Stator for a vehicle electrical motor

Abstract

A stator for a vehicle electrical motor, the stator having a longitudinal axis A and comprises a plurality of stacked laminations 12, each lamination 14 comprises an aperture 28 aligned with a further aperture in the other laminations to form a coolant passage 48. The laminations or groups of laminations (Figure 8: 70, 72) are progressively rotated about the longitudinal axis (Figure 8: A – A) of the stator to progressively offset the apertures forming the coolant passage. The stator may have a plurality of identical laminations, and the laminations or groups of laminations may be progressively rotated about the longitudinal axis through the same incremental angle. The relative angular spacing of adjacent apertures on each lamination may be offset circumferentially with respect to relative nominal aperture positions having an angular spacing equal to the incremental angle, and adjacent apertures may also be offset from one another in the radial direction. The aligned apertures in the laminations may be of the same shape. By progressively offsetting adjacent laminations, or groups of laminations, the coolant passage is caused to follow a snaking or twisting path rather than a straight path, which may increase the overall length of the coolant passage and thereby may increase the cooling effect of the fluid flowing along the passage.

Description

An electric machine motor generally consists of a rotor and a stator. The stator generally consists of stator steel with a plurality of teeth wound with conductorwindings, e.g. copperwire. The windings receive alternating current and generate a rotating magnetic field at the stator teeth, which in turn interact with the rotor. The rotor generally contains a more static magnetic component such as permanent magnets, reluctance variation, inductive bars / cage or direct current windings, which experience a rotating force due to the stator rotating magnetic field. This force, depending on direction, can result in motoring or generation action in the electrical machine. As the stator contains the windings, it is the primary source of heat and requires cooling. A known cooling method is to pass a cooling fluid, such as oil, through channels passing through the stator iron, which acts as a heat sink for the windings. The cooling capability is a function of, amongst other factors, the contact area between the cooling channel and the coolant. 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 of the invention provide a stator for a vehicle electrical motor, a vehicle electrical motor comprising the stator, and to a vehicle comprising a stator or a motor of the invention. According to an aspect of the present invention there is provided a stator for a vehicle electrical motor, the stator having a longitudinal axis and comprising: a plurality of stacked laminations, each lamination comprising an aperture aligned with a further aperture in the other laminations to form a coolant passage; the laminations or groups of laminations are progressively rotated about the longitudinal axis of the stator to progressively offset the apertures forming the coolant passage. By progressively offsetting adjacent laminations, or groups of laminations, the coolant passage is caused to follow a snaking or twisting path rather than a straight path, which increases the overall length of the coolant passage and thereby increases the cooling effect of the fluid flowing along the passage. In an embodiment, the apertures are progressively offset circumferentially. By progressively offsetting the apertures in the circumferential direction of the stator, the coolant passage remains at the same radial position with respect to the longitudinal axis as the coolant passage passes along the stator, which can assist in achieving the desired cooling properties of the coolant channel. In an embodiment, the apertures are progressively offset in a radial direction with respect to the longitudinal axis. By progressively offsetting the apertures in the radial direction of the stator - instead of, or in addition to, progressively offsetting the apertures in the circumferential direction - the coolant passage can move closer to and / or further from the longitudinal axis as it passes along the stator, which can assist in achieving the desired cooling properties of the coolant channel. In an embodiment, the stator may comprise a plurality of identical laminations and the laminations or groups of laminations may be progressively rotated about the longitudinal axis through the same incremental angle. The use of identical laminations assists in minimising costs in the design, manufacture and assembly of the stator. In an embodiment, lamination comprises at least three apertures. By providing at least three apertures, rotation of the laminations or groups of laminations locates successive apertures adjacent to, but offset from, each other to form the coolant passage in which the apertures are progressively offset. In an embodiment, aperture forms part of a respective coolant passage. This improves the cooling of the stator by ensuring that each of the plurality of apertures is in use as part of a cooling passage as the laminations or groups of laminations are progressively rotated. In an embodiment, the apertures are progressively offset circumferentially with respect to the longitudinal axis. By progressively offsetting the apertures in the circumferential direction of the stator, the coolant passage remains at the same radial position with respect to the longitudinal axis as the coolant passage passes along the stator, which can assist in achieving the desired cooling properties of the coolant channel. In an embodiment, the relative angular spacing of adjacent apertures on each lamination is offset circumferentially with respect to relative nominal aperture positions having an angular spacing equal to the incremental angle. If the angular spacing of the apertures were the same as the incremental angle through which the laminations or groups of laminations are progressively rotated, incremental rotation of the laminations or groups of laminations would position adjacent apertures in exactly the same circumferential position and would not result in progressive offsetting of the apertures in the circumferential direction. By ensuring that the angular spacing of adjacent apertures is different from the incremental angle, incremental rotation of the laminations or groups of laminations can cause the apertures in the laminations or groups of laminations to be progressively offset circumferentially. In an embodiment, the apertures are progressively offset in a radial direction with respect to the longitudinal axis. By progressively offsetting the apertures in the radial direction of the stator, the coolant passage can move closer to and / or further from the longitudinal axis as it passes along the stator, which can assist in achieving the desired cooling properties of the coolant channel. In an embodiment, the relative angular spacing of adjacent apertures on each lamination is equal to the incremental angle and wherein adjacent apertures are offset from one another in the radial direction. Maintaining the same circumferential position for the apertures in successive laminations or groups of laminations allows the position of the coolant passage to be progressively offset in the radial direction, which can help in achieving the desired cooling effect of the passage. In an embodiment, the relative angular spacing of adjacent apertures on each lamination is offset circumferentially with respect to relative nominal aperture positions having an angular spacing equal to the incremental angle, and wherein adjacent apertures are also offset from one another in the radial direction. Offsetting the apertures in successive laminations or groups of laminations in both the circumferential and radial direction allows additional freedoms in determining the path of the coolant passage. In an embodiment, adjacent apertures on each lamination are offset circumferentially and radially with respect to nominal relative aperture positions having a circumferential spacing equal to the incremental angle and at the same radial position. If the angular spacing of the apertures were the same as the incremental angle through which the laminations or groups of laminations are progressively rotated, and if all the apertures were also at the same radial position on the laminations, then incremental rotation of the laminations or groups of laminations would position adjacent apertures in exactly the same circumferential and radial positions and there would be no offsetting of the apertures. By ensuring that the angular spacing of adjacent apertures is different from the incremental angle, and that and the radial positions of adjacent apertures are different, incremental rotation of the laminations or groups of laminations can cause the apertures in the laminations or groups of laminations to be progressively offset in both the circumferential and radial directions. In an embodiment, the aligned apertures in the laminations are of the same shape. According to another aspect of the invention, there is provided a vehicle electrical motor comprising a stator in accordance with the invention. According to another aspect of the invention, there is provided a vehicle comprising a vehicle electrical motor in accordance with the invention. 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 is a perspective view of a portion of a stator for an electrical motor for a vehicle; Figure 2 is a front view of the stator of Figure 1; Figure 3 is a front view of the stator of Figure 1, illustrating overlapping, offset coolant fluid apertures in adjacent laminations of the stator;; Figure 4 is a front view of two coolant fluid passages of the stator of Figure 1 identified at A in Figure 3, shown to an enlarged scale; Figure 5 is a schematic exploded perspective view of an embodiment of laminated stator in accordance with the invention, which illustrates a coolant fluid passage of the stator of Figure 1; Figure 6 is a schematic front view of a lamination forming part of an embodiment of laminated stator in accordance with the invention, which is a modification of the stator of Figure 1; Figure 7 is a schematic perspective view of an embodiment of laminated stator in accordance with the invention, which is also a modification of the stator of Figure 1; Figure 8 is a schematic exploded perspective view of the stator of Figure 7; Figure 9 is a schematic exploded front view of a further embodiment of stator in accordance with the present invention; Figure 10 is a schematic exploded front view of a further embodiment of stator in accordance with the present invention; Figure 11 is a schematic exploded front view of a further embodiment of stator in accordance with the present invention; Figure 12 is a schematic exploded front view of a further embodiment of stator in accordance with the present invention; Figure 13 is a schematic exploded front view of a further embodiment of stator in accordance with the present invention; Figure 14 is a front view of an aperture which forms part of a lamination of the stator of Figure 15, shown to an enlarged scale; Figure 15 is a front view of a coolant fluid channel of the stator of Figure 15, shown to an enlarged scale; Figure 16 illustrates the principle of the coolant fluid channel of the stator of Figure 15; Figure 17 is a perspective view of a portion of a further embodiment of stator for an electrical motor for a vehicle; Figure 18 is a front view of the stator of Figure 17; Figure 19 is a view of the portion of the stator identified at A in Figure 18, shown to an enlarged scale; Figure 20 is a front view of a coolant fluid passage of the stator of Figure 17, shown to an enlarged scale; Figure 21 is a perspective view of the fluid coolant channels formed by the coolant fluid passage of Figure 20; and Figure 22 is a schematic exploded perspective view of a further embodiment of stator in accordance with the present invention, which is a modification of the stator of Figure 11. DETAILED DESCRIPTION. Figures 1 to 4 illustrate a portion of an embodiment of stator core 10 of an electric motor (not shown) of a vehicle. The stator core is formed from a stack 12 of annular soft-magnetic steel laminations 14 which are aligned and then bonded together with layers 16 of dielectric adhesive. In the drawings, only five laminations 14 are shown, but in practice many more layers are likely to be used, in order to form a stator of the required dimensions and properties. As shown in the drawings, each lamination 14 is planar and comprises an annular outer ring portion 18, from the inner edge of which forty-eight identical stator tooth portions 20, equally angularly spaced around the inner edge, project radially inwardly. Each of the stator portions is provided with an enlarged head portion 22 at its radially innermost edge and adjacent tooth portions are separated by identical gaps 24. In use, coils are wound around the tooth portions 20, in a conventional manner. As shown in Figures 1 to 4, the annular outer ring portion 18 of each lamination 14 is provided with forty-eight identical circular apertures 28, equally angularly spaced around the lamination. Each aperture 28 is located at the same distance a from the outer peripheral edge of the lamination and each aperture 28 is also aligned in the circumferential direction with a respective gap 24 between two adjacent tooth portions 20. The laminations 14 which form the stator core 10 are identical. However, as shown in Figures 3 and 4, in this embodiment adjacent laminations 14 are rotated by approximately 3° about the longitudinal axis A - A of the stator with respect to each other. As a result of the relative angular offset between adjacent laminations 14, the channels 48 defined by the sub-apertures 30 of the apertures in the stack of laminations 14 which form the stator core 10 are not straight, but instead follow a stepped, quasi-helical path, as shown in Figures 3 and 4. In Figure 3, the apertures 28 of the stacked laminations below the outer lamination 28 are superimposed on the outer face of the outer lamination, but the effect is seen to a larger scale for two of the apertures in Figure 4. Consequently, coolant fluid flowing along each channel 48 will contact part of the face of each lamination 14, thereby increasing the surface of the stator core 10 exposed to the flowing coolant fluid. In addition, the overall length of the helical channels 48 along which the coolant fluid flows is longer than the length of straight channels extending parallel to the longitudinal axis of the stator core 10, which further increases the surface 5 area exposed to the coolant fluid and also increases the time for which the coolant fluid is in contact with the stator core 10. Figure 5 shows schematically the shape of one of the coolant fluid passages of the stator of Figure 1, with the laminations 14 of the stator core shown in exploded view, i.e. separated from each other. Figure 5 is a schematic illustration of the laminations 14 which form the stator core and for ease of illustration and for purposes of clarity, in particular, the Figure 5 omits the radially inwardly extending teeth 20 and the gaps 24 between adjacent teeth and only illustrates one aperture in each lamination 14’. As can be seen in Figure 5, by progressively offsetting the laminations 14’ in the circumferential direction, the apertures 60 form a stepped, generally helical fluid flow channel 62 which precesses around the longitudinal axis A - A of the stator core 10. Figure 6 is a schematic illustration of a modification to the first embodiment. As for Figure 5, Figure 6 is a schematic illustration of the laminations which form the stator core and for ease of illustration and for purposes of clarity in particular, Figure 6 omits the radially inwardly extending teeth 20 and the gaps 24 between adjacent teeth and only illustrates one aperture 60 in each lamination 14’. In addition, only eight apertures 60 have been illustrated. Figure 6(a) is a schematic illustration of a lamination 14’ wherein the apertures 60 are equally angularly spaced around the annular portion 18. A first modification is shown in Figure 6(b), in which the positions of the apertures 66 in the lamination are shown in full lines and the apertures 60 of Figure 6(a) are shown in dotted lines. The apertures 66 are equally angularly spaced around the lamination 14’, but the apertures are progressively offset in a radial direction with respect to adjacent apertures 66. In the example shown in Figure 6(b), a first aperture 66a is shown as having a minimum radial offset, the adjacent aperture 66b in the clockwise direction has a greater radial offset and the aperture 66c at 90° to aperture 66a is displaced still further in the radial direction to a maximum radial offset. The radial offset of apertures 66d and 66e then progressively decreases then progressively increases with apertures 66f and 66g to a maximum radial offset and then progressively decreases via aperture 66h back to the minimum radial offset at aperture 66a. Adjacent laminations 14’ are progressively and successively rotated through an angle corresponding to the angular spacing of adjacent apertures 66. As a result, the fluid coolant passages formed by the stack of laminations 14’ remain at the same circumferential position with respect to the longitudinal axis as the coolant passage passes along the stator, but undulate in the radial direction, increasing the length of the fluid coolant channels. In the modification shown in Figure 6(c), the apertures 68 are approximately equally angularly spaced around the annular portion 18 and are at the same radial distance from the longitudinal axis of the lamination 14”. However, the relative angular spacing of adjacent apertures on each lamination is offset in the circumferential direction with respect to the relative nominal aperture positions 60 having an angular spacing equal to the incremental angle between adjacent apertures 60. In the example shown in Figure 6(c), a first aperture 68a is shown as having a minimum circumferential offset, the adjacent aperture 68b in the clockwise direction has a greater circumferential offset and the aperture 68c at 90° to aperture 68a is displaced still further in the circumferential direction to a maximum circumferential offset. The circumferential offset of apertures 68d and 68e then progressively decreases then progressively increases with apertures 68f and 68g to a maximum circumferential offset and then progressively decreases via aperture 68h back to the minimum circumferential offset at aperture 68a. Adjacent laminations 14” are progressively and successively rotated through an angle corresponding to the angular spacing of the positions of equally angularly spaced apertures 60 having no circumferential offset between adjacent apertures. By progressively offsetting the apertures in the circumferential direction of the stator, the coolant passages formed by the stack of laminations 14” remain at the same radial position with respect to the longitudinal axis as the coolant passage passes along the stator, but undulates in the circumferential direction, increasing the length of the fluid coolant channels. Also envisaged, but not illustrated, is a modification in which both the radial and circumferential position of the apertures 60 is progressively increased between adjacent apertures on the lamination, as compared with the relative nominal position of the apertures 60 having no radial or circumferential offset. In the arrangements described above, adjacent laminations are progressively offset with respect to one another in the circumferential direction (and in the circumferential direction in accordance with one modification). However, as a further modification shown in Figures 7 and 8, the stator core 10b may be formed from a plurality of stacked groups 70, 72, 74, 76, 78, 80 of identical laminations 14’, with the laminations in each group 70, 72, 74, 76, 78, 80 being arranged with the aperture or apertures 60 aligned with one another parallel to the longitudinal axis A - A of the stator core 10b and with adjacent groups of laminations being progressively offset with respect to one another in the circumferential direction. A further embodiment is illustrated in Figure 9, which is a schematic illustration of the laminations which form the stator core 10c and for ease of illustration and, in particular, for purposes of clarity, the tooth portions 20 of each lamination and the gaps 24 between the teeth are omitted so that only the outer annular portion 18 is shown, the size of the apertures in each lamination are shown to a larger scale and the number of apertures in each lamination is smaller than would normally (but not always) be present. In Figure 9, the principle is that the apertures 80, 82, 84, 86 in adjacent stacked laminations 14a to 14c which form the stator core 10 are aligned to form a coolant passage which extends parallel to the longitudinal axis A -A of the stator core 10c, butthat the geometries of the apertures in adjacent laminations - or adjacent groups of laminations, as will be explained - are different. By ensuring that the apertures in adjacent laminations (or groups of laminations) are not the same, the fluid flowing along the coolant channel is forced to come into contact with a greater surface area of the stator, as compared with a straight channel of constant cross-section. In one embodiment, shown in Figure 9, the shapes ofthe apertures in adjacent laminations 14a, 14b, 14c, 14d are of different shapes, for example circular apertures, 80, square apertures 82, star-shaped apertures 84, triangular apertures 86, etc. By placing laminations with differently-shaped apertures adjacent to each other in the lamination stack, the cross-sectional shape of the coolant fluid channel formed by the apertures in the stacked laminations 14a, 14b, 14c, 14d changes along its length, increasing the surface area ofthe channel with which the fluid passing along the channel comes into contact. The variation in the cross-sectional area of the channel also slows down the fluid passing along the channel, increasing the dwell time ofthe fluid and increasing the heat transfer to the fluid. In another embodiment, illustrated in Figure 10, the apertures in the adjacent laminations 14e are all ofthe same shape (in this example, they are circular) but the size ofthe apertures 90a, 90b, 90c, 90d varies between adjacent apertures. This slows down the fluid passing along the channel, increasing the dwell time ofthe fluid and increasing the heat transfer to the fluid. In Figure 10, the size ofthe apertures decreases progressively between adjacent laminations and then increases progressively, and so on, so that the cross-sectional area ofthe coolant fluid apertures varies along their length. The arrangements shown in Figures 9 and 10 require several different laminations to be produced, which is likely to increase the cost. In Figure 11, the same lamination 14e is used to form the stack of laminations, but each lamination 14f is provided with a plurality of apertures of different shapes, each equally angularly spaced around the periphery ofthe lamination. In this example, there are circular, square, star-shaped and triangular apertures 92, 94, 96, 98 spaced apart by 45°, with the pattern of apertures repeating to provide two apertures of each shape, each spaced diametrically opposite to the aperture ofthe same shape. By rotating adjacent laminations consecutively by 45°, apertures of different shapes are arranged next to each other in adjacent laminations for each ofthe coolant fluid channels. It is not necessary for each lamination to have several different apertures. In the embodiment shown schematically in Figure 12, each lamination 14g has several (in this example, eight) non-circular apertures 100 of identical shape equally angularly spaced (in this example by 45°) around the annular portion 18 of the lamination 14f. However, each aperture is progressively rotated with respect to its adjacent apertures through an angle equal to the circumferential angular spacing ofthe apertures. In the example shown, there are eight apertures 100 of identical shape, each spaced apart from its adjacent apertures by 45°. However, each aperture is also progressively rotated through 45° with respect to the adjacent apertures. This results in the apertures all being oriented in the same direction when viewed with respect to any given diameter ofthe lamination (e.g. the diameter D - D). However, by progressively rotating adjacent laminations by an amount corresponding to the angular spacing ofthe apertures (in this case 45°), it will be seen that the facing apertures of adjacent laminations are angularly displaced with respect to one another by an amount corresponding to the relative rotation between adjacent apertures on each lamination (in this case 45°). As a result, the profile of the fluid coolant passages formed by the aligned apertures in the stack of laminations changes between adjacent laminations. Consequently, the coolant fluid flowing through the passages is forced to take a more circuitous route, which increases the area of the laminations exposed to the coolant fluid and increases the cooling. A further modification to the arrangement of Figures is shown in Figures 13 to 15. In this modification, each lamination 14h has several apertures 110 of identical shape equally angularly spaced around the annular portion 18 of the lamination, as for the arrangement of Figure 12. In addition, as for the arrangement of Figure 12, each aperture 110 is progressively rotated with respect to its adjacent apertures through an angle equal to the circumferential angular spacing of the apertures. In the example shown, there are eight apertures 14 of identical shape, each spaced apart from its adjacent apertures by 45°. Each aperture is also progressively rotated through 45° with respect to the adjacent apertures, as for the arrangement of Figures 12. However, the shape of the aperture shown in Figures 13 to 15 is chosen so that when adjacent laminations are progressively rotated as described, there is no straight passage through the apertures forming the coolant fluid channels passing through the stacked laminations. Instead, the shape of the apertures produces meandering, quasi-helical coolant fluid passages in the manner of a “spiral” staircase, as shown in Figure 15. In the example shown the aperture 110 is generally C-shaped, being formed from a generally circular aperture 112 with a central circular solid portion 114 arranged concentrically with the aperture 112 and a radially extending bridge portion 116 joining the circular solid portion 114 to the edge of the generally circular aperture 112. As shown in Figure 15, when the laminations are stacked with adjacent laminations progressively circulated through 45°, the centre of the passage 120 formed by the aligned apertures is blocked, so that there is no straight line passage along the coolant fluid channel formed by the aligned apertures. Instead, the fluid flowing along the coolant fluid channels is forced to follow a quasi-helical path as the profile of the aperture encountered by the fluid changes with adjacent laminations. The principle is illustrated in Figure 16, which shows three identical laminations 14’, each with only three apertures 1,2,3, equally angularly spaced around the lamination at the same radial distance and each aperture being circumferentially aligned with a respective radially extending stator tooth portion 20. As shown in Figure 16(a), the radially extending bridge portion 116 of the first aperture 1 is aligned with the radial direction 111a. However, in Figure 16(b), it will be seen that the radially extending bridge portion 116 of the second aperture 2 is rotated by 240° (or-120°) with respect to the radial direction 111b and in Figure 16(c), it will be seen that the radially extending bridge portion 116 of the third aperture 3 is rotated by 120° (or-240°) with respect to the radial direction 111c. Therefore when several identical laminations 114 are placed adjacent to one another, and adjacent laminations are rotated by an amount equal to the circumferential spacing of the apertures 1,2, 3 (which in this case is 120°), the apertures in each lamination are rotated with respect to the corresponding apertures in the adjacent laminations. Figure 16(d) shows three such laminations which are illustrated superimposed on each other but with adjacent laminations laterally shifted, and Figure 16(e) shows the same laminations fully aligned with each other. A further embodiment is illustrated in Figures 17 to 21, which illustrate a portion of an embodiment of stator core 110 of an electric motor (not shown) of a vehicle. In the same way as for the arrangement of Figure 1, stator core is formed from a stack 112 of annular soft-magnetic steel laminations 114 which are aligned and then bonded together with layers 116 of dielectric adhesive. In the drawings, only three laminations 114 are shown, but in practice many more layers are likely to be used, in order to form a stator of the required dimensions and properties. As shown in Figures 17 to 21, each lamination 114 is planar and comprises an annular outer ring portion 118, from the inner edge of which forty-eight identical stator tooth portions 120, equally angularly spaced around the inner edge, project radially inwardly. Each of the stator portions is provided with an enlarged head portion 122 at its radially innermost edge and adjacent tooth portions are separated by identical gaps 124. In use, coils are wound around the tooth portions 120, in a conventional manner. As shown in Figures 17 to 21, the annular outer ring portion 118 of each lamination 114 is provided with fortyeight identical aperture groups 128, equally angularly spaced around the lamination. Each aperture group 128 is located at the same distance a from the outer peripheral edge of the lamination and each group 128 is also aligned in the circumferential direction with a respective gap 124 between two adjacent tooth portions 120. As shown in Figure 19, in this embodiment each aperture group 128 is formed from five identical, equally angularly spaced sub-apertures 130. Each aperture group 128 effectively comprises a circular aperture indicated by a dashed line 134 in Figure 18 which is separated into the sub-apertures 130 by five identical, equally angularly spaced spider arms 136 which are joined to form a central core 138. Each sub-aperture 130 is therefore generally triangular in shape, although the outer peripheral edge 140 of each sub-aperture 130 is part-circular and lies on the circle 134 of the aperture group. The side edges 142, 144 of each sub-aperture 130 are straight and inclined to one another at approximately 72° and the corners between the edges 140, 142, 144 of each sub-aperture are rounded. The laminations 114 which form the stator core 110 are identical. However, as shown in Figures 19 and 20, in this embodiment adjacent laminations 114 are rotated by 7.5° about the longitudinal axis A - A of the stator with respect to each other (i.e. the same angle as the circumferential spacing of adjacent aperture groups 128). In the absence of any other change to the aperture groups, stacking several laminations 114 to form the stator core 110 and rotating adjacent laminations by an amount corresponding to their angular spacing in the circumferential direction would merely present exactly the same aperture profile for each aperture forming the coolant fluid channels and would result in a series of straight fluid coolant passages extending parallel to the longitudinal axis A - A of the stator core. Therefore, in order to present a different profile to coolant fluid as it passes along the channels 148 formed by the aligned apertures of adjacent laminations, the orientation of adjacent aperture groups 128 around the axis of the aperture group’s central core is also progressively changed in the circumferential direction. The change in the orientation will be in addition to any change as a result of the position of the aperture groups around the circumference of the laminations and may, by way of example, be 3° although other angles are possible. Consequently, rotating adjacent laminations by 7.5° about the longitudinal axis A-A of the stator with respect to each other causes the aligned apertures of adjacent laminations 114 to have a different orientation, as shown in Figure 20. As a result of the progressive change in orientation between adjacent laminations 114, the channels 148 defined by the sub-apertures 130 of the aperture groups 128 in the stack of laminations 114 which form the stator core 110 follow a stepped, quasi-helical path whose longitudinal axes extend parallel to the longitudinal axis A - A of the stator core 110, as shown in Figures 20 and 21. Reference is also made to Figure 16 and the associated description above to explain the principle. Consequently, coolant fluid flowing along each channel 148 will contact part of the face of each lamination 114, thereby increasing the surface of the stator core 110 exposed to the flowing coolant fluid. In addition, the overall length of the helical channels 148 along which the coolant fluid flows is longer than the length of equivalent straight, non-helical channels extending parallel to the longitudinal axis of the stator core 110, which further increases the surface area exposed to the coolant fluid and also increases the time for which the coolant fluid is in contact with the stator core 110. By adjusting the change in orientation between adjacent aperture groups 128, the pitch of the helical channels can be adjusted. In the arrangements described above with reference to Figures 9 to 21, the aligned apertures in adjacent laminations progressively present a different profile with respect to one another. However, as a further modification shown in Figure 22, and in a similar manner to that described above with reference to Figures 7 and 8, the stator core 10i may be formed from a plurality of stacked groups 130, 132, 134, 136, 138, etc. of identical laminations 14f, with the laminations in each group 130, 132, 134,136, 138, etc. being arranged with the aperture or apertures 92, 94, 96, 98 aligned with one another parallel to the longitudinal axis A - A of the stator core 10i and with adjacent groups 130,132,134,136,138, etc. of laminations being progressively offset with respect to one another in the circumferential direction. The embodiment of Figure 22 has been described as a modification to the embodiment of Figure 11, but the principle of forming a stator core from a plurality of stacked groups of identical laminations with the laminations in each group being arranged with the aperture or apertures aligned with one another parallel to the longitudinal axis of the stator core, can be applied to all of the embodiments. 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 for a vehicle electrical motor, the stator having a longitudinal axis and comprising:a plurality of stacked laminations, each lamination comprising an aperture aligned with a further aperture in the other laminations to form a coolant passage;wherein the laminations or groups of laminations are progressively rotated about the longitudinal axis of the stator to progressively offset the apertures forming the coolant passage.

2. A stator as claimed in claim 1, wherein the apertures are progressively offset circumferentially.

3. A stator as claimed in claim 1 or claim 2, wherein the apertures are progressively offset in a radialdirection with respect to the longitudinal axis.

4. A stator as claimed in claim 1, comprising a plurality of identical laminations and wherein the laminations or groups of laminations are progressively rotated about the longitudinal axis through the same incremental angle.

5. A stator as claimed in claim 4, wherein each lamination comprises at least three apertures.

6. A stator as claimed in claim 5, wherein each aperture forms part of a respective coolant passage.

7. A stator as claimed in claim 5 or claim 6, wherein the apertures are progressively offsetcircumferentially with respect to the longitudinal axis.

8. A stator as claimed in claim 7, wherein the relative angular spacing of adjacent apertures on each lamination is offset circumferentially with respect to relative nominal aperture positions having an angular spacing equal to the incremental angle.

9. A stator as claimed in claim 5 or claim 6, wherein the apertures are progressively offset in a radial direction with respect to the longitudinal axis.

10. A stator as claimed in claim 9, wherein the relative angular spacing of adjacent apertures on each lamination is equal to the incremental angle and wherein adjacent apertures are offset from one another in the radial direction.

11. A stator as claimed in claim 5 or claim 6, wherein the relative angular spacing of adjacent apertures on each lamination is offset circumferentially with respect to relative nominal aperture positions having an angular spacing equal to the incremental angle, and wherein adjacent apertures are also offset from one another in the radial direction.

12. A stator as claimed in claim 5 or claim 6, wherein adjacent apertures on each lamination are offset circumferentially and radially with respect to nominal relative aperture positions having a circumferential spacing equal to the incremental angle and at the same radial position.

13. A stator as claimed in any of the preceding claims, wherein the aligned apertures in the laminations are of the same shape.

14. A vehicle electrical motor comprising a stator as claimed in any of the preceding claims.

15. A vehicle comprising a vehicle electrical motor as claimed in claim 14.13

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