US2613238A - Elimination of corona in electrical apparatus - Google Patents

Description

Oct. 7, 1952 R. w. WIESEMAN 2,613,238

ELIMINATION OF CORONA IN ELECTRICAL APPARATUS FiledJuly 27, 1951 Figl. 2.

Fig. Z.

Inventor:

FEobert W. Wieseman.

His Attokney.

Patented Oct. 7, 1952 ELIMINATION OF CORONA IN ELECTRICAL APPARAT s v Robert W. Wieseman, Schenectady, N. Y.,,assignor to General Electric Company, a corporation of New York Application July 27, 1951, Serial No. 238,887

3 Claims. (01. l71252) My invention relates to constructions and processes which have significance in connection with the problem of eliminating corona between an insulated conductor and an adjacent metallic j member.

Heretofore, it has been known to coat the sur face of conductor insulation with a semi-conducting material such as a paint, lacquer or varnish containing dispersed particlesof a conductor such as metal or graphite. Thus it has been known in the past to paint the surface of insulated conductors lying in core slots of large electrical machines with such a-semi-conducting material in order to establish aroundthe conductor a sheath of reasonably uniform potential. While this has proved satisfactory in many installations, for some applications and particularly in the newer machines of larger size and of higher voltage than those formerly known, such a procedure has not entirely eliminated the along the winding slot, nor-entirely eliminated electrical discharges at the end portions where the coil projects beyond the magnetic core. Such corona and discharges are objectionable from the formation of corona adjacent localized points standpoint of creating radio interference, and;- even if they do not reach such proportions as to be damaging to insulation they cause the creation of ozone, a very small trace'of which is readily perceivable by smell and this is sometimes objectionable from the standpoint of customer acceptance.

It is an object of the present invention to provide simple and inexpensive means for overcoming the above-mentioned difiiculties. "A' further object of the presentinvention is to provide an improved method of constructing electrical apparatus so as to prevent formation spaces between coil surfaces and slot surfaces because the slots vary in width a small amount, and the slot portion widths of the coils vary a small amount, and a small clearance space is required to place the coils in the slots without damaging the coil insulation.

0f corona, reduce radio noise and reduce dielec- It is, therefore, a still further object of the present invention to provide in a dynamoelectric machine having a stator comprising interinsulated laminationsprovided with winding slots with aninsulated winding in said slots, means for eliminating corona and ozone formationregardless of manufacturing clearances and consequent small air spaces between coil and slot walls. Another object of the present invention is to providemeans for reducing end discharges which occur when a coil slot sheath is not at ground potential. a In accordance with one aspect of my invention, I completely eliminate or minimize the corona eifect in large high voltage alternating current dynamoelectric machines by not only coating the insulated conductors with a semiconducting material but also scuiiing'the slot sides and thereafter applying semi-conducting material to such sides along the lamination edges to insure that all semi-conducting sheaths are sufficiently grounded by contact with each other and with the core. s

The features of my invention which I believe to be novel are set forth with particularity in ever, both as to 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 connection with the accompanying drawing in V longitudinal section showing a portion of a dynamoelectric machine stator core provided with winding slots with a winding therein and incorporating the present invention.

Referring to the drawing, the stator core is 'made; up of a plurality of punchings ill each provided with a plurality of winding slots H and each provided with interlamination insulating varnish I2. As is well known in the art, the varnish is applied as by dipping the individual punchings after the slots have been formed and the burrs removed, for if the slots were punched out after the varnishing operation, the lamina- -tions would be short circuited.

ductor is first sprayed or otherwise coated with a semi-conducting paint or other semi-conducting material 16 before the insulator conductor is placed in the slot. This semi-conducting coating extends throughout the slot region and for a creepage distance therebeyond at each end of the core as indicated at la in Fig. 2. In accordance with the present inventionjeachsl'ot side is also first prepared before placing the insulated and semi-conductor material coated conductor therein, by scufiing, as with a file, the lamination edges along the slot walls and thereafter, preferably immediately thereafter, applying a semiconducting paint or varnish containing conductive graphite particles or other'semi-conducting material, to the slot walls soscufied to' provide a semi-conducting coating H on the slot walls.

When the insulated conductor is thereafter placed in the slot as shown in the drawing, coating i1 is immediately adjacent the semi-conduct-' conductingrubbers, and it followsthat. the invention is not limited to a particularmeans of applying, as with a brush or spray, since the'semiconducting material may be applied in any other manner, as by winding of otherwise applying a semi-conducting tape or cloth mat to the conductors adjacent the slot portion and to the slot walls themselves. With the process and construction of the invention, the semi-conducting sheaths around the coil conductors are sufiiciently grounded by a satisfactory contactbetween the two coatings and with the core. One explanation of the difficulties encountered with prior art arrangements may bethat the insulating varnish applied .to the core laminations always extended over the slot edges .adjacentthe slot' Walls and had sufiicient ohmic resistance to prevent sufficient electrical contact between the slot surface andthe semi-conducting sheath on thearmature coiljso that the sheath is a fioatingjsheath with a varying potential above ground and, therefore, incapable of completely eliminating corona. With'th'e prior art arrangement, mere sending of theslotiwalls was not enough, possibly because the scuffed portions tendedto oxidize or rust thereby to eventually preventa good contact with the conductor sheath or to provide a sheath ground resistance which varied from slot to slot -.with the result that corona might form' in some slots and not in others. it is, of course, desirable that the ohmic resistance or" a semi-conducting sheath applied to bare laminations be made high In connection with the development of the present invention, tests have rather conclusively indicated that itis not always suflicient to merely iii "semi-conductor, localized corona, may be completely eliminated at all common operating voltages not only in the slot regions but, because the creepage portions of the conductor sheath are .well grounded, at .the end portions of the coils as well. There is thus provided a construction and method of the character described capable of 1 meeting the objects hereinabove set forth,

coat the slot walls with semi-conducting paint" and leave semi-conducting material off of the coil insulation. This is so because if there is. any

small air-space between the-insulation and the While I have illustrated and described a particular embodiment of my invention, modifications thereof will occur to those skilled in the art. I desire it'to be understood, therefore, that my invention is not to be limited to the particular arrangement disclosed and 'I intend in the appended claims to cover all modifications which do not depart from the true spirit and scope of my invention.

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

1. In electrical apparatus having a magnetic core member provided with winding slots, the combination of a coating of a semi-conducting material on the walls of said slots, high voltage insulated conductors'located in said slots, 'and a semi-conducting coating covering the insulation of said conductors adjacent said coating of semiconducting material on the walls of said slots.

.2. In a dynamoelectric machine having a stator core portion comprising varnishinsulated laminationsprovided with winding slots, an insulated windinginsaid slots, said winding having a semiconducting coating applied to .the outer surface of the insulation thereof in the region of .said slots, and said slots having asemi-conducting coating applied to varnish tree edges of said laminations along the'walls .of each slot, said two coatings being in substantial contact with one another to provide a reasonably uniform potential sheathextending around said winding insulation in the slot region thereof with saidsheath-adequately grounded to said slot walls to prevent corona formation between said winding and said slot walls. I

3. In a high voltage dynainoclectric machine having a slotted core structure, an insulated winding extending through the slots in said core structure and projecting therefrom, a semi-conducting coating covering the insulation in each of the slot regions .of said winding and extending therebyond to provide a creepage distance-at each end of said core, and a semi-conducting coating covering the walls of said slots and incontact with said first mentioned coating to prevent the formation of corona-between said conductors and. said slotted core structure. v

ROBERT w. WIESEMAN.-

'No references cited.

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