Grounding device for an electric machine and method for electrically contacting a rotating shaft

Abstract

The invention relates to a grounding device (1) for an electric machine (10), in particular an electric motor, comprising an annular carrier element (2) which is provided for surrounding a shaft (6) of the electric machine (10), and a flexible, electrically conductive discharge strand (7), at least a portion of which is arranged in the interior of the carrier element (2) and which is provided for contacting the shaft (6), wherein a preload can be set between the discharge strand (7) and the shaft (6) in that at least one end of the discharge strand (7) can be fixed to the carrier element (2) in a variable position.

Description

[0001] Earthing device for an electric machine and method for electrically contacting a rotating shaft

[0002] The invention relates to a grounding device for an electrical machine, that is, a generator or an electric motor, which can be designed as a DC or AC motor. Furthermore, the invention relates to a method for electrically contacting a rotating shaft, in particular a shaft of an electric motor or a generator.

[0003] EP 1 523 086 B1 discloses a grounding brush for reducing the electric current in the shaft of an electric motor. This grounding device comprises several electrically conductive threads, which are sufficiently small to induce ionization in the presence of an electric field. The threads are arranged in a channel formed by an annular frame. Furthermore, the annular channel may contain a conductive powder, which is a component of a paste.

[0004] DE 10 2014 109 176 B4 describes an electrical grounding device for dissimilar metals. According to the teaching of DE 10 2014 109 176 B4, the metals, from which, among other things, plate sections are formed, can be aluminum and steel.

[0005] Various possibilities for designing seals as electrically conductive components are described, for example, in documents DE 10 2015 224 042 A1 and EP 3 048 340 B1. In the latter case, the production of an electrically conductive layer made of carbon is proposed.

[0006] The invention is based on the objective of producing more advanced possibilities for manufacturing an electrically conductive connection between a rotating machine element, in particular a shaft, and a non-rotating element, in particular a housing, compared to the aforementioned prior art, whereby an adjustment option should be provided despite a simple, robust design.

[0007] This problem is solved according to the invention by an earthing device suitable for use in an electric machine, having the features of claim 1. Likewise, the problem is solved by a method designed according to claim 9 for electrically contacting, in particular for earthing, a rotating shaft. The embodiments and advantages of the invention explained below in connection with the contacting method also apply mutatis mutandis to the devices, i.e., to the earthing device and to the electric machine equipped therewith, and vice versa.

[0008] The grounding device according to the application comprises an annular support element designed to surround a shaft of the electric machine, and a flexible, electrically conductive conductor, which is arranged at least partially inside the annular support element, i.e., the support ring, and is intended for contacting the shaft. A preload between the conductor and the shaft is adjustable by fixing at least one end of the conductor in a variable position on the support element.

[0009] The ends of the conductor string attached to the support ring, for example, clamped in place, are also referred to as "ends" in cases where the conductor string extends beyond these so-called ends and is, for example, directly connected to the housing of the electric machine at another point. In such cases, the characterization as "ends" refers to that section of the conductor string which is located mostly inside the support ring and does not project beyond it. The invention is based on the consideration that, in particular, frequency-variable controls on AC motors can induce voltages on the motor shaft of the electric motor that carry a potential for damage. If the induced electrical voltages overcome the resistance of the lubricant located in a bearing of the electric motor, a discharge can lead to melt craters, profiling damage, and other damage.

[0010] According to common, unproven concepts, shaft grounding rings can be used to prevent potential differences between the shaft and the housing of an electric motor or generator. Such grounding rings are also designed for use in DC motors, where they can be installed at a single point or at multiple points within the motor. A grounding ring establishes a large-area electrical contact with the shaft of an electric machine, even without adjustments.

[0011] The patented solution deliberately departs from the concept of a grounding ring by proposing an electrical connection between an annular support element and a shaft to be grounded via a shunt conductor. This conductor is attached to the support element on both sides and contacts the shaft. A preload between the shunt conductor and the shaft can be adjusted by moving one end of the shunt conductor around the circumference of the support ring. It has been shown that this method enables a permanent, sufficiently low-resistance contact between a housing-mounted element and the shaft of the electric machine. By setting a moderate preload, wear can be minimized or practically eliminated. Due to the patented grounding device's independence from any shaft bearings, the grounding device constitutes a stand-alone shunt element.

[0012] According to one possible embodiment, the end of the drain line, which can be fixed in a variable position on the support element, is held in a T-nut that is slidable around the circumference of the annular support element. The T-nut can be arranged within a cylinder defined by the cylindrical outer circumferential surface of the support ring. In particular, one surface of the T-nut can be flush with the outer surface of the support ring. A locking screw, for example in the form of a setscrew, can be provided to fix the T-nut.

[0013] Metallic materials are particularly suitable for manufacturing the conductor attached to the T-nut. For example, a section of steel cable is used as the conductor. It is also possible to manufacture the conductor from an electrically conductive non-metallic material or from a combination of different materials. Electrical currents are conducted via the conductor and the support element into the housing of the electric machine.

[0014] Regarding the fastening of the support element in the housing of the electric machine, various fastening options are possible. For example, screw connections, which also serve as electrically conductive connections, are provided for fastening the support ring to the housing of the electric machine. The support ring can be connected to the grounded housing either directly or via intermediate elements, such as metal clamps, sleeves, or other adapters. A friction-fit, detachable connection between the ring element of the grounding device and the housing is also conceivable.

[0015] If a press fit is formed between the carrier ring and the housing, this fit can include a stop geometry that determines the axial position of the carrier ring within the housing. In cases where a material-bonded connection, particularly in the form of a weld, solder, or adhesive bond, is provided between the carrier ring and the housing, this connection can also be used for dissipating electrical charges. In the latter case, an electrically conductive adhesive must be used for this purpose. Optionally, an additional electrically conductive connection between the carrier ring and the housing is established via a separate element, particularly a cable, or an extension of the discharge cable.The patented method for electrically contacting a rotating shaft is generally characterized by the establishment of a potential equalization between the shaft and a non-rotating, electrically conductive support element surrounding the shaft via a flexible, electrically conductive conductor tangent to the shaft, the first end of which is held in a fixed position on the support element and the second end of which is fixed in an adjustable position on the circumference of the support element. In particular, the second end of the conductor is held in a T-nut which is inserted into a groove extending over a section of the circumference of the support ring. For example, the groove extends over an angle of less than 180 degrees, and in particular over an angle of less than 120 degrees but more than 30 degrees, on the circumference of the annular support element.Apart from the T-nut, the support element can be a single piece. In principle, a multi-part construction of the support ring is also possible, in particular a construction consisting of two half-shells, each extending over 180 degrees.

[0016] Several embodiments of the invention are explained in more detail below with reference to a drawing. This drawing shows:

[0017] Fig. 1 Components of an earthing device offering an adjustment option for an electric machine in a first setting,

[0018] Fig. 2 shows the arrangement according to Fig. 1 in a second setting,

[0019] Fig. 3 shows a detail of a component, namely a support ring, of an earthing device for an electric machine.

[0020] Fig. 4 shows a T-nut of an earthing device including the inserted grounding conductor, Fig. 5 shows the T-nut in a perspective view,

[0021] Fig. 6 shows an earthing device for an electric machine with a support ring screwed to the housing of the electric machine,

[0022] Fig. 7 shows an alternative embodiment of an earthing device in a view analogous to Fig. 7.

[0023] Fig. 8 shows a sectional view of a detail of the arrangement according to Fig. 6.

[0024] Unless otherwise stated, the following explanations apply to all embodiments. Corresponding or essentially equivalent parts are marked with the same reference numerals in all figures.

[0025] An earthing device, designated in its entirety by reference numeral 1, is intended for use in an electric machine 10, in this case an electric motor. The electric motor 10 can be designed as a direct current or alternating current motor and can be designed for mobile or stationary applications.

[0026] The grounding device 1 is installed in the electric motor 10 independently of any bearings, such as rolling bearings or sliding bearings, and therefore constitutes a stand-alone shunt element. An annular support element 2 of the grounding device 1 defines an interior space 15, which is penetrated by a shaft 6 of the electric machine 10, i.e., by the motor shaft of the electric motor 10. A cylindrical outer circumferential surface of the support element 2 is designated 16. The support element 2 is made of an electrically conductive material, namely a metal. For grounding the shaft 6, there is a flexible, electrically conductive grounding cable 7, which is attached to the support element 2 on both sides, allowing for adjustment. In all embodiments, the support element 2 is rigidly and electrically connected to a housing 5 of the electric motor 10.

[0027] In the embodiment shown in Figures 1 and 2, the annular support element 2, i.e., the support ring, is pressed into the housing 5. The embodiment shown in Figure 3 differs in that the support ring 3 is electrically connected to the housing 5 by a material-bonded connection, which is not visible in this illustration. In the variants shown in Figures 6 to 8, an electrically conductive and simultaneously mechanically stable connection between the support ring 2 and the housing 5 is established in various ways using screws 4.

[0028] In all embodiments, a T-nut 12 is slidably guided in the support ring 2. The T-nut 12 is located in a groove 8, which extends over a section of the circumference of the support ring 2. A slot 9 is formed in the bottom of the groove 8, which extends at least over most of the length of the groove 8, and in the exemplary embodiments over the entire length of the groove 8. The groove 8 transitions into the slot 9 at strip-shaped shoulders 11.

[0029] The slot 9 is provided for the passage of the grounding conductor 7. An opening in the T-nut 12, designed as a through-hole, in which a first end of the grounding conductor 7 is secured, is designated 14. A further opening 13 in the T-nut 12 is provided for the installation of a fixing screw. By moving the T-nut 12 in the groove 8, the preload with which the grounding conductor 7 contacts the shaft 6 can be adjusted. Once the appropriate position of the T-nut 12 has been found, the T-nut 12 is secured by tightening the fixing screw. The second end of the grounding conductor 7 is initially in a fixed position on the circumference of the support ring 2. The T-nut 12, shown in detail in Figures 4 and 5, is suitable for use in a grounding device 1 according to Figures 1 to 3 as well as for use in a grounding device 1 according to Figures 6 to 8.In a front view, the T-nut 12 has a T-shape, which fits at least approximately flush into the support ring 2. Here, a T-cross piece 17, deviating from the simplified representations in Figures 4 and 5, can have a curved surface adapted to the shape of the outer circumferential surface 16. Radially inward, with respect to the shape of the support ring 2, a rib 18 adjoins the T-cross piece 17, which is oriented in the tangential direction of the support ring 2.

[0030] Regarding possible screw connections between the support ring 2 and the housing 5, reference is made to Figures 6 to 8. In the case of Figures 6 and 8, angled sheet metal mounting clips 3 are attached to the support ring 2 on one side and to the housing 5 on the other by screws 4. Additionally, a clamping connection of the support ring 2 within the housing 5 may be provided. Not shown, a shoulder may be formed in the housing 5 against which the support ring 2 rests. The embodiment according to Figure 7 differs from the embodiment according to Figures 6 and 8 in that the support ring 2 is attached directly to the housing 5 by screws 4.

[0031] List of reference signs

[0032] grounding device

[0033] Support element, support ring

[0034] Mounting bracket

[0035] screw

[0036] Housing

[0037] Wave

[0038] Drainage strand

[0039] Nut

[0040] slot

[0041] electric motor, electric machine

[0042] Paragraph

[0043] T-nut

[0044] Opening for fixing screw

[0045] Opening for the drain line

[0046] interior

[0047] External perimeter area

[0048] T-cross piece

[0049] bar

Claims

Patent claims 1. Earthing device (1) for an electric machine (10), comprising an annular support element (2) which is provided to surround a shaft (6) of the electric machine (10), and a flexible, electrically conductive conductor (7) which is arranged at least partially inside the support element (2) and is provided for contacting the shaft (6), wherein a preload between the conductor (7) and the shaft (6) is adjustable by fixing at least one end of the conductor (7) in a variable position on the support element (2).

2. Earthing device (1) according to claim 1, characterized in that the end of the conductor (7) which can be fixed in a variable position on the support element (2) is held in a T-nut (12) which is displaceable on the circumference of the annular support element (2), i.e. support ring.

3. Earthing device (1 ) according to claim 2, characterized in that the T-nut (12) is arranged within a cylinder described by a cylindrical outer circumferential surface (16) of the support ring (2).

4. Earthing device (1 ) according to one of claims 1 to 3, characterized in that the support element (2) is screwed to a housing (5) of the electric machine (10).

5. Earthing device (1 ) according to claim 4, characterized in that a screw connection (3, 4) between the support element (2) and the housing (5) comprises several fastening clamps (3).

6. Earthing device (1 ) according to claim 4, characterized in that the support element (2) is directly attached to the housing (5) by screws (4).

7. Earthing device (1 ) according to one of claims 1 to 3, characterized in that the support element (2) is connected to a housing (5) of the electric machine (10) via a materially bonded, electrically conductive connection.

8. Earthing device (1 ) according to one of claims 1 to 3, characterized in that the support element (2) is pressed into a housing (5) of the electric machine (10).

9. Method for electrically contacting a rotating shaft (6), wherein a potential equalization between the shaft (6) and a non-rotating, electrically conductive support element (2) surrounding the shaft (6) is established via a flexible, electrically conductive conductor (7) tangent to the shaft (6), the first end of which is in an unchangeable position on the support element (2) is held and its second end is fixed in an adjustable position on the circumference of the support element (2).

10. Method according to claim 9, characterized in that the second end of the drain string (7) is held in a T-nut (12) which is inserted into a groove (8) which extends over a section of the circumference of the carrier ring (2).

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