GB2065383A - Transformers and windings therefor - Google Patents

Abstract

The structure of a transformer is described in which its windings are formed on a bobbin (6) having end-cheeks (8). That winding (9) constituting the most serious hazard by reason of its working voltage is formed first on the bobbin, with its connecting leads passing directly out through apertures (12, 14) in the end-cheeks. A conductive screening layer (15) is laid over the first winding and insulating barriers (18) are formed over the screen layer adjacent the end cheeks. The remaining windings (10) are formed between the barriers and a protective insulating layer (26) encloses the completed windings. Each of the connection leads (20, 24) from the later winding within the barriers are enclosed in an insulating sleeve (20, 23, 24), the sleeve passing into the interior of the bobbin through an aperture (21, 22) in a side cheek of the bobbin and overlying the barrier adjacent that side cheek. <IMAGE>

Description

SPECIFICATION Improvements in or relating to transformers and their construction The present invention relates to transformers and methods of construction thereof, and has particular relevance to smail power transformers.

It has previously been proposed to use bobbins to contain the windings of transformers, the wound bobbins being subsequently placed upon a limb of magnetic material which is associated, usually, with a further magnetic structure to form a complete magnetic circuit about the bobbin. Such a magnetic structure was formerly built up from appropriately shaped laminations, stamped, for example, from a magnetic sheet. More modern forms of transformer, typically for use in association with electronic apparatus have used preformed core units, often moulded from. a sintered magnetic material. In winding the bobbins for such transformers it has been usual to use a conductive wire, usually of copper, which is coated with an insulating layer. With the introduction of miniaturised electronic components for use in such apparatus, the need for similarly miniaturised transformers has become apparent.However, one practical difficulty resulting from the need to make such transformers even smaller has been that although the power and voltage requirements of much of the circuitry of such apparatus has diminished, the input voltage, for example, from which such apparatus is driven has remained substantially the same. In other cases output voltages are required to be maintained at values approaching their former levels. Thus, it has been necessary to cramp conductors carrying relatively high voltages into ever smaller spaces and this has led to additional risks of electrical insulation breakdown.Moreover, for example, where such breakdown takes place between primary and secondary windings, of, say, a mains supply transformer, hazardous voltages are caused to appear on the secondary windings and this also frequently involves consequential damage to components connected to these windings.

Because the relative values of voltage in the different windings will depend upon the respective purposes to which different transformers are put, it is preferred to use the term "most serious hazard" in respect of that winding associated with the greatest value voltage in any given transformer.

In accordance with one aspect of the present invention, a transformer has a winding assembly including a first winding intended in use to present the most serious hazard wound as an inner winding, connection for the first winding each being taken from an end of the winding; a screen layer laid over the first winding; a pair of spaced apart insulating barriers formed over the screen layer; and a second winding wound over the screen layer between the barriers, connections for the second winding each being passed through an insulating sleeve overlying a barrier.

In accordance with another aspect of the invention, a winding assembly for a transformer includes a bobbin comprising an elongate hollow body intermediate a pair of endcheeks; a first winding formed as a first coil from insulated conductive wire wound upon the length of the body between the endcheeks, each external connection from the first coil being taken out of the bobbin through an aperture in one of the end-cheeks; a conductive screen formed by a layer of conductive material laid over the first coil, the screen being provided with a connecting lead passing out of the bobbin through an aperture in an end cheek; an insulating barrier formed at each end of the bobbin within the endcheeks and overlying the screen; and a second coil formed from insulated conductive wire wound upon the conductive screen between the barriers, external connections from the second coil each consisting of a conductive wire contained within an insulating sleeve, the insulating sleeve passing through an aperture in an end-cheek of the bobbin and overlying the barrier adjacent that end-cheek through which it passes.

A transformer and its winding assembly will now be described, by way of example, with reference to the accompanying drawing, in which, Figure 1 shows a partially assembled transformer, and Figure 2 is a sectional view of a wound bobbin for use with the transformer of Fig. 1.

As shown in Fig. 1 a transformer 1 has a pair of magnetic members 2 and 3. The members 2 and 3 are moulded from magnetic material and each has a central boss (not shown) and a pair of walls 4 projecting from an end member 5. The members 2 and 3 are used in association with a bobbin 6 which carries windings as will be explained. The arrangement is conventional and is such that the bobbin 6 is held on the bosses of both members 2 and 3 while the walls 4 of members 2, 3 enclose the bobbin 6 to form a complete magnetic circuit linking the windings on the bobbin 6. The members 3 and 4 are held together by clamps 30.

The arrangement of the windings on the bobbin 6 is shown in more detail in Fig. 2, from which it will be seen that the bobbin 6 has a hollow central body 7 and a pair of endcheeks 8 to form a spool on which windings are formed in a conventional coil-winding operation.

As shown, two windings 9 and 10 are provided on the bobbin 6. The first winding 9 is that which, when the transformer is in use, will constitute the most serious hazard. In the present example the winding 9 is formed as an untapped winding in which one end 11 of the wire to be used in its formation is passed outwards through an aperture 1 2 in one endcheek 8. The winding 9 is then formed by spinning the bobbin 6 while the wire is formed layer on layer to build up the requisite number of turns about the body 7, the end of the winding 1 3 so formed then being fed through a second aperture 14 in the endcheek 8.

A conductive screen 1 5 is formed about the winding 9 by wrapping a foil of copper round the completed winding 9 between the endcheeks 8, and a connection 1 6 is provided from the screen layer 1 5 through an aperture 1 7 in one of the end-cheeks 8 to permit the screen 1 5 to be connected to earth potential.

Before the formation of the second winding 10, barriers 1 8 are formed overlying the screen layer 1 5 at each end, adjacent the side-cheeks 8. The barriers 1 8 are conveniently built up by winding an insulating tape at each end of the screen layer 1 5 until the barrier 1 7 is formed to the desired depth.

The second winding 10 is then formed to overlie the screen layer 1 5 between the barriers 1 8. As a preliminary to the actual formation of the winding 10, the first end of the wire to be used in its formation is enclosed in an insulating sleeve 1 9 which, together with the enclosed wire 20, is passed outwards through an aperture 21 in one end-cheek 8 to form a termination for the winding. The sleeve 1 9 is sufficiently long to pass over the barrier 1 8 adjacent that end-cheek 8 in which the aperture 21 is formed, into the space between the barriers 1 8 in which the winding 10 is to be made.The second winding 10 is then formed in conventional manner. in the example shown, this winding 10 is tapped and the tappings are led out from the winding 10 each through an aperture, such as the aperture 22, in an end-cheek 8. In all cases, such tarlpings are enclosed in insulating sleeve, such as sleeve 23, which extend inwardiy from the end-cheek completeiy over the adjacent barrier 18. In a similar manner, the terminal lead 24 from the winding 10 is also enclosed in an insulating sleeve 25 passing over a barrier 1 8 and thence through an aperture 21 in an end-cheek 8.

Once the windings 9 and 10 have been completed, a protective insulating layer 26 is wound over the entire winding configuration between the end-cheeks 8. Thus, in the completely wound bobbin 6, the winding 10 is surrounded by a protective shield within the bobbin 6, comprising walls formed by the barriers 18, which trap the base layer formed by the screen 1 5 at each end, the outer surface of the winding 10 being enclosed by the insulating layer 26, which again extends over the barriers 1 8. The necessary terminations for the winding 10 are protected by the insulating sleeves 19, 23 and 25 which extend through the end-cheeks 8 and over the barriers 1 8 into the enclosure of the winding 10.

It will be realised that in practice the leads from the bobbin 6 are terminated at suitable connectors and, in practice, such terminations are carried by, for example, a board of insulating material (not shown) which may be secured to the completed transformer by means of a bolt (not shown) passing through the board and a hole 27 formed through the pot core 3, 4, as indicated in Fig. 1.

It is also to be understood that the winding assembly may be formed upon a hollow core without end cheeks. In such a case temporary, supports may be provided during the actual winding operation and then withdrawn after winding is completed. This form of winding is applicable where, for example, the winding assembly is encapsulated in, say, a synthetic resin so that the final assembly is self-supporting. In such a case the various connections described above will actually pass out of the winding assembly at each end, as otherwise described.

Claims (10)

1. A transformer having a winding assembly including a first winding intended in use to present the most serious hazard wound as an inner winding, connections for the first winding each being taken from an end of the winding; a screen layer laid over the first winding; a pair of spaced-apart insulating barriers formed over the screen layer; and a second winding wound over the screen layer between the barriers, connections for the second winding each being passed through an insulating sleeve overlying a barrier.

2. A transformer as claimed in Claim 1, in which the winding assembly includes a bobbin having a hollow central body carrying endcheeks, the first winding being formed over the body of the bobbin between the endcheeks thereof, the connections for the first winding each being taken directly from the winding through an aperture in an end cheek.,

3. A transformer as claimed in Claim 2, in which the insulating barriers are formed adjacent the end cheeks of the bobbin respectively.

4. A transformer as claimed in any of Claims 1 to 3, in which the barriers are formed by winding overlying layers of an insulating tape.

5. A transformer as claimed in any preceding claim, in which the screen layer is conductive.

6. A transformer as claimed in any preceding claim in which a protective layer is formed over the winding assembly.

7. A winding assembly for a transformer, including a bobbin comprising an elongate hollow body intermediate a pair of end cheeks; a first winding formed as a first coil from insulated conductive wire wound upon the length of body between the end-cheeks, each external connection from the first coil being taken out of the bobbin through an aperture in one of the end-cheeks; a conductive screen formed by a layer of conductive material laid over the first coil, the screen being provided with a connecting lead passing out of the bobbin through an aperture in an end-cheek; an insulating barrier formed at each end of the bobbin within the end-cheeks and overlying the screen; and a second coil formed from insulated conductive wire wound upon the conductive screen between the barriers, external connections from the second coil each consisting of a conductive wire contained within an insulating sleeve passing through an aperture in an end-cheek of the bobbin and overlying the barrier adjacent that end-cheek through which it passes.

8. A winding assembly as claimed in Claim 7 in which the barriers are formed by winding an insulating tape in a succession of overlying turns.

9. A transformer having a winding assembly substantially as described with reference to the accompanying drawing.

10. A winding assembly substantially as described with reference to Fig. 2 of the accompanying drawing.