US2717296A

US2717296A - Electrical switch contacts

Info

Publication number: US2717296A
Application number: US379844A
Authority: US
Inventors: Robert T Foley; Orin P Mccarty
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1953-09-14
Filing date: 1953-09-14
Publication date: 1955-09-06
Anticipated expiration: 1972-09-06
Also published as: JPS318220B1

Description

Sept. 6, 1955 R, T FOLEY ET AL 2,717,296

ELECTRICAL SWITCH CONTACTS Filed Sept. 14, 1953 151155 Mfr/1L /l I ML/@7707.5

esaegse Rober? 00P/1m @UPPER ox/f 0,717 R MC @arly United States Patent O ELECTRICAL SWETCH CONTACTS Robert T. Foley, Lanesboro, and Urin P. McCarty, Pittsfield, Mass., assignors to General Electric Company, a corporation of New York Application September 14, 1953, Serial No. 379,844

4 Claims. (Cl. 200-166) This invention relates to electric switch contacts and more particularly to electric switch contacts which operate with a strong applied pressure forcing them tgether. Such contacts are used in tap changing switches .for changing the taps of a transformer. When used in an oil-filled transformer the switch is usually mounted below the surface of the insulating and cooling liquid with which the tank is substantially lled. A typical tap changing switch of the nonarcing duty tap changer type is described in Patent 2,255,501-Blume. While the present switch may be incorporated in a transformer of a type described in the Blume patent it may also be used in certain inert atmospheres as will be more fully described hereinafter.

in a tap changer such as that described in the Blume patent pressure between surfaces in electrical contact is maintained by means of a biasing spring. Since these surfaces are in sliding contact much difficulty has been experienced in the past from rapid contact wear as the result of galling of the surfaces coming into sliding contact with each other. ln our copending application Serial No. 342,422, led March 16, 1953, now Patent No. 2,680,174, dated June l, 1954, and assigned to the same assignee as the present application, we described an electrical contact surface which would not readily gall from sliding Contact. This consisted of a thin film of tin oxide overlying a coating of tin electrodeposited upon the base material of the contact. While the tin oxide coated contacts described and claimed in our copending application are entirely satisfactory we have now discovered a more easily fabricated contact which will also resist galling during pressure sliding contact. This present switch can be used in the same electrical system as the above described Blume type switch.

lt is an object of the present invention to provide non-galling contact surfaces which are easily produced.

lt is another object of the invention to improve the operation of tap changing switches in which the contact members are normally required slidably to engage one another at pressures of from to 75 pounds.

The present invention is based on the discovery that ordinary copper contacts will not gall from sliding contact provided a very thin coating of copper oxide is applied to the contacting surfaces. Briefly stated, in accordance with one of its aspects, the invention is directed to an electrical contact comprising a conducting base, and a layer of copper oxide 0.0020.008 mil in thickness overlying the base, the layer of copper oxide serving as a contacting surface.

Fig. l of the drawing is a broken perspective View partly' in section of a load ratio adjuster switch; Fig. 2 is a side elevation view partly in section of the switch shown in Fig. l; and Fig. 3 is an enlarged sectional view of a single set of switch contacts.

Referring now to the drawing and more particularly to Fig. l, there is shown a portion of a tap changer or load ratio adjuster switch utilized to change the taps 2,717,296 Patented Sept. 6, 1955 of a transformer (not shown). These switches, which normally operate immersed in oil, have a center support post 10 terminating in a stationary circular contact plate 11. Coaxial with the support post 10 is a rotatable crank or selector shaft 12 on which is mounted a crank arm 13 terminating at its outer end in a pair over-and-under

radial slots

14 and 14a, respectively, which maintain a pair of spaced Contact supports 15 in a radial position. The contact supports 15 serve as bases for mounting a pair of clamping contacts 16 at their inner end and clamping contacts 17 at their outer end. The inner clamping contacts 16 are in contact with the contact plate 11 and the outer clamping contacts 17 make selective Contact with one of a plurality of stationary contact terminals 18.

Clamping pressure by the contacts 16 against the contact plate 11 and contacts 17 against a particular one of the terminals 18 is maintained by a spiral compression biasing spring 19 mounted upon a bolt Z1 extending in free movement through the contact supports 15. The spring 19, which desirably exerts a pressure of ten pounds or more against the Contact supports 15, works against the head of the bolt 21 and a washer 22 maintained on the opposite end of the bolt 21 by a Cotter pin 23.

The operation of the switch is as follows: assume that shaft 12 is rotated clockwise as viewed from above. This will cause the crank arm 13 to rotate clockwise as viewed in Fig. l thus causing the assembly which is carried by the crank arm to advance the contacts 17 to the next terminal 18. Since a current in excess of 400 amperes is normally carried through the contacts it has been past practice to provide the contacting surfaces with a very smooth linish to insure low-resistance contact and reduce sliding friction to a minimum. As previously mentioned, such contacts are subject to severe galling. We have discovered that this galling can be greatly minimized by providing the copper Contact surface with a film of copper oxide of controlled thickness. A lm applied in accordance with our invention will not increase the Contact resistance to a noticeable degree.

In Figs. l and 2 the heavy shading of the contact surface indicates a coating of copper oxide. All contacting surfaces are provided with such coatings. These coatings are shown more clearly in Fig. 3 though their thickness has been greatly exaggerated for purposes 0f clarity of illustration. Also shown in Fig. 3 is an underlying layer of copper over a base metal. lf the base metal is composed of copper, as is normally the case, there will be no separate copper layer as shown in Fig. 3. However, the base metal may be a less expensive metal than copper in which case it is necessary to coat it with copper before applying the contacting coating of copper oxide.

lt has always been thought that the best copper contacting surface was one highly polished and subjected to a wiping action during operation which would prevent the formation of a surface film. it was thought that nothing could be done to prevent the galling of such highly polished surfaces during sliding contact under pressure. We have discovered that great resistance to galling is conferred upon copper contacts which are given a copper oxide coating of a thickness of 0.002 mil to 0.008 mil. This thin coating may be described as that which appears on a polished surface as a slight dull bloom. The method of applying the coating is not important as long as the coating furnished is continuous and within the range of thickness set forth above. A chemical bath may be used to provide the oxide coating or the piece to be coated may be exposed for a short period of time to the action of an oxidizing furnace at raised temperature. Both of these means of applying an oxide coating are so well known to those skilled in the art that they will not be described further herein. An oxide coating of the specied thickness has a negligible effect on the Contact resistance; yet such coated contacts have resisted galling at a contact pressure as high as 30 pounds when severe galling occurred with non-oxidized surfaces subjected to a contact pressure of l5 pounds. In view of the small Contact area this represents a pressure in excess of several hundred pounds per square inch.

According to electron diffraction examination our copper oxide films are composed of both cuprous oxide and cupric oxide. While other substances are no doubt present in trace quantities it is desirable to have the oxide film composed substantially of the oxides of copper solely.

A short exposure to air will not seriously affect the contact surfaces and where the contacts are arranged to have sul'icient abrasive action on each other to remove any film which may form thereon prolonged use in air is possible.

The contacts of this invention may be used in inert media such as oil or atmospheres consisting of nitrogen or the rare gases. Such media provide surrounding environments which are neither oxidizing nor reducing to copper. Performance is also satisfactory where the contacts are submerged in a dielectric oil and exposed to an inert atmosphere periodically.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications, can be made without departing from the invention; therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. An electrical contact member comprising a conducting base, and a layer of copper oxide 0002-0008 mil in thickness overlying said base, said layer of copper oxide serving as a contacting surface.

2. In a switch having co-operating contacts adapted to slidably engage one another at pressures of 10 t0 pounds, switch contacts comprising a conducting base, and a layer of copper oxide 0002-0008 mil in thickness overlying said base, saidlayers of copper oxide serving as the contacting surfaces of said switch contacts.

3. An oil immersed switch comprising co-operating switch contacts adapted to slidably engage one an0ther at pressures of 10 to 75 pounds, the surfaces of said contacts which are engaged by one another having thereon a coating of copper oxide having a thickness of from 0002-0008 mil.

4. A nonarcing duty tap changer switch for use in an inert medium comprising, in combination, a plurality of stationary Contact members symmetrically distributed around a common axis, and a movable con tact member for selective slidable contact with stationary contact members at contact pressures of from l0 to 75 pounds, the contact surfaces of said stationary contact members and said movable Contact member having thereon thin surface coatings of copper oxide o a thickness of from 0002-0008 mil.

References Cited in the file of this patent UNITED STATES PATENTS