Axial flux machines with reduced eddy current losses

Abstract

The present invention relates to an axial flux machine (1) having a stator (2) extending along a central axis (100), and at least one rotor (3) arranged next to the stator (2) with respect to the central axis (100), and comprising a return plate (4) on which magnets (5a), in particular permanent magnets, are arranged to form a plurality of rotor poles, wherein one of the two end faces (7a) of the return plate (4) faces the stator (2) and forms an air gap side, characterized in that the air gap side of the return plate (4) is grooved at least in sections or has a surface with increased resistance to suppress eddy currents.

Description

Prior ArtThe present invention relates to an axial flow machine. The axial flow machine has reduced eddy currents and thus a better efficiency.Axial flow machines are known from the prior art. Such machines can be designed with a centrally arranged stator and rotors provided axially adjacent on both sides. The stator is constructed, for example, with single-tooth windings. In these machines, the rotors consist of solid steel plates which, on the one hand, serve as a magnetic return and, on the other hand, also ensure the mechanical strength of the rotor. Magnets are fastened on the surface, which realize the field build-up in the machine.Disclosure of the InventionThe axial flow machine according to the invention has lower losses, since eddy currents are reduced. This allows operation at higher rotational speeds, wherein cooling of the machine is reliably possible on account of the minimized losses.The axial flux machine has a stator and at least one rotor. The stator extends along a central axis, wherein the rotor is arranged next to the stator with respect to the central axis. Thus, the rotor also extends along the central axis.The rotor also has a flux return disk. Magnets are arranged on the return disk for forming a plurality of rotor poles. The magnets are in particular permanent magnets. The magnets can preferably be surface magnets or buried magnets. The magnets are particularly advantageously bonded to the return disk and can additionally be supported by a bandage.One of the two end faces of the return plate faces the stator and forms an air gap side. This air gap side of the flux return disk is formed to be at least partially grooved or surface resistance-increased for suppressing eddy currents, in particular at least in pole sections of the rotor poles.Because of the grooving or an increase in electrical resistance, the formation of eddy currents is made more difficult. Such eddy currents originate from flux fluctuations or air gap harmonics which do not corotate uniformly with the rotor. This results in flux changes in the rotor, which leads to eddy currents in the return flow disks. These eddy currents would lead to considerable losses in the rotor disk without the suppression described above. This prevents the occurrence of excessively large waste heat, which would insufficiently heat up the rotor. As a result, operation at high rotational speeds is possible even over a relatively long time.The dependent claims show preferred developments of the inventionIn the case of surface magnets, the rotor poles of the return disk are preferably grooved in the circumferential direction over the entire rotor pole or over the entire region which faces the air gap. In the case of buried magnets, the return disks are preferably grooved in the pole gaps. Thus, for the respective application cases, the locations relevant for eddy current suppression are correspondingly designed. This enables a reliable suppression of eddy currents.The grooving is particularly advantageously formed by grooves which extend in the circumferential direction and are spaced apart from one another in the radial direction with respect to the central axis. Such grooves can be produced in a simple and low-cost manner. In addition, optimum suppression of eddy currents is achieved by hindering the free propagation of the eddy currents. The grooves of the grooving are particularly preferably of annular configuration in the case of surface magnets.The rotor poles of the return disk are preferably designed to be resistance-increased over the entire surface in the case of surface magnets. In the case of buried magnets, the rotor poles of the return plate are designed to be resistance-increased in the pole gaps. Thus, for the respective application cases, the locations relevant for eddy current suppression are correspondingly designed. This enables a reliable suppression of eddy currents.The surface increase in resistance of the rotor pole or pole section advantageously forms a resistance layer. The resistance layer has a higher specific electrical resistance than the remaining back closure disk. This makes it difficult for eddy currents to form in the resistance layer.The resistance layer particularly advantageously has an electrical conductivity of at most 3.0 MS / m, preferably at most 2.5 MS / m, particularly preferably 2.3 MS / m. This results in a low electrical conductivity, which hinders the formation of eddy currents.The grooves preferably have a depth of less than 1 mm. The resistance layer preferably has a layer thickness of less than 1 mm. This is optimal for suppressing eddy currents. The penetration depth δ of the eddy currents can be calculated using the following equationTherein, ρ is the electric resistivity of the reflux disc, μ r the specific permeability of the reflux disc, μ 0 the absolute permeability in vacuum, and f is the frequency of the flux fluctuations in the reflux disc. The depth of penetration into the rotor disk is preferably very small on account of the very high specific permeability (>100) and the high frequencies at high rotational speeds and is usually significantly below 1 mm. The eddy currents are therefore present only on the surface of the return plate. It is thus optimal for the suppression of eddy currents to use a relatively thin layer with increased resistance or grooves with relatively small depth.The surface resistance increase of the pole or pole section is preferably formed by a surface coating of the return disk or by alloying of the surface of the return disk. In particular, the surface coating or the alloying takes place based on or with carbon, silicon, aluminum or phosphorus. The return plate is thus appropriately machined in order to achieve the surface increase in resistance. Alternatively, the surface resistance increase is formed by a separate cover layer or cover plate. The cover layer is in particular a ferromagnetic cover layer. The cover plate is in particular a cover plate which is particularly preferably formed from electric sheet metal. By using a separate cover layer or cover lamella, no surface treatment is advantageously necessary.In the case of surface magnets, the magnets are preferably arranged on the grooved or surface resistance-increased air gap side of the return disk. Thus, the area below the surface magnets is appropriately machined to suppress eddy currents.The return disk is preferably a solid steel disk. The solid steel of the rotor disk has in particular a specific conductivity of 4.65 MS / m.To increase the stability, the rotor is advantageously surrounded by a bandage. The axial flow machine is designed in particular for higher rotational speeds, whereby the strength-improving bandage is advantageous. Thus, on the one hand, heating up of the rotor at high rotational speeds is minimized and the rotational speed stability of the rotor is maximized.In the case of buried magnets, depressions are preferably formed on the air gap side of the return plate for receiving the buried magnets and for receiving in each case one pole segment covering the buried magnets. The buried magnets are preferably bonded together.The pole segments are preferably made of SMC, i.e. of soft magnetic composite or pressed powdered iron, or as a laminated core.The stator preferably has a multiplicity of individual teeth. Each individual tooth is surrounded in particular by a coil winding. This provides a simple and cost-effective construction. Both due to the teeth in the stator and due to the single-tooth winding and the associated concentrated winding, large harmonics are produced in the machine. These harmonics also reach the rotor yoke or the return disks. These harmonics have a different rotational speed than the rotor. This results in induction fluctuations in the rotor, which leads to eddy currents in the return plates, the propagation of these eddy currents being suppressed.Brief Description of the DrawingsHereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. In the drawing, the following is: FIG. 1 shows a first schematic illustration of an axial flow machine according to a first exemplary embodiment of the invention, FIG. 2 shows a second schematic illustration of the axial flow machine according to the first exemplary embodiment of the invention, FIG. 3 shows a schematic exploded illustration of the axial flow machine according to the first exemplary embodiment of the invention, FIG. 4 shows a schematic illustration of a partial region of a flux return plate of the axial flux machine according to the first exemplary embodiment of the invention, FIG. 5 shows a schematic exploded illustration of an axial flow machine according to a second exemplary embodiment of the invention, FIG. 6 shows a schematic illustration of a return flow disk of an axial flow machine according to a third exemplary embodiment of the invention, FIG. 7 shows a schematic illustration of a return flow disk of an axial flow machine according to a fourth exemplary embodiment of the invention, and FIG. 8 shows a schematic illustration of a return plate of an axial flow machine according to a fifth exemplary embodiment of the invention.Embodiments of the InventionAll the same components, elements and / or units are preferably provided with the same reference numerals in all the figures.FIGS. 1 and 2 each show schematic views of an axial flow machine 1 according to a first exemplary embodiment of the invention. FIG. 3 shows an exploded view of the axial flow machine 1 according to the first exemplary embodiment. The axial flux machine 1 has a stator 2 and two rotors 3. The stator 2 extends along a central axis 100. The stator 2 has a multiplicity of individual teeth 10, which are each surrounded by a coil winding. The rotors 3 are arranged next to the stator 2 with respect to the central axis 100. Each rotor 3 has a return plate 4, on which surface magnets 5 aare attached in order to form a plurality of rotor poles. The surface magnets 5 aare in particular permanent magnets. The return plate 4 is a solid steel plate. To increase the strength, the rotor 3 is enclosed, for example, by a bandage.Each of the return disks 4 has an end face 7 afacing the stator 2, which end face forms an air gap side. This air gap side of the return disks 4 serves to accommodate the surface magnets 5a. It is also provided that the return disks 4 are grooved on the air gap side, i.e. on the end face 7 afacing the stator 2, for suppressing eddy currents. In the exemplary embodiment, the return plate 4 is grooved on the entire end face 7 ain the circumferential direction 300, but it would also be possible for the return plate 4 to be grooved only in pole sections of the rotor poles. It is thus provided that at least the region of the return plate 4 is grooved below the surface magnets 5 a.FIG. 4 shows a detail from the return plate 4 of the axial flux machine 1 according to the first exemplary embodiment. The grooving is formed by grooves 8 which extend in the circumferential direction 300 and are spaced apart from one another in the radial direction 200 with respect to the central axis 100. The grooves 8 are formed annularly in the exemplary embodiment shown. A depth d of the grooves 8 is less than 1 mm. It is to be expected that eddy currents do not penetrate into the return plate 4 more than the said depth d. Thus, generation of eddy currents can be reliably suppressed by the grooves 8. The grooves interrupt the current paths of the current vortices and the eddy current losses can be effectively reduced without substantially weakening the mechanical strength of the return plate 4. Higher rotational speeds of the axial flow machine 1 can thus be achieved.FIG. 5 shows an exploded view of an axial flow machine 1 according to a second exemplary embodiment. The basic construction is identical to the first exemplary embodiment, a difference only consists in the fact that no grooving is present on the return flux plates 4. Instead, the air gap side of the return plate 4 is designed to be resistance-increased in order to suppress eddy currents. This is achieved by applying a separate cover layer 9 or cover plate to the return plate 4. The cover layer is a ferromagnetic cover layer. A ferromagnetic cover plate, for example made of an electric plate, can likewise be used.A thickness d of the cover layer 9 is less than 1 mm. As explained above, generation of eddy currents can be reliably suppressed in this way. The generation of eddy currents is suppressed by the increased electrical resistance. Higher rotational speeds of the axial flow machine 1 can thus be achieved.FIG. 6 schematically shows a flux return plate 4 of an axial flux machine 1 according to a third exemplary embodiment. The third exemplary embodiment is virtually identical to the second exemplary embodiment, with no separate cover layer 9 being provided. Instead, a resistance layer 6 is formed in the surface of the return plate 4. The resistance layer 6 locally increases the electrical resistance of the return plate 4. In particular, the end face 7 aof the return disks 4 facing the stator 2 is designed to be resistance-increased over the entire surface. Alternatively, the return plate 4 can be designed to be surface resistance-increased only in pole sections of the rotor poles.The formation of the resistance layer 6 is effected, for example, by a surface coating or by alloying of the surface of the return plate 4, in particular with carbon, silicon, aluminum or phosphorus. The resistance layer 6 has a higher specific electrical resistance than the rest of the return plate 4. An electrical conductivity of the resistor layer 6 is at most 3.0 MS / m, preferably at most 2.5 MS / m, particularly preferably 2.3 MS / m. Also in this exemplary embodiment, the thickness d of the resistor layer 6 is less than 1 mm. As described above, eddy currents can thus be effectively suppressed, which allows operation of the axial flux machine 1 at higher rotational speeds.FIG. 7 is a schematic illustration of a return plate 4 of an axial flux machine 1 according to a fourth exemplary embodiment of the invention. The axial flux machine 1 according to the fourth embodiment of the invention is almost identical to the first embodiment, with buried magnets 5b being used instead of surface magnets 5a.Buried magnets 5 bare arranged in depressions 7 of the return plate 4 and are covered by pole segments 4 a, for example made of soft magnetic composite (SMC) or formed as a laminated core. Pole gaps 11 are located between the depressions 7, the pole gaps 11 being designed to increase resistance. In the fourth exemplary embodiment, grooving is effected with grooves 8 extending in the circumferential direction 300 in a manner analogous to the first exemplary embodiment. Thus, the same advantages as described above for surface magnets 5 acan also be achieved for buried magnets 5 b.FIG. 8 is a schematic illustration of a return plate 4 of an axial flux machine 1 according to a fifth exemplary embodiment of the invention. The axial flow machine 1 according to the fifth embodiment of the invention is almost identical to the fourth embodiment, wherein instead of grooves 8 for resistance increase, a resistance layer 6 is provided as described in the third embodiment. The formation of the resistance layer 6 thus takes place in the pole gaps 11, whereby also in this exemplary embodiment a reliable suppression of eddy currents and therefore an operation of the axial flux machine 1 at higher rotational speeds is made possible.

Claims

Axial flux machine (1) having - a stator (2) which extends along a central axis (100), and - at least one rotor (3) which is arranged next to the stator (2) with respect to the central axis (100), and comprises a flux return disc (4) on which magnets (5a, 5b), in particular permanent magnets, are arranged for forming a plurality of rotor poles, - wherein one of the two end sides (7a) of the flux return disc (4) faces the stator (2) and forms an air gap side, characterized in that - the air gap side of the flux return disc (4) is formed at least in sections in a grooved manner or with increased surface resistance for suppressing eddy currents.Axial flux machine (1) according to Claim 1, characterized in that the rotor poles of the return plate (4) are grooved in the circumferential direction (300) over the entire rotor pole in the case of surface magnets (5a) or are grooved in the pole gaps in the case of buried magnets (5b).Axial flow machine (1) according to one of the preceding claims, characterized in that the grooving is formed by grooves (8) which extend in the circumferential direction (300) and are spaced apart from one another in the radial direction (200) with respect to the central axis (100), wherein the grooves (8) of the grooving are of annular configuration in the case of surface magnets (5a).Axial flux machine (1) according to Claim 1, characterized in that the rotor poles of the return plate (4) are designed to be resistance-increased over the entire surface in the case of surface magnets (5a) or are designed to be resistance-increased in the pole gaps (11) in the case of buried magnets (5b).Axial flux machine (1) according to one of the preceding claims, characterized in that the surface increase in the resistance of the rotor pole or pole section forms a resistance layer (6) which has a higher specific electrical resistance than the remaining return plate (4).Axial flow machine (1) according to claim 5, characterised in that the resistance layer (6) has an electrical conductivity of at most 3.0 MS / m, preferably at most 2.5 MS / m, particularly preferably 2.3 MS / m.Axial flow machine (1) according to claim 3, 5 or 6, characterised in that the grooves (8) or the resistance layer (6) have a depth or layer thickness of less than 1 mm.Axial flux machine (1) according to one of the preceding claims, characterized in that the surface increase in resistance of the pole or pole section is formed by a surface coating, by alloying of the surface of the return plate (4), in particular with carbon, silicon, aluminium or phosphorus, or by a separate covering layer (9) or covering lamination, in particular a ferromagnetic covering layer or covering lamination, especially in particular made of electrical sheet metal.Axial flux machine (1) according to one of the preceding claims, characterized in that the magnets (5a, 5b) in the case of surface magnets (5a) are arranged on the grooved or surface resistance-increased air gap side of the return disk (4).Axial flow machine (1) according to one of the preceding claims, characterized in that the return disk (4) is a solid steel disk.Axial flow machine (1) according to one of the preceding claims, characterized in that the rotor (3) is surrounded by a bandage.Axial flux machine (1) according to one of the preceding claims, characterized in that, in the case of buried magnets (5b), depressions (7) are formed on the air gap side of the return plate (4) for receiving the buried magnets (5b) and for receiving in each case one pole segment (4a) covering the buried magnets (5b).Axial flux machine (1) according to one of the preceding claims, characterized in that the stator (2) has a multiplicity of individual teeth (10), which are each surrounded by a coil winding.

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