Rotor device and electric motor having same

Abstract

The invention relates to a rotor device (10) for a DC current motor (100), comprising: a rotor shaft (14); a first layer which is formed on the rotor shaft (14) or on a core (30) attached to the rotor shaft; a second layer, wherein the second layer (22) is formed on an outer surface (24) of the first layer and forms a second magnetic layer (22), wherein the second magnetic layer (22) is in the form of a cylindrical jacket, wherein the cylindrical jacket of the second magnetic layer (22) has a thickness ((d2-d1) / 2), measured in the radial direction from a longitudinal axis (Z) of the rotor shaft, which is constant in the circumferential direction (P), and a length (L) measured in the axial direction parallel to the longitudinal axis (Z), wherein a ratio of the length (L) to a difference (d2-d1) between an outer diameter (d2) and an inner diameter (d1) of the second magnetic layer (22) has a value in a range between 4 and 15, inclusive of the limit values.

Description

[0001] ROTOR DEVICE AND ELECTRIC MOTOR HAVING THE SAME

[0002] DESCRIPTION

[0003] Technical area

[0004] Various aspects relate to a rotor device having at least one permanent magnet for an electric motor, in particular a brushless electric DC motor, and to such a motor.

[0005] Technical background

[0006] It is known that brushless direct-current motors (BLDC motors) can be used, among other things, to drive moving components in motor vehicles, such as sunroofs, etc. These generally consist of a rotor equipped with permanent magnets and a stationary stator with an iron core, which may have coils that generate a magnetic field when a controlled current is applied. BLDC motors can be operated in three-phase mode, for example.

[0007] Current BLDC motor designs feature a rotor with concentrically arranged, laminated steel sheets on the shaft leading from the motor to the gearbox. The individual permanent magnets, which are arranged circumferentially with alternating pole orientation, can be mounted on an outer surface of the respective steel sheet package in the case of SPM (surface permanent magnet) motors.

[0008] Alternatively, the permanent magnets of the IPM type (internal permanent magnet) can be inserted into holders arranged within the sheet steel package.

[0009] Magnetic adhesives are typically used to securely fix the permanent magnets in place in both SPM and IPM models by creating a permanent, high-strength bond. At least in the case of SPM models, an additional rotor cover is provided to prevent the permanent magnets from detaching from the rotor due to centrifugal force at high speeds.

[0010] However, the rotor concept described above is subject to several problems and disadvantages. Firstly, a large number of individual parts are required, making the assembly process for the rotor unit complex. This, in turn, increases costs. Furthermore, due to the different material composition, the rotor must be balanced, especially if the motor is designed for high speeds. This entails an additional assembly step and possibly additional counterweight.

[0011] Furthermore, in various applications described above, the number of magnetic poles can hardly be increased due to limitations in the aspect ratio (i.e., the height-to-width ratio) of the sintered magnets used in the BLDC motor. Typically, for example, six slots on the stator and four magnetic poles are used. However, an increase to, for example, eight magnetic poles would be highly desirable, for example, to reduce the influence of cogging torque during operation. Therefore, the conventional design can lead to insufficient efficiency.

[0012] Finally, the adverse noise development must also be considered. As described above, the additional number of parts in the rotor can lead to rotor imbalance, which can result in increased noise if not compensated for.

[0013] Composite or bonded magnets can offer a solution here. "Bonded" does not mean "glued," as described above in connection with sintered magnets. Rather, these are plastic-bonded, injection-molded, or die-cast magnets. They are created, for example, by embedding hard ferrite or rare-earth magnetic powder (in general, all sintered magnetic materials can be used for composite parts (ferrite, NdFeB, SmCo, etc.)) in thermoplastics (matrix material: PA6, PA12, PPS, etc.). The proportion of magnetic powder determines the magnetic and mechanical properties. The magnetic powder content can, for example, range from 84% to 94% (by weight). Bonded magnets offer a wide variety of possible shapes and geometries of magnetization, especially complex shapes.

[0014] JP 4701641 B2 and US 2018 / 109167 A1 describe a rotor magnet (permanent magnet) constructed from bonded layers. When assembled, this magnet extends around a rotor core attached to the rotor shaft. A soft magnetic ferrite layer adjacent to the rotor core has a roughly wave-like or sinusoidal thickness profile in the circumferential direction. A thinner, hard magnetic rare earth layer is located on the outside, which conforms to the wave-like surface on the inside and has a cylindrical shape on the outside. The rare earth layer forms eight regions of greater thickness in the "wave troughs," which, after magnetization, form eight magnetic poles.

[0015] Similarly, CN 115036092 A discloses a two-layer magnet structure with an inner ring made of soft magnetic material and an outer ring made of hard magnetic material. With appropriate external magnetization, the eight poles are provided in sections formed by trapezoidal shapes projecting inward from the outer ring (instead of sinusoidal shapes as above), so that the outer ring is thicker in these areas.

[0016] JP 2013-198220 A discloses a ring magnet constructed from two cylindrical layers for a rotor device for driving a magnetic storage hard disk.

[0017] There is a need to further improve the concept of a permanent magnet extending continuously around the rotor shaft in a ring, in particular to find a balance between excessive costs and adverse influence on magnetization in the assembled state due to the specific structure.

[0018] Presentation of various aspects of the invention

[0019] Aspects of the invention that address this need relate to a rotor device for a DC motor, for example for a roof drive of a vehicle, comprising a rotor shaft, a first layer formed on the rotor shaft or on a core attached to the rotor shaft, and a second layer, the second layer being formed on an outer surface of the first layer and forming a second magnetic layer.

[0020] The second magnetic layer has the shape of a cylindrical shell, wherein the cylindrical shell of the second magnetic layer has a thickness measured in the radial direction from a longitudinal axis of the rotor shaft, which is constant in the circumferential direction, and a length measured in the axial direction parallel to the longitudinal axis. A ratio of the length to a difference (d2 - d1) between an outer diameter d2 of the second magnetic layer and an inner diameter d1 of the second magnetic layer is a value that lies in a range between 4 and 15, including the limit values. The two diameters are measured perpendicular to an axis of the rotor shaft.

[0021] The second magnetic layer therefore extends continuously and uninterruptedly around the rotor shaft – there is no division into individual magnets that would have to be individually attached. The formation of poles, which are inherent in conventional individual magnets, is achieved by magnetization that depends on a position in the circumferential direction. Since aspect ratios of individual magnets are not important in this case, the corresponding restrictions are eliminated and it is easily possible to form eight or more poles in the circumferential direction of the cylindrical shell of the second magnetic layer. Furthermore, the second magnetic layer has a constant thickness in the circumferential direction. In the prior art, which also relates to annular magnetic structures, thickened portions are formed in the respective magnetic layer at the locations of the poles to be formed.This, in turn, allows for savings in valuable rare earth material where no thickening is required, namely between the poles. However, this poses a disadvantage: the corresponding mold and the magnetic field acting on it for magnetization must be perfectly matched and aligned, as misalignment can have significant consequences. Furthermore, imperfections in the mold can result in much more pronounced imbalances if the second magnetic layer has an 8-fold geometric symmetry, which does not occur when forming a layer of constant thickness, the formation of which is inherently easy to control.

[0022] Furthermore, it was found that under the widely varying thermal conditions during operation, the second magnetic layer in particular is subject to considerable stress, for example, due to the different thermal expansion between the first and second layers, as well as the rotor shaft and core. A cylindrical shell-shaped layer of constant thickness contributes to improved durability, avoiding "weak points" between individual thickened portions.

[0023] The present design of a rotor device is based on the special insight that the length of the cylindrical shell of the second magnetic layer in the axial direction of the rotor shaft is related to the alternating field acting from the opposite stator. The longer the cylindrical shell of the second magnetic layer, the stronger the externally applied magnetic field can be designed to achieve torque. This is offset by a certain thickness of the second magnetic layer, which must be kept as low as possible for cost reasons. If it is too thin, the acting magnetic field proves to be comparatively too strong. For this comparison, the hysteresis curve families known in the field should be used.The invention therefore aims to design the thickness of the second magnetic layer in such a way that the directly related difference (d2 - d1) between the outer diameter d2 and the inner diameter d1 of the second magnetic layer is in a ratio to the length that ensures that the high-frequency alternating magnetic fields applied do not lead to excessive demagnetization of the magnetic material as the operating time progresses. The specified value range for the ratio (length divided by twice the thickness) between 4 and 15 (including the limit values) has proven advantageous according to internal studies. At ratio values ​​above 15, the demagnetization is already too high, while at values ​​below 4, the cost of the magnetic material with the desired properties can become too high.

[0024] It should be noted that if we consider the ratio of length to simple thickness, the values ​​found to be advantageous are between 8 and 30.

[0025] Particularly preferred value ranges for the ratio of the length to the difference between the outer diameter and the inner diameter of the second magnetic layer can be specified according to specific embodiments for intervals from 6 to 10, inclusive of the limit values, more preferably for intervals from 7 to 8, inclusive of the limit values.

[0026] According to embodiments of the invention, the first layer and the second magnetic layer in the rotor device are each plastic-bonded using an injection or die-casting process. Without limiting the generality, the plastic binder used can preferably be one from the group comprising: polyamide 6 (PA6), polyamide 12 (PA12), polyamide 66 (PA66), PBT, PET, PPS, or even LCP. With plastic-bonded, initially isotropic bond magnets, at least in the case of the second magnetic layer, there is also relative flexibility in the shaping of the magnetic layers, which is ensured by proven injection or die-casting processes. For many materials (such as NdFeB), additional surface protection is not necessary.The fact that the achievable magnetic flux density is lower, at least due to the plastic content, is accepted, but this can be compensated for by a correspondingly slightly larger thickness of the second magnetic layer. The aforementioned plastic or resin materials have proven to be highly temperature-resistant and durable under the intended mechanical loads (high centrifugal forces, high cogging torques).

[0027] According to a further development, the first layer is a first magnetic layer formed from a material having a first value for a magnetic remanence, i.e. a magnetic flux density remaining after removal or switching off of an external magnetic field magnetizing the layer (remanence flux density BR = IJO MR for H=0, vector symbols omitted), where MR denotes the magnetization and po the magnetic field constant). The second magnetic layer is formed from a material having a second value for a magnetic remanence, wherein the second value for the magnetic remanence is greater than the first value. In particular, the material of the inner first magnetic layer can be a soft magnetic material and the material of the outer second magnetic layer can be a hard magnetic material.

[0028] The second magnetic layer thus maintains a very high residual magnetic flux density after magnetization, while the coercive field strength is also very high due to the material, reducing the risk of gradual demagnetization during operation. Furthermore, the second magnetic layer is positioned further radially outward and can therefore absorb the greater torque during operation. The first magnetic layer allows a magnetic field directed further inward (toward the rotor shaft) and outward, thus supporting the function of the outer magnetic layer.

[0029] In a preferred embodiment, the first magnetic layer is also formed in the shape of a cylinder shell.

[0030] Alternatively, the first layer can also be a non-magnetic layer, preferably a thermoplastic produced by injection or die-casting, but without any magnetic material. This is preferably PA66. This has been found to be a cost-effective alternative that still ensures the high torque and thermal and mechanical durability of the outer second magnetic layer.

[0031] According to a further embodiment of the rotor device, the first magnetic layer may comprise a soft magnetic material in the form of a ferrite material. This is an inexpensive magnetic material that provides the supporting function sufficiently.

[0032] The second magnetic layer can comprise a hard magnetic material in the form of a rare-earth magnet. These materials offer high magnetic energy density and coercivity, as well as a high Curie temperature, particularly in the case of samarium-containing materials. They have also proven particularly suitable for use in motors for driving roof systems (with the corresponding load and temperature requirements) when combined with plastic-bonded injection or die-casting.

[0033] Preferred materials are the rare earth magnet material from the group comprising: SmFeN, NdFeB or SmCo, whereby further alloying additives can be included in each case, with which in particular the temperature resistance of the respective magnetic state can be improved.

[0034] A further development of the rotor device provides that the first magnetic layer is formed directly on the rotor shaft or directly on the core attached to the rotor shaft, wherein the core is formed from layered steel sheets. This represents a particularly practical design.

[0035] According to a special embodiment of the rotor device, the second magnetic layer has a magnetization with at least 8 poles in the circumferential direction. This configuration has proven to be very advantageous. Aspects of the invention also relate to an electric motor having a rotor device configured as in the previously described embodiments. A corresponding stator device interacts with this rotor device. In particular, the stator device can be designed with a basic structure encompassing the rotor device and a number of stator teeth, each for receiving stator windings. For example, six slot sections can be formed between the stator teeth if the rotor unit is equipped with eight magnetic poles.By this structure, the invention provides an electric motor which can provide reduced noise generation due to the specific ratio of slot sections and poles.

[0036] The rotor shaft can be rotatably supported in two or more bearings provided in a motor housing. The rotor shaft can be designed in one section, e.g., an end region, as a worm shaft that interacts with a worm gear in a worm gear to produce a suitable gear ratio for the specific application.

[0037] The electric motor may be a brushless motor, preferably a BLDC motor for driving a roof system in a vehicle, in particular a motor vehicle, the roof system may include a sunroof, a sunroof, a sunblind or the like.

[0038] Aspects of the invention also provide a method for manufacturing a rotor device. This comprises:

[0039] Providing a rotor shaft;

[0040] Providing a first mold enclosing the rotor shaft;

[0041] Injection or pressure casting a first material into the mold to form a cylinder-shell-shaped first layer on the rotor shaft;

[0042] Removing the first mold and providing a second mold enclosing the rotor shaft; injecting or die-casting a second magnetic material into the mold to form a cylindrical second magnetic layer on the first layer;

[0043] Magnetizing the second magnetic layer depending on a position in the circumferential direction after completion of injection or die molding to form a number of poles.

[0044] In a further development of the method, in the step of injection or die-casting the first material, a first magnetic material is used to form a first magnetic layer, wherein the material of the first magnetic layer has a first value for a magnetic remanence, and the second material of the second magnetic layer has a second value for a magnetic remanence, wherein the second value for the magnetic remanence is greater than the first value. Furthermore, during the injection or die-casting of the first magnetic material into the mold, the first magnetic layer is magnetized depending on a position in the circumferential direction in order to support the formation of a number of poles in the second magnetic layer.

[0045] Magnetizing the first magnetic material during the injection or die-casting process and magnetizing the second magnetic material after the injection or die-casting process has proven particularly advantageous. While magnetizing the first magnetic material or the first magnetic layer during the injection or die-casting process eliminates a separate process step and thus saves production time and costs, magnetizing the second magnetic material only after the injection or die-casting process in this case allows for high magnetic isotropy to be achieved, resulting in a comparatively higher magnetic flux density, which is particularly relevant in the case of the second magnetic material or the second magnetic layer.Further advantages, features, and details of the various aspects emerge from the claims, the following description of preferred embodiments, and the drawings. In the figures, like reference numerals designate like features and functions.

[0046] Short description of the drawings

[0047] They show:

[0048] Fig.1 shows in perspective, side and cross-sectional view (from right to left) a section of a rotor device with rotor section and rotor shaft according to a conventional example;

[0049] Fig. 2 shows a perspective sectional view of a rotor device with a rotor section and a rotor shaft according to an embodiment of the invention;

[0050] Fig. 3 is a sectional view through a rotor device arranged in a stator according to an embodiment of the invention with a degree of magnetic flux density shown in grayscale;

[0051] Fig. 4 is a diagram of the magnetic flux density as a function of a position angle in the circumferential direction on the surface of the rotor magnet for three different ratios of an axial length of the rotor magnet to the thickness of an outer hard magnetic layer in the radial direction, according to an embodiment of the invention.

[0052] Preferred embodiment(s) of the invention

[0053] In the following description of a preferred embodiment, it should be noted that the present disclosure of the various aspects is not limited to the details of the construction and arrangement of the components as shown in the following description and in the figures. All embodiments, even those not shown in the figures, can be practiced or carried out in various ways. It should also be noted that the phraseology and terminology used herein is used for the purpose of specific description only and should not be construed as such by one skilled in the art.Furthermore, in the following description, the same reference numerals in the figures denote the same or similar features or objects, so that in some cases a repeated detailed description of the same is omitted in order to maintain the compactness and clarity of the illustration.

[0054] Fig. 1 shows an example of a conventional rotor device 50 for a brushless electric motor (BLDC). A rotor shaft 14 has a core 54, onto whose outer surface a total of four spaced-apart magnets 56 are bonded within recesses. These magnets are sintered magnets. In this exemplary embodiment, the magnets 56 are each formed by a magnetic north pole (N) and a magnetic south pole (S), which, however, are not shown for the sake of simplicity. In total, the arrangement of the four magnets 56 shown thus provides eight poles. The eight poles (N, S) are oriented alternately to obtain a desired magnetic field.In order to prevent the magnets 56 from becoming detached at the high speeds and associated high centrifugal forces often encountered in BLDC motors, a holder 58 is provided on the outside of the rotor device, which holder extends all around and at least partially covers the magnets 56 and holds them in position.

[0055] Fig. 2 shows a schematic representation of an embodiment of a rotor device 10 according to the present invention. The perspective view shows a cross-section perpendicular to a longitudinal axis Z of a rotor shaft of the rotor device 10. A first magnetic layer 20 is formed directly on the rotor shaft 14 by injection molding or die casting. This layer has an outer surface 26 with a radius in the circumferential direction P that is constant with respect to the longitudinal axis Z. The outer surface 24 of the first magnetic layer has an outer diameter d1. The rotor shaft 14 itself is cylindrical, so that the first magnetic layer 20 overall has the shape of a cylinder shell.

[0056] The first magnetic layer 20 has a length L along the longitudinal axis Z. For production, a mold is provided through which the rotor shaft 14 extends. The corresponding mold can be positioned within a magnetization device (not shown), which creates a suitable multi-pole magnetic field that permeates the mold. Meanwhile, a mixture of thermo-elastomers and / or thermoplastic resins and ferrite powder is injected into the mold (injection or die casting). The ferrite particles align themselves in the magnetic field, so that a residual magnetization remains after cooling or curing. One advantage is that the magnetic material can be injection-molded directly onto the rotor shaft 14. Subsequent assembly is no longer necessary.

[0057] A second magnetic layer is formed on the first magnetic layer 20 or on its outer surface 24. It also has a cylindrical outer surface 28 with a constant radius in the circumferential direction P with respect to the longitudinal axis Z. The outer surface 28 of the second magnetic layer has a constant outer diameter d2. The inner diameter d1 of the second magnetic layer 22 corresponds to the outer diameter of the first magnetic layer 20. Due to this structure, the second magnetic layer 22 has the overall shape of a cylindrical shell, similar to the first magnetic layer 20, but is significantly thinner (the thickness is (d2 - d1) / 2).

[0058] Along the longitudinal axis Z, the second magnetic layer 22 has the same length L as the first magnetic layer 20. In principle, however, the lengths can also be different. For the following discussion, however, the length L of the second magnetic layer 22 is relevant. To produce the second magnetic layer 22, a casting mold is again provided through which the rotor shaft 14 with the first magnetic layer 20 formed thereon extends. A mixture of thermoelastomers and / or thermoplastic resins and a metal powder made of rare earth magnet material is injected into the casting mold (injection or die casting). An alloy of SmFeN particles (samarium iron nitride) is preferred here. This material has a high resistance to demagnetization and is therefore particularly suitable.SmFeN / ferrite hybrid magnets, on the one hand, or SmFeN / NdFeB hybrid magnets, on the other, are also possible. The latter offer particularly excellent properties in terms of energy density and coercivity. Blends of thermoelastomers and / or thermoplastic resins with a metal powder containing NdFeB or SmCo particles are also particularly suitable.

[0059] In this case, the desired magnetization is only achieved after cooling or curing. One advantage is that the magnetic material can be injection-molded directly onto the outer surface 24 of the first magnetic layer 20. Subsequent assembly is no longer necessary.

[0060] Fig. 3 shows a cross-section through a brushless electric motor 100 with the rotor device from Fig. 2 and a stator device 40 encompassing it. The grayscale representation shows the results of a simulation with regard to magnetic flux densities during motor operation. Reference numeral 26 designates two of a total of eight magnetic poles established during the magnetization described above. These eight magnetic poles formed in the second magnetic layer are opposite six slots or stator teeth 44 of the stator device 40. The stator device 40 has an outer base structure 42, from which the six stator teeth 44 extend inward toward the longitudinal axis Z of the rotor shaft 14. The stator teeth 44 each have a stator head 46, the head surface of which forms a slot with the outer surface 28 of the second magnetic layer 22.As described, the second magnetic layer 22 has an outer diameter d2 relative to the outer surface 28 and an inner diameter d1 relative to the inner adjacent outer surface 24 of the first magnetic layer 20. The inner diameter d1 of the second magnetic layer 22 is identical to the outer diameter d1 of the first magnetic layer 20, since both coincide. In this case, the thickness of the second magnetic layer 22 is constant (d2 - d1) / 2 in the circumferential direction P.

[0061] In the exemplary embodiment, the second magnetic layer 22 has, in particular, a length L along the longitudinal axis of 22 mm and a difference d2-d1 between the outer diameter d2 and the inner diameter d1 of the second magnetic layer of 3 mm. For geometric reasons, this difference corresponds to twice the thickness of the second magnetic layer 22. A ratio o mbetween the length L and the mentioned difference d2-d1 is in this case rounded to 7.3. It was found that from a value less than o m = 4, the increased costs for manufacturing the second magnetic layer in volume production (essentially material costs for the rare earth material) become too high compared to the improvement in motor service life due to the remaining residual magnetization. Furthermore, it was also found that starting at a value of more than Om = 15, the extent of demagnetization becomes too great due to the thin layer thickness compared to the energy density of the applied magnetic field, compared to the already manageable costs for the rare earth material.

[0062] Fig. 4 shows a plot of the magnetic flux density B determined over the outer circumference (angle θ) of the second magnetic layer 22 after magnetization for three cases. In Fig. 4, in particular, for the ideal case described above, a m = L / (d2-d1 ) = 7.3 (see the curve marked by arrow B), for the lower limit o m = 4 (see the curve marked by arrow A) and for the upper limit o m = 15 (see the curve marked by the arrow C) of the interval found here, the magnetic flux density ß before the start of motor operation is plotted against an angular position in the magnetic layer 22 in the circumferential direction P. It can be seen that for the upper limit o m= 15, ie a relatively thin second magnetic layer compared to the length, only a small budget of magnetic flux density B is available, which is reduced more quickly during operation by a constantly repeated hysteresis loop than in the case of o m = 7.3. From the upper limit at about o m = 15, the second magnetic layer reaches thicknesses which are associated with costs which, at the current stage, are no longer justified by the technical advantage.

[0063] REFERENCE SYMBOL LIST:

[0064] Rotor device

[0065] Rotor shaft first magnetic layer second magnetic layer

[0066] Outer surface (first magnetic layer)

[0067] pole, magnetic pole

[0068] outer surface

[0069] Stator device

[0070] Basic structure

[0071] Stator tooth

[0072] Stator head

[0073] Stator winding

[0074] Rotor device (conventional)

[0075] core

[0076] Sintered magnets

[0077] Electric motor bracket

Claims

Claims: 1 . A rotor device (10) for a DC motor (100), comprising: a rotor shaft (14); a first layer formed on the rotor shaft (14) or on a core attached to the rotor shaft (14); a second layer, the second layer being formed on an outer surface (24) of the first layer, the second layer being a second magnetic layer (22);wherein the second magnetic layer (22) has the shape of a cylindrical shell, wherein the cylindrical shell of the second magnetic layer (22) has a thickness ((d2-d1) / 2) measured in the radial direction from a longitudinal axis (Z) of the rotor shaft, which is constant in the circumferential direction (P) and a length (L) measured in the axial direction parallel to the longitudinal axis (Z), wherein a ratio of the length (L) to a difference (d2-d1) between an outer diameter (d2) of the second magnetic layer (22) and an inner diameter (d1) of the second magnetic layer (22) is a value which lies in a range between 4 and 15, including the limit values; 2. Rotor device (10) according to claim 1, wherein the first layer and the second magnetic layer (22) are each plastic-bonded in an injection or die-casting process using a plastic binder.

3. Rotor device (10) according to claim 1 or 2, wherein the first layer is a first magnetic layer (20) formed from a material having a first value for a magnetic remanence, the second magnetic layer (22) is formed from a material having a second value for a magnetic remanence, wherein the second value for the magnetic remanence is greater than the first value.

4. Rotor device (10) according to claim 1 or 2, wherein the first layer is a non-magnetic layer, preferably a thermoplastic.

5. Rotor device (10) according to one of claims 1 to 4, wherein the value of the ratio of the length (L) to the difference (d2-d1) is in a range between 6 and 10, including the limit values.

6. Rotor device (10) according to one of claims 1 to 5, wherein the value of the ratio of the length (L) to the difference (d2-d1) is in a range between 7 and 8, including the limit values.

7. Rotor device (10) according to one of claims 1 to 6, wherein the first layer also has the shape of a cylinder jacket.

8. Rotor device (10) according to one of claims 3, or 5 to 7, as far as dependent on claim 3, wherein the first magnetic layer (20) comprises a soft magnetic material, preferably a ferrite material.

9. Rotor device (10) according to one of claims 1 to 8, wherein the second magnetic layer (22) comprises a hard magnetic material, preferably a rare earth magnet material.

10. Rotor device (10) according to claim 9, wherein the rare earth magnet material is one of the group comprising: SmFeN, NdFeB or SmCo, including alloying additives.

11. Rotor device (10) according to one of claims 1 to 10, wherein the first layer is formed directly on the rotor shaft (14), or directly on the core (30) attached to the rotor shaft (14), wherein the core (30) is formed from layered steel sheets.

12. Rotor device (10) according to one of claims 1 to 11, wherein the second magnetic layer (22) has a magnetization with at least 8 poles (26) in the circumferential direction (P).

13. An electric motor (100) comprising: a rotor device (10) according to any one of the preceding claims; and a stator device (40).

14. Electric motor (100) according to claim 13, wherein the electric motor (100) is a brushless motor, preferably a BLDC motor for driving a roof system in a vehicle.

15. A method of manufacturing a rotor device, comprising: Providing a rotor shaft; Providing a first mold enclosing the rotor shaft; Injection or pressure casting a first material into the mold to form a cylinder-shell-shaped first layer on the rotor shaft; Removing the first mold and providing a second mold enclosing the rotor shaft; Injection or die-casting a second magnetic material into the mold to form a cylindrical second magnetic layer on the first layer; Magnetizing the second magnetic layer depending on a position in the circumferential direction after completion of injection or die molding to form a number of poles.

16. The method according to claim 15, wherein in the step of injection or pressure molding the first material, a first magnetic material is used to form a first magnetic layer, wherein the material of the first magnetic layer has a first value for a magnetic remanence, and the second material of the second magnetic Layer has a second value for a magnetic remanence, wherein the second value for the magnetic remanence is greater than the first value; and the first magnetic layer is magnetized during injection or compression molding of the first magnetic material into the mold depending on a position in the circumferential direction to assist in the formation of a number of poles in the second magnetic layer.

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