US3025422A - Shuntless brush - Google Patents

Description

March 13, 1962 s. RUSYNYK 3,025,422

SHUNTLESS BRUSH Filed April 24, 1958 Finger Conducrive Cemenf Carbon Brush Braided Cable to Holder Conductive Cement INVENTOR SAMUEL RUS NYK ATTORNEY United States Patent 3,025,422 SHUNTLESS BRUSH Samuel: Rusynylr, Parrna, Ohio, assignor to Union Carbide Corporation, a corporation of New York Filed Apr. 24, 1958, Ser. No. 730,598 8 Claims. (Cl. 310246) This invention relates to a shuntless brush suitable for use in electric motors or generators, particularly on traction motors.

Brushes composed of carbon and graphite or metal and graphite commonly are used in electricity generating or actuated equipment. Ordinarily, such brushes are contained within metal brush holders. In direct current locomotive motors the basic brush system frequently consists of two negative and two positive brush holders. Each holder accommodates in parallel, three brush assemblies. To each brush are available three parallel paths through which current can enter. In one of these current enters from the brush holder directly to the side of the brush. The resistance of this path continually changes as the contact area between the brush and its holder is influenced by vibration, brush and brush holder wear, and change in direction of torque as the motor is reversed. Another current path extends from the brush holder through a steel spring finger bearing on the top of the brush. A solid copper pad normally is attached under the spring finger in contact with the top of the brush. This pad is connected electrically to the brush holder by a short length of braided copper cable. The resistance of this circuit is rather high, owing to the small contact area between the brush and the spring finger pad and the high resistance of the varying metal to carbon contact.

A third possible current path for this arrangement extends from the holder through the brush shunts, and then to the brush. This path is completed by bolting one end of the brush shunt to the holder and tamping the other end securely in to the brush. The resistance of this path is not measurably affected by vibration, torque or brush and holder wear. This last current path is the one possessing the least resistance in the above-described brush system. An average resistance figure in ohms for this brush system is 0.0057 ohm under a 100 ampere test resulting in a power loss of 57 watts.

Another type of brush circuit is the so-called shuntless brush system in which the shunts are eliminated. The elimination of the shunts in addition to reducing the number of parallel paths available to the brush, also excludes the lowest resistance path. In this system, therefore, the total current is carried by the spring finger and the brush holder circuits, both being loose metal-tocarbon contacts. Accordingly, the resistance and power losses of this system are about four times that of the shunted brush system. Despite such disadvantage, there are several advantages existing in the shuntless brush system that have made it useful in particular applications. These advantages are, principally, easier handling during installation, in that shunt troubles such as frayed-off cable and brush-cable connection pull-outs are eliminated. A typical resistance figure encountered with prior art shuntless brush systems is one of about 0.196 ohm in a 100 ampere test equivalent to a power loss of 195.2 watts.

A more recently developed shuntless brush systeni has proposed the replacement of the solid copper pad under the spring finger by a braided cable pad in an attempt to decrease the contact resistance between spring finger and brush. In essence, the braided wire pad can be considered a continuation of the braided cable which electrically connects the solid copper pad to the brush holder. With this modification, however, an undesirably high metal-to-carbon resistance was still present.

3,025,422 Patented Mar. 13, 1962 Typical resistance figures obtained with this modification were 0.0184 ohm under a ampere test, giving a power loss of 184.0 watts.

With a view to departing from the above enumerated practices of the prior art, it is an object of the present invention to provide novel connection means for carbon brushes and the like, which means possess reduced resistance and power loss.

It is another object of the invention to provide a novel brush construction in which current is carried to the brush by a shunted spring finger pad, which is in contact with a short length of braided wire cable cemented with conductive cement into the top of the brush.

Further and related objects and advantages of the present invention will be apparent as the description thereof proceeds, especially when taken in connection with the accompanying drawing in which:

FIG. 1 is a side elevation in vertical section, and partly broken away, of a split carbon brush featuring the connection of the invention; and

FIG. 2 is a front elevational vertical section of the brush shown in FIG. 1.

Referring to the drawing, the carbon brush 10 is provided with a cavity 12 formed therein of a size somewhat larger than that of the braided wire pad 14 to be embedded centrally therein. The cavity is filled with a conductive cement, and the braided cable previously folded in two to fit into each half of the split brush is inserted in the cavity so that the ends of the cable protrude inch beyond the top of the brush. Surmounting the braided cable insert is the copper spring finger plaque '16, to which wire cable 18 is attached. This cable goes to a connection on the brush holder (not shown).

It will be noted that the embedded conductor 14 has the major portion of its length disposed in each cavity 12 of the split brush. The portion of the conductor which extends beyond the brush is sufficient only to make contact with the underside of copper plaque 16, which is riveted to the spring arm, and to prevent the plaque from making direct contact with the carbon brush top. The spring arm, usually made of steel, is integral with and in extension of the coiled spiral spring (not shown), which is mounted on the bronze casting constituting the brush holder (not shown). The copper plaque thus is held in contact with the top ends of the embedded wire pads by virtue of the spring arm pressure.

The multi-strand cable insert should consist of a bundle, or a Weaving, or a twist, of small diameter high conductivity metal strands of the kind generally used in the formation of brush pigtails. Preferably the strands should consist of oxygen-free high conductivity fully annealed copper. However, any metal strand composite possessing a suificiently high circular mil current-carrying capacity for the individual brush current rating involved, is suitable. Individual strands having diameters as high as 0.010 inch, if sufliciently flexible, can be used. Preferably strands having a diameter of 0.005 inch should be used. Tinned copper strands may be used if a maximum protection against oxidation or chemical corrosion is desired. Brass, bronze and other copper or silver-bearing alloys are also suitable. In the case of a thicker single thickness brush, only one embedded pad is needed instead of the two shown.

The conductive cement employed in securing the multistrand cable pad in the carbon brush top suitably may be of the type disclosed and claimed in the co-pending application of K. R. Matz, U.S. Serial No. 653,765, now US. Patent 2,849,631. This cement consists of 30 to 60 percent by weight of silver flake particles, the largest dimension of which does not exceed 65 microns, 4 to 12 percent by Weight of a finely divided quartz filler, up to 20 percent by weight of an alkaline thermoset catalyst,

the balance being a thermosetting resin such as an epoxy resin, a phenolic resin, a urea formaldehyde resin or a melamine aldehyde resin.

A number of split brushes of the construction described were experimentally subjected to vibration and to current loading conditions considerably more severe than those encountered in actual brush operation. All of the constructions remained mechanically sound, and showed almost no change in electrical resistance. Even after these tests, the average resistance of the assemblies was only 0.0073 ohm, giving a power loss at the rated 100 ampere load of 73 Watts.

The advantages of the construction of the invantion are the following:

(1) The multi-strand insert provides a low resistance metal-to-metal multiple point contact.

(2) Use of the conductive cement makes possible a low interface resistance at the inside surface of the insert cavity.

(3) A comparatively large cross-sectional interface area in the cavity holds power losses to a minimum, and provides more uniform current distribution throughout the brush than is obtained when conventional shunt connections are used.

The metallic cement embedded conductor disclosed herein is provided with a mechanically tight large area of contact with the carbon. The mechanically loose part of the contact is constituted of metal-to-metal areas. For this reason it is superior to the construction discussed wherein a braided copper pad was provided under the spring arm, which does not provide a connection contact as mechanically secure and unvarying as the cemented connection. Stated another way, mechanical movement within the metal-to-metal contact in a brush system is not detrimental because a lower voltage drop exists than is the case between carbon and metal. At this lower potential, the creation of an arc is much more unlikely. Consequently, the likelihood of flashovers is decreased.

What is claimed is:

1. A shuntless brush comprising a carbonaceous body having a vertical cavity formed therein at the top thereof, said cavity extending substantially centrally over the 4 width of said body, a flexible inulti-strand metal pad having a portion disposed in said cavity Within said body, secured thereto by means of an electrically-conductive cement, and spring finger means in contact with said metal pad for conveying electrical current to said brush.

2. The brush of claim 1 wherein said metal pad consists of a plurality of strands of oxygen-free, high-conductivity, annealed copper.

3. The brush of claim 1 wherein said metal pad consists of a plurality of strands of tinned copper.

4. The brush of claim 1 wherein said metal pad consists of a plurality of strands of brass.

5. The brush of claim 1 wherein said metal pad consists of a plurality of strands of bronze.

6. The brush of claim 1 wherein said electrically conductive cement contains from 30 to percent by Weight of silver flake particles and a thermosetting resin binder.

7. In combination in a, shuntless brush for use on traction motors, a carbonaceous body having a cavity formed therein at the top thereof, said cavity extending substantially centrally over the width of said body, a flexible pad composed of a plurality of metal strands having a portion disposed in said cavity, a conductive cement securing said pad to said carbonaceous body, and a spring finger in contact with said metal pad for conveying electrical current to said carbonaceous body.

8. The combination of claim 7 wherein the diameter of said metal strands ranges from 0.005 inch to 0.010 inch.

References Cited in the file of this patent UNITED STATES PATENTS 849,511 Speirs Apr. 9, 1907 852,686 Thornton May 7, 1907 1,102,119 Aichele June 30, 1914 1,506,740 Eaton Sept. 2, 1924 2,301,991 Zollner Nov. 17, 1942 2,631,252 Falcettoni Mar. 10, 1953 2,849,631 Matz Aug. 26, 1958 FOREIGN PATENTS 4,258 Great Britain of 1892 22,269 Great Britain of 1903

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