US2555997A - Sliding contact of electric machines - Google Patents

Description

June 1951 r F. PORTAIL SLIDING CONTACT 0F ELECTRIC MACHINES Filed June 16, 1947 Patented June 5, 1951 SLIDING CONTACT OF ELECTRIC MACHINES Fernand Portail, Paris, France, assignor to Societe: Le Carbone Lorraine, a French corporation Application June 16, 1947, Serial No. 754,867 In France June 3, 1942Section 1, Public Law 690, August 8, 1946 Patent expires June 3, 1962 4 Claims. i

The present invention relates to sliding contacts of electric machines.

I have found that under the influence of an electric field, some hydrocarbons or substitution fatty bodies, such as Vaselines or chlorinated naphthalenes, become conductors of electricity and can even acquire a high conductivity. This modification of the properties of liquid or semiliquid insulating bodies takes place only when they are spread in a sufiiciently thin layer and when they are subjected to a suflicientiy high field of potential.

The object of the present invention is to take advantage of these phenomena for providing improved sliding and rotating contacts for electric machines, and in particular commutators and brushes cooperating with these commutators.

When two conductive bodies are applied against each other, whatever be the perfection of adjustment of their contacting surfaces and the high degree of polish that has been obtained for these surfaces, a continuous uninterrupted contact does not exist between them. Contact exists only between the most projecting portions of the contacting surfaces, and there is always left an extremely thin layer of air between said surfaces. Of course, this air layer constitutes a dielectric which opposes the passage of electric current. This dielectric layer therefore creates a resistance which can be overcome only by an expense of energy. If the two surfaces in contact are movable with respect to each other, the points of contact change constantly, and so change the thicknesses of the air layer at the different points of the contact area, which is a cause of perturbation in the flow of current. Furthermore, if, as in the case of commutators, a commutation takes place, sparks are unavoidably produced in frequent repetition. These sparks not only involve a loss of energy but also gradually destroy the solid surfaces in contact. Finally, the friction that takes place between the parts in relative movement produces a rapid wear. One has endeavoured to reduce this wear and tear by slightly lubricating the brushes sliding on the commutators, for instance by making use of brushes impregnated with a small amount of oil. 1

In view of the conditions of operation, it is not possible to make commutators and brushes of any material whatever. In the case of collectors, the only material that is being practically used is, copper, a heavy and expensive metal. The use of oxidable metals is impossible under the existing, conditions...

The object of the present invention is to obviate all these drawbacks.

With this object in view, I apply between two contacting surfaces, between which passes an electric current, and at least one of which is movable, for instance between the collector and brushes of an electric machine, a thin film of a liquid or semi-fluid dielectric body, in particular a hydrocarbon, or a substituted hydrocarbon which normally is an insulator, but is capable of becoming a conductor of electricity when exposed to the influence of the electric current.

According to my invention, and in particular when applied to the commutators and brushes of electric machines, this thin layer of a body such as a hydrocarbon plays an entirely new role and ensures a very useful practical result from a technical point of view, for the following reasons:

First, the electric contact is improved, as a consequence of the phenomena above mentioned, by eliminating the detrimental space filled with an air layer which opposes the flow of current, and also because a body such as a hydrocarbon,

' which, under normal conditions, has insulating properties, becomes a conductor of electricity under the efiect of the electric field existing between the brush and the commutator bars.

Secondly, the layer of hydrocarbon, or other body, exerts a constantly renewed lubricating action, which reduces the detrimental effects of friction between the parts in contact.

As a consequence of these two advantages, the wear of the parts is greatly reduced, first because the production of sparks is almost entirely eliminated between the parts and secondly by the important reduction of the mechanical friction between said parts.

Another important technical advantage consists therein that the surface of the commutator bars is no longer in contact with air, so that it becomes possible to make use of oxidizable metals for said commutator bars, which had not been possible up to now. The present invention makes it possible to make these parts of iron, aluminium, or other metals which are cheap or easily obtainable.

It is significant that the film substance becomes a conductor of electricity only during the periods of time when it is spread between the contacting parts, such for instance as a brush and the commutator bars that slide past said brush. After it has left the space between the brush and the commutator, this substance again becomes an insulator .and acts,.over the remainder of the collector area, only as a protective coating against oxydation and, accessorily, against dust.

Another advantage afforded by the present invention is the possibility of running electric machines at higher speeds than it had been possible up to the present time. As a matter of fact, speed limitation was due to the arising of vibrations which, in addition to the mechanical. perturbations they caused, involved poor electric contact, and the formation of sparks, etc. The presence of a layer of lubricating material prevents these vibrations from taking place and makes it possible to increase the speed of revolution to a considerable degree.

Most of the known lubricant bodies can be utilized for forming the thin film with which the collector is to be coated. Preferably, I make use of bodies having a high specific inductive capacity. Furthermore, it is necessary to make use of substances having a power of adhesion to the commutator greater than the centrifugal force at the speeds that are considered. Among these substances, I may cite, in particular, hydrocarbons and substitution products such as chlorine compounds of benzene, naphthalene, and the like.

By way of example, I. may say that the substances which, according to my experiments, can be employed with advantage consist chiefiy of oils such as Vaseline oils or parafiin oils, or more fluid hydrocarbons, such as kerosene (lamp oil) to which there may be added, if advisable, an oil of the fatty kind.

Another object of the present invention is to provide devices for forming a thin film of the lubricating substance on the surface of a commutator or a similar part.

According to a first embodiment, I make use of the brushes of the electric machine for feeding the lubricant.

I may for instance make these brushes sufficiently porous to make them capable of conveying the desired amount of the liquid. Or I provide these brushes with wicks housed in a conduit or conduits provided in the mass of the brush. In both cases, the head of the brush should be provided with a small reservoir containing a certain amount of the dielectric liquid on the surface of the collector, and the output of the whole must be sufiicient for maintaining the desired thickness of the oil film.

According to another embodiment of my invention, the lubricating dielectric substance is fed directly onto the outer surface of the collector for instance through a wick connected with a small reservoir. Or I make use of a roller in contact with the commutator and fed with oil, or other body to be distributed, through means analogous to the devices used in ofiset printing presses.

According to a third embodiment of my invention, the oil is fed through the interior of the commutator. For this purpose, I provide in the axis of the commutator means for the. inflow of oil, hydrocarbon, etc. Such means communicate with distributing channels or conduits, or equivalent devices for conveying the body that is utilized to each commutator bar. These bars may be provided with small channels for the passage of the liquid, but preferably they are made of porous structure. Owing to this porosity, the liquid reaches the outer surface of the commutator, where it forms the desired thin fiow of liquid dielectric material.

Preferred embodiments of the devices accord- 4 ing to the present invention will be hereinafter described with reference to the accompanying drawings, by way of example, in which:

Fig. 1 is a diagrammatic sectional view of part of a commutator and brush system made according to the present invention;

Fig. 2 is a similar view of another embodiment of the device according to the invention;

Fig. 3 shows, in a similar way, a modification;

Fig. 4 is a similar View of a fourth embodiment of the invention.

In the embodiment of Fig. 1, the dielectric substance which is to form a thin film l I on the surface of commutator l is fed through brushes such as 2. Each brush is fitted, in any suitable manner, with a small reservoir 3.

In the example illustrated by Fig. 1, I have provided smallchannels containing wicks 4 connecting the reservoir with the space between the brush and the commutator. These channels and wicks are unnecessary if the brush is made of porous material, so that the dielectric substance can travel directly through said brush.

In the embodiment illustrated by Fig. 2, the dielectric substance is fed directly to the surface of the commutator by means ofwicks 5 rubbing against said surface and one end of which is immersed in areservoir 6 containing the liquid.

According to a modification of this embodiment, illustrated by Fig. 3, the film of dielectric lubricating substance is fed by means of asmall roller 1 running on the surface of commutator l and dipping in the liquid contained in areservoir 6.

According to the third embodiment above referred to, the dielectric substance is fed through the commutator from the inside. A feed channel 8 (Fig. 4) extending axially of the commutator, communicates throughradial conduits 10 with the commutator bars 9 which, in this example, are supposed to be made of porous material, so that the liquid fed through conduits I0 passes to the outer surface of the bars 9 and therefore of the commutator.

It should be further pointed out that, with arrangements as above described, the working of the commutators and analogous parts is made independent of the atmospheric pressure and of the hygrometric conditions, which is very important for instance for the operation of electric machines at high altitudes.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of the present invention, it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the appended claims.

What I claim is:

1. The combination, with two contacting conductive parts of an electric device, one of which is a commutator, of a thin film between said parts of a substance which normally is an insulator, but becomes electrically conductive when exposed to an electrical field, porous bars in said commutator and means for feeding the substance to said bars through an axially and radially extending conduit.

2. The: combination ofclaim 1 wherein at least one of the contacting conductive parts is made ct an oxidizable' metal, and. whereinv said substance is sufllciently adhesive to maintain the thin filmon said oxidizable part as a layer to protect it from oxidation.

3. The combination ofclaim 2 wherein the substance is a chlorinated hydrocarbon.

4. The combination ofclaim 1 wherein the 5 substance is a chlorinated hydrocarbon.

FERNAND PORTAIL.

REFERENCES CITED 10 The following references are of record in the file of this patent:

Number 6 UNITED STATES PATENTS Name Date Craig Nov. 27, 1888 Neville et a1 Apr. 21, 1908 Wiswall Nov. 10, 1908 Gabel Mar. 30, 1909 Ketchum Apr. 15, 1913 MacGregor Nov. 3, 1914 Schweitzer July 13, 1915 Ster Nov. 13, 1917 Wappler Jan. 10, 1928

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