Patent Application
20070132334
The technology described in this patent document relates generally to electric motors and generators. More particularly, systems and methods are described for providing electrical contact with a rotating element of a machine.
The earliest electrostatic motors and generators dating back to the early 19th century relied upon filamentary metal brushes to transfer electric charge from the stationary power source to rotating members of the machine. Multiple fine hair-like whiskers constituted the so-called “brushes.” Each individual metallic fiber was individually suspended independent of the surrounding fibers. Accordingly, numerous contact points were afforded by the collection of discrete conductors acting in concert to conduct electric current between surfaces in relative motion with respect to one another.
Although the label “brushes” has remained in the lexicon of electrical engineering, the physical form of the brush changed radically with the advent of magnetically-based rotating machinery requiring electric currents thousands of times higher than those found in electrostatic machines. Solid blocks of graphite replaced the filamentary representation of the brush. Although a poor conductor compared to most metals, graphite nevertheless offered several unique features favoring serviceability above all other methods of current transfer between sliding surfaces: 1) graphite has natural lubrication properties; 2) graphite forms a protective film on the ring surface in the presence of atmospheric humidity and oxygen which shifts wear from the ring to the easily-replaceable brush; and 3) graphite has a peculiar thermal characteristic which causes it to vaporizes at very high temperature rather than melt (sublimation), which extends the operational life of the brush.
Electrical wear in conventional solid brushes accounts for about half of the total wear, the other half arising from dry mechanical friction. Electrical wear results from vaporization of graphite at several scattered contact points which randomly move across the interface surface. Extremely high current density creates local temperatures exceeding 5000° F., above the vaporization temperature of graphite.
The complex phenomena occurring at the interface of the slip-ring and graphite brush has been exhaustively studied for over a century. Based on decades of empirical practice, the combined effects of materials selection, brush pressure, surface speed, current density, atmospheric oxygen and humidity has been reduced to predictable and reproducible performance under specified conditions. In short, the operational characteristics of the graphite brush have been investigated seemingly to the theoretical limit. Nevertheless, demands of modern high current machine processes exceed the capability of the most advanced technologies of current collection based on conventional graphite brush technology.
Within the past decade there has been a revival of the basic filamentary brush concept. Known as “metal fiber brushes,” this modification of the original free-fiber brush incorporates numerous metal fibers bound together into a solid block somewhat resembling a standard graphite brush. Fibers are fused or bonded to one another within a proprietary matrix material. This device excels in many respects compared to graphite brushes, but remains inadequate to the demands of many industrial applications.
In accordance with the teachings herein, systems and methods are described for providing electrical contact with a rotating element of a machine. In one example system, a nozzle may be used to emit a stream of a molten metal material, the nozzle being configured to transmit electric current from a power source to the stream of molten metal material. The rotating element may be supported within the machine to rotate relative to the nozzle, and may include a current collector ring. The nozzle may be operable to emit the stream of molten metal material onto a localized portion of the current collector ring to transfer the electric current from the power source to the current collector ring, the localized portion of the current collector ring being a portion of the current collector ring that is less than an entire circumference of the current collector ring.
One example method for providing electrical contact with a rotating element of a machine may include the following steps: charging a molten metal material with an electric potential; and emitting a stream of the electrically charged molten metal material from a stationary element of the machine onto a localized portion of a current collector ring attached to the rotating element of the machine; the stream of electrically charged molten metal material supporting a transfer of electrical current from the stationary element of the machine to the rotating element of the machine.
In operation, the nozzle 14 emits the stream of molten metal material 16 onto a localized portion of the current collector ring 20 to transfer electric current from the power source 18 to the current collector ring 20. As referred to herein, a localized portion of the current collector ring 20 is a portion of the current collector ring 20 that is less than an entire circumference of the current collector ring 20. Also illustrated in
Preferably, the molten metal material 16 is a metal having a low melting temperature relative to the surrounding structure of the pump 24, nozzle 14 and current collector ring 20. Example metals that may be used include eutectic alloys, such as alloys of bismuth, cadmium and lead, which have melting points in the range of about 107° to 180° F. These example low melting point metals may be readily accommodated within the example system of
As described above, the molten metal material 16 acts as a conductive intermediary between the nozzle 14 and the current collector ring 20. Electrical and thermal conductivity between these components is facilitated by wetting at the liquid-solid interface on the current collector ring 20. In many cases, however, the wetting phenomenon may be accompanied by dissolution of the solid ring material by a liquid eutectic metal. Thus, the current collector ring 20 may preferably be made of a material that is wetted, but not dissolved, by a eutectic liquid, such as an iron-nickel alloy, steel or stainless steel. It should be understood, however, that other materials could also be used for the current collector ring 20.
The system 10 depicted in
Electric wear in conventional systems may also be partially due to the formation of localized hot spots around a small number of restrictive contact points at the brush-ring interface. These concentrated areas of conductivity create pin-point areas of high current density, ultimately leading to electric arcs that comprise the primary current-carrying mechanism between brush and ring during normal operation. The extremely high temperature of the electrical arcs ablates the graphitic brush material by evaporation. In the system 10 depicted in
Mechanical wear in conventional systems may arise from the friction of solid surfaces sliding relative to one another. In the system 10 of
The cooling system 48 may be used, in some examples, to remove excess heat from the molten metal material 42, 44. Typical bismuth eutectic alloys, which may be used for the molten metal material 42, 44, have an electrical conductivity around 5% that of pure copper. Electric current flowing through an eutectic metal therefore generates about 20 times more heat than copper under similar circumstances. Stream resistance is provided by the portion of the liquid metal stream between the nozzle tip and the current collector ring. The relatively short stream length that participates in current conduction represents a low value of absolute electrical resistance. However, heat generation may nevertheless be significant at very high current densities. Accordingly, the stream flow rate should be high enough to move heat away from the conduction zone and prevent undue increases in the localized temperature of the liquid metal.
The flowing liquid metal stream provides its own inherent cooling mechanism inasmuch as the heated liquid metal is continuously replaced by cooler material flowing into the heat generation region between the nozzle and the current collector ring. The cooling system 48 may be included to remove heat from the molten metal before it is returned to the nozzles 34, 36 by the pump 50. In some examples, an active cooling system could be used, such as cooling coils. In other examples, however, the return path 52 between the rotating element 32 and the pump 50 may act as the cooling system by providing sufficient time and conditions for the molten metal cool to a desired temperature range.
In addition, the cross-sectional area (thickness) of the liquid metal stream 82 emerging from the nozzle 84 should be sufficient to minimize electrical resistance and the attendant heat generation. The preferred thickness of the liquid metal stream 82 may be dependent on the voltage-current characteristics of the system 80. For example, in a system utilizing high current and low voltage, a thicker liquid metal stream 82 may be preferable. Conversely, in a system utilizing high voltage and low current, a thinner liquid metal stream 82 may be preferred.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.
