A rotor, an electric machine comprising the rotor, and a vehicle comprising the electric machine

Abstract

A rotor (100) is presented, which comprises: - a rotor hub (110) of a first material; - a plurality of magnet holders (120) of a second material, different from the first material, and the plurality of magnet holders (120) being attached to the rotor hub (110) about an axis of rotation (115) of the rotor; - a plurality of magnets (130) arranged about the axis of rotation (115), wherein each magnet (130) is arranged between two magnet holders (120) and held in place by the two magnet holders (120); wherein - each magnet holder (120) is attached to the rotor hub (110) by a joint (121) 10 comprising a male joint portion (122) of the magnet holder (120) fitted into a female joint portion (112) of the rotor hub (110); and - the male joint portion (122) comprises one or more cavities (125) confined within the male joint portion (122).

Description

[0001] A ROTOR, AN ELECTRIC MACHINE COMPRISING THE ROTOR, AND A VEHICLE COMPRISING THE ELECTRIC MACHINE

[0002] Technical field

[0003] The present invention relates to a rotor, and more specifically to a rotor to be used in an electric machine. The present invention further relates to an electric machine comprising the rotor, and to a vehicle comprising the electric machine.

[0004] Background

[0005] The following background description constitutes a description of the background to the present invention, which does not, however, necessarily have to constitute prior art.

[0006] An electric machine comprises a stator and a rotor, where the stator is configured to be stationary and the rotor is configured to rotate within the stator. Electric energy flows through the stator to or from the rotating rotor. In an electric motor, electric energy is converted into mechanical energy by usage of interacting magnetic fields and current carrying conductors / windings / coils. The stator provides a rotating magnetic field that drives the rotor. More in detail, alternating currents supplied to windings of the stator energizes these windings to create a rotating magnetic field. The rotating magnetic field in the air gap between the stator and the rotor causes a current to run through the windings of the rotor. The rotating magnetic field and the produced current provides a torque causing the rotor to rotate. Conversely, in an electric generator, the stator converts a rotating magnetic field from the rotor into an electric current instead.

[0007] For some rotor types, the magnets of the rotor have an orientation such that a portion of the rotating magnetic field, which is supposed to run through the air gap between the stator and the rotor, goes through a hub of the rotor instead, causing so-called leakage flux. This reduces the rotating magnetic field in the air gap, and thus causes a reduced torque provided by the electric motor.

[0008] Brief description of the invention

[0009] Conventional solutions for reducing the portion of the magnetic field going through the hub of the rotor involves utilizing non-magnetic materials for the hub of the rotor. However, the usage of such non-magnet materials for the hub of the rotor may cause problems of stability and / or integrity for the rotor because conventional solution rotors generally lack sufficient mechanical strength. Conventional solutions may therefore result in a rotor that cannot cope with the forces experienced when the rotor is rotating, whereby the rotor may break.

[0010] It is therefore an objective of the present invention to provide a rotor such that these problems are at least partly solved.

[0011] According to an aspect of the present invention, this objective is achieved through the above-mentioned rotor. The rotor comprises:

[0012] - a rotor hub of a first material;

[0013] - a plurality of magnet holders of a second material, the second material being different from the first material, and the plurality of magnet holders being attached to the rotor hub about an axis of rotation of the rotor;

[0014] - a plurality of magnets arranged about the axis of rotation, wherein each magnet is arranged between two magnet holders and is held in place by the two magnet holders; wherein

[0015] - each magnet holder is attached to the rotor hub by a joint, the joint comprising a male joint portion of the magnet holder fitted into a female joint portion of the rotor hub; and

[0016] - the male joint portion comprises one or more cavities confined within the male joint portion.

[0017] Since the male joint portion is provided with the one or more cavities, it is made elastically deformable, which gives the male joint portion, and the joint as a whole, an improved mechanical strength and robustness in comparison to conventional solutions. The risk for the male joint portion being plastically deformed and / or for the male joint portion to break is hereby considerably reduced, both at insertion of the male joint portion into the female joint portion and at rotation of the rotor when the electric machine is in use.

[0018] The elastic deformation of the male joint portion, which is provided by the one or more cavities confined within the male joint portion, causes a resilience of the male joint portion and the joint. This resilience is used for making sure that the magnets are kept intact with, i.e. always stay in contact with, the magnets, such that the magnets are securely fixed to the magnet holders and thus securely attached to the rotor hub. Without such resilience, i.e. for very stiff magnet holders, there is a risk that the slots / pockets between the magnet holders, in which the magnets are arranged, become larger than required, i.e. larger than the magnets, due to plastic deformation. Such plastic deformation could lead to decreased performance of the electric machine and / or to the magnet holders and / or the magnets breaking.

[0019] The improved mechanical strength of the joint results in that all parts of the rotor, including the rotor hub, the magnet holders and the magnets, function as one integral part, although being composed of multiple assembled parts, possibly being of differing materials. Since it is possible to provide an improved integrity for the rotor by utilization of the improved mechanical strength of the joint, the one or more materials of the rotor hub may be chosen such that the magnetic field through the rotor hub is minimized. This minimized magnetic field through the rotor hub in turn results in a maximized magnetic field through the air gap between the rotor and the stator. Hereby, an improved torque may be provided by the electric machine comprising the stator and the rotor.

[0020] The presented rotor further eliminates the need of an additional flexural support, e.g. a ring-formed support, for the magnet holders and / or the magnets, which is often needed in the conventional solutions. The presented rotor therefore simplifies production of the rotor and reduces costs for its manufacturing.

[0021] According to an embodiment of the present invention, the one or more cavities are void of solid material and are arranged to be elastically shrinkable.

[0022] Since no solid material is arranged inside the one or more cavities according to this embodiment, the size of the cavities may be reduced when the male joint portion is put under pressure, e.g. at insertion of the male joint portion into the female joint portion and / or during rotation of the rotor. The hereby provided elastic deformation / shrinkage of the one or more cavities results in an elastic deformation of the male joint portion, such that the volume of the outer surface of the male joint portion may be elastically reduced, whereby resilience and increased mechanical strength is provided. According to an embodiment of the present invention, the one or more cavities comprise one or more in the group of:

[0023] - at least one surface cavity of the male joint portion; and

[0024] - at least one confined cavity inside a surface of the male joint portion.

[0025] The one or more cavities may be configured / arranged depending on the form and size of the male joint portion and / or on the present implementation of the rotor. Thus, depending on the implementation, the number of cavities and / or placement of the cavities may be chosen to provide an elastic deformation and / or a cooling suitable for that implementation.

[0026] According to an embodiment of the present invention, the one or more cavities extend along the axis of rotation, from a first axial end of the male joint portion to a second axial end of the male joint portion.

[0027] Thus, the one or more cavities are arranged along the whole axial length of the male joint portion, i.e. along the whole axial length of the magnet holder, and thus essentially along the axial length of the rotor. Hereby, a cooling agent may be guided through at least one of the cavities, such that cooling of the rotor is provided. Also, to have the one or more cavities run along the whole length of the male joint portion, i.e. along the length of the magnet holder, the manufacturing of the magnet holders is simplified.

[0028] According to an embodiment of the present invention, at least one of the one or more cavities guides a cooling agent from the first axial end to the second axial end.

[0029] Hereby, the one or more cavities provide the combined effects of improved mechanical strength of the joint and cooling of the rotor.

[0030] According to an embodiment of the present invention, at least one of the one or more cavities is filled with air.

[0031] Hereby, a controlled and predictable elastic deformation of the male joint portion is provided.

[0032] According to an embodiment of the present invention,

[0033] - a cross section perpendicular to the axis of rotation of the male joint portion has a male dovetail form; and

[0034] - a cross section perpendicular to the axis of rotation of the female joint portion has a female dovetail form.

[0035] The dovetail joint is a strong joint because of the way the tails and pins of the male and female portions of the joint are shaped. The joint is thus difficult to pull apart when it has been fitted. By usage of a dovetail joint with one or more cavities arranged in the male portion of the dovetail joint, the effect of a strong and robust joint is provided, also when different materials are used for the rotor hub and the magnet holders.

[0036] According to an embodiment of the present invention,

[0037] - a cross section perpendicular to the axis of rotation of the male joint portion has a partially circular male form; and

[0038] - a cross section perpendicular to the axis of rotation of the female joint portion has a partially circular female form.

[0039] For some implementations, a partially circular form joint is suitable for attaching the magnet holders to the rotor hub. By usage of a partially circular joint with one or more cavities arranged in the male joint portion, the combined effect of a strong and robust joint is provided, also when different materials are used for the male and female joint portions.

[0040] According to an embodiment of the present invention, the male joint portion is attached to the female joint portion by an interference fit.

[0041] The interference fit, also known as press fit or friction fit, utilizes the friction caused by the interference between the male joint portion and the female joint portion after the male joint portion has been pushed / pressed into the female joint portion. A disconnection of the male joint portion and the female joint portion is counteracted by the contact forces created by the interference fit, such that a secure and strong joint is provided.

[0042] According to an aspect of the present invention, the one or more cavities are arranged to provide one or more in the group of:

[0043] - an elastic deformation of the male joint portion when the male joint portion is being inserted into the female joint portion; and

[0044] - a spring functionality between the male joint portion and the female joint portion when the rotor is rotating.

[0045] Since the male joint portion comprises the one or more cavities, it is made elastically deformable at the insertion into the female joint portion, which gives the male joint portion itself, as well as the whole joint, improved mechanical strength and robustness. The risk for the male joint portion being plastically deformed and / or for the male joint portion to break due to stiffness is considerably reduced since it is made elastically deformable by the one or more cavities.

[0046] The one or more cavities confined within the male joint portion further causes a spring functionality of the joint. The one or more cavities thus provide for a resilience of the male joint portion, which is utilized for robustly and securely fixing the magnets to the magnet holders. The resilience considerably reduces the risk for the magnets being loosely held by the magnet holders due to plastic deformation of the magnet holders, causing slots / pockets between the magnet holders being too large for the magnets. The provided resilience thus reduces the risk for degraded performance of the rotor, and the risk for the magnet holders and / or the magnets breaking at rotation of the rotor due to stiffness.

[0047] According to an aspect of the present invention,

[0048] - the first material is at least partly low-magnetic or non-magnetic; and

[0049] - the second material is at least partly ferromagnetic.

[0050] By choosing the first material of the rotor hub to be an at least partly low-magnetic or non-magnetic material and the second material of the magnet holders to be an at least partly ferromagnetic material, the concentration of the magnetic field in the air gap between the rotor and its surrounding stator is maximized. This is because the low-magnetic or non-magnetic material of the rotor hub efficiently reduces the portion of the magnetic field running through the rotor hub.

[0051] According to an aspect of the present invention,

[0052] - the rotor is a spoke rotor; and

[0053] - the magnets are permanent magnets. In spoke rotors, the magnetic field density in the air gap between the stator and the rotor is higher than for many other types of rotors. However, there still is a risk that some of the magnetic field, due to the orientation of the magnets, creates a loop through the rotor hub. When the herein described one or more cavities are arranged on the male joint portion of a magnet holder of a spoke rotor, the magnetic field density in the air gap is further increased. Thus, spoke rotors utilizing the herein described rotor provide a large amount of the useful magnetic field such that it may be used for providing the rotation of the rotor.

[0054] According to an aspect of the present invention, each magnet is glued to the two magnet holders between which the magnet is arranged.

[0055] Hereby, the magnets are further fixed to their adjacent magnet holders, providing an even more integrated rotor, functioning as one integrated part although being composed of a plurality of parts.

[0056] According to an aspect of the present invention, the objective is achieved through an electric machine, comprising:

[0057] - a stator; and

[0058] - a rotor as herein described, the rotor being arranged radially inside the stator.

[0059] The electric machine has the advantages mentioned for the rotor above.

[0060] According to an aspect of the present invention, the objective is achieved through a vehicle comprising a herein described electric machine.

[0061] The vehicle has the advantages mentioned for the rotor above.

[0062] Brief list of figures

[0063] Embodiments of the invention will be illustrated in more detail below, along with the enclosed drawings, where similar references are used for similar parts, and where:

[0064] Figure 1 schematically illustrates an example vehicle, in which embodiments of the present invention may be implemented, Figure 2 schematically illustrates an electric machine, in which embodiments of the present the invention may be implemented,

[0065] Figure 3 schematically illustrates a rotor according to some embodiments of the present the invention,

[0066] Figures 4a-f schematically illustrate examples of magnet holders according to various embodiments of the present invention, and

[0067] Figure 5 schematically illustrates a magnet holder according to an embodiment of the present invention.

[0068] Description of preferred embodiments

[0069] Figure 1 schematically shows an exemplary heavy vehicle 500, such for example a truck or a bus, which will be used to explain the herein presented embodiments. The embodiments are, however, not limited to use in vehicles as the ones shown in figure 1 , but may also be used in lighter vehicles, such as cars or other types of vehicles.

[0070] A vehicle 500, as shown schematically in Figures 1 , comprises multiple wheels, of which at least one pair is drive wheels 503, 504. The vehicle 500 furthermore comprises a drivetrain 502 configured to transfer a torque between at least one power source, such as e.g. at least one electric machine 501 , or a combination of a combustion engine and at least one electric machine 501 , implementing a so-called hybrid drive, to the at least one pair of drive wheels 503, 504. The at least one electric machine 501 is provided with electrical energy by at least one battery 510 coupled to the at least one electric machine 501 , and is controlled by at least one control unit / device / system 511.

[0071] The torque provided by the power source may be provided to the at least one pair of drive wheels 503, 504 via a central gear, such as e.g. a customary differential, and drive shafts connected with the central gear. One or more electric machines 501 may also be arranged essentially anywhere in the vehicle 500, as long as the produced torque is provided to the drive wheels 503, 504, e.g. adjacent to one or more of the drive wheels 503, 504, as is understood by a skilled person. The vehicle may be braked by utilizing the electric machine 501 , i.e. by utilizing regenerative braking. The vehicle 500 may further include at least one braking arrangement arranged at each one of the wheels of the vehicle, where the at least one braking arrangement may be included in a braking system.

[0072] The control unit / device / system 511 may be configured for controlling the one or more electric machines 501 , and one or more of the at least one battery 510, the braking system and other suitable systems and / or components of the vehicle 500. However, in figure 1 , only the units / devices / entities of the vehicle useful for understanding the present invention are schematically illustrated.

[0073] Figure 2 schematically illustrates an electric machine 501 . The electric machine 501 comprises a stationary stator 200 and a rotor 100 configured to rotate within the stator 200. The rotor 100 rotates R around an axis of rotation 115 of the rotor 100. The axis of rotation 115 is in this document a longitudinal axis through a centre of the rotor 100, around which the rotor 100 rotates. The stator 200 and the rotor 100 are separated by an air gap 201 .

[0074] When the electric machine 501 is used as an electric motor, the stator 200 provides a rotating magnetic field that drives the rotor 100 to rotate. The rotating magnetic field in the air gap 201 between the stator 200 and the rotor 100 then provides a torque causing the rotor to rotate. Conversely, when the electric machine 501 is used as an electric generator, e.g. during regenerative braking, the stator 200 converts a rotating magnetic field provided by the rotor 100 via the air gap 201 into an electric current.

[0075] Figure 3 schematically illustrates an end view, i.e. a view perpendicular to the axis of rotation 115, of a rotor 100 according to an embodiment of the present invention. The rotor 100 comprises a rotor hub 110, multiple magnets 130 and multiple magnet holders 120. The multiple magnet holders 120, and thus also the multiple magnets 130, are here arranged here around the rotor hub 110 as spokes of a wheel, and have radial orientations outwards from the rotor hub 110. Rotors 100 configured this way, i.e. rotors 100 comprising multiple spoke-like magnets 130 and / or magnet holders 120 arranged around the rotor hub 110, are often denoted spoke-rotors. For spoke-rotors, there is a risk that a portion of the rotating magnetic flux / field is leaked via the magnet holders 120 and runs through the hub 110 of the rotor instead of through the air gap 201 radially outside of the rotor 100, i.e. between the rotor 100 and the stator 200, as illustrated in figure 2.

[0076] The rotation R of the rotor 100 is driven by the magnetic field in the air gap 201 between the rotor and the stator 200, as mentioned above. Thus, if some of the magnetic field is leaked and runs through the hub 110 of the rotor, instead of through the air gap 201 , the rotational force / torque on the rotor 100 is reduced.

[0077] Therefore, in order to reduce the portion of the magnetic field running through the rotor hub 110, the rotor hub 110 comprises, i.e. is at least partially produced of, a first material, which according to various embodiments is at least partly low-magnetic or non-magnetic. The plurality of magnet holders 120 comprises, i.e. are at least partially produced of, a second material, which is different from the first material of the rotor hub 110. According to an embodiment, the second material of the magnet holders is at least partly ferromagnetic, such as for example iron or electric steel.

[0078] The differing first and second materials of the rotor hub 110 and the magnet holders 120 may cause stability and / or integrity problems for the rotor 100, where these problems are related to a lack sufficient mechanical strength of the magnet holders 120 and their attachments to the rotor hub 110.

[0079] According to the embodiment schematically illustrated in figure 3, the plurality of magnet holders 120, which may also be called rotor teeth, are attached to the rotor hub 110 about 115 the axis of rotation 115 of the rotor. According to an embodiment, the plurality of magnet holders 120 are attached to an outer periphery of the rotor hub 110 and are there equally spaced around / about the axis of rotation 115.

[0080] The plurality of magnets 130 are also arranged around / about the axis of rotation 115. According to an embodiment, the magnets are also equally spaced around / about the axis of rotation 115. Each one or these magnets 130 is arranged between two adjacent magnet holders 120 and is held in place by the two magnet holders 120. Each magnet 130 is thus fixed axially, radially and tangentially to the rotor hub 110 by the two adjacent magnet holders 120 between which it is arranged. According to an embodiment, the two adjacent magnet holders 120 form a slot / pocket corresponding to the size and shape of the magnet 130, in which the magnet 130 is fitted / held in place. According to some embodiments, the plurality of magnets are permanent magnets, for example when the rotor is a spoke rotor.

[0081] According to the embodiment, each magnet holder 120, being of the second material, is attached to the rotor hub 110, being of the first material, by a joint 121 . The joint 121 comprises a male joint portion 122 of the magnet holder 120, being of the second material, which is fitted / attached / pressed into a female joint portion 112 of the rotor hub 110, being of the first material.

[0082] In this document, the male joint portion 122 of the magnet holder 120 is the inner radial end portion of the magnet holder, which is enclosed / comprised / held in / by the female joint portion 112 when they are fitted / engaged / attached together. The male joint portion 122 may also be defined as being confined radially inside of a waist 126 of the inner radial end portion of the magnet holder 120, as schematically illustrated in figure 4a.

[0083] The male joint portion 122 according to the embodiment comprises one or more cavities / recesses / voids 125 confined within the male joint portion 122, as illustrated in figure 3. Thus, each of the inner radial end portions of the magnet holders 120 comprises one or more cavities 125. These one or more cavities 125 are arranged on the magnet holder 120 such that they will be enclosed / encapsulated within the joint

[0084] 121 , i.e. within the female joint portion 112 when the magnet holder 120 is attached / engaged / fitted to the rotor hub 110.

[0085] The one or more cavities 125 in the male joint portion 122 provides for a slight elastic deformation of the male joint portion 122 and / or a resilience of the male joint portion

[0086] 122. Hereby, the risk for the male joint portion 122 to break, e.g. at the waist 126, is considerably reduced.

[0087] Further, as schematically illustrated in figure 3, the rotor 100 comprises a plurality of magnets 130, in this non-limiting example 16 magnets 130, possibly being permanent magnets, that are equally spaced around the rotation axis 115 in a circumferential direction and extend radially out from the rotor hub 110. In a cross section perpendicular to the rotation axis 115, each of the magnets has an essentially rectangular shape. In the axial / longitudinal direction of the rotor 100, the magnet 130 extends from a first axial end to a second axial end of the magnet 130, corresponding to an axial length L of the rotor 100.

[0088] The rotor 100 further comprises a plurality of magnet holders 120, in this non-limiting example 16 magnet holders 120, being essentially fan-shaped in the cross section perpendicular to the rotation axis 115, and being arranged between the magnets 130 to radially extend out from the rotor hub 110. Thus, there are as many magnet holders 120 as there are magnets 130, and each magnet 130 is in the circumferential direction arranged between two magnet holders 120. In the axial / longitudinal direction of the rotor 100, the magnet holders 120 extend from a first axial end to a second axial end of the magnet holders 120, essentially corresponding to the axial length L of the rotor 100. The second material of the magnet holders 120 may be a suitable ferromagnetic material, such as e.g. electrical steel or iron, and the magnet holders 120 may be made / produced of stacked / laminated layers of the ferromagnetic material. Alternatively, each of the magnet holders 120 may be made of a solid piece of the ferromagnetic material.

[0089] As illustrated in figure 3, any two adjacent magnet holders 120 are placed / arranged such that they have one magnet 130 in between them. Each magnet 130 is thus sandwiched between two adjacent magnet holders 120, for example in a slot / pocket formed by the magnet holders. According to an embodiment, each magnet 130 is glued to the two magnet holders 120 between which the magnet 130 is arranged / sandwiched. Hereby, the magnets 130 are held firmly in place by the magnet holders 120.

[0090] According to an embodiment, the rotor hub 110 comprises a centrally arranged base part 111 , e.g. a shaft, which is of a magnetic or non-magnetic material. The rotor hub further comprises an engagement part 113, which is arranged radially outside of the base part / shaft 111 and is of an at least partly low-magnetic or non-magnetic material. The female joint portion 112 of the rotor hub 110 is arranged in the outer surface of the engagement part 113.

[0091] By the configuration of the rotor 100 schematically illustrated in figure 3, the plurality of magnet holders 120 are firmly attached to the rotor hub 110 by the secure and flexible joints 121 . Further, the plurality of magnets 130 are firmly attached to the magnet holders 120, for example by being glued to them. Hereby, the whole rotor 100 functions as one integral part, although it is composed of multiple parts, possibly of different materials. Furthermore, the rotor 100 comprises very few parts, since various positioning rings and other fastening arrangements needed in conventional solutions are not necessary to the secure the attachment of the plurality of magnet holders 120 and the plurality of magnets 130 to the rotor hub 110. Thus, the improved mechanical strength and the resilience provided by the one or more cavities 125 of the male joint portion 122 make such additional fastening arrangements superfluous.

[0092] Figures 4a-f show magnet holders 120 according to various embodiments. Each of these figures show a cross section of one magnet holder 120, where the cross section is transversal / perpendicular to the axis of rotation 115 of the rotor. The male joint portion 122 is arranged at the inner radial end portion of the magnet holder 120. As mentioned above, the male joint portion 122 will be received / enclosed / encapsulated within the joint 121 , i.e. within the female joint portion 112, when the magnet holder 120 is attached / engaged / fitted to the rotor hub 110. One or more cavities / recesses / voids 125 are confined within the male joint portion 122 of the magnet holder 120.

[0093] According to an embodiment, the two magnet holders 120 adjacent to a magnet 120 form a slot / pocket, in which the magnet is held in position. The slot / pocket is here formed by the waist 126 at the male joint portion 122, an outer edge 117, and a side wall 118 of each or the two magnet holders 120, illustrated in figure 4a. The magnet 120 is thus held in place in the radial direction by the waists 126 and the outer edges 117 of the magnet holders 120, and is held in place in the tangential direction by the side walls 118 of the magnet holders 120.

[0094] According to an embodiment, the cross section of the male joint portion 122 shown in figures 4a-f has a male dovetail form 129. As understood by a skilled person, a corresponding cross section of the female joint portion 112 then also has a corresponding female dovetail form 119, as shown in figure 3. Thus, when attaching the magnet holder 120 to the rotor hub 110, the dovetail-formed male joint portion 122, 129 is pressed into and received by the corresponding dovetail-formed female joint portion 112, 119.

[0095] The above mentioned one or more cavities 125 confined within the dovetail-formed male joint portion 122, 129 may, according to various embodiments, comprise at least one surface cavity 125a arranged on the surface 127 of the of the male joint portion 122, 129. Some non-limiting examples of such surface cavities 125a are schematically shown in figures 4a-e as cross-sectional views of the magnet holder 120 and its male joint portion 122. There may be essentially any number of surface cavities 125a, and they may have essentially any cross-sectional shape. As nonlimiting examples, figure 4a shows three surface cavities 125a having a partially triangular shape in the cross section. Figure 4b shows three surface cavities 125a having a partially rectangular shape in the cross section. Figure 4c shows two surface cavities 125a being essentially finger-shaped in the cross section. Figure 4d shows three surface cavities 125a having a partially circular form. Figure 4e shows two surface cavities 125a having a lobed channel form, e.g. partially having an asymmetric U-shape or partially having an asymmetric V-shape.

[0096] The above mentioned one or more cavities 125 confined within the dovetail-formed male joint portion 122, 129 may, according to various embodiments, also comprise at least one confined cavity 125b inside a surface of the male joint portion 122. Thus, the at least one cavity 125 may then comprise at least one closed void, i.e. a hole, of a body 128 of the male joint portion 122 in the cross section perpendicular to the axis of rotation 115. Essentially any number of confined cavities 125b of essentially any form may be arranged in the male joint portion 122. As a non-limiting example, figure 4e discloses one round hole / cavity 125b in the cross section of the male joint portion 122 of the magnet holder 120. Figure 4f shows a confined cavity 125b in form of one irregular polygon, here exemplified as a trapezoid, in the cross section of the male joint portion 122. As exemplified in figure 4e, at least one surface cavity 125a and at least one confined cavity 125b may be arranged in one male joint portion 122.

[0097] Figure 5 schematically shows a side view of a magnet holder 120. According to an embodiment, the herein described one or more cavities 125 extend along, i.e. in the same direction as, the axis of rotation 115, from a first axial end 123 of the male joint portion 122 to a second axial end 124 of the male joint portion 122. Thus, the male joint portion 122 extends along the whole axial length L of the magnet holder 120, which often essentially corresponds to the axial length of the rotor 100, and the one or more cavities 125 also run / extend that whole length L.

[0098] According to an embodiment, the one or more cavities 125, for example the at least one surface cavity 125a and / or the at least one confined cavity 125b are void of solid material, i.e. are empty of solid material. The one or more cavities 125 are hereby arranged to be elastically shrinkable. This causes the male joint portion 122 to be possible to elastically shrink, i.e. to be able to elastically be changed / reshaped to a smaller volume, if it is put under pressure / force from its outside / surroundings.

[0099] At least one of the one or more cavities 125 may, according to an embodiment, be filled with air, such that controlled and predictable reshaping of the cavities is possible to provide.

[0100] At least one of the one or more cavities 125 may, according to an embodiment, be arranged to guide / hold a cooling agent, such that the cooling agent may be transported from the first axial end 123 of the male joint portion 122 to the second axial end 124 of the male joint portion 122. Thus, the one or more cavities 125 may be utilized for cooling of the rotor 100.

[0101] In figures 3, 4a-f and 5, the joints 121 , and their male 122 and female 112 joint portions, are illustrated as a dovetail-formed joints. However, according to an embodiment, the male joint portion 122 has a partially circular male cross-sectional form, where the cross section is transversal / perpendicular to the axis of rotation 115 of the rotor. The female joint portion 112 then has a corresponding partially circular female cross-sectional form, such that the male joint portion 122 fits into the female joint portion 112.

[0102] The herein presented embodiments are generally applicable to essentially any type of male and female joint, whereof the herein mentioned dovetail and circular forms are only two non-limiting examples. Thus, the herein described one or more cavities may be confined within a male joint portion 122 of a magnet holder 120, where male joint portion 122 may have essentially any form and / or shape, to be is fitted into a correspondingly shaped female joint portion 112.

[0103] The magnet holder 120 may, according to various embodiments, comprise two or more stacked / laminated axial layers and / or two or more stacked / laminated radial layers of the ferromagnetic material. Alternatively, the magnet holder 120 may be made of a solid piece of the ferromagnetic material.

[0104] Further, the magnet holder 120 may, according to an embodiment, comprise two or more circumferential portions in the cross section perpendicular to the rotation axis 115. For example, the magnet holder 120 may comprise two mirrored circumferential portions in the cross section, configured such that the two portions together form the magnet holder 120 when both of the two portions are attached to the rotor hub 110. As a non-limiting example, a magnet holder 120 as the one shown in figure 4a could, according to this embodiment, be divided vertically in the middle into a left portion and a right portion, being mirrored versions of each other.

[0105] According to an embodiment, the male joint portion 122 is attached to the female joint portion 112 by an interference fit. Thus, the magnet holders 120 are then attached to the rotor hub 110 by the interference fit of the joints 121 , such that the magnet holders 120 and thus also the magnets 130 are firmly attached to the rotor hub 110. Cooling of the male joint portion 122 to reduce its size before insertion into the female joint portion 112 may here be utilized in some embodiments.

[0106] When the male joint portion 122 is fitted / forced into the female joint portion 112, the one or more cavities 125 confined within the male joint portion 122 are arranged to provide an elastic deformation of the male joint portion 122 at the insertion of the male joint portion 122 into the female joint portion 112. Thus, the male joint portion 122 is, thanks to the one or more cavities 125, elastically deformed, i.e. is deformed such that it can return to its initial form / shape. Hereby the male joint portion 122 is held very tightly in the female joint portion 112 after its insertion therein, and the mechanical strength of the joint 121 and the male joint portion 122 is improved.

[0107] When the rotor 100 is rotating, i.e. when the rotor 100 is used, the one or more cavities 125 confined in the male joint portion 122 are arranged to provide a spring functionality between the male joint portion 122 and the female joint portion 112. This spring functionality during the rotation of the rotor 100 is caused by the one or more cavities 125 being squeezed and released when the magnet holder 120 experiences tangential forces, such that also the male joint portion 122 elastically changes its form / shape according to the squeezing and releasing of the cavities 125. The male joint portion 122, and thus also the magnet holder 120, is hereby arranged to be resilient, such that the risk for the male joint portion 122 and / or the rest of the magnet holder 120 to break or becoming more loose is considerably reduced. Hereby, it is secured that the magnet 130 is always tightly held in place by its adjacent magnet holders 120, and that there are no gaps between the magnet holders 120 and the magnet 130.

[0108] The present invention is not limited to the above described embodiments. Instead, the present invention relates to, and encompasses all different embodiments being included within the scope of the independent claims.

Claims

Claims1. A rotor (100) comprising:- a rotor hub (110) of a first material;- a plurality of magnet holders (120) of a second material, the second material being different from the first material, and the plurality of magnet holders (120) being attached to the rotor hub (110) about an axis of rotation (115) of the rotor;- a plurality of magnets (130) arranged about the axis of rotation (115), wherein each magnet (130) is arranged between two magnet holders (120) and is held in place by the two magnet holders (120); wherein- each magnet holder (120) is attached to the rotor hub (110) by a joint (121 ), the joint(121 ) comprising a male joint portion (122) of the magnet holder (120) fitted into a female joint portion (112) of the rotor hub (110); and- the male joint portion (122) comprises one or more cavities (125) confined within the male joint portion (122).

2. The rotor (100) as claimed in claim 1 , wherein the one or more cavities (125) are void of solid material and are arranged to be elastically shrinkable.

3. The rotor (100) as claimed in any one of claims 1-2, wherein the one or more cavities (125) comprise one or more in the group of:- at least one surface cavity (125a) of the male joint portion; and- at least one confined cavity (125b) inside a surface of the male joint portion (122).

4. The rotor (100) as claimed in any one of claims 1-3, wherein the one or more cavities (125) extend along the axis of rotation (115), from a first axial end (123) of the male joint portion (122) to a second axial end (124) of the male joint portion(122).

5. The rotor (100) as claimed in claim 4, wherein at least one of the one or more cavities (125) guides a cooling agent from the first axial end (123) to the second axial end (124).

6. The rotor (100) as claimed in any one of claims 1-4, wherein at least one of the one or more cavities (125) is filled with air.

7. The rotor (100) as claimed in any one of claims 1 -6, wherein- a cross section perpendicular to the axis of rotation (115) of the male joint portion (122) has a male dovetail form (129); and- a cross section perpendicular to the axis of rotation (115) of the female joint portion (112) has a female dovetail form (119).

8. The rotor (100) as claimed in any one of claims 1-6, wherein- a cross section perpendicular to the axis of rotation (115) of the male joint portion (122) has a partially circular male form; and- a cross section perpendicular to the axis of rotation (115) of the female joint portion (112) has a partially circular female form.

9. The rotor (100) as claimed in any one of claims 1 -8, wherein the male joint portion (122) is attached to the female joint portion (112) by an interference fit.

10. The rotor (100) as claimed in any one of claims 1 -9, wherein the one or more cavities (125) are arranged to provide one or more in the group of:- an elastic deformation of the male joint portion (122) when the male joint portion (122) is being inserted into the female joint portion (112); and- a spring functionality between the male joint portion (122) and the female joint portion (112) when the rotor (100) is rotating.11 . The rotor (100) as claimed in any one of claims 1 -10, wherein- the first material is at least partly low-magnetic or non-magnetic; and- the second material is at least partly ferromagnetic.

12. The rotor (100) as claimed in any one of claims 1-11 , wherein- the rotor (100) is a spoke rotor; and- the magnets (130) are permanent magnets.

13. The rotor (100) as claimed in any one of claims 1-12, wherein each magnet (130) is glued to the two magnet holders (120) between which the magnet (130) is arranged.

14. An electric machine (501 ), comprising:- a stator (200); and- a rotor (100) as claimed in any one of claims 1-13, the rotor being arranged radially inside the stator (200).

15. A vehicle (500) comprising an electric machine (501 ) according to claim14.

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