Field of the invention

The invention relates to a slipring brush, which may be used in an electrical pow- er transfer device like a slipring or rotary joint, or in the collector of an electrical motor. Sliprings are used to transfer electrical power between rotating parts of machines like wind power plants, CT scanners or electrical generators. There a brush, mainly comprising of electrically conductive material like carbon is sliding on a rotating cylindrical track of conductive material. Common materials for such tracks are steel or brass.

Description of the related art

Slipring brushes must have a low contact resistance to the track and a low contact noise during a long lifetime. Furthermore, wear of the track must be low, as replacement of the track is complex and expensive.

The US patent 2,153,049 discloses a slipring brush having holes essentially parallel to the wearing face sliding on the track. When material of the brush is abraded by wear, the parallel rows get opened and form grooves extending diagonally across the wearing face. This should increase the electric contact by preventing build up of vacuum or pressure, which tends to develop under portions of the face of the brush and also by foreign particles of matter which attempt to pass under the brush.

A different solution is disclosed in the US patent 6,091,178. This patent tries to reduce hydrodynamic forces between a brush and the track to obtain a good contact by providing a gap at a side of the brush. The Chinese patent application publication 102082381 A discloses to provide a hole through a slipring brush for providing an exhaust duct for gases and particles.

Summary of the invention

The problem to be solved by the invention is to provide a slipring brush with good contact properties, long lifetime and low track wear.

Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention. Longtime observations of slipring tracks and brushes running thereon have shown that the current density in a slipring brush is critical for good contact characteristics, long lifetime and low track wear.

In a first embodiment, a slipring brush with an essentially rectangular cross section has at least one blind hole in its contact surface. Preferably, the diameter and/or cross-section of the blind hole is adapted to the current flowing through the brush to obtain a specific current density. It is essential, that this blind hole ends within the brush and does not penetrate through the brush. A through hole would, as already observed in the prior art, lead to an exhaust of air and gases which adversely affect the thermal balance in the brush. It furthermore leads to an excessive current density within the contact area between the contacting wire and the brush due to the limited cross-section of the brush in the contact area. Such a through hole may be useful when operating a slipring brush and track at comparatively high power levels, where signal quality is not critical. When operating at moderate or lower power levels, transmission quality and lifetime can significantly be increased by carefully optimizing thermal and current balance. The blind hole in this embodiment defines a usable cross-section for the current flowing through the brush, resulting in a specific current density. Due to this specific density the wear of the track and of the brush are lower than with lower or higher current densities. Preferably, the current density is in the range of 1 - 100 A/mm ² of brush cross-section. Is further preferred to limit the current density to a range of 3 - 30 A/mm ² . Here, a lower current density is even more critical to wear than a higher current density. As the blindfolded ends before the contact area between the contacting wire and the brush, the current density can be lowered within this area to provide for a long-term reliable contact.

The advantage of this embodiment is that the outer size, which fits into a stand- ard brush holder, must not be changed to adapt the brush to a variety of currents. Therefore, the outer size remains the same, while the cross-section of a different current can be individually adapted to the current to keep the current density in a predetermined range. This simplifies modification of existing systems as well as replacing existing brushes. The blind hole must not necessarily be a round hole. Instead, it may have any shape, as long as it maintains a required cross-section of the surrounding brush body. It may for example be elliptical, triangular, or rectangular. Of course, there may also be a plurality of holes within the slipring brush body.

In a further embodiment, the blind hole is filled with insulating material. This insulating material prevents current flowing through the area of the blind hole and therefore keeps the current density as described above. Preferably, the insulating material is not only electrically insulating, but also thermally insulating to reduce an increase of the brush temperature. It may be PTFE (Teflon) or any other plastic material. Tests have shown, that the best results are obtained, when the holes are empty, forming a hollow space. Although holes filled with an insulator may be an alternative which is still an improvement over the prior art. Description of Drawings

In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings.

Figure 1 shows a brush with a blind hole. Figure 2 shows a cross-section of the brush. Figure 3 shows a bottom view of the brush. Figures 4 to 7 show further embodiments of the brush. Figures 8 to 11 show embodiments with twin blind holes. Figure 12 shows a perspective view of the brush. Figure 13 shows a brush body filled with an insulating material. Figure 14 shows a sectional view showing the current density distribution.

In figure 1, a preferred embodiment of a brush is shown. A brush body 10 of electrically conductive material, which may comprise at least one of carbon material, metal powder like silver, lubricant, is connected by a contact wire 30 to a contact plate 40. The contact wire preferably is a Litz wire. The contact plate 40 serves for the electric contact of the brush and may for example be contacted to a cable. For applying mechanical force to the brush body and pressing the brush to a sliding track, which is not shown here, a spring 20 may be provided. The brush body 10 has a contact surface 12 which contacts with sliding track. A wear marker may be provided at the surface of the brush to indicate a position to which the brush may be worn. When the wear marker has reached the sliding track or even has been worn off, the brush must be replaced. Furthermore, a blind hole 50 is provided within the brush body 10.

In figure 2, a cross-section of the brush is shown. It can be seen, that the blind hole 50 ends within the brush body and does not penetrate the brush. This is essential as described above.

Figure 3 shows a bottom view of the brush. Here also the blind hole can be seen.

The figures 4 to 7 show further embodiments of the brush. Figure 4 shows a brush with a blind hole having a rounded end, while the brush of figure 6 has a blind hole with a conical end. Figure 5 shows a bottom view of the brush of figure 4, while figure 7 shows a bottom view of the brush of figure 6.

The figures 8 to 11 show further embodiments of the brush with twin blind holes. As shown in figure 8, there may be two blind holes 51, 52 having approximately the same length. The respective bottom view is shown in figure 9. In fig- ure 10 a brush with displaced twin blind holes 51, 52 is shown. The respective bottom view is shown in figure 11.

Figure 12 shows a perspective view of the brush.

Figure 13 shows a brush body filled with an insulating material 55, which may be PTFE or any other plastic material. Figure 14 shows a sectional view showing the current density distribution. The dashed line 13 shows the wear limit up to which the brush may be worn. This may correspond to the wear marker 11. In the area 60 besides of the hole and most preferably below the wear limit, there is a constant and relatively high current density colleges necessary to achieve good their characteristics. Above this area close to the contact area 31, where the contact wire 30 contacts the brush body 10, there is a region 61 with reduced current density.

List of reference numerals

10 brush body

11 wear marker

12 contact surface

13 wear limit

20 spring

30 contact wire

31 contact area

40 contact plate

50 blind hole

51, 52 double blind holes

55 insulating material

60 area of higher current density

61 area of lower current density