Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-04-23
2004-06-22
Zimmerman, John J. (Department: 1775)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S236000, C428S687000, C029S596000
Reexamination Certificate
active
06753635
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the management of so-called contact spots through which, on a micro-scopic scale, electrical currents are conducted across interfaces of solids, whether between the two sides of switches or between sliding as well as stationary electrical brushes and their substrates, being mostly but not exclusively slip rings and commutator bars.
The electrical brushes at issue include fiber brushes disclosed in the above-noted U.S. Pat. Nos. 4,358,699 and 4,415,635, and in U.S. Pat. No. 6,245,440. Additionally, they include foil brushes as described in the publication “Production and Performance of Metal Foil Brushes,” P. B. Haney, D. Kuhlmann-Wilsdorf and H. G. F. Wilsdorf, WEAR, 73 (1981), pp. 261-282, which is also incorporated by reference, and ordinary monolithic brushes made of graphite or graphite-metal mixtures. The invention is also applicable to electrical switches for the reduction of resistance and sticking forces, as well as to devices for efficient heat transfer.
The present invention includes the use of various technologies referenced and described in the above-noted U.S. Patents and Applications, as well as described in the references identified in the appended APPENDIX and cross-referenced throughout the specification by reference to the corresponding number, in brackets, of the respective references listed in the APPENDIX, the entire contents of which, including the related patents and applications listed above and the references listed in the APPENDIX, are incorporated herein by reference.
2. Discussion of the Background
Sliding electrical contacts, i.e., “brushes”, conduct electrical current between solids, very preponderantly metals, in relative motion. Brushes are in widespread use in various types of electric motors and generators and are also widely used in less common but numerous special applications, e.g. telemetry devices and rotating antennae. Even while to date the traditional “monolithic” (i.e., in the form of a solid piece) graphite-based (i.e., including compacted graphite or various metal-graphite mixtures) brushes are overwhelmingly frequent, they have a number of technological limitations. Specifically, monolithic graphite-based brushes cannot be reliably used, over extended periods of time, at current densities above about 30 Amp/cm
2
, nor at sliding speeds above about 25 m/sec. Further, as a coarse estimate, they waste about one watt per ampere conducted across the brush-substrate interface, i.e. the equivalent of one Volt, in terms of Joule and friction heat. Further, they emit significant intensities of electromagnetic waves (i.e., they are electrically very noisy so as to interfere with radio and similar signal reception), and finally they wear into a powdery debris that can be highly detrimental in electrical machinery, especially aboard submarines.
As a result of these shortcomings of traditional monolithic brushes, a number of otherwise very attractive technological developments are stymied for lack of electrical brushes which will conduct reliably over extended time periods, much higher current densities at low losses up to much higher speeds. Most importantly impacted are so-called “homopolar” motors and generators. They have potentially very high power densities and would be excellent for Navy ship drives, among others, but typically require current densities in excess of one hundred Amperes per cm
2
to be conducted across interfaces of metal parts relatively moving at sustained speeds up to of 30 m/sec or even more while producing or requiring EMF's of only 20V or so. The requirements of homopolar machinery in terms of current densities and speeds can thus not be fulfilled by monolithic brushes, and in any event a loss of 2 Volts per monolithic brush pair, i.e., in and out, is prohibitive for homopolar machines.
In previous inventions, particularly in the Patent Application “Continuous Metal Fiber Brushes, [1]” the capabilities of metal fiber brushes, including multitudes of essentially parallel hair-fine metal fibers, are outlined. They are intrinsically capable of easily conducting the desired current densities and to do so up to at least 70 m/sec with a total loss in the order of 0.1 Volt per brush. At the same time such brushes are electrically very quiet. These superior qualities derive from large numbers of separate electric “contact spots”, namely at the fiber ends at the brush “working surface” sliding along the brush-substrate interface, through which the current is physically conducted on a microscopic scale. That current is conducted across solid interfaces only through a restricted number of contact spots, whose total area amounts to only fractions of one percent of the macroscopic area of contact, is a well-known general physical phenomenon. To a large extent the poor qualities of monolithic brushes arise from their small number of contact spots, namely in the order of ten per brush. As a result, the current flow lines in monolithic brushes are not rather uniformly distributed, as they are in metal fiber brushes, but they are “constricted [2]” at the few contact spots. This causes the corresponding “constriction resistance” that represents in the order of one third the resistance of monolithic brushes.
The superiority of metal fiber brushes does not only derive from their thousands of evenly distributed contact spots, but also from the fact that at their contact spots bare metal meets bare metal, ideally separated only by a double monomolecular layer of adsorbed water vapor. Fortuitously, this most favorable type of lubrication, which prevents cold-welding and accommodates the relative motion between brush and substrate at a “film resistivity” of only &sgr;
F
−1×10
−12
&OHgr;m
2
and average friction coefficient (&mgr;) of about 0.3, establishes itself automatically at any modest ambient humidity, provided that undue contamination with oils, etc., is avoided. By contrast, monolithic brushes deposit a lubricating graphitic layer through which the current must flow at much higher electrical film resistivity. Further, the body resistance of graphitic brushes can be significant while it is always negligible for metal fiber brushes. Finally, monolithic brushes are hard and “bounce”. At increasing speed, that “brush bounce” must be counteracted by an increasingly strong pressure between brush and substrate at the correspondingly increased friction power loss. Practically speaking, this syndrome limits the sliding speed of monolithic brushes to about 25 m/sec, as already indicated, whereas metal fiber brushes are intrinsically flexible (i.e., have a much larger “mechanical compliance”). Therefore, they can and should be mechanically only lightly loaded and can be operated to high speeds at only minor friction heat loss.
Metal foil brushes [3] closely resemble metal fiber brushes except that they are composed not of substantially parallel fibers but of thin parallel foils. Consequently they typically have many fewer, but otherwise the same kind of, contact spots. Thus metal foil brushes are very similar to metal fiber brushes but cannot match their attainable current densities, sliding speeds and low power losses. At any rate, foil brushes are based on the same principle as metal fiber brushes, namely electrical contact to the substrate at a large number of microscopically small, bare metal-metal contact spots, optimally lubricated by a double monomolecular layer of adsorbed water. Hence, also, in terms of number of contact spots per unit working surface area (i.e., “contact spot density”), and mechanical load per contact spot, exactly the same theory applies to metal foil as to metal fiber brushes [4-6].
As stressed, on account of their different geometry, foil brushes comprise a substantially smaller density of contact spots than well-constructed metal fiber brushes. By way of numerical example, the working surface of a typical metal fiber brush constructed of d=50 &mgr;m cop
Hi Per Con
Zimmerman John J.
LandOfFree
Management of contact spots between an electrical brush and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Management of contact spots between an electrical brush and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Management of contact spots between an electrical brush and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3365243