US 2783405 A
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Feb. 26, 1957 DYNAMO-ELECTRIC MACHINE BRUSH Filed Feb. 24, 1953 IN V EN T ORS Unitedr States Patent Oce 2,783,405 DYNAMO-ELECTRIC MACHINE BRUSH Bernard Roy Atkins, Peter Stuart Maer, and Michael James Lanham, Portslade, Sussex, England Application February 24, 1953, Serial No. 338,582 3 Claims. (Cl. S10-228) This invention relates to dynamoelectric machines, and more particularly to improvements in the manufacture and operation of the current-collecting brushes and commutating elements in such machines.
One object of the invention is to improve commutation in dynamoelectric machines.
Another object is to provide means of reducing brush wear.
A further object of the invention is to provide a method and means of improving commutation by reducing brush wear.
Yet another object of the invention is to improve commutation and reduce brush wear particularly where the machines are operated at high altitude and/or when the brushes are required to pass exceptionally high currents.
According to the invention, in dynamoelectric machines, there are provided current-collecting brushes and/or commutating elements having a substance or substances introduced either into the matrix or onto the surface or surfaces thereof, wherein such substance or substances have the effect of improving commutation and/or brush wear especially at high altitudes and/ or under conditions of heavy current passage.
ln dynamoelectric machines, electric current is transferred to or from a moving commutator or slip-ring by means of current collecting brushes which may consist of carbon, graphite or a metal or of a mixture of two or more of these substances. It has been found that the rate of wear of such brushes is aggravated by the operation of the machines at high altitudes and/ or when the brushes are required to pass exceptionally large currents. There have previously been proposed various means of overcoming excessive brush wear and these have generally included the incorporation into the brush material of various substances which have been found to reduce the wear. The efficiency of many of these previously proposed substances is impaired either by their tendency to deposit undesirable products by decomposition under the prevailing thermal and electrical conditions, or by volatilisation under the said conditions.
Fig. l illustrates a block of electrically conductive material 1 with rods of polytetrafluoroethylene 2 therein, and
Fig. 2 illustrates another embodiment of alternate layers of the electrically conductive material 3 and the polytetrafluoroethylene 4.
According to a feature of the invention various proportions of polyuorethylene are incorporated into the structure of a brush (which may consist of carbon, graphite or a metal or a mixture of 2 or more of these substances) which proportions depend upon the conditions which will exist during the operation of the brush. The exceptionally high dissociation temperature of this compound prevents its ready volatilisation and moreover, the mode of its decomposition under prevailing conditions at the brush surface is such that any accumulation of undesirable products is insuicient to impair commutation.
2,783,405 Patented Feb. 26, 1957 One method of introducing the polytetralluorethylene is to drill holes into the commutating surface of the brush and to insert rods of the substance in these holes. An alternative method, is to fabricate the brush of alternate layers of carbon, for example, and polytetrailuorethylene. A preferred embodiment of this method is to form the brush such that the layers form a structure laminated across its width.
It has previously been proposed to reduce brush wear, especially at high altitudes or when the brushes are required to pass exceptionally high currents, by applying various oils and/or other substances to the slip rings or commutators in an endeavour to improve the operation of the machines. Many of these oils or other substances are unsatisfactory owing to the accumulation of undesirable products due to the decomposition of the substances under the thermal and electrical conditions associated with the operation of the machine.
According to another feature of the invention, a film consisting of one or more organo-silicon oxide polymers (generally known as silicones) is applied to the surface of the commutator or slip ring. The molecules of these silicones consist of oxygen atoms joined to other oxygen atoms by atoms of silicon to which organic radicals are linked through carbon atoms in the following manner:
examples of R1 and R2 being organic radicals such as methyl, propyl and phenyl. The substance or mixture of substances may be applied either as a liquid or as a solid but is preferably in the form of a grease-like consistency. It is to be noted that, owing to the high temperatures existing on the sliding surfaces of the brushes, it is undesirable that the substance or substances should be applied via these surfaces because of the resultant incidence of increased decomposition.
It has been common practice to build up a commutator structure from segments of solid metal, especially copper, in strip or bar form.
According to a further feature of the present invention, commutator segments having a porous metallic structure are provided and the porous structure may be obtained by the well known methods of powder metallurgy. The object of improving commutation by reducing brush wear is achieved according to yet another feature of the invention by including in the porous structure of the commutator segments suitable proportions of a substance or a mixture of substances which have the effect of reducing brush wear. These substances may be introduced into the segment structure before or after sintering. The substances which may be mixed with the metal powder from which the porous commutator segment is made, may include for example, where the said metal powder is copper (as will usually be the case), other powdered metals such as zinc and/or lead and may include other powdered substances such as carbon. Certain or all of the additional substances may be in such proportions as to alloy with the commutator segment metal or not. In addition or alternatively, substances which may be introduced into the commutator segments after sintering, may, for example, be such as graphite, silicones, polylluorethylene, metallic soaps, esters of dibasic organic acids, silicon or selenium or a mixture of these or other substances having the desired effect. The introduction of these substances after sintering may be by any well known and convenient methods or may, according to another feature of the invention, be effected by wearing-in to the commutator surface by application of the substances or substance or mixture thereof in a manner similar to that in which a lubricant might be applied or alternatively may be allowed to seep stance or substances to a surface or surfaces other than the commutating surface and thereafter allowing the penetration of the commutator to take place as a result of centrifugal force on the particles of the substances during the operation of the commutator.
The application of the various features of the invention O are illustrated in the examples which follow:
Example l Along the centre line of the rubbing surface of a graphite brush, one inch in length, one, inch wide and live sixteenths inch thick, were drilled three holes three sixteenths inch in diameter and one. eighth inch apart and having a depth of halfv inch. RodsV of polytetrauorethylene, three sixteenths inch in diameter andhalt inch long were inserted into these holes.
Example 2 The, method of manufacturing sintered commutator segments is now illustrated as follows:
The following constituents were mixed together in a trough mixer for 30 minutes:
95 ozs. copper powder (passing 6G B. S. S.)
5 ozs. paraffin wax ozs. benzene The mixture was then compressed at 20 tons/sq. in. in a mouldto produce segments of the desired shape and size. dered graphite and baked to a temperature of 850 C. The sintered products are then removed from the powdered graphite and placed in a vacuum chamber from which the air has been pumped and an aqueous suspension of graphite (so-called colloidal graphite) is admitted to the chamber. After sufcient time has been allowed for the suspension to enter the porous structure ofthe segments, these are removed and slowly dried. Finally, the segments are subjected to a sizing operation by being pressed in a die at 50 tons/sq. in. The segments are then ready for assembly into a commntator.
Example 3 In another example of the manufacture of sintered commutator segments, the following constituents were blended together for some 45 minutes:
These segments are then covered with powozs. of electrolytic copper powder (150-240 B. S. S.)
2 ozs. tin powder (100 B. S. S.) 1 oz. lead powder (100 B.. S. S.) 2 ozs. flake graphite (200 B. S. S.)
The mixture is then compressed at 20 tons/sq. in. in a mould heated to 250 C., 11/2 minutes is allowed for the heat to properly penetrate the moulding composition. The segments thus produced are ready without further treatment for assembly into commutators.
l. An electrical Contact element, comprising a body of electrically conductive material having incorporated therein bodies of polytetrailuoroethylene and being coated with a film essentially consisting of at least one organo-silicon oxide polymer.
2. An electrical contact element, comprising a body of electrically conductive material having incorporated therein bodies of p olytetrauoroethylene and being coated with a ilm essentially consisting of at least oneorgano-silicon oxide polymer having a grease-like consistency.
3. An electrical contact element, comprising a body of electrically conductive material selected from the group consisting of carbon, graphite, metals `and mixtures thereof having incorporated therein bodies of polytetratluoroethylene and being coated with a lm essentially consisting of at least one organo-silicon oxide polymer having a grease-like consistency.
References Cited in the le of this patent UNITED STATES PATENTS 427,674 Daft May 13, 1890 2,172,045 'Burr Sept. 5, 1939 2,224,724 Elsey Dec. 10, 1940 2,310,108 Moberly Feb. 2, 1943 2,393,816 Savage Ian. 29, 1946 2,425,046 Radanoff Aug. 5, 1947 2,531,007 Strom Nov. 2l, 1950 2,656,475 Diehl Oct. 20, 1953 2,662,065 Berry Dec. 8, 1953 2,689,380 Tait Sept. 21', 1954 2,736,830 Savage Feb. 28, 1956 FOREIGN PATENTS 614,372 Great Britain Dec. 14, 1948'
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