Method for producing slip ring brush

Abstract

A method of producing a composite material for a slip ring brush, the method comprising: a) mixing copper powder having a particle size of 10-500 [mu] m with graphene to obtain a copper-graphene mixture, the content of graphene in the copper-graphene mixture being in the range of 0.5-5 wt.% of the total weight of the copper-graphene mixture, and b) sintering the copper-graphene mixture.

Description

Technical Field

[0001] The present invention generally relates to a composite material for a brush of a slip ring unit. Background Art

[0002] A slip ring unit is used to conduct electric current to or from a rotating shaft of an electric machine. The slip ring unit includes a slip ring typically made of copper, bronze or stainless steel and a carbon brush as a mating member typically made of graphite or a metal-graphite mixture.

[0003] When the carbon brush is stationary, the slip ring is mounted on the rotor shaft. The carbon brush is pressed against the slip ring surface by a spring to ensure good electrical contact.

[0004] Graphite has two functions. The first is to provide sufficient electrical conductivity to transfer the required current to the slip ring, and the second is to act as a solid lubricant to provide low friction to keep the slip ring intact.

[0005] The main disadvantage of existing carbon brushes is that they wear over time and need to be replaced to ensure proper function. Depending on the application, carbon brushes typically need to be replaced every 6 to 12 months, resulting in high maintenance costs over the entire life of the electric machine.

[0006] In addition, due to the wear of carbon brush carbon, dust is generated, which can cause, for example, clogging or electrical bridging.

[0007] A common practice to extend the brush life is to limit the contact pressure between the brush and the slip ring to a very low level, such as 20 - 25 kPa. Since the contact resistance is inversely proportional to the contact pressure, the electrical loss in the contact area is relatively high. Another limitation of commercial graphite-containing brushes is that the friction performance is sensitive to humidity, which limits their application in lower humidity environments.

[0008] CN 112981159B discloses a preparation method of a graphene-reinforced copper-based composite material. The method includes mixing copper powder and an organometallic carbon source and spark plasma sintering to obtain a three-dimensional graphene / carbide or oxide co-reinforced copper-based composite material. Summary of the Invention

[0009] A general object of the present disclosure is to provide a method for producing a slip ring brush that solves or at least alleviates the problems of the prior art.

[0010] Accordingly, a method for producing a slip ring brush is provided according to a first aspect of the present disclosure, the method comprising: producing a composite material, comprising: a) mixing copper powder having a particle size of 10 - 500 μm with graphene to obtain a copper-graphene mixture, wherein the content of graphene in the copper-graphene mixture is 0.5 - 5 wt.% of the total weight of the copper-graphene mixture, b) sintering the copper-graphene mixture; and shaping the composite material into a slip ring brush in or after step b).

[0011] Compared with the electric brush containing commercial graphite, the composite material exhibits excellent tribological properties, including low friction and high wear resistance. The electrical contact resistance is 1 to 2 orders of magnitude lower than that of the commercial electric brush. Due to the low wear property of the copper-graphene composite material produced by this method, the contact pressure between the electric brush and the slip ring can be increased. This will further reduce the contact resistance and voltage drop, thereby minimizing the electrical loss during application.

[0012] Therefore, the composite material provides low friction, good electrical conductivity and much less wear, which results in an extended life of the brush part. The reduced voltage drop and contact resistance also open up the possibility of increasing the current-carrying capacity. Different from the commercial brush containing graphite which is sensitive to humidity changes, the composite material of the present invention is robust and can be operated over a wide range of humidity and temperature.

[0013] Regarding CN 112981159B, the composite material of the present invention has better electrical conductivity because it does not contain oxides and carbides.

[0014] Herein, the term graphene is commonly used for carbon atoms in a 2D-honeycomb lattice, which is in the form of a single-layer sheet, a bilayer sheet, a few (3 - 5 layer) layer sheets or a nanosheet having a thickness of at most 50 nm (e.g., in the range of 1 to 50 nm).

[0015] According to one embodiment, the sintering is spark plasma sintering.

[0016] The high speed of the spark plasma sintering process ensures that it can densify powders with small particle sizes while avoiding coarsening associated with standard densification processes such as non-spark plasma sintering techniques. The composite material samples produced by this method show a compaction density of 80 - 99% of the theoretical density of pure copper.

[0017] Due to the rapid nature of the spark plasma sintering process, which is characterized by a high heating rate and a short sintering time, typically only a few minutes, the composite material retains the structure of the graphene material and, compared with other sintering techniques that take much longer time, it does not aggregate, resulting in oxidation or agglomeration of the graphene sheets, deteriorating the lubricating and electrical properties of the graphene.

[0018] According to one embodiment, step b) is carried out at a temperature of 650 - 950 °C and at a pressure of 10 - 100 MPa.

[0019] According to one embodiment, step b) is carried out in an inert atmosphere. This eliminates the risk of oxidation of the composite material. Oxidation reduces the electrical conductivity of the material.

[0020] According to one embodiment, step b) is carried out for at most 20 minutes, such as at most 15 minutes, such as at most 10 minutes.

[0021] According to one embodiment, step b) is carried out for at least 2 minutes, such as at least 3 minutes. Step b) can be carried out for 5 minutes, for example.

[0022] According to one embodiment, the content of graphene in the copper - graphene mixture is in the range of 1 - 3 wt.% of the total weight of the copper - graphene mixture. It has been found that this range provides the best compromise between cost and lubricity and wear resistance, taking into account the risk of graphene turning into graphite.

[0023] According to one embodiment, in addition to mixing the copper powder with graphene, step a) involves mixing an additive with the copper powder and graphene to obtain a copper - graphene mixture.

[0024] According to one embodiment, the additive includes a stabilizer and a binder.

[0025] According to one embodiment, the additive consists of a stabilizer and a binder.

[0026] According to one embodiment, the content of the additive is less than 5 wt.% of the total weight of the copper - graphene mixture.

[0027] According to one embodiment, the copper - graphene mixture consists of copper powder, graphene, and an additive.

[0028] According to one embodiment, the composite material consists of a sintered copper - graphene mixture.

[0029] According to one embodiment, the content of copper in the copper - graphene mixture is in the range of 90 - 99 wt.% of the total weight of the copper - graphene mixture.

[0030] According to one embodiment, the content of copper in the copper - graphene mixture is higher than 90 wt.% of the total weight of the copper - graphene mixture.

[0031] According to one embodiment, the graphene is in the form of graphene particles, each having a surface area of 100 - 750 m 2 / g.

[0032] According to one embodiment, the copper powder has a particle size of at least 15 μm, such as at least 30 μm, at least 50 μm, at least 100 μm or at least 200 μm.

[0033] The particle size of the copper powder may be of a quasi-spherical morphology.

[0034] According to one embodiment, the mixing in step a) is mechanical mixing. Mechanical mixing is dry mixing. The mixing can be carried out, for example, in a high-speed vibrator with satisfactory results. Other mixing methods can also be used, such as ball milling.

[0035] According to one embodiment, step b) is the only sintering step for producing the composite material.

[0036] By only carrying out spark plasma sintering, the density of the composite material will be high enough to be used as a brush in a slip ring unit. If the density is further increased by an additional primary sintering step after spark plasma sintering, the hardness of the material will increase, and this will provide too much wear on the slip ring.

[0037] According to one embodiment, the graphene is graphene nanosheets. Graphene nanosheets are low-cost materials that meet the purpose of producing the composite material.

[0038] According to one embodiment, the slip ring brush does not contain graphite.

[0039] According to a second aspect, it can be obtained by means of the method of the first aspect.

[0040] In general, all terms used in the claims will be interpreted according to their ordinary meaning in the technical field, unless otherwise defined herein. Unless otherwise stated, all references to "an / the element, apparatus, component, device, etc." shall be construed broadly as referring to at least one instance of the element, apparatus, component, device, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Specific embodiments of the inventive concept will now be described by way of example with reference to the accompanying drawings, in which:

[0042] Figure 1 is a flowchart of a method for producing a slip ring brush; and

[0043] Figure 2 is a side view of a slip ring unit including a slip ring brush manufactured by the method in Figure 1 ; DETAILED DESCRIPTION

[0044] The concept of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. However, the concept of the present invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided as examples so that this disclosure will be thorough and complete and will fully convey the scope of the concept of the present invention to those skilled in the art. Throughout the specification, the same reference numerals denote the same elements.

[0045] Figure 1 A method of producing a slip ring brush is shown.

[0046] The method generally includes producing a composite material shaped or formed as a slip ring brush.

[0047] In step a), copper powder having a particle size of 10 - 500 μm is mixed with graphene to obtain a copper-graphene mixture.

[0048] According to some examples, the particle size of the copper powder can be at least 15 μm, such as at least 30 μm, at least 40 μm, at least 50 μm, at least 100 μm, at least 200 μm or at least 300 μm.

[0049] The content of graphene in the copper-graphene mixture is in the range of 0.5 - 5 wt.% of the total weight of the copper-graphene mixture. The content of graphene in the copper-graphene mixture can be, for example, in the range of 1 - 3 wt.% of the total weight of the copper-graphene mixture.

[0050] Graphene as a 2D material can be in the form of graphene particles, each having a surface area in the range of 100 - 750 m 2 / g.

[0051] The content of copper in the copper-graphene mixture can be in the range of 90 - 99 wt.% of the total weight of the copper-graphene mixture, for example, in the range of >90 wt.%.

[0052] The mixing step b) is preferably mechanical mixing.

[0053] The copper powder and graphene can be vigorously mixed, for example, in a high-speed vibrator. According to one example, the vibration speed can be 700 rpm and the mixing time can be 150 seconds. In the test, a paint vibrator model SK35 from Fast&Fluid was used. The mixing time varies according to the amount of material being mixed.

[0054] Graphene can be, for example, graphene nanosheets, graphene nanopowder or graphene flakes.

[0055] Step a) may further include mixing an additive with the copper powder and the graphene. The additive may include, for example, a stabilizer and a binder. According to one example, the additive consists of a stabilizer and a binder.

[0056] The content of the additive may be less than 5 wt.% of the total weight of the copper-graphene mixture.

[0057] In one example, the copper-graphene mixture obtained in step a) may consist of copper powder, graphene, and an additive.

[0058] In step b), the copper-graphene mixture is sintered. Thereby, a composite material is obtained.

[0059] The sintering in step b) may be spark plasma sintering.

[0060] The sintering in step b) may be carried out at a temperature of 650 - 950 °C and at a pressure of 10 - 100 MPa (e.g., 10 - 75 MPa).

[0061] The composite material is formed into a slip ring brush in step b), or it may be formed into a slip ring brush after step b). The slip ring brush thus obtained consists of the composite material obtained according to the method. The composite material may consist of a sintered copper-graphene mixture.

[0062] Step b) is preferably carried out in an inert atmosphere.

[0063] Step b) is preferably the only sintering step carried out to obtain the composite material.

[0064] Step b) can generally be carried out for at most 20 minutes, such as at most 15 minutes, such as at most 10 minutes. Step b) can generally be carried out for at least 2 minutes, such as at least 3 minutes. Thus, generally, step b) is carried out in the range of 2 - 20 minutes, for example, in the range of 3 - 15 minutes or 3 - 10 minutes. According to one example, step b) is carried out for 5 minutes.

[0065] The slip ring brush obtained by this method preferably does not contain graphite.

[0066] Figure 2 The slip ring unit 1 is shown. The slip ring unit 1 is mounted on the shaft 9. The shaft 9 may be, for example, the rotor shaft of an electric machine.

[0067] The slip ring unit 1 includes a plurality of slip rings 3 arranged in a support 5. The support is attached to the shaft 9.

[0068] The slip ring unit 1 includes a plurality of slip ring brushes 7 manufactured by the above method.

[0069] Each slip ring brush 7 is arranged to make mechanical and electrical contact with a corresponding slip ring 3. The slip ring brush 7 is radially inwardly pressed against the corresponding slip ring 3 by a force member 11 such as one or more mechanical springs. The force can be higher than 25 kPa, such as 30 kPa or higher than 30 kPa, for example equal to or higher than 35 kPa or equal to or higher than 40 kPa, such as equal to or higher than 45 kPa. According to some examples, the force can be equal to or higher than 100 MPa, for example equal to or higher than 250 MPa.

[0070] The slip ring brush 7 is fixedly arranged and the slip ring 3 rotates simultaneously with the rotation of the shaft 9.

[0071] The concept of the present invention has been mainly described above with reference to several examples. However, it is readily understood by those skilled in the art that other embodiments besides the embodiments disclosed above are equally possible within the scope of the concept of the present invention defined by the appended claims.

Claims

1. A method for producing a slip ring brush (7), comprising: Producing a composite material, comprising: a) Mixing copper powder with a particle size of 10 - 500 μm with graphene to obtain a copper-graphene mixture, wherein the content of graphene in the copper-graphene mixture is in the range of 0.5 - 5 wt.% of the total weight of the copper-graphene mixture; b) Sintering the copper-graphene mixture; and Forming the composite material into a slip ring brush (7) in or after step b).

2. The method according to claim 1, wherein the sintering is spark plasma sintering.

3. The method according to claim 1 or 2, wherein step b) is carried out at a temperature of 650 - 950 °C and under a pressure of 10 - 100 MPa.

4. The method according to any one of the preceding claims, wherein step b) is carried out in an inert atmosphere.

5. The method according to any one of the preceding claims, wherein step b) is carried out for at most 20 minutes, such as at most 15 minutes, such as at most 10 minutes.

6. The method according to any one of the preceding claims, wherein step b) is carried out for at least 2 minutes, such as at least 3 minutes.

7. The method according to any one of the preceding claims, wherein the content of graphene in the copper-graphene mixture is in the range of 1 - 3 wt.% of the total weight of the copper-graphene mixture.

8. The method according to any one of the preceding claims, wherein step a) involves, in addition to mixing the copper powder with the graphene, mixing an additive with the copper powder and the graphene to obtain the copper-graphene mixture.

9. The method according to claim 8, wherein the additive comprises a stabilizer and a binder.

10. The method according to claim 9, wherein the additive consists of the stabilizer and the binder.

11. The method according to any one of claims 8 to 10, wherein the content of the additive is less than 5 wt.% of the total weight of the copper-graphene mixture.

12. The method according to any one of claims 8 to 11, wherein the copper-graphene mixture consists of the copper powder, the graphene, and the additive.

13. The method according to claim 12, wherein the composite material consists of the sintered copper-graphene mixture.

14. The method according to any one of the preceding claims, wherein the content of copper in the copper-graphene mixture is in the range of 90 - 99 wt.% of the total weight of the copper-graphene mixture.

15. The method according to any one of the preceding claims, wherein the content of copper in the copper-graphene mixture is higher than 90 wt.% of the total weight of the copper-graphene mixture.

16. The method according to any one of the preceding claims, wherein the graphene is in the form of graphene particles, each of the graphene particles having a surface area in the range of 100 - 750 m 2 / g.

17. The method according to any one of the preceding claims, wherein the copper powder has a particle size of at least 15 μm, such as at least 30 μm, at least 50 μm, at least 100 μm, or at least 200 μm.

18. The method according to any one of the preceding claims, wherein the mixing in step a) is mechanical mixing.

19. The method according to any one of the preceding claims, wherein step b) is the only sintering step for producing the slip ring brush (7).

20. The method according to any one of the preceding claims, wherein the graphene is graphene nanosheets.

21. The method according to any one of the preceding claims, wherein the slip ring brush (7) does not contain graphite.

22. A slip ring brush (7) obtainable by means of the method according to any one of the preceding claims.

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