising the stator core and a stator winding. According to a further aspect of the present invention there is provided an electric machine comprising a stator as described herein. The electric machine may be an electric traction motor for propelling a vehicle, such as an automobile. According to a further aspect of the present invention there is provided a vehicle comprising a stator core as described herein. The stator core may be provided in an electric machine. The electric machine may be incorporated into an electric drive unit. According to a further aspect of the present invention there is provided a stator core for a stator of an electric machine, the stator core comprising: a central stator axis; a plurality of winding slots in the stator core; and at least one cooling duct in the stator core, the at least one cooling duct extending substantially parallel to the central stator axis and being radially separated from the winding slots by regions of the stator core; wherein the or each cooling duct has a profile in transverse section comprising or consisting of a polygon composed of a plurality of edges and corners. The profile may comprise a first edge disposed in a radially innermost position and extending substantially perpendicular to a radial axis of the stator core. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, byway of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle comprising an electric machine having a stator core in accordance with an embodiment of the invention; Figure 2 shows a transverse section through the stator core shown in Figure 1; Figure 3 shows a longitudinal section through the electric machine shown in Figure 1; Figure 4A shows a segment of the stator core in accordance with an embodiment of the present invention; Figure 4B shows a computational representation of the magnetic flux in the stator core shown in Figure 4A; Figure 5 shows an enlarged view of a portion of the stator core shown in Figure 4A; Figure 6A shows a segment of the stator core in accordance with a further embodiment of the present invention; Figure 6B shows a computational representation of the magnetic flux in the stator core shown in Figure 6A; Figure 7 shows an enlarged view of a portion of the stator core shown in Figure 6A; Figure 8 shows the spacing between the first and second cooling ducts in the stator core shown in Figure 6A; Figure 9A shows a segment of the stator core in accordance with a further embodiment of the present invention; Figure 9B shows a computational representation of the magnetic flux in the stator core shown in Figure 9A; Figure 10 shows an enlarged view of a portion of the stator core shown in Figure 9A; 5 Figure 11A shows a segment of the stator core in accordance with a further embodiment of the present invention; Figure 11B shows a computational representation of the magnetic flux in the stator core shown in Figure 11A; Figure 12 shows an enlarged view of a portion of the stator core shown in Figure 11 A; and Figures 13A, 13B, 13C and 13D illustrate other possible profiles of the cooling ducts formed in the stator core in accordance with an embodiment of the present invention. DETAILED DESCRIPTION A stator core 1 in accordance with an embodiment of the present invention will now be described with reference to the accompanying Figures. The stator core 1 is suitable for a stator 3 of an electric machine 5. As described herein, the stator 3 is an assembly comprising the stator core 1 and a plurality of stator windings. As shown schematically in Figure 1, the electric machine 5 is configured to be used in an electric drive unit EDU1 of a vehicle V. The vehicle V is a road vehicle having a plurality of wheels W-n. The electric machine 5 is configured, in use, to generate torque to drive one or more of the wheels W-n. The electric machine 5 may be referred to as a traction motor or a drive motor. The vehicle V comprises one or more traction battery BTT1 for storing electrical energy. The vehicle V may be a battery electric vehicle (BEV), a hybrid electric vehicle (HEV) or a plug-in hybrid electric vehicle (PHEV). The electric drive unit EDU1 comprises one or more controller 7 and at least one inverter 9 for converting direct current (DC) supplied from the traction battery BTT1 to alternating current (AC) for supply to the electric machine 5. The electric machine 5 is a three-phase machine in the present embodiment. In use, the electric machine 5 generates a torque which is output to an axle of the vehicle V to drive one or more of the wheels W-n. One or more of the electric machines 5 may be used in the vehicle V. The vehicle V in the present embodiment is a passenger vehicle, such as an automobile. The electric machine 5 may be used in other types of vehicles, such as a utility vehicle or a sports utility vehicle. A transverse section through the electric machine 5 is shown in Figure 2; and a longitudinal section through the electric machine 5 is shown in Figure 3. The stator 3 is configured to form a plurality of magnetic poles which, in use, are selectively energized to cause a rotor 11 to rotate about a rotational axis X. The stator core 1 has a central stator axis X1 which is coincident with the rotational axis X. The stator core 1 is composed of a plurality of steel laminations, typically electrical steel. The stator core 1 comprises a radially inner portion STIN and a radially outer portion STOUT, as shown in Figure 3. The radially outer portion STOUT may be referred to as a stator back-iron region (or yoke) of the stator core 1. The stator core 1 comprises an inner surface 13 in the form of a right circular cylinder; and an outer surface 14 in the form of a right circular cylinder. A gap (not shown) is maintained between the inner surface 13 of the stator core 1 and the rotor 11. The outer surface 14 of the stator core 1 is not necessarily a right circular cylinder and other shapes are contemplated. The stator core 1 comprises a plurality of winding slots 15. The winding slots 15 are formed in the radially inner portion STIN of the stator core 1. The winding slots 15 are each configured to receive a stator winding 17 (shown 6 schematically in Figure 2) made up of wound coils. The stator 3 is a stator assembly comprising the stator core 1 and stator windings 17. The winding slots 15 extend in a longitudinal direction substantially parallel to the central stator axis X1 along a length of the stator core 1. The winding slots 15 each have a central slot axis SY extending in a radial direction substantially perpendicular to the central stator axis X1. Adjacent slot axes SY are offset from each other by an angular spacing (pitch) which is substantially uniform around the stator core 1. The winding slots 15 each comprise: a slot opening 19 open to the inner surface 13 of the stator core 1; and a slot base or end wall 20. The slot end wall 20 is disposed at a radially outermost end of the winding slot 15. The stator core 1 comprises a plurality of stator teeth 21. The stator teeth 21 are formed between the winding slots 15 and project radially inwardly from the radially outer portion STOUT of the stator core 1. Each of the plurality of stator teeth 21 comprises a central tooth axis TY extending in a radial direction substantially perpendicular to the central stator axis X1. The stator teeth 21 are symmetrical about the respective tooth axis TY. The stator teeth 21 are formed integrally with the stator core 1 and each have a radially outer end 21A and a radially inner end 21B. A first lateral projection 23 and a second lateral projection 25 are formed on the radially inner end 21B of each stator tooth 21. The first and second lateral projections 23, 25 extend in first and second circumferential directions which are opposite to each other. The first and second lateral projections 23, 25 have respective first and second radially outer surfaces 27, 29 oriented into the winding slots 15. In the present embodiment, the first and second radially outer surfaces 27, 29 are inclined at an acute angle to the central tooth axis TY. The radially inner end 21B of each stator tooth 21 has a part-cylindrical inner surface 30. The plurality of part-cylindrical inner surfaces 30 collectively form the inner surface 13 of the stator core 1. The stator teeth 21 in the present embodiment each comprise first and second tooth tips extending in opposite circumferential direction. The first and second tooth tips partially close the winding slots 15. In a variant, each of the winding slots 15 may be closed by a bridge section extending in a circumferential direction between adjacent stator teeth 21. In a further variant, the tooth tips may be omitted. At least one cooling duct 33 is formed in the stator core 1. The at least one cooling duct 33 comprises an aperture or a channel extending in a longitudinal direction within the stator core 1. The at least one cooling duct 33 is formed in the radially outer portion STOUT of the stator core 1. The at least one cooling duct 33 is formed radially outwardly of the winding slots 15. The at least one cooling duct 33 is separated from the winding slots 15 by regions of the stator core 1. The or each cooling duct 33 extends substantially parallel to the central stator axis X1. In the present embodiment, the stator core 1 comprises a plurality of the cooling ducts 33. The cooling ducts 33 are configured to receive a cooling fluid to promote cooling of the stator core 1. In use, the cooling fluid is pumped through the cooling ducts 33 to promote heat rejection from the stator core 1. The cooling fluid in the present embodiment is a liquid coolant. The stator core 1 comprises at least one cooling fluid inlet port 35 and at least one cooling fluid outlet port 37. The at least one cooling fluid inlet port 35 may comprise an annular inlet chamber; and the at least one cooling fluid outlet port 37 may comprise an annular outlet chamber. In use, the cooling fluid is introduced into the annular inlet chamber through the at least one cooling fluid port 35. The cooling fluid flows from the annular inlet chamber through the or each cooling duct 33 and enters the annular outlet chamber. The cooling fluid is discharged through the at least one cooling fluid outlet port 37. The cooling fluid is passed through a heat exchanger to reject heat and is then recirculated through the stator core 1. As shown in Figure 3, the at least one cooling fluid inlet port 35 and the at least one cooling fluid outlet port 37 may be provided at respective first and second ends of the stator core 1. In use, the cooling fluid is introduced 7 through the at least one cooling fluid inlet port 35 at the first end of the stator 1 and flows through the cooling ducts 33 before exiting through the at least one cooling fluid outlet port 37 at the second end of the stator 1. The cooling fluid is passed through a heat exchanger (not shown), such as a radiator, and re-circulated through the stator core 1. In this arrangement, the cooling fluid flows through the cooling ducts 33 in a first direction. In a variant, the at least one cooling fluid inlet port 35 and the at least one cooling fluid outlet port 37 may both be provided at the first end of the stator core 1. The cooling fluid flow direction of the cooling fluid may be reversed in some of the cooling ducts 33. At least one flow reversal channel may be provided at the second end of the stator core 1. In use, the cooling fluid is introduced through the at least one cooling fluid inlet port 35 at the first end of the stator 1. The cooling fluid flows through at least one first cooling duct 33 in a first direction and is then re-directed by the at least one flow reversal channel to flow through at least one second cooling duct 33 in a second direction. The first and second directions are opposite to each other in this arrangement. The cooling fluid exits through the at least one cooling fluid outlet port 37 at the first end of the stator 1. The first and second cooling ducts 33 may be offset from each other in a radial direction and / or a circumferential direction. The configuration of the cooling ducts 33 according to an embodiment of the present invention will now be described with referenced to Figures 4A, 4B and 5. In the present embodiment, the cooling ducts 33 each have a line of (reflection) symmetry. The line of symmetry of each cooling duct 33 is at least substantially aligned with a radial axis of the stator core 1 extending substantially perpendicular to the central stator axis X1. Each of the cooling ducts 33 have a profile comprising or consisting of a polygon in transverse section (i.e., in a plane perpendicular to the central stator axis X1). The cooling ducts 33 may, for example, have a profile comprising or consisting of a triangle, a rhombus (diamond), a kite, a trapezoid, a rectangle, a square, a pentagon or a hexagon. The polygonal profile of each cooling duct 33 is composed of a plurality of edges 41-n and a plurality of corners 43-n. The edges 41-n may be planar or may be curved, for example the edges 41-n may be concave or convex. The corners 43-n are preferably rounded, for example comprising or consisting of a substantially continuously curved profile. The polygon may be equiangular and / or equilateral. Each cooling duct 33 may be a regular polygon in transverse section. In the present embodiment, each cooling duct 33 has a profile comprising or consisting of a triangle in transverse section. One of the edges 41-n forms a base of the triangle. The corners 43-n are rounded to form a rounded triangle. In the present embodiment, each cooling duct 33 has a profile comprising or consisting of an equilateral triangle. In a variant, each cooling duct 33 may have a profile comprising or consisting of an isosceles triangle. Each cooling duct 33 is in the form of an equilateral triangle in transverse section. In a variant, the cooling ducts 33 may have a profile comprising or consisting of a circle, a part-circle or an ellipse in transverse section. The cooling ducts 33 may have a profile comprising or consisting of a stadium in transverse section. Other profiles of the cooling ducts 33 are contemplated. In the present embodiment, the cooling ducts 33 comprise a plurality of first cooling ducts 33A and a plurality of second cooling ducts 33B. The first cooling ducts 33A and the second cooling ducts 33B are spaced apart from each other in the stator core 1. In the present embodiment, the first cooling ducts 33A and the second cooling ducts 33B are radially and circumferentially offset from each other. The first and second cooling ducts 33A, 33B have first and second geometric centres C1, C2 respectively. The first geometric centre C1 of each said first cooling duct 33A is disposed at a first radial distance R1 from the central stator axis X1. The second geometric 8 centre C2 of each said second cooling duct 33B is disposed at a second radial distance R2 from the central stator axis X1. The second radial distance R2 is greater than the first radial distance R1 in the present embodiment. In a variant, the second radial distance R2 may be less than or substantially equal to the first radial distance R1. The first cooling ducts 33A are aligned with the winding slots 15. In particular, the first geometric centre C1 of each first cooling duct 33A is disposed on a corresponding central slot axis SY. The second cooling ducts 33B are aligned with the stator teeth 21. In particular, the second geometric centre C2 of each second cooling duct 33B is disposed on a corresponding central tooth axis TY. The position of the first and second cooling ducts 33A, 33B relative to the winding slots 15 and the stator teeth 21 may be reversed. The first and second cooling ducts 33A, 33B have substantially like profiles in transverse section. However, the profiles of the first and second cooling ducts 33A, 33B have different orientations. The first and second cooling ducts 33A, 33B are arranged in respective first and second orientations. In the present embodiment, the first cooling ducts 33A are angularly offset from the second cooling ducts 33B by an angular rotation of approximately 180°. The first and second cooling ducts 33A, 33B are oriented in opposite directions. Other angular offsets are contemplated, for example 30°, 60°, 90° or 120°. The first and second cooling ducts 33A, 33B each have a profile comprising or consisting of a triangle in transverse section. The first cooling duct 33A may have a first profile, and the second cooling duct 33B may have a second profile, the first and second profiles may be the same as each other or may be different from each other. For example, the first cooling ducts 33A may have a first profile comprising or consisting of a triangle; and the second cooling ducts 33B may have a second profile comprising or consisting of a rhombus. These profiles may be reversed such that the first cooling ducts 33A have a first profile comprising or consisting of a rhombus; and the second cooling ducts 33B may have a second profile comprising or consisting of a triangle. Other combinations are contemplated. With reference to Figure 5, each first cooling duct 33A comprises a first edge 41 A-1, a second edge 41A-2 and a third edge 41A-3. The first cooling duct 33A comprises a first corner 43A-1, a second corner 43A-2 and a third corner 43A-3. The second and third edges 41 A-2, 41A-3 are inclined at an acute angle relative to the central slot axis SY. The second and third edges 41 A-2, 41A-3 are tapered towards each other in a radially outwards direction along the central slot axis SY. The first edge 41 A-1 is oriented substantially perpendicularto the central slot axis SY. The first edge 41 A-1 of the first cooling duct 33A is disposed in a radially innermost position along the central slot axis SY. The first corner 43A-1 is disposed in a radially outermost position along the central slot axis SY. The first corner 43A-1 is directed radially outwardly along the central slot axis SY of a corresponding stator slot 15. The first edge 41 A-1 is disposed closest to the winding slot 15. In particular, the first edge 41 A-1 is disposed closest to the slot base 20. The first edge 41 A-1 is presented towards the winding slot end wall 20. At least in certain embodiments, the positioning of the first edge 41 A-1 (rather than a corner of the cooling duct) closest to the winding slot 15 may provide improved heat transfer properties when in use. The first edge 41 A-1 of the first cooling duct 33A may be orientated so as to be substantially parallel with the winding slot end wall 20, so as to optimise heat transfer from the stator winding coil 17 into the cooling fluid carried by the cooling duct 33A in use. With reference to Figure 5, each second cooling duct 33B comprises a first edge 41B-1, a second edge 41B-2 and a third edge 41B-3. The second cooling duct 33B comprises a first corner 43B-1, a second corner 43B-2 and a third corner 43B-3. The second and third edges 41B-2, 41B-3 are inclined at an acute angle relative to the central tooth axis TY. The second and third edges 41B-2, 41B-3 are tapered inwardly towards each other in 9 a radially inwards direction along the central tooth axis TY. The first edge 41B-1 is oriented substantially perpendicular to the central tooth axis TY. The first edge 41B-1 of the second cooling duct 33B is disposed in a radially outermost position along the central tooth axis TY. The first corner 43B-1 is disposed in a radially innermost position along the central tooth axis TY. The first edge 41B-1 is disposed in a radially outermost position and extends substantially perpendicular to the central tooth axis TY. The first edge 41B-1 is disposed closest to the outer surface 14 of the stator core 14. The first corner 43A-1 of the first cooling duct 33A is disposed in a radially outermost position and the second and third edges 41A-2, 41A-3 of the first cooling duct 33A open outwardly in a radially inwards direction. The first corner 43B-1 of the second cooling duct 33B is disposed in a radially innermost position and the second and third edges 41B-2, 41B-3 of second first cooling duct 33B open outwardly in a radially outwards direction. The first and second cooling ducts 33A, 33B are oriented in opposite directions. This alternating arrangement is repeated around the stator core 1. At least in certain embodiments, this inter-locking (or tessellated) arrangement of the first and second cooling ducts 33A, 33B may help to distribute the flux more uniformly within the stator core 15. The second edge 41A-2 of each first cooling duct 33A is oriented substantially parallel to the second edge 41B-2 of an adjacent second cooling duct 33B. The third edge 41A-3 of each first cooling duct 33A is oriented substantially parallel to the third edge 41B-3 of an adjacent second cooling duct 33B. A bridge 45 is formed integrally in the stator core 1 between each pair of adjacent first and second cooling ducts 33A, 33B. A plurality of the bridges 45 is formed around the stator core 1 to form a continuous (closed) loop. The bridges 45 each have a central axis 47 oriented at a non-zero angle a relative to a radial axis Y1 of the stator core extending substantially perpendicular to the central stator axis X1. The bridges 45 each have a width corresponding to a (shortest) separation distance SD between the first and second cooling ducts 33A, 33B. In the present embodiment, the separation distance SD is measured perpendicular to the opposing edges of the first and second cooling ducts 33A, 33B which are arranged parallel to each other. The bridges 45 have a substantially constant width along their length. In the present embodiments, all of the bridges 45 formed between the first and second cooling ducts 33A, 33B have at least substantially the same width. In the present embodiment, the plurality of bridges 45 comprise a first bridge 45A formed between the opposing second edges 41A-2, 41B-3 of the first and second cooling ducts 33A, 33B; and a second bridge 45B formed between the opposing third edges 41A-3, 41B-3 of the first and second cooling ducts 33A, 33B. The first and second bridges 45A, 45B have respective first and second central axis 47A, 47B oriented at non-zero first and second angles a1, a2 relative to a radial axis Y1 of the stator core extending substantially perpendicular to the central stator axis X1. The first and second central axis 47A, 47B are symmetrical about the radial axis Y1. The first and second angles a1, a2 are of equal magnitude but opposite signs (+ve and -ve). As shown in Figure 5, the first and second bridges 45A, 45B are arranged in a zigzag pattern around the stator core 1. The first and second central axis 47A, 47B are symmetrical about a corresponding central slot axis SY. The first and second central axis 47A, 47B are inclined at substantially equal angles a1, a2 to the central slot axis SY. The alternating orientation of the first and second bridges 45A, 45B may be implemented in respect of first and second cooling ducts 33A, 33B having different profiles in transverse section. For example, this arrangement may be replicated in first and second cooling ducts 33A, 33B having a profile comprising or consisting of a rhombus and / or a triangle. As outlined above, the first and second geometric centres C1, C2 of the first and second cooling ducts 33A, 33B are disposed at first and second radial distances R1, R2 from the central stator axis X1, respectively. The second radial distance R2 from the central stator axis X1. In the present embodiment, the second radial distance R2 is greater than the first radial distance R1. The offset between the first and second radial distances R1, R2 may be modified to tune the flux characteristics in the stator core 1, for example to provide improved uniformity of the flux within the stator core 1. A variant having a smaller offset between the first and second radial distances R1, R2 is shown in Figures 6A, 6B and 7 by way of example. The positioning of the first and second stator ducts 33A, 33B within the stator core 1 will now be described with reference to Figure 8. The radial position of the first cooling duct 33A is defined herein with respect to the slot end wall 20. The radial position of the first cooling duct 33A is defined herein with respect to the slot end wall 20. A first radial separation 11 is defined between the slot end wall 20 and the first geometric centre C1 of the first cooling duct 33A. The radial position of the second cooling duct 33B is defined herein with respect to an (effective) electromagnetic radius of the stator core 1. A second radial separation I2 is defined between the electromagnetic radius of the stator core 1 and the second geometric centre C2 of the second cooling duct 33B. The electromagnetic radius is a minimum distance from the centre of the stator core 1 to any of the external surfaces 14 of the stator core 1. In the present example, the stator core 1 comprises a right cylinder and the electromagnetic radius is equal to the radius of the stator core 1. The first and second radial separations 11, I2 are be defined by the following equation: Il = x.I2 (1) where x is a scale factor in the range 1 to 1.5 inclusive. The scale factor x is preferably in the range 1.1 to 1.2 inclusive. In the present embodiment, the scale factor x is approximately 1.17. A minimum value of the separation distance SD between adjacent first and second cooling ducts 33A, 33B in the stator core 1 is defined by the equation: SD >12 / n (2) Where n a constant defined as follows: Total number of coolinq channels n =-------------------------------- (3) Number of phases * Number of rotor poles In the illustrated example, the electric machine 5 is a three-phase machine and the stator core 1 has 96 cooling channels, and has 8 rotor poles. This results in a constant n of 4. It will be understood that other configurations of the electric machine 5 are contemplated. The minimum value of the separation distance SD is measured as the shortest distance between adjacent the first and second cooling ducts. As outlined above, the separation distance SD corresponds to a width of each of the bridges 45. It has been determined that providing one or more internal fin 51-n in each of the plurality of cooling ducts 33 improves cooling performance of the stator core 1 at least in certain embodiments. The one or more internal fins 51-n may increase an internal “wet” area of the cooling ducts 33, thereby promoting heat rejection from the stator core 1 into the cooling fluid circulated through the cooling ducts 33. The one or more internal fins 51-n may thereby promote heat exchange. The cooling ducts 33 described herein may be modified to incorporate one or more internal fins 51-n. The one or more internal fins 51-n may be formed along one of more of the edges 41-n of the cooling duct 33. Alternatively, or in addition, the one or more internal fins 51-n may be formed at one of more of the corners 43-n of the cooling duct 33. The one or more internal fins 51-n may subdivide the or each cooling duct 33 into a plurality of chambers which are preferably maintained in fluid communication with each other. The or each cooling duct 33 may comprise a plurality of the internal fins 51-n. The internal fins 51-n in each cooling duct 33 may have the same length as each other and / or the same width as each other. Alternatively, the internal fins 51-n in each cooling duct 33 may have different lengths from each other; and / or different widths from each other. An embodiment of the stator core 1 comprising a plurality of internal fins 51-n in each of the cooling ducts 33 is shown in Figures 9A, 9B and 10. This embodiment is a modification of the embodiment shown in Figures 6A and 6B comprising first and second cooling ducts 33A, 33B. The description herein focuses on the differences between these embodiments. Like reference numerals are used for like components. The first and second cooling ducts 33A, 33B have like profiles. The first and second cooling ducts 33A, 33B have first and second orientations which are different from each other. The orientation of the first and second cooling ducts 33A, 33B is substantially unchanged from the arrangement described herein. The description of the present embodiment will focus on the profiles of the first and second cooling ducts 33A, 33B. The first cooling duct 33A comprises a first edge 41 A-1, a second edge 41A-2 and a third edge 41A-3. In the present embodiment, each of the first, second and third edges 41 A-1,41 A-2, 41 A-3 is curved outwardly to form a convex profile. The first cooling duct 33A comprises a first corner 43A-1, a second corner 43A-2 and a third corner 43A-3. A first internal fin 51 A-1 is formed at the first corner 43A-1; a second internal fin 51 A-2 is formed at the second corner 43A-2; and a third internal fin 51 A-3 is formed at the first corner 43A-3. The first, second and third internal fins 51 A-1,51 A-2, 51 A-3 project inwardly into the first cooling duct 33A. The first, second and third internal fins 51 A-1,51 A-2, 51 A-3 are separate from each other such that their respective distal (free) ends are spaced apart from each other. This arrangement helps to reduce or avoid the formation of flux paths through the first, second and third internal fins 51 A-1, 51 A-2, 51 A-3. In the present embodiment, the first, second and third internal fins 51 A-1,51 A-2, 51 A-3 are oriented towards the first geometric centre C1 of the first cooling duct 33A. The first, second and third internal fins 51 A-1, 51 A-2, 51 A-3 subdivide the first cooling duct 33A into the three (3) sub-chambers which are open to each other along the length of the first cooling duct 33A. An enlarged view of the first cooling duct 33A is shown in Figure 10. The first, second and third internal fins 51 A-1, 51 A-2, 51 A-3 are substantially the same length as each other. The second cooling duct 33B comprises a first edge 41B-1, a second edge 41B-2 and a third edge 41B-3. In the present embodiment, each of the first, second and third edges 41B-1,41B-2, 41B-3 is curved outwardly to form a convex profile. The second cooling duct 33B comprises a first corner 43A-1, a second corner 43A-2 and a third corner 43A-3. A first internal fin 51 B-1 is formed at the first corner 43B-1; a second internal fin 51B-2 is formed at the second corner 43B-2; and a third internal fin 51B-3 is formed at the third corner 43B-3. The first, second and third internal fins 51 B-1,51 B-2, 51 B-3 project inwardly into the second cooling duct 33B. The first, second and third internal fins 51 B-1,51 B-2, 51 B-3 are separate from each other such that their respective distal (free) ends are spaced apart from each other. This arrangement helps to reduce or avoid the formation of flux paths through the first, second and third internal fins 51 B-1,51 B-2, 51 B-3. In the present embodiment, the first, second and third internal fins 51 B-1, 51 B-2, 51 B-3 are oriented towards the first geometric centre C1 of the second cooling duct 33B. The first, second and third internal fins 51 B-1, 51 B-2, 51 B-3 subdivide the second cooling duct 33B into the three (3) chambers which are open to each other along the length of the first fooling duct 33B. An enlarged view of the second cooling duct 33B is shown in Figure 10. The first, second and third internal fins 51 B-1,51 B-2, 51 B-3 are substantially the same length as each other. The convex profile of the first and second edges 41A-1,41A-2 of the first cooling duct 33A, and the convex profile of the first and second edges 41 B-1,41 B-2 of the second cooling duct 33B forms first and second bridges 45A, 45B having a concave profile. The width of each of the first and second bridges 45A, 45B is smallest at or proximal to their mid-points. In the present embodiment, the first and second bridges 45A, 45B are symmetrical about their respective first and second axis 47A, 47B. A further embodiment of the stator core 1 is shown in Figures 11A, 11B and 12. The present embodiment is a modified version of the present embodiment. In particular, the arrangement of the internal fins 51 in the first and second cooling ducts 33A, 33B have been modified. The arrangement of the first and second cooling ducts 33A, 33B will now be described. Like reference numerals are used for like components. The first cooling duct 33A comprises a first edge 41 A-1, a second edge 41A-2 and a third edge 41A-3. In the present embodiment, each of the first, second and third edges 41 A-1,41 A-2, 41 A-3 is curved outwardly to form a convex profile. The first cooling duct 33A comprises a first corner 43A-1, a second corner 43A-2 and a third corner 43A-3. A first internal fin 51 A-1 is formed at the first corner 43A-1; a second internal fin 51 A-2 is formed at the second corner 43A-2; and a third internal fin 51A-3 is formed at the first corner 43A-3. A fourth internal fin 51A-4 is formed at a mid-point of the first edge 41 A-1; a fifth internal fin 51 A-5 is formed at a mid-point of the second edge 41 A-2; and a sixth internal fin 51A-6 is formed at a mid-point of the third edge 41 A-3. Each of the internal fins 51A-1 to 51 A-6 projects inwardly into the first cooling duct 33A. As in the previous embodiment, the internal fins 51 A-1 to 51 A-6 are separate from each other such that their respective distal (free) ends are spaced apart from each other. In the present embodiment, each of the internal fins 51A-1 51A-6 are oriented towards the first geometric centre C1 of the first cooling duct 33A. The first, second and third internal fins 51A-1,51A-2, 51 A-3 subdivide the first cooling duct 33A into the six (6) sub-chambers which are open to each other along the length of the first fooling duct 33A. An enlarged view of the first cooling duct 33A is shown in Figure 12. The first, second and third internal fins 51 A-1, 51 A-2, 51 A-3 are substantially the same length as each other. The fourth, fifth and sixth internal fins 51A-4, 51A-5, 51A-6 are substantially the same length as each other. In the present embodiment, the first, second and third internal fins 51 A-1,51 A-2, 51 A-3 are longerthan the fourth, fifth and sixth internal fins 51A-4, 51A-5, 51A-6. The second cooling duct 33B comprises a first edge 41B-1, a second edge 41B-2 and a third edge 41B-3. In the present embodiment, each of the first, second and third edges 41B-1,41B-2, 41B-3 is curved outwardly to form a convex profile. The second cooling duct 33B comprises a first corner 43B-1, a second corner 43B-2 and a third corner 43B-3. A first internal fin 51 B-1 is formed at the first corner 43B-1; a second internal fin 51B-2 is formed at the second corner 43B-2; and a third internal fin 51B-3 is formed at the third corner 43B-3. A fourth internal fin 51B-4 is formed at a mid-point of the first edge 41 B-1; a fifth internal fin 51B-5 is formed at a midpoint of the second edge 41 B-2; and a sixth internal fin 51B-6 is formed at a mid-point of the third edge 41 B-3. Each of the internal fins 51 B-1 to 51B-6 projects inwardly into the second cooling duct 33B. As in the previous embodiment, the internal fins 51 B-1 to 51B-6 are separate from each other such that their respective distal (free) ends are spaced apart from each other. In the present embodiment, each of the internal fins 51 B-1 51B-6 are oriented towards the first geometric centre C1 of the second cooling duct 33B. The first, second and third internal fins 51 B-1, 51 B-2, 51 B-3 subdivide the second cooling duct 33B into the six (6) sub-chambers which are open to each other along the length of the first fooling duct 33A. An enlarged view of the second cooling duct 33B is shown in Figure 12. The first, second and third internal fins 51 B-1, 51 B-2, 51 B-3 are substantially the same length as each other. The fourth, fifth and sixth internal fins 51B-4, 51B-5, 51B-6 are substantially the same length as each other. In the present embodiment, the first, second and third internal fins 51 B-1, 51 B-2, 51 B-3 are longer than the fourth, fifth and sixth internal fins 51 B-4, 51 B-5, 51 B-6. The convex profile of the first and second edges 41A-1,41A-2 of the first cooling duct 33A, and the convex profile of the first and second edges 41 B-1,41 B-2 of the second cooling duct 33B forms first and second bridges 45A, 45B having a concave profile. The width of each of the first and second bridges 45A, 45B is smallest at or proximal to their mid-points. In the present embodiment, the first and second bridges 45A, 45B are symmetrical about their respective first and second axis 47A, 47B. The first and second cooling ducts 33A, 33B have been described herein as having a profile comprising or consisting of a triangle. It will be understood that the first and second cooling ducts 33A, 33B may have different profiles. Further variants having different profiles are illustrated byway of example in Figures 13Ato 13D. In the arrangement illustrated in Figure 13A the first and second cooling ducts 33A, 33B each have a profile comprising or consisting of a rhombus in transverse section. The first cooling ducts 33A comprise a first rhombus which is symmetrical about a first major axis which is at least substantially coincident with the central slot axis SY. The second cooling ducts 33B comprise a second rhombus which is symmetrical about a second major axis which is at least substantially coincident with the central tooth axis TY. The first rhombus and the second rhombus each comprises rounded corners. In the illustrated example, the first and second cooling ducts 33A, 33B have like profiles. In a variant, the first and second cooling ducts 33A, 33B may comprise different profiles. The first and second cooling ducts 33A, 33B may have different sizes and / or different shapes. For example, the first and second cooling ducts 33A, 33B may comprise first and second rhombuses respectively having different cross-sectional areas. As illustrated in Figure 13B, the first and second cooling ducts 33A, 33B shown in Figure 13A may be modified to incorporate internal fins 51 A, 51B. In the illustrated arrangement, internal fins 51 A, 51B are provided on each edge 41 A, 41B of the first and second cooling ducts 33A, 33B. The first cooling ducts 33A each comprise four 14 internal find 51 A. The first cooling ducts 33A may each have less than or more than four (4) internal fins 51 A. The second cooling ducts 33B may each have less than or more than four (4) internal fins 51 A. Alternatively, or in addition, internal fins 51 A, 51B may be provided at each corner 43A, 43B of the first and second cooling ducts 33A, 33B. The first and second cooling ducts 33A, 33B in the illustrated example have the same number of internal fins 51 A, 51B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51B. In the arrangement illustrated in Figure 13C the first and second cooling ducts 33A, 33B each have a profile comprising or consisting of a circle in transverse section. The first cooling ducts 33A each have a profile comprising or consisting of a first circle having three (3) internal fins 51 A. The first cooling ducts 33A may each have less than or more than three (3) internal fins 51 A. The first cooling duct 33A has a first axis of (reflection) symmetry which is at least substantially coincident with the central slot axis SY. The second cooling ducts 33B each have a profile comprising or consisting of a second circle having three (3) internal fins 51B. The second cooling duct 33B may each have less than or more than three (3) internal fins 51B. The second cooling duct 33B has a second axis of (reflection) symmetry which is at least substantially coincident with the central tooth axis TY. The first and second cooling ducts 33A, 33B in the illustrated example have the same number of internal fins 51 A, 51B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51B. The first and second cooling ducts 33A, 33B in the illustrated example have the same diameter. In a variant, the first and second cooling ducts 33A, 33B may have different diameters. In a further example illustrated in Figure 13D, the first cooling ducts 33A each comprise a first circle having six (6) internal fins 51 A. The first cooling ducts 33A may each have less than or more than six (6) internal fins 51 A. The first cooling duct 33A has a first axis of (reflection) symmetry which is at least substantially coincident with the central slot axis SY. The second cooling ducts 33B each comprise a second circle having six (6) internal fins 51B. The first cooling ducts 33A may each have less than or more than six (6) internal fins 51 A. The second cooling duct 33B has a second axis of (reflection) symmetry which is at least substantially coincident with the central tooth axis TY. The first and second cooling ducts 33A, 33B in the illustrated example have the same number of internal fins 51 A, 51B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51B. The first and second cooling ducts 33A, 33B in the illustrated example have the same diameter. In a variant, the first and second cooling ducts 33A, 33B may have different diameters. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A stator core for a stator of an electric machine, the stator core comprising:a central stator axis;a plurality of winding slots in the stator core; andat least one first cooling duct in the stator core, the at least one first cooling duct extending substantially parallel to the central stator axis and being radially separated from the winding slots by a region of the stator core;wherein the or each first cooling duct has a first profile in transverse section comprising a first edge disposed in a radially innermost position and extending substantially perpendicular to a radial axis of the stator core;wherein the first profile comprises a first polygon composed of a plurality of edges and a plurality of corners, the first edge being one of the plurality of edges, the first polygon is a triangle having one or more rounded corners;wherein the or each first cooling duct is associated with one of the winding slots, the or each first cooling duct being disposed radially outboard of the associated winding slots and aligned with the associated winding slot;and wherein the stator core comprises at least one second cooling duct extending substantially parallel to the central stator axis and being radially separated from the winding slots by a region of the stator core.

2. A stator core according to claim 1, wherein at least one of the edges of the first polygon is rectilinear; and / or at least one of the edges of the first polygon is curved.

3. A stator core according to any one of the preceding claims, wherein the or each first cooling duct comprises at least one internal fin.

4. A stator core as claimed in any one of the preceding claims, wherein the or each first cooling duct comprises a line of symmetry, the line of symmetry being at least substantially aligned with a radial axis of the stator core.

5. A stator core according to any preceding claim, wherein the or each second cooling duct has a second profile in transverse section comprising a first edge disposed in a radially outermost position and extending substantially perpendicular to a radial axis of the stator core.

6. A stator core according to any preceding claim comprising a plurality of stator teeth, wherein the or each second cooling duct is associated with one of the stator teeth, the or each second cooling duct being disposed radially outboard of the associated stator tooth.

7. A stator core according to any preceding claim; wherein the first and second cooling ducts are offset from each other in a radial direction and / or a circumferential direction in the stator core.

8. A stator core according to claim 7, wherein adjacent edges of the first and second cooling ducts are oriented at least substantially parallel to each other.

9. A stator core according to any preceding claim, wherein a bridge is formed in the stator core between adjacent pairs of the first and second cooling ducts, each bridge having a central axis oriented at an acute angle to a radial axis of the stator core.

510. A stator comprising a stator core as claimed in any one of the preceding claims.

11. An electric machine comprising a stator as claimed in claim 10.10 12. A vehicle comprising a stator core according to any one of claims 1 to 9.

Images