US3503126A - Method of making electrostatic screens,especially for toroidal transformers - Google Patents

Description

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- March 31; 1970 B. M. FALLER METHOD OF MAKING ELECTROSTATIC SCREENS, ESPECI FOR TOROIDAL TRANSFORMERS Filed March 15, 1967 I nvenlor m QM ttornefi United States Patent 3,503,126 METHOD OF MAKING ELECTROSTATIC SCREENS, ESPECIALLY FOR TOROIDAL TRANSFORMERS Brian Michael Faller, Hertfordshire, England, assignor to Hawker Siddeley Dynamics Limited, Hatfield, Hertfordshire, England Filed Mar. 15, 1967, Ser. No. 623,327 Claims priority, application Great Britain, Mar. 17, 1966, 1 1,827 66 Int. Cl. .Hlllf 7/ 06' US. Cl. 29-602 6 Claims ABSTRACT OF THE DISCLOSURE A technique is described for making an electrostatic screen in the form of an open-circuited turn, between two windings laid one upon another on a toroidal transformer. In this technique, a metallic paint coating is applied over the end winding, leaving a peripheral gap all around the toroid ring. A strip of insulating tape-is then applied so that it overlaps one margin only of the applied metallic coating and extends part way only across the width of the gap. After this, the screen is completed by continuing the paint coating across the outer surface of the insulating tape strip, so that this further application of conductive paint units ,with the margin of the previous conductive coating not covered by the tape, and overlaps but does not touch the margin of the previous paint coating that is covered by the tape.

DESCRIPTION OF INVENTION This invention relates to electrostatic screens, particularly but not exclusively for toroidal transformers.

The usual method of making an electrostatic screen between two windings on a toroidal core in order to reduce capacitive coupling between the windings, is to. wind a single close wound layer of wire around the core, with the start and finish of this' winding open-circuited. This suffers from the disadvantages that the screen is far from perfect either due to the gaps between turns on the outer periphery of the winding if the wire is close wound on the inner periphery, or there is a large build-upof wire on the inner periphery if the wire is close wound on the outer periphery.

Also, in order to reduce the volume of the screen, a fine gauge of wire must be used. This leads to highvoltages being generated between the ends of the winding due to the large number of turns required to cover one layer, and these voltages lead to high capacitance currents which tend to nullify the prime purpose of the screen. Also, where two or more screens are required using this method, the voltages generated by each screen should be equal if the screens are to be effective. To achieve this, each screen must be wound with a different gauge of wire since the winding length, i.e. the inner periphery of the winding, is changing due to the buildup of the windings. To ensure that the screen winding always exactly fits the winding length a very tight control is required on the build-up and this is very difiicult to achieve in production due to the normal manufacturing tolerances on cores, tape and wire.

It is an object of the invention to produce a more efficient screen of electrical length very close to one turn and which is also easier to construct and occupies less volume than the conventional methods.

According to the present invention, an open-circuited turn suitable as an electrostatic screen is formed by the application of a conductive layer of, for example metallic paint to a suitable insulating backing. The backing may be a form of insulating tape.

3,503,126 Patented Mar. 31, 1970 reference being had to the accompanying diagrammatic drawings. The drawings illustrate the application of the invention to the production of an electrostatic screen over toroidal windings on an annular core, as in a toroidal transformer. In the drawings:

FIGURE 1 is a diagrammatic pictorial view of the completed screened winding with layers broken away to show the build-up in detail,

FIGURE 2, 3 and 4 are diagrammatic cross sections illustrating three stages in the build-up of the screen, and

FIGURE 5 is a diagram of an interleaving detail of the screen.

It will be understood that, in the drawings, the thicknesses of the various layers are exaggerated for the sake of clarity.

The winding or windings 1, which will be underneath the screen, are completed and taped up with a suitable type of tape 2, e.g. cotton (FIGURE 2). The winding is then fully impregnated with a suitable varnish; this is necessary since the screen, when completed, is virtually impervious to impregnating varnish.

The impregnated winding is next wound with a single half lap layer of a non-porous conformable type of insulating tape 3, e.g. 0.003" unsintered etched polytetrafiuorethylene. The entire winding with the exception of a narrow strip 6 encircling the outer periphery, is then coated with a conductive paint layer 4 (FIGURE 3) which will eventually form the screen. A suitable metallic paint is Aquadag Silver dispersion 915 in M.I.B.K. A simple method of doing this is to apply a mask consisting of a narrow strip of pressure sensitive tape 5 to the outer periphery before coating the winding with paint. After the paint has dried the tape can be removed leaving a narrow unpainted circle around the outer periphery.

When the paint film 4 is dry a length of self-adhesive polyester tape 7, e.g. Scotch Boy 56, of a suitable width is applied around the outer periphery of the winding such that one edge of the tape lies halfway across the width of-the encircling gap 6 in the paint film (FIGURES 4 and 5). The remainder of the encircling gap 6, which is not covered by the polyester tape, is pain-ted with the metallic paint, the paint film 8 being continued part of the way across the width of the polyester tape 7 so that the paint film efi'ectively overlaps the paint film under the tape 7 without causing a short-circuited turn.

The connection to the screen may be made by a suitable conductor held in contact, such as a strip of copper foil 9 (FIGURE I) wound around the outer periphery, or by means of a conductor attached by conductive cement, e.g. Aquadag type 1624.

When the paint has been fully cured, which with the paint mentioned requires typically 15 minutes at C., the screen is covered by one or two half lap layers of a suitable tape 10, e.g. cotton. The winding is then ready for either further screens or the rest of the windings. Where multiple screens are being made, the tape interlayers may require impregnation with a suitable varnish to prevent sponginess.

The above method results in a very elfective electrostatic screen of low resistance and very low induced voltage with a maximum thickness at the inner periphery of the toroidal winding of about 0.015". This means that triple screens are possible with less than one sixteenth of an inch build-up even on small toroidal windings.

The method may be employed for all types of electrostatic screening required on toroidal windings, and for single turn windings where very close coupling is required. By the use of a suitable permeable metallic paint, magnetic screening of the completed winding may be achieved. Moreover, the use of the 'invention is not restricted to toroids but may be extended to screening for other electrical apparatus.

I claim:

1. A method of making an electrostatic screen between two windings in the form of an open-circuited turn, comprising the steps of applying a layer of insulating material over a winding, coating the insulating material with an electrically-conducting paint leaving a narrow uncoated strip along the whole length of the winding, applying a length of insulating tape along the uncoated strip such that one edge of said tape lies within the width of the uncoated strip while the other edge overlaps the conductive paint coating, and then painting over with said electrically-conducting paint the remaining uncovered portion of said unpainted strip, the conductive paint coating being continued far enough across the width of said length of tape for said paint coating to overlap the paint coating under said further length of tape, thereby to complete an open-circuited turn having only one single break at the overlap.

2. A method according to claim 1, wherein the narrow uncoated strip is masked, during the painting of said insulating material layers, with a narrow strip of pressure-sensitive tape which is removed afterwards.

3. A method according to claim 1, wherein said insulating material layer comprises a non-porous conformable type of insulating tape wound in a single half lap layer, and said paint is a metallic paint.

4. A method according to claim 3, wherein said Winding is first taped with a textile or porous tape and impregnated before the non-porous insulating tape is applied.

5. A method according to claim 1, wherein an electrically-conductive strip is attached to the paint coating for making electrical connection to the screen afforded by said coating.

6. A method according to claim 1, comprising the further steps of curing the paint and applying at least one half lap layer of porous tape over the paint layer.

References Cited UNITED STATES PATENTS 1,320,980 11/ 19.19 Bowman 336-84 1,586,889 6/1926 Elmer 29-607 X 3,032,729 5/1962 Fluegel 336-84 3,063,135 11/1962 Clark 336-84 X 3,143,720 8/1964 Rogers 336-84 X 3,149,296 9/1964 Cox 336-84 3,292,127 12/ 1966 Dormaier 336-84 CHARLIE T. MOON, Primary Examiner C. E. HALL, Assistant Examiner US. Cl. X.R. 174-35; 336-84

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