Rotor-wound electric machine

Abstract

Rotor-Wound Electric Machine The present invention relates to an electric machine (EM) equipped with at least: - a housing (1), - a stator, - a wound rotor comprising a rotor body fixed to a shaft rotating (3) about an axis of rotation (X), - a bearing (2) mounted in the housing of the electric machine (EM) and in which one end of the rotating shaft (3) is rotatably mounted, the end of the rotating shaft (3) comprising at least one electrical track (10, 16), - an excitation system comprising at least one brush (51, 52) disposed in contact with the electrical track (10, 16), the electric machine (EM) being characterized in that the electrical track (10, 16) comprises a contact surface with the brush (51, 52), disposed orthogonally to the axis of rotation (X). (Figure 1)

Description

Title of the invention: Rotor-excited electric machine

[0001] The present invention relates to the fields of mechanics and electrotechnics and concerns a wound-rotor electric machine using a ring and brush excitation system, finding particular application in motor vehicles.

[0002] The use of a wound rotor in an electric or hybrid vehicle electric motor eliminates the need for rare-earth magnets. Such a wound-rotor electric motor typically uses a system of conductive rings arranged around a rotating shaft of the wound rotor and connected to the rotor windings. These conductive rings receive an electric current from radially arranged, fixed carbon brushes that rub against the rotating conductive rings.

[0003] However, in certain vehicle powertrains incorporating an electric traction or propulsion machine, the rotor of the electric machine is connected at one end of its rotating shaft to a gear in a gearbox. This gear generates tooth forces that can cause the rotating shaft to resonate at speeds of approximately 5000 revolutions per minute. Under the effect of this resonance, the ends of the rotating shaft can then move radially by about twenty microns due to shaft deflection. In this case, the use of a conventional system of slip rings and brushes is unsuitable because the brushes are no longer in continuous contact with the conductive rings, which risks generating electrical arcs and damaging the electric machine.

[0004] Another drawback of conventional slip-ring and brush excitation systems is the temperature rise of the conductive rings due to the high current flowing through them and the significant differential speed between the brushes and these rotating conductive rings. The heat generated must be dissipated to prevent damage to these materials and the performance of the electrical machine.

[0005] Furthermore, carbon brushes generate carbon dust that must be kept from coming into contact with the live parts of the electric machine. A seal is therefore generally arranged between a wound rotor excitation system, comprising the carbon brushes, and a wound rotor body. This seal rubs against the rotating shaft of the wound rotor, causing it to heat up. The heat produced by friction on the seal must also be dissipated to prevent it from deteriorating too quickly.

[0006] Usually, the heat produced at the carbon brushes, conductive rings, and sealing gasket is dissipated by natural air convection. The rotor is in thermal contact on one side with these elements and on the other with metallic parts of the electrical machine, such as a metal casing that may include a cooling jacket. For this cooling to be sufficient, the amount of material in these metallic parts, and in particular their thickness, must be relatively large, which makes the wound rotor expensive.

[0007] The sealing gasket is, for example, arranged in an opening in a wall of the electric machine's housing, being radially wedged in the opening between the housing and an axial extension of an inner ring of a bearing in which the rotating shaft is mounted. Since the bearing is connected to an electrical ground via its outer ring, the carbon dust, kept away from the rotor body by the sealing gasket, must not touch the inner ring against which it rubs, to avoid creating a short circuit. A minimum electrical insulation distance from the carbon brushes must therefore be maintained, which necessitates a substantial axial dimensioning of the rotating shaft of the wound rotor.

[0008] The present invention aims to remedy at least in part the aforementioned drawbacks by providing a wound-rotor electric machine, using an excitation system based on conductive rings and brushes, in which the brushes are mounted axially with respect to a rotating shaft of the wound rotor.

[0009] To this end, the invention proposes an electrical machine equipped with at least: - a casing, - a stator, - of a wound rotor comprising a rotor body fixed to a shaft rotating around an axis of rotation, - of a bearing mounted in the casing of the electrical machine and in which one end of the rotating shaft is rotatably mounted, the end of the rotating shaft having at least one electrical track, - of an excitation system comprising at least one brush disposed in contact with the electric track, the electric machine being characterized in that the electric track comprises a contact surface with the brush, disposed orthogonally to the axis of rotation.

[0010] Thanks to the invention, the brush remains in contact with the electrical track, even when the latter is subjected to radial displacement due to the rotating shaft entering resonance. Indeed, in this case, the axial displacement of the ends of the rotating shaft is much less than their radial displacement. The invention reduces the risk of shaft damage due to this resonance phenomenon by a factor of ten.

[0011] It should be noted that in this patent application, the term "axial" refers, unless otherwise stated, to a direction parallel to the axis of rotation of the rotor of the electric machine. Similarly, the term "radial" refers, unless otherwise stated, to a direction orthogonal to the axis of rotation of the electric machine rotor, and secant to this axis of rotation, while the terms "angular" or "ortho-radial" refer, unless otherwise stated, to a direction orthogonal to the axial direction and to a radial direction, this orthogonal direction being in fact rotating around the axis of rotation of the rotor.

[0012] In the invention, the brush is mounted axially or radially with respect to the rotating shaft, but is in contact with the contact surface arranged orthogonally to the axis of rotation. By "orthogonally" is meant with a tolerance of a few percent with respect to a direction orthogonal to the axis of rotation.

[0013] Preferably, the excitation system comprises two brushes and two electrical tracks arranged at the same end of the rotating shaft, but alternatively, each brush and each track is arranged at different ends of the rotating shaft. In another embodiment, the excitation system does not use a slip ring and brush for one of the supply terminals, but, for example, a capacitive coupling.

[0014] In one embodiment of the invention, the electric machine comprises two brushes and two electrical tracks, the end of the rotating shaft comprises a first portion of a first diameter, comprising a first axial end surface, and a second portion of a second diameter smaller than the first diameter, projecting from the first portion and comprising a second axial end surface, a first of the electrical tracks being brought onto the first axial end surface and a second of the electrical tracks being brought onto the second axial end surface.

[0015] This embodiment allows for a smaller footprint of the excitation system at the end of the rotating shaft. Preferably, the brushes are mounted axially with respect to the rotating shaft, each axial end surface being in contact with one of the brushes.

[0016] The contact surface of the first electrical track takes, for example, the form of a disk with a hole in its center, while the contact surface of the second electrical track takes the form of a solid disk. It is understood that the contact surface of the first electrical track is located on the first axial end surface and that the contact surface of the second electrical track is located on the second axial end surface.

[0017] The excitation system comprises, for example, first brushes in contact with the first electrical track, and a single second brush in contact with the second electrical track. This embodiment is possible because the first electrical track is much wider angularly than the second electrical track.

[0018] To ensure secure brush retention, the excitation system is, for example, equipped with a housing comprising a first support proximal to the rotor. This first support has a flat annular surface facing the first electrical track. This flat annular surface is perforated by holes for the passage of the first brushes. The first support forms a well in the center of the flat annular surface for the passage of the second brush. The housing comprises a second support distal to the rotor, on which metal cages are fixed, in which the first and second brushes are mounted in translation. The supports of the excitation system housing are preferably made of electrically insulating material. The first support of the housing forms, for example, a base for the housing, and the second support of the housing forms, for example, a cover that fits into the base.

[0019] In order to supply current to the brushes, the excitation system includes, for example, on the one hand, a first electrical connector passing through the second support of the housing and in electrical contact inside the housing with a metal ring in contact with the metal cages of the first brushes, and on the other hand, a second electrical connector passing through the second support of the housing and in electrical contact inside the housing with the metal cage of the second brush.

[0020] According to an optional and advantageous feature of this embodiment of the invention, the housing contains a heat sink and thermally conductive and electrically insulating material blocks, in contact on one side with the metal cages of the first brushes and on the other with the heat sink. The heat sink is, for example, a radiator with cooling fins. The material blocks efficiently dissipate the heat emitted by the current flowing through the first brushes and the friction of the first brushes to the heat sink. These material blocks are known as "thermal pads" or "thermal paste" in the electronics field. Some formulations are silicone-based, for example with graphite, while some thermal pastes are synthetic and guaranteed silicone-free. All these thermal pastes are dielectric.The blocks of material are glued or deposited onto the heat sink and onto the metal cages. Alternatively, they are not in direct contact with the first brushes and the heat sink, but via other elements with good thermal conductivity.

[0021] According to another optional and advantageous feature of this embodiment of the invention, the first housing support has at least one hole adapted to allow the passage of dust generated by at least one of the brushes, the hole being located under at least one of the contact surfaces of the first or second electrical tracks with the first brushes or, respectively, the second brush. The first housing support has, for example, a hole under the contact surface of the first electrical track. with the first brushes, and a hole under the contact surface of the second electrical track with the second brush, this latter hole being made in the well formed by the first support of the housing. By "under the contact surface" is meant under a horizontal plane located below this contact surface, so that dust falls by gravity into the hole located below this contact surface.

[0022] According to another optional and advantageous feature of this embodiment of the invention, a wall of the housing separates the rotor body from the excitation system and has an opening in which a sealing device is fixed in contact with the end of the rotating shaft, the bearing being fixed in a counterbore of the opening and disposed axially between the rotor body and the sealing device, the first electrical track having a cylindrical part, connected to the contact surface of the first electrical track on the side opposite the first brushes, and coming into contact with the sealing device.

[0023] The opening in which the bearing is mounted is, for example, located in a wall of the bottom or cover of the electrical machine housing, or in an intermediate wall of the electrical machine housing. The sealing device is, for example, a seal such as a lip seal made of elastomeric material. This is, for example, fixed between the cylindrical part of the first electrical track and a metal ring covering an internal surface of the excitation system housing made of insulating material, this internal surface also being fixed in the opening of the housing wall. Indeed, such an elastomeric sealing device must be clamped between two metal parts, preferably insulated from the ground.

[0024] Thanks to this arrangement of the sealing device, dust generated by the brushes does not penetrate the active parts of the electrical machine and cannot generate a short circuit, even when the brushes are close to the sealing device. This is because the device is not in contact with a metallic part connected to ground, but rather with the insulating housing or the first electrical track, which is itself insulated from ground. Furthermore, the heat generated by the friction of the seal is efficiently dissipated by the distribution of this heat across the plurality of first brushes and by the thermally conductive blocks of material in thermal contact between these first brushes and the heat sink.

[0025] In this embodiment of the invention, the end of the rotating shaft comprises, for example, a cylindrical steel end cap having at least two axial grooves in which are fitted branches of an electrically insulating cap on which are arranged the first and second electrical tracks, the cap including electrical conductors connected each by one of their ends to the first or to the second electrical track, and by the other of their ends to the windings of the rotor.

[0026] The cap overmolds, for example, the electrical conductors. This design is simple to implement, in particular it does not require any passages to be cut inside the rotating shaft for the passage of the electrical conductors.

[0027] The invention also relates to an electric or hybrid vehicle comprising the electric machine according to the invention. The vehicle according to the invention has advantages similar to those of the electric machine according to the invention.

[0028] Other features and advantages of the invention will become apparent from the following description on the one hand, and from several illustrative and non-limiting examples of embodiments given with reference to the accompanying schematic drawings on the other hand, in which:

[0029] [Fig. 1] represents in axial section a portion of an electrical machine according to the invention, in one embodiment of the invention,

[0030] [Fig.2] is a perspective view of a cylindrical steel end piece of a shaft rotating of a rotor of the electric machine of the [Fig.l],

[0031] [Fig.3] is a perspective view of an insulating cap comprising conductors electrical and electrical tracks and intended to be attached to the cylindrical end of the [Fig.2], and

[0032] [Fig.4] is a perspective view of an excitation system mounted on one end of the rotating shaft of the electric machine in [Fig.1], in which a cover of the excitation system has been removed.

[0033] According to an embodiment of the invention shown in [Fig. 1], an electric machine ME according to the invention is of the radial flux electric machine type. It comprises a stator (not shown) and a wound rotor, the latter comprising a rotor body and a rotating shaft 3 to which it is fixed, the wound rotor being mounted to rotate about an axis of rotation X. The stator and the wound rotor are housed in a casing 1 of which only a portion of the wall is shown in [Fig. 1].

[0034] The rotor body is, for example, formed from a stack of magnetic steel laminations, shrink-fitted onto the rotating shaft 3, the laminations forming teeth around which electrical wires are wound to form rotor poles. Of course, the wound rotor may have a different design from that described here; the rotor body may be assembled onto the rotating shaft by a means other than shrink-fitting, and may not consist of a stack of laminations but rather a different assembly to form the rotor teeth.

[0035] Figure 1 shows in axial section only a portion of the end of the electric machine ME centered at an opening in the wall of the housing 1, in which one end of the rotating shaft 3 is mounted. The wall of the housing 1 is, for example, a cover or bottom wall of the housing 1, or an intermediate wall of the housing 1, i.e., integral with a fixed part of the electric machine ME. The housing 1 is, for example, made of aluminum.

[0036] The rotating shaft 3 comprises a steel body, and an insulating cap 12 covering a cylindrical end 30 of the steel body of the rotating shaft 3.

[0037] Fig. 2 shows the cylindrical steel end cap 30 of the rotating shaft 3, and Fig. 3 represents the insulating cap 12 designed to fit onto the cylindrical end cap 30.

[0038] The cylindrical steel end piece 30 comprises a first bearing and a second bearing respectively, extending from a larger diameter central portion having grooves for shrink-fitting the stack of laminations forming the rotor body. Two axial grooves 18 are cut on either side of these bearings, i.e., at 180° to each other, so as to receive each one insulating arm 120 of the insulating cap 12. Each axial groove 18 forms the same flat surface in both bearings, on which one of the insulating arms 120 can slide.

[0039] These insulating branches 120 are flush with the surface of the second bearing and have a radial extension at the level of the first bearing so as to reach, at the surface of the first bearing, an insulating ring 126 of the insulating cap 12, connecting the ends of the insulating branches 120 and bearing against the shoulder formed by the first bearing.

[0040] The insulating cap 12 rests against an axial end surface of the second bearing of the cylindrical steel end piece 30, to form the end portion of the rotating shaft.

[0041] Returning to [Fig. 1], the rotating shaft 3 comprises:

[0042] - the cylindrical steel tip 30 equipped with insulating branches 120, - a first end portion 122 formed by a first cylindrical portion of the insulating cap 12 from which the insulating branches 120 extend towards the rotor body, this first end portion 122 having a first diameter equal to that of the second bearing of the steel body, and

[0043] - a second end portion 124, formed by a second portion cylindrical of the insulating cap 12 and protruding from the first end portion 122 of the rotating shaft 3, this second end portion 124 being of a second diameter strictly smaller than the first diameter.

[0044] The insulating cap 12 serves as a support for a first electrical track 10 and a second electrical track 16, made, for example, of copper. The first electrical track 10 is in the form of a cylindrical cup with a hole in its center. More precisely, the first electrical track 10 comprises a cylindrical portion 101 attached to a cylindrical surface of the first end portion 122 of the rotating shaft 3, and a flat portion 102 attached to an axial end surface of the first end portion 122 of the rotating shaft 3.

[0045] The second electrical track 16 is in the form of a metallic disc in contact with an axial end surface of the second end portion 124 of the rotating shaft 3, this second portion 124 protruding from the first portion 122 of the end of the rotating shaft 3 through the central hole of the cylindrical cup formed by the first electrical track 10.

[0046] The insulating cap 12 allows the ends of the rotor windings to be connected to the first and second electrical tracks 10, 16 in a manner electrically isolated from the steel body of the rotating shaft 3.

[0047] As seen in [Fig.1], two ends 13 of winding wires are soldered, one into a positive electrical lug 14 and the other into a negative electrical lug 15, these electrical lugs 14, 15 protruding from the insulating ring 126, in which they are connected to electrical conductors 37 and 36 respectively, overmolded each into one of the insulating branches 120 of the insulating cap 12.

[0048] The electrical conductor 37 is connected at one end to the positive electrical terminal 14 and at the other end to the second electrical track 16, which is therefore a positive supply terminal for the wound rotor.

[0049] The electrical conductor 36 is connected at one end to the negative electrical terminal 15 and at the other end to the first electrical track 10, which is therefore a negative supply terminal for the wound rotor. The electrical conductors 36 and 37 are, for example, made of copper and the insulating cap 12, for example, made of polyamide.

[0050] A ball bearing 2 is mounted in the opening of the wall of the housing 1, the outer ring of the bearing 2 being disposed against the housing 1 and the inner ring of the bearing 2 being disposed against the cylindrical steel portion 30 of the rotating shaft, on certain angular portions thereof, and against the two arms 120 of the insulating cap 12, on the other angular portions of the cylindrical steel portion 30.

[0051] The electric machine ME also includes an excitation system housed in a rigid insulating material casing 25, the latter being formed of a base 4 and a cover 5. The base 4 of the casing 25 has a first cylindrical wall mounted tightly in the opening of the casing wall 1, on the side opposite the rotor body with respect to the casing wall 1. The ball bearing 2 is arranged axially between the rotor body and the first cylindrical wall of the base 4 of the casing 25.

[0052] A metal ring is fixed to the inner surface of the first cylindrical wall of the base 4, to receive a sealing device 9. The sealing device 9, here a lip seal, is glued by its cylindrical portion to the metal ring, and its lip comes into contact with the cylindrical part 101 of the first electrical track 10. The sealing device 9 prevents the cooling oil of the active parts of the electric machine ME from reaching further into the housing 25 of the excitation system of the wound rotor.

[0053] The base 4 of the housing 25 also has a flat wall opposite the flat part 102 of the first electrical track 10, this flat wall having passage holes for first brushes 51 coming into contact with the flat part 102 of the first electrical track 10. This flat wall has an overall ring shape and is connected at its center to a well formed by the base 4 of the housing 25, at the bottom of which is arranged a passage hole for a second brush 52 coming into contact with the second electrical track 16. The base 4 of the housing 25 therefore has an overall shape complementary to the first and second end portions 122, 124 of the rotating shaft 3, this shape being achieved at least in part by the first cylindrical wall of the base 4, the flat wall of the base 4 and the well formed by the base 4.

[0054] The base 4 also includes a second cylindrical wall for receiving the cover 5 of the housing 25, coaxial with the first cylindrical wall and located opposite it with respect to the flat wall of the base 4. This second cylindrical wall is open towards the outside of the housing 1.

[0055] It is therefore understood that the brushes 51, 52, which are for example carbon brushes, are arranged, in this embodiment of the invention, axially with respect to an axis of rotation X of the rotating shaft.

[0056] The brushes 51, 52 are mounted movable in translation each in a metal cage 7, in which an axially mounted pressure spring 6 maintains the contact between the brushes 51, 52 and the corresponding electrical track 10 or 16.

[0057] The metal cages 7 each have an axial end for fixing to the cover 5 of the housing 25 of the excitation system, on the side opposite the contact surfaces of the brushes 51, 52 with the electrical tracks 10, 16. The axial fixing ends of the metal cages 7 are conductive and each welded to one end of a metal braid 21 extending inside the pressure spring 6 present in the corresponding metal cage 7, the other end of the metal braid 21 being in contact with the corresponding carbon brush.

[0058] This fixing of the metal cages 7 to the cover 5 of the housing 25 is carried out via electrical connectors 23 and 24.

[0059] The first electrical connector 23 passes through the cover 5, is fixed to it, and forms a first power supply terminal for the excitation system. This first electrical connector 23 is itself fixed, for example made of material, to a metal ring 22 fixed to the axial fixing ends of the metal cages 7 of the first brushes 51.

[0060] Similarly, the second electrical connector 24 passes through the cover 5, is secured to it, and forms a second power supply terminal for the excitation system. This second electrical connector 24 is secured, on the inner side of the housing 25, to the axial mounting end of the metal cage 7 of the second brush 52, for example by welding. The second electrical connector 24 here takes the form of a rivet.

[0061] The fact that the brushes 51, 52 and the electrical connectors 23, 24 are integral with the cover 5 of the housing 25 greatly facilitates the assembly of the excitation system in the ME electric machine, and in particular the positioning of the brushes 51, 52.

[0062] The contact surfaces of the electrical tracks 10, 16 with the brushes 51, 52 respectively are respectively on the one hand the flat surface facing the first brushes 51, of the flat wall 102 of the first electrical track 10, and on the other hand the flat surface of the second electrical track 16 facing the second brush 52. These contact surfaces being orthogonal to the axis of rotation X of the rotor, they allow the first brushes 51 and the second brush 52 to always be in contact with their respective electrical tracks 10 and 16, despite a possible radial displacement of the end portion of the rotating shaft 3 due to the entry into resonance of the rotating shaft 3.

[0063] Furthermore, it should be noted that the electrical tracks 10, 16 are concentric but not coplanar in order to respect a minimum electrical isolation distance between them.

[0064] By way of example, the tracks 10 and 16 are axially offset from each other by approximately 9 mm (millimeters) in this embodiment of the invention, in order to reduce the risk of contamination and short circuits due to wear dust from the carbon brushes 51, 52. The second electrical track 16 has a significantly smaller surface area than the first electrical track 10, and a second diameter of approximately 8.5 mm. This is due to the fact that the differential speed of the second electrical track 16 with the second brush 52 is lower than that of the first electrical track 10 with the first brushes 51, and to the fact that the center of the second brush 52 is theoretically always in contact with this first, smaller electrical track 10. The dimensions of the electrical tracks 10, 16 are, of course, related to the current flowing through them, which in this embodiment is approximately 25 A (amperes).

[0065] The first electrical track 10 is larger than the second electrical track 16 because its dimensions take into account, in addition to the current flowing through it, the amount of heat to be dissipated, this amount of heat being greater than for the second electrical track 16. Indeed, the first electrical track 10 is subject to greater brush friction due to the greater differential speed between the first brushes 51 and the first electrical track 10 than between the second brush 52 and the second electrical track 16. Furthermore, the first electrical track 10 must also dissipate the heat due to the friction of the sealing device 9 against the cylindrical part 101 of the first electrical track 10. This sealing device notably prevents the cooling oil from entering the track. of the ME electric machine to mix with the wear dust of the brushes 51, 52, such a mixture being able to give rise to a conductive paste which could generate current leaks in the excitation system.

[0066] In this embodiment, as shown [Fig.4], the excitation system comprises three first brushes 51 angularly and regularly distributed around the axis of rotation X of the rotor, in order to increase the thermal conductivity between the first electrical track 10 and a heat sink 11 disposed in the excitation system, and thus to better dissipate heat.

[0067] The heat sink 11 is here a radiator attached to the base 4 of the housing 25. The radiator is for example made of aluminum and has a cylindrical portion 110 surrounding the first brushes 51, fixed to the second cylindrical wall of the base 4 of the housing 25, and cooling fins 112 positioned between the first brushes 51.

[0068] Blocks of material 8 (visible [Fig.1]) that are thermally conductive and electrically insulating, for example silicone, are glued on one side to the metal cages 7 of the first brushes 51 and on the other side to the cylindrical portion 110 of the heat sink 11. The glue used is for example a silicone-based glue.

[0069] The heat generated by the friction of the lip seal against the first electrical track 10 is thus evacuated via the first brushes 51 and the blocks of material 8, into the heat sink 11, itself cooled by the natural convection of the air. The second brush 52 is not cooled in a manner similar to the first brush 51 in this embodiment of the invention, its cooling requirement being less because the second electrical track 16 is only subjected to the friction of the second brush 52. Alternatively, it is cooled by means other than natural air convection, for example by means of another block of material 8 that is thermally conductive and electrically insulating in contact on one side with the metal cage 7 of the second brush 52 and on the other side with the heat sink 11 or with a cooling plate.

[0070] Furthermore, the housing 25 of the excitation system has two holes 19 located in a lower part of the base 4 of the housing, for the passage of wear dust from the first brushes 51 and the second brush 52. One of the holes 19 is located below the contact surface between the first brushes 51 and the first electrical track 10, in a substantially horizontal portion of a wall of the base 4 connecting the first cylindrical wall of the base 4 to the flat wall of the base 4 having the passage holes for the first brushes 51. A second of the holes 19 is located in a cylindrical wall of the base 4 forming the passage well for the second brush 52, below the contact surface between the second brush 52 and the second electrical track 16.

[0071] Finally, a reverse osmosis membrane 20 is arranged in the upper part of the cover 5 of the housing 25, to prevent the pressure of the air from rising in the housing while preventing the passage of water into the housing.

[0072] Of course, the invention is not limited to the examples just described, and many modifications can be made to these examples without departing from the scope of the invention. In particular, the characteristics of the different embodiments of the invention envisaged in this application can be combined to carry out the invention, provided that these embodiments are not incompatible with each other.

Claims

Demands

1. An electrical machine (EM) equipped with at least: - a housing (1), - a stator, - a wound rotor comprising a rotor body integral with a shaft rotating (3) about an axis of rotation (X), - a bearing (2) mounted in the housing of the electrical machine (EM) and in which one end of the rotating shaft (3) is rotatably mounted, the end of the rotating shaft (3) comprising, on the one hand, a first portion (122) of a first diameter, comprising a first axial end surface, and on the other hand, a second portion (124) of a second diameter smaller than the first diameter, projecting from the first portion (122) and comprising a second axial end surface, the rotating shaft comprising a first electrical track (10) attached to the first axial end surface and a second electrical track (16) attached to the second axial end surface,- of an excitation system comprising a first brush (51) disposed in contact with the first electrical track (10), and a second brush (52) disposed in contact with the second electrical track (16), the first and second electrical tracks (10, 16) each having a contact surface with the first or second brush (51, 52), disposed orthogonally to the axis of rotation (X), the contact surface of the first electrical track (10) taking the form of a disk with a hole in its center, and the contact surface of the second electrical track (16) taking the form of a solid disk, the electrical machine (EM) being characterized in that the excitation system comprises several first brushes (51) in contact with the first electrical track (10), and a single second brush (52) in contact with the second electrical track (16).

2. An electric machine (EM) according to claim 1, wherein the excitation system is provided with a housing (25) having a first support (4) proximal to the rotor, the first support (4) having a flat annular surface opposite the first electrical track (10), this flat annular surface being perforated by passage holes for the first brushes (51), the first support (4) forming, at the center of the flat annular surface, a well for the passage of the second brush (52), the housing (25) comprising a second support (5) distal to the rotor on which are fixed metal cages (7) in which the first and second brushes (51, 52) are mounted in translation.

3. Electric machine (EM) according to claim 2, wherein the excitation system comprises on the one hand a first electrical connector (23) passing through the second support (5) of the housing and in electrical contact inside the housing (25) with a metal ring (22) in contact with the metal cages (7) of the first brushes (51), and on the other hand a second electrical connector (24) passing through the second support (5) of the housing and in electrical contact inside the housing (25) with the metal cage (7) of the second brush (52).

4. Electric machine (EM) according to claim 2 or 3, in which the housing (25) accommodates a heat sink (11) and thermally conductive and electrically insulating blocks of material (8), in contact on one side with the metal cages (7) of the first brushes (51) and on the other side with the heat sink (11).

5. Electric machine (EM) according to any one of claims 2 to 4, wherein the first support (4) of the housing has at least one hole (19) suitable for allowing dust generated by at least one of the brushes (51, 52) to pass through, the hole (19) being disposed under at least one of the contact surfaces of the first or second electrical tracks (51, 52) with the first brushes or respectively the second brush (51, 52).

6. Electric machine (EM) according to any one of claims 1 to 5, wherein, a wall of the casing (1) separating the rotor body from the excitation system and having an opening in which a sealing device (9) is fixed in contact with the end of the rotating shaft (3), the bearing (2) being fixed in a counterbore of the opening and disposed axially between the rotor body and the sealing device (9), the first electrical track (10) has a cylindrical part (101), connected to the contact surface of the first electrical track (10) on the side opposite the first brushes (51), and coming into contact with the sealing device (9).

7. An electric machine (EM) according to any one of claims 1 to 6, wherein the end of the rotating shaft (3) has a cylindrical steel end cap (30) having at least two grooves axial (18) in which are fitted branches (120) of an electrically insulating cap (12) on which are arranged the first and second electrical tracks (10, 16), the cap (12) including electrical conductors (36, 37) each connected by one of their ends to the first or to the second electrical track (10, 16), and by the other of their ends to the windings of the rotor.

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