US2897386A - Self-cleaning insulator for brush holder stud - Google Patents

Description

- July 2s,1959 s. 1 JONES 2,897,386

SELF-CLEANING INSULATOR FOR BRUSH HOLDER STUD Filed Aug. 6, 1953 Inventor": Samuel PJor es,

y His Attorney- United States 2,897,386 SELF-CLEANIN G INSULATOR FOR BRUSH HOLDER STUD Samuel P. Jones, Erie, Pa., assignor to General Electric Company, a corporation of New York Application August 6, 1953, Serial No. 372,727 2 Claims. (Cl. 310-239) My invention relates to insulating members and has particular reference to insulating assemblies subjected to high current arcs or flashovers across an exposed insulator surface.

In insulator assemblies where an insulating member is spaced between conductive members across which a high potential may exist, it is important not only that the insulating member itself have a high dielectric strength, but that its exposed surfaces provide a high resistance leakage path thereacross despite exposure of the insulating surface to dirt, water vapor, and other foreign matter which may collect thereon. In direct current rotating machinery, such as that designed for railway locomotives, where the insulator assembly is employed to support the brush holders from the grounded machine frames, the problem is particularly acute. In such cases, the insulating studs commonly employed are subject to severe high current flashovers across their exposed surfaces with the result that the carbonized material left on the insulator by an arc provides a low resistance leakage path. Since in the course of normal duty the insulating surfaces of the studs are also likely to collect oil, grime and other foreign matter, the likelihood of flashover and the amount of carbonized material are both increased, thus requiring frequent cleaning or replacement of the brush holder studs. Inasmuch as a severe flashover may cause a locomotive to stall, cleaning of the insulators is often required while a train is enroute. In addition, very severe flashovers may not only leave a carbonized deposit but may also result in the spewing of molten metal on the insulator or breakage or melting of the insulator.

It is therefore an object of my invention to provide for installations where exposure to grime and oil and other foreign matter is involved or encountered, an insulator which is substantially self-cleaning upon a high current flashover or arc thereacross.

It is another object of my invention to provide an insulating brush holder stud for rotating machines having an exposed surface whose leakage resistance is not adversely affected by high current arcing thereacross.

It is a still further object of my invention to provide an insulating brush holder stud for high voltage rotating machines for operation under conditions where exposure to dirt, grime or oil is required.

Briefly, in accordance with my invention, a high voltage insulating member having an exposed surface subject to high current flashovers or arcs thereacross has a coating or outer surface comprising polytetrafiuoroethylene or polytrifluorochloroethylene. I have discovered that this material has the property of being selfcleaning when exposed to a high current flashover. The heat of a breakdown are vaporizes a thin outer layer of the coating, thus exposing a clean new surface instead of leaving a carbonized deposit. A surface exposed to collection of grime or other foreign matter which would decrease the leakage resistance and increase the likelihood of a flashover is thus rendered essentially selfcleaning by the action of the flashover arc.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in con nection with the accompanying drawing in which Fig. 1 is a view of a portion of a large high voltage direct current motor illustrating the use of an insulating stud incorporating my invention to support and insulate a brush holder with respect to a motor frame, and Fig. 2 is a sectional view of the insulating stud of Fig. 1.

Referring now to Fig. 1 a portion of the grounded steel frame 1 of a large direct current motor is shown from which is insulatingly supported a brush holder assembly 2 containing a brush 3 in contact with the bars of a commutator 4. An insulator assembly 5 commonly termed a brush holder stud, is employed to both support the brush holder and insulate it from the frame. Both the mechanical and electrical duty requirements of this insulating support are severe since especially in a locomotive motor, the brush holder stud must be very strong to withstand shock and provide maximum service while at the same time the insulation must be adequate to withstand potentials in the order of thousands of volts without breakdown. However, even if the dielectric strength of the insulator is adequate to prevent breakdown therethrough, the exposed surface of the insulator provides a relatively long leakage path whose resistance is materially decreased by the collection of foreign matter on the exposed surface. Thus when a high current flashover or are is initiated between the brush holder and the frame due either to abnormally high voltages at the commutator or breakdown under normal voltage (600 volts in most railway traction motors) due to the decreased resistance of the leakage path along the insulator surface, the carbonized deposit on the insulator left by the arc must be removed before the motor can function properly.

In Fig. 2 the construction of the brush holder stud 5 is shown in detail. As may be seen, a steel stud 6 to which the brush holder 2 is firmly fastened by threaded engagement therewith is provided with a cup-shaped end portion. A threaded nut 7 for accommodating a frame bolt 8 (shown in Fig. 1) is positioned within the cup-shaped opening of the stud 6 but spaced therefrom. The nut 7 is preferably provided with a radially extending circular flange designed to bear against the motor frame when it is tightened in place. An insulator body 9 between the mounting members shown as the stud 6 and nut 7 bonds them together and provides the desired insulation, this insulator body suitably being made of a glass-bonded mica insulating material or other mechanically strong insulator, such as a ceramic, glass, hard rubber, or a plastic material.

In accordance with my invention, the outer surface of the cylindrical insulating body 9 is provided with a cylindrical cover or jacket 10 of a solid fluorinated polymer, preferably polytetrafluoroethylene. This material is marketed under the trademark Teflon by E. I. du Pont de Nemours & Co. The material is preferably applied by shrinking the tubular section 10 of extruded polymer on the cylindrical surface of the insulator 9, the tubing being formed with a diameter somewhat less than that of the insulator 9 and then heated to a temperature in the neighborhood of 250 C. and allowed to cool in position. No further bond between the jacket 10 and the insulator 9 is required so long as the fit between them is sufliciently tight to keep the jacket in place. While the polymer has a very high dielectric strength, it is the dielectric strength of the insulator 9 which is employed to prevent breakdown therethrough between the opposing ends of the stud 6 and the nut 7, as well as to provide the mechanical strength of the brush holder stud assembly.

According to my invention, the function of the jacket It is principally restricted 'to providing a substantially self-cleaning exposed insulating surface or leakage path or high resistance leakagefpath between the brush holder and the motor "frame. other means of applyin the Teflon material to form a cover or jacket may also be employed, "such as by molding a sleeve from the powdered polymer and fusing it on the stud assembly, by Windinga'polymer tape around the stud or a mandrel and fusing the turns together, 'or by brushing a polymer enamel over the cylindrical "surface of the insulator 9 and fusing it. The thickness of the polymer layer is not critical, in the sense that a few thousandths of an inch will affect its dielectric strength or se'lf 'cleani'n'g properties. However, the thickness must be sufficient to Withstand the heat of a fiash'over 'without vaporization of the entire layer along the arc path. Since the material is relatively expensive, it is usually desirable from the standpoint of economy to limit the amount of material used. Under conditions to 'be subsequently described in the following paragraph, thicknesses from to A; have been found entirely satisfactory.

In operation, the exposed surfaces of the polymer jackets 10 are subject to collection of soot, oil, carbon, dust, or other foreign matter. When aihigh currentfl'ashover occurs across the ex osed surface of the jacket 10 between the brush holder and the frame, no carbonized path 'is formed by the intense heat of the arc, however, and instead the arc vaporizes a thin layer or the fluorinated polymer and actually cleans offth'at portion of the exposed surface adjacent the arc. I "have 'found, for example, that a T200 ampere arc usually removes only a :001" layer from the surface of the jacket 1'0, -thus makinga sin'glejstudholdcr assembly reusable after man 'fiashovers "without renewal 'or cleaning of the polymer jacket. Additional economies may be effected by r0- tatingth'e studs a few degrees after a number of flashovers since the flashovers usually occur between given points on the frame and brush holder. The advantages of 'such a self-cleaning insulator surface are-obvious and especially so in such applications such as that of the brush holders on railway motors where the problem of cleaning or replacement after fiashovers has been a particularly troublesome one. "It is 'to be emphasized that the vaporization of an outer layer of the jacket '10 is not accompanied by failureof thepolymer as a dielectric or are puncture thereof since the short-'circuiting arc does not occur through the jacket but only across its exposed surface. Thus thepolymer itself need not have a high degree of purity or cleanliness since it is the va- :p'orization of the material .rather than its dielectric strength which is relied upon.

Surprisingly enough, when a stud is not jacketed and the insulating body 9, which is designed to withstand lngher temperatures than the polymer 10, is exposed to the flashover arcs, extensive damage is likely to occur. Thus, when the exposed insulator is aglassbondedmica material, such as the insulating material marketed as :Myca'lex by the Mycalex Corporation of America, and is subjected 'to'such duty, the high current arc melts the surface of the insulator to form small nodules that :ruin the insulating stud. Similarly, when porcelain is used as a jacket "the carbonized material .left by an arc must be sanded off and in some cases the porcelain .is cracked and broken by the arc. With the polymer sleeves, however, no sanding .is needed and the motorais not-stalled despitemany 'flas'hovers.

While solid ,polytetrafiuoroethylene is preferred as the sleeve solid polytrifiuorochloroethylene may also be substituted, this material being marketed under-the name Kel-F by the N. W. Kellogg Co. The latter material, however, is not as desirable because for most applications it would-have 'to be molded in place and is subject 'tothe disadvantage of a lower melting temperature. Y i 7' if While I-have illustrated anddescribed a particular embodiment of my invention and modifications thereof, various insulators having an exposed solid fluorinated polymer surface subject to -highcurrent arcs thereacross and other modifications will occur to those skilled in the art. It is to be understood, therefore, that my invention is not to be limited to the particular embodiment disclosed, and '-I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim as newand desire to secure by Letters Patent of the United States is:

1. A self-cleaning insulator for a brush holder stud arranged inside .a dynamoelectric machine between a brush holder and a grounded metal frame member of the machine and subject to repeated high current flashovers in the :presenceofoil and dirt accumulations on its surface during operation of the machine, comprising a mechanically strong cylindrical insulator fastened at opposite ends to the brush .holder and the frame .member respectively and of a suflicient dielectric strength and length to prevent dielectric failures therethrough at voltages normally present during operation and thereover in the absence of such .dirt and oil accumulations, and comprising further .a ;polytetrafluoroethylene :sleeve circumferentially covering at least part of the length of the outer surface of the stud, said :sleeve being atleast approximately inch thick to prevent destruction by a fiashover whereby said fiashover over the surface will vaporize only a surface layer which, ,prior to the flash- -over, would support any detrimental dirt accumulations thereon and thus the vaporization will provide a clean polymer surface of :renewed high creepage resistance.

2. A .dynamoelectric --maohine :having a commutator and astationaryframe member with :a brush supported on the stationary :member for engaging both mechanically and electrically the commutator and a self-cleaning brush .holder stud-arranged inside the dynamoelectric machine between a holder :for the brush and a portion of the stationary frame for "supporting the brush, the brush holder being subject to repeated :high current flashovers in the presence-of oil and dirtaccumulations on its surface during operation of the machine and comprising, a mechanical strong cylindrical insulator secured at opposite ends to the brush holder -:-and the frame member respectively and \of a sufficient dielectric strength and length to prevent dielectric failures therethrough eat the voltages normally present during the operation of the machine and of a suflicient length topreventfiashovers there across in the absence of dirt and 'oil accumulations thereon, and comprising further a sleeve of solid material selected from the ,group consisting of polytetra-- fluoroethylene and ;polytrifiuorochloroethylene circumferentially covering at .least -a portion --of the length of the outer surface of the stud,=sai'd sleeve being at least about inch thick =to {prevent destruction \by :flashover whereby each flashover over the surface will vaporize only a surface layer which prior 'to the :flashover would support any detrimental dirt accumulations thereon and thus will provide 'a clean polymer surface of renewed high creepageresistance.

References Cited in the file of-this patent STATES PATENTS 728,152 Batchel'der "May 12, 1903 2,451,969 Mueller et al. Oct. 19, "1.948 2,615,954 'Mungovan et al. Oct. '28, I952 FOREIGN PATENTS 843,561 Germany July ID, 1952 OTHER REFERENCES Publication; Modern fPl'astics Encyclopedia, 1947, vol. ll-P g 154..

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