Axial flux electric machine

Abstract

Axial Flux Electric Machine (8) Axial flux electric machine, the electric machine comprising: - a rotor (10) free to rotate about an axis of rotation (Y), the rotor comprising a first ring of magnets (11) and a second ring of magnets (12) axially distant from the first ring, each ring of magnets being centered on said axis of rotation, and - a stator (9, 9'), without an iron core, the stator extending between the first ring of magnets and the second ring of magnets, the stator comprising at least one induction circuit (20, 31, 32, 33) formed by a set of interlaced electrical conductors. Figure for the abbreviation: Figure 2

Description

Title of the invention: Axial flux electric machine Technical field of the invention

[0001] The invention relates to an axial flux electric machine. The invention further relates to a light vehicle, in particular an electrically assisted bicycle comprising such an electric machine. The invention also relates to a method for manufacturing such an electric machine. Prior art

[0002] Electrically assisted bicycles include an energy storage means such as an electrochemical battery and an electric machine, in particular an electric motor, connected to the electrochemical battery. The electric machine is configured to convert electrical energy supplied by the electrochemical battery into mechanical energy providing assistive torque that facilitates pedaling. Electric machines configured to convert mechanical pedaling energy into electrical energy suitable for recharging the electrochemical battery are also known. Such electric machines can be used during the braking phases of the bicycle to recharge the electrochemical battery, and thus increase its range.

[0003] The stator of electric machines for electrically assisted bicycles traditionally has windings of electrical wire arranged around an iron core. One advantage of such a machine is that the reluctance is independent of the rotor's position, which limits torque oscillations and the resulting losses. However, such electric machines exhibit reduced efficiency due to losses related to the generation of eddy currents in such cores.

[0004] Furthermore, such electric motors are generally heavy and bulky, making their integration into a bicycle quite difficult. Electric motors are usually integrated into a bottom bracket axle or the rear wheel of the electrically assisted bicycle. The space available at these locations is particularly limited. Presentation of the invention

[0005] The object of the invention is to provide an electric machine for an electrically assisted bicycle remedying the above disadvantages and improving upon known electrical machines of the prior art.

[0006] More specifically, a first object of the invention is an electrical machine that is particularly compact relative to its power. Summary of the invention

[0007] The invention relates to an axial flux electrical machine, the electrical machine comprising: - a rotating rotor that rotates around an axis of rotation, the rotor comprising a first ring of magnets and a second ring of magnets axially distant from the first ring, each ring of magnets being centered on said axis of rotation, and - a stator, without an iron core, the stator extending between the first ring of magnets and the second ring of magnets, the stator comprising at least one induction circuit formed by a set of interlaced electrical conductors.

[0008] The stator may include: - an active part extending opposite the first ring of magnets and the second ring of magnets, - an external protrusion comprising an internal diameter strictly greater than an external diameter of the first and second rings of magnets, and - an internal protrusion comprising an outer diameter strictly less than an inner diameter of the first ring of magnets and the second ring of magnets.

[0009] The stator may include: - an active part extending opposite the first ring of magnets and the second ring of magnets, - an external protrusion comprising a thickness strictly greater than the distance separating the first ring of magnets from the second ring of magnets, and - an internal protrusion comprising a thickness strictly greater than the distance separating the first ring of magnets from the second ring of magnets.

[0010] The stator may comprise: - an active part extending opposite the first ring of magnets and the second ring of magnets, - an external protrusion comprising a thickness at least equal to twice the thickness of the active part, and - an internal protrusion comprising a thickness at least equal to twice the thickness of the active part.

[0011] The electrical machine may further include a casing, and a thermal bridge arranged between the outer protrusion and the casing.

[0012] The stator may comprise at least three induction circuits, each induction circuit being formed by a set of interlaced electrical conductors, each induction circuit being offset from other induction circuits, in particular by an angle of 120°, so as to form a multi-phase stator, in particular three-phase.

[0013] The electrical conductors of the stator can be impregnated with a thermally conductive resin so that the stator forms a rigid block.

[0014] The electrical conductors of the stator can be formed by windings of stranded Litz wires.

[0015] The electrical conductors of the stator can be formed by unwrapped electrical wires.

[0016] The invention also relates to a method for manufacturing an electrical machine according to one of the preceding claims, the method comprising a stator manufacturing step, the stator manufacturing step comprising; - the placement of electrical conductors using a matrix, the matrix comprising a set of salient elements around which the electrical conductors are guided so as to form a set of interlaced electrical conductors, then - a step of impregnating the electrical conductors with a resin.

[0017] The invention also relates to a light vehicle comprising an electric machine as defined above, the electric machine being intended to provide a power output between 1,000W and 10,000W inclusive.

[0018] The invention also relates to an electrically assisted bicycle, the bicycle comprising an energy storage means and an electric machine as defined above, the electric machine being electrically connected to the energy storage means. Presentation of the figures

[0019] These objects, features and advantages of the present invention will be described in detail in the following description of a particular embodiment, given by way of non-limiting example, with reference to the accompanying figures, among which:

[0020] Fig. 1 is a schematic view of a bicycle equipped with an electric machine according to one embodiment of the invention.

[0021] Fig. 2 is a schematic cross-sectional view of the electrical machine.

[0022] Figure 3 is a schematic perspective view of a rotor of the machine electric.

[0023] Fig. 4 is a schematic front view of a first embodiment of an induction circuit of a stator of the electric machine.

[0024] Fig. 5 is a schematic front view of a second embodiment of an induction circuit of the stator of the electric machine.

[0025] Fig. 6 is a schematic perspective view of a matrix used to manufacture an embodiment of an induction circuit for the stator of the electric machine. Detailed description

[0026] Figure 1 schematically illustrates an electrically assisted bicycle 1 according to an embodiment of the invention. The bicycle 1 comprises a frame 2 and an energy storage means 3, such as an electrochemical battery and / or a supercapacitor, supported by the frame 2. The energy storage means 3 may, in particular, be arranged inside a housing provided in the frame 2. The bicycle 1 also comprises a crankset 4 equipped with cranks and pedals, and a transmission means 5, for example a chain or a belt, arranged between the crankset 4 and a rear wheel 6 of the bicycle, for transmitting a mechanical force exerted by a user of the bicycle into a rotational torque of the rear wheel. The bicycle 1 also comprises an electrically assisted device 7.The electric assist device 7 is configured to convert electrical energy supplied by the energy storage means 3 into mechanical energy providing assistive torque that facilitates pedaling. In particular, the electric assist device 7 comprises an electric machine 8, notably an electric motor 8. According to the embodiment presented, the electric machine 8 is integrated into the rear wheel 6 of the bicycle, specifically into a hub of the rear wheel of the bicycle. The electric machine 8 includes a housing enabling it to be integrated between two rows of spokes of the rear wheel 6. The electric machine 8 is thus particularly compact in the direction of the axis of rotation of the rear wheel 6. According to an alternative embodiment, the electric machine 8 could also be integrated at the level of a bottom bracket axle 4.

[0027] The electric machine 8 is schematically illustrated in [Fig. 2]. The electric machine 8 can be configured to supply a power output between 1,000 W and 10,000 W. The electric machine 8 comprises a stator 9 and a rotor 10 that rotates relative to the stator about an axis of rotation Y. The axis of rotation Y preferably coincides with an axis of rotation of the rear wheel 6. The stator 9 is secured to the frame 2, for example, by fixing screws. The electric machine 8 is an axial flux machine, that is, an electric machine in which the magnetic flux between the stator 9 and the rotor 10 is substantially parallel to the axis of rotation Y of the rotor. An axial flux electric machine has a smaller footprint and lighter weight compared to a radial flux electric machine for equivalent power output.

[0028] The rotor 10 comprises a first ring of magnets 11 and a second ring of magnets 12 axially distant from the first ring. Each ring of magnets 11, 12 extends in a plane substantially perpendicular to the axis of rotation Y and is centered on said axis of rotation, that is to say that the center of each crown is positioned on the axis of rotation Y.

[0029] We define by el, the distance separating the first ring of magnets 11 from the second ring of magnets 12 along the axis of rotation Y. The distance el is non-zero and sufficiently large to integrate an active part 13 of the stator 9. The stator 9 thus extends mainly between the first ring of magnets 11 and the second ring of magnets 12.

[0030] Each ring of magnets 11, 12 comprises an inner circular contour and an outer circular contour. The inner diameter of the inner circular contour of the first ring of magnets 11 and the inner diameter of the inner circular contour of the second ring of magnets 12 are preferably equal. This inner diameter is referred to as d1 in [Fig. 2]. Similarly, the outer diameter of the outer circular contour of the first ring of magnets 11 and the outer diameter of the outer circular contour of the second ring of magnets 12 are preferably equal. This outer diameter is referred to as d2 in [Fig. 2]. The two rings of magnets 11, 12 preferably have the same shape.

[0031] Figure 3 illustrates, by way of perspective view, the two rings of magnets 11 and 12. Each ring of magnets comprises a plurality of magnets 14 distributed radially around the axis of rotation Y, in particular according to an alternation of north and south poles. Each magnet 14 may have a roughly trapezoidal shape. The first ring of magnets 11 and / or the second ring of magnets 12 preferably comprise the same number of magnets, and in particular each an even number of magnets. The magnets may all be identical and arranged according to an alternation of polarity with the adjacent magnets. The two rings of magnets 11,12 are fixed relative to each other, and arranged so that each magnet of the first ring of magnets 11 faces a magnet of the second ring of magnets 12. The magnets of each ring of magnets can in particular be arranged according to a Halbach lattice.

[0032] The magnet rings 11, 12 are held respectively by a first support 15 and a second support 16. The supports 15 and 16 are fixed rigidly to a shaft 17 of the electric machine 8. The shaft 17 extends along the axis of rotation Y and is fixed to the rear wheel 6. The shaft 17 is supported by two bearings 18, in particular two roller bearings. The bearings 18 are arranged between a housing 19 of the electric machine and the shaft 17. The housing 19 may have the overall shape of a cylinder of revolution.

[0033] The stator 9 extends perpendicularly to the axis of rotation Y, between the first ring of magnets 11 and the second ring of magnets 12. The stator is a coreless stator, that is, without an iron or iron alloy element intended to guide a Magnetic field. One advantage of using a stator without an iron core is the elimination of losses associated with the generation of eddy currents in such cores. Indeed, such currents would be dissipated as heat due to the internal resistance of the iron cores. Such currents would lead to a temperature rise in the electrical machine and a degradation of its efficiency.

[0034] With reference to [Fig. 4], the stator 9 comprises at least one induction circuit 20. The induction circuit 20 is electrically connected to the energy storage means 3. When an electric current flows through the induction circuit 20, the stator 9 produces a magnetic field that interacts with the magnets of the rotor 10 to generate a rotational torque that assists the pedaling effort. Conversely, when the rotor is driven to rotate, for example by the inertia of the bicycle or by the pedaling force of its user, an electric current is generated in the induction circuit 20. This electric current can then recharge the energy storage means 3.

[0035] The induction circuit 20 is formed by a set of interlaced electrical conductors. "Interlaced" means that the electrical conductors are interwoven or entangled, in particular braided together. The interlacing of the electrical conductors reduces electromagnetic interference and capacitive coupling between adjacent electrical conductors, thereby improving the performance and reliability of the electrical machine. Furthermore, this configuration facilitates the installation of the electrical conductors by reducing their size and allowing for better organization of the conductors.

[0036] Electrical conductors are preferably wires made of conductive material, for example, copper wires. Advantageously, the stator's electrical conductors are formed by windings of stranded Litz wires. Stranded Litz wire windings offer several advantages over traditional solid copper wires. First, they reduce skin effect losses: at high frequencies, electric currents tend to flow mainly on the surface of the electrical conductor, a phenomenon known as the skin effect. Litz wires are designed to reduce this effect by dividing the conductor into several thinner wires, thus allowing for better current distribution over the entire cross-section of the wire. In addition, stranded Litz wire windings reduce losses caused by magnetic fields generated by currents flowing in adjacent conductors.Stranded Litz wires minimize these losses by distributing the conductors in a way that reduces interactions between them. Furthermore, stranded Litz wires are generally more flexible than solid wires, making them easier to handle and install. In addition, the twisting process strengthens the wire structure, thus improving its mechanical strength. The use of windings... using stranded Litz wires thus improves the efficiency, performance and durability of the electrical machine 8.

[0037] Preferably, the stator's electrical conductors are formed by unsheathed electrical wires. The absence of individual sheaths around the wires frees up space, which is then used to house more electrical conductors. This increases the number of conductors in the active part of the induction circuit.

[0038] To ensure their electrical insulation, the electrical conductors are impregnated with a thermally conductive resin after being shaped. The stator thus forms a rigid block. The electrical conductors are therefore not at risk of moving relative to each other during the use of the bicycle, for example under the effect of vibrations. The electric machine 8 thus offers stable and durable performance.

[0039] As can be clearly seen in [Fig. 2], the induction circuit 20 comprises said active part 13, but also an external protrusion 22, or external head 22, and an internal protrusion 23, or internal head 23. The electrical conductors of the induction circuit preferably extend radially at the level of the active part 13. The electrical conductors comprise a loop, for example in the shape of a U, at the level of the external protrusion 22 and at the level of the internal protrusion 23. The electrical conductors make substantially a half-turn at the level of the external protrusion 22 and the internal protrusion 23.

[0040] The stator 9 is fixed to the housing 19 via a thermal bridge 24. The thermal bridge 24 is arranged between the outer protrusion 22 and the housing 19. The thermal bridge 24 may be ring-shaped and is advantageously made of a heat-conducting material. The heat produced by the Joule effect during the flow of an electric current in the induction circuit can thus be dissipated to the outside of the electric machine. Another advantage of having a relatively large outer protrusion is to increase the heat exchange surface area of ​​the stator: the heat can thus be dissipated more efficiently to the housing 19. The induction circuit 20 is electrically insulated from the housing 19 at least by the thermally conductive resin of the second ring of magnets 12 along the axis of rotation Y.

[0041] The outer protrusion 22 and the inner protrusion 23 do not extend opposite the magnet rings 11 and 12. In particular, on the one hand, the outer protrusion 22 has an inner diameter d3 strictly greater than the outer diameter d2 of the first magnet ring 11 and the second magnet ring 12. The outer protrusion 22 thus extends into a free volume between the housing 19 and the outer edge of the supports 15 and 16. On the other hand, the inner protrusion 23 has an outer diameter d4 strictly less than to an inner diameter of the first ring of magnets and the second ring of magnets. The inner protrusion 23 extends in a free volume between the shaft 17 and the inner circumferences of the rings of magnets 11, 12.

[0042] Since the protrusions 22 and 23 do not extend opposite the rings of magnets 11 and 12, they can have thicknesses strictly greater than the distance e1 separating the first ring of magnets from the second ring of magnets along the axis of rotation Y. The outer protrusion 22 has a thickness e3 strictly greater than a distance e1. Similarly, the inner protrusion 23 has a thickness e4 strictly greater than the distance e1. This greater thickness of the outer protrusion 22 and the inner protrusion 23 can notably be caused by the rigidity of the electrical conductors that make up the induction circuit 20: indeed, when the electrical conductors are bent to make a half-turn, they generally have a radius of curvature greater than or equal to a given value which depends in particular on the thickness of the electrical conductors.Although the use of Litz wires reduces this radius of curvature, it remains significant. In particular, the thickness e3 of the outer protrusion 22 can be at least twice the thickness e2 of the active part 13. Similarly, the thickness e4 of the inner protrusion 23 can be at least twice the thickness e2 of the active part 13. Furthermore, the thickness e3 of the outer protrusion 22 can be strictly greater than the thickness e4 of the inner protrusion 23.

[0043] Fig. 4 illustrates, in front view, the induction circuit 20. The induction circuit 20 comprises an annular crenellated line shape. The induction circuit 20 includes, in particular, radial segments 25 extending into the active part 13, external tangential segments 26 extending into the outer protrusion 22, and internal tangential segments 27 extending into the inner protrusion 23. The induction circuit preferably includes as many external or internal tangential segments 26, 27 as there are magnets 13 in the first ring of magnets 11 or in the second ring of magnets 12. The induction circuit 20 also includes two terminals 28, 29, at each of its two ends, through which the stator is connected to the energy storage means 3.

[0044] Figure 5 illustrates a front view of a stator 9' according to an embodiment of the invention. According to this embodiment, the stator 9' comprises three induction circuits 31, 32, 33 such as the induction circuit 20 described previously. Each induction circuit is therefore also formed by a set of interlaced electrical conductors. Each induction circuit 31, 32, and 33 is offset from the other induction circuits by an angle of 120° so as to form a three-phase stator. Generally, the stator can be multi-phase, that is, it can have m phases, where m is an integer. Preferably, the stator is three-phase with m = 3. Another embodiment is a six-phase stator with m = 6. Another embodiment is a five-phase stator with m = 5.

[0045] To manufacture the stator 9, 9' as defined above, a die 34 is advantageously used, comprising a set of projecting elements 35 around which the electrical conductors are guided and positioned, so as to form a set of interlaced electrical conductors. An example of such a die is shown in [Fig. 6]. The electrical conductors can then be pressed and compacted against the die 34. The die 34 can then be removed, and the electrical conductors can be impregnated with a thermally conductive resin so as to form a single unit. The stator 9, 9' thus manufactured can then be assembled with the other components of the electrical machine 8.

[0046] Thanks to the invention, we have an electric machine 8 that is particularly compact relative to its power. Such a machine is therefore more easily integrated into a bicycle, especially into a bicycle's rear wheel. Furthermore, the resulting electric machine efficiently dissipates the heat it generates. Although very compact, the electric machine 8 thus maintains relatively low operating temperatures.

[0047] The electric machine 8 described above in relation to an electrically assisted bicycle 1 could more generally be integrated into any type of light vehicle suitable for transporting people and / or goods, for example a tricycle or even more generally any pedal-powered vehicle, or an electric scooter, or even an electrically assisted cart. The electric machine 8 could advantageously be used to provide a power output between 1,000 W and 10,000 W inclusive.

Claims

Demands

1. Axial flux electric machine (8), the electric machine comprising: - a rotor (10) movable in rotation about an axis of rotation (Y), the rotor comprising a first ring of magnets (11) and a second ring of magnets (12) axially distant from the first ring, each ring of magnets being centered on said axis of rotation, and - a stator (9, 9'), without an iron core, the stator extending between the first ring of magnets and the second ring of magnets, the stator comprising at least one induction circuit (20, 31, 32, 33) formed by a set of interlaced electrical conductors.

2. Electric machine (8) according to the preceding claim, characterized in that the stator (9, 9') comprises: - an active part (13) extending opposite the first ring of magnets (11) and the second ring of magnets (12), - an external protrusion (22) comprising an inner diameter (d3) strictly greater than an outer diameter (d2) of the first ring of magnets and the second ring of magnets, and - an inner protrusion (23) comprising an outer diameter (d4) strictly less than an inner diameter (dl) of the first ring of magnets and the second ring of magnets.

3. An electric machine (8) according to any one of the preceding claims, characterized in that the stator (9, 9') comprises: - an active part (13) extending opposite the first ring of magnets (11) and the second ring of magnets (12), - an external protrusion (22) comprising a thickness (e3) strictly greater than a distance (el) separating the first ring of magnets from the second ring of magnets, and - an internal protrusion (23) comprising a thickness (e4) strictly greater than the distance (el) separating the first ring of magnets from the second ring of magnets.

4. Electric machine (8) according to any one of the preceding claims, characterized in that the stator (9, 9') comprises: - an active part (13) extending opposite the first ring of magnets (11) and the second ring of magnets (12), - an external protrusion (22) comprising a thickness (e3) at least equal to twice a thickness (e2) of the active part, and - an internal protrusion (23) comprising a thickness (e4) at least equal to twice a thickness (e2) of the active part.

5. Electric machine (8) according to any one of claims 2 to 4, characterized in that it further comprises a casing (19), and a thermal bridge (24) arranged between the external protrusion (22) and the casing.

6. Electric machine (8) according to any one of the preceding claims, characterized in that the stator (9') comprises at least three induction circuits (31, 32, 33), each induction circuit being formed by a set of interlaced electrical conductors, each induction circuit being offset from the other induction circuits, in particular by an angle of 120°, so as to form a multi-phase stator, in particular three-phase.

7. Electric machine (8) according to any one of the preceding claims, characterized in that the electrical conductors of the stator (9, 9') are impregnated with a thermally conductive resin so that the stator forms a rigid block.

8. Electric machine (8) according to any one of the preceding claims, characterized in that the electrical conductors of the stator (9, 9') are formed by windings of stranded Litz wires.

9. Electric machine (8) according to any one of the preceding claims, characterized in that the electrical conductors of the stator (9, 9') are formed by unwrapped electrical wires.

10. A method for manufacturing an electrical machine (8) according to any one of the preceding claims, characterized in that it comprises a stator manufacturing step (9, 9'), the stator manufacturing step comprising: - the placement of electrical conductors by means of a matrix (34), the matrix comprising a set of salient elements (35) around which the electrical conductors are guided so as to form a set of interlaced electrical conductors, and then - a step of impregnating the electrical conductors with a resin.

11. A light vehicle, characterized in that it comprises an electric machine (8) according to any one of claims 1 to 9, the machine electric being intended to provide a power output between 1,000W and 10,000W inclusive.

12. Electric assist bicycle (1), characterized in that it comprises an energy storage means (3) and an electric machine (8) according to any one of claims 1 to 9, the electric machine being electrically connected to the energy storage means.

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