A stator

Abstract

We describe flow restrictors and their use in stators for axial flux machines. In use, one or more flow restrictor are placed within the stator housing either between an outer cylindrical wall of the stator housing and the stator coil stack of the stator located in the stator housing, or between an inner cylindrical wall of the stator housing and the stator coil stack of the stator located in the stator housing. The flow restrictor restricts the flow of at least a portion of the cooling fluid flowing from one side of the flow restrictor to the other side of the flow restrictor. Some cooling fluid may be permitted to pass from one side of the flow restrictor to the other side of the flow restrictor. Each of the one or more flow restrictors comprises a first radial end in contact with a respective stator coil stack, and a second radial end in contact with a respective inner or outer cylindrical wall of the stator housing. One or both of the first radial end and the second radial end may be bifurcated to provide respectively a first and second contact portions, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall of the stator coil stack.

Description

[0001] A Stator

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a stator for use in an axial flux machine, and a flow restrictor for use in the stator.

[0004] BACKGROUND OF THE INVENTION

[0005] As demand for high-power electric motors continues to rise in various industrial, applications, the need for efficient and reliable cooling mechanisms becomes increasingly vital. Electric motors used in applications such as electric vehicles (land, sea and air), industrial machinery, and renewable energy systems are subjected to intense operational conditions, leading to significant heat generation within the motor stators. Inadequate cooling can lead to thermal issues, reduced performance, and ultimately, motor failure.

[0006] Flow path guided cooling with flow diverters is an innovative cooling technique used for high-power electric motor stators. In electric motors, stator segments generate significant heat during operation mainly due to l2R losses in coils, and effective cooling is crucial to maintain optimal performance and prevent overheating.

[0007] Flow path guided cooling involves the strategic placement of flow diverters within a heat generating electrical or electronic device to guide the flow of coolant along specific paths. These flow diverters act as guides to develop flow directions that use device walls and components to form virtual channels that direct coolant to areas dissipating heat. By controlling coolant flow to flow evenly over all heat dissipating areas this reduces variability of heat dissipation and consequential hotspots and so prevent localised overheating.

[0008] For a multi-toothed / stator-bar motor, stator power handling is only as good as the weakest link. Frequently the weakest link is where local temperature rise degrades stator structural integrity, e.g. through wire burnout, or material bond failure through excessive temperature. Ideally optimised cooling is where all parts of a stator uniformly rise in temperature as power output increases.

[0009] 15286993-1A flow path guided cooling system ensures uniform cooling distribution throughout the stator, optimising the motor's overall thermal management. This technique when applied to electric machines helps improve their efficiency, reliability, and longevity, making it particularly suitable for high-power electric motors used in various industrial applications, e.g., electric vehicles and electric aerospace applications.

[0010] Using flow path guided cooling with flow diverters, engineers can design electric motors capable of handling higher power densities, reducing the risk of thermal-related failures and enhancing the motor's overall performance and lifespan.

[0011] Key features that affect positioning of flow diverters is coolant fluid dynamics and thermal behaviour, e.g., heat capacity and viscosity, and the impact of different flow diverter configurations on cooling effectiveness. Manufacturing consistency i.e., placement is among the practical implications of implementing this cooling approach, including potential placement manufacturing challenges and integration within existing motor designs.

[0012] A traditional way of directing stator coolant flows is to use motor components themselves, particularly housing walls and stator bars / teeth. EP2396873B1 Figure 1 shows this traditional route for an axial flux machine, in which the predominant coolant flow paths are in the annular cavities 152b and 152a. Each cavity being fed from coolant inlet 156 with flow blockers 158 ensuring flow circulation. A problem with this approach is that flow between annular cavities 152b and 152a through gaps 155 is inconsistent from one pair of coils to the next, and from one motor to the next due to variations in gaps 155 between coils.

[0013] US2004145252A1 shows a similar approach for a radial flux machine in which gaps between stator teeth are partially filled with resin forming an axial coolant passage between coils and thereby removing heat from the coils. Coolant flows back and forth along the stator being returned in the opposite direction by U-turn cover plates. The teaching of US’252 solves part of the problem with radial flux machines and teaches axial coolant channel size can be adjusted to accommodate loss of pressure and so maintain / ‘uniformize’ coolant flow rate from inlet to outlet. However, the teaching does not attend

[0014] 15286993-1to the commonly unsolved challenge in radial machines of cooling end windings and is not a submersed coolant system.

[0015] It is usual for submerged cooling of electrical and electronic devices to rely on device components and casing walls to guide coolant flow from inlet to outlet. However, this approach does not provide coolant sufficiently evenly to avoid overheating when such devices are driven at their highest power densities over significant periods for instance 10s of minutes.

[0016] A solution offered to more evenly distributing coolant is seen with reference to EP2396873B1 Figures 11a,b, Figures 4 and 5 herein, which show schematic drawings of an axial flux machine in which motor stator components i.e., housing, stator bars, coils and the gaps between these are not the final arbiter of coolant flow. With reference to Figure 5, there is shown coolant inlet 156 and coolant outlet 160. Coolant fluid flows in a meandering path between inlet 156 and outlet 160 caused by blockers 158b1 , 158b2 and 158b3. Blockers 158b1 , 158b2 and 158b3 provide a new direction to coolant flow which forces coolant to flow between and around coils 122 a-d, where without such blockers, flow would be arbitrary, and some coils would see more cooling than others.

[0017] This new teaching of EP’873 is a significant advantage over previous cooling approaches, but building exact replica motors i.e., clones, with the same cooling profile from one motor to the next is challenging in a manufacturing setting. This is because variations in inter-coil spacing 155 (Figure 4) and segment 122a-d placement with respect to inner 146 and outer 102 stator housing walls, makes placement of blockers 158b1 , 158b2 and 158b3 a problem in manufacture, leading to variation in blocker effectiveness, inconsistency of placement and risk of dislodgement.

[0018] In reviewing prior-art approaches to address optimised and evenly cooled stator coils, we have appreciated there are problems of manufacturing implementation as yet unresolved and there is need for a better solution to guide coolant flow in a simple, effective and reproducible fashion suitable for large scale manufacture. Other objects and features of the present invention will become apparent from the following description with reference to the accompanying drawings.

[0019] 15286993-1SUMMARY OF THE INVENTION

[0020] The present invention is defined by the independent claims appended hereto. Further advantageous embodiments are also defined by the dependent claims, also appended hereto.

[0021] We describe a stator for an axial flux machine, comprising: a plurality of stator bars extending axially and disposed circumferentially at intervals around an axis of the stator, each of the stator bars having a set of windings wound therearound to form a stator coil stack for generating a magnetic field generally parallel to the axis of the stator, the plurality of stator coil stacks being arranged to provide a hollow region at the centre of the axis of the stator; a housing for enclosing the plurality of stator bars to form a chamber flooded with a cooling fluid, the housing comprising: an inner cylindrical wall at an inner radius and an outer cylindrical wall at an outer radius, the inner and outer cylindrical walls extending axially and having respectively axially first and second ends; a first radial wall extending between the first axial ends of the inner and outer cylindrical walls, and a second radial wall extending between the second axial ends of the inner and outer cylindrical walls; and an inlet for receiving the cooling fluid and an outlet for expelling the cooling fluid; and one or more flow restrictors located within the housing for restricting the flow of at least a portion of the cooling fluid, each of the flow restrictors having a height that extends in the axial direction of the stator between the first and second radial walls, a width that extends in the circumferential direction of the stator and a depth that extends in the radial direction of the stator between the inner cylindrical wall and a respective stator coil stack, or between the outer cylindrical wall and a respective stator coil stack, wherein each of the one or more flow restrictors comprises a first radial end in contact with a respective stator coil stack, and a second radial end in contact with a respective inner or outer cylindrical wall , and wherein one or both of the first radial end and the second radial end is bifurcated to provide respectively a first and second contact portion, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall .

[0022] The flow restrictors provide a clip-fit, adaptable flow restrictor that is applicable to cooling of electric machines, high power electronics e.g., power inverters, submersed memory banks and similar complex device topologies where there are no appropriate internal

[0023] 15286993-1walls to optimally restrict the flow of the cooling fluid in the stator housing between the stator housing and respective stator coil stacks in order to guide coolant towards temperature sensitive and / or high heat dissipating components and to prevent stagnant flow leading to localised hot spots within the stator.

[0024] Two such arrangements include a Y-shaped flow restrictor having one bifurcated portion in contact with the stator coil stack or stator housing, and a X-shaped flow restrictor having two bifurcated portions respectively contacting the stator coil stack and the stator housing.

[0025] A portion of the one or more flow restrictors between the first radial end and the second radial end may comprise one or more through holes that extend through the width of the flow restrictor for receiving interconnects of the stator there through. The flow restrictor may also comprises a channel for each of the through holes, each channel extending from the respective through hole to the first end of the flow restrictor.

[0026] Providing through holes and channels enables the flow restrictor to be placed around stator coil stack interconnects within the stator, which provides flow restriction to be enabled even in areas of the stator where it is often difficult to provide due to the arrangement of the interconnects.

[0027] The flow restrictor may also comprise one or more cut-out portions extending at least a portion between adjacent through holes and adjacent channels, and the cut-out portions extending at least a portion of the width of the flow restrictor. This enables a more flexible flow restrictor to be provided.

[0028] Each of the one or more flow restrictors may be formed of a separate coil part and a wall part separated between the first and second ends. The coil part may comprise the first radial end and a contact end opposed the first radial end, and the wall part may comprise the second radial end and a contact end opposed the second radial end. The contact end of the coil part and the contact end of the wall part are shaped and arranged to engage with one another.

[0029] 15286993-1The contact end of the wall part may comprise a recessed portion along at least a portion of the height of the wall part, and wherein the contact end of the coil part comprises a correspondingly shaped projection to engage with the recessed portion of the wall part.

[0030] A portion of the coil part may comprise one or more through holes that extend through the width of the coil part for receiving interconnects of the stator there through. The coil part may comprises a channel for each of the through holes, each channel extending from the respective through hole to the contact end of the end of the coil part.

[0031] Both of the first radial ends and the second radial ends may be bifurcated to provide respectively the first and second contact portions, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall.

[0032] The first radial end of the flow restrictor may be bonded to the stator coil stack.

[0033] The wall part may be formed of a material having a lower modulus than that of the modulus of the coil part.

[0034] Each of the first and second contact portions may comprise tapered edges. Furthermore, the bifurcated portion may be configured to flex. This enables the flow restrictor to conform to the dimensions between the stator coil stack and the stator housing irrespective of the tolerances achieved within each machine.

[0035] The flow restrictor may comprise a locating feature disposed on an axial end of the flow restrictor between the first and second radial ends that is configured to engage with a correspondingly shaped locating feature in the respective radial wall of the stator housing. The locating feature on the flow restrictor may be a projection projecting away from the axial end, and the locating feature on the respective radial wall of the stator housing comprises a detent configured to receive the projection. Each axial end of the flow restrictor may comprise a respective locating feature, and each of the first and second radial walls comprises respective correspondingly shaped locating features.

[0036] The stator may comprise one or more support structures arranged radially inward or radially outward of a respective stator coil stack for supporting one or more interconnects of the stator. A respective support structure may be located adjacent a respective flow

[0037] 15286993-1restrictor. The respective flow restrictor and support structure may be arranged within the stator such that the coolant impinges on the respective flow restrictor, and the support structure is located on the opposing side of the flow restrictor to the impinging flow of the cooling fluid.

[0038] When there are three or more flow restrictors, the flow restrictors may be arranged circumferentially around the stator alternately between a respective stator coil stack and circumferentially outward wall, and a respective stator coil stack and circumferentially inner wall.

[0039] The flow restrictor may permit at least a portion of the cooling fluid to pass from one side of the flow restrictor to the other side of the flow restrictor. This prevents stagnant cooling fluid at the interface with the flow restrictor, which would otherwise cause localised hot spots within the stator.

[0040] We also describe an axial flux machine, comprising: a stator as described above; a rotor comprising a set of permanent magnets and mounted for rotation about the axis of the machine, the rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and rotor.

[0041] The axial flux machine may comprise a second rotor comprising a set of permanent magnets and mounted for rotation about the axis of the machine, the second rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and second rotor, and the second rotor being disposed on a side of the stator opposed to the rotor. The machine may be a motor or a generator.

[0042] We also describe a flow restrictor for a stator for an axial flux machine, the flow restrictor being configured in use to restrict the flow of at least a portion of a cooling fluid in a stator housing, the flow restrictor comprising: a first end for contacting a respective stator coil stack of a stator; a second radial end for contacting a respective inner or outer cylindrical wall of a stator, wherein one or both of the first and second ends is bifurcated to provide a respective first and second contact portions, separated by a gap, for contacting a respective stator coil stack, or inner or outer cylindrical walls of a stator.

[0043] 15286993-1Two such arrangements include a Y-shaped flow restrictor having one bifurcated portion for contacting the stator coil stack or stator housing, and a X-shaped flow restrictor having two bifurcated portions respectively for contacting a stator coil stack and a stator housing.

[0044] A portion of the flow restrictor between the first end and the second end may comprise one or more through holes that extend through a width of the flow restrictor for receiving interconnects of a stator there through. The flow restrictor may comprise a channel for each of the through holes, each channel extending from the respective through hole to the first end of the flow restrictor. The flow restrictor may comprise one or more cut-out portions extending at least a portion between adjacent through holes and adjacent channels, and the cut-out portions extending at least a portion of the width of the flow restrictor.

[0045] The flow restrictor may be formed of a separate coil part and a wall part separated between the first and second ends. The coil part may comprise the first radial end and a contact end opposed the first radial end, and the wall part may comprise the second radial end and a contact end opposed the second radial end. The contact end of the coil part and the contact end of the wall part may be shaped and arranged to engage with one another.

[0046] The contact end of the wall part may comprise a recessed portion along at least a portion of the height of the wall part, and wherein the contact end of the coil part may comprise a correspondingly shaped projection to engage with the recessed portion of the wall part.

[0047] A portion of the coil part may comprise one or more through holes that extend through the width of the coil part for receiving interconnects of the stator there through. The coil part may comprise a channel for each of the through holes, each channel extending from the respective through hole to the contact end of the end of the coil part.

[0048] Both of the first radial ends and the second radial ends may be bifurcated to provide respectively the first and second contact portions, separated by a gap, for contacting the respective stator coil stack, or inner or outer cylindrical wall of a stator.

[0049] 15286993-1The wall part may be formed of a material having a lower modulus than that of the modulus of the coil part.

[0050] Each of the first and second contact portions may comprise tapered edges. The bifurcated portion may be flexible.

[0051] The flow restrictor may comprise a locating feature disposed on an axial end of the flow restrictor between the first and second ends that is configured to engage with a correspondingly shaped locating feature in a radial wall of a stator housing. Each axial end of the flow restrictor may comprise a respective locating feature.

[0052] The one or more flow restrictors may comprise an elastic material. The one or more flow restrictors may comprise a material having a durometer hardness of between Shore A 80 to 100 and a Shore D 40 to 60 .

[0053] LIST OF FIGURES

[0054] The present invention will be described, by way of example only, and with reference to the accompanying figures, in which:

[0055] Figure 1 shows a schematic illustration of a yokeless and segmented armature machine;

[0056] Figure 2 shows a schematic illustration of a yokeless and segmented armature machine;

[0057] Figure 3 shows a schematic illustration of a yokeless and segmented armature machine;

[0058] Figure 4 shows a schematic representation an axial flux machine stator;

[0059] Figure 5 shows a simplified schematic representation an axial flux machine stator having blockers and coolant flow;

[0060] Figures 6a to 6f show a flow restrictor;

[0061] 15286993-1Figure 7 shows a simplified illustration of an axial flux stator housing comprising the flow restrictors;

[0062] Figures 8a to 8c show an arrangement of flow restrictors and stator interconnects;

[0063] Figure 9 shows an alternative form of the flow restrictor;

[0064] Figures 10a to 10d show an alternative flow restrictor;

[0065] Figure 11 shows the alternative flow restrictor with features for use with stator interconnects;

[0066] Figure 12 shows an alternative view of the flow restrictor of figure 11 ;

[0067] Figures 13a and 13b show an alternative flow restrictor configured to sit between a stator coil stack and the inner circumferential wall;

[0068] Figures 14a and 14b show the flow restrictor parts of figures 13a and 13b assembled together;

[0069] Figure 15 shows the flow restrictor of figures 13 and 14 in place within a stator between a stator coil stack and the inner circumferential wall;

[0070] Figures 16a and 16b show an alternative flow restrictor configured to sit between a stator coil stack and the outer circumferential wall;

[0071] Figures 17a and 17b show the flow restrictor parts of figures 16a and 16b assembled together; and

[0072] Figure 18 shows the flow restrictor of figures 16 and 17 in place within a stator between a stator coil stack and the outer circumferential wall.

[0073] 15286993-1DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] In brief, the present invention relates to flow restrictors and their use in stators for axial flux machines. In use, one or more flow restrictor are placed within the stator housing either between an outer cylindrical wall of the stator housing and the stator coil stack of the stator located in the stator housing, or between an inner cylindrical wall of the stator housing and the stator coil stack of the stator located in the stator housing. The flow restrictor restricts the flow of at least a portion of the cooling fluid flowing from one side of the flow restrictor to the other side of the flow restrictor. Some cooling fluid may be permitted to pass from one side of the flow restrictor to the other side of the flow restrictor. Each of the one or more flow restrictors comprises a first radial end in contact with a respective stator coil stack, and a second radial end in contact with a respective inner or outer cylindrical wall of the stator housing. One or both of the first radial end and the second radial end may be bifurcated to provide respectively a first and second contact portions, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall of the stator coil stack.

[0075] The flow restrictors provide a clip-fit, adaptable flow restrictor that is applicable to cooling of electric machines, high power electronics e.g., power inverters, submersed memory banks and similar complex device topologies where there are no appropriate internal walls to optimally guide coolant towards temperature sensitive and / or high heat dissipating components.

[0076] The flow restrictors provide a manufacturing solution applicable to many submersed coolant flow applications where a preferred coolant flow direction and / or flow rate can be enabled during assembly with consistency that overcomes manufacturing variations, particularly in the dimensions of the stator housing and stator coil stacks.

[0077] For context, we will first describe axial flux machines. Referring first to Figures 1 , 2 and 3, which are taken from WO2012 / 022974, Figure 1 shows a schematic illustration of a yokeless and segmented armature machine 10.

[0078] The machine 10 comprises a stator 12 and two rotors 14a, b. The stator 12 is a collection of separate stator bars 16 spaced circumferentially about a rotation axis 20 of the rotors 14a, b. Each bar 16 has its own axis (not shown) which is preferably, but not essentially, disposed parallel to the rotation axis 20. Each end of each stator bar is provided with a

[0079] 15286993-1shoe 18a, b which serves a physical purpose of confining a coil stack 22, which stack 22 is preferably of square / rectangular section insulated wire so that a high fill factor can be achieved. The coils 22 are connected to an electrical circuit (not shown) that, in the case of a motor, energizes the coils so that the poles of the resultant magnetic fields generated by the current flowing in the coils is opposite in adjacent stator coils 22.

[0080] The two rotors 14a,b carry permanent magnets 24a, b that face one another with the stator coil 22 between (when the stator bars are inclined - not as shown - the magnets are likewise). Two air gaps 26a, b are disposed between respective shoe and magnet pairs 18a / 24a, 18b / 24b. There are an even number of coils and magnets spaced around the axis of rotation 20 and, preferably, there are a different number of coils and magnets so that the coils do not all come into registration with the corresponding magnet pair at the same time and at the same rotational position of the rotor with respect to the stator. This serves to reduce cogging.

[0081] In a motor the coils 22 are energized so that their polarity alternates serving to cause coils at different times to align with different magnet pairs, resulting in torque being applied between the rotor and the stator. The rotors 14a,b are generally connected together (for example by a shaft, not shown) and rotate together about the axis 20 relative to the stator 12. The magnetic circuit 30 is provided by two adjacent stator bars 16 and two magnet pairs 24a, b and a back iron 32a, b for each rotor links the flux between the back of each magnet 24a, b facing away from the respective coils 22. The stator coils 16 are enclosed within a housing that extends through the air gap 26a, b and which defines a chamber supplied with a cooling medium.

[0082] Turning to Figure 3, a stator 12a is shown in which the stator coils are located between plastic material clam shells 42a, b. These clamshells have external cylindrical walls 44, internal cylindrical walls 46, and annular radially disposed walls 48. In the prior art example of Figure 3 the radial walls 48 include internal pockets 50 to receive the shoes 18a,b of the stator bars 16 and serve to locate the stator coil assemblies 16, 22, 18a,b when the two clam shell housings 42a, b of the stator 12a are assembled together. The stator housing 42a, b defines spaces 52 internally of the coils 22 and externally at 54 around the outside of the coils 22 and there are spaces 56 between the coils. The spaces 52,54,56 are interlinked defining a cooling chamber. Although not shown in Figure 3, when assembled, the stator housing 42a, b is provided with ports that allow cooling

[0083] 15286993-1medium such as oil to be pumped into the spaces 52, 54, 56 to circulate around the coils and cool them.

[0084] The coil cores may be laminated with the inter-lamination insulation parallel to the desired flux direction. However, the coil cores may also be formed from soft-iron particles coated with electrical insulation and moulded to a desired shape (soft magnetic composites - SMC), being bound together by the insulation matrix. Conveniently the shoes and stator bar may be formed separately and subsequently assembled.

[0085] Yokeless and segmented armature 22 (YASA) axial flux machines are of particularly high-power density partly because there is no iron based magnetic return yoke for the stator which instead is segmented and manages magnetic return path by way of neighbouring stator segments 22 and through rotors 42a, b that oppose either side of stator 12. Each stator segment 22 is mounted on a non-magnetic, electrically, and thermally insulating substrate (not shown) which forms the stator walls.

[0086] With reference to Figure 4, there is shown a YASA stator 100 with outer stator walls 102 and 146 with stator segments 156 comprising stator bars 200, and coils 122. Stator segments 156 are distributed in a clockwise fashion within cavity 152 defining two annular volumes 152b and 152a. For maximum power density a trade-off is defined between the distance of separation i.e., gaps 155 between stator segments 156 and maintaining access for coolant to flow between segment coils 122. This trade-off which defines gaps 155 between stator segments 156 leads to gaps being small, typically of the order of 1mm.

[0087] For many power consuming arrayed components, e.g., memory chips on a PCB, power switches in an inverter, segmented armatures in an axial flux motor, or teeth of a radial flux machine, the maximum power output is often limited by the weakest link in the array and in many instances, this will be the component that is not cooled sufficiently. Small differences in cooling can lead to large temperature differences between components.

[0088] Turning to Figure 5, which is a simplified schematic representation is shown of a prior art axial flux stator from WO2012 / 022974 in which coolant inlet 156 provides a source of coolant which is guided in a meandering path back and forth between stator cores, by blockers 158b on outer and inner walls of the stator towards outlet 160. The blockers are

[0089] 15286993-1provided to entirely block the flow of the coolant either between the outer cylindrical wall of the stator housing and the stator coil stack, or between the inner cylindrical wall of the stator housing and the stator coil stack. The meandering path of the coolant flow is as a result of the alternating position of the blockers around the periphery of the stator housing. However, a problem with such an arrangement lies in the manufacturing and assembling of the stator coil stacks and blockers to enable the flow to be blocked and thus diverted between the stator coil stacks for improved cooling.

[0090] The gaps between the stator coil stacks and respective walls of the stator housing are relatively small, and the associated tolerances of those dimension makes for a complex and difficult assembly process, sometimes involving additional sealant. Furthermore, during development, it has become apparent that ‘flow blockers’ that are fully effective cause the downstream side of a flow blocker to become a stagnant / linear flow region in which cooling is poor in comparison to turbulent faster flow regions. ‘Flow Blockers’ in this instance can inadvertently add to inconsistent cooling.

[0091] Figures 6a to 6f show an arrangement of a coolant flow restrictor 410. The flow restrictor is shown set against contact surfaces 420, 430 to which they may be required to form an interference fit, in practice contact surfaces 420, 430 may be an outer surface of the stator coil stack, an inner circumferential wall of the stator housing, or an outer circumferential wall of the stator housing.

[0092] During development, it has become apparent that so long as consistency is maintained in manufacturing of motors, modifying flow paths using ‘flow restrictors’, has the same and in some instances provides more consistent and effective cooling than complete blocking of flow. Partly this is because in general, coolant flow rate in submersed cooled motor stators is high and leakage paths past ‘blockers’ though inconsistent is usually a negligible difference from motor to motor.

[0093] With reference to Figure 6a there is seen a schematic drawing of a Y-shaped flow restrictor 410. The flow restrictor 410 comprises a first end 406 for contacting a respective stator coil stack of a stator; and a second end 406 for contacting a respective inner or outer cylindrical wall of a stator. In the arrangement show, one of the first or second ends of the flow restrictor 410 comprises a bifurcated portion to provide a

[0094] 15286993-1respective first 406a and second 406b contact portions, separated by a gap, for contacting a respective stator coil stack, or inner or outer cylindrical walls of a stator.

[0095] In use, the stator of an axial flux machine according to the invention comprises one or more flow restrictors located within the housing for restricting the flow of at least a portion of the cooling fluid.

[0096] For completeness and the sake of clarity, the stator comprises a plurality of stator bars extending axially and disposed circumferentially at intervals around an axis of the stator, each of the stator bars having a set of windings wound therearound to form a stator coil stack for generating a magnetic field generally parallel to the axis of the stator. The plurality of stator coil stacks are arranged to provide a hollow region at the centre of the axis of the stator.

[0097] The stator comprises a housing for enclosing the plurality of stator bars to form a chamber flooded with a cooling fluid, where the housing comprising an inner cylindrical wall at an inner radius and an outer cylindrical wall at an outer radius, the inner and outer cylindrical walls extending axially and having respectively axially first and second ends. The housing also comprises a first radial wall extending between the first axial ends of the inner and outer cylindrical walls, and a second radial wall extending between the second axial ends of the inner and outer cylindrical walls. Furthermore, the housing also comprises an inlet for receiving the cooling fluid and an outlet for expelling the cooling fluid.

[0098] Relative to the dimensions of the stator housing, each of the flow restrictors has a height that extends in the axial direction of the axial flux stator between the first and second radial walls, a width that extends in the circumferential direction of the stator and a depth that extends in the radial direction of the stator between the inner cylindrical wall and a respective stator coil stack, or between the outer cylindrical wall and a respective stator coil stack. The depth extends between the first 406 and second 406 ends of the flow restrictor.

[0099] As mentioned above, the one or more flow restrictors comprise a first radial end in contact with a respective stator coil stack, and a second radial end in contact with a respective inner or outer cylindrical wall.

[0100] 15286993-1In the instance shown in figures 6a to 6c, edges 406, 406a, 406b interfere with and contact stator components (e.g. stator coil stacks) and inner and outer circumferential walls of the stator housing to restrict flow of the coolant from one side of side of the flow restrictor to the other, and provide a higher pressure region forcing flow to take a different path within the stator housing.

[0101] With reference to Figure 6a, which shows a perspective drawing of a Y-shaped flow restrictor 410, there is shown tapered edges 406 to the bifurcated portions, which add compliance to edges to better contact stator surfaces effecting sealing.

[0102] Alternatively, and again with reference to Figure 6a, one or more sealing edges 406 may be coated with a solid epoxy thermal curing adhesive formulation that once placed and heated undergoes thermosetting chemical curing which attaches and seals the flow restrictor to chosen contact surfaces.

[0103] With reference to Figure 6b, an aspect of the Y-shaped flow restrictor is the resilient, spring characteristic of the ‘V’ profile which under no strain has span width ‘h’, and takes up the form shown in Figure 6b, with ‘V’-angle <t> and which when confined in a smaller gap than ‘h’ the ‘V’ angle opens to > <t> through spring compliance and exerts a force F along the direction of h as shown in Figure 6c. Still referencing Figure 6c, where is shown a section of stator housing wall 432 and the top i.e., plan view of a flat wound coil stack 434. Flow restrictor 410 is wedged between flat wound coil stack 434 and stator housing wall 432, causing the spring arms of the flow restrictor to splay to an angle of ‘V’ > <$> and so shorten in length to a value <h where h is the span width of an unconstrained flow restrictor.

[0104] Still referencing Figure 6c, there is shown a right facing arrow ‘C’ which denotes impingement force due to coolant flow. Flow impingement force C has a component C’ which acts on flow restrictor 410 as shown in Figure 6c. Flow force component C’ tends to weaken the flow restrictor spring contact force F at point 435 where the flow restrictor 410 contacts coil 434 on the flow impingement side. Countering weakening of spring force F at Y-shaped flow restrictor contact point 434 is transfer of spring force F to the downstream leg of contact restrictor 410 at contact point 436. Additionally, components

[0105] 15286993-1of coolant flow force C are also transferred to contact point 436. Higher flow pressure adds to Y-shaped flow restrictor spring force F through points of contact 436 and 437.

[0106] Figure 6d to 6f illustrate the compliance of the bifurcated portions 406a, 406b to accommodate different distances between surfaces 420 and 430 such that the bifurcated portions 406a and 406b remain in contact with surface 420 despite the decreasing dimension. The bifurcated portions 406a 406b splay further apart in order to accommodate the reducing dimension. Examples of materials for Y-shaped flow restrictors 410 include, silicone rubbers, fluoropolymer rubbers and similar materials. A preferred material is a form of fluorosilicone as this has the right combination of hardness / flexibility, temperature range compatibility and coolant compatibility. Typical durometer hardness may vary from Shore A 80 to 100 and Shore D 40 to 60.

[0107] Though the flow restrictors described above are suitable for flow restriction and diversion, they are also suitable for most applications where there are situations e.g., of high flow force, complexity of sealing surface, where spring compliant sealing ‘V’s may be advantageously available at both ends of a flow restrictor.

[0108] Referring back to Figure 4, which shows a schematic diagram of an axial flux machine, the Y-shaped flow restrictor 410 contact surfaces 406, 406a, 406b may be in contact with the stator cavity wall surface 147 of inner housing wall 146 or stator cavity wall surface 103 of outer housing wall 102. Axial inward facing 600b and axial outward facing radial surfaces 600a of stator segments 156 are also suitable interference surfaces for restrictor 410 shown in Figure 6a. Stator segment axial radial surfaces 600a, b, are surfaces of coils 122 on the coil stacks. Coils may be coated with drip, vacuum pressure impregnating resin or similar resin binding and coating, providing coil robustness against magnetic and vibration forces. Resin application may smooth coil outward facing surfaces. Nevertheless, stacked coil 122 surfaces may retain an undulating surface and at least tips 402 and 404 of restrictors 410 shown in Figure 6a to 6c are of a sufficiently flexible material that they can accommodate such non-planar surfaces.

[0109] The Y-shape of flow restrictors 410 when made from spring / elastic materials provide flexibility and sealing contact reaction force pressure through splaying of the upper half ‘V’-shape of the Y-shaped restrictor. Depending on Y-shaped restrictor dimensions Y-shaped flow restrictors can accommodate gap dimension differences of up to 5mm,

[0110] 15286993-1allowing different placement options and gaps to be filled with the same basic shaped and size restrictor. Manufacturing variations which are typically of the order of <1mm are readily accommodated.

[0111] With reference to Figure 7, which shows a simplified illustration of an axial flux stator housing comprising the flow restrictors, the axial flux stator comprises representative LIVW segment stator bars wound with stator coils 122. The flow restrictors 410a, 410b, 410c, 41 Od and 41 Oe have been placed in contact with respective stator coil stacks and either the inner circumferential wall of the stator housing or the outer circumferential wall of the stator housing. In the example illustrated in figure 7, flow restrictor 410a, 410c and 41 Od are located between the respective stator coil stack and the outer circumferential wall. Flow restrictors 41 Ob and 41 Oe are located between the respective stator coil stack and the inner circumferential wall of the stator housing. There may be more or fewer flow restrictors within the stator housing, as such the example shown in figure 7 is for illustrative purposes only.

[0112] Cooling fluid entering the stator housing via inlet 156 thus flows clockwise (in this example illustration) around the stator housing, and is forced to meander between the inner and outer radius of the stator housing around the whole of the stator housing as it encounters the flow restrictors until it reaches the outlet 160 where the cooling fluid exits to the stator housing before being cooled external to the machine.

[0113] As is shown in figure 7, flow restrictors 41 Od and 41 Oe are located on the same stator coil stack. Such an arrangement prevents the cooling fluid entering the stator housing from “short circuiting” and immediately exiting the stator housing via the outlet 160. As such, the cooling fluid is forced to flow around the whole of the stator housing.

[0114] Whilst the examples described above are shown to have the bifurcated portions contacting the stator housing, they may instead be arranged such that the respective bifurcated portions contact the stator coil stacks.

[0115] Also shown in Figure 7 are busbars or interconnects 800. These are conductors located in the stator housing that conduct current between electrically connected coil stacks, and transfer power around the axial flux machine. As can be seen, these interconnects run around portions along the inner and / or outer periphery of the stator coil stacks between

[0116] 15286993-1the stator coil stacks and respective inner or outer circumferential walls of the stator housing. As such, there is a need for the flow restrictors to be able to accommodate such interconnects.

[0117] With reference to figures 6a, and 8a, 8b and 8c, a portion of the flow restrictors 410 between the first radial end and the second radial end comprise one or more through holes 408 that extend through the width of the flow restrictor for receiving interconnects 800 of the stator. In order to facilitate assembly of the axial flux machine (since the interconnects are often in place before the flow restrictors are assembled into the machine) the flow restrictor comprises a channel 409 for each of the through holes 408, each channel extending from the respective through hole 408 to the first end of the flow restrictor 406.

[0118] Through hole passages 408 thus accommodate stator coil busbar interconnections 800 and inlets 409 enable Y-shaped flow restrictors 410 to be placed after coil busbar connections (not shown) have been fixed in place.

[0119] With reference to Figure 6a, three busbar passages 408 are shown to accommodate busbar coil interconnections (although they may be more or fewer). In some instances, the Y-shaped restrictors 410 may be placed where only two or perhaps one busbar needs accommodating. Rather than fill or use a different Y-shaped restrictor with fewer through hole features, the same restrictors 410 may be used in all instances and the small passage of coolant flow that passes through non-occupied busbar holes and around non-sealing edges permits a portion of the cooling fluid to flow from one side of the flow restrictor to the other, which prevents stagnant flow regions downstream of the flow restrictors.

[0120] Whilst these figures show the channels 409 extending from the non-bifurcated end of the Y-shaped flow restrictor 410 to the through holes 408, it is possible for the Y-shaped flow restrictor 410 to be provide instead with channels 409 extending from the bifurcated end 406a, 406b to the through holes 408.

[0121] Also shown in figures 8a and 8b are support structures 500, which are arranged radially inward or radially outward of a respective stator coil stack 122 for supporting one or more interconnects 800 of the stator. Preferably a respective support structure 500 is located

[0122] 15286993-1adjacent a respective flow restrictor 410 and the respective flow restrictor 410 and support structure 500 are arranged within the stator such that the cooling fluid impinges on the respective flow restrictor, and the support structure is located on the opposing side of the flow restrictor to the impinging flow of the cooling fluid.

[0123] The support structure thus may serve more than one purpose. Primarily the support structures help to retain the interconnects 800 in place and prevent movement axially within the stator housing. When located adjacent, and downstream of, a respective flow restrictor, the support structure 500 thus also provides a supporting function to the flow restrictor. A force applied to the flow restrictor by the flowing cooling fluid may be transferred to the support structure to help keep the flow restrictor in place.

[0124] Figure 9 shows an alternative form of the flow restrictor. In order to help with assembly of the flow restrictor into the stator housing in the presence of the interconnects 800, the flow restrictor may be provided with one or more cut-out portions 510 extending at least a portion between adjacent through holes 408 and adjacent channels 409, and the cutout portions extending at least a portion of the width of the flow restrictor. When provided with these cut-out portions, the flow restrictor may be more pliable, to aid with assembly of the flow restrictor into the stator housing.

[0125] Referring to figures 6a, 8c, and 9, the flow restrictor 410 may comprise a locating feature 412 disposed on an axial end of the flow restrictor between the first and second radial ends. The locating feature 412 is configured to engage with a correspondingly shaped locating feature in the respective radial wall of the stator housing (not shown). In the arrangement shown in these figures, the locating feature 412 on the flow restrictor 410 is a projection projecting away from the axial end, and the locating feature on the respective radial wall of the stator housing could take the form of a correspondingly-shaped detent configured to receive the projection. Other arrangements are possible, so long as they provide the same function of helping to retain the flow restrictor in place once the stator is assembled.

[0126] Arrangements are also envisaged where each axial end of the flow restrictor 410 comprises a respective locating feature 412, and each of the first and second radial walls comprises respective correspondingly shaped locating features.

[0127] 15286993-1The locating feature 412 as described above may be used in conjunction with the support structure 500 also described above. However, it is also considered possible to use either of these features in isolation.

[0128] With reference to Figures 10a to 10d, we will briefly describe an alternative flow restrictor 470. These flow restrictors 470 take the form of X-shaped flow restrictors 470 as can be seen by cross section of the flow restrictors 470. Figures 10a to 10d show these X-shaped flow restrictors 470 at various stages of compression between sealing surfaces 480, 490. That is, instead of only one of the radial ends comprising a bifurcated portion, in this arrangement both radial ends comprise bifurcated portions to provide an X-shaped flow restrictor. As for the Y-restrictors described above, X-shaped flow restrictors can be made from the same range of materials providing spring compliant wedge and clip-fit flow restrictor devices with similar features adding compliance and sealing / adhesive functions as previously described.

[0129] With reference to Figure 10a there is shown a schematic drawing of an unconstrained X-shaped flow restrictor 470 with height h which in Figure 10b when constrained to height <h causes unconstrained ‘V’ angles <$> to increase to > <$>. Spring force Fi is supplied by the two V-profiles of X-shaped flow restrictor 470 and in similar fashion to transfer of flow force C to X-restrictor contact points there is added resistance to movement.

[0130] The X-shaped flow restrictors 470 may be provided with the same features with those of the Y-shaped flow restrictors 410 above, and used in conjunction with one or more of the locating features 412 and support structures 500 as described above. It would be appreciated that arranging the through holes 408 and channels 409 in such a structure has its challenges, although the same principles as described would apply.

[0131] Figures 11 and 12 show an example arrangement in which the alternative X-shaped flow restrictor 470 may comprise the locating feature 412 and work with the stator interconnects 800 as described above. Whilst figures 11 and 12 show both of these features in the same X-shaped flow restrictor 470, the X-shaped flow restrictor 412 may instead be provided one or other of the locating feature 412 or the features 408, 409 for use with the stator interconnects. The locating feature 412 operates in the same way as described above with reference to the Y-shaped flow restrictor 410. However, we will

[0132] 15286993-1describe in more detail the features 408 and 409 for use with the stator interconnects 800 below.

[0133] In figures 11 and 12, the X-shaped flow restrictor 470 is shown having two bifurcated ends 406, each having respective contact portions 406a, 406b, 406c, and 406d for contacting surfaces 480 and 490 respectively.

[0134] A portion of the flow restrictors 470 between the first radial end and the second radial end comprises one or more through holes 408 that extend through the width of the flow restrictor for receiving interconnects 800 of the stator. In order to facilitate assembly of the axial flux machine (since the interconnects are often in place before the flow restrictors are assembled into the machine) the flow restrictor comprises a channel 409 for each of the through holes 408, each channel extending from the respective through hole 408 to the first end of the flow restrictor 406. In this case, as can be seen in figures 11 and 12, the channels 409 extend through the bifurcated portions and through contact surfaces 406c, 406d.

[0135] Through-hole passages 408 thus accommodate stator coil busbar interconnections 800 and inlets 409 enable X-shaped flow restrictors 470 to be placed after coil busbar connections have been fixed in place.

[0136] Three busbar passages 408 are shown to accommodate busbar coil interconnections (although they may be more or fewer). In some instances, the X-shaped restrictors 470 may be placed where only two or perhaps one busbar needs accommodating. Rather than fill or use a different X-shaped restrictor with fewer through hole features, the same restrictors 470 may be used in all instances and the small passage of coolant flow that passes through non-occupied busbar holes and around non-sealing edges permits a portion of the cooling fluid to flow from one side of the flow restrictor to the other, which prevents stagnant flow regions downstream of the flow restrictors.

[0137] Preferably, the interconnects 800 are substantially straight at the point where they sit in the through-holes 408, which enables a simpler X-shaped flow restrictor to be manufactured. In cases where it is not possible to locate the X-shaped flow restrictor 470 at a point where the interconnects 800 run substantially straight (at the location adjacent the stator coil stack), X-shaped flow restrictors having angled through-holes 408 may be

[0138] 15286993-1used, or the X-shaped flow restrictor 470 shown in figures 11 and 12 may be adapted to accommodate such interconnects 800 by extending the through-holes by cutting or otherwise modifying the flow restrictor.

[0139] Whilst in figures 11 and 12 the bifurcated end having contact portions 406c and 406d is shown as being shorter than the bifurcated end having contact portions 406a and 406b, this need not be the case in all embodiments. Alternatively, the bifurcated end having contact portions 406c and 406d may be longer than the bifurcated end having contact portions 406a and 406b, or alternatively, the bifurcated end having contact portions 406c and 406d may be the same length as the bifurcated end having contact portions 406a and 406b.

[0140] With reference to Figures 13 to 18, we will describe an alternative flow restrictors 472, 476. The arrangement shown in figures 13 to 15 relate to a flow restrictor 472 configured to be seated between the stator coil stack and the inner circumferential wall of the stator housing, and the arrangement shown in figures 15 to 18 relate to a flow restrictor 476 configured to be seated between the stator coil stack and the outer circumferential wall of the stator housing. However, both arrangements share several features.

[0141] Generally speaking, these flow restrictors 472, 476 take the form of X-shaped flow restrictors. Importantly, the main difference between the flow restrictors 472, 476 shown in figures 13 to 18 to those shown in earlier figures are that flow restrictors 472, 476 are formed of two separate pieces that engage together with one another when arranged between the stator coil stack and the respective wall of the stator housing.

[0142] Forming the flow restrictors 472, 476 out of two pieces maximises sealing potential while allowing easy installation of each part which aligns with preferred assembly process of the stator. Furthermore, this arrangement enables each of the two pieces to be formed from the same material or different materials, depending on the required performance of that piece in the intended environment.

[0143] Referring first to Figures 13a, 13b, 14a, 14b and 15, the first arrangement shows a flow restrictor 472 formed of a coil part 474a (show in isolation in Figure 13a) and a wall part 474b (shown in isolation in Figure 13b). Following the labelling convention of the earlier

[0144] 15286993-1flow restrictors, the coil part 474a and wall part 474b are separated between the first and second ends of the flow restrictor 472.

[0145] As shown, the flow restrictor 472 has two bifurcated ends 406, each having respective contact portions 406a, 406b, 406c, and 406d for contacting the stator coil stack and the surface of the inner circumferential wall of the stator housing.

[0146] The coil part 474a comprises a contact end 540 that is formed at an end of the coil part 474a that is opposed to the first contact end 406. The contact end 540 may be formed for example as a protrusion, although other forms may be possible. A correspondingly-shaped recess 550 is formed in the wall part 474b at an end of the wall part 474b that is opposed to the second end 406, into which the contact end of the contact end 540 of the coil part 474a sits to form the assembled flow restrictor 472. Whilst shown along its full height of the flow restrictor 472, each of the contact ends 540, 550 may instead be formed along a portion of the height of the flow restrictor 472.

[0147] A portion of the coil part 474a comprises one or more through holes 408 that extend through the width of the coil part 474a for receiving interconnects of the stator there through. The coil part 474a comprises a channel 409 for each of the through holes, each channel extending from the respective through hole to the contact end 540 of the end of the coil part 474a.

[0148] Referring now to Figures 16a, 16b, 17a, 17b and 18, the second arrangement shows a flow restrictor 476 formed of a coil part 478a (show in isolation in Figure 16a) and a wall part 478b (shown in isolation in Figure 16b). Following the labelling convention of the earlier flow restrictors, the coil part 478a and wall part 478b are separated between the first and second ends of the flow restrictor 476.

[0149] As shown, the flow restrictor 476 has two bifurcated ends 406, each having respective contact portions 406a, 406b, 406c, and 406d for contacting the stator coil stack and the surface of the outer circumferential wall of the stator housing.

[0150] The coil part 478a comprises a contact end 540 that is formed at an end of the coil part 476a that is opposed to the first contact end 406. The contact end 540 may be formed for example as a protrusion, although other forms may be possible. A correspondingly-

[0151] 15286993-1shaped recess 550 is formed in the wall part 478b at an end of the wall part 478b that is opposed to the second end 406, into which the contact end of the contact end 540 of the coil part 478a sits to form the assembled flow restrictor 476. Whilst shown along its full height of the flow restrictor 476, each of the contact ends 540, 550 may instead be formed along a portion of the height of the flow restrictor 476.

[0152] A portion of the coil part 478a comprises one or more through holes 408 that extend through the width of the coil part 478a for receiving interconnects of the stator there through. The coil part 478a comprises a channel 409 for each of the through holes, each channel extending from the respective through hole to the contact end 540 of the end of the coil part 478a.

[0153] As can be seen, these X-shaped flow restrictors 472, 476 may be provided with the same features with those of the Y-shaped flow restrictors 410 above, and used in conjunction with one or more of the locating features 412 and support structures 500 as described above.

[0154] Figures 15 and 18 show an example arrangement in which the alternative X-shaped flow restrictor 472, 476 may comprise the locating feature 412 and work with the stator interconnects 800 as described above. Whilst figures 15 and 18 show both of these features in the same X-shaped flow restrictor 472, 476, the X-shaped flow restrictor may instead be provided one or other of the locating feature 412 or the features 408, 409 for use with the stator interconnects. The locating feature 412 operates in the same way as described above with reference to the Y-shaped flow restrictor 410. However, we will describe in more detail the features 408 and 409 for use with the stator interconnects 800 below.

[0155] In figures 15 and 18, the X-shaped flow restrictor 472, 476 are shown having two bifurcated ends 406, each having respective contact portions 406a, 406b, 406c, and 406d for contacting respective coil and stator wall surfaces.

[0156] A portion of the flow restrictors coil parts 474a, 478a between the first radial end 406 and the respective contact end 540 comprises one or more through holes 408 that extend through the width of the flow restrictor for receiving interconnects 800 of the stator. In order to facilitate assembly of the axial flux machine (since the interconnects are often

[0157] 15286993-1in place before the flow restrictors are assembled into the machine) the flow restrictor comprises channels 409 for each of the through holes 408, each channel extending from the respective through hole 408 to the contact end 540. Forming the flow restrictors 472, 476 as separate parts enables easier construction, assembly and placement of the flow restrictors in the stator housing as compared to the above examples.

[0158] Through-hole passages 408 thus accommodate stator coil busbar interconnections 800 and inlets 409 enable X-shaped flow restrictors 472, 476 to be placed after coil busbar connections have been fixed in place.

[0159] Two busbar passages 408 are shown in the first arrangement of the flow restrictor 472, and four busbar passages 408 are shown in the second arrangement of the flow restrictor 476 to accommodate busbar coil interconnections (although they may be more or fewer). In some instances, the X-shaped restrictors 470 may be placed where only two or perhaps one busbar needs accommodating. Rather than fill or use a different X-shaped restrictor with fewer through hole features, the same restrictors 472, 476 may be used in all instances and the small passage of coolant flow that passes through non-occupied busbar holes and around non-sealing edges permits a portion of the cooling fluid to flow from one side of the flow restrictor to the other, which prevents stagnant flow regions downstream of the flow restrictors.

[0160] Whilst in figures 13 to 18 the bifurcated end having contact portions 406c and 406d is shown as being wider than the bifurcated end having contact portions 406a and 406b, this need not be the case in all embodiments. Alternatively, the bifurcated end having contact portions 406c and 406d may be narrower than the bifurcated end having contact portions 406a and 406b, or alternatively, the bifurcated end having contact portions 406c and 406d may be the same length as the bifurcated end having contact portions 406a and 406b.

[0161] As described above, an advantage of providing the flow restrictor 472, 476 as two separate parts is that the different parts may be made from the same, similar, or different materials. In the case when different materials are provided, one such example combination could be to form the coil part 474a, 478a from a harder polymer such as Polyetheretherketone (PEEK), in particular an unfilled PEEK. Temperature resistant

[0162] 15286993-1properties are advantageous since the coil part contacts the coil, which can reach temperatures in excess of 200°C.

[0163] Conversely, the wall part 474b, 478b need not be made of the same material. Instead, the wall part may be made from Nylon, for example an unfilled nylon. The temperature requirements of the wall part is lower than that of the coil part as the wall part is in the coolant flow and away from the coil.

[0164] With regards to the flexibility of the materials used for each of the coil part and the wall part, in broad terms it is preferable for the wall part to have a lower modulus than that of the coil part.

[0165] During assembly, it is preferable for the first radial end of the coil part 474a, 478a of the flow restrictor 472, 476 to be bonded to the stator coil stack. Bonding to the coil stack is a further assembly aid as this prevents the part moving around and ensures good axial positioning of the part relative to the pole bar.

[0166] Throughout the description, we have been referencing an axial flux machine. In practice, this may be used as a motor or generator.

[0167] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

[0168] 15286993-1

Claims

28CLAIMS:1 . A stator for an axial flux machine, comprising:a plurality of stator bars extending axially and disposed circumferentially at intervals around an axis of the stator, each of the stator bars having a set of windings wound therearound to form a stator coil stack for generating a magnetic field generally parallel to the axis of the stator, the plurality of stator coil stacks being arranged to provide a hollow region at the centre of the axis of the stator;a housing for enclosing the plurality of stator bars to form a chamber flooded with a cooling fluid, the housing comprising:an inner cylindrical wall at an inner radius and an outer cylindrical wall at an outer radius, the inner and outer cylindrical walls extending axially and having respectively axially first and second ends;a first radial wall extending between the first axial ends of the inner and outer cylindrical walls, and a second radial wall extending between the second axial ends of the inner and outer cylindrical walls; andan inlet for receiving the cooling fluid and an outlet for expelling the cooling fluid; andone or more flow restrictors located within the housing for restricting the flow of at least a portion of the cooling fluid, each of the flow restrictors having a height that extends in the axial direction of the stator between the first and second radial walls, a width that extends in the circumferential direction of the stator and a depth that extends in the radial direction of the stator between the inner cylindrical wall and a respective stator coil stack, or between the outer cylindrical wall and a respective stator coil stack, wherein each of the one or more flow restrictors comprises a first radial end in contact with a respective stator coil stack, and a second radial end in contact with a respective inner or outer cylindrical wall, andwherein one or both of the first radial end and the second radial end is bifurcated to provide respectively a first and second contact portion, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall.

2. A stator according to claim 1 , wherein a portion of the one or more flow restrictors between the first radial end and the second radial end comprise one or more through holes that extend through the width of the flow restrictor for receiving interconnects of the stator there through.15286993-13. A stator according to claim 2, wherein the flow restrictor comprises a channel for each of the through holes, each channel extending from the respective through hole to the first end of the flow restrictor.

4. A stator according to claim 3, wherein the flow restrictor comprises one or more cut-out portions extending at least a portion between adjacent through holes and adjacent channels, and the cut-out portions extending at least a portion of the width of the flow restrictor.

5. A stator according to claim 1 , wherein each of the one or more flow restrictors is formed of a separate coil part and a wall part separated between the first and second ends, the coil part comprising the first radial end and a contact end opposed the first radial end, and the wall part comprising the second radial end and a contact end opposed the second radial end, and wherein the contact end of the coil part and the contact end of the wall part are shaped and arranged to engage with one another.

6. A stator according to claim 5, wherein the contact end of the wall part comprises a recessed portion along at least a portion of the height of the wall part, and wherein the contact end of the coil part comprises a correspondingly shaped projection to engage with the recessed portion of the wall part.

7. A stator according to claim 5 or 6, wherein a portion of the coil part comprises one or more through holes that extend through the width of the coil part for receiving interconnects of the stator there through.

8. A stator according to claim 7, wherein the coil part comprises a channel for each of the through holes, each channel extending from the respective through hole to the contact end of the end of the coil part.

9. A stator according to any one of claims 5 to 8, wherein both of the first radial ends and the second radial ends are bifurcated to provide respectively the first and second contact portions, separated by a gap, that contact the respective stator coil stack, or inner or outer cylindrical wall.15286993-110. A stator according to any one of claims 5 to 9, wherein the first radial end of the flow restrictor is bonded to the stator coil stack.

11. A stator according to any one of claims 5 to 10, wherein the wall part is formed of a material having a lower modulus than that of the modulus of the coil part.

12. A stator according to any preceding claim, wherein each of the first and second contact portions comprise tapered edges.

13. A stator according to any preceding claim, wherein the bifurcated portion is configured to flex.

14. A stator according to any preceding claim, wherein the flow restrictor comprises a locating feature disposed on an axial end of the flow restrictor between the first and second radial ends that is configured to engage with a correspondingly shaped locating feature in the respective radial wall of the stator housing.

15. A stator according to claim 14, wherein the locating feature on the flow restrictor is a projection projecting away from the axial end, and the locating feature on the respective radial wall of the stator housing comprises a detent configured to receive the projection.

16. A stator according to claim 14 or 15, wherein each axial end of the flow restrictor comprises a respective locating feature, and each of the first and second radial walls comprises respective correspondingly shaped locating features.

17. A stator according to any preceding claim, comprising one or more support structures arranged radially inward or radially outward of a respective stator coil stack for supporting one or more interconnects of the stator.

18. A stator according to claim 17, wherein a respective support structure is located adjacent a respective flow restrictor.

19. A stator according to claim 18, wherein the respective flow restrictor and support structure are arranged within the stator such that the coolant impinges on the respective15286993-1flow restrictor, and the support structure is located on the opposing side of the flow restrictor to the impinging flow of the cooling fluid.

20. A stator according to any preceding claim, wherein, when there are three or more flow restrictors, the flow restrictors are arranged circumferentially around the stator alternately between a respective stator coil stack and circumferentially outward wall, and a respective stator coil stack and circumferentially inner wall.21 . A stator according to any preceding claim, wherein the flow restrictor permits at least a portion of the cooling fluid to pass from one side of the flow restrictor to the other side of the flow restrictor.

22. An axial flux machine, comprising:a stator according to any preceding claim;a rotor comprising a set of permanent magnets and mounted for rotation about the axis of the machine, the rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and rotor.

23. The axial flux machine according to claim 22, comprising a second rotor comprising a set of permanent magnets and mounted for rotation about the axis of the machine, the second rotor being spaced apart from the stator along the axis of the machine to define a gap between the stator and second rotor, and the second rotor being disposed on a side of the stator opposed to the rotor.

24. The axial flux machine according to claim 22 or 23, wherein the machine is a motor or a generator.

25. A flow restrictor for a stator for an axial flux machine, the flow restrictor being configured in use to restrict the flow of at least a portion of a cooling fluid in a stator housing, the flow restrictor comprising:a first end for contacting a respective stator coil stack of a stator;a second radial end for contacting a respective inner or outer cylindrical wall of a stator,15286993-132wherein one or both of the first and second ends is bifurcated to provide a respective first and second contact portions, separated by a gap, for contacting a respective stator coil stack, or inner or outer cylindrical walls of a stator.

26. A flow restrictor according to claim 25, wherein a portion of the flow restrictor between the first end and the second end comprise one or more through holes that extend through a width of the flow restrictor for receiving interconnects of a stator there through.

27. A flow restrictor according to claim 26, wherein the flow restrictor comprises a channel for each of the through holes, each channel extending from the respective through hole to the first end of the flow restrictor.

28. A flow restrictor according to claim 27, wherein the flow restrictor comprises one or more cut-out portions extending at least a portion between adjacent through holes and adjacent channels, and the cut-out portions extending at least a portion of the width of the flow restrictor.

29. A flow restrictor according to claim 25, wherein the flow restrictor is formed of a separate coil part and a wall part separated between the first and second ends, the coil part comprising the first radial end and a contact end opposed the first radial end, and the wall part comprises the second radial end and a contact end opposed the second radial end, and wherein the contact end of the coil part and the contact end of the wall part are shaped and arranged to engage with one another.

30. A stator according to claim 29, wherein the contact end of the wall part comprises a recessed portion along at least a portion of the height of the wall part, and wherein the contact end of the coil part comprises a correspondingly shaped projection to engage with the recessed portion of the wall part.

31. A flow restrictor according to claim 29 or 30, wherein a portion of the coil part comprises one or more through holes that extend through the width of the coil part for receiving interconnects of the stator there through.15286993-13332. A flow restrictor according to claim 31 , wherein the coil part comprises a channel for each of the through holes, each channel extending from the respective through hole to the contact end of the end of the coil part.

33. A flow restrictor according to any one of claims 29 to 32, wherein both of the first radial ends and the second radial ends are bifurcated to provide respectively the first and second contact portions, separated by a gap, for contacting the respective stator coil stack, or inner or outer cylindrical wall of a stator.

34. A flow restrictor according to any one of claims 5 to 33, wherein the wall part is formed of a material having a lower modulus than that of the modulus of the coil part.

35. A flow restrictor according to any one of claims 25 to 34, wherein each of the first and second contact portions comprise tapered edges.

36. A flow restrictor according to any one of claims 25 to 35, wherein the bifurcated portion is flexible.

37. A flow restrictor according to any one of claims 25 to 36, wherein the flow restrictor comprises a locating feature disposed on an axial end of the flow restrictor between the first and second ends that is configured to engage with a correspondingly shaped locating feature in a radial wall of a stator housing.

38. A flow restrictor according to claim 37, wherein each axial end of the flow restrictor comprises a respective locating feature.15286993-1

Images