A stator for an electrical motor

Abstract

A stator for an electrical motor comprising a plurality of stacked laminations 14, each lamination comprising a plurality of apertures 28, 30 which cooperate with corresponding apertures in the other

Description

TECHNICAL FIELD The present disclosure relates to a stator for an electrical motor. Aspects of the invention relate to an electrical motor comprising a stator in accordance with the present invention and a method of forming a stator for an electrical motor. BACKGROUND It is known to form a stator for an electrical motor from a stack of metal laminations which are aligned and secured together (typically by bonding them together with layers of dielectric adhesive / varnish) to form the stack. Typically, each lamination has a plurality of apertures which cooperate with corresponding apertures in the other laminations to define a plurality of coolant passages extending through the stator, and through which oil is circulated to control the temperature of the stator. However, under elevated oil pressure, the stator lamination stack can exhibit seepage of oil from the stator lamination stack oil channels, radially through the laminations, and onto the external surfaces of the lamination stack. The oil seepage may lead to degradation of the cooling and lubricating capacity or eventual failure of the electric drive unit. 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 for an electrical motor, an electrical motor comprising a stator in accordance with the present invention and a method of forming a stator for an electrical motor. According to an aspect of the present invention, there is provided a stator for an electrical motorthat comprises: a plurality of stacked laminations, each lamination comprising a plurality of apertures which cooperate with corresponding apertures in the other laminations to define a plurality of coolant passages extending through the stator, one or more of the coolant passages comprising a plurality of projections and / or recesses; wherein one or more of the coolant passages comprises an electrically insulating lining which is an interference fit with the coolant passage in which it is located. The lining of the coolant passage helps to prevent or reduce the seepage of coolant, such as oil, radially from the coolant passage between adjacent laminations. The plurality of projections and / or recesses increases the surface area of the coolant passage and assists in the transfer of heat to the coolant passing along the lined coolant passage. The use of an electrically insulating lining ensures that there is no electrical connection between the stacked laminations, which could otherwise result in the generation of undesirable eddy currents and the interference fit between the lining and its coolant passage helps to assist in the transfer of heat from the stator laminations into the coolant flowing through the lined coolant passage. In an embodiment, the lining may extend along the whole length of the coolant passage in which it is located. This may assist in preventing or reducing coolant seepage between adjacent laminations along the whole length of the coolant passage. In an embodiment, the coolant passages may extend parallel to a longitudinal axis of the stator. In an embodiment, the external surface of the lining may be shaped complementarily with the shape of the coolant passage in which it is located. This may help to transfer heat from the stator laminations to the lining and in turn to the coolant passing along the lined coolant passage. In an embodiment, the inner surface of the lining may be non-circular. A non-circular inner surface of the lining may increase its surface area, thus, providing more surface area for the transfer of heat to the coolant. In an embodiment, the inner surface of the lining may comprise a plurality of projections and / or recesses. In an embodiment, the lining may comprise a tube. A tube inserted in a coolant passage may form a continuous, unbroken lining which may help to reduce or to prevent the seepage of coolant between adjacent stator laminations. In an embodiment, the tube may be expanded into contact with the wall of the coolant passage. This may allow a very close contact to be formed between the wall of the coolant passage and the external surface of the tube, which may assist in the bettertransfer of heatto the coolant passing along the lined coolant passage. In an embodiment, the tube may comprise a thermoplastic material. This may allow to have a tube having a diameter slightly smaller than that of the coolant passage to be inserted and expanded beyond its yield point, for example, by application of heat and / or pressurised liquid or gas, to form a sealing interface between the exterior of the tube and the coolant passage. In an embodiment, the stator may comprise a plurality of identical coolant passages. In an embodiment, the stator may comprise a plurality of identical laminations. The use of a plurality of identical laminations may facilitate manufacture and assembly of the stator. According to another aspect of the present invention, there is provided an electrical motor comprising a stator as claimed in any preceding claim. According to another aspect of the present invention, there is provided a method of forming a stator for an electrical motor, comprising the steps of: forming a stack of laminations, each lamination comprising a plurality of apertures which cooperate with corresponding apertures in the other laminations to define a plurality of coolant passages extending through the stator, wherein one or more of the coolant passages comprises a plurality of projections and / or recesses; securing the laminations together; and inserting an electrically insulating tube into one or more of the coolant passages. In an embodiment, the method may further comprise the step of: expanding the or each electrically insulating tube into contact with the coolant passage into which it is inserted. In the method of the present invention, an electrically insulating tube having a diameter slightly smaller than that of the coolant passage which it is intended to line is inserted into the coolant passage and can be expanded beyond its yield point, fore.g. by application of heat and / or pressurised liquid or gas, to form a sealing interface between the exterior of the tube and the coolant passage. In an embodiment, the tube may comprise a thermoplastic material. The use of a tube of thermoplastic material may allow heat to be applied to the tube to form a continuous, sealing interface with the coolant passage. 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 perspective view of an embodiment of a stator for an electrical motor, in accordance with the present invention; Figure 2 shows a front view of the stator of Figure 1; Figure 3 shows a partially exploded perspective view of the stator of Figure 1; Figure 4 shows a perspective view of another embodiment of stator for an electrical motor, in accordance with the present invention; Figure 5 shows a front view of the stator of Figure 4; Figure 6 shows a partially exploded perspective view of the stator of Figure 4; Figure 7 illustrates an end view, to a larger scale, of an example of a coolant passage extending through the stator of Figure 4; Figure 8 illustrates an end view, to a larger scale, of another variation of a coolant passage extending through the stator of Figure 4; and Figure 9 is a flow diagram illustrating a method of forming a stator for an electrical motor, in accordance with the present invention. DETAILED DESCRIPTION Figures 1 to 3 illustrate an embodiment of stator 10 of an electric motor (not shown) of a vehicle. The stator 10 consists of a stator core and coils of insulated wire or windings (not shown). The stator core is formed from a stack 12 of identical annular soft-magnetic steel laminations 14 (illustrated schematically and not to scale) which are aligned and bonded together with layers 16 of dielectric adhesive / varnish. Each lamination 14 is formed from a sheet of electrical steel, for example, each lamination may be 0.2 mm thick, and is typically stamped from the sheet. Each lamination 14 may also be pre-coated on both sides with an electrically insulating bonding varnish, for example Backlack™ varnish, although other insulating bonding varnishes could be used, if desired. As shown in the drawings, each lamination 14 may be planar and may comprise an annular outer ring portion 18, from the inner edge of which stator tooth portions 20, may angularly be spaced around the inner edge, may project radially inwardly. It should be noted that even though the drawings depict forty-eight identical stator tooth portions, the present invention is not limited to forty-eight and there may be any suitable number of stator tooth portions as per the requirements or the application. Also, the stator tooth portions may be identical, nonidentical ora variation of both. Further, depending on the requirement, the stator tooth portions may be equally spaced around the inner edge of the outer ring. Each of the stator portions may be provided with an enlarged head portion 22 at a radially innermost edge and adjacent tooth portions are separated by gaps 24, which may be identical. In use, coils (not shown) wound around the tooth portions 20, in a conventional manner. As illustrated in an embodiment of Figure 3, the annular outer ring portion 18 of each lamination 14 may be provided with a radially inner ring of apertures 28 and a radially outer ring of apertures 30. In this embodiment, the apertures 28, 30 are shown of circular shape and of same size. However, the apertures 28, 30 may be of any different shape and / or size. Further, the apertures 30 may be of different shape and / or size to that of the apertures 28. The radially inner and outer rings of apertures 28, 30 may be arranged coaxially with the annular outer ring portion 18 and may be configured such that the apertures 28 of the innerring are positioned between two apertures 30 of the outer ring and vice versa. With such arrangement of apertures, the adjacent apertures may be staggered radially in the circumferential direction. In this embodiment, the apertures 28, 30 are depicted as circular in cross-section and shown as equally angularly spaced around the annular outer ring portion 18. In order to form the stator core, several identical laminations 14 (in examples, there may be approximately 800, or 750, or 700. In one example there are 760 laminations) may be assembled into a stack with the apertures 28, 30 of the laminations being aligned. The assembled stack may then be compressed (e.g. at a pressure of 4MPa) and cured (e.g. at a temperature of 175° C) to bond the laminations together to form the stator core. For illustration, Figures 1 to 3 show the aligned apertures 28, 30 in the laminations 14 form a stator core having ninety-six identical coolant passages 40 of circular cross-section (only two of which are fully illustrated in Figure 3), extending parallel to the longitudinal axis A-A of the core. In other examples, there may be approximately 100, approximately 95 or 90-100 coolant passages 40. As shown in Figures 1 to 3, each coolant passage 40 may receive a straight elongate tube 44 formed from a polymer or elastomer such as polyphthalamide (PPA), polyphenylene sulphide (PPS) and polyamide (PA). The tube 44 may be of the same length as the coolant passages 40. Further, the tube 44 may have a smooth cylindrical outer surface of the same external diameter as (or slightly smaller than) the internal diameter of the coolant passages 40. All elongate tubes 44 may be identical. The tubes 44 may be pushed manually into the coolant passages 40 of the stator core and are intended to be an interference fit with the coolant passages 40. In use, coolant liquid is fed through the elongate tubes 44 from one end to the other in order to reduce the temperature of the stator 10. The elongate tubes 44 form a lining in each coolant passage 40, which helps to prevent or reduce the seepage of coolant radially from the coolant passage into the adjacent laminations of the stator 10. The elongate tubes comprise an electrically insulating material. In addition, the use of an electrically insulating lining (which in this embodiment comprises a tube 44) ensures that there is no electrical connection between the stacked laminations 14, which could otherwise result in the generation of undesirable eddy currents. The interference fit between the lining formed by the tubes 44 and the coolant passages 40 helps to assist in the transfer of heat from the stator laminations 14 into the coolant flowing through the coolant passages 40. Thereby, reducing the temperature of the stator 10. Another embodiment (which is a variation of the embodiment of Figures 1 to 3) is shown in Figures 4 to 6. This embodiment shares most of the features of the embodiment of Figures 1 to 3, and are identified in Figures 4 to 6 with the same reference numerals as in Figures 1 to 3. The only difference from the embodiment of Figures 1 to 3, and as shown in Figure 5, is that the tubes 44 of the embodiment of Figures 1 to 3 which have a cylindrical inner surface are replaced with tubes 46 whose internal surface is formed from inwardly-projecting lobes, for e.g. six inwardly projecting lobes 48a, 48b, 48c, 48d, 48e, 48f. As shown in Figure 7, the projecting lobes define bights or recesses 50a, 50b, 50c, 50d, 50e, 50f between adjacent lobes, which increases the internal surface area of the tubes 46 as compared with a tube having a circular profile and the same external diameter. This further increase the heat transfer coefficient. As for the arrangement shown in Figures 1 to 3, in this embodiment, the external surface of each tube 46 may also be smooth and cylindrical and the elongate tubes may be pushed manually into the coolant passages 40 of the stator core 10. Further, the tubes 46 may be intended to be an interference fit with the coolant passages 40. Similarly in this embodiment, in use, coolant liquid is fed through the elongate tubes 46 from one end to the other in order to reduce the temperature of the stator 10. The elongate tubes 46 may form a lining in each coolant passage 40, which may help to prevent or reduce the seepage of coolant radially from the coolant passage which might otherwise take place between adjacent laminations of the stator 10. The projections and / or recesses formed by the inwardly-projecting lobes 48a, 48b, 48c, 48d, 48e, 48f increases the surface area of the coolant passage as compared with a cylindrical passage. This may assist in the better transfer of heat from stator to the coolant passing along the lined coolant passages. In another variation, and as shown in Figure 8, the circular apertures 28, 30 in the laminations 14 are replaced with profiled apertures 58, 60. Each formed from inwardly projecting lobes for e.g. six inwardly-projecting lobes 62a, 62b, 62c, 62d, 62e. The projecting lobes define bights or recesses 64a, 64b, 64c, 64d, 64e between adjacent lobes, and forming elongate coolant passages having that cross-section. In addition, the straight elongate tubes 44 having a smooth cylindrical outer surface are replaced with straight elongate tubes 70 having a profiled outer surface which is complementarily-shaped and sized with the shape and size of the interior surface of the elongate coolant passages and whose external profile generally follows the profile of the internal surface. As for the previous embodiments, the tubes 70 may be fitted manually into the coolant passages and are intended to be an interference fit with the coolant passages. The provision of contoured coolant passages formed by the inwardly-projecting lobes 62a, 62b, 62c, 62d, 62e and the bights or recesses 64a, 64b, 64c, 64d, 64e increases the surface area of the coolant passages as compared with a coolant passage having a circular cross-section. Further, it may increase the heat transfer coefficient and may assist in the better transfer of heat from stator to the coolant passing along the lined coolant passages. In a further variation, the elongate tubes 44, 46, 70 as referred to above may be sized to be a loose fit within their associated coolant passages 40. Further, after being inserted into the passages 40, the elongate tubes 44, 46, 70 may be expanded outwardly into contact with the wall of the coolant passage 40 and beyond the yield point to form an interference fit with the interior surface of the coolant passage. Thus, forming a sealing interface between the exterior of the tube and the coolant passage. This variation is illustrated in the method or process of Figure 9. In Figure 9, and in the following description, “Step” is abbreviated to “S”. In Figure 9, steps S100 to S106 are as follows: S100: Forming a stack of laminations having coolant passages S102: Securing the laminations together S104: Inserting an electrically insulating tube into one or more coolant passages S106: Expanding the tube(s) into contact with the coolant passage(s) The above steps are explained in more detail below. At S1OO, a stack of laminations 14 having apertures 28, 30 is formed, for example as described above, with the apertures 28, 30 in the laminations all being aligned with each other. At S102, the assembled stack is secured together, for example using applied pressure and or an elevated temperature, as described previously, to form a stator core 10 with elongate coolant passages 40 extending longitudinally through the stator core, also as described previously. At S104, electrically insulating tubes 44, 46, 70 are inserted into one or more of the coolant passages 40 of the stator core 10. The method further comprises S106. At S106, the ends of the insulating tubes 44, 46, 70 may be sealed and each tube may be expanded beyond its yield point into an interference fit with the interior surface of the associated coolant passage 40. Heat and / or pressure may also be applied as part of the process. This produces a stator having coolant passages which are lined with an electrically insulating material. This configuration helps to prevent or reduce the seepage of coolant radially from the coolant passage between adjacent laminations. The interference fit between the lining and its associated coolant passage helps to assist in the transfer of heat from the stator laminations into the coolant flowing through the lined coolant passage. It should be noted that although the above embodiments include identical laminations, the present invention is not limited to that. The laminations might not be identical. For example, there may be differences, particularly on the outer diameter of the laminations, to provision for timing slots or other features required to support manufacturing processes. Moreover, the above embodiments referto bonded lamination stacks, but it is not necessary forthe laminations to be bonded. For example, the lamination stacks may be welded rather than bonded. Further, with the present invention, as the coolant channels are sealed, it does not rely on the inter-lamination bonding to be sealed. Furthermore, the number of lobes, slots, coolant channels and the like may differ from the numbers mentioned in the above embodiments. It is envisaged that the overall group of coolant channels may comprise a mixture of shapes, such as some circular cross-sectional channels, as described, and some multi-lobed cross-sectional channels, as later described, within a single stator arrangement. 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 an electrical motor, comprising:a plurality of stacked laminations, each lamination comprising a plurality of apertures which cooperate with corresponding apertures in the other laminations to define a plurality of coolant passages extending through the stator, one or more of the coolant passages comprising a plurality of projections and / or recesses;wherein one or more of the coolant passages comprises an electrically insulating lining which is an interference fit with the coolant passage in which it is located.

2. The stator as claimed in claim 1, wherein the lining extends along the whole length of the coolant passage in which it is located.

3. The stator as claimed in claim 1 or claim 2, wherein the coolant passages extend parallel to a longitudinal axis of the stator.

4. The stator as claimed in any preceding claim, wherein the external surface of the lining is shaped complementarily with the shape of the coolant passage in which it is located.

5. The stator as claimed in any preceding claim, wherein the inner surface of the lining is non-circular.

6. The stator as claimed in any preceding claim, wherein the inner surface of the lining comprises aplurality of projections and / or recesses.

7. The stator as claimed in any preceding claim, wherein the lining comprises a tube.

8. The stator as claimed in claim 7, wherein the tube is expanded into contact with the wall of the coolantpassage.

9. The stator as claimed in claim 7 or claim 8, wherein the tube comprises a thermoplastic material.

10. The stator as claimed in any preceding claim, comprising a plurality of identical coolant passages.

11. The stator as claimed in any preceding claim, comprising a plurality of identical laminations.

12. An electrical motor comprising a stator as claimed in any preceding claim.

13. A method of forming a stator for an electrical motor, comprising the steps of:forming a stack of laminations, each lamination comprising a plurality of apertures which cooperate with corresponding apertures in the other laminations to define a plurality of coolant passages extending through the stator, wherein one or more of the coolant passages comprises a plurality of projections and / or recesses;5 securing the laminations together; andinserting an electrically insulating tube into one or more of the coolant passages.

14. The method as in claim 13, further comprising the step of: expanding the or each electrically insulating tube into contact with the coolant passage into which it is inserted.10 15. The method as claimed in claim 13 or claim 14, wherein the tube comprises a thermoplastic material.s

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