An electric machine

Abstract

An electric machine, comprising a stator comprising a first end face, a second end face, and stator windings extending through respective stator slots that extend between the first and second end face

Description

07 11 25 FIELD OF INVENTION The invention relates to an electric machine and in particular a high performance electric machine with optimised indirect cooling combined with a minimal housing structure. BACKGROUND The climate emergency is driving global governments to impose demanding targets for the decarbonisation of economies. Industry, including transportation, is making progress in achieving those targets resulting in increasing levels of electrification. End users and manufacturers of electric products face ever increasing challenges in the field of electric machines. These include increasing performance requirements including power to weight ratios, power to volume ratios and efficiency; evolving noise, vibration and environmental requirements; increasing material scarcity including rare earth materials; and a drive for cost reduction: both recurring and non-recurring. The electric machine industry is working hard on tackling these challenges with the development of high performance axial flux machines, oil cooled radial flux machines, and high performance rare earth free machines, including induction, switched reluctance and synchronous reluctance machines. These technologies will be appropriate for a number for applications. However, they may not always meet end user needs. Concentrated wound permanent magnet radial flux machines including rare earth material content have the benefit of being able to achieve very high rotational speeds and high-power density. However, these materials are expensive and require significant cooling. A typical way to cool an electric machine is with oil directly on the windings. The challenges associated with this method of cooling involve sealing the overall machine to contain the oil; requiring compatibility of materials used in the machine with oil; preventing metallic contamination of the oil from debris from the gearbox (which causes short circuits); and limited ability to absorb heat (low specific heat capacity). 07 11 25 SUMMARY According to an aspect of the present disclosure there is provided an electric machine. The electric machine comprises a stator comprising a first end face, a second end face, and stator windings extending through respective stator slots that extend between the first and second end faces. The machine further comprises a rotor arranged at least partially within the stator. A plurality of cooling elements are arranged within respective stator slots. Each of the plurality of cooling elements comprise a fluid inlet and a fluid outlet. A manifold is configured to deliver and receive fluid from the plurality of cooling elements. A plurality of fluid couplings fluidly couple respective cooling elements to the manifold. Each of the plurality of fluid couplings comprise a first fluid channel extending between the manifold and the inlet of the respective cooling element, and a second fluid channel extending between the manifold and the outlet of the respective cooling element. Each fluid coupling is configured to electrically isolate the respective cooling element from the manifold. The fluid couplings may electrically isolate the plurality of cooling elements from one another. Each of the plurality of cooling elements may comprise a proximal end and a free distal end, and the fluid inlet and outlet are arranged at the proximal end. Each fluid coupling may comprise a first pipe defining at least part of the first fluid channel and a second pipe defining at least part of the second fluid channel. Each fluid coupling may be formed at least in part of an electrically insulating material. The electrically insulating material may comprise a polymeric material. Each cooling element may comprise a cooling rod comprising an outer conduit, and an inner conduit arranged within the outer conduit such that an axially extending channel is formed therebetween; and a flow path is defined through the cooling element from the inlet, along the inner conduit, into the outer conduit, and along the axial channel to the outlet. The outer conduit of the cooling rod may comprise a closed distal end, the inner conduit of the cooling rod comprises an open distal end, and the open distal end of the inner conduit is axially spaced away from the closed distal end of the outer conduit to enable fluid to pass from the inner conduit into the outer conduit. The inner conduit of the cooling rod may be formed as a tube having a central longitudinal axis aligned with the inlet of the cooling element, and the outlet of the cooling element is radially spaced away from the longitudinal axis of the inner conduit. 07 11 25 The stator windings may extend across the first end face of the stator to define a first plurality of end windings; and each cooling element further comprises a cooling block configured to cool at least one of the first plurality of end windings. Each cooling block may include a manifold-facing side connected to the inlet and the outlet of the cooling element, and a stator-facing side connected to the cooling rod. At least one concave surface may be formed on the stator-facing side, the shape of at least one concave surface conforms to the shape of one of the first plurality of end windings. Each cooling block may comprise a channel fluidly coupling the outlet of the cooling element to the axial channel of the cooling rod. The stator windings may extend across the second end face of the stator to define a second plurality of end windings; and the machine further comprises a cooling loop arranged to cool the second plurality of end windings, the cooling loop comprising an inlet and an outlet coupled to the manifold by one of the plurality of fluid couplings. The cooling loop may comprise a curved tubular portion arranged at the second end face of the stator. The electric machine may further comprise a cooling jacket circumferentially surrounding the stator, wherein the cooling jacket is in fluid communication with the manifold, and the cooling jacket is electrically isolated from the manifold. The cooling jacket may comprise an inlet coupled to the inlet of the cooling loop to enable fluid communication therebetween, and an outlet coupled to the outlet of the cooling loop to enable fluid communication therebetween. The cooling jacket may comprise a plurality of cooling segments connected together by joints having fluid channels defined therethrough. The electric machine may further comprise a first end plate and a second end plate arranged respectively at opposing end faces of the stator, and a plurality of tie rods arranged circumferentially around the stator and extending axially between the first and second end plates; wherein the plurality of tie rods are configured to retain the stator in position between the first and second end plates. The joints of the cooling jacket may comprise slots configured to receive at least a portion of a tie rod such that the channels of the joints are arranged radially outwards of the slots and extends over the tie rods. 07 11 25 The cooling jacket may comprise a cooling coil formed around the stator, the cooling coil having a channel defined therethrough to enable passage of fluid from the inlet of the cooling jacket to the outlet of the cooling jacket. The present disclosure relates to an electric machine that increases performance, reduces material content, and achieves a cost reduction by integrating a highly effective liquid cooling network with a minimal cost-effective housing. In an example, a segmented indirect water-cooling circuit is used with coolant circulated in thermally conductive pipes, such as metal pipes, embedded within the windings in the machine's slots, the end windings and around the stator outer diameter. This arrangement reduces the thermal resistance between the coolant and the conductors. This approach can take benefit of the improved thermal performance of water / glycol compared to that of oil. Using a self-contained indirect liquid cooling network obviates the need for a traditional expensive cast or machined housing. This conventional arrangement is replaced with two drive end plates and a plurality of beams (i.e. tie rods) that hold the stator, with the stator outer diameter liquid cooling being fitted around the outer diameter of the stator and beams. The electric machines may be a high-performance electric motor that may be less than 5MW. The motor may concentrated wound. The cooling arrangement of the present invention allows the use of water and other specialist compounds (e.g. EGW 50 / 50, 'super coolants') that cannot be used in conventional oil cooling arrangements. Additionally, by isolating this coolant within an indirect coolant circuit, rather than on the windings of the machine, the challenges associated with direct oil cooling are addressed. The use of coolants with increased specific heat capacity results in increased cooling ability. This increased cooling ability allows accommodation of the increased heat produced as the current density of an electric machine is increased. As increased current density can result in a smaller machine (crude approximation F = BH), the cooling arrangement of the present disclosure allows for a reduction in superconductor use. Compared to conventional direct oil cooled machines, the arrangement of the present disclosure reduces the content of active electromagnetic materials (such as laminations, windings and magnets) by at least 10% to 20%, by virtue of increased cooling. The cooling circuit architecture minimises thermal resistances between the cooling media and the conductors, minimises eddy losses within its structure and is low cost to produce. 07 11 25 The liquid cooling network is cooling media agnostic making it particularly useful in future cryogenic cooling of superconducting machines. Cooling fluid can be directed through the liquid cooling network to locations that require it whilst keeping the cooling fluid separated and segregated from lubrication networks and dynamic components. Without the requirement for a traditional housing, the material content and energy needed to produce the housing structure, with burst containment / environmental sealing if necessary, is significantly reduced. The requirement for casting, machining, 5 axis milling of a complex machine body may be replaced with simple casting or turning operations. The electric machine of the present disclosure is a concentrated wound machine comprising a segmented water cooling network independent of the housing structure. The cooling network architecture, which includes slot cooling elements, one or more manifolds and may include end winding cooling, is configured to achieve required flow rates whilst minimising losses from eddy currents. A cooling jacket may be provided structurally independently of the housing body. The machine housing structural design / stator interface may use beams to connect and constrain machine ends, and may be unconstrained by the cooling or lubrication network. A customisable machine protection sleeve or cover may be provided that independent of the machine housing structure. That is to say the housing may not be a structural element of the electric machine. General statement to machine including tie rods such that don't need to use cast body = lower cost BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic rear view of an embodiment of the invention showing all cooling conduits, manifolds, and pipes without the machine support structure; Figure 2 is a diagrammatic front view of an embodiment of the invention showing all cooling conduits, manifolds, and pipes without the machine support structure; Figure 3 is a diagrammatic view of a slot cooling pipe according to an embodiment of the invention; 07 11 25 Figure 4 is a diagrammatic cross section view through the inlet / outlet chamber of a slot cooling pipe according to an embodiment of the invention; Figure 5 is a diagrammatic cross section view through the inlet / outlet chamber of a slot cooling pipe according to an embodiment of the invention; Figure 6 is diagrammatic view of an embodiment of the invention without an outer sleeve; Figure 7 is a cross sectional view of an embodiment of the invention showing the coolant flow path; Figure 8 is a diagrammatic rear view of an embodiment of the invention showing all cooling conduits, machined manifold, and pipes without the machine support structure; Figure 9 is a diagrammatic front view of an embodiment of the invention showing all cooling conduits, machined manifold, and pipes without the machine support structure; Figure 10 is a diagrammatic view of an embodiment of the invention with a continuous pipe cooling jacket; Figure 11 is a diagrammatic view of an embodiment of the invention with a cast segmented flat top cooling jacket; Figure 12 is a diagrammatic view of an embodiment of the invention with a cast segmented cylindrical cooling jacket; and Figure 13 is a diagrammatic view of an embodiment of the invention with a cast segmented cylindrical cooling jacket with integrated burst containment shields. DESCRIPTION OF EMBODIMENTS The following description presents exemplary embodiments and, together with the drawings, serves to explain principles of the disclosure. The scope of the disclosure is not intended to be limited to the precise details of the embodiments or exact adherence with all method steps. Variations will be apparent to a skilled person and are deemed also to be covered by the description. Terms for features used herein should be given a broad interpretation that also encompasses equivalent functions and 07 11 25 features. In some cases, several alternative terms (synonyms) for structural features have been provided but such terms are not intended to be exhaustive. Descriptive terms should also be given the broadest possible interpretation; e.g. the term "comprising" as used in this specification means "consisting at least in part of" such that interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. Directional terms such as "vertical", "horizontal", "up", "down", "upper" and "lower" are relative terms that may be used for convenience of explanation usually with reference to the illustrations and are not intended to be ultimately limiting if an equivalent function can be achieved with an alternative dimension and / or direction. The description herein refers to embodiments with particular combinations of configuration steps or features. However, it is envisaged that further combinations and cross-combinations of compatible steps or features between embodiments will be possible. The description of multiple features in relation to any specific embodiment is not an indication that such features are inextricably linked, and isolated features may function independently from other features and not necessarily require implementation as a complete combination. Referring to Figure 1 and 2, the cooling networks of the electric machine comprise a manifold 14, cooling loop 19, a cooling jacket 7, and a plurality of slot cooling elements 16. The manifold 14 can supply fluid to, and receive fluid from, the cooling jacket, and cooling elements sequentially or simultaneously (i.e. in series or in parallel). This segmented water-cooling network is independent of any housing structure of the electric machine. The manifold is electrically isolated from the cooling elements 16 by hoses 13 attached to a manifold 14. Each hose 13 has a fluid channel defined therethrough. Isolation from the inlet / outlet manifold by isolating hose 13 provides protection from short circuits and eddy losses in the metallic fluid conduits. The hose 13 is formed of electrically insulating materials, such as polymeric material. The cooling loop 19 may comprise simple pipe members or may be configured to conform to the geometry of the end winding. The cooling loop 19 comprises a curved tubular portion 54 to which an inlet 47 and outlet 48 are connected. The inlet 47 and outlet 48 are each electrically isolated from the manifold by a hose 13. The cooling jacket 7 is designed to circumferentially surround, and fit closely to, a stator to minimise thermal resistance between the coolant and the stator and maximise heat transfer. The cooling jacket 7 is segmented and includes bridging connectors or joints 18, with slots 43 for the receipt of beams 6 07 11 25 to allow the cooling jacket conduit to extend over the beams 6, maximising fluid flow over the stator surface and minimising thermal resistance. The cooling jacket 7 comprises an inlet 51 coupled to the inlet 47 of the cooling loop to enable fluid communication therebetween, and an outlet 52 coupled to the outlet 48 of the cooling loop 19 to enable fluid communication therebetween. By virtue of connecting to the inlet 47 and outlet 48 of the cooling loop 19 the cooling jacket is also electrically isolated from the manifold 14. Referring to Figures 3 to 5, each winding cooling elements 16 includes a cooling rod comprising an internal pipe 24 extending along the centre of the cooling rod and an external pipe 23 located radially outwards and about the internal pipe 24 such that an axially extending channel 22 is formed between the internal pipe 24 and external pipe 23. The outer pipe 23 has a closed distal end 83, the inner pipe has an open distal end 85 that is axially spaced away from the closed distal end 83. A flow path is thereby facilitated forwardly through the centre of the inner pipe 24, then into the axial space between the open distal end 85 and the closed distal end 83, then rearwardly in a return direction along the axial channel 22 between the outer surface of the inner pipe 24 and the internal surface of the outer pipe 23. It will be appreciated that the opposite flow path could also be utilised i.e. the outward flow may be via the external pipe and the return flow may be via the internal pipe. The winding cooling elements 16 each include an inlet 65 in fluid communication with the inner pipe 24 and an outlet 67 in fluid communication with the outer pipe 23. The inlet 65 and outlet 67 pipes are each connected to the exterior manifold 14 by an isolating hose 13. The inner pipe 24 has a central longitudinal axis 81 aligned with the inlet pipe 65 of the cooling element 16, and the outlet pipe 67 of the cooling element 16 is radially spaced away from the longitudinal axis 81 of the inner pipe 24. Each winding cooling element 16 comprises a cooling block 61. The cooling block 61 functions as a manifold, with an internal channel connecting the outlet pipe 67 and the outer pipe 23. The cooling block 61 is also configured to cool the stator end winding. Each cooling block 61 includes a manifoldfacing side 63 connected to the inlet 65 of the cooling element 16 and the outlet 67 of the cooling element 16. The cooling blocks 61 also include a stator-facing side 69 connected to the cooling rod. Concave surfaces 71, 73 are formed on the stator-facing side 69, the concave surfaces shaped to conform to the shape of the adjacent stator end winding. Each cooling block 61 comprises a channel 75 fluidly coupling the outlet 67 of the cooling element 16 to the axial channel 22 of the cooling rod. Circular pipe conduits are preferred for the inner and outer pipes of the winding cooling elements 16, as this allows the use of stock pipe rather than requiring expensive bespoke parts. However, the 07 11 25 winding cooling elements 16 can be formed in any shape depending on electromagnetic requirements, for example rectangular or wedge shaped. Referring to Figure 6 and 7, the cooling network of Figure 1 and 2 is installed inside an electric machine in the form of an electric motor having a rotor 45 and a stator 25. Stator windings 77 extend through stator slots 2 and over first and second end faces of the stator 25 to form respective first 78 and second 79 pluralities of end windings. The cooling jacket 7 is circumferentially arranged about the outer diameter of the stator 25. The cooling rods of the cooling elements 16 extend within the stator slots 2 for cooling the stator windings. End winding cooling loop 19 is arranged to cool the second plurality of end windings of the machine. At the opposing end of the stator 25 the cooling elements 16 are inserted into the stator slots, with each winding cooling elements 16 being received independently in a respective slot i.e. one per slot. There is insulation / isolation provision between each slot cooling element 16 and the first end plate 8 to mitigate the transfer of eddy currents. Structural beams 6 (i.e. tie rods) are used to hold the stator 25, drive end 5 and non-drive end 8 plates together. The beams 6 are arranged circumferentially around the stator 25 and extend axially between the first 8 and second end 5 plates. The beams 6 are round in cross section, with enlarged ends that act together to hold the stator 25 axially in place. The beams can be sized or shaped to withstand differing torsional load requirements. The beams 6 can be slightly depressed into the lamination material 27 of the stator 25 to provide radial restraint without impacting electromagnetic performance. The beams 6 can be keyed or pass straight through the lamination material of the stator 25 although this is typically unnecessary. The drive end 5 and non-drive end 8 plates comprise steel inserts that serve as bearing housings or liners. The drive end plate 5, and non-drive end plate 8 are each connected to the beams 6 by threaded joints 26. Burst containment protection means, in the form of a tube or wrapped sheet 9, are arranged over the beams. The wrapped sheet 9 may be formed from mesh or solid material. The machine may be provided without such protection enabling improved passive heat dissipation so that function can be delivered by external means such as a laboratory, engine bay or nacelle. This solution allows for the containment of high energy debris in high speed / high energy applications. The burst protection means can be of any suitable material including, metal, plastic, or composite. The protection means can satisfy various protection requirements including ATEX, waterproofing, dust proofing through appropriate jointing with the DE plate 5 and NDE plate 8. 07 11 25 In an alternative embodiment, the liquid cooling system shown in Figure 8 and Figure 9, comprises an end winding cooling machined manifold 28 or formed pipe (inlet) connected directly (by brazing, press fit with o-rings or by threaded joint) to slot cooling pipes 29, typically manufactured from Titanium. Those pipes at the opposing end are connected to isolating hoses 13 using hose clips which are then connected to a manifold 14 or formed pipe (the outlet). Typically, the opposing end windings to the inlet are cooled by a separate cooling circuit / pipe similar to 19. The cooling circuit arrangement can be organised such that the manifolds are positioned axially inside or outside the drive end / non drive end covers or if required radially. The arrangement can also be reversed with the outlet acting as the inlet. A typical arrangement comprises a machined manifold 28 mounted on the drive end windings with an appropriate inlet, connected by thread to titanium pipes 29 which pass through the slots and through insulators in the non-drive end cover. Isolator hoses 13 are connected to the pipes which are connected to a machined manifold 14 with appropriate outlet, that manifold being attached to the non-drive end cover by supports. The non drive end windings are cooled by a separate circuit and the cooling jacket is as previously described. This arrangement optimises axial length, provides a low pressure drop across the cooling network with unidirectional flow allowing higher flower rates, provides minimal and robust connections and facilitates a level of segmentation and isolation within the cooling network. Whilst pressure drops are lower than the previously described cooling arrangement of Figures 1 and 2 the connection of the cooling elements 19 to two manifolds potentially leads to greater eddy losses within the cooling network. In an alternative embodiment, as shown in Figure 10, the cooling jacket may be provided in the form of cooling coil 30 that encircles the stator 25 in a helical manner. The cooling coil 30 has a channel 40 defined therethrough to enable passage of fluid from the inlet of the cooling jacket to the outlet of the cooling jacket. In an alternative embodiment, as shown in Figure 11, the cooling jacket may be provided in the form of annularly arranged plates 31 with inner surfaces that conform to the outer geometry of the stator 25. The plates 31 each have a respective inlet and outlet aperture, where pipes (not shown) are used to connect the inlet of one plate to the outlet of the adjacent plate. In an alternative embodiment, as shown in Figure 12, the cooling jacket may be provided in the form of annularly arranged curved plates 32 that conform to the outer geometry of the stator 25. The curved plates each have a respective inlet and outlet aperture, where pipes (not shown) are used to connect the inlet of one curved plate to the outlet of the adjacent curve plate. In an alternative embodiment, as shown in Figure 13, the cooling jacket has a similar form to the cooling jacket of Figure 12 except for the provision of axially extending integral burst protection means 33. In certain application, integrally forming the burst protection means with the cooling jacket obfuscates the need for a wrapped sheet burst protection means. It will be appreciated that in further embodiments various modifications to the specific arrangements described above and shown in the drawings may be made. 07 11 25

Claims

1. An electric machine, comprising:a stator comprising a first end face, a second end face, and stator windings extending through respective stator slots that extend between the first and second end faces;a rotor arranged at least partially within the stator;a plurality of cooling elements, each arranged within a respective stator slot, and each comprising a fluid inlet and a fluid outlet;a manifold configured to deliver and receive fluid from the plurality of cooling elements; anda plurality of fluid couplings, each fluidly coupling a respective cooling element to the manifold, and each comprising a first fluid channel extending between the manifold and the inlet of the respective cooling element, and a second fluid channel extending between the manifold and the outlet of the respective cooling element; whereineach fluid coupling is configured to electrically isolate the respective cooling element from the manifold.

2. An electric machine according to claim 1, wherein the fluid couplings electrically isolate the plurality of cooling elements from one another.

3. An electric machine according to any preceding claim, wherein each of the plurality of cooling elements comprises a proximal end and a free distal end, and the fluid inlet and outlet are arranged at the proximal end.

4. An electric machine according to any preceding claim, wherein each fluid coupling comprises a first pipe defining at least part of the first fluid channel and a second pipe defining at least part of the second fluid channel.

5. An electric machine according to any preceding claim, wherein each fluid coupling is formed at least in part of an electrically insulating material.

6. An electric machine according to claim 5, wherein the electrically insulating material comprises a polymeric material.

7. An electric machine according to any preceding claim, wherein each cooling element comprises a cooling rod comprising an outer conduit, and an inner conduit arranged within the outer conduit such that an axially extending channel is formed therebetween; and a flow path is defined through the cooling element from the inlet, along the inner conduit, into the outer conduit, and along the axial channel to the outlet8. An electric machine according to claim 7, wherein the outer conduit of the cooling rod comprises a closed distal end, the inner conduit of the cooling rod comprises an open distal end, and the open distal end of the inner conduit is axially spaced away from the closed distal end of the outer conduit to enable fluid to pass from the inner conduit into the outer conduit.

9. An electric machine according to claim 7 or 8, wherein the inner conduit of the cooling rod is formed as a tube having a central longitudinal axis aligned with the inlet of the cooling element, and the outlet of the cooling element is radially spaced away from the longitudinal axis of the inner conduit.

10. An electric machine according to any one of claims 7 to 9, wherein the stator windings extend across the first end face of the stator to define a first plurality of end windings; and each cooling element further comprises a cooling block configured to cool at least one of the first plurality of end windings.

11. An electric machine according to claim 10, wherein each cooling block includes a manifoldfacing side connected to the inlet and the outlet of the cooling element, and a stator-facing side connected to the cooling rod.

12. An electric machine according to claim 11, wherein at least one concave surface is formed on the stator-facing side, the shape of at least one concave surface conforms to the shape of one of the first plurality of end windings.

13. An electric machine according to any one of claims 10 to 12, wherein each cooling block comprises a channel fluidly coupling the outlet of the cooling element to the axial channel of the cooling rod.

14. An electric machine according to any one of claims 10 to 13, wherein the stator windings extend across the second end face of the stator to define a second plurality of end windings; and the machine further comprises a cooling loop arranged to cool the second plurality of end windings, the cooling loop comprising an inlet and an outlet coupled to the manifold by one of the plurality of fluid couplings.

15. An electric machine according to claim 14, wherein the cooling loop comprises a curved tubular portion arranged at the second end face of the stator.

16. An electric machine according to claim 14 or 15, further comprising a cooling jacket circumferentially surrounding the stator, wherein the cooling jacket is in fluid communication with the manifold, and the cooling jacket is electrically isolated from the manifold.

17. An electric machine according to claim 16, wherein the cooling jacket comprises an inlet coupled to the inlet of the cooling loop to enable fluid communication therebetween, and an outlet coupled to the outlet of the cooling loop to enable fluid communication therebetween.

18. An electric machine according to claim 16 or 17, wherein the cooling jacket comprises a plurality of cooling segments connected together by joints having fluid channels defined therethrough.

19. An electric machine according to claim 18, further comprising a first end plate and a second end plate arranged respectively at opposing end faces of the stator, and a plurality of tie rods arranged circumferentially around the stator and extending axially between the first and second end plates; wherein the plurality of tie rods are configured to retain the stator in position between the first and second end plates.

20. An electric machine according to claim 19 wherein the joints of the cooling jacket comprise slots configured to receive at least a portion of a tie rod such that the channels of the joints are arranged radially outwards of the slots and extends over the tie rods.

21. An electric machine according to any one of claims 17 to 20, wherein the cooling jacket comprises a cooling coil formed around the stator, the cooling coil having a channel definedtherethrough to enable passage of fluid from the inlet of the cooling jacket to the outlet of the cooling jacket.

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