Shaft grounding, electric machine and electrically operated axle drive train

Abstract

The invention relates to a shaft grounding device (1) for dissipating an electrical charge and / or voltage from a shaft (4), in particular for a drive train (40) of an electrically operated motor vehicle (3), comprising a retaining ring (5) that can be positioned coaxially to the shaft (4) and a discharge element (7) fixed to the retaining ring (5), which can be electrically contacted with an outer circumferential surface (8) of the shaft (4), wherein the discharge element (7) is designed in an annular disk shape and is arranged coaxially to the shaft (4) on the retaining ring (5), wherein a plurality of contact tabs (9) extend radially towards the shaft (4) from the discharge element (7) such that, in the assembled state of the shaft grounding device (1), they bear against the shaft (4) in a spring-like manner in the radial direction.In addition, the retaining ring (5) has a plurality of retaining tabs (10) which extend axially out of the retaining ring (5) and which clamp the ring-shaped deflecting element (7) to the retaining ring (5).

Description

The present invention relates to a shaft ground for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operable motor vehicle, comprising a retaining ring positionable coaxially with the shaft and a discharge element fixed to the retaining ring, which can be electrically conductively contacted with an outer lateral surface of the shaft. The invention further relates to an electric machine and an electrically operable axle drive train.Electric machines, in particular electric traction machines, are electrically charged during operation by induced wave voltages. In addition, circulating high-frequency currents can arise in motors of high power classes. Such electric machines generally have a rotor and a rotor shaft, which are mounted in a housing via rolling bearings. Owing to electrical discharges or the circulating high-frequency currents, damage to the rolling bearings can occur. In order to avoid this, dissipation of this electrical energy is necessary. For this purpose, for example, grounding brushes or radially acting grinding elements (carbon brushes) are known, which serve for grounding the rotor shaft. Furthermore, axially acting systems for shaft grounding are known.The publication DE 10 2020 119 719 A1, for example, discloses a discharge device for a motor for discharging an electrical charge and / or voltage from a rotor via a shaft to a housing, having a first contact module for electrical and mechanical connection to the shaft, having a second contact module for electrical and mechanical connection to the housing, wherein the first and the second contact modules are rotatable relative to one another and are electrically connected to one another in an interior space sealed with respect to a housing space, wherein the interior space is filled with an electrically conductive liquid, wherein the first and the second contact module are electrically conductively connected to one another via the electrically conductive liquid.For the bearings of an electric motor, in addition to the mechanical wear due to shaft rotation, electric currents flowing from the rotor shaft to ground via the bearings are therefore a further cause of wear. Stresses of some magnitude can overcome the isolation properties of bearing lubrication, creating sparks which can lead to pitting, corrugated surface, fusion craters and eventually to premature failure of the bearings and motor. A bearing voltage, i.e. the voltage between the two bearing shells, usually correlates with a shaft voltage, i.e. the voltage between the shaft and ground.Due to capacitive coupling between the stator winding and the rotor and the high-frequency actuation of the stator phases, a voltage potential is formed on the rotor bearings, which voltage potential can damage the bearings by means of so-called discharge currents via the bearings. As countermeasures, for example, the bearings are insulated (for example by means of ceramic balls) or the shaft is grounded by means of a sliding ground contact (electrically parallel to the bearings).Furthermore, DE 10 2017 109 049 A1 already discloses a device and a method for compensating a wave voltage on a wave. In this case, by means of a compensation circuit for generating a compensation voltage, the compensation voltage is coupled directly or indirectly to the rotor shaft via a coupling element. A detection of the shaft voltage takes place directly or indirectly on the rotor shaft via a tapping element.EP 1 455 434 A1 discloses an electric motor having a stator and a rotor which is rotatably mounted in or on the stator by means of a bearing arrangement. For electrical insulation, at least one sleeve and / or plastic layer electrically insulating the bearing arrangement with respect to the stator or rotor and / or the rotor shaft with respect to the rotor is provided. As a result, capacitive displacement currents through the bearings are prevented, which primarily arise at high pulse or switching frequencies. These can lead to a reduction in the service life of the bearingsDE 20 2017 107 755 U1 describes a device for reducing harmful bearing voltages in an electric machine fed by an intermediate circuit voltage of an intermediate circuit, comprising a stator having windings and insulated from earth and a rotor and a motor shaft, wherein furthermore a rotor-side bearing and a stator-side bearing are each designed insulated from earth and wherein the rotor and the stator are electrically connected to one another via a bypass capacitance with a predetermined capacitance.DE 10 2017 009 360 A1 shows a shaft ground (without any example) for the dissipation of an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operable motor vehicle.DE 10 2018 208 823 A1 discloses a contacting element for electrically contacting a shaft of an electric drive unit of a motor vehicle, formed as an electrically conductive body, which can be electrically coupled to a component external to the shaft.DE 10 2014 115 291 A1 discloses an earthing device for avoiding rotor-induced bearing currents in an electric motor comprising an electrically conductive ring element with holding means arranged thereon, to which fiber bundles are fastened.DE 10 2017 122 466 A1 relates to a drive device which has a rotatable rotary shaft, a rotor which is connected to the rotary shaft in a rotationally fixed manner, and at least one balancing disk. In order to make it possible to produce the drive device without external alignment devices, it is proposed that the balancing disk has direct contact with the rotary shaft and is connected thereto in a rotationally fixed manner, wherein the balancing disk and the rotor lie next to one another with at least one partial region in a fixing plane orthogonal to the rotary shaft.In the field of electric mobility, such electric machines are also frequently integrated into an axle drive train, which forms a structural unit consisting of power electronics (inverter), electric machine and transmission. This structural unit must be connected to the vehicle ground for potential compensation. This is generally effected via a conductor which is located between the body and the housing of the structural unit.In such final drive trains, usually also a grounding element is located in the gear set of the transmission and / or directly on the rotor shaft of the electric machine, in order to be able to secure the bearings located there particularly well against harmful currents or discharges with the structural proximity. These grounding elements are usually electrically conductively connected to the housing of the axle drive train.In principle, such grounding elements have the problem that dry running grounding elements generate electrically conductive abrasion, which can thus lead to short circuits in the electric machine. Wet running grounding elements, on the other hand, often have the problem that they do not function efficiently under oil at all operating points.It is therefore the object of the invention to reduce or completely avoid the disadvantages known from the prior art and to provide a shaft grounding which is of simple construction and has a high operational reliability and also durability. It is also the object of the invention to realize an improved electric machine and an improved electrically operable axle drive train.This object is achieved by a shaft grounding for discharging an electrical charge and / or voltage from a shaft, in particular for a drive train of an electrically operable motor vehicle, comprising a retaining ring positionable coaxially with the shaft and a discharge element secured to the retaining ring, which discharge element can be electrically conductively contacted with an outer lateral surface of the shaft, wherein the discharge element is of ring-disk-shaped design and is arranged coaxially with the shaft on the retaining ring, wherein a plurality of contact lugs extend from the discharge element in the radial direction toward the shaft in such a way that, in the mounted state of the shaft grounding, they bear resiliently on the shaft in the radial direction.This achieves the advantage that the voltage (or voltage potential) present via the bearing is reduced, so that the probability of spark discharges is reduced and the energy / power of the discharges still occurring is limited, with the result that the bearing damage due to electrical discharges is reduced.As a result, a radially acting system for shaft grounding can be provided, which functions efficiently in dry and wet electric machines and at all operating points. In this case, an existing installation space, for example in an electric machine or a transmission arrangement, can be used without enlarging the electric machine axially or radially. The wave grounding can also be integrated later in existing applications in a simple manner. In this way, on the one hand, bearing damage to the rotatably mounted shaft can be prevented and, on the other hand, electromagnetic compatibility (EMC) can be improved in order to reduce or prevent the interference with other devices.Furthermore, it is provided that the retaining ring has a plurality of retaining lugs which extend out of the retaining ring in the axial direction and which clamp the annular disc-shaped discharge element to the retaining ring. By using a plurality of holding tabs, the holding force is distributed uniformly on the discharge element, whereby a firm and reliable hold is achieved. This is especially important when the discharge element is subjected to strong loads, for example vibrations, shock loads or high speeds. In addition, the use of a plurality of retaining lugs makes it possible to mount and disassemble the discharge element more easily. It may not be necessary to use special tools or fasteners, as the diverter element may be easily positioned and clamped between the retaining tabs. The retaining tabs can also be stamped out of the retaining ring in a particularly simple manner from the manufacturing standpoint, for example by punching methods. This also has the advantage that the retaining lugs are connected integrally to the retaining ring and are therefore not releasable therefrom, which likewise contributes to increasing the installation reliability.A further advantage of the wave grounding according to the invention is that the wave grounding can be constructed as an independent system and thus be offered as a prefabricated unit.The radial tolerance chain between the shaft and a connection structure of the shaft ground is furthermore compensated by the diverting element.The wave grounding according to the invention can be used as dry-running and / or wet-running wave grounding.A further advantage of the wave grounding according to the invention is that the individual components of the wave grounding are simple to produce and have no geometries that make it possible to make a manufacturing effort, which also has a positive impact on the cost price of the wave grounding.The retaining ring and / or the annular disk-shaped discharge element can be embodied to be circumferentially closed or open. The retaining ring and / or the annular disk-shaped discharge element can also be embodied in multiple parts, for example by ring segments.Further advantageous embodiments of the invention are specified in the dependent claims. The features listed individually in the dependent claims can be combined with one another in a technically expedient manner and can define further embodiments of the invention. In addition, the features specified in the claims are more precisely described and explained in the description, further preferred embodiments of the invention being presented.In a simplest embodiment, the contact-making lugs can be of substantially cuboidal configuration. It is also conceivable for the free ends of the contacting tabs to have a surface treatment and / or shape, by means of which the electrical conductivity of the contacting tabs can be improved. It is thus conceivable, for example, for the contact lugs to have a bend at their free end which deviates from the rectangular parallelepiped shape or to be hardened and / or to be provided with a coating which is particularly electrically conductive.According to a further preferred development of the invention, it can also be provided that at least two of the contact-making lugs, preferably all contact-making lugs, have a mutually different longitudinal extent, such that the contact points of the contact-making lugs on the circumferential surface of the shaft are arranged axially offset from one another. It can be achieved in this way that the lateral surface is not locally overloaded and worn by contact points lying on a common circular ring.Furthermore, according to a likewise advantageous embodiment of the invention, it can be provided that the contacting tabs, in the assembled state of the shaft grounding, are arranged at their free end contacting the lateral surface axially offset with respect to the discharge element. As a result, a radial spring elasticity can be generated in a particularly simple manner, which thereby enables a sufficiently good contact between shaft and contact lugs and can also provide a tolerance compensation in the radial direction.Preferably, the number of contact-making lugs is equal to the number of retaining lugs. According to a further particularly preferred embodiment of the invention, it can be provided that in each case one contact-making tab is positioned between two retaining tabs in the circumferential direction of the discharge element. The placement of the contacting tab between two holding tabs ensures that the contact surface of the respective contacting tab is pressed tightly against the surface of the holding ring. This allows an optimum electrical connection between the retaining ring and the discharge element, whereby the resistance is minimized and an effective discharge of electrical currents takes place. The placement of the contacting tab between the retaining tabs also contributes to the stability and safety of the connection. The flap is held in position by the adjacent retaining flaps and protected from undesired movements or loosening. This is especially important in environments where vibrations, shocks or other mechanical stresses may occur. In addition, the positioning of the contacting tab between the holding tabs enables a better distribution of the contact load. Since the contacting tab is supported by the holding tabs from both sides, the pressure on the contact surface is evenly distributed, which ensures higher load capacity and durability.Furthermore, the invention can also be further developed to the effect that the retaining tabs are arranged equidistantly over the circumference of the retaining ring, which can likewise contribute to improved retention of the discharge element by a clamping effect distributed uniformly over the circumference.In a likewise preferred embodiment variant of the invention, it can also be provided that the retaining ring is produced from a metal sheet by means of forming methods. The use of sheet metal material and forming methods allows for cost effective production compared to other manufacturing methods. Sheet metal is furthermore a comparatively light material, which can contribute to a weight reduction of the retaining ring. This may be advantageous in particular in applications in which low weight plays a part, for example in vehicles, in order to reduce the overall weight and to optimize the energy consumption. Moreover, forming methods offer great flexibility with regard to the shape of the retaining ring. The sheet metal may be formed into various shapes and configurations to meet the specific requirements and geometric characteristics of the retaining ring. This also allows custom manufacturing for different applications. During the forming process, additional functions or features may be incorporated into the retaining ring.For example, mounting holes, fasteners, or other specific design features may be directly incorporated into the retaining ring, which facilitates mounting and integration into the overall system, for example an electric machine or an electrically operable axle drive train of a motor vehicle.It can also be advantageous for the same reasons to further develop the invention to the effect that the discharge element is produced from a metal sheet by means of forming methods.It is understood that the retaining ring and the discharge element are manufactured from an electrically conductive material. It can be provided that the retaining ring and the discharge element are manufactured from different materials. By using different materials for the retaining ring and the discharge element, their material properties can each be matched to their specific requirements and functions. For example, the retaining ring can be made of a material with high strength and rigidity in order to ensure robust fastening, while the diverting element can be made of a material with high conductivity for the efficient diverting of electrical currents. The use of different materials may further allow optimizing the weight of the wave grounding. If the discharge element is produced from a lighter material than the retaining ring, for example, this can lead to a weight reduction of the shaft ground, which is advantageous in particular in applications in which weight plays a part.The retaining ring can preferably have an L-shaped contour in cross section, wherein the leg of the retaining ring extending in the axial direction can then be coupled to a connection structure, for example in an electric machine or in an axle drive train, and the leg of the retaining ring extending in the radial direction can be coupled to the discharge element. The L-shaped contour can contribute to an efficient use of the available installation space for the shaft grounding. The combination of the axial and radial legs furthermore creates a stable structure which can effectively absorb the loads and forces acting on the retaining ring. This is especially important in applications where vibrations, shock loads or other mechanical stresses may occur. By connecting the axial leg to the connecting structure and the radial leg to the discharge element, a particularly reliable grounding and discharge of electrical currents is additionally also achieved by providing correspondingly large contact surfaces.The object of the invention can furthermore be achieved by an electric machine, in particular for a drive train of an electrically operable motor vehicle, comprising a stator and a rotor with a rotor shaft which is rotatably mounted relative to the stator, wherein the electric machine has a shaft grounding according to one of the preceding claims 1-8, the retaining ring of which is connected in a rotationally fixed manner to a connection structure of the electric machine and the contact lugs of which bear against the outer lateral surface of the rotor shaft.Finally, the object of the invention can also be achieved by an electrically operable axle drive train comprising an electric machine comprising a stator and a rotor, which is rotatably mounted relative to the stator and has a rotor shaft, and a transmission arrangement coupled to the rotor of the electric machine in a torque-transmitting manner, wherein a shaft grounding according to one of the preceding claims 1-8 is arranged on a shaft lying in the torque flow of the axle drive train.The invention will be explained in more detail below with reference to figures without limiting the general concept of the invention.It shows: FIG. 1 shows a shaft ground in a perspective view, FIG. 2 shows a shaft grounding in a perspective axial sectional representation, FIG. 3 shows an electrically operable axle drive train in a schematic block switching illustration.FIGS. 1-2 show a shaft ground 1 for the dissipation of an electrical charge and / or voltage from a shaft 4, in particular for a drive train of an electrically operable motor vehicle 3, as is also outlined in FIG. 3.The shaft ground comprises a retaining ring 5 positionable coaxially to the shaft 4 and a discharge element 7 fixed to the retaining ring 5, which makes electrical contact with an outer lateral surface 8 of the shaft 4.The discharge element 7 is of annular disc-shaped design and is arranged on the retaining ring 5 coaxially with respect to the shaft 4. In the radial direction toward the shaft 4, a plurality of contact lugs 9 extend out of the diverting element 7 in such a way that, in the assembled state of the shaft grounding 1, they bear on the shaft 4 in a spring-elastic manner in the radial direction.The retaining ring 5 has a plurality of retaining lugs 10 which extend out of the retaining ring 5 in the axial direction and which firmly clamp the annular disc-shaped discharge element 7 on the retaining ring 5. The diverting element 7 is centered on the retaining ring 5 via the retaining lugs 10 and clamped in place by the folding over of the retaining lugs 10. During assembly, the retaining tabs 10 can initially extend out of the retaining ring in the axial direction and are then pivoted radially inward after the insertion of the diverting element 7, in order to produce the corresponding centering and clamping of the diverting element 7 with respect to the retaining ring 5. In the exemplary embodiment shown, the retaining lugs 10 are substantially identical and their pivot points with the retaining ring 5 lie on a common pitch circle, which can facilitate the centering of the diverting element 7. The clamping force of the retaining lugs 10 is set in such a way that a rotation of the diverting element 7 relative to the retaining ring 5 is prevented by the entrainment torque on the respective retaining lugs 10 which occurs during operation of the shaft grounding 1.The retaining ring 5 has an L-shaped contour in cross section, wherein the leg of the retaining ring 5 extending in the axial direction can then be coupled to a connection structure, for example in an electric machine 30 or in an axle drive train 40, and the leg of the retaining ring 5 extending in the radial direction can be coupled to the discharge element 7.In order that the strip tracks of the contacting lugs 9 do not overlap on the circumferential surface 8 of the shaft 4, the contacting lugs 9 have a mutually different longitudinal extent, so that the contact points of the contacting lugs 9 on the circumferential surface 8 of the shaft 4 are arranged axially offset from one another. The enveloping circle of the respective contacting lugs 9 is not for this purpose than the outer diameter of the shaft 4, so that when the shaft ground 1 is placed on or the shaft 4 is inserted into the shaft ground 1, the contacting lugs 9 are pivoted out in the axial direction from the diverting element 7 and then exert a radial force which acts against the enveloping surface 8 of the shaft 4. This radial spring force ensures permanent mechanical and electrical contact between the guide element 7 and the shaft 4, and thus ensures the conduction of current both during standstill and during the rotating operation of the shaft 4.In the assembled state of the shaft grounding 1, the contact lugs 9 are arranged at their free end 11 contacting the lateral surface 8 axially offset with respect to the discharge element 7. In the exemplary embodiment shown, a contacting tab 9 is positioned in each case in the circumferential direction of the discharge element 7 between two retaining tabs 10, wherein the retaining tabs 10 are in turn arranged equidistantly over the circumference of the retaining ring 5.The retaining ring 5 and the discharge element 7 are each produced from a metal sheet by means of forming methods.FIG. 3 shows an electric machine 30 in a drive train of an electrically operable motor vehicle 3, comprising a stator 31 and a rotor 32 with a rotor shaft 33, which rotor is rotatably mounted relative to the stator 31, wherein the electric machine 30 has a shaft ground 1, the retaining ring 5 of which is connected in a rotationally fixed manner to the rotor shaft 33 and the sleeve 8 of which is connected to a connection structure 34 of the electric machine 30.Finally, FIG. 3 also shows an electrically operable axle drive train 40 comprising the electric machine 3 with a stator 31 and a rotor 32 with a rotor shaft 33 rotatably mounted relative to the stator 31, as well as a transmission arrangement 41 coupled to the rotor 32 of the electric machine 3 in a torque-transmitting manner, wherein a shaft ground 1 is arranged on a shaft 42 lying in the torque flow of the axle drive train 40. In the example shown, the transmission arrangement 41 is designed as a planetary transmission.The invention is not limited to the embodiments shown in the figures. The foregoing description is, therefore, not to be considered as limiting, but illustrative. The following claims should be understood to mean that a said feature is present in at least one embodiment of the invention. This does not exclude the presence of further features. If the patent claims and the above description define "first" and "second" features, this designation serves to distinguish two features of the same type without specifying a ranking.List of reference characters1 Shaft grounding 3 Motor vehicle 4 Shaft 5 Retaining ring 7 Diverting element 8 Lateral surface 9 Contact-making lugs 10 Retaining lugs 11 End 30 Electric machine 31 Stator 32 Rotor 33 Rotor shaft 34 Connecting structure 40 Axle drive train 41 Transmission arrangement 42 Shaft

Claims

Shaft grounding (1) for discharging an electrical charge and / or voltage from a shaft (4), in particular for a drive train (40) of an electrically operable motor vehicle (3), comprising a retaining ring (5) positionable coaxially with respect to the shaft (4) and a discharge element (7) fixed to the retaining ring (5) which can be electrically conductively contacted with an outer lateral surface (8) of the shaft (4), wherein the discharge element (7) is of annular disc-shaped design and is arranged coaxially with respect to the shaft (4) on the retaining ring (5), wherein a plurality of contact lugs (9) extend out of the discharge element (7) in the radial direction toward the shaft (4) in such a way that, in the mounted state of the shaft grounding (1), they bear resiliently against the shaft (4) in the radial direction, characterized in that the retaining ring (5) has a plurality of retaining lugs (10), which extend out of the retaining ring (5) in the axial direction and which clamp the annular disc-shaped discharge element (7) on the retaining ring (5).The shaft grounding according to claim 1, characterized in that at least two of the contact-making lugs (9), preferably all contact-making lugs (9), have a mutually different longitudinal extension, so that the contact points of the contact-making lugs (9) on the lateral surface (8) of the shaft (4) are arranged axially offset from one another.The shaft grounding (1) according to one of the preceding claims, characterized in that the contacting tabs (9) are arranged axially offset with respect to the discharge element (7) at their free end (11) contacting the circumferential surface (8) in the assembled state of the shaft grounding (1).The shaft grounding (1) according to one of the preceding claims, characterized in that a contacting tab (9) is positioned between two holding tabs (10) in the circumferential direction of the discharge element (7).The shaft grounding (1) according to one of the preceding claims, characterized in that the retaining lugs (10) are arranged equidistantly over the circumference of the retaining ring (5).The shaft grounding (1) according to one of the preceding claims, characterized in that the retaining ring (5) is produced from a metal sheet by means of forming methods.The shaft grounding (1) according to one of the preceding claims, characterized in that the discharge element (7) is produced from a metal sheet by means of forming methods.Electric machine (30), in particular for a drive train (40) of an electrically operable motor vehicle (3), comprising a stator (31) and a rotor (32), which is rotatably mounted relative to the stator (31) and has a rotor shaft (33), wherein the electric machine (30) has a shaft grounding (1) according to one of Claims 1 - 7, the retaining ring (5) of which is connected in a rotationally fixed manner to a connection structure (34) of the electric machine (30) and the contact lugs (9) of which bear against the outer lateral surface (8) of the rotor shaft (33).Electrically operable axle drive train (40) comprising an electric machine (3) comprising a stator (31) and a rotor (32), which is rotatably mounted relative to the stator (31) and has a rotor shaft (33), and a transmission arrangement (41), which is coupled to the rotor (32) of the electric machine (3) in a torque-transmitting manner, wherein a shaft grounding (1) according to one of Claims 1 - 7 is arranged on a shaft (42) lying in the torque flow of the axle drive train (40).

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