Axial flux machine with integrated braking device

Abstract

The invention relates to an axial flux machine with an integrated braking device, wherein a disk-shaped stator 1 with stator teeth 2a, 2b and electrical windings 3 arranged around the stator teeth are provided and a rotor 14 is arranged along a central axis next to the stator and is designed to be rotatable about the axis, and the rotor 14 has a magnet 6 and a rotor yoke 7, made in particular from a soft magnetic material, wherein the magnet 6 of the rotor is arranged at least partially on the side of the rotor facing the stator and the rotor yoke is arranged at least partially on the side of the rotor facing away from the stator.In this case, a brake body 10 is provided, wherein the brake body 10 is suitable for braking the rotating rotor and between the rotor and the, in particular stationary, brake body 10 a suitably sealed space 40 is provided for receiving a magnetorheological fluid 11, so that the rotor 14 can be braked via the magnetorheological fluid 11 on the brake body 10.

Description

The invention relates to an axial flux machine with an integrated braking device, wherein at least one disk-shaped stator with stator teeth and electrical tooth windings is provided and a rotor is provided which is arranged along a central axis next to the stator and is designed to be rotatable about the axis, and the rotor has at least one magnet and a rotor return circuit, in particular made of a soft magnetic material, wherein the magnet of the rotor is arranged at least partially on the side facing the stator and the rotor return circuit is arranged at least partially on the side of the rotor facing away from the stator.Various concepts for axial flow machines are known from the prior art. In some documents, reference is also made to the possibility of combining an axial flow machine with a brake. By way of example, mention may be made here of the publication US 2020 / 0343788A1, which shows an electric motor with an integrated brake.The solutions known from the prior art do not meet the requirements of compact and highly integrated applications. In particular, many applications require high active and passive torques with a simultaneously very high torque density. In particular, continuously variable control of the brake is desirable. The solutions from the prior art are technically more complicated.It is therefore an object of the present invention to develop an axial flow machine with an integrated brake device which solves the problems from the prior art and operates efficiently and reliably.The object is achieved by a device according to claim 1 and a method according to claim 7.According to a particular embodiment of the invention, an axial flux machine with an integrated brake device is provided, wherein a disk-shaped stator with stator teeth and electrical windings arranged around the stator teeth are provided. Along a central axis, a rotor arranged around the axis and designed to be rotatable is provided next to the stator. The rotor has at least one magnet, in particular at least one magnetic pole pair, and a rotor return circuit, in particular made of a soft magnetic material, connected in a rotationally fixed manner to the at least one magnet, in particular the at least one magnetic pole pair, wherein the at least one magnet of the rotor is arranged on the side facing the stator and the rotor return circuit is arranged on the side of the rotor facing away from the stator. Furthermore, a brake body is provided, wherein the brake body is suitable for braking the rotating rotor and a suitably sealed space for accommodating a magnetorheological fluid is provided between the rotor and a fixed brake body, so that the rotor can be braked on the brake body via the magnetorheological fluid.According to one embodiment, the invention makes it possible to realize a drive machine which simultaneously also contains a brake.According to a special embodiment, the properties of the magnetorheological fluid and thus a braking effect are controlled by influencing and utilizing the rotor leakage flow.According to a particular embodiment, no further windings or turns are required for the realization of the brake, as in the prior art.According to a particular embodiment of the invention, the stator has two stator units which are configured separately from one another and the stator teeth of the stator have windings which are each arranged around the stator teeth. The rotor has two rotor units arranged in the axial direction, between the two stator units, wherein each rotor unit has in each case, as part of the rotor return of the rotor, a rotor return and in each case, as part of the magnet of the rotor, at least one magnet connected in a rotationally fixed manner to the respectively assigned rotor return. The rotor units are mounted rotatably with respect to one another.According to a particular embodiment of the invention, each rotor unit has a radial outer surface and / or a radial inner surface and the two rotor units are arranged such that, in the axial direction between the magnet of the first rotor unit and the magnet of the second rotor unit, the rotor return circuit of the first rotor unit and the rotor return circuit of the second rotor unit are arranged back-to-back, separated by a gap, for example by an air gap. The two rotor units can be rotated with respect to one another about the axial axis and the sealed space for accommodating a magnetorheological fluid is provided in the region of the rotor units on the radial outer surface and / or radial inner surface adjacent to the rotor return of the rotor, in particular adjacent to at least one of the rotor return of the rotor units.According to a particular embodiment of the invention, the sealed space is arranged as a substantially annular space between the brake body and at least one of the rotor units.According to a particular embodiment of the invention, the rotor return circuit is configured in segmented fashion, so that a plurality of air gaps are provided between the segments of the rotor return circuit in the circumferential direction. The segments in the return path can be arranged, for example, between the individual magnetic poles.According to a particular embodiment of the invention, the air gaps between the segments of the rotor return circuit extend in the tangential direction.According to a particular embodiment of the invention, a method for operating an axial flow machine with an integrated brake device is provided. In this case, a disk-shaped stator with stator teeth and electrical tooth windings is provided, wherein the stator teeth each have one of the electrical windings and a rotor is provided which is arranged along a central axis next to the stator and is designed to be rotatable about the axis. The rotor has at least one magnet, in particular at least two magnetic poles, and a rotor return circuit, in particular made of a soft magnetic material, wherein the at least one magnet of the rotor is arranged on the side facing the stator and the rotor return circuit is arranged on the side of the rotor facing away from the stator. A brake body and a magnetorheological fluid are furthermore provided and, by suitable energization of the windings of the stator, the magnetorheological fluid is changed in its rheological property in such a way that, on the other hand, the rotor is braked by friction between the rotor and the magnetorheological fluid and friction between the brake body and the magnetorheological fluid.According to a particular embodiment of the invention, the stator has two stator units and the rotor has two rotor units and the two rotor units are arranged between the two stator units along the central axis. Each rotor unit is arranged in the axial direction and the rotor return is connected in a rotationally fixed manner in each case to the at least one magnet or magnetic pole pair. The rotor return circuit is made of a soft magnetic part. Each rotor unit has a radial outer surface and / or a radial inner surface, and the two rotor units are arranged such that, in the axial direction between the magnet of the first rotor unit and the magnet of the second rotor unit, a soft magnetic part of the first rotor unit and a soft magnetic part of the second rotor unit are arranged back-to-back separated by an air gap. The two rotor units are rotatable relative to one another about the axial axis and the sealed space for accommodating a magnetorheological fluid is provided in the region of the rotor units and adjacent to the soft magnetic parts of the rotor on the radial inner and / or outer surface. The energization of the two stator units for adjusting the brake takes place in such a way that the two rotor units rotate with respect to one another and the magnetic poles of the two rotor units therefore likewise shift with respect to one another.According to a particular embodiment of the invention, the two rotor units are rotated relative to one another between two positions, wherein the first position corresponds to a first magnetic orientation of the permanent magnets of the first rotor unit and of the second rotor unit arranged in the axial direction and the second position corresponds to a second magnetic orientation of the permanent magnets of the first rotor unit and of the second rotor unit arranged in the axial direction and corresponds to north-south and south-north or vice versa.The invention is explained in more detail below with reference to non-limiting possible embodiments. The following are shown: FIG. 1 is a general schematic view of an axial flow machine FIG. 2 is another general schematic view of an axial flow machine FIG. 3 shows a schematic view of a detail of a stator of an axial flux machine FIG. 4 shows a schematic view of a first embodiment of an axial flow machine with a magnetorheological brake FIG. 5 shows a schematic view of a second embodiment of an axial flow machine with a magnetorheological brake FIG. 6 shows a schematic view of a third embodiment of an axial flow machine with a magnetorheological brake FIG. 7 shows a schematic view of a detail of an axial flow machine with a magnetorheological brake FIG. 8 shows a schematic view of a detail of a second embodiment of an axial flow machine with a magnetorheological brakeBy way of introduction, it should be noted that, in the embodiments described differently, identical parts are provided in part with identical reference symbols or identical component names, it being possible for the disclosures contained in the entire description to be transferred analogously to identical parts with identical reference symbols or identical component names. The position information selected in the description, such as, for example, top, bottom, side, etc., also relates to the directly described and illustrated figure. These position information should be transferred analogously to the new position in the event of a change in position.FIGS. 1 to 3 show an exemplary configuration of an axial flow machine, the features shown are not restrictive for the scope of protection. FIG. 1 shows a first variant embodiment of an axial flux machine comprising at least one stator 1 and at least one rotor 4.Magnets 6, for example permanent magnets, are arranged on the rotor 4 or the rotors 4. The at least one stator 1 and the at least one rotor 4 are arranged in an optionally present motor housing, as is indicated in dashed lines in FIG. 1.Variants of the axial flux machine with multiple stators can also be used. Such an embodiment is schematically shown in FIG. 2. Here, two stators 1a and 1b are used. The stators 1a and 1b have respective associated stator teeth 2a and 2b. In the embodiment variant shown in FIG. 2, only a single rotor 4 is used. The rotor 4 is arranged between the two stators 1 aand 1 b, as viewed in the axial direction. The rotor 4 is equipped with a plurality of magnets 6 (permanent magnets) on both sides. The two stators 1 aand 1 bare arranged such that return elements or a stator yoke are each arranged on the side of the respective stator 1 aand 1 bfacing away from the rotor 4.FIG. 3 shows a further possible variant embodiment of a stator 1 for an axial flux machine. The stator 1 has a plurality of stator teeth 2. The stator teeth 2 have end portions in a direction along a rotation axis 42 that form a tooth tip and a tooth root. The stator teeth 2 are each wound around by a coil winding 3 (indicated only schematically in FIG. 3 ). The tooth bodies lie in the axial direction along the axis of rotation 42 on an, in particular plate-shaped, stator return 9 or are at least partially inserted into the stator return 9, for which purpose the stator return 9 has corresponding recesses in which the tooth bodies are preferably received in a form-fitting manner. The stator return 9 is preferably formed in one piece. According to a further embodiment, the stator return 9 is preferably formed integrally with the stator teeth 2. According to a further possible embodiment, the stator return coil 9 is wound from an electric sheet.According to one possible embodiment, an electrical insulation, not shown in any more detail, is arranged at least between the coil winding 3 and the tooth body of the stator teeth 2. The number of stator teeth 2 shown in the figures is not to be understood as limiting. Rather, the number thereof depends on the respective circumstances in the use of the stator 1 on the desired performance characteristics of the application.The stator teeth 2 are arranged uniformly distributed over the circumference of the stator 1. The stator teeth 2 have, for example, an at least approximately trapezoidal cross-sectional area as viewed in the direction of the axis of rotation 42.In FIG. 3, flux collection elements 6 aare shown or removed at a location in order to allow a better view of the stator teeth 2.The stator return path 9, in particular an annular stator return path, of the stator 2 can consist of a material customary for this purpose. Conclusions are made in particular from soft magnetic materials. According to a particular embodiment, the stator teeth 2 can be embodied integrally or integrally with the stator return 9, which is also sometimes referred to by the person skilled in the art as a stator yoke.Both in FIG. 4 and in FIG. 5, axial flow machines are shown which are configured for actuating a magnetorheological brake for a passive torque request. The electric motor used, which is designed as an axial flow machine, actively provides torques.The energization of the stator and / or the permanent magnets produces a magnetic field which is guided in a magnetic circuit comprising rotor and stator. By changing the magnetic circuit, the field across the magnetorheological fluid can be changed. As a result, the magnetorheological fluid in the gap between the rotor and the stator changes its properties. In this case, the shear stress in the fluid can be regulated.In this respect, FIG. 4 shows an embodiment of an axial flow machine with a magnetorheological brake. A stator 1 with stator teeth 2 aand stator return / stator yoke 2 band electrical windings 3 arranged around the stator teeth 2 aare illustrated. The components flux collector / pole shoe 9, stator teeth and stator return 2 bare manufactured in particular from soft magnetic material. The rotor 4 of the E-machine is spaced apart from the stator by an air gap 5 and has magnets 6 and a rotor return circuit 7 made of soft magnetic material. The rotor and the stator together form an electric machine in the construction of an axial flux machine. The rotor 4 is designed to be rotatable and has a suitable drive torque of the axial flux machine. The rotor 4 of the drive machine is connected via a shaft rotatable about the axis of rotation 42 to a rotor 8 of the brake and is set in rotation together therewith. Furthermore, a brake body 10 or a separating plane or capsule for a magnetorheological fluid 11, in particular a liquid, which is arranged between brake body 10 and rotor 8 of the brake, is provided. Between the brake body 10 and the rotor 8 of the brake, a space 40 is thus provided for receiving the magnetorheological fluid 11 which is correspondingly sealed, in particular between the rotating rotor 8 of the brake and the brake body 10.According to a particular embodiment, the brake body 10 is connected to the stator return 2 bin a rotationally fixed manner, for example in a positive-locking manner. The magnetorheological fluid or magnetorheological fluid 11 is provided between the stationary brake body 10 and the rotor of the brake 8. The magnetorheological fluid can be a liquid or else a powder mixture or another liquid, gaseous or solid / pulverulent medium. By suitable energization of the stator 1 of the axial flux motor, in particular by suitable energization of the windings 3, the magnetic field is changed via the magnetorheological gap of the axial flux machine, as a result of which the rheological properties of the magnetorheological fluid 11, in particular the toughness and / or the flowability, of the fluid in turn change. This change in the properties of the magnetorheological fluid 11 is optimized to the effect that in particular the toughness of the fluid 11 is increased, as a result of which the rotor 4 can be braked by friction of the magnetorheological fluid 11 between the rotor of the brake 8 and the brake body 10. A suitable braking torque is thereby generated at the rotor shaft 4.FIG. 5 shows a further possible embodiment of an axial flow machine with an integrated brake device.A realization of such a system enables both a motor operation and a brake operation, in that the axial flux motor is embodied with two stators 12, 13 and a rotor 14 arranged therebetween. The rotor is generally fastened, for example rotationally fixedly, at least partially to a rotor shaft 33 rotatable about a rotation axis 43. A magnetorheological fluid 16 is arranged on the radial rotor shell surface 15. Between the brake body 10 and the rotating rotor 14, the space 37 for receiving a magnetorheological fluid 16 is sealed by means of suitable seals 37 a. The rotor has two rotor units 17, 18. Each of the rotor units 17 and 18 has magnets 19, 20 and a soft magnetic part 21, 22 as a rotor return circuit. The two rotor units 17, 18 are arranged (back to back) such that the magnets 19, 20 face the respective stator 12 or 13. The first rotor unit 17 has on its soft magnetic component 21 of the rotor return circuit a, in particular plate-shaped and / or plate-shaped and / or cup-shaped, projection 17 a,which together with the brake body 10 defines a space 37 for receiving a magnetorheological fluid 16, for example a magnetorheological fluid or a powder. According to a particular embodiment, the first rotor unit 17 is connected to the shaft in such a way that torque can be transmitted to a downstream component and / or assembly. According to a particular embodiment, the fastening is designed to be rotationally fixed. According to a particular embodiment, the second rotor unit 18 is partially rotatably mounted on the shaft 33 or connected thereto. For motor operation, the rotor arrangement is selected such that the following components are arranged in the axial direction along the axis 43: magnets 19 of the first rotor unit 17 oriented with polarity NS (north-south) adjacent to the soft magnetic component 21 of the first rotor unit. A gap 23 is then arranged in the axial direction to the second rotor unit 18 and then adjacent to the soft magnetic component 22 of the second rotor unit and adjacent thereto the magnet 20 of the second rotor unit 18 likewise with polarity NS. This embodiment is shown schematically in FIG. 5 with the opposing magnetic poles in a detailed illustration. The magnetic circuit closes again from one stator 12 via the two rotor units 17, 18 to the second stator 13 and via the next pole. For the actuation of the magnetorheological fluid, the change of polarity is necessary at a rotor unit 17 or 18 so that the two rotor units are rotated relative to each other and thus the magnetic circuit closes via the soft magnetic components of the rotor 14, via the magnetorheological fluid, to the housing or to the brake body 23. The relative rotation of the two rotor units is achieved by a different actuation of the two stators 12, 13. In particular, by rotatably mounting the second rotor unit 18 on the shaft 33, the rotor unit 17 and the rotor unit 18 are rotated with respect to one another by suitable actuation of the stators. Suitable mechanical stops (not shown in the schematic drawings) of the second rotor unit 18 on the shaft 33 allow, according to a particular embodiment, the rotation of the second rotor unit 18 on the shaft 33 and, on the other hand, a transmission of torque to the shaft 33 is possible by a coupling to the shaft, in particular the mechanical stops. The rotor components are oriented by the rotation in the axial direction, for full actuation of the magnetorheological brake, as follows: magnet / magnets 19 of the first rotor unit with polarity NS, adjacent thereto the soft magnetic components 21 of the first rotor unit and subsequently an air gap 23 and subsequently the soft magnetic components 22 of the second rotor unit and magnet / magnets 20 of the second rotor unit 18 with polarity SN. This embodiment is likewise schematically illustrated in FIG. 5 with the opposite magnetic poles in a detailed illustration. The differential axial polarity of the magnets urges the magnetic field lines outwardly into the magnetorheological fluid 16 thereby causing a change in the rheological properties of the fluid. The controlled magnetic flux, which, as already explained in connection with FIG. 5, is conducted through the magnetorheological fluid, controls the rheological properties, i.e. in particular the flowability and / or the shear strength and / or toughness, of the fluid. When the toughness is increased, a braking torque can be generated by braking the rotor and the rotating rotor can be braked by the brake body via the magnetorheological fluid.FIG. 6 shows a further embodiment which has a special embodiment of the rotor yoke or of the rotor return circuit 24. The axial flux machine has a first stator 27 and a second stator 28. In this case, a first rotor unit 29 and a second rotor unit 30 are provided, which are spaced apart by an air gap 31. Each rotor unit has its own rotor return circuit. Each rotor return of the two rotor units is divided into individual rotor return segments 24 a, 24 b. The segmentation produces pie-piece-like segments having gaps or slots arranged between the segments, which extend in the radial direction with respect to the axis of rotation 44. In this way, segments separated over the circumference are produced which, with respect to the axis of rotation 44, have boundary surfaces in radial and axial extension. Between the segments of the rotor yoke, air gaps 26 are formed by the segmentation for guiding the magnetic flux. These air gaps are distributed over the circumference and are designated by reference numerals only by way of example. The air gaps are arranged tangentially. The rotor units 29, 30 are partially mounted rotationally fixedly on a separate shaft(s), as is known to the person skilled in the art from the prior art. An example of this rotationally fixed bearing would be the embodiment via stops, so that a certain angle of rotation between the rotor units 29, 30 is possible. These details are not referenced for clarity. The magnetic flux is additionally influenced by the segmentation, so that the magnetic flux closes via the magnetorheological fluid mainly across the gap. As a result, the magnetorheological fluid is activated even more effectively than without a gap. With the same magnet volume, a higher braking torque can be achieved with this. The characteristic, i.e. in particular the response behavior, of the brake can be influenced by the width of the slots, i.e. the distance between the segments of the rotor return. This embodiment of the rotor yokes thus reinforces and significantly influences the functional principle of the magnetorheological brake. Without segmentation, in the braking mode, the magnetic flux would close mainly within the rotor return 24 from one magnetic pole to the next magnetic pole. This would result in a weaker effect on the magnetorheological fluids / liquid or the powder. As described in the other exemplary embodiments, the rotors are rotated with respect to one another by suitable energization of the stators. Not shown in this illustration are the space for receiving a magnetorheological fluid and the brake bodies, these construction spaces being indicated by dashed lines.FIG. 7 shows an axial view of the rotor 4 of a further possible embodiment of an axial flow machine with an integrated brake device in a schematic illustration. The segments 24 aof the rotor return circuit 24 are shown here. On the segments 24a, a ring 32 (shown here in section) is connected to the segments 24a, for example by using a welding process. The ring can be designed similar to the projection 19a in FIG. 5. On the outside, a fixed brake body 10 is arranged, on which the first rotor unit 17 ais braked. Between the brake body 10 and the ring 32, a suitable space 40 for receiving the magnetorheological fluid and the magnetorheological fluid 11 are arranged. On the inner diameter, the rotor return is connected, for example, rotationally fixedly to a rotor shaft 33.FIG. 8 shows an embodiment similar to FIG. 7. However, here the ring 32, the space 40 and the magnetorheological fluid 11 arranged therein and the brake body 10 are arranged on the inner diameter. The ring 32 is connected in a rotationally fixed manner to the segments 24 aof the rotor return circuit 24. The rotor is connected to the rotor shaft, for example, outside the sectional plane.As magnetorheological fluid or magnetorheological fluid, liquids, pastes, powders or others known from the prior art are used. For example, carbonyl iron powders or similar fine materials are used directly or as magnetically polarisable particles dispersed in a carrier liquid.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedUS 2020 / 0343788A1

[0002]

Claims

Axial flux machine with integrated braking device, wherein a disk-shaped stator with stator teeth and electrical windings arranged around the stator teeth are provided and a rotor arranged along a central axis next to the stator and designed to be rotatable about the axis is provided, and the rotor has at least one magnet and a rotor return circuit, in particular made of a soft magnetic material, wherein the at least one magnet of the rotor is arranged at least partially on the side facing the stator and the rotor return circuit is arranged at least partially on the side of the rotor facing away from the stator, characterized in that a brake body is provided, wherein the brake body is suitable for braking the rotating rotor and a suitably sealed space for accommodating a magnetorheological fluid is provided between the rotor and the, in particular fixed, brake body, such that the rotor can be braked on the brake body via the magnetorheological fluid and the magnetorheological fluid is controlled in its viscosity by the magnetic field which is impressed at least partially on the magnetorheological fluid by the axial flow machine, that is to say by the stator and / or rotor.Axial flux machine according to Claim 1, characterized in that the stator has two stator units which are designed separately from one another, and the rotor has two rotor units which are arranged in the axial direction, between the two stator units, each rotor unit having in each case, as part of the rotor return of the rotor, a rotor return and in each case, as part of the at least one magnet of the rotor, a magnet which is connected in a rotationally fixed manner to the respectively assigned rotor return.Axial flux machine according to claim 2, characterised in that each rotor unit has a radial outer surface and / or a radial inner surface and the two rotor units are arranged such that the rotor return of the first rotor unit and the rotor return of the second rotor unit are arranged back-to-back separated by an air gap in the axial direction between the magnet of the first rotor unit and the magnet of the second rotor unit, wherein the two rotor units are rotatable relative to one another about the axial axis and the suitably sealed space for accommodating a magnetorheological fluid is provided in the region of the rotor units on the radial outer surface and / or radial inner surface adjacent to at least one of the rotor return of the rotor units.Axial flow machine according to one of the preceding claims, characterized in that the space for receiving a magnetorheological fluid is arranged as a substantially annular space between the brake body and at least one of the rotor units.Axial flux machine according to one of the preceding claims, characterized in that the rotor return circuit is configured in segmented fashion, so that a plurality of air gaps are provided between the segments of the rotor return circuit in the circumferential direction.Axial flux machine according to Claim 5, characterized in that the air gaps between the segments of the rotor return circuit run in the radial and axial directions.Method for operating an axial flux machine with an integrated braking device, wherein a disc-shaped stator with stator teeth and electrical tooth windings is provided and a rotor is provided which is arranged along a central axis next to the stator and is designed to be rotatable about the axis, and the rotor has at least one magnet and a rotor return circuit, in particular made of a soft magnetic material, wherein the magnet of the rotor is arranged on the side of the rotor facing the stator and the rotor return circuit is arranged on the side of the rotor facing away from the stator, characterized in that a braking body and a magnetorheological fluid are provided and, by suitable energization of the windings of the stator, the magnetorheological fluid is changed in its rheological property in such a way that, on the other hand, the rotor is braked by friction between the rotor and the magnetorheological fluid and friction between the braking body and magnetorheological fluid.Method for operating an axial flux machine with integrated brake device according to Claim 7, characterized in that the stator has two stator units and the rotor has two rotor units, and the two rotor units are arranged between the two stator units along the central axis, and each rotor unit is arranged in the axial direction and has a rotor return and connected in a rotationally fixed manner to the rotor return, one of the at least one magnets and each rotor unit has a radial outer surface and / or a radial inner surface, and the two rotor units are arranged in such a way that, in the axial direction between the magnet of the first rotor unit and the magnet of the second rotor unit, a rotor return of the first rotor unit and a rotor return of the second rotor unit are arranged back-to-back separately by an air gap, wherein the two rotor units are rotatable relative to each other about the axial axis and a suitably sealed space for accommodating a magnetorheological fluid is provided in the region of the rotor units on the radial inner and / or outer surface of at least one of the rotor units and the energization of the windings of the two stator units for adjusting the brake takes place such that the two rotor units rotate relative to each other.Method according to claim 8, characterised in that the two rotor units are rotated relative to one another between two positions, wherein the first position corresponds to a first magnetic orientation of the permanent magnets arranged in the axial direction of the first rotor unit in north-south and the second rotor unit in north-south and the second position corresponds to a second magnetic orientation of the permanent magnets arranged in the axial direction of the first rotor unit in north-south and the second rotor unit in south-north or vice versa.

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