Rotor assembly

Abstract

A rotor assembly for an axial flux motor is herein discussed, comprising: a disc shaped base element having a first surface and an opposite second surface connected by a circumferential sidewall, an eddy current structure (ECS) made from a soft magnetic metal joined directly onto the first surface, wherein the ECS is made by laser-melt additive manufacturing and configured to suppress eddy currents in the rotor assembly.

Description

[0001] TITLE OF INVENTION

[0002] ROTOR ASSEMBLY

[0003] TECHNICAL FIELD

[0004] The present invention relates to a motor assembly for an axial flux motor. Further, the present invention relates to an axial flux motor comprising such a rotor assembly and a method for fabricating such a rotor assembly.

[0005] BACKGROUND

[0006] Common axial flux motors comprise a stator having one or more coils formed from a plurality of windings for generating an axial flux in use. The stator is fixed and, for example, connected to a housing of the axial flux motor. Axially next to the stator is arranged a rotor assembly comprising a base element and one or more magnets attached to the base element interacting with the magnetic field generated by the coils of the stator for rotating the rotor assembly when powered. Therein, the gap between the rotor assembly and the stator is along a plane perpendicular to the axis of rotation and therefore the direction of the magnetic flux between the rotor and the stator is aligned parallel with the axis of rotation.

[0007] To suppress eddy currents induced into the base element of the rotor assembly, the base element has a laminated structure of interchanging layers of conductive and non- conductive material that contribute to suppressing the circulation of eddy currents in the base element. However, the laminated base element is limited in its design and flexibility to adapt the lamination stack for optimal suppression, either by the specific application, i.e., a specific shape of the axial flux motor and / or by the specific design specifications needed by the tailored magnetic flux in the rotor assembly.

[0008] Hence, it is an object of the present invention to provide a rotor assembly and a method for manufacturing of such a rotor assembly providing an improved flexibility in the design of the rotor assembly.

[0009] Traditionally, axial flux rotors are constructed by assembly of the main constituent parts, magnets, rotor back iron and the rotor disc as separate parts, and typically joined by glueing or by brazing (or similar techniques). Such processes are expensive, time consuming, bad for the environment, and usually hard to scale to mass production with sufficient quality. The glued joints are also very difficult to assemble strongly enough, and consequently glued joints failure is a recurring (and critical) problem both during assembly and use, as the rotor may spin at high speed (e.g. 8000 rpm), and often is has a large diameter (e.g. 280 mm), which results in very high centrifugal forces. Furthermore, operation is at elevated temperatures (e.g. above 100°C), strongly contributing to glue failure.

[0010] The present invention addresses the above problems and others by providing a base element comprising a back iron, preferably made from conventional high strength steel, whereon an eddy current structure has been joined by additive manufacturing, preferably by additively manufacturing from a soft magnetic steel as for example Fe(i-X)Sixor Fe(i-x-y)AlySix, and configured for suppressing eddy currents in the powered rotor assembly. In the context of the present application, there is distinguished between two structures being joined together and two structures being connected to each other. In the methods of assembly currently in use in the industry, such as gluing or brazing, the individual parts of the rotor assembly are considered in the present context as connected to each other and can be separated into the constituent parts willfully or by fault during use. Contrary to this, in the context of the present application by joining or like uses of the verb to join, or of the noun joint, there is meant that the formed connection, the joint, between the joined elements is permanent and that separation of the joined elements cannot occur without destroying the same elements.

[0011] To achieve this effect, the aforementioned one or more eddy current structures are manufactured by melt-assisted 3D- printing (effectively laser spot 3D-printing, laser-melt assisted additive manufacturing, selective laser melting) onto the back iron, such that during materials addition a melt zone is formed wherein spot melting and additive fusion of the constituent steels of back iron and eddy current structure will create a permanent joint between these two constituent parts. Inside this joint (i.e. the combination melt zone), the chemical composition of the joint will differ from the chemical composition of the starting back iron and the subsequently formed eddy current structure. By itself, the joint may not be thicker than the focus depth of the laser, but this is not of importance to the present invention, as the joint in all circumstances will be considerably smaller than the relevant structures influencing the magnetic field of the rotor assembly.

[0012] A welcome advantage of using additive manufacturing is, that it is possible to create secondary structural elements, apart from the primary structural element, the eddy current structure (13), wherein such secondary structural elements can be, and are in preferred embodiments of the present invention, a bedding structure comprising a recess for receiving a magnet and a circumferential wall (e.g., pockets, indentations, grooves or similar features) for providing an enclosed bedding for a magnet for holding and enclosing the same a magnet inside the bedding structure. Such bedding structures for magnets has the advantage that when a magnet is placed in a such bedding structure and connected to the rotor assembly either by gluing or brazing at least a part of the centrifugal forces experienced by the magnet placed in the bedding structure will be taken up by the bedding structure and consequently reduce the demands on glue or brazing strength needed for securing a fault free operation of the rotor assembly of the present invention.

[0013] Additive manufacturing of component for electrical machines have been tried, but typically the focus is the complete soft magnetic parts (stator + rotor) or the stator only. For the stator the additively manufactured soft magnetic materials cannot compete with traditional laminations due to high losses. For radial flux machines is does not typically make sense to manufacture just the rotor by additive manufacturing as the rotor parts are typically stamped out from the stator, which would then need to be scrapped.

[0014] However, the present inventors herein make beneficial use of the fact that for axial flux motors however, there is no such dependence and the soft magnetic material for the rotor and stator can be chosen independently, as the magnitude of the variations of the flux is typically much lower in the rotor than in the stator and consequently a resulting flux loss at the level of additively added soft magnetic materials can be accepted in light of the present advantages.

[0015] In embodiments of the present invention, there is herein below suggested one or more eddy current structures joined to a back iron, preferably joined to a back iron by additive manufacturing, wherein the one or more eddy current structures can be respectively concentric rings or one or more spirals, the one or more eddy current structures separated by an air gap.

[0016] In the art, cf. DE102020113047 Al, eddy current structures made from respectively concentric rings or spirals and attached to a carrier part have been suggested for use with axial flux motor stators. In the prior art, however, the gap between the eddy current structures is a ceramic, reflecting the manufacturing process, wherein stacked layers of alternating soft magnetic metals and ceramics (both mixed in with filler polymers) are assembled to form the eddy current structure with ceramic gap filling. Herein the ceramic gap filling is necessary to maintain structural integrity during assembly. Subsequently the complete layer structure with eddy current structure and ceramic gap filling is first burned to release the filler polymers and subsequently sintered to attach the formed eddy current layer(s) with ceramic gap filling to the carrier part. However, no joint between carrier part and eddy current structure or the ceramic is thereby formed due to the lower sintering temperature compared to melt formation.

[0017] The presently suggested rotor assemblies are consequently advantageous over the rotor assemblies of the prior art by being simple to manufacture yet having a higher robustness in use compared to the known art. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1: A top view of a schematic rotor assembly according to the present invention.

[0019] Figure 2: A top view of a schematic rotor assembly according to the present invention.

[0020] Figure 3: A rotor assembly according to a further embodiment.

[0021] Figure 4: A rotor assembly according to a further embodiment.

[0022] Figure 5: A rotor assembly according to a further embodiment.

[0023] Figure 6: A flow diagram of a manufacturing method according to the present invention.

[0024] It is to be understood, that the embodiments shown in the figures are for illustration of the present invention and cannot be construed as being limiting on the present invention. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure.

[0025] DETAILED DESCRIPTION

[0026] In accordance with the present invention there is herein disclosed a rotor assembly (10) for an axial flux motor having an axis of rotation (18) when assembled and comprising: a disc shaped base element (12) having a first surface (12a) and an opposite second surface (12b) connected by a circumferential sidewall (12c), where onto the first surface (12a) an eddy current structure (13) has been joined using a laser-melt assisted additive manufacturing process, the eddy current structure (13) built from a soft magnetic metal using the laser-melt assisted additive manufacturing process and configured to suppress eddy currents in the powered rotor assembly (10) by presenting a plurality of separated elements (14) separated by one or more gaps (15) to prevent flow of an eddy current.

[0027] In preferred embodiments of the rotor assembly (10) the base element (12) is made from steel, preferably the base element (12) is a back iron for an axial flux motor, and even more preferably the base element (12) is an off-the-shelf back iron for an axial flux motor.

[0028] According to the invention a rotor assembly is provided for an axial flux motor, in particular for use in an automotive application. The rotor assembly according to the present invention comprises a disc shaped base element, such as a back iron, having a first surface and an opposite second surface connected by a circumferential sidewall. Further, an eddy current structure (ECS) made from a soft magnetic metal is joined to the first surface. Therein, the ECS is made by additive manufacturing and configured to suppress eddy currents in the rotor assembly when this is powered. As the ECS is created using a laser-melt assisted additive manufacturing process, complex structures with a high degree of flexibility in the design can be created permitting tailoring of the shape of the ECS to the specific needs of the rotor assembly, such as being adapted to the overall shape of the axial flux motor or for improving the magnetic flux between the rotor assembly and the stator of the axial flux motor.

[0029] Preferably, the base element is made from steel and is at the same time the substrate for the eddy current structure. More preferably, the base element is a back iron for a rotor assembly, and even more preferably, an off-the-shelf back iron. The fact that it is possible to use an off-the-shelf back iron as base element in the present invention is an integral part of the advantages of the present invention as production cost thereby is lowered.

[0030] In accordance with the invention, the ECS is joined onto the disc shaped base element by the laser-melt assisted additive manufacturing process. Hence, the connection between the base element and the ECS is intrinsically provided by the additive manufacturing in the form of a joint. No further means are necessary for attaching the ECS to the base element.

[0031] Preferably, the ECS is made from a silicon steel, such as a soft magnetic steel of type Fe(i-x-y-Z)MgzAlySix, Fe(1-0-y)AlyS1x, or Feu-X)Sixor most preferably of a soft magnetic steel of type Fe(1-x)Six, wherein x, respectively y and z, denote the mass-fraction of the atomic element in the steel. Manganese and aluminum can independently be added up to 0.5 wt%. In particular, the amount of Si in the silicon steel should be more than 2 wt%, preferably more than 3 wt%, preferably more than 4 wt%, more preferably more than 5 wt% or most preferably more than 6 wt%, but not exceeding 10 wt%, preferably not exceeding 9 wt%, more preferably not exceeding 8 wt%, and most preferably not exceeding 7 wt%.

[0032] It has been shown that silicon steels are able to be used in an additive manufacturing process and at the same time provide sufficient magnetic properties in order to guide the magnetic flux in the axial flux motor.Silicon steels (c.f. W.M. Garrison Jr, in Encyclopedia of Materials: Science and Technology, 2001) are ferritic alloys of iron and silicon that have magnetic properties which make them useful in motors and transformers. The silicon additions improve magnetic softness and increase the electrical resistivity. They also have the undesirable effects of decreasing the Curie-temperature, reducing the saturation magnetization, and of embrittling the alloy when the silicon additions exceed about 2 wt.%. The embrittling effects of silicon make it difficult to produce silicon steels with more than about 3 wt.% silicon, however higher mass-content of silicon has improved magnetic properties. In industry, silicon steels are produced in two forms, highly textured grain-oriented alloys and alloys in which the grains are not oriented and wherein grain orientation is carried out to align the magnetic easy axis.

[0033] It is a surprising discovery of the present inventors that when using laser-melt assisted additive manufacturing, the normal impediments to using high silicon content steels as soft magnetic materials are not present, such as brittleness and reduction in saturation magnetization due to the formation of Bz and DOs chemically ordered phases during slow cooling. The present inventors consider this, without being bound be this theory, to be related to the small cross- sectional size of the eddy current structures suggested herein, which are small enough to prevent slow cooling after laser-melting thereby avoiding formation of these phases. Rather, the small cross-sectional size of the eddy current structures of the present invention combined with the small melt zone formed by the laser serve to permit rapid quenching of the molten zone during laser-melt additive manufacturing, thereby reducing the negative impact of silicon on the soft magnetic steels without reducing its positive impact. Contrary to this, when bulk sintering of the entire eddy current structure and filling layers, such as done e.g., in the prior art, slow cooling, and consequently, formation of Bz and DOz phases, cannot readily be avoided. In a preferred embodiment, an eddy current structure of the invention configured for suppressing eddy currents in a powered rotor assembly comprises a plurality of separated elements (14), separated by gaps (15) to prevent flow of the eddy currents. The gaps and the eddy current structure elements are therefore integral to the eddy current structures of the invention and therefore, together, form the eddy current structures of the preferred embodiments of the present invention. Thereby the gaps must be of a size such that created eddy currents cannot pass these gaps and large eddy currents are prevented.

[0034] To determine if two or more elements of the ECS can be considered separated elements (14) in the context of the present invention, a cross-section (such as shown in Figure 2) of the rotor assembly (1) of the invention can be laid across the rotor assembly (1) and if two, neighboring or further separated metallic elements (14) are not touching, but presenting at least one gap (15) therein between, the two elements (14) are considered to be separated elements (14). In this respect, whether the eddy current structure (13) is formed from one or more spirals concentrically formed on the base element (12), concentric rings, hexagonal or other structure, it is not relevant to the assessment of whether the structure formed on the base element has a directional element away from the cross-section plane, as the eddy currents will propagate in the aforementioned crosssection plane and therefore always experience a mixture of separated elements (14) and gaps (15) and become suppressed thereby.

[0035] Preferably, a gap (15) between adjacent separated elements (14) of the eddy current structure (13) are smaller (D) than 1 mm, preferably smaller than 0.5 mm, or more preferably smaller than 0.1 mm, but not smaller than 0.01 mm, preferably not smaller than 0.05 mm.

[0036] Preferably, an ECS for automotive use has a height (H) in the axial direction of more than 5 mm, preferably more than 8 mm and more preferably more than 10 mm. In general, the ECS is limited upwards by its intended use and the height in the axial direction is therefore determined as a design parameter dependent on the intended use, rather than being determined by other limitations of the present invention. Rarely, the height may be up to 20 mm, but usually the height will not be in excess of 15 mm.

[0037] Preferably, a separated element (14) of the eddy current structure (13) has a width (W) of between 0.1 mm and 2 mm, preferably between 0.3 mm and 1.5 mm and more preferably between 0.5 mm and 1 mm.

[0038] In the following there is referred to a rotor assembly (10) according to the present invention as shown in Figures 1 and 2. The rotor assembly (10) is rotated in the electric motor around axis (18) as the axis of rotation. At the same time, the rotor assembly (10) may have a symmetry around the axis (18). The rotor assembly (10) comprises a disc shaped base element (12) having a first surface (12a), an opposite second surface (12b) and a circumferential sidewall (12c) connecting the first surface (12a) with the second surface (12b). The disc shaped base element (12) may be made for example from steel. Joined to the second surface (12a) is an eddy current structure (ECS) (13) made from a soft magnetic metal by additive manufacturing, preferably laser-melt assisted additive manufacturing. The ECS shown in Figure 1 presents a plurality of separated elements (14), in the form of concentric ring elements, which are separated by gaps (15). Therein, the ECS may be made from a silicon steel, which is able to be used in laser-melt assisted additive manufacturing and, in particular, in selective laser melting in order to create small structures as exemplified in the figures. In the example of Figure 1, the ECS (13) presents five concentric rings as separated elements (14) nested radially to each other and separated by the aforementioned gaps (15). In the example of Figure 2, the ECS (13) has sixteen separated elements (14), which could be e.g., concentric rings nested radially to each other and separated by the aforementioned gaps (15).

[0039] Therein, in the example of Figures. 1 and 2 the ECS (13) comprises a plurality of concentric rings nested radially to each other and separated by gaps (15). However, due to the use of additive manufacturing different structures are also possible. Thus, the ECS may comprise a Hilbert structure providing a space filling Hilbert curve of a higher order separated from each other for suppressing or preventing a flow of eddy currents. Alternatively, a slotted structure or hexagonal shaped structure can be provided by the additive manufacturing in order to prevent flow of the eddy currents. In an alternative embodiment, one or more spiral structures are be joined to the base element, preferably concentrically nested thereon.

[0040] Accordingly, in embodiments of the present invention, the ECS comprises a plurality of rings as separated elements nested radially to each other and each separate by gaps. Due to the separated rings propagation of the eddy currents are limited to the respective rings and cannot flow across the

[0041] ECS. Preferably, in embodiments of the present invention, the ECS is built as a Hilbert structure. A Hilbert structure is also called a Hilbert curve, wherein the ECS comprises separated elements following the Hilbert structure / curve. In particular, the Hilbert structure is a Hilbert structure of a higher order, i.e. second order or higher. By the Hilbert structure a space filling curve or elements are provided, which are separated from each other by gaps to prevent flow of eddy currents.

[0042] Preferably, in embodiments of the present invention, the ECS comprises a slotted and / or hexagonal shaped structure. Due to the slots of the slotted structure separated elements are created, wherein by the gaps flow of the eddy current is prevented. Similar, the ECS may have slots in a hexagonal shape separating the ECS in hexagons preventing spread of the eddy current across the whole ECS limiting the eddy currents to individual hexagons.

[0043] Accordingly, in an embodiment there is herein disclosed a rotor assembly (10), wherein the eddy current structure (13) comprises a plurality of rings (14) nested radially and concentrically with respect to the axis of rotation (18) of the rotor assembly (10).

[0044] Further, in an embodiment there is herein disclosed a rotor assembly (10), wherein the eddy current structure (13) comprises one or more spirals (14) formed concentrically with respect to the axis of rotation (18) of the rotor assembly (10). Further, in an embodiment there is herein disclosed a rotor assembly (10), wherein the eddy current structure (13) is a Hilbert structure.

[0045] Further, in an embodiment there is herein disclosed a rotor assembly (10), wherein the eddy current structure (13) has a slotted and / or hexagonal shaped structure.

[0046] Preferably, in embodiments of the present invention, the base element (12) comprises a coolant channel connected to one or more of the gaps in the ECS to allow coolant flow from the coolant channel through the ECS. Thus, in addition to preventing eddy currents by the gaps, the gaps also serve as coolant channel such that a coolant can flow through the gaps and dissipate heat from a rotor assembly. Therein, the coolant can flow from the coolant channel through the ECS or through the ECS towards the coolant channel in the base element.

[0047] Accordingly, in an embodiment there is herein disclosed a rotor assembly (10), wherein the base element (12) comprises a coolant channel connected to one or more of the one or more gaps (15) for permitting a coolant to traverse the eddy current structure (13) by flowing. In an embodiment thereof, there is herein disclosed a rotor assembly (10), wherein the rotor assembly (10) further comprises an entry coolant channel connecting to the eddy current structure (13) and an exit coolant channel connecting to the eddy current structure (13) elsewhere from the entry coolant channel such that a flowing coolant reaching the eddy current structure (13) via the entry coolant channel can exit the eddy current structure (13) via the exit coolant channel. Cooling of rotors in electrical machines is generally a challenge as they tend to be thermally isolated from the surroundings and any coolant. This is especially difficult in axial flux machines as rotor (magnet) losses tend to be higher. One solution commonly used is air cooling, either forced or natural convection. Such forced air cooling improves cooling to some extent but usually the rotor cooling is still the weakest point when trying to maximize the cont. power / torque. Having a fan forcing an air flow all time when needed or not also has the drawback of additional losses, and the fan and fan blades also comes with an additional cost, space and weight.

[0048] By additive manufacturing of the back iron or base element, oil cooling channels can be embedded into the axial rotor which significantly increases the cooling power without the drawbacks above. The oil flow can be diverted from the stator cooling oil flow typically present.

[0049] Referring to Figure 4, there is therein shown a preferred embodiment of the rotor assembly comprising at least one cooling channel (20). In the embodiment of Figure 4 a coolant channel (20) is provided in the base element (12) such that a coolant can flow along the arrows (22,23) through the gaps (15) of the ECS (13). Naturally also an opposite flow direction first through the ECS (13) towards the coolant channel (20) is possible and encompassed by the present invention. Thus, the gaps (15) of the ECS serve at the same time two purposes, firstly, to suppress eddy currents in the ECS and secondly, as cooling channels (20) for a coolant in order to dissipate heat from the rotor assembly (10). With the hitherto applied additive manufacturing methods, such double benefits are not achievable, as there are no gaps (or voids) between the separated elements (14) of the eddy current structures of the prior art, but rather solid (ceramic) connectors instead of gaps.

[0050] The coolant channel (20) of the invention must further comprise a connection to a reservoir for being in fluid connection with the said reservoir contained within the base element (12) for permitting a coolant to flow from said reservoir to the ECS (13) or return the coolant from the ECS towards the said reservoir. This connection may have any shape suitable. In Figure 4 the coolant channel (20) is shown to be opened towards the center of the base element. Other configurations are also possible such that the coolant channel (20) may be opened towards the radial outer surface, circumferential sidewall (12c) or the second surface (12b).

[0051] Preferably, in embodiments of the present invention, one or more magnets are attached to the ECS (13) opposite to the base element (12) in the direction of the axis of rotation (18). In particular, the magnets may be attached by glue, welding, brazing, clamping or other fastening means such as screws, bolts or the like.

[0052] Preferably, in embodiments of the present invention, the one or more magnets are at least partially embedded into the ECS. Due to additive manufacturing of the ECS, an increased degree of freedom in the design of the ECS is provided such that the position of the magnets can be tailored to the specific needs of the application. Hence, for compactness of the device, magnets may be embedded or at least partially embedded into the ECS. In particular, the surface of the ECS and a surface of the magnet embedded in the ECS are flat such that the magnet does not protrude from the ECS. Alternatively, the magnet is fully embedded in the ECS, surrounded by the ECS at all its sides Embedding the magnets in the rotor will protect them to a certain degree from the highly variable fields in the air gap and thus reduce the magnet losses. In traditional axial motor designs using a lamination coil this cannot be done due to manufacturing limitations.

[0053] In accordance therewith, the rotor assembly (10) according to the present invention may in embodiments comprise secondary structural elements, apart from the eddy current structure (13) (the primary structural element, wherein such secondary structural elements can be, and are in preferred embodiments of the present invention, a bedding structure (cf. Figure 3) comprising a recess (24) for receiving a magnet and, optionally a circumferential wall (not shown) such as e.g., pockets, indentations, grooves or similar features, for providing an enclosed bedding for a magnet for holding and enclosing the same a magnet inside the bedding structure. Such bedding structures for magnets has the advantage that when a magnet is placed in a such bedding structure and connected to the rotor assembly either by gluing or brazing at least a part of the centrifugal forces experienced by the magnet placed in the bedding structure will be taken up by the bedding structure and conseguently reduce the demands on glue or brazing strength needed for securing a fault free operation of the rotor assembly of the present invention.

[0054] A further development of embedded magnets would be to arrange them in a V-Shaped pattern similar to what is typically done in traditional V-shaped IPMs for automotive traction applications. This magnet arrangement will increase the reluctance component of the torque and give better high-speed efficiency and performance, which is a high priority for many automotive traction machine applications.

[0055] Preferably, at least one magnet is tilted relative to the first surface of the base element or tilted relative to a plane perpendicular to an axis of rotation of the rotor assembly. Due to the increased flexibility in the design provided by the additive manufacturing, arrangement of the magnet is not limited anymore to the top surface of the ECS and can be freely selected, i.e. tilted relative to a first surface of the base element and / or tilted relative to a plane perpendicular to an axis of rotating of the rotor assembly in order to improve a magnetic flux or adapt the magnetic flux of the rotor assembly to the specific need of the application.

[0056] Referring again to Figures 1 and 2, it is contemplated in preferred embodiments that to a surface of the ECS (13) one or more magnets (16) shall be attached for example by gluing, welding, brazing or the like. Thus, in the example Figures 1 and 2 the one or more magnets (16) extend from the surface of the ECS (13). In principle, the same magnets may be formed by additive manufacturing, however this is not currently cost-effective.

[0057] Referring to Figure 3 showing only a first half-section of the rotor assembly, a different configuration for the magnets (16) is shown which are embedded or at least partially embedded in the ECS (13). Therefore, by the elements (14) of the ECS (13) a recess (24) is created, which receives the magnets (16). Therein, in a preferred embodiment, a surface (16a) of the magnets (16) flushes with the axial surface of the ECS (13) opposite to the base element (12). Thus, the magnets (16) do not protrude from the ECS (13) Referring to Figure 5 showing a further embodiment, wherein the magnets (16) are fully embedded into the ECS (13), wherein the separated elements (14) of the ECS (13) are omitted for simplicity. The magnets (16) are therein tilted by an angle (30) with respect to a plane (26) perpendicular to the axis of rotation (18). Therein, the angle (30) may be determined between a center axis (28) of the magnet, an upper surface of the magnet (16) facing away from the base element (12) or a lower surface of the magnet 16 facing towards the base element. Due to the versatility of the additive manufacturing process alignment of the magnets 16 within the ECS can be freely selected in order to improve magnetic flux in the axial flux motor and shapes the magnetic flux to the specific needs of the respective application.

[0058] In particular, the magnets (16) in the embodiment of Figure 5 can be inserted in a radial direction into the ECS (13). Therefore, the ECS (13) may have openings in a radial direction in order to receive the magnets (16).

[0059] Of course, different embodiments of the present invention can be freely combined. In particular, the embodiment of Figure 3 can be freely combined with the embodiment of Figures 1 and 2, 4 and 5. Other technical details from the embodiments can be freely selected and can be freely combined by the skilled person based on the disclosure herein.

[0060] In another aspect the present invention an axial flux motor is provided comprising a rotor assembly as described before. Further, the axial flux motor comprises a stator interacting with the rotor assembly such that the rotor assembly is rotated. In another aspect of the present invention a method for fabricating a rotor assembly in particular for an axial flux motor is provided. The method includes: Providing a disc shaped base element having a first surface and an opposite second surface connected by a circumferential sidewall, and adding by an additive manufacturing process an eddy current structure, ECS, made from a soft magnetic metal joined directly onto the first surface, wherein the structure is configured to suppress eddy currents in the rotor assembly.

[0061] Accordingly, in an aspect of the present invention, there is herein disclosed a method of manufacturing from a soft magnetic metal a rotor assembly (10) according to any of the herein disclosed embodiments comprising an eddy current structure (13) on a base element (12) by using a laser-melt assisted additive manufacturing process, wherein the eddy current structure (13) is joined onto the base element (12), comprising:

[0062] - providing a disc shaped base element (12) having a first surface (12a) and an opposite second surface (12b) connected by a circumferential sidewall (12c),

[0063] - forming the eddy current structure (13) by addition of the soft magnetic metal onto the base element (12) using the laser-melt assisted additive manufacturing process.

[0064] In a preferred embodiment thereof, there is herein disclosed a method of manufacture, wherein the laser-melt assisted additive manufacturing process is selective laser melting.

[0065] In embodiments of the herein disclosed method, after formation of the eddy current structure (13), one or more magnets (16) are attached onto the eddy current structure (13) opposite to the base element (12), wherein preferably attachment is by gluing, welding or by brazing.

[0066] In particular, the ECS is joined directly onto the base element such that the base element serves as substrate for the additive manufacturing process. Preferably, the additive manufacturing process is selective laser melting (SLM).

[0067] Preferably, after adding the ECS to the base element one or more magnets are attached to a surface of the ECS opposite to the base element. Alternatively, one or more magnets are inserted into the ECS. In particular, the magnets may be at least partially embedded into the ECS. Therein, when fabricating the ECS, the ECS may comprise respective recesses to receive the magnet. This will lead to an increased accuracy in positioning and inherently improved fixturing due to abutting of the magnet against sidewalls of the respective recess. Thus, the one or more magnets may be inserted into the ECS such that the ECS partially surrounds or fully surrounds the respective magnets. Fixturing the magnets to the ECS may be by gluing, welding, brazing, or any other fixing means such as screws, bolts or the like.

[0068] Preferably, one or more magnets are inserted to the ECS in a radial direction. Thus, by the additive manufacturing process openings in the circumferential sidewall can be provided such that the magnets can be inserted into these openings to precisely position the magnets in the ECS.

[0069] Preferably, the method is further built along the features as described before with respect to the rotor assembly. In particular, the method for manufacturing according to the present invention results in a rotor assembly as previously described In the following referral is made to Figure 6 showing a flow diagram of the method for manufacturing respective rotor assembly. The method for fabricating the rotor assembly for an axial flux motor includes the following steps:

[0070] - In step SOI, a disc shaped base element is provided having a first surface and an opposite second surface connected by circumferential sidewall.

[0071] - In step S02, an eddy current structure (ECS) is added by a laser-melt assisted additive manufacturing process joined directly onto the first surface of the base element, wherein the ECS is made from a soft magnetic metal and configured to suppress eddy currents in the rotor assembly.

[0072] In particular, by the method as exemplified in Figure 6 a rotor assembly according to any embodiment of Figures 1 to 5 as described before is fabricated. In particular, by the method of Figure 6 a rotor assembly is fabricated as described and defined in the appended claims.

[0073] REFERENCE LIST

[0074] 10 rotor assembly

[0075] 12 base element

[0076] 13 eddy current structure

[0077] 14 element

[0078] 15 gap

[0079] 16 magnet

[0080] 18 axis of rotation / center axis

[0081] 20 coolant channel

[0082] 22 coolant flow

[0083] 23 coolant flow

[0084] 24 recess

[0085] 26 plane

[0086] 28 center axis of magnet

[0087] 30 angle

[0088] 12a first surface

[0089] 12b second surface 12c sidewall

[0090] 16a surface

[0091] CLOSING COMMENTS

[0092] Although the present invention has been described in detail for purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art in practicing the claimed subject matter, from a study of the drawings, the disclosure, and the appended claims.

[0093] The term "comprising" as used in the claims does not exclude other elements or steps. The indefinite article "a" or "an" as used in the claims does not exclude a plurality. A reference sign used in a claim shall not be construed as limiting the scope.

Claims

CLAIMS1. A rotor assembly (10) for an axial flux motor having an axis of rotation (18) when assembled and comprising: a disc shaped base element (12) having a first surface (12a) and an opposite second surface (12b) connected by a circumferential sidewall (12c), where onto said first surface (12a) an eddy current structure (13) has been joined using a laser-melt assisted additive manufacturing process, said eddy current structure (13) built from a soft magnetic metal using said laser-melt assisted additive manufacturing process and configured to suppress eddy currents in the powered rotor assembly (10) by presenting a plurality of separated elements (14) separated by one or more gaps (15) to prevent flow of an eddy current.

2. A rotor assembly (10) according to claim 1, wherein said base element (12) is made from steel, preferably wherein said base element (12) is a back iron for an axial flux motor, more preferably wherein said base element (12) is an off-the-shelf back iron for an axial flux motor.

3. A rotor assembly (10) according to either claim 1 or claim 2, wherein said soft magnetic metal is a soft magnetic steel of type Fe(i-x-y-z)MgzAlySix, wherein x, respectively y and z, denote the mass-fraction of the atomic element in the soft magnetic metal, and wherein magnesium (Mg) is present from 0 to 0.5 wt%, aluminum (Al) is present from 0 to 0.5 wt%, and silicon (Si) is present from 2.0 wt% to 10 wt%, and wherein the total content of x+y+z does not exceed 0.1.

4. A rotor assembly (10) according to claim 3, wherein said soft magnetic metal is Fe (i-x-y)AlySix, or Fea-X)Six, preferably wherein said soft magnetic metal is Fe(i-xjSix.

5. A rotor assembly (10) according to claim 3 or claim 4, wherein the amount of Si in the soft magnetic metal is from 3 wt%, preferably from 4 wt%, more preferably from 5 wt% or most preferably from 6 wt%, but not exceeding 10 wt%, preferably not exceeding 9 wt%, more preferably not exceeding 8 wt%, and most preferably not exceeding 7 wt%.

6. A rotor assembly (10) according to any preceding claim, wherein said eddy current structure (13) comprises a plurality of rings (14) nested radially and concentrically with respect to said axis of rotation (18) of said rotor assembly (10).

7. A rotor assembly (10) according to any preceding claim, wherein said eddy current structure (13) comprises one or more spirals (14) formed concentrically with respect to said axis of rotation (18) of said rotor assembly (10).

8. A rotor assembly (10) according to any preceding claim, wherein said eddy current structure (13) is a Hilbert structure.

9. A rotor assembly (10) according to any preceding claim, wherein said eddy current structure (13) has a slotted and / or hexagonal shaped structure.

10. A rotor assembly (10) according to any preceding claim, wherein a separated element (14) of the eddy current structure (13) has a width (W) of between 0.1 mm and 2mm, preferably between 0.3 mm and 1.5 mm, and more preferably between 0.5 mm and 1 mm.

11. A rotor assembly (10) according to any preceding claim, wherein a gap (15) between adjacent separated elements (14) of the eddy current structure (13) are smaller (D) than 1 mm, preferably smaller than 0.5 mm, or more preferably smaller than 0.1 mm, but not smaller than 0.01 mm, preferably not smaller than 0.05 mm.

12. A rotor assembly (10) according to any preceding claim, wherein said base element (12) comprises a coolant channel connected to one or more of said one or more gaps (15) for permitting a coolant to traverse said eddy current structure (13) by flowing.

13. A rotor assembly (10) according to claim 12, wherein said rotor assembly (10) further comprises an entry coolant channel connecting to said eddy current structure (13) and an exit coolant channel connecting to said eddy current structure (13) elsewhere from said entry coolant channel such that a flowing coolant reaching said eddy current structure (13) via said entry coolant channel can exit said eddy current structure (13) via said exit coolant channel.

14. A rotor assembly (10) according to any preceding claim, wherein one or more magnets (16) are attached to said eddy current structure (13) opposite said base element (12).

15. A rotor assembly (10) according to claim 14, wherein said one or more magnets (16) are at least partially embedded into said eddy current structure (13)16. A rotor assembly (10) according to either claim 14 or claim 15, wherein at least one of said one or more magnets (16) is tilted relative to said first surface (12a) and / or tilted relative to a plane perpendicular to said axis of rotation (18) of said rotor assembly (10).

17. A rotor assembly (10) according to any of the claims 14 to 16, wherein a bedding structure is comprised in said eddy current structure (13), said bedding structure comprising a recess (24) and, optionally, a circumferential wall, for receiving a magnet (16) therein.

18. An axial flux motor comprising a rotor assembly (10) according to any of the claims 1 to 17 and a stator interacting with said rotor assembly (10) when said axial flux motor is powered.

19. A method of manufacturing from a soft magnetic metal a rotor assembly (10) according to any of the claims 1 to 17 comprising an eddy current structure (13) on a base element (12) by using a laser-melt assisted additive manufacturing process, wherein the eddy current structure (13) is joined onto the base element (12), comprising:- providing a disc shaped base element (12) having a first surface (12a) and an opposite second surface (12b) connected by a circumferential sidewall (12c),- forming said eddy current structure (13) by addition of said soft magnetic metal onto said base element (12) using said laser-melt assisted additive manufacturing process.

20. A method according to claim 19, wherein the laser-melt assisted additive manufacturing process is selective laser melting.

21. A method according to either claim 19 or claim 20, wherein after formation of said eddy current structure (13) said one or more magnets (16) are attached onto said eddy current structure (13) opposite to said base element (12), preferably wherein attachment is by gluing, welding or by brazing.

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