STATOR HOUSING FOR AN AXIAL FLUX MACHINE

Abstract

Described herein is a method of manufacturing a housing for the stator of an axial flux permanent magnet machine. The housing has a cylindrical wall including a metal outer ring lined with a polymer inner ring. The method includes positioning the metal outer ring in an injection moulding machine, and with the injection moulding machine injection moulding a polymer resin onto an inner surface of the metal outer ring to fabricate the polymer inner ring. The polymer inner ring includes a gripping surface arranged to grip a portion of the outer ring, for example moulded around a formation on an inner surface of the metal outer ring. The housing is manufactured using the metal outer ring and the polymer inner ring.

Claims

1. A method of manufacturing a housing for the stator of an axial flux permanent magnet machine, the machine having a stator comprising a set of coils wound on respective stator bars and disposed circumferentially at intervals about an axis of the machine, and a rotor bearing a set of permanent magnets and mounted for rotation about the axis, and wherein the rotor and stator are spaced apart along the axis to define a gap therebetween in which magnetic flux in the machine is generally in an axial direction, the housing having a cylindrical wall comprising a metal outer ring lined with a polymer inner ring, the method comprising: positioning the metal outer ring in an injection moulding machine; with the injection moulding machine, injection moulding a polymer resin onto an inner surface of the metal outer ring to fabricate the polymer inner ring, the polymer inner ring comprising a gripping surface arranged to grip a portion of the metal outer ring; and manufacturing said housing using the metal outer ring and the polymer inner ring.

2. The method of claim 1, wherein the injection moulding comprises cooling the metal outer ring and the polymer inner ring to shrink the gripping surface of the polymer inner ring relative to the gripped portion of the metal outer ring to secure the polymer inner ring to the metal outer ring.

3. The method of claim 1, wherein the injection moulding comprises forming the gripping surface around a formation on an inner surface of the metal outer ring.

4. The method of claim 3, wherein the formation comprises a raised portion raised above an inner surface of the metal outer ring, wherein the formation extends circumferentially around the inner surface, wherein the formation has two edges, and wherein each edge has an overhang to lock the polymer inner ring onto the metal outer ring.

5. The method of claim 4, wherein the raised portion has a recess under an edge of the raised portion defining the overhang, and wherein an inner part of the recess is curved and an outer edge of the raised portion of the formation is curved, such that there is a smooth transition from the inner surface of the outer metal ring to an upper surface of the raised portion of the formation.

6. The method of claim 1, wherein the injection moulding comprises outsert injection moulding using the metal outer ring as an outsert mould.

7. The method of claim 1, wherein the injection moulding comprises, with an insert, forming a port in the polymer inner ring arranged in alignment with a corresponding port in the metal outer ring.

8. The method of claim 1, wherein the injection moulding comprises forming a rib on an upper and/or on a lower surface of the polymer inner ring.

9. The method of claim 8, further comprising laser welding respective end wall plates to the rib on the upper and lower surface of the polymer inner ring to hermetically seal a chamber between the polymer inner ring and the respective end wall plates.

10. The method of claim 1, wherein each end of each stator bar is provided with a shoe, and wherein the end wall plates mount the shoes.

11. The method of claim 1, wherein a radial thickness of the polymer inner ring is between 1.5 mm and 3.0 mm.

12. The method of claim 1, comprising: applying a corona treatment to the inner surface of the metal outer ring before fabricating the polymer inner ring thereon.

13. The method of claim 1 wherein the axial flux permanent magnet machine comprises two rotors, one on each side of the stator, the method further comprising attaching a metal cover to each side of the metal outer ring, wherein the metal covers cover the housing for the stator and also the rotors.

14. The method of claim 1 wherein the polymer resin comprises a glass-reinforced polyamide resin.

15. The method of claim 9, wherein the machine is a yokeless and segmented armature machine having a pair of the rotors, one to either side of the stator, wherein the housing defines a chamber for coolant for the coils of the stator, and wherein the end plates hold the stator bars in position during operation of the machine.

16. An axial flux permanent magnet machine comprising: a stator comprising a set of coils wound on respective stator bars and disposed circumferentially at intervals about an axis of the machine; a rotor bearing a set of permanent magnets and mounted for rotation about said axis, wherein said rotor and stator are spaced apart along the axis to define a gap therebetween in which magnetic flux in the machine is generally in an axial direction; and a stator housing; wherein the stator housing comprises a metal outer ring and a polymer inner ring lining an inner surface of the metal outer ring, and wherein the polymer inner ring comprises a gripping surface arranged to grip a portion of the metal outer ring.

17. The axial flux permanent magnet machine of claim 16, wherein the gripped portion of the metal outer ring comprises a formation on an inner surface of the metal outer ring.

18. The axial flux permanent magnet machine of claim 17, wherein the formation comprises a raised portion raised above an inner surface of the metal outer ring, wherein the formation extends circumferentially around the inner surface, wherein the formation has two edges, and wherein each edge has an overhang to lock the polymer inner ring onto the metal outer ring.

19. The axial flux permanent magnet machine of claim 18, wherein the raised portion has a recess under an edge of the raised portion defining the overhang, wherein an inner part of the recess is curved and an outer edge of the raised portion of the formation is curved such that there is a smooth transition from the inner surface of the outer metal ring to an upper surface of the raised portion of the formation.

20. The axial flux permanent magnet machine of claim 16, wherein the stator housing comprises respective end wall plates secured to respective ribs of an upper surface and a lower surface of the polymer inner ring.

21. (canceled)

22. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

[0038] FIGS. 1a to 1c show, respectively, a general configuration of a two-rotor axial flux machine, example topologies for axial permanent magnet machines including a schematic side view of a yokeless and segmented armature (YASA) machine, and an enumerated drawing of a YASA machine.

[0039] FIG. 2 shows a perspective view of the YASA machine of FIG. 1c.

[0040] FIG. 3 shows a perspective exploded view of a stator and stator housing for a YASA machine.

[0041] FIG. 4a shows an exploded perspective view of a cylindrical wall of a stator housing of an axial flux permanent magnet machine according to an embodiment.

[0042] FIG. 4b shows a perspective view of a polymer inner ring of the cylindrical wall of FIG. 4a.

[0043] FIG. 4c shows a zoomed in view of the polymer inner ring of FIG. 4c.

[0044] FIG. 5a shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating a step of a method according to an embodiment.

[0045] FIG. 5b shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating a step of a method according to an embodiment.

[0046] FIG. 5c shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating a step of a method according to an embodiment.

[0047] FIG. 5d shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating a step of a method according to an embodiment.

[0048] FIG. 5e shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating a step of a method according to an embodiment.

[0049] FIG. 6a shows a portion of a cylindrical wall of a stator housing of an axial flux permanent magnet machine illustrating retention features.

[0050] FIG. 6b shows a cross section of part of a cylindrical wall of a stator housing of an axial flux permanent magnet machine having retention features illustrated in FIG. 6a, illustrating a step of a method according to an embodiment.

[0051] Like elements are indicated by like reference numerals.

DETAILED DESCRIPTION

[0052] FIGS. 1c, 2 and 3, which are taken from WO2012/022974, 1c show details of an example yokeless and segmented armature (YASA) machine 10. The machine 10 may function either as a motor or as a generator.

[0053] The machine 10 comprises a stator 12 and, in this example, two rotors 14a,b. The stator 12 comprises a collection of separate stator bars 16 spaced circumferentially about a machine axis 20, which also defines an axis of the rotors 14a,b. Each bar 16 carries a stator coil 22, and has an axis which is typically disposed parallel to the rotation axis 20. Each end 18a,b of the stator bar is provided with a shoe 27, which helps to confine coils of the stator coil 22 and may also spread the magnetic field generated by the stator coil. The stator coil 22 may be formed from square or rectangular section insulated wire so that a high fill factor can be achieved. In a motor the stator coils 22 are connected to an electrical circuit (not shown) that energizes the coils so that poles of the magnetic fields generated by currents flowing in the stator coils are opposite in adjacent stator coils 22.

[0054] 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 inclined. Gaps 26a,b are present between respective shoe and magnet pairs 17/24a, 27/24b. In an example motor the stator coils 22 are energized so that their polarity alternates to cause coils at different times to align with different magnet pairs, resulting in torque being applied between the rotor and the stator. As described above, the housing must accordingly be structurally strong enough to withstand the forces thereon resulting from the torque between the rotor and stator. In FIGS. 1c, 2 and 3, the structural strength of the stator housing is achieved by providing a suitably thick layer of polymer, for example greater than 10 mm and bolting the polymer to an aluminium outer housing.

[0055] The rotors 14a,b are generally connected together, for example by a shaft (not shown), and rotate together about the machine axis 20 relative to the stator 12. In the illustrated example a magnetic circuit 30 is formed by two adjacent stator bars 16, two magnet pairs 24a,b, and two back plates 32a,b, one for each rotor, linking the flux between the backs of each magnet pair 24a,b facing away from the respective coils 22. The back plates 32a,b may be referred to as back irons and comprise a magnetic material, typically a ferromagnetic material although not necessarily iron. This magnetic material is not required to be a permanent magnet. The stator coils 16 are enclosed within a housing which defines a chamber for the rotors and stator, and which may be supplied with a cooling medium.

[0056] FIG. 3 shows a stator 12a in which the stator coils are located between plastics material clam shells 42a,b. These clamshells have external cylindrical walls 44, internal cylindrical walls 46, and annular end walls 48. In FIG. 3 the end 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 FIG. 3, when assembled the stator housing 42a,b is provided with ports that allow cooling medium such as oil to be pumped into the spaces 52, 54, 56 to circulate around the coils and cool them.

[0057] 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. An example SMC may comprise glass-bonded iron particles, a thin layer (typically <10 μm) of glass bonding and mutually electrically insulating the iron particles, leaving some residual porosity. The shoes 27 may be moulded from SMC, e.g. using a high-temperature, high-pressure compaction process. Conveniently the shoes and stator bar may be formed separately and subsequently assembled.

[0058] FIGS. 4a, 4b and 4c respectively show an exploded perspective view of a cylindrical wall 401 of a stator housing of an axial flux machine according to an embodiment, a perspective view of a polymer inner ring 403 of the wall 401 according to an embodiment, and a zoomed in view of the polymer inner ring 403 showing a gripping surface 404 of the polymer inner ring 403 according to an embodiment.

[0059] The generally cylindrical wall 401 comprises a metal outer ring 402 lined with a polymer inner ring 403. The polymer inner ring 403 comprises a gripping surface 404 arranged to grip a portion of the metal outer ring 402. The cylindrical wall 401 comprises a number of ports 405, for example a port for an inverter interface, sealed inspection holes for busbar connections, sensor ports, and coolant ports for circulating coolant around the inside of the stator housing.

[0060] An upper and/or lower surface of the inner polymer ring is provided with one or more ribs 406 configured for receiving an end wall plate (not shown) thereon and for being securely laser welded to said end wall plate to provide a hermetically sealed chamber (apart from the ports) defined by the wall 401 and the end wall plates. The metal outer ring 402 is provided with a plurality of bolt holes 407 for bolting the cylindrical wall 401 to one or more other components of the axial flux machine, for example to an axial flux machine mount. The metal outer ring 402 is further provided with an electronics interface housing 408 for housing an interface to electronics of the axial flux machine, for example an inverter interface and/or one or more busbars. The gripping surface 404 of the polymer inner ring 403 clamps down onto a formation of the metal outer ring 402.

[0061] FIGS. 5a, 5b, 5c, 5d and 5e respectively illustrate steps of a method for manufacturing a housing for the stator of an axial flux machine according to an embodiment, the housing having a generally cylindrical wall comprising a metal outer ring lined with a polymer inner ring.

[0062] In FIG. 5a, a cross section of part of a cylindrical metal outer ring 501, for example an aluminium outer ring, is shown. The metal outer ring 501 comprises an outer surface 502a, an inner surface 502b, an upper surface 503a and a lower surface 503b. The inner surface 502b comprises a formation 504. The formation 504 comprises a raised portion raised above an inner surface 502b of the metal outer ring 501, wherein the formation extends circumferentially (not necessarily continuously, though this may be preferred) around the inner surface 502b of the metal outer ring. The formation 504 has two edges 505a, 505b, one towards each of the edges of the metal outer ring and being approximately parallel thereto.

[0063] Each edge 505a, 505b has an overhang 505c, 505d to lock the polymer inner ring onto the metal outer ring 501. The raised portion has a recess 505e, 505f under an edge of the raised portion defining the overhang 505c, 505d. An inner part of the recess 505e, 505f is curved such that there is a smooth transition from the inner surface of the outer metal ring to an upper surface 505g of the raised portion of the formation 504. As previously described this ensures there are no sharp circumferential edges, resulting in less stress in the polymer, improved polymer resin melt flows, and better shrinkage onto the formation 504. As described above, a corona treatment may be applied to the metal outer ring 501 prior to or after positioning in the injection moulding machine.

[0064] In FIG. 5b, the metal outer ring 501 is positioned in an injection moulding machine. In positioning the metal outer ring 501 in the injection moulding machine, the metal outer ring 501 is used as an outsert, for example used as one of the walls of a mould 506 having a space 508 therein into which polymer resin may be injected. The polymer resin may be, for example, a 35% glass reinforced polyamide resin such as Zytel™ HTN 51G35 by DuPont™ materials although other polymer resins may be used. Typically the polymer resin has a higher temperature dependent rate of shrinkage than the metal of the metal outer ring e.g. aluminium.

[0065] One or more upper surfaces of the formation 504 of the metal outer ring 501 may be shaped to engage with one or more other walls of the mould 506, thereby sealing against them prior to the injection moulding process.

[0066] The mould 506 may be provided with rib-shaped structures 507 thereon. Thus, the ring-like space 508 into which polymer resin is to be injected may be defined by the protrusion 504, the rib-shaped structures 507 and an inner wall 509 of the mould 506. The injection moulding process is then performed and a polymer resin is injected into the space 508, thereby fabricating a polymer inner ring 510 on the metal outer ring 501.

[0067] As part of the injection moulding process, the polymer resin melt may be heated to around 300-350° C., e.g. around 325° C., and injected under a maximum pressure of e.g. around 155-205 MPa, e.g. 180 MPa, with an injection moulding machine, into the mould 506 over around e.g. 1-3 seconds, e.g. 2 seconds, using the metal outer ring 501 as an outsert, held at e.g. around 125-175° C., e.g. 150° C. Once injected, the polymer resin is left to pack into the mould 506 under a maximum pressure of e.g. around 125-175 MPa, e.g. 150 MPa to allow air bubbles to vent over around e.g. 10-30 seconds, e.g. 20 seconds.

[0068] The polymer resin melt fills the space 508 forming a shape defined by the formation 504, rib-like structures 507 and the inner surface 509 of the mould 506, thus forming the polymer inner ring 510 having a gripping surface 511 around the formation 504 of the metal outer ring 501. As the polymer resin melt fills around the raised portion of the formation 504, portion 512 of the gripping surface 511 flows around raised portion and curves back onto itself resulting in an engagement between the gripping surface 511 and the formation 504 without any sharp edges therebetween.

[0069] The polymer resin melt is then allowed to cool over e.g. around 10-30 seconds, e.g. around 18 seconds until the temperature is e.g. around 205-255° C., e.g. 230° C. As described above, it is during the cooling that the gripping surface 511 of the polymer inner ring 510 shrinks to a greater extent than the metal outer ring 501, thereby causing it to grip the metal outer ring 501. In particular, the cooling causes shrinking not only in a radially inwards direction but also in a direction parallel to the central axis of the cylindrical ring. Accordingly, the gripping surface 511 has shrinkage having both a horizontal and vertical component relative to the central axis of the cylindrical ring. The portion 512 of the gripping surface 511 that formed around the raised portion of the formation 504 thus clamps down onto the raised portion and also pulls it radially inwards as it cools, thereby providing a stronger and more positionally stable securing of the polymer inner ring 501 to the metal outer ring 510 with less structural stress than would be possible without the raised portion. For example, the polymer inner ring 510 is more securely attached to the metal outer ring 501 and with less structural stress than if the formation 504 had perfectly flat surfaces and sharp corners and/or edges. As described above, when the polymer resin melt fills the rib-shaped structures 507, the polymer inner ring 510 is provided with ribs 513 for subsequent securing of end plates to said ribs with, for example, laser welding.

[0070] In FIG. 5c, once the polymer resin is cooled to e.g. around 205-255° C., e.g. 230° C., the mould 506 is opened over e.g. around 5 seconds, and the injection moulded polymer inner ring 510 and the corresponding metal outer ring 501 against which the polymer resin melt has been injected is ejected from the injection moulding machine. Once ejected, the polymer inner ring 510 fabricated on the metal outer ring 501 is allowed to cool further to room temperature, for example e.g. around 25° C. This cooling may be performed in a controlled environment to control the cooling profile and time or may occur naturally without any specific cooling profile.

[0071] FIG. 5d shows a cut away view of the cylindrical wall 515 of the stator housing after the injection moulding process is complete. The cylindrical wall 515 in FIG. 5d is similar to the cylindrical wall 401 shown in FIG. 4a. In the view of FIG. 5d, the polymer inner ring 510 is shown in a plane into the page to illustrate that one or more ports 514 may be provided thereon, for example, a port for an inverter interface, sealed inspection holes for busbar connections, sensor ports, and coolant ports for circulating coolant around the inside of the stator housing in a similar manner to in FIG. 4a. The ribs 513 are provided in a continuous circumferential manner around the upper and lower surfaces of the polymer inner ring 510.

[0072] In FIG. 5e, the manufacture of the stator housing is continued by laser welding respective end wall plates 516 to the ribs 513 on the upper and lower surfaces of the polymer inner ring 510 cylindrical wall 515 of the stator housing to hermetically seal (apart from the ports) a chamber 517 between the polymer inner ring 510 and the inner surfaces of the end wall plates 516. As shown in FIG. 5e, in laser welding the ribs 513 to the polymer inner ring 510 the outer surfaces of the end wall plates 516 may be made flush with the upper surface of the metal outer ring 501 to provide a more vertically compact stator housing than would be possible had bolting of the polymer to the metal been required.

[0073] Whilst not shown, where the axial flux permanent magnet machine comprises two rotors, one on each side of the stator, a further step may be performed in manufacturing the housing of the stator. This may comprise attaching a metal cover to each side of the metal outer ring, the metal cover enclosing the housing for the stator and also the rotors. As described above this can provide a complete metal outer shell made up of the metal outer ring and the metal covers enclosing a complete polymer inner shell made up of the polymer end wall plates and the polymer inner ring. Thus, a double housing arrangement is provided having advantageous structural strength and hermetic sealing without the disadvantages of high space requirements, high assembly cost and assembly time.

[0074] Whilst not shown, each end of each stator bar may be provided with a shoe and the end wall plates may be configured to mount the shoes during assembly of the axial flux permanent magnet machine. For example the end wall plate may have recesses in which the shoes may be supported or fastened. As described above, the assembly steps of mounting the shoes in the end wall plate have very tight manufacturing tolerances and require accurate positioning of the end wall plates. This in turn depends on accurate positioning and positional stability of the inner polymer ring onto which the end wall plates are secured. The methods described herein, and the axial permanent magnet machine assembled therewith, can ensure that the mounting of the shoes is accurate.

[0075] Optionally the end wall plates may be provided with an opening in their central axis, for example to allow a component such as an axle of the axial flux machine to pass through. For example the stator housing may have a generally toroidal shape with an inner wall (not shown).

[0076] In some other implementations different configurations of the formation on the inner surface of the metal outer ring may be used. For example, the formation on the inner surface of the metal outer ring need not be a single, circumferentially continuous formation and may instead comprise a plurality of formations spaced apart along the inner surface.

[0077] FIGS. 6a and 6b show cross-sectional views of a portion of a cylindrical wall 601 of a stator housing. In the same way as the formation on the metal outer ring 602 described above, each of the plurality of formations 604 comprises a raised portion raised above an inner surface 605 of the metal outer ring 602 and in this case has four edges, one 606a, 606b towards each edge of the metal outer ring 602 and now also one edge 606c, 606d in each circumferential direction along the inner surface 605 of the metal outer ring 602.

[0078] Different to the formation described above, it is the edges 606c, 606d in the circumferential directions that have an overhang to lock the polymer inner ring 603 onto the metal outer ring 602. The raised portion thus has a recess under an edge 606c, 606d of the raised portion defining the overhang. As with the continuous formation, an inner part of the recess is curved and an outer edge of the raised portion (for example all the edges 606a, 606b, 606d, 606d) of the formation 604 is curved, such that there is a smooth transition from the inner surface 605 of the outer metal ring to an upper surface of the raised portion of the formation 604.

[0079] This implementation of the formation 604 has the same advantages as the earlier implementation of the formation described herein. The plurality of formations 604 on the inner surface 605 of the metal outer ring 602 may be manufactured by, for example cutting out recesses on the inner surface of the metal outer ring, leaving a formation 604 between each of the cut outs. Thus, when the injection moulding process is performed, the polymer resin melt flows, for example axially, into the space left by the cut outs around the formations 604, and in the same manner as described above, locks the polymer inner ring 603 onto the metal outer ring 602 as it cools. The inner polymer ring 603 is also provided with ribs 607 in the same manner as the other implementations thereof described herein.

[0080] The terms upper and lower surface, and the horizontal and vertical directions as used herein are used to describe the relative positioning of said surfaces and directions relative to each other and are not intended to limit the present disclosure to any given orientation in a coordinate system. The terms upper and lower, and horizontal and vertical are used for convenience of illustration relative to the figures provided herein. Thus, the upper surface is on an opposite side of a feature to the lower surface. Similarly, the inner surface is on an opposite of a feature to the outer surface regardless of the orientation of the feature in the coordinate system.

[0081] 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.