Abstract
An electrical machine (1) comprising: a rotatable drive shaft having a rotational axis (15); a rotor assembly (2) connected to the drive shaft, the rotor assembly 2 arranged to generate a static rotor magnetic field; a primary stator assembly (4), comprising a plurality of stator coils (5a, 5b) arranged to generate a rotating stator magnetic field for interacting with the static rotor magnetic field of the rotor assembly (2) such as to rotate the rotor assembly (2) along the rotational axis (15), and a secondary stator assembly (7) arranged to generate a static stator magnetic field; wherein the electrical machine (1) comprises a magnetic torsion spring (9) formed by the interaction of the static stator magnetic field with the static rotor magnetic field.
Claims
1. An electrical machine (1) comprising: a rotatable drive shaft having a rotational axis (15); a rotor assembly (2) connected to the drive shaft, the rotor assembly (2) arranged to generate a static rotor magnetic field; wherein the rotor assembly is arranged to rotate at least 360° around the rotational axis, a primary stator assembly (4), comprising a plurality of stator coils (5a, 5b) arranged to generate a rotating stator magnetic field for interacting with the static rotor magnetic field of the rotor assembly (2) such as to rotate the rotor assembly (2) along the rotational axis (15), and a secondary stator assembly (7) arranged to generate a static stator magnetic field; wherein the electrical machine (1) comprises a magnetic torsion spring (9) formed by the interaction of the static stator magnetic field with the static rotor magnetic field, wherein the magnetic torsion spring applies a pulsating torque profile on the rotor assembly, the pulsating torque profile having a periodicity of at least one period per revolution of the rotor assembly.
2. The electrical machine (1) of claim 1 wherein the static stator magnetic field and the static rotor magnetic field are movable relative to one another and are forming a plurality of stable equilibrium positions (13) and a plurality of unstable equilibrium positions (14), said plurality of stable and unstable equilibrium positions (13, 14) located around the rotational axis (15) and said plurality of unstable equilibrium positions (14) located interspersed between the stable equilibrium positions (13), wherein the rotor assembly (2) is rotatable towards and past any given one of said equilibrium positions (13, 14) in a springing manner via magnetic forces created by said interaction of the static rotor and static stator magnetic fields.
3. The electrical machine (1) according to claim 1 wherein the static rotor magnetic field comprises a first number of alternating magnetic poles distributed along the rotor assembly (2) circumference defined by the surface of the rotor assembly (2) delimiting the airgap between the rotor assembly (2) and the secondary stator assembly (7), and wherein the static stator magnetic field comprises a second number of alternating magnetic poles distributed along the secondary stator assembly (7) circumference defined by the surface of the secondary stator assembly (7) delimiting the airgap between the rotor assembly (2) and the secondary stator assembly (7).
4. The electrical machine (1) according to claim 1, wherein stable equilibrium positions (13) are formed at angular positions of the rotor assembly (2) with respect to the secondary stator assembly (7) where the static rotor magnetic field and the static stator magnetic field have at least one overlapping magnetic pole of the opposite magnetic polarity and wherein unstable equilibrium positions (14) are formed at angular positions of the rotor assembly (2) with respect to the secondary stator assembly (7) where the static rotor magnetic field and the static stator magnetic field have at least one overlapping magnetic pole of the same magnetic polarity.
5. The electrical machine (1) according to claim 3, wherein the alternating magnetic poles of the static rotor magnetic field and the alternating magnetic poles of the static stator magnetic field are evenly distributed along respectively the rotor assembly (2) circumference defined by the surface of the rotor assembly (2) delimiting the airgap between the rotor assembly (2) and the secondary stator assembly (7), and the secondary stator assembly (7) circumference defined by surface of the secondary stator assembly (7) delimiting the airgap between the rotor assembly (2) and the secondary stator assembly (7).
6. The electrical machine (1) according to claim 3, wherein the first number of alternating magnetic poles equals the second number of alternating magnetic poles.
7. The electrical machine (1) according to claim 1, wherein the secondary stator assembly (7) comprises a set of PMs (8a, 8b) arranged annularly around the rotational axis (15), wherein the set of PMs (8a, 8b) of the secondary stator assembly (7) is arranged to generate the static stator magnetic field.
8. The electrical machine (1) according to claim 1 wherein the primary stator assembly (4) and the secondary stator assembly (7) are distinct stator assemblies.
9. The electrical machine (1) according to claim 8, wherein the primary stator assembly (4) is positioned radially outward with respect to the rotor assembly (2) and wherein the secondary stator assembly (7) is positioned radially inward with respect to the rotor assembly (2).
10. (canceled)
11. The electrical machine (1) according to claim 1 wherein the electrical machine (1) is a pulsating torque electrical machine.
12. The electrical machine (1) according to claim 1, wherein the rotor assembly (2) comprises a single set of magnetic pole forming means (3) arranged annularly around the rotational axis (15), wherein the single set of magnetic pole forming means (3) is arranged to generate the static rotor magnetic field, and wherein the single set of magnetic pole forming means (3) are permanent magnets.
13. The electrical machine (1) according to claim 1, wherein the rotor assembly (2) comprises a first set of magnetic pole forming means (20) arranged annularly around the rotational axis (15) and arranged along the surface (16) of the rotor assembly (2) facing the primary stator assembly (4), wherein the rotor assembly (2) comprises a second set of magnetic pole forming means (21) arranged annularly around the rotational axis (15) and arranged along the surface (17) of the rotor assembly (2) facing the secondary stator assembly (7), wherein the first and second sets of magnetic pole forming means (21, 22) are arranged to generate the static rotor magnetic field together, and wherein the first and second sets of magnetic pole forming means (21, 22) are permanent magnets.
14. The electrical machine (1) according to claim 1 wherein the rotor assembly (2) comprises a first annular part (22) interconnected to a second annular part (23) by an interconnection means (24), wherein the first annular part (22) comprises the first set of magnetic pole forming means (20) of the rotor assembly (2) and wherein the second annular part (23) comprises the second set of magnetic pole forming means (21) of the rotor assembly (2) and wherein the first annular part (22) has a different axial length than the second annular part (23).
15. (canceled)
16. A kit of parts for assembling the electrical machine (1) according to claim 0, the kit of parts comprising the following parts: the primary stator assembly (4), the first annular part of the rotor assembly (2), the second annular part of the rotor assembly (2) and the secondary stator assembly (7).
17. (canceled)
Description
DRAWINGS
[0048] FIG. 1 depicts a cross-sectional view of an electrical machine according to one embodiment;
[0049] FIG. 2 shows that the pulsating torque profile that the magnetic torsion spring produces onto the driven shaft, is chosen such as to substantially cancel out the pulsating torque profile of the pulsating torque industrial application driven via the driven shaft.
[0050] FIG. 3 depicts a graph of the torque produced by the electrical machine according to one embodiment of the present invention as a function of the rotation of the rotor assembly with respect to the secondary stator assembly.
[0051] FIG. 4 depicts a cross-sectional view of an electrical machine according to a further embodiment.
[0052] FIG. 5a-5d show four embodiments of the electrical machine wherein the modularity of the electrical machine is illustrated in a cross-section taken along an axial plane.
[0053] FIGS. 6a-6c show three embodiments of the electrical machine wherein the modularity of the electrical machine is illustrated in a cross-section taken along a plane comprising the rotation axis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention. Furthermore, the terms first and second, in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.
[0055] Furthermore, the various embodiments, although referred to as “preferred” are to be construed as exemplary manners in which the invention may be implemented rather than as limiting the scope of the invention.
[0056] The term “comprising”, used in the claims, should not be interpreted as being restricted to the elements or steps listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising A and B” should not be limited to devices consisting only of components A and B, rather with respect to the present invention, the only enumerated components of the device are A and B, and further the claim should be interpreted as including equivalents of those components.
[0057] FIG. 1 depicts a cross-sectional view of an electrical machine 1 according to one embodiment. The electrical machine 1 is illustrated in a cross-section taken along an axial plane (i.e. a plane perpendicular to the axial direction). The electrical machine 1 is a pulsating torque electrical machine 1, for example a pulsating torque electromotor for driving an industrial device exhibiting reciprocating movement such as a weaving loom. The electrical machine 1 comprises a rotor assembly 2 connected to a rotatable drive shaft (not shown) having a rotational axis 15. The rotor assembly 2 comprises an annular component formed by a set of PMs 3a, 3b arranged annularly around the rotational axis 15, wherein the set of PMs 3a, 3b of the rotor assembly 2 are arranged to generate a static rotor magnetic field. Each PM 3a, 3b is radially magnetized, thereby creating a magnetic pole pair wherein the magnetic north pole and the magnetic south pole are radially disposed with respect to each other. Neighboring PMs 3a, 3b have an opposite magnetization vector. The magnetization direction of the PM 3a is directed radially outward, whilst the magnetization direction of the neighboring PM 3b is directed radially inward. The static rotor magnetic field emanates from the PMs 3a, 3b of the PM rotor assembly 2 and travels to first 11 and second 12 airgaps provided adjacent to respectively the outer 16 and inner 17 radial surfaces of the rotor assembly 2. Along the circumference of the inner radial surface 17 of the rotor assembly 2 alternating magnetic poles are present, i.e. a sequence of alternating magnetic north and magnetic south poles are present, for example observed in the second airgap 12 adjacent the inner radial surface 17 of the rotor assembly 2. The alternating magnetic poles of the rotor assembly 2 are evenly distributed along the circumference of the inner radial surface 17 of the rotor assembly 2. The eighteen alternating magnetic poles along the inner radial surface 17 of the rotor assembly 2 thus each span an arc of 20°. The electrical machine 1 further comprises two physically distinct stator assemblies, in particular a primary stator assembly 4 and a secondary stator assembly 7. Both stator assemblies 4, 7 are positioned concentrically with the rotor assembly 2. The primary stator assembly 4 is positioned radially outward from the outer radial surface 16 of the rotor assembly 2. The secondary stator assembly 7 is positioned radially inward from the inner radial surface 17 of the rotor assembly 2. The primary stator assembly 4 is separated from the outer radial surface 16 of the rotor assembly 2 by the first airgap 11. The secondary stator assembly 7 is separated from the inner radial surface 17 of the rotor assembly 2 by the second airgap 12. The airgaps 11, 12 enable the rotor assembly 2 to rotate with respect to the stator assemblies 4, 7. The primary stator assembly 4, the secondary stator assembly 7 and the rotor assembly 2 are nested i.e. the rotor assembly 2 lies in between the two stator assemblies 4, 7. The stator assemblies 4, 7 are static, i.e. are not arranged to rotate in use. The primary stator assembly 4 comprises a primary stator yoke 18 provided with a multitude of stator teeth 6a, 6b. Around each stator tooth 6a, 6b a stator coil 5a, 5b is wounded. The stator coils 5a, 5b are wound in a 3 phase winding pattern and are arranged to, in use, generate a rotating stator magnetic field. The rotating stator magnetic field, in use, is present within the first airgap 11 and interacts with the static rotor magnetic field of the rotor assembly 2 such as to rotate the rotor assembly 2 along the rotational axis 15. The interaction of the static rotor magnetic field and the rotating stator magnetic field for example forms a conventional electromotor 10. The secondary stator assembly 7 comprises an annular component formed by a second set of PMs 8a, 8b arranged annularly around the rotational axis 15, wherein the set of PMs 8a, 8b of the secondary stator assembly 7 are arranged to generate a static stator magnetic field. Each PM 8a, 8b is radially magnetized, thereby creating a magnetic pole pair wherein the magnetic north pole and the magnetic south pole are radially disposed with respect to each other. Neighboring PMs 8a, 8b have an opposite magnetization vector. The magnetization direction of the PM 8a is directed radially outward, whilst the magnetization direction of the neighboring PM 8b is directed radially inward. The static stator magnetic field generated by the PMs 8a, 8b of the secondary stator assembly 7 is present in the second airgap 12. Along the circumference of the outer radial surface 19 of the secondary stator assembly 7, alternating magnetic poles are present, i.e. a sequence of alternating magnetic north and magnetic south poles is present, for example observed in the second airgap 12 adjacent the outer radial surface 19 of the secondary stator assembly 7. The alternating magnetic poles of the secondary stator assembly 7 are evenly distributed along the circumference of the outer radial surface 19 of the secondary stator assembly 7. The eighteen alternating magnetic poles along the outer radial surface 19 of the secondary stator assembly 7 thus each span an arc of 20°. The static stator magnetic field, in use, is present within the second airgap 12 and interacts with the static rotor magnetic field of the rotor assembly 2 such as to create a magnetic torsion spring 9.
[0058] FIG. 2 shows that the pulsating torque profile that the magnetic torsion spring 9, such as the magnetic torsion spring 9 of FIG. 1, exerts onto the drive shaft, is chosen such as to substantially cancel out the pulsating torque profile of the pulsating torque industrial application driven by the drive shaft. The graph depicts the torque, in Nm, along the vertical axis, in function of the degrees of rotation of the rotor assembly 2 with respect to the secondary stator assembly 7 along the horizontal axis. The solid line depicts the torque exerted by the pulsating torque industrial device on the drive shaft, and thus on the rotor assembly 2. The dashed line depicts the torque exerted on the drive shaft by the magnetic torsion spring 9 due to the interaction of the static rotor and static stator magnetic fields. The electrical machine 1 of FIG. 1 is shown at 0° rotation of the rotor assembly 2 with respect to the secondary stator assembly 7. The positive degrees of rotation are defined as a clockwise rotation of the rotor assembly 2 with respect to the secondary stator assembly 7. Positive torque is defined as the torque accelerating the rotor assembly 2 along the positive direction of rotation.
[0059] In the graph of FIG. 2, the solid line follows a regular wave pattern, crossing the zero torque axis at regular intervals. The shape of the wave, for example when the wave is sinusoidal its amplitude and frequency, is determined by the industrial device that is to be driven by the electrical machine 1. As shown in the first half wavelength of the solid line, the industrial device exerts positive torque to the drive shaft and thus to the rotor assembly 2 for example when the industrial device releases potential energy. As an example, a weaving loom can drop down from an elevated position to a lower position, thereby releasing its potential energy and generating positive torque on the drive shaft and thus on the rotor assembly 2. Subsequently, as shown in the second half wavelength of the solid line, the industrial device exerts a negative torque on the rotor assembly 2 for example when the industrial device stores potential energy. As an example, the weaving loom has to be lifted from the lower position towards the elevated position, thereby storing potential energy and generating negative torque on the drive shaft and thus on the rotor assembly 2. The reciprocating movement of the pulsating industrial device is repeated in a regular manner.
[0060] In the graph of FIG. 2, the dashed line follows a regular wave pattern, crossing the zero torque axis at regular intervals. In the present example, the angular positions of the rotor assembly 2 with respect to the secondary stator assembly 7 where the dashed line crosses the zero torque axis are referred to as equilibrium positions 13, 14. These equilibrium positions 13, 14 occur at angular positions of the rotor assembly 2 with respect to the secondary stator assembly 7 where a substantial number of the PMs 3a, 3b of the PM rotor assembly 2 coincide with the PMs 8a, 8b of the secondary stator assembly 7. In the electrical machine 1 of FIG. 1 the equilibrium positions occur every 20°. The rotor assembly 2 is rotatable towards and past any given one of said equilibrium positions 13, 14 in a springing manner via magnetic forces created by said interaction of the static rotor and static stator magnetic fields. The magnetic torsion spring 9 is arranged to transduce kinetic energy into potential energy and vice versa. Stable 13 and unstable 14 equilibrium positions are for example formed were the potential energy stored in the magnetic torsion spring 9 is respectively at a minimum and at a maximum. Referring to the electrical machine 1 of FIG. 1, the stable equilibrium positions 13 occur where the PMs 3a, 3b of the rotor assembly 2 with a magnetization vector respectively directed radially outward and radially inward coincide with the PMs 8a, 8b of the rotor assembly 2 with a magnetization vector respectively directed radially outward and radially inward. The unstable equilibrium positions 14 occur where the PMs 3a, 3b of the rotor assembly 2 with a magnetization vector respectively directed radially outward and radially inward coincide with the PMs 8a, 8b of the rotor assembly 2 with a magnetization vector respectively directed radially inward and radially outward.
[0061] The shape of the dashed line, is chosen such as to oppose the shape of the solid line which is determined by the industrial device to be driven. In particular the shape of the dashed line is obtained by inverting the values of the vertical axis of the solid line. The shape of the dashed line can be adapted by the person skilled in the art by adapting the characteristics of the magnetic torsion spring 9, for example by changing the number, positioning and strength of the PMs 3a, 3b of the rotor assembly 2 and of the PMs 8a, 8b of the secondary stator assembly 7. It has been found that by opposing the shape of the dashed line with respect to the solid line, the torque exerted on the rotor assembly 2 is substantially averaged out. Discrepancies in the solid and dashed lines, as well as frictional losses, have to be compensated by the interaction of the first static field and the second rotating field, i.e. by the electromotor 10.
[0062] In practice, the pulsating torque industrial device will require an offset torque component to be driven, i.e. to drive a given load at a given speed, in addition to the pulsating torque component. Therefore, the apparatus of the present invention is an electrical machine 1 such as an electromotor 10 with an integrated magnetic torsion spring 9, wherein the magnetic torsion spring 9 has a pulsating torque profile for example as described in FIG. 2. FIG. 3 depicts an illustrative graph of the torque produced by the electrical machine 1 according to one embodiment of the present invention as a function of the rotation of the rotor assembly 2 with respect to the secondary stator assembly 7. The torque profile produced by the electrical machine 1 comprises a substantially constant offset torque component superimposed with a pulsating torque component.
[0063] FIG. 4 depicts an electrical machine according to a further embodiment illustrated in a cross-section taken along an axial plane (i.e. a plane perpendicular to the axial direction). Corresponding features between the electrical machine 1 depicted in FIG. 4 and the electrical machine 1 depicted in FIG. 1 have been given the same reference number. The electrical machine depicted in FIG. 4 differs from the one depicted in FIG. 1 principally by the construction of the rotor assembly 2. The rotor assembly 2 in FIG. 1 comprises a single set of annularly arranged magnetic pole forming means, i.e. permanent magnets 3, which generates the static rotor magnetic field. The rotor assembly in FIG. 4 comprises two sets of annularly arranged magnetic pole forming means 20, 21, i.e. permanent magnets. The first set of PMs 20 is provided along the circumference of the rotor assembly 2 defined by the surface of the rotor assembly 2 delimiting the airgap 11. In the present embodiment, the term ‘along the surface of the rotor assembly’ comprises being provided on top of the surface of the rotor assembly. This first set of PMs 20 is arranged in proximity, i.e. substantially adjacent, to the primary stator assembly 4, such that the part of the static rotor magnetic field generated by the first set of PMs 20 optimally interacts with the rotating stator magnetic field generated by the plurality of stator coils 5 of the primary stator assembly 4. The second set of PMs 21 is provided along the circumference of the rotor assembly 2 defined by the surface of the rotor assembly 2 delimiting the airgap 12. In the present embodiment, the term ‘along the surface of the rotor assembly’ comprises being provided on top of the surface of the rotor assembly. This second set of PMs 21 is arranged in proximity, i.e. substantially adjacent, to the secondary stator assembly 7, such that the part of the static rotor magnetic field generated by the second set of PMs 21 optimally interacts with the static stator magnetic field generated by the magnetic pole forming means, i.e. the permanent magnets 8 of the secondary stator assembly 7.
[0064] The rotor assemblies 2 of the electrical machines 1 depicted in FIGS. 1 and 4 are formed of a single annular part. It is however possible to provide the rotor assembly 2 out of a first radially outward annular part and a second radially inward annular part interconnected to the first annular part 22 and a second annular part 23. This is shown in the FIGS. 5a-5d.
[0065] FIG. 5a-5d show four embodiments of the electrical machine 1 is illustrated in a cross-section taken along an axial plane (i.e. a plane perpendicular to the axial direction). The electrical machines 1 from the FIGS. 5a-5d differ from the electrical machine 1 from FIG. 4 in at least that the rotor assembly 2 comprises two interconnected annular parts, i.e. a first annular part 22 interconnected to a second annular part 23. The first annular part 22 is provided radially outward from the second annular part 23. The radially inward surface of the first annular part 22 is positioned adjacent to the radially outward surface of the second annular part 23, and is attached to the radially outward surface of the second annular part 23 by an interconnection means 24 such as an adhesive layer or a mechanical interconnection. The interconnection means 24 has a reluctance which is higher than the reluctance of the core material of the first annular part 22 and of the second annular part 23, thereby ensuring the optimal confinement of the magnetic fluxes to each of the annular parts 22, 23. The first annular part 22 comprises the interaction component, in the present embodiment being the first set of magnetic pole forming means 20, in particular the first set of permanent magnets 20. This first set of PMs 20 is annularly arranged around the rotation axis 15 along the radially outward surface of the first annular part 22 i.e. the surface facing towards the primary stator assembly 4 i.e. the surface of the first annular part 22 delimiting the airgap between the primary stator assembly 4 and the rotor assembly 2. In particular, the first set of PMs 20 is provided on, i.e. on top of, the radially outward surface of the first annular part 22. The part of the static rotor magnetic field generated by the first set of PMs 20 is optimally confined in the first annular part 22 due to the relatively high reluctance of the interconnection means 24. The part of the static rotor magnetic field generated by the first set of PMs 20 is referred to as the interacting magnetic field of the rotor assembly, as it is this part of the static rotor magnetic field that optimally interacts with the rotating stator magnetic field thereby causing the rotation of the rotor assembly 2. The second annular part 23 of the rotor assembly 2 comprises the second set of magnetic pole forming means 21, in particular the second set of permanent magnets 21. This second set of PMs 21 is annularly arranged around the rotation axis 15 along the radially inward surface of the second annular part 23 i.e. the surface of the second annular part 23 facing towards the secondary stator assembly 7 i.e. the surface of the second annular part 23 delimiting the airgap between the secondary stator assembly 7 and the rotor assembly 2. In particular, the second set of PMs 21 is provided on, i.e. on top of, the radially inward surface of the second annular part 23. The part of the static rotor magnetic field generated by the second set of PMs 21 is optimally confined in the second annular part 23 due to the relatively high reluctance of the interconnection means 24. The part of the static rotor magnetic field generated by the second set of PMs 21 optimally interacts with the static stator magnetic field thereby creating the magnetic torsion spring. The electrical machines 1 shown in FIGS. 5a-5d depict the modularity of the electrical machines 1 of the present invention. It has been found that the requirements of a pulsating torque electrical machine for a particular industrial application can be categorized based on two dominant parameters, i.e. the required angular velocity (the amount of RPM) and the required periodicity of the pulsating torque profile per revolution of the rotor assembly 2. The required dominant parameters of the electrical machine 1 of the present invention can be easily selected by providing the electrical machine with the correct first annular part 22 and the correct second annular part 23. The first annular part 22, comprising the interaction component, can be selected to provide the required angular velocity of the electrical machine 1. By providing a first annular part 22 wherein the interaction component comprises a low amount of PMs 20 of alternating polarity, one provides an electrical machine 1 with a high angular velocity and vice versa. The electrical machines 1 depicted in FIGS. 5c and 5d comprise a first annular part 22 selected for high angular velocity i.e. with a low amount of PMs 20, whereas the electrical machines 1 depicted in FIGS. 5a and 5b comprise a first annular part 22 selected for low angular velocity i.e. with a higher amount of PMs 20. In any case, the primary stator assembly 4 must be selected such as to optimally interact with the first annular part 22 of the rotor assembly 2, i.e. by creating the corresponding amount, for example the same amount, of alternating magnetic poles in the rotating stator magnetic field. Additionally, the yoke 18 of the primary stator assembly 4 in the ‘low angular velocity’ electrical machines 1 depicted in FIGS. 5a and 5b can be made thinner than the yoke 1 of the primary stator assembly 4 in the ‘high angular velocity’ electrical machines 1 depicted in FIGS. 5c and 5d′. The second annular part 23, comprising the second set of PMs 21, can be selected to provide the required periodicity of the pulsating torque profile per revolution of the rotor assembly 2. By providing a second annular part 23 with a low amount of PMs 21 a low frequency torque pulsation can be achieved and vice versa. The electrical machines 1 depicted in FIGS. 5b and 5c comprise a second annular part 23 selected for low frequency torque pulsations i.e. with a low amount of PMs 21, whereas the electrical machines 1 depicted in FIGS. 5a and 5d comprise a second annular part 23 selected for high frequency torque pulsations i.e. with a higher amount of PMs 21. In any case, the secondary stator assembly 7 must be selected such as to optimally interact with the second annular part 23 of the rotor assembly 2, i.e. by creating the corresponding amount, for example the same amount, of alternating magnetic poles in the static stator magnetic field. In particular as shown in the FIGS. 5a-5d, the amount and angular positions of the PMs 8 of alternating polarity provided on the secondary stator assembly 7 are identical to the amount and angular positions of the PMs 21 of alternating magnetic polarity provide on the second annular part 23 of the rotor assembly 2.
[0066] FIGS. 6a-6c show three embodiments of the electrical machine 1 illustrated in a cross-section taken along a plane comprising the rotation axis 15. The electrical machine 1 depicted in FIGS. 6a-6c is for example any one of the electrical machines shown in FIG. 5. FIGS. 6a-6c shows the electrical machine 1 enclosed by a stator housing 25 for example comprising cooling means for cooling the primary stator assembly 4. In the electrical machines 1 depicted in FIGS. 6a-6c, the primary stator assembly 4, the rotor assembly 2 and the secondary stator assembly 7 are radially nested, i.e. upon advancing in the radial direction starting from the rotational axis 15 one would first encounter the secondary stator assembly 7, one would subsequently encounter the rotor assembly 2, and one would finally encounter the primary stator assembly 4. The rotor assembly 2 comprises two interconnected annular parts 22, 23. The primary stator assembly 4 is radially nested with the rotor assembly 2, in particular with its first annular part 22, over a given axial length, referred to as the axial overlap between the primary stator assembly 4 and the rotor assembly 2. The secondary stator assembly 7 is radially nested with the rotor assembly 2, in particular with its second annular part 23, over a given axial length, referred to as the axial overlap between the secondary stator assembly 7 and the rotor assembly 2. In the electrical machine 1 depicted in FIG. 6a, the axial overlap between the primary stator assembly 4 and the rotor assembly 2 is equal to the axial overlap between the secondary stator assembly 7 and the rotor assembly 2. In the electrical machine 1 depicted in FIG. 6b, the axial overlap between the primary stator assembly 4 and the rotor assembly 2 is smaller than the axial overlap between the secondary stator assembly 7 and the rotor assembly 2. The electrical machine 1 depicted in FIG. 6b therefore exhibits an increased influence of the pulsating torque of the magnetic torsion spring and a decreased influence of the average torque of the electromotor in comparison to the same electrical machine 1 provided with an axial overlap as depicted in FIG. 6a. In the electrical machine 1 depicted in FIG. 6c, the axial overlap between the primary stator assembly 4 and the rotor assembly 2 is larger than the axial overlap between the secondary stator assembly 7 and the rotor assembly 2. The electrical machine 1 depicted in FIG. 6c therefore exhibits a decreased influence of the pulsating torque of the magnetic torsion spring and an increased influence of the average torque of the electromotor in comparison to the same electrical machine 1 provided with an axial overlap as depicted in FIG. 6a. The axial lengths of the first annular part 22 and the second annular part 23 are substantially equal to respectively the axial lengths of the primary stator assembly 4 and the secondary stator assembly 7. The first annular part 22 axially overlaps with the primary stator assembly 4 over its entire axial length. The second annular part 23 axially overlaps with the secondary stator assembly 7 over its entire axial length. It is clear that the present embodiment can be easily obtained by interconnecting first and second annular parts 22, 23 having different axial lengths. In a further embodiment (not shown), the desired axial overlaps are obtained by merely selecting different axial lengths of the first annular part 22 and the second annular part 23, i.e. whilst providing the primary stator assembly 4 with an axial length substantially equal to the axial length of the secondary stator assembly 7.
