Electrical machine for a vehicle

Abstract

An electric machine for a vehicle, comprising: a stator (100, Fig. 3) formed of laminations having cooling channels (140, Fig. 3) extending from one end to the other; and a resiliently deformable comp

Description

TECHNICAL FIELD The present disclosure relates to an electrical machine for a vehicle. Aspects of the invention relate to an electrical machine for a vehicle, an electric drive unit and a vehicle comprising such an electrical machine or drive unit. BACKGROUND Vehicles propelled by electric drive units are increasingly common. The performance of such vehicles is also steadily improving, and continuing this trend requires greater efficiencies in the components used. Electric drive units are based around electrical machines (electric motors) which comprise a stator, and a rotor (bearing a shaft) which rotates within the stator under the application of electrical energy. Heat in generated in the various parts of the electrical machine during use, but particularly in the stator, which houses electrical windings which are selectively energised and generate heat in use. It is known to provide a cooling system for electrical machines, but existing cooling systems, for example water cooling systems, have limitations due to being unable to convey the water too close to the electrical machine. 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 an electrical machine for a vehicle, an electric drive unit and a vehicle comprising such an electrical machine or drive unit as claimed in the appended claims. According to an aspect of the present invention there is provided an electrical machine for a vehicle, comprising: a stator, formed of a plurality of laminations, and having a plurality of cooling channels extending through the laminations from one end of the stator to the other; and a resiliently deformable compression element, configured to apply a compression force to one end of the stator to compress the laminations together. In this way, cooling fluid (preferably oil) can be driven through the cooling channels of the stator without risk (or with a reduced risk) of the oil forcing its way between the laminations and towards and ultimately leaking into an airgap between the stator and a rotor mounted within the stator. In particular, the compression element, by applying an axially directed force to compress the laminations together, enables oil to be pumped through the stator under pressure (which increases the rate of cooling) without compromising the integrity of the stack. As the element is resiliently deformable, it is able to move to accommodate expansion due to heating and cooling of the stator, while maintaining a consistent compression force on the laminations. The axial compression force is preferably substantially constant across an operating temperature range of the stator. The electrical machine may comprise a housing. The compression element may act (directly or indirectly) between the housing and the stator. In one implementation, the compression element is directly engaged with the housing. In another implementation, the electrical machine comprises a circumferential plate mounted into the housing proximate the one end of the stator, wherein the compression element is engaged with the plate. The housing preferably comprises a shoulder at the opposite end of the stator from the compression element, wherein the compression element urges the stator against the shoulder to compress the stator between the shoulder and the compression element. In one implementation, the electrical machine comprises an oil seal radially inwardly of the cooling channels, wherein the compression element is configured to apply the compression force via the oil seal. In this way, the compression element serves both to improve the sealing pressure of the oil seal, and to compress the stator laminations together. In some implementations electrical insulation is disposed between the compression element and the stator. This avoids or at least reduces interference with the electrical properties of the end lamination against which the compression element presses. In some implementations the compression element contacts or applies the compression force to the stator radially outwardly of the cooling channels. In this case the compression element is typically disposed inside a sealed chamber or manifold used to inject oil into the cooling channels. The resiliently deformable compression element may be a spring, or formed of a compressible material. Preferably, the compression element is annular. The compression element may for example be a Belleville washer. The laminations may be bonded together using a backlack coating. This, combined with the axial pressure provided by the compression element, reduces the risk of gaps being present, or opening, between the laminations, through which oil can leak (sweat) into the air gap. The electrical machine may comprise a cooling system, configured to pump oil through the cooling channels of the stator. The oil may serve a dual use of lubricating gears and other moving parts of the electrical machine and associated transmission, and also to cool the stator. According to another aspect of the invention, there is provided an electric drive unit comprising the electrical machine according to the above. According to another aspect of the invention, there is provided a vehicle comprising one or more electrical machines according to the above, or one or more electric drive units according to the above. 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 anyway 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 schematic representation of an automobile having electric drive units; Figure 2 shows a schematic representation of a stator for an electric drive unit; Figure 3 shows a schematic representation an electrical machine including a compression element; Figures 4A and 4B show a schematic representation of a Bonneville washer suitable for use as a compression element; Figure 5 shows a schematic representation of a portion of an electrical machine with a compression element engaged directly with a housing; Figure 6 shows a schematic representation of a portion of an electrical machine with a compression element engaged with a plate mounted into the housing; and Figure 7 shows a schematic representation of a portion of an electrical machine with the compression element acting on an oil seal. DETAILED DESCRIPTION A vehicle in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1. As shown in Figure 1, a vehicle 1 is provided, which comprises a set of road wheels 10a, 10b, 10c, 10d, of which road wheels 10a and 10b are provided on a front axle 12, and of which road wheels 10c and 10d are provided on a rear axle 14. A first electric drive unit 16 is configured to provide propulsion torque to the front axle 12 (and thus the wheels 10a, 10b), while a second electric drive unit 18 is configured to provide propulsion torque to the rear axle 14 (and thus the wheels 10c, 10d). The first and second electric drive units 16, 18 are powered by a vehicle battery 20. The first and second electric drive units 16, 18 each comprise a cooling system (not shown), which is configured to pump oil from an oil sump (not shown, provided at the base of the electric drive unit) through portions of the electric drive units to regulate the temperatures thereof. The first and second electric drive units 16,18 each comprise an electrical machine (motor) having a stator, and a rotor mounted within the stator. Figure 2 shows a cross-sectional view of a stator 100. The stator 100 can be seen to be annular in cross section, and to have winding slots 120 extending through it in a direction parallel to a longitudinal axis of the stator 100. The winding slots are provided proximate an internal diameter of the stator 100. Radially outwards of the winding slots 120 are a set of cooling channels (conduits) 140, which also extend through the stator 100 in a direction parallel to the longitudinal axis of the stator 100. When the stator 100 is assembled into an electrical machine (such as the motors 16, 18), windings are provided within the winding slots 120, and connected to the vehicle battery 20, which is used to selectively energise the windings to control the rotation of the rotor (not shown) within the stator 100. The cooling channels 140 are configured, in use, to receive cooling oil, to maintain a desired temperature for the stator 100. The cooling oil is pumped or otherwise urged through the cooling channels 140 and thus through the stator 100, carrying away heat from the stator 100. Figure 3 shows a cross-sectional view through part of an electrical machine (some parts are omitted in the interests of clarity, but such parts may be conventional in nature). The parts shown in Figure 3 (and the subsequent figures) are not to scale, and are intended to be indicative only of the relative position and function of those parts. The electrical machine can be seen to comprise the stator 100, which is mounted within a housing 200. A rotor 300 is disposed to rotate within the stator 200. A shaft 400 is disposed within (and mounted for rotation with) the rotor 300. In some implementations the shaft 400 may be hollow, or include a through bore, via which cooling oil may be passed from one side of the electrical machine to the other to permit recirculation of the oil. The cooling channels 140 can be clearly seen within Figure 3, and extend entirely through the stator 100, from one end face (for example at the right hand side of Figure 3) to the other (for example at the left hand side of Figure 3). An air gap 350 is provide between the stator 100 and the rotor 300. For efficient operation this airgap 350 should be kept clear. To one side of the stator 100 and provided around the entrance to the cooling channels 140 is a first cooling chamber 160. This is a region defined between one end face of the stator 100, the housing 200, and a chamber wall 165, into which cooling oil is pumped for delivery through the cooling channels 140. To the other side of the stator 100 and provided around the exit from the cooling channels 140 is a second cooling chamber 170. This is a region defined between the other end face of the stator 100, the housing 200, and a chamber wall 175, into which the cooling oil from the cooling channels 140 is provided, and from which the cooling oil is extracted, for example for return to an oil sump, or for direct recirculation to the first cooling chamber 160. As can be seen from Figure 3, the stator 100 is formed of a series of laminations, stacked together. The laminations are relatively thin, typically 0.02mm in thickness. The laminations are typically adhered together, and may be individually covered with, and bonded together by, a backlack coating (enamel resin). Since the cooling oil is driven under pressure through the cooling channels 140, there is a risk that the oil may force its way between the laminations, degrading the backlack coating, and eventually intruding into the airgap 350 between the stator 100 and the rotor 300, thereby causing drag and degrading efficiency of the electrical machine. To address this problem, a resiliently deformable compression element 500 is provided, which is configured to apply a compression force to one end of the stator 100 to compress the laminations together. In particular, the compression element 500 applies an axially directed force to compress the laminations together, which reduces the risk of the oil driven through the channels 140 of the stator 100 forcing its way between the laminations and towards and ultimately into the airgap 350 between the rotor300 and stator 100. The axial force prevents or at least inhibits delamination of the laminated stator stack. As a result of the element being resiliently deformable, it is able to move to accommodate expansion of the stator due to heating and cooling, while maintaining a generally consistent compression force on the laminations (by Hooke’s Law). It will be appreciated that this would not be possible if a non-resiliently deformable structure were to be used to urge the laminations together. It will be understood that the consistency of the compression force need only apply within a certain temperature range relevant to operation of the electrical machine. In particular, the axial compression force should be substantially constant across an operating temperature range of the stator 100, but not necessarily outside of that temperature range. The housing 200 can be seen to comprise a shoulder 250 at the opposite end of the stator 100 from the compression element 500. The compression element 500 urges the stator 100 against the shoulder 250 to compress the stator 100 between the shoulder 250 and the compression element 500. There are various mechanical components capable of providing a suitable resiliently deformable compression to the stator 100. For example, the resiliently deformable compression element may be a spring, or may be formed of a compressible (e.g. elastomeric) material. Preferably, given the shape of the stator 100 and housing 200, the compression element 500 is annular, so that the compression can be applied equally to a circular region of an end face of the stator 100. In the present implementation as shown, the compression element is a Belleville washer. Figures 4A and 4B show a Belleville washer suitable for use as the compression element 500. Figure 4A shows a 3D view of a Belleville washer, while Figure 4B shows a side view of the Belleville washer. The Belleville washer is frustoconical in shape, having a large diameter end 510 and a small diameter end 520, with a sloped surface 530 extending from the large diameter end 510 to the small diameter end 520. In use, the large diameter end 510 engages (directly or indirectly) with the housing 200, while the small diameter end 520 engages (directly or indirectly) with an end face of the stator 100. Figure 5 shows one implementation, in which the compression element 500 is directly engaged with the housing 200. In particular, the housing 200 comprises a recess 260 (which in practice may be a groove in the housing 200 having a circular shape and extending proximate the circumference of the stator 100), into which the large diameter end 510 of the Bonneville washer extends. The small diameter end 520 of the Bonneville washer contacts an end face of the stator 100 (and in particular contacts a first, end lamination, of the set of laminations of the stator 100), urging the stator 100 against the shoulder 250 to compress the laminations together. The compression element 500 contacts the face of the stator 100 radially outwardly of the cooling channels 140. It will be appreciated that the compression element 500, when mounted to compress the stator laminations together, is required to be in a deflected (or deformed, or compressed) state. Figure 5 also shows the equilibrium (or free, or at rest) state of the compression element 500 (dashed outline of the compression element), to illustrate how the compression element is deformed away from its equilibrium state to apply the axial compressive force on the stator laminations. The same principle applied to Figures 4, 6 and 7. Figure 6 shows another implementation, in which a circumferential plate 280 (which may be an annular component such as a washer) is mounted to the housing 200 proximate the end of the stator. In this case, the compression element is engaged with and between the plate (at the large diameter end 510) and the stator 100. In this case, because the plate 280 is able to extend some distance radially inwardly of the housing 200, the compression element 500 may contact the face of the stator 100 radially inwardly of the cooling channels 140, which may provide a preferably location for applying pressure to the stator 100 in some implementations. Figure 7 shows another implementation, in which an oil seal forming part of the first cooling chamber serves as an intermediate element in the application of compressive force on the stator 100. In Figure 7, the chamber wall 165 can be seen to terminate in an annular seal 167 (for example a gasket, or large ‘O’ ring), which abuts the end of the stator 100 radially inwardly of the cooling channels to provide the sealed chamber 160. In Figure 7 the compression element 500 presses against the wall 165 to urge the annular seal 167 against the stator 100. As well as providing for the compression of the stator laminations together, this has the additional benefit of improving the integrity of the seal and eliminating or at least reducing oil leakage through the seal. In Figure 7 the compression element 500 is engaged between the wall 165 and a recess 290 (similar to the recess 260 of Figure 5), but it will be appreciated that it could instead be engaged with a structure (plate) similar to that shown in Figure 6. Typically, the compression element may be a steel Belleville washer. Where such a washer is directly applying pressure to the stator end face (for example as per Figures 4 to 6), electrical insulation may be disposed between the compression element and the stator, to reduce the impact of the compression element on the electromagnetic properties of the end lamination. This could be in the form of a coating on one or both of the end lamination of the stator 100 and the compression element 500. Alternatively, the compression element 500 could be formed of a non-conductive material, or may be a composite structure having a non-conducting part which contacts the stator 100. In terms of assembly, the deformable nature of the compression element also facilitates this. In particular, the stator may be press-fitted first into the housing 200, to contact the shoulder 250, and then the compression element 500 is inserted into the housing and against the stator 100. The outer edges of the compression element may then be urged past the recess 260 or plate 280, causing it to deform and deflect 5 inwardly, whereupon it then expands outwardly to become trapped between the stator 100 and the recess 260 or plate 280 once it has been urged past the recess 260 or plate 280. It will be appreciated that the present technique can be applied to electrical machines serving as either (or both) of a motor and a generator. 10 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. 15

Claims

1. An electrical machine for a vehicle, comprising:a stator, formed of a plurality of laminations, and having a plurality of cooling channels extending through the laminations from one end of the stator to the other; anda resiliently deformable compression element, configured to apply a compression force to one end of the stator to compress the laminations together.

2. The electrical machine according to claim 1, wherein the axial compression force is substantially constant across an operating temperature range of the stator.

3. The electrical machine according to claim 1 or claim 2, comprising a housing, wherein the compression element acts between the housing and the stator.

4. The electrical machine according to claim 3, wherein the compression element is directly engaged with the housing.

5. The electrical machine according to claim 3, comprising a circumferential plate mounted into the housing proximate the one end of the stator, wherein the compression element is engaged with the plate.

6. The electrical machine according to any one of claims 3 to 5, wherein the housing comprises a shoulder at the opposite end of the stator from the compression element, wherein the compression element urges the stator against the shoulder to compress the stator between the shoulder and the compression element.

7. The electrical machine according to any preceding claim, comprising an oil seal radially inwardly of the cooling channels, wherein the compression element is configured to apply the compression force via the oil seal.

8. The electrical machine according to any preceding claim, comprising electrical insulation disposed between the compression element and the stator.

9. The electrical machine according to any preceding claim, wherein the compression element contacts or applies the compression force to the stator radially outwardly of the cooling channels.

10. The electrical machine according to any preceding claim, wherein the resiliently deformable compression element is a spring, or is formed of a compressible material.

11. The electrical machine according to any preceding claim, wherein the compression element is annular.

12. The electrical machine according to claim 11, wherein the compression element is a Belleville washer.

13. The electrical machine according to any preceding claim, wherein the laminations are bonded 5 together using a backlack. coating.

14. An electric drive unit comprising the electrical machine according to any preceding claim.

15. A vehicle comprising the electrical machine according to any one of claims 1 to 13, or the electric10 drive unit according to claim 14.10

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