US2439114A - Electrical apparatus collector conditioning - Google Patents

Description

April 6, 1948. F.P. VACHA 2,439,114

ELECTRI CAL APPARATUS COLLEC TOR CONDIT ION ING Filed Sept. 16, 1945 Patented Apr. 6, 1948 ELECTRICAL APPARATUS .COIJLEOTOR CONDITIONING Fred P. 'Vacha, Atlantic, Mass.,,ass'ignor, hymesne assignments, to First Industrial Corporation, Wihnington, Del.,-a corporation ofDelaware Application September 16, 1943, Serial N 0.1502590 2 Claims.

The present invention relates to electrical apparatus, and is more particularly concerned with the operation of brushes in conjunction with collectors. By a collector is meant a rotating metallic member which is in continuous contact with current-carrying brushes, and includes such devices as commutators and slip rings.

Under some circumstances, examples of which will be given presently, considerable difiicu-lty has been experienced in rapid wear or dusting of brushes. It has been observed that satisfactory operation of an electrical collector and brush assembly depends on the formation of a thin film, presumably an oxide or other compound of copper, on the collector. This film, which gives the familiar bronzed appearance to the collector, provides lubrication between the collector and the brushes. The theory of film formation and its effectiveness in preventing brush wear is not fully understood. but it appears that certain at- .mospheric conditions are an important factor in film maintenance under normal conditions. The film normally tends to wear away by the [Eriction between the brushes and the collector surface. but it is continually renewed at a corresponding rate, in the presence of oxygen and moisture in the atmosphere. Probably this renewal action depends on oxidation which is accelerated by the mechanical rubbing contact between the collector and the carbon brushes, and to some extent on electrical disintegration of the mo sture film that forms on the collector surface.

Whatever the explanation of satisfactory norma] performance may be, the rapid wear of brushes is a serious hazard when there is a substantial departure from ideal conditions. One importantexample is in the operation of electrical equipment of aircraft at high altitudes. Under low barometric pressure and low humidity, such as are encountered at altitudes in excess of about 23,000 feet, the film is not maintained in ordinary operation, that is to say, theabsenee of oxygen and moisture prevents reformation of the film at a sufliciently rapid rate. Under these circumstances the brushesare actually ground away by con-tact with the collector, and may not last more than a few minutes.

Another example of unsatisfactory operation is in some electro-plating generators which use metal brushes in order to limit the voltage drop through the brushes. With metal brushes, even under normal atmospheric conditions, the lubri eating film does not form, or at least it does not form asrapidly as with carbon brushes, nd hence does not maintain itself sufficiently to .afiord satisfactory protection against excessive brush wear.

Abnormal brush wear has also been noted under conditions of less apparent severity than those described above. The presence of chemical fumes and vapors in the atmosphere surrounding generators and motors has been ob served to result in increased wear. Also, rotary converters, in which conditions of commutation are especially severe, may exhibit poor commutation and undue brush wear, even under only moderate departure ,from ideal atmospheric conditions. In such cases, unsatisfacotory operation follows destruction of the protective film.

As a protective measure for high-altitude operation, it has been proposed to impregnate the current-carrying brushes with various compounds designed to produce a lubricating effect. These expedients have, in general, proved unsuccessful, because they affect not only the elec trical characteristics ,for which the brushes are particularly designed, but also are relatively short lived due to the inherently high operating te1nperatures of the conducting brushes and consequent rapid vaporization of irnpregnant. This rapid vaporization .of impregnant from the conducting brushes is particularly serious where heavy starting or surge currents are encountered in normal operation.

According to the present invention, I provide a non-current-carrying member, which is in contact with the collector for the purpose of conditioning the collector in a manner to provide for relative movement between the collector and ourrent-carrying brushes with a minimum of friction, whereby brush wear is materially reduced.

One embodiment of the invention comprises a block of porous material such as wood, impregnated with a liquid suitable for properly conditioning-the surface of the collector. The liquid maybe an organic material such as glycerine or ethylene glycol. The effectiveness of the member has been found to depend not only on the lubrication afiorded by the liquid itself, but mainly .on some chemical reaction with the collector, probably because of the hygroscopic prop erties of the liquid.

Another embodiment of the invention contemplates the use of a. conditioning member of solid material. For this purpose a hard rubber compound containing an excess of sulphur has been found satisfactory. It is believed that the effectiveness of the rubber is due to the presence of sulphur, which acts with the collector to form a 3 thin film of copper-sulphur compound which presents a similar appearance and has substantially the same conditioning effect as the oxide film formed under normal conditions. This form of the invention is preferable to the first form in that it does not require the use of liquids which they do not melt or soften under the high temperatures to which the brushes are frequently subjected.

In the accompanying drawings, Fig. 1 is a diagrammatic end elevation of a motor or generator having a commutator with a conditioning member of the present invention applied thereto and Fig. 2 is a perspective view of a brush with incorporated conditioning elements.

In Fig. 1 a, commutator 6 of usual form is provided with a single pair of current-carrying brushes 8 mounted for engagement therewith in any suitable manner. Although a single pair of brushes is shown for simplicity, it will be understood that the invention is equally applicable to motors or generators employing a larger number of brush pairs.

Mounted for engagement with the commutator at a convenient point between the brushes 3 is a conditioning member ID, to be presently described in detail. The member ID is mounted in a brush carrier i2 of any suitable form and is pressed toward the commutator by a spring I i. The member ID may be termed an electrically idle brush, that is to say, it carries no current, and its purpose is to condition the commutator for proper action with respect to the current-carrying brushes 8.

In one form of the invention, the member it comprises a soft wood block impregnated with a conditioning liquid, preferably glycerine or ethylene glycol. The impregnation is carried out under carefully controlled conditions. The blocks are submerged for 48 hours in the impregnating fluid at high temperature, in the neighborhood of 200 C., and under high vacuum, preferably in excess of 29 inches of mercury. Under these conditions the air entrapped in the cells of the block is largely removed and the cells are substantially filled with the liquid. The blocks are then allowed to drain While still hot under high vacuum, and are then dipped individually in pyroxylin to provide a protective coating which is preferably not more than 0.005 thick. The purpose of the coating is to resist evaporation of liquid from the block.

After the protective coating is thoroughly dried at room temperature, the blocks are preferably packaged in individual moisture-proof cellophane bags. When the block is to be put into service, it is removed from the bag and mounted in the brush carrier l2. After a few revolutions of the commutator, the pyroxylin coating at the surface of engagement with the commutator is worn off, thereby exposing the impregnant to the commutator. This exposure permits some evaporation of the impregnant, especially at high altitudes, but'it has been found that the block may remain carrying brushes, the member in service satisfactorily for a month or more. The presence of the pyroxylin coating on all sides of the block except that in contact with the commutator limits the evaporation so that reasonable service can be obtained from a block before necessity of replacement.

In operation a small amount of impregnant is continuously applied to the commutator. At low altitudes, where an oxide film is normally formed by the action of the commutator and the current-- It] does not affect the operation. At high altitudes, where the barometric pressure and the humidity are low, the commutator is conditioned by the application of the impregnant. The reasons for the satisfactory operation are not fully understood. It is believed that the action does not depend entirely on the lubricating action of the liquid itself, but involves some reaction with the commutator, possibly affected by ionization or other electrical conditions existing at the areas of contact between the commutator and the current-carrying brushes. The hygroscopic qualities of glycerine or ethylene glycol also appear to be of primary importance, since whatever moisture exists is taken up by the liquid, or perhaps moisture taken up at low altitudes is available for conditioning the commutator under the low moisture conditions prevailing at high, altitudes. In any event, a film is formed and maintained even at high altitudes, this film having an appearance quite similar to the normal oxide film which is formed in ordinary operation at low altitudes. No difiiculties due to carbonization or chemical break-down of the impregnating fluid have been observed. The commutator is so conditioned that the wear on the current-carrying brushes 8 is reduced to a satisfactory minimum comparable to normal wear under ordinary conditions.

Another form of conditioning member ID comprises a block of hard rubber compound, containing free sulphur. This has been found entirely satisfactory in most instances, and is generally preferable to the impregnated wood block because there is no problem of evaporation. It is believed that the sulphur is the medium which is effective in properly conditioning the commutator. Experiments with sulphur alonehave indicated the utility of sulphur as a conditioning agent, except that the presence of sulphur in large quantities induces a problem of corrosion. A member of hard rubber'compound presents to the commutator a sumcient amount of sulphur for proper conditioning of the commutator as the member gradually wears down, but without such an excess of sulphur as would result in corrosive tendencies. It is believed that the block interacts with the collector to produce a film in the form of a chemical compound of copper and sulphur, possibly copper sulphide or sulphite, which film is entirely effective in preventing wear on the current-carrying brushes. This film presents a bronze-d appearance similar to the normal oxide film and is capable of being maintained at high altitudes so long as the conditioning member remains in contact with the commutator.

In the preferred composition of the block, a non-abrasive heat-resisting inert filler is used. This filler is preferably tale. It gives body to the block and maintains its against distortion at high temperatures. The proportions of rubber, sulphur and talc may be varied within wide limits. A composition of one-third rubber, one-third sulphur and one-third talc has been found satisfactory for most purposes. After vulcanization,

this leaves about 6 to 10% of free sulphur which is adequate for proper conditioning of the collector under practically all circumstances.

The conditioning member or members may be directly associated with or incorporated in one or more of the current-carrying brushes. This may be done in various ways, one construction being as shown in Fig. 2, wherein a currentcarrying brush it} is drilled, and plugs of conditioning-material iii are inserted and cemented in the holes. The plugs may be staggered as shown, in order to condition substantially the entire path of contact. Plugs of hard rubber compound, as above described, are preferably used. The rate of wear of the conditioning material should correspond with that of the brush material, and is readily controlled by the proportion of inert filler (talc). In general, the proportion of filler will be somewhat less than in the case of the physically separate conditioning element, usually about 25% of the total weight, but depending on the character of the brush with which the conditioning element is associated.

The device shown in Fig. 2 differs in form and function from the use of impregnants in currentcarrying brushes. The elements l8 are electrically idle, as is the member I!) of Fig. 1, and do not affect the current-carrying properties of the brush [6.

The construction of Fig. 2 is most advantageous in small equipment where difficulty might be experienced in mounting a separate brush carrier. However, the use of a separate conditioning element, as shown in Fig. 1, is preferred under conditions wherein the current-carrying brushes may reach excessively high temperatures, as under starting conditions in some motors.

Conditioning agents other than sulphur may be used. 'In general, any substance capable of a limited chemical reaction with the copper may be used. Iodine, nitrosylsulphuric anhydride, arsenic sulphide and oxalic acid are examples of satisfactory agents. In any case, the agent in powdered form may be mixed with a suitable binder, or may be vulcanized into a hard rubber compound. It will be apparent, however, that free sulphur in a hard rubber compound will usually be preferred because of its cheapness.

Although, for purposes of explanation, particular reference has been made to high-altitude operation, it will be understood that the invention is not limited to such service, but may be used under other circumstances, for example, in rotary converters, in generators using metal current-carrying brushes, in equipment which is operated in the presence or" chemical vapors, and under other conditions wherein excessive brush wear would be likely to develop.

ving thus described the invention, I claim:

The combination with a coliector and ourrent-carrying brushes therefor, or a solid conditioning member comprising a hard rubber compound containing free sulphur and a nonabrasive inert filler.

2. ihe combination with a collector and ourrent-carrying brushes therefor, of a solid conditioning member comprising a hard rubber compound containing free sulphur and talc.

FRED P. VACHA.

REFERENCES CITED The following references are of record in the his of this patent:

UNITED STATES PATENTS Number Name Date 338,197 Snowberger Mar. 16, 1886 427,674 Daft May 13, 1890 1,121,960 Whitney Dec. 22, 1914 1,231,716 Trood Oct. 15, 1918 1,546,729 Davis July 21, 1925 1,564,397 Armstrong Dec. 8, 1925 1,895,756 Fuller Jan. 31, 1933 2,995,809 Elsey Oct. 12, 1937 2,105,038 Helwig Jan. 11, 1938 2,409,818 Wikle Oct. 22, 1946 PATENTS Number Country Date 8,656 England Oct. 7, 1911 249,864 Germany Aug. 1, 1912

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