US2949592A - Adjustable transformer with stabilized contact track - Google Patents

Description

Aug- 16, 1960 G. SMILEY 2,949,592

ADJUSTABLE TRANSTORMER WITH STABILIZED CONTACT TRACK Filed April 19, 1951 3 Sheets-Sheet 1 /5 QQ o (o 0 o Fig. 2.

byw um Attorneys Aug. 16, 1960 ADJUSTABLE Filed April 19, 1951 G. SMILEY 2,949,592 TRANSFORMER WITH STABILIZED CONTACT TRACK 5 Sheets-Sheet 2 Attorneys Aug, 16, 1960 G. SMILEY ADJUSTABLE TRANSFCRMER WITH sTABTLrzED CONTACT' TRACK Filed Apr-i1 19, 1951 3 Sheets-Sheet 3 Fig. 4.

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Afro/'nays United States Patent Oihce ADJUSTABLE TRANSFORMER WITH STABILIZED CONTACT TRACK 1 Claim. (Cl. 336-4148) Mass., assignor to General Cambridge, Mass., a corporation of The present invention relates to methods of and apparatus for rendering stable, over long periods of time, the resistance between movable brush contacts and portions of conductors engaged by the brush contacts, and, more particularly, the resistance between a resistance brush and the winding of continuously adjustable transformers and the like.

A continuously adjustable auto-transformer with which the present invention is particularly useful is described, for example, in United States Letters Patent 2,009,013, issued on July 23, 1935, to Karplus and Tuttle. While the invention will hereinafter be described as applied to such a transformer, it is to be understood that the invention is not limited thereto. These transformers are in lwide use `for many different purposes such as, for example, for the smooth control of voltage in both lowand high-voltage alternating-current electric circuits. Predetermined adjacently disposed regions of the successive turns of Ia conductor winding of the transformers form a commutating surface or track, with adjacent portions of which a carboniferous or graphitic resistive brush may engage to effect the desired voltage variation in the winding. Because of its superior physical properties for these purposes, in combination with low cost, copper is commonly used as the electrically conducting winding material. As explained in the said Letters Patent, for proper operation, the brush-to-track resistance should be greater than about one-third and not less than about three times the voltage effectively short-circuited by the brush between adjacent turns of the winding divided by the safe value of load current above which the winding would 'become damaged. The transformer is initially designed to provide a satisfactory brush-to-track resistance within these resistance limits. It has been found, however, that when the apparatus is in continuous use, particularly at elevated temperatures and in regions where industrial vapors or other corrosive iniiuences are present, the copper track detrimentally oxidizes, contaminates `and corrodes, markedly and rapidly increasing, and thus rendering unstable, the resistance of the brush-to-coppertrack interface. With long periods of continuous use, indeed, a progressively destructive cycle is often irritated in which the increasing oxidation, contamination and corrosion of the track during the use of the instrument increases the brush-to-track resistance, which, in turn, further increases the temperature at the contact between the brush and track, which still further increases the oxidation, contamination and corrosion, until failure or improper operation of the instrument results `from the high temperature.

Several techniques have Ibeen proposed for overcoming these disadvantageous features of such copper-wound continuously adjustable transformers though the actual factors underlying the same were not heretofore fully appreciated. Electrical conductors free from these disadvantageous properties of copper may, 4for example, be utilized. It is unfortunate, though, tha-t such conductors, including cert-ain of the noble metals, such as gold and 2,949,592 Patented Aug. 16, 1960 the platinoids, which have been found to resist oxidation, contamination and other attack by corrosive influences encountered in industrial atmospheres, are not only prohibitively costly, but, also, are -far inferior to copper in volume conductivity. Silver, however, is superior to copper in volume conductivity. The use of solid silver conductors, on the other hand, is not economically justiable for normal commercial instruments, except in certain rare cases. Other electrical conductors that might compete with copper on a conductivity versus cost basis, such as aluminum and magnesium, are even more subject to progressive oxidation, contamination and corrosion than copper. The problem can not, therefore, be solved from a commercial point of view by substituting a diierent metal than copper for the winding. Since the bulk of the applications of such instruments is commercial, this approach to the solution of the problem is not successful.

Another proposal to overcome the above-described diffculties has been periodically to clean the brush track. This is subject to the disadvantage that the instrument cannot 4be used over prolonged periods of time without shut down for the cleaning operation. Such a proposal is disadvantageous, also, `from the point of view of requiring disassembly of the instrument to clean or scrape the track in order to remove what is now known to be high-temperature-produced oxide or other contamination thereupon.

An object of the present invention is to provide a new and improved method of and apparatus for solving this problem attendant upon the use of copper and similar conductors with resistive brushes.

A further object is to provide a new and improved conductor coating adapted for continuous operation with a resistive brush over long periods of time and that maintains a substantially stable brush-to-conductor resistance.

Still another object is to provide a new and improved continuously variable or adjustable transformer that is not subject to the above-described disadvantages.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claim.

In summary, from its broadest aspect, the invention relates to conductors along predetermined longitudin-ally disposed regions vonly thereof there are integrally bonded coatings selected from the group consisting of gold, platinum, palladium, rhodium, silver and nickel. The conductors, in the preferred embodiment of the invention, are in the rform of a winding of a variable-impedance element, such as an auto-transformer, insulated except along the said predetermined regions. A resistive brush engages the coated regions. Preferred constructional details Iand a prefer-red method of preparing the coating are hereinafter described in detail.

The invention will now be described in connection with the accompanying drawings Fig. l of which is a sectional view taken upon the line 1--1 of Fig. 2, but upon an enlarged scale, looking in the direction of the arrows and illustrating a preferred embodiment of the invention; Fig. 2 is a perspective view, partly cut away, showing a continuously adjustable auto-transformer constructed in accordance with the present invention; Fig. 3 is a graph illustrating typical experimentally obtained results with various types of brush-track coatings; Fig. 4 is a longitudinal cross-section of the transformer Winding assembly of Figs. l and 2, illustrating the formation of the track; and Fig. 5 is a vertical section of a plating tank, showing the winding assembly in end elevation.

A continuously adjustable auto-transformer is shown comprising a copper-conductor, single-layer winding 1, wound in preferably substantially toroidal form, upon an insulator yframe 2 disposed about a circular or cylindrical annular iron core 8, Fig. 4. A dial or knob 15, Fig. 2,` permits the rotation of a Contact arm 16 about an axis 17 disposed in alignment with the axis of the core 8. At the free end of the arm 16 is carried a resistive brush contact 4,L such asa carboniferous or graphitic brush,v for engaging adjacently disposed predetermined regions of the successive turns of the winding .along the thickest or uppermost portion of the frame 2. The copper conductor 1 is provided with insulation 3, such as an enamel or plastic surfacing as of a polyvinyl formal product, more particularly shown in Fig. l, except at the predetermined longitudinally disposed regions 7 with which` the brush is to engage. The insulation prevents the successive turns of the winding.4 from short-circuiting one another. The regions 7' are flattened somewhat during the process of removing the enamel' insulation 3 therefrom, as by grindingy with the, aidv of a rotatable, disc 9, Fig. 4, carrying an abrasive surface such as sandpaper, secured to its` front face. In actual practice, satisfactory results are obtained'` by grinding. olf about ten percent of the diameter of the adj'acently disposed regions 7 of the. successive turns of the conductor winding 1. The flattened and insulation-baredv regions 7 provide a uniform commutating surface or track for engagement with the substantially planar lower face 18 Fig. 1, of the resistive brush 4.

It is at these predetermined. regions 7` forming the track for engaging the brush 4, that the above-described progressive oxidation, contamination and corrosion occur, particularly with continuous use of the transformer at elevatedv temperatures. As an illustration, the transformer may be initially designed to have a proper brushto-track resistance of about 0.075 ohm, lying within the previously described resistance limits discussed in the said Letters Patent. The transformer may be operated with a brush 4 of the EG-3 Morganite type, comprising graphitized carbon with a small percentage of ash and having a volume resistancev of about 0.0013 ohm per cubic inch, engaging the track with a pressure of about fourteen ounces. During continuous use of the transformer with a substantially constant current of about ten amperes passed from an external electric circuit, not shown, through they winding and the brush, the beforementioned' deleterious results occurrapidly and erratically raising the brush-to-track resistance to a value far in excess of the designed value. As illustrated in thegraph labelled Copper in Fig. 3, failure of the device may occur shortly after but two. days of continuous use.

Sincethe copper track 7 isthe only portion of the winding 1 from which the insulation 3 has been removed, the oxidation, contamination and, other corrosive processes only take place therealong. The present invention provides, therefore, for the treatment of the track 7 with appropriate integrally bonded coatings or patches 5, Fig. l, in such a manner that a stable, predictable surface is presented to the brush 4, which surface, unlike the bare copper, yields substantially reproducible results and substantially uniform brush-to-track resistances over extended periods of operation.

It has, of course,.long been known that extremely lowresistance contacts may be obtained by the use of coatings or plates upon the contacts ofv such materials as silver, nickel and the like. Such techniques have been applied to electrical contactors, switches, circuit breakers, discharge gaps, circuit interrupters and the like. This use of such coatings, however, has nothing to do either with the problem, or the solution lof the problem, involved in the present invention. Letters Patent, in the type of apparatus with which the present invention is concerned, such yas the continuously -adjustable auto-transformer, there must be suicient nite resistance in the circuit when the brush 4 is in contact with two or more adjacent turns of the Wire 1 at the predetermined regions 7v rforming, the track, to limit the current in the effectively short-circuited turnsA to a safe In fact, as discussed in the saidV value. The present invention is, therefore, not concerned with reducing the brush-to-track resistance by using any one of the well-known metal coats upon the contact surface. Where, indeed, the brush-to-track resistance should happen to become reduced by a particular coating 5, which, as before stated, is something that is not desired in the present invention, the brush-to-track resistance must then be correspondingly increased again,

- as by using a more resistive brush 4, to keep within the proper resistance limits. The present invention, on the contrary, is concerned with maintainingl a nite value of brush-to-track resistance at all times, and, more important, maintaining this value substantially constant no matter how long the instrument may be operated in an external electric circuit.

Experiments with a large number of different types of coatings have shown that a stable, coated surface 5 can, in general, be secured with some of those metals which inherently resist oxidation and the like, particularly gold, platinum, rhodium and palladium, among the noble metals. Referring again to Fig. 3, the curve labelled Gold illustrates that acoating 5 of gold upon the copper track 7, under the same operating conditions before described, will maintain the brush-to-track resistance substantially constant at a value of approximately 0.08 ohm over long periods of continuous use. Similarly, the curve labelled Palladium demonstrates the stable character of a palladium coating upon the brush track, maintaining thebrush-to-track resistance substantially constant at a value of about 0.06 ohm during continuous operation of the instrument. These curves are to be contrasted with the rapidly failing tendency of the bare. copper 7, shown in the curve labelled Copper, before discussed. Rhodium and platinum similarly provide coatings 5 that have been. found to produce a stable brush-to-track. resistance over long periods of continuous operation.

There does not appear, however, to be any correlation in the periodic table or in any other known classification of metals or elements or their known properties, that would enable one to predict whether one metal or another would be suitable as a coating for the stableresistance purposes ofthe present invention. Aside from the noble metals, above discussed, for example, nickel has been found to produce reasonably satisfactory results, though the performance of nickel is by no means the sameV as that of the noble metals. Referring to the curve` labeled Nickel in Fig. 3, it will be observed that nickel, unlikev gold and palladium, does not maintain a constant resistance over, for example, the first'four days of' continuous use. The, nickel coating, on the contrary, produces successively increased valuesv of brush-to-track resistance over the first four days of'operation. The value of the resistance reaches an asymptote, however, after four days, and thereafter remains substantially constant. Nickel. can therefore be usedv forv theY purposes of the present invention. The other elements in the periodic table corresponding to nickel, such as iron and chromium, on. the other hand, have been found to be extremely poor for this purpose. Cobalt, also, has produced inferior results even though; it, too, is one of the ferromagnetic group of metals.

Asv further illustrations, the soft metals, such as zinc and lead, have been found, also, to be unstable for this purpose. Tin, likewise, is unsatisfactory. Cadmium and mercury amalgam coatings, as still further examples, produce unstable resistance characteristics closely paralleling-those illustrated by the curve identified by Copper in Fig. 3.

lt'has been found, also, that a silver coating 5 is quite satisfactory in the absence of sulphur contamination in the atmosphere. The performance of silver in maintaining a substantially constant brush-to-track resistance is shown by the` substantially horizontal lineV labeled Silver in Fig. 3. Since the silver, however, so markedly reduces the contact resistance between the track 7 and the brush 4, it is necessary to compensate for this undesired effect by using a brush 4 of higher resistance than that used with, for example, the nickel, gold and palladium coatings. A brush of the HM6782 Morganite type, composed of natural graphite and a comparatively high resistance resin binder and having a volume resistivity of about 0.0049 ohm per cubic inch, has been found satisfactory for use with a silver coating 5, to maintain sucient resistance in the brush-to-track Contact, as discussed in the said Letters Patent, for proper performance of the auto-transformer.

It is, indeed, coincidence, therefore, that some of the coatings that have been found satisfactory for use in accordance with the present invention for the purpose of maintaining a substantially stable brush-to-track resistance over long periods of time during the repeated and continuous use of the instrument at high temperatures, may also have found previous application in other types of systems for the entirely diiferent and undesired purpose, insofar as the present invention is concerned, of providing low-resistance contacts.

Surface coatings or patches 5 of gold, platinum, rhodium, palladium, silver and nickel, integrally bonded upon the copper track 7, may be of thickness of the order of from about one ten-thousandth of an inch to about one thousandth of an inch to withstand the normal erosion due to friction wear caused by the operation of the brush 4 and, possibly, some slight erosion or pitting due to the passage of current, for the duration of the reasonable operating life expectancy of the transformer. Alloys containing substantial percentages of these metals may also be used, such as, for example, an alloy of 85% silver and 15% zinc, producing results similar to those represented by the curve labeled Silver in Fig. 3. Such a silver-alloy coating, furthermore, unlike pure silver, will resist tarnish in an atmosphere containing sulphur vapors. These successful coatings, moreover, may be used, also with aluminum, magnesium or similar conductors that, like copper, are subject to the before-described difficulties.

It remains to explain the technique for integrally bonding the coating 5 to the brush track 7. In other applications, coatings have been applied to metal Surfaces by rolling, bonding, welding or other techniques. Such operations are not particularly suited to the problems of the present invention since, among other reasons, they require high temperatures which would destroy the enamel or plastic insulation 3 of the winding.

Still other techniques for applying the coating to the brush track include various types of metal spraying, but these require special masking operations and the like. It might be expected, moreover, that an electroplating process would be unsatisfactory, also, because of the possibility of attack by the bath and leakage of the bath into the transformer. A highly commercial process has been found, however, for electroplating a satisfactory coating upon the brush track.

The transformer winding is first completely assembled. The winding 1 is anchored to the supporting insulator frame 2 by a moisture-repellent varnish 6, Fig. 1, such as a resin distillation product of furfnral alcohol. After the track 7 of the winding is formed, as by the grinding technique previously discussed in connection with Fig. 4, the free ends 11, Fig. 5, of the winding are also bared and connected to a negative or cathode terminal of a source of plating potential. The whole transformer winding is then immersed in a tank 14 containing an appropriate plating bath 13, such as any Well-known silver plating solution in which, for example, a silver anode 12 is inserted. The desired coating will be deposited only upon the longitudinally disposed flat regions 7 of clean copper, for example, forming the brush track. This is because the remaining portions of the winding 1 are coated with the insulation 3 and will not therefore receive any electro-deposit. The varnish 6, moreover, has been found to exclude the plating bath from the interior of the transformer so that the bath does not seep into the core. After the coating 5 has been plated, the transformer is dried out by a simple baking process. The instrument is now clean and ready for linal assembly with its brush 4, and then, for immediate use.

Modfcations will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the present invention as defined in the appended claimt.

What is claimed is:

A variable-impedance auto-transformer having, in cornbination, a copper-wire single-layer substantially toroidal winding wound in successively disposed turns about an annular core to provide along the exterior of the winding a track extending across the successively disposed turns, there being bonded to the turns along the track coatings selected from the group consisting of gold, platinum, palladium, rhodium, silver and nickel, means for connecting the winding to a source of voltage, the winding being adapted to be connected also with a load circuit to exchange current with the load circuit at values not greater than a predetermined safe Value above which the winding would become damaged by such exchange of current, and a carboniferous or graphitic resistive brush actuable along the track in contact with the coatings and adapted for connection with the load circuit, the width of the brush being greater than the distance between two successive turns of the winding in order that the brush may establish contact with the coating bonded to a turn before breaking contact with the coating bonded to the adjacently disposed turn with which it last contacted, thereby to prevent interruption of the current in the load circuit into the brush, means for connecting a point of the winding and the brush to the load circuit, and the brush being rotatable about the axis of the toroidal winding along the said track in engagement with the coatings of the successively disposed turns, the coatings maintaining the resistance between the brush and the track, during passage of the current ofthe predetermined safe value between them, substantially constant.

References Cited in the file of this patent UNITED STATES PATENTS 93,609 Chadwick Aug. 10, 1869 380,158 Acheson et al. Mar. 27, 1888 1,189,882 Sommer July 4, 1916 1,339,505 Fahrenwald May 11, 1920 1,570,420 Yngve Jan. 19, 1926 1,892,755 Scheppmann Jan. 3, 1933 2,009,013 Karplus et al. July 23, 1935 2,023,603 Lodge Dec. 10, 1935 2,089,434 Schemerhorn Aug. 10, 1937 2,157,933 Hensel et al. May 9, 1939 2,204,623 Ruben June 18, 1940 2,303,497 Reeve Dec. l, 1942 2,354,081 Weder July 18, 1944 2,398,333 Shoemaker Apr. 9, 1946 2,418,710 Hensel Apr. 8, 1947 2,427,727 Huntley Sept. 23, 1947 FOREIGN PATENTS 620,284 Great Britain Mar. 22, 1949 886,118 France lune 21, 1943 OTHER REFERENCES Materials and Methods Manual, June 1948, Electroplated Coatings, pp. 94-104.

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