GB2348741A - Insulation for a microwave oven transformer - Google Patents

Abstract

Insulation for a transformer (1) of a kitchen microwave oven of the type having an E-shaped core (2) with three rectangular-section columns (2a, 2b, 2c) and including a primary (3), secondary (4) and auxiliary (5) winding. A tubular-shaped liner (11), made from semi-rigid insulating paper, wraps around the central column of the core (2b) and is closed by the overlapping of its two ends (11a). The bottom end of the liner (11) has a folded edge (11b), which is positioned between the secondary winding (4) and the bottom (2d) of the openings (2c) between the columns. Further insulation may also be provided by wrapping C-shaped liners (8, 9) around the primary and secondary windings (3, 4) and placing a insulating sheet (10) between the secondary and auxiliary windings (4, 5).

Description

2348741 INSULATION FOR A MICROWAVE OVEN TRANSFORMER This invention relates

to a high-voltage transformer, such as is used for a domestic microwave-heatel oven, and, in particular, to its electrical insulation.

High-voltage transformers are known, for example, for powering microwave ovens, of the type that comprises an "E- III-shaped core having a central column of rectangular crosssection, on which a secondary winding of a high-voltage, such as 2500 V, an auxiliary winding, and a primary winding, also of rectangular outline, are mounted in that order, wherein the long sides of the windings are wrapped in an individual insulation liner madeota thin material, such as semi-rigid paper, in order to take up less space in the transformer. Said long sides of the- windings are fitted in the openings between the core columns, pressing against the central column and against the bottom of the core openings. During the installation of thewiijdings on the central column, the insulating liner is subjected to friction with the sharp edges of the core, with the consequent risk of the insulation being damaged. The diffe-rent.pieces of insulating paper making up the insulation liner are joined together with adhesive tape. To protect the vertices of the secondary winding and, in addition, to isolate it from the sharp edges of the core and the -bottom of the said openings, a known solution is to wipd-a strip of adhesive tape around the corners of the winding. The shaping of the pieces of paper for making the liner of the high-voltage secondary winding and their positioning and joining on each side of the winding make automated transformer assembly impossible.

EP 294 297 discloses a solution for insulation of the primary and secondary windings of a transformer for microwave ovens which, to facilitate automatic installation of the windings, dispenses with all the adhesive tapes and avoids the cutting and folding-Of numerous insulating pieces. The 2 solution for this is to coil the windings on an insulating frame having an end flange at each end to facilitate the electrical connections, to which a cap is attached that completes the insulating cover all the way round the winding. 5 US-5440286 describes a transformer of a known type, which has a tubular- shaped insulating paper surrounding a substantial length of a central column of the core for the insulation of the inner surfaces of the primary and secondary windings and two insulating pieces of "C"-shaped crosssection for the insulation of the outer surfaces of each winding.

The present invention provides a high-voltage transformer for kitchen microwave ovens comprising an IIEIIshaped transformer core having three columns of rectangular cross-section, at least one secondary winding, an auxiliary winding and a primary winding, all of rectangular crosssection and mounted around the central column of the core, and a tubular insulator of semi-rigid material, which insulator encircles the central column and has an outwardly folded edge at one end in contact with the bottom of the transformer core. The folded edge lies between the lower winding and the bottom of the openings between the columns of the core to protect the winding.

The present invention further provides a high-voltage transformer in accordance with claim 1 or claim 3.

The present invention further provides a transformer comprising a core having a central column and primary, secondary and auxiliary windings each mounted around the central column, wherein the inner surfaces of the windings are insulated from the column by an interposed insulating means that is formed from a semi-rigid sleeve of material and has an outwardly extending folded edge at one end of the sleeve. The insulating means may be formed from a single strip of semi-rigid material, which may be die-stamped, for 3 example, and made from insulating paper. on one side of the central column the ends-of the strip may overlap one another, so as to facilitate the installation of the insulating means on the column. 5 The transformer will-usually be a high-voltage transformer intended- fora domestic appliance and will usually have an "E"-shaped core. The central and outer columns of such cores will usually be rectangular in crosssection.

The invention is especially directed to transformers for domestic microwave-heated ovens and, in particular, those with "Ell-shaped cores.

The insulation for the transformer is preferably built entirely of insulating pieces made from a strip of semi-rigid insulating paper, thus dispensing with adhesive tapes for the insulation.

In a preferred embodiment, the electrical insulation comprises the forming of a small number of pieces, two of them, preferably of "C"- shaped cross-section, to insulate the outer surfaces of the winding and a tubular-shaped central piece to insulate the inner surfaces and vertices. An economical low-cost insulation is thereby achieved that complies with the standard for safety separation distances, withstands friction with the edges of the core, and simplifies the operations for positioning the insulating liners in place on the windings.

The pre-forming of the tubular central piece overcomes the problem of folding a semi-rigid insulating paper to obtain the edges of its rectangular outline.

At the same time, one aim of the insulation according to the invention is to keep the windings covered, especially in the area of contact-with-the edges of the bottom of the core, 4 without using end flanges at the edges of the tubular central piece, which would make it necessary to start from a highcost, rigid, moulded insulating material. To achieve the objectives described, in the solution according to the invention, the only primary insulating material used is a semi-rigid sleeve which is preferably a strip of semi-rigid paper.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective exploded view of a preferred transformer according to the invention; FIG. 2 is a perspective view of the central insulating liner of the core of the transformer of figure 1; FIG. 3 is a plan view from below of the transformer of figure 1, showing the insulation of the secondary winding; and FIG. 4 is a cross-sectional view according to line IV-IV of figure 3.

With reference to figures 1-4, a preferred transformer 1 according to the invention (fig. 1) comprises an "E"-shaped ferromagnetic core 2, a rectangular-shaped primary winding 3 insulated from the core 2 and supplied at 220V-50Hz, a rectangular-shaped secondary winding 4 insulated from the core, which has an output voltage of, for example, 250OV, an auxiliary winding 5 mounted on the secondary winding 4, a flat magnetic shunt core 6 inserted between the primary 3 and secondary 4 windings, which is also electrically insulated, and a closing yoke 21 for the "I"shaped core, which yoke presses on the windings 3 and 4 when joined to the "Ell core 2.

The core 2 has three columns 2a, 2b, 2a (fig. 1) and is made of diestamped, "E"-shaped plates with right-angled vertices and sharp edges 7. The primary 3 and secondary 4 windings have a respective central opening 31 and 41, also of a rectangular shape, for their insertion around the central column 2b of the core -during assembly of the transformer. The coil pack making up each winding 3,4 has a rectangular-shaped crosssection and, after its insertion in the core 2, the respective long sides 3a, 4a of both windings (fig. 4) fit into the openings 2c between the core columns, and the sides 4a of the secondary Winding against the bottom of the opening, the latter being subject to friction with the sharp edges 7 of the core 2.- The windings 3 and 4 are inserted on the core 2 once each of the sides 3a, 4a have been covered with a respective liner 8, 9 of semi-rigid electrical insulating paper, such as the type made with a resin binder, which is formed from a strip of paper by means of die stamping. A lamination or sheet 10 of the same insulating paper is fitted between the auxiliary winding 5 and the secondary winding 4 to maintain the required safety separation between windings. Both insulators 9 and 10 are attached to the winding 4 by means of an adhesive tape 12. The insulating liners 9 of the secondary winding 4 are individual liners for each side 3a and have a "C"-shaped cross section which covers the three outer surfaces of each wire pack, leaving the area of the central opening 41 of the winding uncovered.

For the insulation of the inner surfaces of the secondary winding 4 there is a separately mounted tubular shaped liner 11, also of insulating paper (fig. 2), which surrounds the central column 2b of the core up to a height occupied by the secondary winding 4 and the auxiliary winding 5 (f ig.4). The central- liner 11 has an outline edge that tightly conforms to the rectangular cross-section of the central column 2b of the -core, but is open, so facilitating its insertion over the central column (2b), for which purpose its two ends 11a overlap and form a completely closed rectangular shape. When the winding 4 is inserted over the 6 central column 2b, it has to rub against the central liner 11, while the latter surrounds the central column 2b tightly. The central liner 11 (fig. 2) has at its lower end a folded edge lib that is inserted between the winding 4 and the bottom 2d of the openings 2c between columns. Thus, the winding 4 is also isolated from the sharp edges 7 of the said openings 2c. The folded edge lib and the liner 9 of the long sides 4a thus overlap (fig. 3-4) covering the whole contour of the coil pack. The tubular-shaped central liner 11 is made from a strip of semirigid paper by moulding, including the folds of the rectangular contour and the fold of the edge lib, as well as the flat deformation with curvature of the vertices llc of the edge lib.

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Claims (9)

1. A high-voltage transformer for kitchen microwave ovens comprising:

- an "Ell-shaped transformer core, its three columns being of rectangular section and a core yoke closing the magnetic circuit; - at least one highvoltage secondary winding, an auxiliary winding and a primary winding, all of rectangular outline, and mounted in this order around the central column of the core and fitting tightly on it; - a tubular insulator formed from a die-stamped strip of semi-rigid material, which encircles the core central column entirely and at its tubular end has an edge folded outwards in contact with any sharp edges of the bottom of the central column.

2. The transformer of claim 1, wherein said tubular insulator is formed of a single piece from a strip of paper made with resin binder by means of moulding and deformation of said folded edge, which extends all the way round the four sides.

3. A high-voltage transformer of the type comprising an "E"-shaped core having a central column of rectangular crosssection, on which at least one secondary winding, an auxiliary winding and a primary winding, also of rectangular shape, are mounted in that order, wherein the central column of the core is covered-with a tubular insulator and the long sides of the windings are wrapped in an individual insulating liner and fitted tightly in the openings between the columns of the core, characterised in that said tubular insulator is of rectangular cross- section and is formed from a strip of semi-rigid insulating material the ends of which overlap to form a closed shape and which has at one end a folded edge in contact with the edges of the bottom of the openings of the core for the insulationof the secondary winding.

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4. A high voltage transformer for kitchen microwave ovens comprising an "E"-shaped transformer core having three columns of rectangular crosssection, at least one secondary winding, an auxiliary winding and a primary winding, all of rectangular cross-section and mounted around the central column of the core, and a tubular insulator of semi-rigid material, which insulator encircles the central column and has an outwardly folded edge at one end in contact with the bottom of the transformer core.

5. An appliance including a transformer as claimed in any one of the preceding claims.

6. An insulation kit for a transformer for a kitchen microwave oven comprising a sleeve-shaped semi-rigid liner for mounting on the central column of the transformer formed of insulating paper and having an outwardly folded edge at one end of the sleeve and one or more optional semi-rigid insulating paper liners of "C"-shaped cross-section for insulating the windings of the transformer.

7. An insulation kit as claimed in claim 6 wherein the sleeve-shaped liner is formed from a strip of insulating paper having two edges that overlap to form a closed tube.

8. A liner for insulating the central column of a high voltage transformer, substantially as hereinbefore described with reference to figures 1 to 4 of the drawings.

9. A transformer substantially as hereinbefore described with reference to figures 1 to 4 of the drawings.

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