US2318074A - Corona elimination in generator end windings - Google Patents

Description

May 4, 1943. c. F. HILL ETAL 2,318,074

CORONA-ELIMINATION IN GENERATOR END-WINDING Filed Aug. 6, 1941 v l 4 e i l i i 43 30 43 1:::1-11 :5:: :1:11 :1 IH'l WITNESSESI NVENTORS TTORNEY Patented May 4, 1943 CORGNA ELIMINATION IN GENERATR END WINDINGS Charles F. Hill, Edgewood, and Leo J. Berberich,

Forest Hills,

Pa., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a

corporation of Pennsylvania Application August 6, 1941, Serial No. 405,572

(Cl. Til-206) Claims.

Our invention relates to methods and means for eliminating corona in the end-windings of high-voltage alternating-current generators or other electrical machines having windings disposed in air or other gaseous medium.

The dangers of corona, in high-voltage machines, are now well known, and there have been proposed many diiferent corona-preventing means, mostly imperfect er of limited application. As explained in a patent of Charles F. Hill, No. 1,784,989, corona is a gaseous-ionization phenomenon which occurs in a gaseous medium when it is subjected to an electrostatic voltagegradient high enough te accelerate the ionized electrons (which are always present) to a sumciently high velocity, during the free path of travel of such electrons, so they will have a sufciently high velocity, by the time when they strike against a neutral molecule of the gas, to ionize that molecule as a result of the force of the collision. In high-voltage machines, corona can be so destructive as to destroy enough of certain organic insulation to start an arc which can cause a short-circuit within a matter of a few months. The corona-hazard has heretofore limited the practical design and use of high-voltage dynamo-electric machines.

As set forth in the Hill patent just mentioned, the elimination of corona in the small gaseous spaces existing Within the slots 0f a tric machine has been successfully effected by coating the straight slot-lying-portions or coilsides of the insulated windings with aquadag, or a point which will be referred to, in our present specication, as a conducting paint. This conducting paint constitutes an adhering-surface or layer, which adheres closely against the surface of the insulation surrounding the conductors of the winding, so that there is no air-gap therebetween, and hence no corona therebetween; and this conducting paint has a sunicient amount of electrical conductivity so that all portions of the surface of said aquadag or conducting paint are at very nearly the same electrical potential, the resistance-drop within the painted coating, along the length of the insulated conductor, being of a very small order of magnitudes with respect to the Voltage-gradient which is necessary to produce corona in air or whatever other gaseous medium is utilized within the machine. This so-called conducting coating of aquadag, or the equivalent, necessarily has, however, a certain amount of resistivity, which may be of the order of from 100 to 10,000 ohms surface-resistivity, more or less, which is dynamo-elecneeded in order to prevent such coating from short-circuiting the laminations of which the iron core-member of the machine is composed.

The end-windings of high-Voltage generators, or the portions of the windings and the windingterminals which extend out beyond the slotted core-iron, still present diiiiculties from the standpoint of corona. There are several reasons, which need not be discussed herein, why it is not generally desirable or practical to coat the entire surface of the insulated end-windings with aquadag or other painted coating having as high a conductivity as has been successfully utilized in the slot-enclosed straight-portions or coil-sides of the winding. We utilize a fairly high resistivity, as set forth in this application, in order to prevent corona where the paint ends on the terminal-leads of the Winding, and to reducethe voltage-stress on numerous taped joints near the loops of the end-winding portions of the coils.

The problem of the elimination of end-winding corona is aggravated, also, by the limited available space, which results in close spacing between insulated conductors having a relatively high potential-difference therebetween, and by the practical necessity, in any actual machine, for utilizing Various spacing-blocks and cross-lashings for holding the various elements of the end-windings in rigid, predetermined spaced relation with respect to each other, and for securing certain portions of the end-windings to some sort of supporting-member which must be secured to the stator-frame or other grounded portion of the machine.

It should be understood that the solid insulation surrounding the .end-winding conductors (considered, for a moment, as single conductors in space), operates as a capacitance dielectric which is operated upon, by the alternating voltages of the conductors, to produce capacitance charging-currents which, if not carried away, will produce a surface-voltage, on the surface of the insulation, nearly equal to, but possibly out of phase with, the voltage of the conductor inside of the insulation. It should also be understood that, at al1 points, wherever the surface of an insulated high-voltage conductor is spaced, in a gaseous medium, from a surface at any other potental (whether the surface of another insulated conductor` carrying alternating currents, or a grounded surface), there is danger of corona, and this danger must be carefully eliminated when machines are designed for higher-than-normal alternating-current voltages.

It is an object of our invention to provide a 2 novel corona-eliminating means in which the entire surface of the end-windings, including their associated spacing-blocks, cross-lashing, supporting-blocks, and supporting-fastenings to grounded frame-parts, are all coated with an adherent semi-conducting paint or compound having a preferred range of surface-resistivity (for the thickness of the semi-conducting sheath which we use, ranging probably from 2 mils to 15 mils, more or less), said surface-resistivity ranging from 5 to 20 megohms, or any other high value (perhaps in the range from the order of 1 megohm or less, up to 10o megohms or more), or such other value as is necessitated by the magnitudes of the alternating voltages which are present in the winding-conductors, and by the number and extent of the interconnections between the surfaces of the different insulated endwinding conductors of the same or different phases, and between such insulated-conductor surfaces and adjacent ground-connected conductors. It is necessary that, at no point, shall there be a corona-producing potential-gradient, either in an air-space extending away from a painted surface and between conductors at different potentials, or linearly along any painted surface. At the same time, the resistivity of the semi-conducting coating or painted surface must, at all places, be suiciently high to prevent the development of any objectionable ohmic heating-losses in the coating, while the coating is carrying the capacity-induced charging-currents which flow therein.

It is a further object of my invention to utilize the same semi-conducting coating to cover all of the surface of the insulated conductors comprising the high-voltage winding of a dynamo-electric machine, including the straight coil-sides which lie within the slots of the core, aswell as the entire end-windings, and the spacers, abutments, lashings, and supports thereofthis being done either with, or without, the addition of an extra coating of relatively good-conducting material (such as aquadag) on the straight coilsides which lie Within the slots.

It should be understood that the terms conducting and ,-s'emi-conducting, as here used, are relative terms, so used because the resistivity of one coating is of the order of 1000 or 100,000 times that of the other. It should be understood, at the same time, that the resistivity of the socalled conducting or aquadag coating is many times that of a metal sheath, say copper, of comparable thickness.

A still further object of our invention is to provide a novel means and manner for providing a semi-conducting coating on the insulation of an insulated electrical conductor, comprising the process-steps and the resultant product of applying, first, a painted coat of a semi-conducting paint-compound, and while the painted coat is still wet, wrapping the painted insulated conductortightly with a porous tape which is preferably of woven glass, but which may be of cotton, asbestos, or other porous materials, applied so tightly that the Wet, paint penetrates through the pores of the tape, and while the paint is still wet, finally coating the taped surface with a final coat of the same, or a similar paint.

A still further object of our invention is to provide a corona-eliminating means and method for treating the end-winding region of a dynamo- `electric machine, in which only one grade or kind or resistivity of semi-conducting paint-compound need be utilized, for all parts and portions ofthe amount of treated surfaces, which enormously simplifies the shop-practices necessary in the manufacture of the machine.

With the foregoing and other objects in View, our invention consists of the materials, parts, structures, combinations, and methods hereinafter described and claimed, and illustrated in the accompanying drawing, in which:

Figure l is a fragmentary longitudinal sectional View of an end-portion of a high-voltage three-phase synchronous generator embodying our invention in a preferred or illustrative form of embodiment,

Fig. 2 is a developed plan-view of a portion of three coils Yof a high-voltage stator-winding, as viewed from the inside of the stator-bore, on a somewhat larger scale than Fig. l, with the paint which'is applied after the winding is in place shown in section.

Fig. 3 is a longitudinal sectional view, on a still larger scale, of the end-windings, approximately on the plane indicated by the line II-III of Fig. 2, with the paint which is applied after the windings are in place shown in section, and

Fig. 4 is an enlarged transverse sectional view of the conductor, with its various coatings, the coatings being exaggerated*considerably in thickness, for clarity of illustration and explanation, the section-plane being indicated, for example, bythe line IV-IV in Figs. l, 2 and 3.

In Fig. l, we have shown a portion of a threephase high-voltage generator comprising a statormember V5 and a rotor-member 6,-the latter being mounted upon a shaft 1. The .rotor-member 6 is, or may be, the usual rotor-member of a synchronous generator, having the usual direct-current `windings and damping windings winch need not be described, as they are conventional and constitute no particular part of our invention, except that a synchronous machine is provided. The stator-member 5 is shown-as comprising a core- .inember 8 comprising a plurality of bundles of iron or steel laminations 9 which are Vcentrally bored,

Yas indicated at Il, to provide the air-gap within which the rotor-member rotates. The statorcore Bis provided with a plurality of slots lil, at its bore il, for receiving Ythe straight coil-side portions l5 of the insulated high-voltage windings of the machinasaid windings being indicated in their entirety by the numeral fit, and

comprising the slot-lying, straight coil-.sides l5, and the end-windings Il.

It has heretofore been known, to brace and space .the various portions ofthe end-,windings by means of suitable bracing, spacing, lashing, and supporting-means which have taker. a number of diiferent forms. A representative type of bracing is shown, in Figs. l, 2, and 3, with certain-modiiications and improvements in accordance with my invention, as will subsequently be described. Thus, the straight coil-sides iextend a certain distance out beyondthe end of the iron core '3, before the conductors are bent, as indicated at 20 (Fig'Z), and thesestraight-side eX- Ytensions l5, Vat points closeto the end of the core Byare'bracedby means of split, channelshaped, insulating blocks or wedges 2i, which are driven into place and are lashed to the respective coil-sides by lashings 22. Theselashings maybe glass-rope or cord, or any suitable porousinsulating cordage Vor taping, which may be utilized for all of the lashings which will be referred to in the-subsequent description. The

straight-portionextensions i5 of `the coil-.sides may also'be'cross-lashed, as indicated at 23,'at

point between the blocks 2l and the rst endturn bend 29, as shown more clearly in Fig. 3, which shows the cross-lashing 23 between the top and bottom conductors lying in any slot.

At various points along the end-windings I'l, a plurality of laterally disposed spacing-blocks 25 are disposed between the insulated conductors coming from adjacent slots, said conductors being lashed together, with said blocks 25 in between, by means of a suitable lashing 25, as shown in 2. In some instances, also, as shown in Fig. l, additional insulating spacing-blocks are interposed between the conductors coming from the top and bottom layers of the windings lying in the various slots I4 of the stator-core 8, these last-mentioned spacing-blocks being indicated at 21, and being suitably lashed to the respective end-winding conductors, as indicated at 28.

The stator end-turns l1 are usually provided with cross-connections 29 for the coils.

It is universally customary, also, to support the end-windings I7 in some fashion, various means being utilized for this purpose, the particular supporting-means illustrated in Figs. l

and 3 comprising a plurality of insulating pieces z or brackets 33 which extend out from, and are supported by, the end-portions 3l of the statorcore 8, or the end-plates which clamp the core in place. In some instances, the end-winding supports, corresponding to the brackets 3Q, are

made of conducting material suitably insulated from the end-windings I1. In any case, there must be a supporting connection from the endwindings Il to some grounded conducting portion (such as 3l) of the stator-frame 5. In the illustrated form of embodiment, various portions of the outer layer of the end-windings I1 are lashed to the insulating supporting-brackets 3U, by suitable lashings 32.

Some, or all, of the bracing, lashing, and supporting-means, 2l to 32, which have just been described, or their equivalents, are commonly utilized in large generators of the type to which our invention applies; and we do not mean to imply that these features are broadly new. In

accordance with our present invention, however, we have introduced modifications in these features, and in the high-voltage winding-conductors I6 themselves, and these modifications will now be described.

As shown in Fig. 1i, the high-voltage winding I5 is made up of insulated conductors 34 comprising an inner copper conductor 35 which is surrounded by a compactly applied, usually impregnated, insulation 35 of suitable thickness for withstanding the voltage of the conductor 35, with a suitable factor of safety. The conductor 35, with its insulating cover Sti, together constitute what we refer to as the insulated conductor. In accordance with our present invention, the surface of this insulated conductor 35-36 is specially treated, preferably along the entire length of the conductor, including the straight, slot-lying coil-side portions l5, the end- Winding portions Il, the cross-connections for the coils, and the terminal-leads of the machine. our method of treatment of the surface of these insulated conductors 35-3S is to apply a semiconducting, adherent, painted-on layer or coating, which may be applied in many di'erent ways, and which may be a single coating or a plurality of coatings or any desired thickness.

We prefer, however, to apply the semi-conducting coating in a manner which is illustrated,

in an exaggerated manner, in Fig. 4, wherein the thicknesses of the semi-conducting coating-portions are considerably exaggerated, (out of proportion to the size of the conductor 35 and the thickness of the conductor-insulation 36), in order to facilitate the showing, without making the figure too large. In our preferred method of fabrication for the semi-conducting coating, we first coat the surface of the conductor-insulation with a semi-conducting paint-compound 40, and then, while the painted layer 40 is still wet, we preferably tightly apply a layer of porous tape 4|, which is preferably a braided glass tape, or perhaps 10 mils thickness, but which may be made of other porous insulating materials, said tape 4i being applied so tightly that the fibers or threads of the tape are pressed into the wet paint-layer 40 so that the semi-conducting paint 40 oozes through the pores or interstices of the tape, and then, while the paint is still Wet, we preferably apply a final layer 42 of this same semi-conducting paint-compound. In this manner, the porous tape serves to fix or hold the semi-conducting paint, so as to reinforce the same against any possible tendency to split or crack away from the surface to which it is applied.

The semi-conducting paint-compound which we utilize for our painted layers 40 and 42, and for other painted semi-conducting layers to be subsequently described, is a compound which is made up of a large proportion, by volume, of a finely powdered semi-conducting substance, and a relatively small proportion, by volume, of a suitable paint-medium or varnish which serves as a binder which is capable of drying out after the paint-compound has been applied. We need to utilize a large proportion of the powdered semiconducting material, so that we will have approximately particle-to-particle contact of the grains of the powder, in the finished painted semi-conducting surface, as distinguished from a construction in which the conducting or semiconducting particles are so few in number that they will be separated and insulated from each other by relatively thick layers of the paint or varnish-material in the finished coating. The disadvantage of a semi-conducting paintcom pound, which has only a relatively small proportion of powdered conducting or semi-conducting material in it, is that, when high voltages are applied to the finished semi-conducting surfaces, after the paint has dried, the insulating layers of paint or varnish, which separate the conducting or semi-conducting grains, will become punctured by electrical stress or flashovers, so as to cause the conductivity of the coating as a whole to in crease or vary in an erratic and undesirable fashion. We believe it is to be necessary, therefore in fabricating our semi-conducting paintcompound, to utilize a relatively large amount of semi-conducting powdered material, which is held together with not too large an amount of paintbinder or varnish, and to control the conductivity of the resulting paint-compound by controlling, or properly choosing, the conductivity of the powdered material, rather than by varying the amount of the powdered material.

For the powdered material which we utilize in our paint-compound as just described, we may utilize a suitably controlled wood-char, similar to that which is described in a patent to Leon McCulloch, No. 2,050,357, patented August l1, 1936, and assigned to the Westinghouse Electric 8a Manufacturing Company. Such wood-chars are very imperfectly carbonized cellulose-materials, the degree of carbonization of which, and

hence the amount of conductivity, is determined by a proper choice of the firing-temperature, as described in the McCulloch patent.

It is highly desirable, however, to utilize a semiconducting paint-compound which will be highly stable, during a life-time of use, without gradually increasing in resistivity, or otherwise changing its resistivity, during the life of the machine to which the compound is applied. While we lack conclusive life-tests on various semi-conducting paint-materials which could be utilized for our purpose, there is room for the belief that more stable results could probably be obtained Iby the use of some inorganic semi-conducting powdered material, of the proper conductivity or semi-conductivity, in place of the Wood-char just nientioned.

One such semi-conducting powdered material which we are considering, as the semi-conducting part of our paint-compound, is a specially treated, powdered'titanium dioxide, which has been partially reduced in a hydrogen furnace, the amo-unt of reduction being controlled by the temperature of the furnace and the time in the furnace, so as to produce a partially reduced titanium-dioxide powder having the desired degree of conductivity or semi-conductivity. This powder would then be mixed with a suitable quantity of paint-binder or varnish, usualy requiring a paint-volume at least equal to the powder-volume. A suitable ratio of powder to varnish, in mixing the semiconducting paint-compound wlnch we utilize in the manner described herein (Whatever the nature of the semi-conducting powder), can be determined by adding suicient powder, to the varnish, to produce a dull matt surface after the varnish has dried or the solvent has been removed. A shiny surface indicates that too much varnish has been added. r'ihe particular paintcompound, utilizing partially reduced titaniumdioxide powder, which has just been referred to, will be made the subject of another application, assigned to the Westinghouse Electric & Manufacturing Company.

In accordance with our invention, the entire assembly of Vspacing-blocks, lashings, coil crossconnections, and supports, 2| to 32, which are applied after the high-voltage winding i has been inserted in place on or in the dynamo-electric machine, are all painted over, with one or more coatings d3 of a semi-conducting paint-compound such as has just been described, so as to make, in effect, a plurality of semi-conducting charging-current-carrying paths, between a plurality of different portions of the conductor-coat.- ing 42 of the end-windings, and other adjacent surfaces, whether surfaces of diiferent painted in.. sulated conductors, or surfaces of grounded metal members. Preferably, the various lashings 22, 23, 25, and 32 are applied in the manner already described for the tape 4I; namely, by first painting the surface to be lashed, with our semiconducting paint-compound, then applying the lashing, and then repainting while the first paintcoat is still wet. Preferably, also, the insulating spacing-blocks, such as 2 l 25, and 27, are painted before being applied, and again afterwards; although, at least in the case of the split wedgeblocks 2l, it has been found undesirable to paint the wedge-surfaces which have to slide against one another, while the blocks are being pounded into place, this being merely a matter of avoiding excessive friction which would hamper the application of the blocks. v

'Regardless of the specific preferred details of Cil application, the essential feature, however, is that the semi-conducting paint shall make a semiconducting electrical connection between the various surfaces which are to be joined, as previously described, so that the various capacitor charging-currents are conducted away from the several surfaces in such manner as to avoid the possibility of obtaining a corona-inducting potential-gradient at any point.

A very considerable variation is tolerable, in the permissible or desirable resistivity or resistivities of the semi-conducting coatings. One rule which we have found helpful from the designers standpoint, as guiding us toward the correct order of magnitude of conductivity to choose, or a desirable order of conductivity, is to choose a semiconducting coating having a surface-resistivity such that, before any spacing or supporting blocks are applied, and before any lashing or cross-lashing is applied, the resistivity of the semi-conducting coating on the end-windings Il is such that there will be a relatively small difference in voltage between the internal conductor-proper 3% and the semi-conducting coating til-52, at the central loops ifi (Fig. 2) at the ends of the endwindings. This relationship is subsequently altered, of course, when the cross-connections are applied, but the rule just indicated aiords a basis for a very approximate rule-of-thumb method for determining one suitable value for the eurface-resistivity of the semi-conducting coating. The general range of values of the surface-conductivities has already been indicated, in the preliminary portions of this specification.

The semi-conducting layer @ii-t2, as above described, may be applied only to the end-windings il, and to the various blocks, lashings, and supporting-means or adjacent grounded surfaces in the vicinity thereof, but we prefer, as previously indicated, to extend this semi-conducting coating over the entire insulated winding lil, including the straightcoil-sides l5 which will be thus coated before the coil is inserted in place in the stator-slots I. We believe that the semi-conducting coating l-2, as described, is of sufiicient conductivity, and is satisfactory in every way, for preventing corona in the stator-slots Ill. If desired, however, and as an extra precaution, a relatively conducting coating, of aquadag, or equivalent coating as previously discussed, may be applied to the straight side-portions which are to lie in the stator-slots la, as indicated at It,

' in Figs. 2 and 4. In inserting the coil-sides in the slots la, it is necessary to make sure that there is an electrical contact between the coated layer of each coil-side and the iron of the slot, as

pointed out in the previously mentioned Hill patent.

While we have described our invention in a form of embodiment which we now prefer, and while We have explained the invention in accordance with our present understanding or" the saine, we desire it to be understood that such illustration and explanation are intended oniy by way of suggestion, and not altogether by way of limitation; and we desire it to be understood that many changes may be made, by way of additions, alterations and omissions, without departing from the essential spirit and objects of our invention. We desire, therefore, that the appended claims Vshall be accorded the broadest construction consistent with their language.

We claim as our invention:V 1. A high-voltage alternating-current dynamo- .electric machine comprising astator-niemberand a rotor-member operating in a gaseous mediuml one of said members having a slotted core-member, and an insulated high-voltage winding comprising slot-lying coil-side portions and end-Winding portions, a plurality of diiierent insulating blocking-means and insulated lashings between different insulated conductors of said end-windings, a plurality of grounded insulating supporting-means connected to a plurality of portions of the end-windings, and a semi-conducting adherent coating covering the surfaces of said endwindings and the surfaces of all of said blockingrneans, lashings and insulated supporting-means.

2. A high-voltage alternating-current dynamo-- electric machine comprising a stator-member and a rotor-member operating in a gaseous medium, one of said members having a slotted core-member, and an insulated high-voltage winding comprising slot-lying coil-side portions and endwinding portions, a plurality of different insulating blocking-means and insulated lashings between diferent insulated conductors of said endwindings, a plurality of grounded insulating supporting-means connected to a plurality of portions of the end-windings, a semi-conducting adherent coating covering the surfaces of said end- Windings and the surfaces of all of said blockingmeans, lashings and insulated supporting-means, and a more-conducting adherent coating covering the surfaces of the slot-lying coil-side portions.

3. A high-voltage alternating-current dynamoelectric machine comprising a stator-member and a rotor-member operating in a gaseous medium, one of said members having a slotted core-member, and an insulated high-voltage Winding comprising slot-lying coil-side portions and end-Winding portions, a plurality of different insulating blocking-means and insulated lashings between different insulated conductors of said end-windings, a plurality of grounded insulated supporting-means connected to a plurality of portions of the end-windings, and a semi-conducting adherent coating covering the entire surface of all of the insulated conductors of both the slot-lying coil-side portions and the end-Winding portions of the insulated high-voltage Winding, and also covering the surfaces of all of said blocking-means, lashings and insulated supporting-means.

4. The invention as defined in claim 1, characterized by said semi-conducting adherent coating comprising a single grade or resistivity of semi-conducting paint-compound for the entire coating aforesaid.

5. The invention as defined in claim 1, characterized by said semi-conducting adherent coating having a surface-resistivity of the order of a megohm or more.

6. The invention as deiined in claim 3, characterized by the portion of the semi-conducting adherent coating, which covers the surfaces of i the insulated conductors of the high-voltage winding, comprising a iirst coat of a semi-conducting paint, a taped layer of porous tape sinking down into said first coat, and a second coat of semi-conducting paint over said taped layer.

7. A high-voltage alternating-current dynamoi electric machine comprising a stator-member and a rotor-member operating in a. gaseous medium, one of said members having a slotted core-member, and an insulated high-voltage winding comprising slot-lying coil-side portions and end- Winding portions, a semi-conducting adherent coating substantially covering the surfaces of at least said end-Winding portions, a plurality of means, at a plurality of different points, for semi-conductingly blocking apart, and semiconductingly lashing together, diiierent coated conductors of said end-windings, and a plurality of grounded means for semi-conductingly supporting a plurality of portions of the end-Windings.

8. The invention as defined in claim '7, chai'- acterized by the coating on said end-Winding portions comprising a first coat of a semi-conducting paint, a taped layer of porous tape sinking down into said first coat, and a second coat of semi-conducting paint over said taped layer.

9. The invention as dened in claim 7, characterized by said semi-conducting adherent coating having a surface-resistivity of the order of a megohm or more.

10. A high-voltage alternating-current dynamo-electric machine comprising a stator-member and a rotor-member operating in a gaseous medium, one of said members having a slotted coremember, and an insulated high-voltage winding comprising slot-lying coil-side portions and endwinding portions, a semi-conducting adherent coating substantially covering the surfaces of the insulated conductors of said winding, a plurality of means, at a plurality of different points, for semi-conductingly blocking apart, and semiconductingly lashing together, diiierent coated conductors of said end-windings, and a plurality of grounded means for semi-conductingly supporting a plurality of portions of the end-Windings.

11. The invention as defined in claim 10, characterized by the coating on the slot-lying coilside portions being more conducting than that on the end-winding portions.

l2. The invention as defined in claim 10, characterized by the coating on said end-Winding portions comprising a first coat of a semi-conducting paint, a taped layer of porous tape sinking down into said Iirst coat, and a second coat of semi-conducting paint over said taped layer.

13. A high-voltage alternating-current dynamo-electric machine comprising a stator-member and a rotor-member operating in a gaseous medium, one of said members having a slotted coremember, and an insulated high-voltage Winding comprising slot-lying coil-side portions and end- Winding portions, a semi-conducting adherent coating covering the entire surface of all of the insulated conductors of both the slot-lying coilside portions and the end-winding portions of the insulated high-voltage Winding, a plurality of means, at a plurality of different points, for semi-conductingly blocking apart, and semi-conductingly lashing together, different coated conductors of said end-windings, and a plurality of grounded means for semi-conductingly supporting a plurality of portions of the end-windings.

14. The invention as deiined in claim 13, characterized by the coating on the slot-lying coilside portions being more conducting than that on the end-winding portions.

l5. The invention as deiined in claim 13, characterized by the coating on said end-winding portions comprising a Iirst coat of a semi-conducting paint, a taped layer of porous tape sinking down into said first coat, and a second coat of semi-conducting paint over said taped layer.

CHARLES F. HILL. LEO J. BERBERICH.

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