Patent Application
20060082243
The present invention generally relates to brushes for an electric motor and more particularly to a slip-ring submerged in an liquid medium in order to decrease wear of the brushes, and, increase current density and heat removal.
Filamentary metal brushes have been used to transfer electrical energy from a power source to the rotating member of a machine. The brushes comprise a plurality of fine hair-like metallic fibers that are individually suspended independent of the surrounding fibers. Each of the fibers provides a discrete contact point with a metallic ring surface of the rotating member of the electrical machine. Accordingly, numerous contact points are established and act in concert to conduct electrical current between the brushes and the rotating member of the electrical machine.
As the amount of current has increased in electrical machines, the format of the brushes has changed. In order to handle large amounts of current, the hair-like brushes have been replaced by solid blocks of graphite which provide greater serviceability. Graphite has been used because it provides natural lubrication and vaporizes at a high temperature rather than melts. Furthermore, the formation of a protective film on the ring surface is formed from the presence of atmospheric humidity and oxygen. The protective film shifts wear from the ring to the brush which is easily replaced during routine machine maintenance.
The graphite brushes wear out from both electrical wear and mechanical wear. The electrical wear results from the vaporization of the graphite at scattered contact points of the ring that randomly move across the interface surface. Mechanical wear results from the dry mechanical friction between the graphite and the surface of the metallic ring. In order to ensure proper operation of the electrical motor, inspection and servicing of the graphite brushes are required.
Recently, metal fiber brushes have been incorporated into electrical motor designs. These brushes incorporate numerous metal fibers bound together into a solid block that resembles a standard graphite brush. The fibers are fused or bonded together using a matrix material. However, these brushes cannot handle the high current density required for industrial applications.
In accordance with the present invention there is provided a slip ring assembly for transferring electrical current to an electrical device such as a rotating machine. The slip ring assembly has a housing that is attachable to the electrical device. Disposed within the housing is a rotatable slip ring and a flexible conductor. The flexible conductor is configured to conform to the shape of the slip ring and conduct an electric current. A fluid is contained within the housing. The fluid forms a conductive film between the slip ring and the conductor when the slip ring rotates through hydrodynamic forces. The conductive film is operative to transfer electrical current between the slip ring and the conductor while also preventing wear of the slip ring and flexible conductor.
The slip ring is attached to a rotatable shaft of the electrical device. In this respect, the shaft extends through the housing and is supported by a seal and/or bearing. In order to attach the slip ring to the shaft, the assembly can include first and second insulators attached to respective first and second sides of the slip ring and the shaft. The insulators electrically isolate the slip ring from the shaft.
It is possible to have two slip rings disposed within the housing and attached to the shaft. In this configuration, a second flexible conductor is used with the second slip ring. The second slip ring may be attached to the shaft with respective insulators and a center separator may be used to separate the first and second slip rings from one another.
Typically, the slip ring has a groove formed in the outer circumference thereof. The groove is sized and configured to receive the flexible conductor. The flexible conductor may be a braided strap which partially wraps around the slip ring.
In accordance with the present invention, there is provided a method of conducting electrical current between a power source and an electrical device with the slip ring assembly. The method comprises rotating the slip ring within the housing to form a conductive film with fluid between the flexible conductor and the slip ring. Next, electrical current is transferred between the flexible conductor and the slip ring. The conductive film formed by the fluid prevents wear to the slip ring and flexible conductor.
These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein:
Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same,
Attached to an end of the housing 12 opposite the flange 14 is an end plate 20 that is secured with tie-rod nuts 22. The end plate 20 is tightened up against the housing 12 with the nuts 22 to prevent fluid from leaking from the housing 12. A washer or seal can be inserted between end plate 20 and the housing 12 to further prevent fluid from leaking from the housing 12.
The end plate 20 further includes a shaft bearing and seal 24 for supporting a rotating shaft 26 of the motor. As will be further explained below, the shaft 26 supports the metallic slip rings of the slip ring assembly 10. The shaft bearing and seal 24 supports the shaft 26, while allowing rotation thereof. Furthermore, the shaft bearing and seal 24 prevents fluid from leaking from between the shaft 26 and the end plate 20.
Referring now to
Referring to
The second slip ring 30b is similarly supported by the second end insulator 32b and the second center insulator 34b. In this regard, the second slip ring 30b is sandwiched between end insulator 32b and center insulator 34b. The slip ring 30b, end insulator 32b and center insulator 34b may be keyed in order to facilitate alignment. In order to further isolate the slip rings 30a, 30b and center insulators 34a, 34b, the slip ring assembly 10 further includes a center separator 36 that is mounted to the shaft 26. The center separator 36 is fabricated from an electrically insulating material and is attached to both of the center insulators 34a, 34b, as well as the shaft 26. In this respect, the center separator 36 spaces the slip rings 30a, 30b, as well as the center insulators 34a, 34b axially along the shaft 26. The end insulators 32, center insulators 34 and center separator 36 may be bolted together in order to sandwich the slip rings 30 into position.
Even though, there are two slip rings 30a, 30b shown in
Each of the slip rings 30 has a channel or groove 38 formed on the outer circumference thereof. The groove 38 of each slip ring 30 is sized and configured to receive a flexible braided conductor 40 which partially wraps around the outer circumference of the slip ring 30. In this respect, the conductor 40 may be a strap which can conform to the circular shape of the circumference of the slip ring 30. The braided conductor 40 is fabricated from an electrically conductive material such as copper. As seen in
Each of the slip rings 30 is attached to a wire conductor (not show) that carries electric current from the slip ring 30 to the motor. Specifically, the wire conductors are soldered or otherwise attached to the slip ring 30 and run through the shaft 26 to the motor. Each wire conductor is insulated to prevent electrical current from being transferred to other members of the slip ring assembly 10.
In operation, the housing 12 serves as a containment vessel for a conductive fluid into which each of the slip rings 30 and braided conductors 40 are immersed. The fluid is prevented from escaping the housing 12 with the seals 24. The housing 12 is filled by pouring the fluid through the fill port 18. The fluid is of a non-metallic composition such that hazardous metals such as mercury and low-melting-temperature eutectic alloys are not required. Organic liquids such a hydrocarbon oils degrade through carbonization with the passage of electric current and therefore are not suitable as a conductive medium despite their high lubricity. Various aqueous-based liquid solutions of salts and acids provide the ionic electrical conduction as well as necessary viscosity for use in the slip ring assembly 10. This type of fluid creates a very thin film between the braided conductor 40 and the respective slip ring 30 such that the overall electrical resistance of the fluid is very low. A typical fluid can be water.
As the shaft 26 rotates, hydrodynamic forces from the fluid contained within the housing 12 develop between the each of the braided conductors 40 and respective slip rings 30. The hydrodynamic forces prevent direct physical contact between the braided conductors 40 and slip rings 30 such that mechanical wear of the slip rings 30 and conductors 40 is prevented. Because the fluid is an electrically conducting medium, electrical energy is still transferred from the braided conductors 40 to a respective slip ring 30. The electrical conduction is facilitated by the ionic properties of the liquid film separating the braided conductor 40 and a respective slip ring 30. Furthermore, electrical wear is mitigated by the thermal quenching of the fluid such that localized hot spots, pitting and vaporization of the sliding surfaces is prevented.
It will be appreciated by those of ordinary skill in the art that the concepts and techniques described here can be embodied in various specific forms without departing from the essential characteristics thereof. For example, the slip ring assembly 10 may be used in various other formats including linear and rotary elements through which electric current is to be conducted. The presently disclosed embodiments are considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced.
