Electric machine with a function for dissipating electrical charges induced in the rotor.

Abstract

An electric machine (EM) comprising a stator and a rotor (9) mounted for rotation about a machine axis (X), the electric machine comprising a first stranded ring (B1), stationary, equipped with a plurality of radial first strands (11) with free first ends (P1), a second stranded ring (B2), rotating, equipped with a plurality of radial second strands (12) with free second ends (P2) opposite the free first ends, the free first ends generally being located on a first circle (C1), the free second ends generally being located on a second circle (C2), the second circle being located in the vicinity of the first circle, so that electric charges can pass between the free first ends and the free second ends to form an electrical coupling, practically without mechanical contact, between the rotor and the electrical ground of the electric machine. (Short figure: Fig. 2)

Description

Title of the invention: Electric machine with function for discharging electrical charges induced in the rotor.

[0001] The present invention relates to an electrical machine incorporating a function for dissipating electrical charges induced in the machine's rotor. This is also referred to in the trade as electrically "grounding" the rotor, although establishing a permanent conductive path between the rotor and a static entity is not necessary to perform said function of dissipating induced electrical charges.

[0002] The rotor of an electrical machine can accumulate electrical charges as it rotates. This is true for asynchronous machines, and for synchronous machines with permanent magnet rotors as well as for wound rotors.

[0003] The electrical charges induced by the rotation of the rotor can be discharged via the bearings but the small electrical arcs caused by the discharge of these electrical charges through the bearings damage the balls and / or the raceways of these bearings (the so-called 'pitting' phenomenon).

[0004] It is therefore preferable to be able to dissipate the electrical charges induced by the rotation of the rotor by a path other than that of the rotor bearings.

[0005] Among the known solutions, there is a stationary eyelash ring whose end of the eyelashes rubs against a rotating part such as the rotor shaft in order to create a path for dissipating electrical charges.

[0006] The tips of the eyelets rub against a cylindrical surface formed by an outer bearing surface of the rotor shaft. To prevent eyelet wear and friction detrimental to the energy efficiency of the electric machine's operation, if the free end of the eyelets is placed in the immediate vicinity of the rotor shaft bearing surface, the point effect at the free ends also makes it possible to create a satisfactory path for dissipating electrical charges.

[0007] Furthermore, there is a trend towards using dielectric oil as a cooling fluid for electrical machines. This dielectric oil circulates and lubricates many internal parts of the machine, including the area where the eyelets of the eyelet ring rub against the rotor shaft.

[0008] The presence of this dielectric oil in the ring area forms an oil film covering the rotor shaft bearing surface. The presence of this oil film is reinforced by the rotation of the rotor shaft. The dielectric oil can also coat the blades, and in particular their tips.

[0009] This film is unfavorable to the passage of an electric current which allows the electrical charges accumulated in the rotor to be evacuated.

[0010] It should be noted here that the dielectric strength of air is 3 kV / mm while the dielectric strength of oil is much greater, and is close to 20 kV / mm.

[0011] The creation of an electric arc that passes through a film of oil is therefore made more difficult.

[0012] The inventors sought to propose a solution which at least partially solves the problem described above.

[0013] To this end, an electric machine is proposed comprising a stator and a rotor mounted to rotate about a machine axis, the electric machine comprising: - a first stranded ring (B1), stationary, equipped with a plurality of first radial strands (11) with first free ends (PI), - a second stranded ring (B2), rotating, equipped with a plurality of second radial strands (12) with second free ends (P2), the first free ends generally being located on a first circle (C1), the second free ends generally being located on a second circle (C2), the second circle being located in the vicinity of the first circle, so that electric charges can pass between the first free ends and the second free ends to form an electrical coupling between the rotor and the electrical ground of the electric machine.

[0014] The free ends can be in the form of points that are more or less pointed. Advantageously, the second points are located opposite the first points.

[0015] Thanks to the arrangements proposed above, the formation of electric arcs between the first and second points is promoted for the rotor grounding function. In practice, each of the first points is located near at least one second point, whether the rotor is stationary or not rotating.

[0016] Regarding the terminology "in the vicinity" in the phrase "the second circle being located in the vicinity of the first circle", it must be understood that the distance separating the two circles is small, generally less than 0.5 mm, in the radial direction and in the axial direction, most often generally less than 0.25 mm, and preferably on the order of 0.1 mm.

[0017] The first stationary stranded ring is connected rigidly to the stator, by a direct or indirect mounting.

[0018] The second stranded, rotating ring is mounted on the rotor, for example on the rotor shaft.

[0019] The first stranded ring and the second stranded ring together form a pair of stranded rings ensuring the function of dissipating the electrical charges induced in the rotor.

[0020] The area where arcs occur between the points is not affected by a film of oil; the arc occurs in air, which has a lower dielectric strength than the dielectric strength of a layer of oil.

[0021] The rotor's "grounding" function prevents the "pitting" phenomenon of the bearings, caused by electrical / electrostatic discharges.

[0022] According to one embodiment, the first radial strands extend at least radially inwards, and the second radial strands extend at least radially outwards.

[0023] This is a very classic internal rotor configuration, namely that the rotor is located inside the stator.

[0024] According to an alternative embodiment, the first radial strands extend at least radially outwards, and the second radial strands extend at least radially inwards.

[0025] This is a particular external rotor configuration, namely that the rotor is located outside the stator.

[0026] According to one embodiment, the rotor includes windings and at least two annular excitation tracks are provided on the side of the second axial end of the rotor shaft, configured to allow excitation of the rotor windings.

[0027] Wherefore, the control of the current flowing in the windings is pulsed in pulse width modulation and promotes the excitation of parasitic capacitances which exist between the different parts of the rotor, the accumulation of electrical charges is thus promoted.

[0028] It is noted that the stranded rings can be arranged near the excitation tracks and together they form a complete electrical equipment electrically linking the rotating crew and the static crew.

[0029] According to an alternative embodiment, the rotor can be of the permanent magnet type. In this case, there is greater freedom in positioning the pair of stranded rings relative to the rotor shaft.

[0030] According to one embodiment, the strands are rigid. Since the design provides that the tips do not touch, it is not necessary to provide flexibility to the strands as in the case of eyelashes of known art.

[0031] According to an alternative embodiment, the strands can be flexible.

[0032] On the rotor side, the strands can lengthen (e.g., straighten by bending) by passing into a purely radial position under the effect of centrifugal force. This ensures that there is no contact between the tips at low speeds, by means of a clearance distance between the first and second tips.

[0033] At high speed, the guard distance is reduced, which promotes the generation of electric arcs between the first and second points. It should be noted that this is at At high speeds, the phenomenon of electrical charge accumulation in the rotor occurs. In other words, the phenomenon of electrical charge accumulation in the rotor increases with the rotor's rotational speed.

[0034] It is further noted that at high rotational speeds, a film of dielectric oil forms between the balls and the inner and outer rings of the bearing, which isolates the rotor from the stator and prevents any charge transfer except by arcing. The stranded rings then act as discharge bridges without the discharges passing through the bearing balls.

[0035] According to one embodiment, the strands are metallic wires.

[0036] It is noted that it is possible to use the manufacturing technology of metal brushes to manufacture the wire-bristled ring. Such an assembly is simple and robust.

[0037] For example, metal wires can be obtained by cutting with shears, which makes it possible to create a sharp edge favorable to the phenomenon of point effect and therefore favorable to the creation of an electric arc at that point.

[0038] According to one embodiment, the strands can be polymer wires coated with a conductive layer.

[0039] According to one embodiment, the strands can be carbon fibers.

[0040] According to one embodiment, the first radial strands 11 are arranged in a ring discoid centered on the axis, and the second radial strands are arranged in a discoid ring centered on the axis.

[0041] According to one embodiment, several rows of strands are provided, each row being arranged in a discoidal ring centered on the axis. This allows for the distribution of electric arcs over a very large number of strands. The wear on each strand can thus be reduced.

[0042] According to one embodiment, the lengths of the first radial strands and the second radial strands are close or identical. Thus, the first circle and the second circle can be positioned medianally in the interval between the body of the first stranded ring and the body of the second stranded ring.

[0043] According to one embodiment, the pair of stranded rings can be positioned on the internal side of the machine relative to the first rotating rotor bearing. The pair of rings is thus protected within the internal space of the electric machine.

[0044] The present invention also relates to a motor vehicle, comprising at least one electrical machine as described above.

[0045] The vehicle in question may be an electric or hybrid vehicle.

[0046] The invention will be further detailed by describing non-limiting embodiments, and based on the accompanying figures illustrating variants of the invention, in which: - [Fig-1] schematically represents in axial section an example of an electrical machine in which the present invention is implemented; - [Fig.2] schematically represents in cross-section to the axis an example of a rotor grounding device with a stationary strand ring and a rotating strand ring, the respective tips of the strands being arranged opposite each other, only a part of the radial strands having been represented; - [Fig.3] schematically represents in partial axial section an area of ​​implantation of the stranded rings; - [Fig.4] schematically represents in partial axial section another example of stranded rings with several rows; - [Fig.5] schematically represents in partial axial section another example of stranded rings with several axially offset rows.

[0047] In the various figures, the same reference numerals designate identical or similar elements. For the sake of clarity, some elements are not necessarily shown to scale.

[0048] Fig. 1 shows an electric MEL machine which can be used as a drive element in an electric or hybrid vehicle powertrain.

[0049] It is noted that this electric machine can be used as an electric motor in traction mode but it can also be used as a generator in a regenerative braking circumstance.

[0050] Furthermore, the invention is not limited to electrical machines for electric vehicle powertrains. The invention can be used on any type of electrical machine, for example, any type of motor or any type of generator.

[0051] The electric machine includes a housing CM, composed here of three parts: a main housing C0 in the shape of a sleeve, a first end housing C3 and a second end housing C4.

[0052] The electric machine comprises a stator ST and a rotor 9. The illustrated electric machine is radial flux, but the invention can also be applied to an axial flux electric machine.

[0053] The rotor comprises a rotor shaft 1, for example of tubular shape (this shape not being limiting). At least one transmission shaft (not shown) may pass through the rotor shaft.

[0054] The rotor shaft 1 is connected to the housing by a mounting for rotation around the machine axis X by means of two bearings RI and R2. The electric machine generally extends from a first axial end denoted El to a second axial end denoted E2.

[0055] The RI bearing is mounted in an inner seat of the first end housing C3. The R2 bearing is mounted in an inner seat of the second end housing C4.

[0056] Near the first bearing RI is the rotor electrical discharge device, or in other words, the rotor electrical grounding device. The rotor electrical discharge device is generally designated 3.

[0057] Turning to figures 1 to 5, the rotor electrical charge evacuation device comprises a first stranded ring Bl, equipped with a plurality of first radial strands 11, and a second stranded ring B2, equipped with a plurality of second radial strands 12.

[0058] The first ring with strands Bl is stationary, it is connected to the CM housing of the machine, and therefore it is connected directly or indirectly to the stator ST.

[0059] The first radial strands 11 each extend from an anchoring end 41 on a ring body 21 to a tip also generally called the first free end PI.

[0060] The second stranded ring B2 is mounted to rotate, connected directly or indirectly to the rotor. In practice, the second stranded ring B2 is mounted on the rotor shaft.

[0061] The second radial strands 12 each extend from an anchoring end 42 on a ring body 22 to a tip also generally called the second free end P2.

[0062] The first free end, otherwise called tip PI, is located opposite a second free end, otherwise called tip P2, whether in static or dynamic mode at any speed

[0063] It is indeed envisaged a plurality of first strands distributed all around the conference of the body 21 and the first free ends PI are generally found on a first circle Cl.

[0064] The same applies to the second strands which are distributed all around the conference of the body 22 and the second free ends P2 are generally found on a second circle C2.

[0065] There may be a single row of strands or several rows of strands one after the other along the axis. When there are several rows of first strands, the tips of these first strands are generally located on a cylinder centered on the axis, by extension of the first circle CL. And the same is true for the tips of the second strands, which are generally located on a cylinder centered on the axis, by extension of the second circle C2.

[0066] Each row of strands is arranged according to a discoid ring centered on the axis, the discoid ring extending in a plane transverse to the axis.

[0067] The first radial strands 11 extend at least radially inwards, and the second radial strands 12 extend at least radially outwards. The rotor is located inside the stator. It should be noted that the reverse configuration is also possible for the application of the present invention, namely, the configuration of the rotor external to the stator.

[0068] The strands can be inclined in a purely radial direction, and moreover they are not necessarily straight, there may be a bend in the middle of the strand as illustrated in [Fig.2].

[0069] The strands can be flexible or rigid.

[0070] The cross-section of each strand can be constant over the length of the strand.

[0071] According to another option, the section can go decreasing by going from the rootstock towards the free end, that is to say the tip.

[0072] Generally, the cross-section of each strand is small, for example between 0.05 mm2 and 0.2 mm2, these values ​​not being limiting.

[0073] In the example illustrated in [Fig.1], the rotor is wound, it comprises windings electrically activated via excitation tracks T1, T2. The excitation tracks are arranged in an annular fashion at the second end E2 of the rotor shaft 1.

[0074] The two excitation tracks T1, T2 are supported by an insulating ring. Opposite each track, a friction pad system is provided, with one or more pads rubbing on the excitation tracks.

[0075] In the illustrated example, the electrical charge evacuation device of the present invention is arranged at the first end El while the excitation tracks of the wound rotor are arranged at the 2nd end E2.

[0076] In an alternative embodiment, the electrical charge dissipation device of the present invention is arranged at the same end as the excitation tracks of the wound rotor. These elements together form a complete electrical assembly electrically connecting the rotating assembly and the static electrical assembly.

[0077] According to an alternative, the rotor may be without windings and excitation tracks, the rotor may for example be of the permanent magnet type.

[0078] The distance EZ which separates the first circle Cl and the second circle C2 can be on the order of 0.1 mm, these values ​​not being limiting.

[0079] Generally speaking, this distance EZ will be less than 0.5 mm and preferably less than 0.2 mm.

[0080] A tolerance interval is managed to ensure that there is no mechanical contact between the first and second points in order to minimize losses by friction inside the electrical machine and avoid the generation of electrically conductive particles that would disperse in the dielectric oil.

[0081] In Figures 3 and 4, the first points PI and the second points P2 face each other, that is, they are in the same axial position. Even taking into account the rotation of the second ring relative to the stationary first ring, there is always a first point PI in the immediate vicinity of the second point P2, which allows an electric arc to be established if necessary, given the electrical charge of the rotor.

[0082] In [Fig. 5], the rows of first points are offset relative to the rows of second points. In other words, the first and second points are arranged in a staggered pattern in an axial section.

[0083] Excluding the anchoring portion, the length of the first strands is denoted L1, and the length of the second strands is denoted L2. L1 and L2 can be between approximately 2 mm and 5 mm, these values ​​not being limiting.

[0084] According to one example, L1 and L2 have similar or identical values, which optimizes the use of space in the interval between the first and second rings. However, it should be noted that the lengths L1 and L2 could be quite different.

[0085] The axial dimension of the ring body is denoted Wl. W1 can be between 3 millimeters and 5 millimeters in the case of a single row of strands, and Wl can be larger in the case of multiple rows. The values ​​mentioned above are not, however, limiting.

[0086] The radial thickness of the body of the first ring is denoted Hl, that of the first ring is denoted H2.

[0087] Hl and H2 can be between approximately 2 mm and 4 mm, these values ​​not being limiting.

[0088] The inner diameter D2 of the body 22 of the second ring B2 corresponds to the diameter of the rotor shaft at the mounting point. D2 can range from approximately 20 to 40 mm, these values ​​not being limiting. Note that the diameter D2 can be smaller than the diameter of the inner ring of the bearing.

[0089] The first and second circles Cl, C2 are both located substantially at diameter D3.

[0090] D5 represents the inner diameter of the body 21 of the first ring Bl.

[0091] The outer diameter D6 of the body 21 of the first ring Bl corresponds to the diameter of the bearing housing in the case, as illustrated in [Fig. 3], where the pair of rings B1, B2 is placed in parallel with the bearing RL

[0092] It is noted that the diameter D6 can be greater than the diameter of the outer ring of the bearing.

[0093] According to one embodiment, the strands are metallic wires. It should be noted that it is possible to use the manufacturing technology of metal brushes to form the plurality of radial strands.

[0094] According to an alternative example, the strands can be polymer wires coated with a conductive layer.

[0095] It may be envisaged to form a subset of strands in the form of a garland before assembling the subset into a ring body.

[0096] The strands can be embedded in the ring body in various ways. The strands can be welded to the ring body, which is made of metal, for example in one piece or in two pieces joined together, for example by shrink fitting.

[0097] The ring body can be made of conductive composite material and enclose the strand feet.

[0098] The anchoring of the strands in the ring body is solid; this anchoring is designed, with regard to the rotating part, to withstand the effects of the centrifugal force induced by high rotor rotation speeds, in practice exceeding 10,000 revolutions per minute.

Claims

Demands

1. Electric machine (EM) comprising a stator and a rotor (9) mounted to rotate about a machine axis (X), the electric machine comprising: - a first stranded ring (B1), stationary, equipped with a plurality of first radial strands (11) with first free ends (PI), - a second stranded ring (B2), rotating, equipped with a plurality of second radial strands (12) with second free ends (P2), the first free ends generally being located on a first circle Cl, the second free ends generally being located on a second circle C2, the second circle C2 being located in the vicinity of the first circle Cl, so that electric charges can pass between the first free ends and the second free ends to form an electrical coupling between the rotor and the electrical ground of the electric machine.

2. An electric machine according to claim 1, wherein the first radial strands (11) extend at least radially inwards, and the second radial strands (12) extend at least radially outwards.

3. An electric machine according to any one of claims 1 to 2, wherein the rotor comprises windings and at least two annular excitation tracks (T1,T2) are provided on the side of the second axial end of the rotor shaft, configured to permit excitation of the rotor windings.

4. An electric machine according to any one of claims 1 to 3, wherein the strands are rigid.

5. An electric machine according to any one of claims 1 to 4, wherein the strands are metallic wires.

6. Electric machine according to any one of claims 1 to 5, wherein the strands are polymer wires coated with a conductive layer.

7. An electrical machine according to any one of claims 1 to 6, wherein the first radial strands (11) are arranged in a discoid ring centered on the axis, and the second radial strands (12) are arranged in a discoid ring centered on the axis.

8. An electrical machine according to any one of claims 1 to 7, wherein several rows of strands are provided, each row being arranged in a discoidal ring centered on the axis.

9. An electric machine according to any one of claims 1 to 8, wherein the length (L1) of the first radial strands (11) and the length (L2) of the second radial strands (12) are close or identical.

10. Motor vehicle, preferably electric or hybrid, comprising an electric machine according to any one of claims 1 to 9.

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