EP4627699A1

EP4627699A1 - Stator core

Info

Publication number: EP4627699A1
Application number: EP23817961.8A
Authority: EP
Inventors: Jose MALUMBRES RUIZ; Enric SORO JUAN; Matthew CROUCH; Mario VILA MILLAN
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2022-12-02
Filing date: 2023-11-27
Publication date: 2025-10-08
Also published as: GB2625063A; GB2625063B; WO2024115375A1; GB202218121D0

Abstract

Aspects of the present invention relate to a stator core (1) for a stator (3) of an electric machine (5). The stator core (1) has a central stator axis (X1); and a plurality of winding slots (15). At least one cooling duct (33A, 33B) is provided in the stator core (1). The at least one cooling duct (33A, 33B) extends substantially parallel to the central stator axis (X1) and is radially separated from the winding slots (15) by regions of the stator core (1). At least in certain embodiments, the or each cooling duct (33A, 33B) includes at least one internal fin (51-n). Aspects of the present invention also relate to a stator (3), an electric machine (5); and a vehicle (V).

Description

STATOR CORE

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 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 comprises at least one internal fin.

At least in certain embodiments, the at least one internal fin projects into the cooling duct. In use, a cooling fluid is introduced into the or each cooling duct. The at least one internal fin increases an internal (wet) surface area of the or each cooling duct. At least in certain embodiments, this may improve the heat rejection from the stator core into the cooling fluid. 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 or each internal fin is preferably formed integrally with the stator core. The cooling fluid may be a liquid coolant. The or each internal fin extends from a peripheral surface of the cooling duct. The or each internal fin may project into a central region of the duct. In certain embodiments, the or each internal fin may extend towards a geometric centre of the cooling duct.

At least in certain embodiments, the or each internal fin extends partway across the cooling duct. The or each internal fin preferably do not extend completely across the cooling duct. The or each internal fin comprises a distal end disposed in the cooling duct.

The or each internal fin may comprise one or more branches. For example, the or each internal fin may be bifurcated.

The or each cooling duct may comprise a plurality of the internal fins. The plurality of the internal fins may project into the or each cooling duct. The internal fins in the or each cooling duct are preferably separate from each other. The distal ends of the internal fins in the or each cooling may be spaced apart from each other. This arrangement helps to reduce or avoid the formation of magnetic flux paths through the internal fins. At least in certain embodiments, this may reduce or avoid flux saturation which may otherwise result in localised heating. The internal fins projecting into the or each cooling duct may comprise at least one first internal fin having a first length and at least one second internal fin having a second length. The first and second lengths may be different from each other. For example, the first length may be greater than the second length.

The or each cooling duct extends in a longitudinal direction substantially parallel to the central stator axis.

The or each internal fin extends in a longitudinal direction substantially parallel to the central stator axis. The at least one internal fin may comprise a uniform profile in the longitudinal direction. At least in certain embodiments, the stator core may have a uniform profile along its length. The or each internal fin extends at least partway along the length of the stator core. At least in certain embodiments, the or each internal fin extends along the length of the stator core.

The or each cooling duct may comprise a central duct axis. The central duct axis may be aligned with a geometric centre of the cooling duct. The central duct axis may extend at least substantially parallel to the central stator axis. The at least one internal fin may extend radially inwardly in a direction substantially perpendicular to the central duct axis of the or each cooling duct.

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 cooling duct may comprise a line of (reflection) symmetry. The 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 slot axis of one of the winding slots formed in the stator core. In this arrangement, the line of symmetry of the cooling duct may be aligned with the central slot axis. Alternatively, the radial axis may be a central tooth axis of one of the stator teeth formed in the stator core. In this arrangement, the line of symmetry of the cooling duct may be aligned with the central tooth axis.

The or each cooling duct comprises a 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 cooling duct which faces inwardly. The at least one internal fin may be provided on the first edge to promote heat transfer. Two or more internal fins may be formed on the first edge. For example, internal fins may be disposed at opposing ends of the first edge (for example at the corners of the cooling duct) and optionally at a mid-point of the first edge. The first profile may comprise other edges which have fewer internal fins and/or smaller internal fins. The other edges may be free of internal fins.

The or each cooling duct may comprise a closed curve in transverse section. The closed curve may be in the form of a circle or an ellipse. The at least one internal fin may project inwardly from the closed curve.

The or each cooling duct may comprise a polygon in transverse section. The polygon may be symmetrical about a line of (reflection) symmetry. The line of symmetry may be aligned with a radial axis of the stator core.

The polygon may be equilateral and/or equiangular. The polygon may be a regular polygon. The at least one internal fin may project from the polygon.

The polygon may be composed of a plurality of edges and corners. The corners of the polygon may be rounded. 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.

The polygon may comprise one or more of the following: a triangle, a rhombus, a trapezoid, an isosceles trapezoid.

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 at least one internal fin may extend substantially perpendicular to the associated edge of the cooling duct.

One said internal fin may be disposed on each edge of the cooling duct. Two or more said internal fins may be disposed on each edge of the cooling duct. In a variant, a different number of said internal fins may be disposed on different edges of the cooling duct. For example, a first edge of the cooling duct may have one internal fin; and a second edge of the cooling duct may have two internal fins.

The stator core may comprise a plurality of the cooling ducts. The plurality of cooling ducts may comprise a first cooling duct and a second cooling duct. The first and second cooling ducts may be disposed adjacent to each other in the stator core. The first and second cooling ducts may be offset from each other in a circumferential direction and/or a radial direction.

At least in certain embodiments, the opposing edges of the adjacent first and second cooling ducts may be oriented at least substantially parallel to each other. The opposing 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. The stator core may comprise a bridge extending between opposing edges of the adjacent first and second cooling ducts. A plurality of the bridges may be formed in the stator core. The plurality of bridges may extend around the stator core, for example in an annular region. Each bridge may have a substantially uniform width along its length. Alternatively, each bridge may have a concave or a convex profile.

Each of the bridges may comprise a central axis oriented at a non-zero angle to a radial axis of the stator core. First and second bridges located adjacent to each other may have respective first and second central axes. The first and second central axes may be angularly offset relative to each other. The first central axis may be oriented at a first (non-zero) angle to the radial axis of the stator core; and the second central axis may be oriented at a second (non-zero) angle to the radial axis of the stator core. At least in certain embodiments, the first and second angles may be explementary angles. The sum of the first and second angles may be a complete angle (i.e., 360°). The first and second angles may alternate with each other around the stator core. The bridges may be arranged in a uniform zigzag pattern around the stator core.

The stator core may comprise a first said cooling duct and a second said cooling duct. The first and second cooling ducts may be oriented in opposite directions. The first and second cooling ducts may be arranged in an interlocking arrangement.

The first cooling duct may comprise a first profile in transverse section. The first profile may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge of the first cooling duct may be disposed in a radially innermost position.

The second cooling duct may comprise a second profile in transverse section. The second profile may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge of the second cooling duct may be disposed in a radially outermost position.

The first and second ducts may have different orientations. The first and second ducts may be oriented in opposite directions to each other. This may form an interlocking arrangement of the first and second ducts.

The cooling ducts may each have a profile comprising or consisting of one of the following: a triangle, for example an equilateral or isosceles triangle; a rhombus; a kite; a trapezoid, for example an isosceles trapezoid; a rectangle; a square; a pentagon; and a hexagon. The first and second cooling ducts may have like profiles. For example, the first and second cooling ducts may each comprise a triangle, a rhombus, a kite or a trapezoid. Alternatively, the first and second cooling ducts may have different profiles. For example, the first cooling duct may comprise a triangle; and the second cooling duct may comprise a rhombus.

The stator core may comprise a plurality of first cooling ducts and a plurality of second cooling ducts. The first cooling ducts may each have a first geometric centre disposed at a first radial distance (II) from a slot end wall of the winding slots. The second cooling ducts may each have a second geometric centre disposed at a second radial distance (12) from a cylindrical outer surface of the stator core. The relationship between the first and second radial distances (II, 12) may be defined by the equation:

Il = x. 12 where x is a scale factor in the range 1 to 1.5 inclusive.

The first and second cooling ducts may be offset from each other in a radial direction. The first cooling ducts have a first radial position; and the second cooling ducts have a second radial position. The scale factor x defines the relative (radial) positioning of the first and second cooling ducts within the stator core.

The scale factor x may be in the range 1.1 to 1.2 inclusive.

The scale factor x may be approximately 1.17. A minimum value of a separation distance (13) between adjacent said first and second cooling ducts in the stator core is defined as follows:

13 > I2/n

Where n a constant defined as follows:

Total number of cooling channels Number of phases * Number of rotor poles

The minimum value of the separation distance (13) is measured as the shortest distance between adjacent the first and second cooling ducts.

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 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 polygon may comprise one or more line of (reflection) symmetry. The or each cooling duct may be oriented such that the line of symmetry is aligned with a radial axis of the stator core extending from the central stator axis. Alternatively, the or each cooling duct may be oriented such that the line of symmetry is substantially perpendicular to a radial axis of the stator core extending from the central stator axis.

The polygon may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge of the polygon may be disposed in a radially innermost position or a radially outermost position. The first edge of the polygon may be rectilinear. Alternatively, the first edge of the polygon may be curved, for example concave or convex.

The polygon may comprise one or more of the following: a triangle, a rhombus, a trapezoid, an isosceles trapezoid, a kite, a rectangle, a square, a pentagon and a hexagon.

The polygon may be an equilateral polygon and/or an equiangular polygon. The polygon may be a regular polygon. Alternatively, the polygon may be an irregular polygon.

The stator core may comprise a first cooling duct and a second cooling duct. The first and second cooling ducts may be disposed adjacent to each other in the stator core. The first and second cooling ducts may be offset from each other in a circumferential direction and/or a radial direction. The first and second cooling ducts may at least partially overlap each other in a radial direction.

The first cooling duct may have a first profile comprising or consisting of a first polygon; and the second cooling duct may have a second profile comprising or consisting of a second polygon. The first and second cooling ducts may have like profiles. The first and second profiles may be at least substantially the same as each other. The first and second polygons may have the same orientation as each other. Alternatively, the first and second polygons may have the different orientations. For example, the first and second polygons may be oriented in opposite directions. The first polygon may be a reversed or mirror image of the second polygon about an axis extending perpendicular to a radial axis of the stator core. At least in certain embodiments, this may form an interlocking arrangement of the first and second cooling ducts.

The first polygon may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge of the first polygon may be disposed in a radially innermost position. The second polygon may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge of the second polygon may be disposed in a radially outermost position.

The adjacent edges of the first and second cooling ducts may be at least substantially parallel to each other.

The opposing 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.

The first and second cooling ducts may comprise respective first and second triangles. The first and second triangles may, for example, be equilateral triangles or isosceles triangles. The first and second triangles may be oriented in opposite directions. The first triangle may be a reversed or mirror image of the second triangle about an axis extending perpendicular to a radial axis of the stator core. This may form an interlocking arrangement of the first and second cooling ducts.

The first cooling duct may have a first profile comprising or consisting of a first triangle. The first triangle may be a rounded triangle. The first triangle may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge may be disposed in a radially innermost position. The first triangle may comprise a first corner disposed opposite the first edge. The first corner may be disposed in a radially outermost position.

The second cooling duct may have a second profile comprising or consisting of a second triangle. The second triangle may be a rounded triangle. The second triangle may comprise a first edge extending substantially perpendicular to a radial axis of the stator core. The first edge may be disposed in a radially outermost position. The second triangle may comprise a second corner disposed opposite the second edge. The second corner may be disposed in a radially innermost position.

The first and second cooling ducts may comprise respective first and second rhombuses. The first and second rhombuses may each comprise a regular rhombus. The first and second rhombuses may be offset from each other in a radial direction and/or a circumferential direction. The first rhombus may be a reversed or mirror image of the second rhombus about an axis extending perpendicular to a radial axis of the stator core. The first and second rhombuses may be arranged to form an interlocking arrangement of the first and second cooling ducts.

The first and second cooling ducts may comprise respective first and second trapezoids. The trapezoids may be isosceles trapezoids. The orientation of the first and second trapezoids may be reversed to form an interlocking arrangement of the first and second cooling ducts.

The or each cooling duct may comprise at least one internal fin. The first cooling duct may comprise one or more first internal fins. The second cooling duct may comprise one or more second internal fins. The or each cooling duct may comprise at least one internal fin disposed at one or more corners of the cooling duct. Alternatively, or in addition, the or each cooling duct may comprise at least one internal fin disposed on one or more edges of the cooling duct.

The corners of the polygon may be rounded. This may reduce localised stresses in the stator core. The polygon may, for example, comprise a rounded triangle.

One or more of the edges of the polygon may be rectilinear. Alternatively, or in addition, one or more of the edges of the polygon may be curved. The one or more edges of the polygon may be concave or convex. The polygon may comprise a combination of rectilinear and curved edges.

The or each cooling duct may have reflective symmetry. The or each cooling duct may comprise a line of (reflection) symmetry. The line of symmetry may be at least substantially aligned with a radial axis of the stator core. At least in certain embodiments, the radial axis extends substantially perpendicular to the central stator axis.

According to a further aspect of the present invention there is provided a stator core for an electric machine, the stator comprising: a cylindrical outer surface; a plurality of winding slots, each of the plurality of winding slots having a slot end wall disposed at a radially outermost end of the winding slot; and a plurality of first cooling ducts in the stator core and a plurality of second cooling ducts in the stator core, the first and second cooling ducts extending substantially parallel to the central stator axis and being radially separated from the winding slots by regions of the stator core; the first cooling ducts each have a first geometric centre disposed at a first radial distance (II) from the slot end wall of the winding slots; the second cooling ducts each have a second geometric centre disposed at a second radial distance (12) from an electromagnetic radius of the stator core; wherein the relationship between the first and second radial distances (II, 12) is defined by the equation:

Il = x. 12 where x is a scale factor in the range 1 to 1.5 inclusive.

The electromagnetic radius is a minimum distance from the centre of the stator core to an external surface the stator core. By way of example, if the stator core comprises a right cylinder, the electromagnetic radius is equal to the radius of the stator core. If the stator core is non-circular in transverse section, the electromagnetic radius may be equal to a maximum radius of an inscribed circle having centre coincident with a central axis of the stator core. The stator core may, for example, have a hexagonal or octagonal profile in transverse section. The electromagnetic radius may correspond to the maximum radius of a circle inscribed inside hexagonal or octagonal profile of the stator core. The stator core may have other polygonal profiles in transverse section.

At least in certain embodiments, the stator core may comprise or consist of a right cylinder. An outer surface of the stator core may comprise or consist of a right cylindrical surface.

The first and second cooling ducts may be offset from each other in a radial direction. The first cooling ducts have a first radial position; and the second cooling ducts have a second radial position. The scale factor x defines the relative (radial) positioning of the first and second cooling ducts within the stator core. The scale factor x may be in the range 1.1 to 1.2 inclusive.

The scale factor x may be approximately 1.17.

A minimum value of a separation distance (13) between adjacent said first and second cooling ducts in the stator core is defined as follows:

13 > I2/n

Where n a constant defined as follows:

Total number of cooling channels Number of phases * Number of rotor poles

According to a further aspect of the present invention there is provided a lamination for forming the stator core described herein. At least in certain embodiments, the stator core may comprise a plurality of like laminations arranged in a stack.

According to a further aspect of the present invention there is provided an electric machine comprising a stator core of the type described herein.

According to a still aspect of the present invention there is provided an electric machine comprising a stator as described herein. The stator is an assembly comprising the stator core and one or more stator windings.

According to a yet further aspect of the present invention there is provided a vehicle comprising one or more electric machines of the type described herein.

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, by way 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;

Figure 11A shows a segment of the stator core in accordance with a further embodiment of the present invention;

Figure 11 B shows a computational representation of the magnetic flux in the stator core shown in Figure 11 A;

Figure 12 shows an enlarged view of a portion of the stator core shown in Figure 11A; 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 BTT 1 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 BTT 1 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 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 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 21 B. A first lateral projection 23 and a second lateral projection 25 are formed on the radially inner end 21 B 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 21 B 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 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 recirculated 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 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 41A-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, 41 A-3 are inclined at an acute angle relative to the central slot axis SY. The second and third edges 41 A-2, 41 A-3 are tapered towards each other in a radially outwards direction along the central slot axis SY. The first edge 41A-1 is oriented substantially perpendicular to the central slot axis SY. The first edge 41A-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 41A-1 is disposed closest to the winding slot 15. In particular, the first edge 41A-1 is disposed closest to the slot base 20. The first edge 41A-1 is presented to the winding slot 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.

With reference to Figure 5, each second cooling duct 33B comprises a first edge 41 B-1 , a second edge 41 B-2 and a third edge 41 B-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 41 B-2, 41 B-3 are inclined at an acute angle relative to the central tooth axis TY. The second and third edges 41 B-2, 41 B-3 are tapered inwardly towards each other in a radially inwards direction along the central tooth axis TY. The first edge 41 B-1 is oriented substantially perpendicular to the central tooth axis TY. The first edge 41 B-1 of the first cooling duct 33A 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 41 B-1 is disposed in a radially outermost position and extends substantially perpendicular to the central tooth axis TY. The first edge 41 B-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, 41 A-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 41 B-2, 41 B-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 41 A-2 of each first cooling duct 33A is oriented substantially parallel to the second edge 41 B-2 of an adjacent second cooling duct 33B. The third edge 41 A-3 of each first cooling duct 33A is oriented substantially parallel to the third edge 41 B-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, 41 B-3 of the first and second cooling ducts 33A, 33B; and a second bridge 45B formed between the opposing third edges 41 A-3, 41 B- 3 of the first and second cooling ducts 33A, 33B. The first and second bridges 45A, 45B have respective first and second central axis 47 A, 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 47 A, 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 47 A, 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, 12 are be defined by the following equation:

Il = x. /2 (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: )

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 41 A-2 and a third edge 41 A-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 51A-2 is formed at the second corner 43A-2; and a third internal fin 51A-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 , 51A-2, 51A-3 are substantially the same length as each other.

The second cooling duct 33B comprises a first edge 41 B-1, a second edge 41 B-2 and a third edge 41 B-3. In the present embodiment, each of the first, second and third edges 41 B-1, 41 B-2, 41 B-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 51 B-2 is formed at the second corner 43B-2; and a third internal fin 51 B-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 41 A-1 , 41 A-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 47 A, 47B.

A further embodiment of the stator core 1 is shown in Figures 11A, 11 B 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 41 A-2 and a third edge 41 A-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. A fourth internal fin 51A-4 is formed at a mid-point of the first edge 41 A-1; a fifth internal fin 51A-5 is formed at a mid-point of the second edge 41A-2; and a sixth internal fin 51A-6 is formed at a mid-point of the third edge 41A-3. Each of the internal fins 51A-1 to 51A-6 projects inwardly into the first cooling duct 33A. As in the previous embodiment, the internal fins 51A-1 to 51A-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 51 A-1, 51 A-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 , 51A-2, 51A-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 longer than the fourth, fifth and sixth internal fins 51A-4, 51A-5, 51A-6.

The second cooling duct 33B comprises a first edge 41 B-1, a second edge 41 B-2 and a third edge 41 B-3. In the present embodiment, each of the first, second and third edges 41 B-1, 41 B-2, 41 B-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 51 B-2 is formed at the second corner 43B-2; and a third internal fin 51 B-3 is formed at the third corner 43B-

3. A fourth internal fin 51 B-4 is formed at a mid-point of the first edge 41 B-1; a fifth internal fin 51 B-5 is formed at a mid-point of the second edge 41 B-2; and a sixth internal fin 51 B-6 is formed at a mid-point of the third edge 41 B-3. Each of the internal fins 51 B-1 to 51 B-6 projects inwardly into the second cooling duct 33B. As in the previous embodiment, the internal fins 51 B-1 to 51 B-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 51 B-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 51 B-

4, 51 B-5, 51 B-6 are substantially the same length as each other. In the present embodiment, the first, second and third internal fins 516-1, 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 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 by way of example in Figures 13A to 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, 51 B. In the illustrated arrangement, internal fins 51 A, 51 B are provided on each edge 41 A, 41 B of the first and second cooling ducts 33A, 33B. The first cooling ducts 33A each comprise four internal find 51A. The first cooling ducts 33A may each have less than or more than four (4) internal fins 51A. The second cooling ducts 33B may each have less than or more than four (4) internal fins 51A. Alternatively, or in addition, internal fins 51 A, 51 B 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, 51 B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51 B.

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 51A. The first cooling ducts 33A may each have less than or more than three (3) internal fins 51A. 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 51 B. The second cooling duct 33B may each have less than or more than three (3) internal fins 51 B. 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, 51 B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51 B. 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 51A. The first cooling ducts 33A may each have less than or more than six (6) internal fins 51A. 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 51 B. The first cooling ducts 33A may each have less than or more than six (6) internal fins 51A. 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, 51 B. In a variant, the first and second cooling ducts 33A, 33B may have different numbers of internal fins 51 A, 51 B. 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; 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 comprises at least one internal fin.

2. A stator core according to claim 1 , wherein the or each cooling duct comprises a plurality of the internal fins, the internal fins in the or each cooling duct being separate from each other.

3. A stator core according to claim 2, wherein the internal fins in the or each cooling duct comprise at least one first internal fin having a first length and at least one second internal fin having a second length, the first length being greater than the second length.

4. A stator core according to any one of the preceding claims, wherein the or each internal fin extends in a longitudinal direction substantially parallel to the central stator axis.

5. A stator core according to any one of the preceding claims, wherein the or each cooling duct comprises a central duct axis; the at least one internal fin extending radially inwardly in a direction substantially perpendicular to the central duct axis of the or each cooling duct.

6. A stator core according to any one of the preceding claims, wherein the or each cooling duct comprises a transverse section in the form of a closed curve comprising the at least one internal fin.

7. A stator core according to any one of claims 1 to 5, wherein the or each cooling duct comprises a transverse section in the form of a polygon comprising the at least one internal fin, the polygon being composed of a plurality of edges and corners.

8. A stator core according to claim 7, wherein the or each cooling duct comprises 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.

9. A stator core according to claim 7 or claim 8 comprising a first said cooling duct and a second said cooling duct; the first and second cooling ducts being disposed adjacent to each other in the stator core.

10. A stator core according to claim 9, wherein opposing edges of the first and second cooling ducts are oriented at least substantially parallel to each other.

11. A stator core according to claim 9 or claim 10, wherein the first cooling duct comprises a first profile having a first edge extending substantially perpendicular to a radial axis of the stator core and being disposed in a radially innermost position; and the second cooling duct comprises a second profile having a first edge extending substantially perpendicular to a radial axis of the stator core and being disposed in a radially outermost position.

12. A stator core according to any one of the preceding claims, wherein the stator core comprises a plurality of the cooling ducts, the plurality of the cooling ducts comprising first cooling ducts and second cooling ducts; the first cooling ducts each having a first geometric centre disposed at a first radial distance from a slot end wall of the winding slots; and the second cooling ducts may each have a second geometric centre disposed at a second radial distance from an electromagnetic radius of the stator core; wherein the relationship between the first and second radial distances, II, 12, is defined by the equation:

Il = x. 12 where x is a scale factor in the range 1 to 1.5 inclusive.

13. A stator core according to claim 12, wherein the scale factor x is in the range 1.1 to 1.2 inclusive.

14. A stator core according to claim 12, wherein the scale factor x is approximately 1.17.

15. A stator core according to any one of claims 12, 13 or 14, wherein a minimum value of a separation distance, 13, between adjacent said first and second cooling ducts in the stator core is defined as follows:

13 > I2/n

Where n is a constant defined as follows:

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

17. An electric machine comprising a stator as claimed in claim 16.

18. A vehicle comprising a stator core according to any one of claims 1 to 15.