US2427749A - High-voltage dynamoelectric machine winding - Google Patents

Description

gut-Hun p 23, 1947. E. 1.. SCHULMAN 2,427,749

HIGH VOLTAGE DYNAMO-ELECTRIC MACHINE WINDING Filed June 30, 1943 Organic l'nsu/at/bn 60/(6 figs/fiance Pal/It WITNESSE INVENTOR A4;

Patented Sept. 23, 1947 Owl bl! I.

HIGH-VOLTAGE DYNAMOELECTRIC MACHINE WINDING Earl L. Schulman, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 30, 1943, Serial No. 492,847

2 Claims. 1

This invention relates to resistance compositions for application to electrical apparatus in order to prevent corona.

The formation of corona in electrical apparatus limits the permissible voltages since the corona tends to destroy organic insulation on high-voltage members exposed to air or other gaseous medium. Corona is produced in electrical apparatus when the surface of the electrical insulation thereof exposed to air or other gas is at such an electrostatic voltage gradient that the molecules of air or other gas are rendered highly active chemically and electrically. When subjected to these voltage gradients, air, for example, will form ozone and nitrous oxides, which react with water vapor to form oxidizing acids that are destructive to organic insulation. Since organic insulation constitutes the major portion of insulation in most present-day electrical apparatus, corona becomes a limiting factor in the permissible voltage employable in the electrical conductors. The use of voltages above the threshold values at which corona is initiated may result in such destructive action upon the organic insulation of conductors that in some cases in only a few months the insulation, if entirely organic, may be destroyed or impaired so greatly that it will short-circuit and become ineffective for its intended purpose. If inorganic material such as mice. is employed in combination with organic substances as insulation, the destruction is limited to the organic portion of the insulation. However, this necessitates refinishing the apparatus at frequent intervals to replace the deteriorated organic substance.

In building high voltage electrical apparatus resistance compositions have been applied as a coating to the exterior surface of the electrical conductors of the apparatus in order to grade or to reduce the electrical potential on the surfaces exposed to air or other gaseous medium to prevent the development of potentials causing generation of corona. While, in theory, the appllcation of resistance materials appears simple and easy to carry out, unexpected difliculties are met with in practice. Resistance compositions must be prepared having certain specific resistivity when applied as a surface film on conductors such that the potential is graded or grounded whereby corona is not produced. Furthermore, the most effective resistivity of a resistance coating applied to a conductor depends upon its location with respect to the apparatus in which it is placed. For example, a coil in a high voltage generator has portions disposed in contact with slots in the magnetic core and the coil has end windings which are relatively remote from such magnetic core. A resistance composition for the end windings should have a high resistance in order to grade the potential along the end winding to prevent the build-up of voltages suflicient to produce corona without, however, permitting heavy currents to flow along the surface of the insulated conductors. On the other hand, for that portion of the insulated conductor within the slots or closely adjacent thereto, it has been found that the resistivity of a coating suitable for preventing corona, at this place should be about one thousand times less than the coating applied to the re remote end winding.

From research and experience it has been found that while carbonaceous substances such as graphite and wood chars may be applied in combination with a vehicle to insulated conductors as resistance coatings, there are encountered difliculties in securing a predetermined resistivity suitable for any given application by using these materials. Furthermore, even when a given initial resistivity in the coating upon the insulated conductor has been procured, the resistivity is not constant over a period of time. These carbonaceous substances in such coatings ordinarily change in resistivity with time at such a rate that in a few years the resistance coating is ineffective for its intended purpose,

The object of this invention is to provide a resistance composition having a predetermined resistivity when applied as a paint on high voltage electrical conductors whereby to reduce or prevent corona thereof.

A further object of this invention is to provide for combining coke and a binder to produce a composition for use as a paint on electrical members to prevent corona.

A still further object of this invention is to provide for treating coke to reduce it to a state in which it may be applied to electrical conductors in order to eliminate corona thereof.

Other objects of the invention will, in part,

be obvious, and will, in part, appear hereinafter. For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing in which Figure 1 is a fragmentary cross-sectional view, partly broken away, or a high voltage conductor in a slot, and

Fig. 2 is a sectional view taken along the line IIII of Fig. 1.

According to this invention, it has been discovered that coke, when properly treated and reduced to a suitable physical condition, has unexpectedly good characteristics when applied in a paint to electrical conductors. Such paint produces coatings which, when dry, have a low resistivity rendering them particularly satisfactory for treating those portions of coils disposed within the slots of magnetic cores of dynamoelectric machines.

Ordinary coal coke free from dirt and consisting essentially of high temperature carbonized coal may be treated to produce a paint by a relatively simple milling process. Other cOkes, such, for example, as petroleum coke, may be employed in the practice of the invention if they are heat-treated to eliminate substantially all of the volatile matter therefrom. Ordinarily, petroleum coke having about volatile matter may be fired for from 1 to 2 hours at temperature of about 650 C. to 700 C. to render the product suitable for the particular invention disclosed herein. A sample of petroleum coke having 10% volatile matter with an original electrical resistance of 100 megohms, after firing at 700 C. for 7 hours, had a resistivity of 50,000 ohms when measured in a given cell. The lower resistance is highly desirable for the purpose of producing a resistance composition.

Coke has an advantage over wood chars in that coke is more easily processed and is not as dusty to handle. The resistivity of wood chars is critically dependent upon the temperature of heat treatment, a few degrees may mean changes of resistivity of the order of 10%. Further advantages from the standpoint of satisfactory results of the apparatus to which it may be applied are believed due to the low volatile content and the higher resistivity of coke over that of graphite. Other advantages will be pointed out hereinafter.

Coal coke or heat-treated petroleum coke or the equivalent is subjected to milling in a ball or tube mill or the equivalent for a prolonged period of time of as much as 24 hours in order to reduce the coke to particles of colloidal dimensions. A better coke suspension in a vehicle is produced by milling to this degree, as well as facilitating the making of a resistance composition that has other highly desirable characteristics.

The milling is conducted in the presence of a liquid such as water or other solvent for the resinous binder which is to be employed in combination with the finely divided coke in producing a resistance composition. Water in an amount of about 1 to 1 times the weight of coke is introduced into the ball mill containing a number of steel balls and the whole set in operation for a predetermined period of time. Similar proportions of toluene, solvent naphtha or other solvent may be employed.

One suitable test for determining whether the coke has been reduced to a suitable fineness is to remove a sample or the coke-water mixture from the ball mill and disperse in sumcient added water to give a dispersion having 0.01% by weight of coke. The dispersion so prepared is introduced into the cell of a photoelectric colorimeter. Light transmission values of from A of 1% to 10% are indicative of suiiicient oomminution of the coke. A 5% light transmission value is believed to correspond to an average particle size of under 1 micron.

When the coke has been comminuted to the proper degree of fineness, there may be added to the mixture in the ball mill a resinous substance to be employed as the binder for the coke when applied as a coating. One composition may consist of one part of coke with two parts of alkyd varnish having 10% diamyl phthalate as a plasticizer, solvent naphtha or petroleum spirits being employed as the liquid in which the coke is being ball-milled. Operating the ball mill for half an hour or thereabouts will produce a homogeneous paint-like mixture of coke, resin binder and solvent suitable for application to electrical members.

A paint-like mixture having certain desirable properties may be prepared from coke ball-milled in water by adding a water soluble organic film forming substance to the coke. Gum arabic has been found to be particularly satisfactory for this purpose. Gum tragacanth and gum gatti are likewise suitable. In some cases polyvinyl alcohol which has been derived by hydrolizing a polyvinyl ester to the degree that the polyvinyl alcohol is soluble in water may be employed with the finely divided coke. Caesin glue is another example of a suitable water soluble organic material of the kind which may be employed in the practice of this invention.

As an example of the preparation of the composition, the gum arable coke composition and its preparation will be described in detail. The gum arabic is added to the coke water suspension after the suspension has been removed from the ball mill and strained through a fine mesh fabric in order to remove large particles therefrom. Gum arable is added to the colloidal coke suspension over a range of from parts to 10 parts by Weight of gum arabic to from 25 parts to parts by weight of the coke. Sufiicient water should be added to dissolve the gum arabic and form a uniform coke suspension. Ordinarily 2 parts of water or more per part of coke and resin is sufficient. To stabilize the suspension and prevent deterioration of the gum arabic, 1% of ammonia based on the weight of the gum arabic is admixed. Other stabilizers may be substituted for the ammonia. If the composition is to be applied to asphaltic impregnated conductor insulation to which a water suspension may not be an entirely satisfactorily adherent, a low molecular weight alcohol such as ethyl alcohol may be added in the proportion of about 10% of the amount of the water. The mixture of the colloidal coke, gum arabic, water and ammonia, and ethyl alcohol if required, is thoroughly mixed by running through a gear pump or by use of a stirrer until a, smooth paint-like suspension is produced. For most purposes a viscosity of from 50 to seconds No. 1 Demmler cup is satisfactory for brushing upon conductors.

The coke may be employed with numerous resinous binders depending upon the requirements of the application. Oil-modified glycerophthalate resins, cumarone resins, maleic anhydride-glycol resins, asphalts and numerous other binders may UUUI U be used with satisfactory results in combination with the coke.

Binders which do not polymerize subsequently or otherwise react are preferable for practicing the invention. Resins which harden by evaporation of a solvent are most desirable. Gum arabic is a non-reactive binder whose advantages are typical of this class. Immediately upon drying gum arabic-coke coatings attain a substantially constant resistance. Polymerizing resins may take months at apparatus operating temperatures to reach a reasonable steady state. Coatings including a polymerizing resin may easily change twenty-fold in resistance in 30 days at 100 C.

In most cases, the resinous binder is employed with a relatively large proportion of the coke so that the electrical properties are not greatly afiected by the type of binder employed. The resistance of the coke is so high that a large quantity may be employed with the result that the resistivity of a film produced therefrom is of the order of from 500 to 15,000 ohms per square inch of 2 or 3 mil thick film. An advantage due to the use of coke is that these values are not appreciably changed if the percentage of coke in the binder varies a few per cent.

Where a low resistance material, such as graphite is employed in producing a resistance composition, it is necessary to work within very close limits with relatively minute quantities of graphite to secure an equivalent resistivity per square of film 2 or 3 mils thick. In the case of graphite it is believed that the resistivity is obtained by separating the particles to such an extent that the contact resistance primarily controls the film resistivity and not the particle resistance. In the case of coke the contact resistance is not critical since from 15% to 90% by weight of th film consists of the finely divided coke. Under these conditions aging, shrinking of the resinous binder, and other changes with time do significantly alter the film resistivity. Consequently commercial quality control of the composition and coatings produced therewith may be easily achieved and the aging characteristics are far superior to those obtained with prior art resistance substances.

For an example of the application of the composition, reference should be made to Figs. 1 and 2 of the drawing showing a dynamoelectric apparatus with a conductor having the composition applied thereto. The magnetic core l0 consists of a plurality of magnetic laminations in which a slot or cell I: has been produced with a pair of grooves H at the upper part of the slot for the application of a wedge IS. A conductor 18 forming part of a coil is disposed within the slot l2 and maintained tightly therein by the wedge [8. The conductor It! comprises metallic straps or wires 20 for conducting electrical current with electrical insulation 22 disposed thereabout. In high voltage apparatus operating at 6600 volts or higher, the irsulation 22 is frequently composed of a plurality of layers of mica sheet impregnated with a cementitious binder such as an insulating varnish or an asphalt. When properly impreghated, the insulation 22 is substantially completely filled without voids or spaces being present therein.

The insulation 22 will not prevent corona being produced upon its exterior surface since potentials may exist sufficient to cause the gases of the atmosphere to ionize and thus produce corona.

In order to prevent this taking place, the surface a layer of inorganic tape 24, such, for example, as glass tape, and resistance composition applied to the surface of the tape, Upon the exterior surface of that portion of the conductor in contact with the magnetic material I0 and immediately adjacent thereto, a relatively low resistance coating 26 having a, resistivity of the order of 500 to 15,000 ohms per square inch in films of from 2 to 3 mils thick is applied. For this purpose the coke-gum arabic composition disclosed herein is most satisfactory. The glass tape may be painted with the composition or the conductor dipped in the composition in order to impregnate the glass tape therewith. Upon drying, a coating comprising coke and gum arabic binder will be present thereon.

For that portion of the conductor remote from the magnetic core, such winding comprising in general the end windings, a coating of much higher resistivity is required. End windings will operate with good results if the resistance coating has a resistivity of from 1 megohm to megohms or more per square inch. Such high resistance coating prevents the flow of undue current without, however, permitting the building up of high potentials. As shown in the drawing, the relatively low resistance coke coating 26 and the higher resistance coating 28 are overlapped, in order to secure an electrical contact therebetween and to completely cover the exterior surface of the insulation to prevent the building up of a. high potential sufficient to produce corona.

The high resistance coating 28 found most desirable for the practice of this invention conveniently may be the finely divided anthracite coal and binder disclosed by Barker et a1. Patent 2,399,314.

Inasmuch as the coke coating may dust when handled, it has been found that a thin coating of a Protective finish such as a good grade insulating paint may be applied thereover without any detrimental results. The insulating paint applied as a. coating as shown at 30 in the figures of the drawing should be as thin as reasonably possible since it is desirable to secure an electrical contact between the walls of the slot l2 and the resistance coating 26. The operation of inserting the conductor l8 within the slot I2 to secure a tight fit will be suificient to cause the magnetic laminations to abrade through the insulating coating 30 and attain this electrical contact.

After a number of coils l8 have been assembled within the slots l2, they are spaced apart at their end windings by means of spacer blocks and wedges and lashed together with ties. The spacer blocks are preferably made of a semiconducting or resistance material in order to grade the potential from conductor to conductor and prevent the build-up of excessive electrical potentials therebetween. A suitable material for this purpose is the anthracite coal-phenol-formaldehyde laminate disclosed in the patent application of Atkinson et a1. Serial No. 467 ,7 28, filed December 3, 1942. Likewise, the cross ties and other lashings with which the end windings are suitably tied together may be treated with a high resistance composition to further prevent the building up of excessive potential differences between adlacent conductors. By such means the potential at any point relative to any other point will be reduced to a value below that at which corona will be formed. After the coil has been placed within the slot l2 and the wedge l8 inserted into the grooves H, the entire coil and end windings Ohms Coke Resistance Coating Ohms Graphite Resistance Coating Days at 200 C.

The graphite resistance coating is that sold under the trade name of Aquadag. The change of resistance of the coke coating is approximately 30% while the graphite coating has been-reduced to a value of less than 1 2% of its initial value, It will be appreciated that the coke coatin maintains its predetermined resistivity far better than the other coating. In the accelerated aging test, one day at 200 C. is believed equal to one year at 100 C. for apparatus. e

Other benefits of the coke resistance composition is that a predetermined resistance may be secured by varying the quantity of coke within the range of 25% to 90% based on the total weight and resin, whereas graphitic resistance compositions cannot be so readily varied to give satisfactory results. A further advantage of the coke resistance composition is the relatively low cost of the coke and its reduction to a state in which it may be employed in producing the resistance composition.

The coke composition may be applied directly to the surface of any conductor by brushing, spraying, dipping, or similar process without the use of the glass tape shown in the figures of the drawing. The composition may be dried in air or in ovens whichever is most convenient. The conductors or coils produced therewith may be put into use as soon as dried.

Since certain obvious changes may be made in the above procedures and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or taken in connection with the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. Dynamoelectric apparatus comprising, in

combination, a body of magnetic material, a slot in the magnetic material, an electrical member in the slot, the member comprising an electrical conductor, electrical insulation disposed about the conductors, a resistance coating applied to the exterior surface of the insulation of that portion of the electrical member in and adjacent to the slot, the resistance coating composed of from 25 to 90 parts by weight of finely divided coke and from to 10 parts by weight of a resinous binder, and a substantially uniform thickness of a higher resistance coating applied to the remaining exterior surface of the insulation and applied in contact with the other resistance coating, and a protective finish applied to the electrical member.

2. Dynamoelectric apparatus comprising, in combination, a body of magnetic material, a slot in the magnetic material, an electrical member in the slot, the member comprising an electrical conductor, electrical insulation disposed about the conductors, a resistance coating applied to the exterior surface of the insulation of that portion of the electrical member in and adjacent to the slot, the resistance coating composed of from 25 to parts by weight of finely divided coke of colloidal dimensions and from 75 to 10 parts by weight of a resinous binder, the resinous binder being gum arabic, and a higher resistance coating applied to the remaining exterior surface of the insulation, semiconducting spacers and ties being applied to the portion of the electrical member outside of the slot, and a protective finish applied to the electrical member.

EARL L. SCHULMAN.

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

UNITED STATES PATENTS Number Name Date 573,558 Voss Dec. 22, 1896 653,468 Burke July 10, 1900 1,987,508 Johns et al Jan. 8, 1935 2,322,702 Peterson June 22, 1943 1,784,989 Hill Dec. 16, 1930 2,042,208 Calvert May 26, 1936 2,050,357 McCulloch Aug. 11, 1936 2,328,894 Cranmer Oct. 24, 1941 1,987,508 Johns Jan. 8, 1935 2,293,677 Slayter Aug. 18, 1942 2,344,334 Vrenneau Mar. 14, 1944 FOREIGN PATENTS Number Country Date 756 Great Britain 1880 455,193 Great Britain Oct. 15, 1936

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