GB2125228A - Audio transformers for safety isolation - Google Patents

Abstract

A transformer comprises a laminated core 10 having a figure-of- eight configuration, the central limb of which carries a rigid insulating bobbin 12 on which are concentrically wound the primary and secondary windings 14 and 16. The core 10, bobbin 12 and windings 14 and 16 are designed so that, in the assembled transformer, there are spaces between the outer winding and the core 10. These spaces are filled with a semi-rigid curable insulating compound 26 in order to prevent breakdown between the winding and the core 10 of the transformer which, in use, are at different potentials. The concentric windings 14 and 16 are isolated from one another by a layer of insulating tape 30. The tape used is extremely extensible so that its longitudinal margins can be stretched to extend radially outwards and over the edges of the end flanges 20 of the bobbin 12. The tape insulation 30 thus extends around the outer winding 16 without breaks or interstices. Such transformers can withstand high voltages and can, therefore, be used for providing safety isolation, using a standard bobbin in a wide range of sizes. <IMAGE>

Description

SPECIFICATION Audio transformers for safety isolation This invention relates to improvements in audio transformers, particularly those used to provide safety isolation.

It is becoming increasingly desirable to take a direct audio feed from commercial equipment to act as an input to sound mixing equipment used in broadcasting. An audio transformer is used to provide unbalanced-to-balanced conversion and to prevent hum loops which might otherwise result from the interconnection of the earths of two separate systems. It is not always possible to ensure that the commercial equipment which provides the audio feed is safe and it must be assumed that, in the worst case possible, the feed may become live, for example, at 250V a.c. mains potential. Under these circumstances, it is necessary that the audio transformer should provide a safety barrier to isolate the live feed.

The U.K. Health and Safety at Work Act, 1 974 places responsibility for safety upon employers and its requirements regarding safety in the circumstances described above may be met in one of two ways. The Class I safety mains specification requires that an additional safety earth be run to the transformer interwinding safety screen, as the programme earth (screen) lines of broadcasting equipment are not generally capable of carrying fault currents. However, the provision of a seaprate safety earth is in general impracticable.

The Class II safety mains specification requires the provision of reinforced insulation at the transformer such that it will meet the requirements of BS 3456 (part 101, 1978, 16.3), that is, such that it will withstand a test voltage of 3750V a.c. r.m.s. without breaking down.

In conventional audio transformers, primary and secondary windings surround a common ferromagnetic core. When the transformer is in use, the primary or secondary windings may be at a high potential relative to the other winding(s) or to the core. Consequently, breakdown between components at different potentials is likely unless the components are adequately insulated from one another. If the transformer is to be suitable for safety isolation purposes, the insulation must be capable of withstanding the test voltage set out in BS 3456.

The windings of such transformers are usually supported on a bobbin, which also serves to insulate the windings from the central core. In order to minimise the flux leakage, the cores used in audio transformers are of a rectilinear figure-ofeight configuration having the windings surrounding the central cross-limb. To obtain the optimum performance from the transformer, the windings should idealiy fill the spaces between the central and outer cross-limbs of the core but this brings the outermost windings into very close proximity with the core so that breakdown is likely to occur at the outer surface of the windings.

This problem has been overcome to a certain extent in existing transformers by enclosing the windings in several layers of insulating tape. A single layer of tape would provide insufficient insulation between the windings and the core and, although the insulating properties are improved by using several layers of tape, this form of insulation is relatively inefficient as pockets of air can easily be trapped between the layers and breakdown can occur around the edges of the tape. We have not found it possible using this form of insulation to provide a transformer which can withstand the test voltages required for safety isolation use.

U.K. Patent Application 2,037,087 describes a transformer which has primary and secondary windings side-by-side on a twin bobbin so that the two windings are separated by a central dividing disc or limb. The transformer is covered with a coat of resin, and the problems of using thermosetting resin are overcome by potting the transformer in a thermoplastic resin which has approximately the same thermal expansion coefficient as the thermoplastic material of the bobbin, and has a good chemical affinity therefor.

The drawings of that application show that the central dividing limb of the bobbin is not as high as the end walls of the bobbin, and thus the windings do not completely reach the opposed part of the figure-of-eight core and some of the potting resin fills the resultant space. For audio safety isolation purposes this transformer construction is not preferred as the leakage inductance is greater by a factor of typically 3 to 5 as compared witha conventional single bobbin having concentric windings and the common mode rejection ratio is inferior. The bobbin construction is also more complex.

In accordance with this invention in a first aspect there is provided an electrical transformer comprising a ferro-magnetic core having a plurality of limbs arranged to define at least one substantially closed loop, an insulating bobbin, having a tubular portion which surrounds a limb of the core and annular end walls which extend radially outwardly from each end of the tubular portion, a first coil disposed around the tubular portion and between the end walls of the bobbin and a second coil arranged concentrically around the first coil and between the end walls of the bobbin; the coils together filling a part of the space defined by the or each loop of the core; the remainder of the space defined by the or each loop being filled by a layer of curable insulating compound.

In a transformer constructed in this way, the core, bobbin and windings are designed so that a considerable gap exists between the outermost windings and the outer cross-limbs of the core.

The conventional tape insulation is omitted, although a single layer of tape may be retained to hold the windings in place, and the spaces between the core and the outer surface of the windings is filled with a curable insulating material. The insulating material is preferably introduced into the spaces under vacuum so as to avoid the formation of air pockets and produce an homogeneous layer.

The performance of the transformer is degraded to a small extent because the windings do not fill the spaces between the central and outer crosslimbs of the core though to a much lesser extent than would be the case if a split bobbin were used.

However, the insulation between the windings and the core is greatly improved, and transformers constructed in this way may be used to provide isolation.

The invention provides in another aspect an electrical transformer comprising a ferro-magnetic core having a plurality of limbs arranged to define at least one substantially closed loop, an insulating bobbin, having tubular portion which surrounds a limb of the core and annular end walls which extend radially outwardly from each end of the tubular portion, and first and second coils disposed around the tubular portion and between the end walls of the bobbin so as to fill a part of the space defined by the or each loop of the core; the remainder of the space defined by the or each loop being filled by a layer of curable insulating compound which is semi-rigid when cured to provide insulation between the windings and core while accommodating stress in the compound.

An embodiment of the invention will be described in detail, by way of example, with reference to the drawings, in which: FIGURE 1 is a side view showing the general shape of a transformer of the type being considered; FIGURE 2 is a section taken on the line lI-Il of Figure 1 in a conventional transformer; and FIGURE 3 is a corresponding section through a transformer embodying the invention.

Figures 1 and 2 show a conventional audio transformer. The transformer comprises a laminated magnetic core 10 on which is mounted a rigid plastics bobbin 12 carrying the primary and secondary windings 1 4 and 1 6.

The core 10 is of a figure-of-eight configuration having two rectangular loops joined by a common cross-limb 17. The bobbin 12 has a central tubular portion 1 8 which, in the assembled transformer, surrounds the cross-limb 1 7 of the core 10. At either end of the tubular portion 18, a generally circular flange 20 extends radially outwards. Thus, as seen ih Figure 2, the bobbin has a U-shaped half-section. The primary and secondary windings 14 and 1 6 are wound concentrically on the bobbin 12 so that the secondary winding 1 6 surrounds the primary 14. Only a single secondary winding is shown, though there may of course be more than one.

When the transformer is in use, either the primary winding 14 or the secondary winding 1 6 may be at a high potential with respect to the other winding or to the core 10. Consequently there may be breakdown between the components which are at different potentials.

Breakdown is most likely to occur between the primary and secondary windings 14 and 1 6 where they are close to one another as at A in Figure 2 and between the core 10 and secondary winding 1 6 where they are close together at the outermost part of the windings as indicated at C in Figure 2.

If the transformer is used or installed so that the secondary winding 1 6 is at the same potential as the core 10 the possibility of breakdown between the secondary 1 6 and the core 10, that is, at C, it removed. However, this constraint leads to a loss of design flexibility and the use of a transformer constrained in this way must be carefully controlled if safety requirements are to be met. A transformer without this constraint is inherently safer.

In conventional transformers the likelihood of breakdown between the winding 1 6 and the core 10, has been reduced by covering the outer surfaces with layers of insulating tape, as mentioned above. The transformer shown in Figure 3 is provided with improved insulation in these areas.

The core 10, bobbin 12 and the windings 14 and 1 6 of the transformer of Figure 3 are thus designed so that there are considerable spaces between the outer surface of the winding 1 6 and the outer cross-limbs 24 of the core 10. That is to say, the windings do not completely fill the bobbin. The conventional tape insulation around the outer surface of the secondary winding is omitted, and the spaces are filled with a curable insulating compound 26.

The transformer assembly may be enclosed within a mumetal can (not shown) and, in this instance, it is convenient to completely encapsulate the transformer assembly within the can with the insulating compound 26. The compound is introduced into the can under vacuum by means of a flexible burette. In addition to having good insulating properties the compound used must have low viscosity so that all the air inside the can will be expelled as it is filled with the compound and an homogeneous layer of insulating material 26 will be produced between the windings 14 and 1 6 and the core 10.

One suitable chemically-cured compound is General Electric silicon RTV flexible moulding compound used with Beta-one curing agent in the ratio 10:1. The use of such a semi-rigid material gives a degree of resistance to stress resulting from mechanical abuse, and can accommodate thermal stresses resulting from thermal expansion of all the transformer components with which it is in contact. No heating is required in order to cure this insulating material.

The dimensions of the layers of insulating material 26 are chosen according to the particular material used: the better the insulating properties, the thinner the layer of material needed to provide adequate insulation. The layer of insulating compound may, for example, be in the range 2-3 mm thick.

As the insulating layer 26 does not include air pockets its insulating properties are far better than those of the tape insulation used in conventional transformers and this form of insulation can be designed to withstand test voltages of the magnitude required in safety isolation applications. Where the transformer assembly is completely encapsulated within a can, the insulating compound also anchors the lead-out wires from the windings and prevents flashover from unused lead-out holes formed in the bobbin, and permits the transformer to satisfy harsher safety humidity tests.

Although the use of the insulating compound 26 prevents breakdown between the secondary windings 1 6 and the core 10 in the transformer of Figure 3 it is also necessary to provide improved insulation between the primary and secondary windings 14 and 1 6 if the transformer is to withstand high voltage sufficient to meet the requirements of B.S. 3456. In existing transformers this insulation has been provided by insulating tape placed around the primary winding 14 with its longitudinal edges slit at intervals to allow the tape to continue up the flanges 20 of the bobbin 12. However, the slitting of the edges of the tape greatly reduces the efficiency of the insulation.

This problem can be overcome by using an extremely extensible insulating tape, that is, a tape which can be stretched to at least 1.5 times its former length. A piece of tape 30 whose width is sufficient to extend across the primary winding 14 and up and over the edges of the flanges 20 of the bobbin 12 is wrapped around the primary 14 as shown in Figure 3. The elastic properties of the tape 28 enable its edges to be stretched so that it can be moulded to follow the surface of the winding 14 and continue up the flanges 20 without any need for slitting the edges. Because there are no slits, the tape 28 provides continuous insulation which has no breaks and which partially encloses the secondary windings 1 6.

Using a conventional inextensible tape with slit edges, the creepage distance in a medium-sized audio transformer was small, of the order of 0.5 mm using the stretched tape of Figure 3 the creepage distance is increased to 5 mm.

One tape which is suited to this application is BlCC/Rotunda Polyisobutylene tape, which can be stretched to three times its original length.

The transformer illustrated has the advantage that it can be constructed using standard bobbins in a wide range of sizes.

Claims (8)

1. An electrical transformer comprising a ferromagnetic core having a plurality of limbs arranged to define at least one substantially closed loop, an insulating bobbin, having a tubular portion which surrounds a limb of the core and annular end walls which extend radially outwardly from each end of the tubular portion, a first coil disposed around the tubular portion and between the end walls of the bobbin and a second coil arranged concentrically around the first coil and between the end walls of the bobbin; the coils together filling a part of the space defined by the or each loop of the core; the remainder of the space defined by the or each loop being filled by a layer of curable insulating compound.

2. A transformer according to claim 1 in which the dimensions of the layer of insulating compound are such that it can withstand voltages greater than 3750V a.c. r.m.s.

3. A transformer according to claim 2, in which the thickness of the layer of the insulating material is not less than 2 millimetres.

4. A transformer according to claim 2 in which the thickness of the layer of insulating compound is not greater than 3 millimetres.

5. A transformer according to any preceding claim including a canister which encloses the core, bobbin and coils; the space within the canister around the core, bobbin and coils being substantially completely filled with the insulating compound.

6. A transformer according to any preceding claim, in which the curable insulating compound is semi-rigid when cured.

7. An electrical transformer comprising a ferromagnetic core having a plurality of limbs arranged to define at least one substantially closed loop, an insulating bobbin, having a tubular portion which surrounds a limb of the core and annular end walls which extend radially outwardly from each end of the tubular portion, and first and second coils disposed around the tubular portion and between the end walls of the bobbin so as to fill a part of the space defined by the or each loop of the core; the remainder of the space defined by the or each loop being filled by a layer of curable insulating compound which is semi-rigid when cured to provide insulation between the windings and core while accommodating stress in the compound.

8. A transformer according to claim 6 or 7, in which the cured insulating compound is substantially less rigid than the bobbin.