GB2125227A - Transformer insulation - 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 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. 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. 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 Transformer insulation This invention relates to improvements in electrical transformers, and particularly in audio transformers 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, 1974 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 broadcastiny equipment are not generally capable of carrying fault currents. However, the provision of a separate safety earth is in general impracticable.

The Class li 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.

The primary and secondary windings may be positioned elther side by side along the length of the bobbin, or concentricelly so that the secondary surrounds the primary. The latter arrangement Is 'efrred Wince It reduces the flux leakage and rovos p tho the efflcloncy of the transformer giving batter frequency response and common mode rejection ratlo, However, it is more difficult to provide adequate insulation between the windings when the windings are arranged concentrically. In transformers where the windings are side-by-side, the insulating bobbin on which the windings are arranged can be shaped so that it forms a barrier which separates and isolates the two sets of windings from each other.Where the windings are arranged concentrically, it is not possible to adapt a conventional bobbin so that it forms an insulating barrier to separate the windings.

In existing transformers with concentric windings, insulating tape is wrapped over the inner winding to provide insulation between the windings. In order to increase the creepage distance, that is, the shortest distance in air over the insulation between conducting components, the tape used has been sufficiently broad that the edges of the tape extend up the end walls of the bobbin to partly enclose the outer windings.

However, because the circumference of the end walls increases with increasing radius, it is necessary to slit the edges of the tape. This produces gaps in the insulation and random gaps persist even when several layers or tape are used.

In accordance with this invention, there is provided an electrical transiormer comprising a ferromagnetic core, an insulating bobbin having a tubular portion which surrounds the core and annular end walls which extend radially outwardly from each end of the tubular portion, and first and second coils disposed concentrically around the tubular portion and between the end walls of the bobbin; the first and second coils being separated by one or more layers of extensible insulating tape, each layer of tape extending around the circumferential surface of the inner coil and radially outwardly of the circumferential edge of at least one end wall of the bobbin sathat the tape forms a continuous layer of insulating material over the surface of the inner coil and the said end wall or walls.

By using such extensible tape, the need to slit the edges is avoided and adequate insulation can be provided using a single layer of tape.

Furthermore, by using extensible tape in this way, it is possible to make a transformer in which the insulation between the windings will withstand 3750V a.c. r.m.s., namely the test voltage of BS 3456.

It is, of course, also necessary to provide adequate insulation between the outermost winding and the core. Preferably, the first and second coils are 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.

An embodiment of the invention will be described in detai;, 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 Il-Il of Figure 1 in a conventional transformer; Figure 3 is a corresponding section through a transformer embodying the invention; Figure 4 is a part-sectional perspective view of the bobbin in the transformer of Figure 3 with its outer windings and insulating tape partly removed; and Figure 5 is a section through a partiallyfabricated modified transformer in accordance with 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 in 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 1 2 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 16 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.

In order to meet the requirements of BS 3456 it is necessary inter alia to provide improved insulation between the primary and secondary windings 14 and 16. 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, as explained above, the slitting of the edges of the tape greatly reduces the efficiency of the insulation.

In accordance with this invention, 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 Figures 3 and 4. 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 as shown in Figure 4 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 winding 16.

Using a conventional inextensible tape with slit edges, the creepage distance in a medium-sized audio transformer was small, of the order of 0.5mm. Using the stretched tape of Figures 3 and 4 the creepage distance is increased to 5mm.

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

To make applying the tape 30 easier, it may be convenient in some instances to use two separate layers of tape 30, one of which extends across the primary winding 14 and up and over the edge of one flange 20 and the other of which extends across the primary winding 14 and up and over the edge of the other flange 20 of the bobbin 12, as shown in Figure 5.

Transformers incorporating the tape insulation described in relation to Figures 3 to 5 can be constructed so that the insulation between the primary and secondary windings is capable of withstanding the test voltage of 3750V a.c. r.m.s.

prescribed in BS 3456 (part 101, 1978, 16.3).

If the transformer is used or installed so that the secondary winding 16 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, is 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, and so it is necessary to provide adequate insulation between the outermost secondary winding 16 and the core 10.

In conventional transformers the likelihood of breakdown between the winding 1 6 and the core 10 has been reduced by covering the outer surfaces of the winding with layers of insulating tape. A single layer of tape would provide insufficient insulation between the winding and the core and, although the insulating properties are improved by using several layers of tape, this form of insulation is reiatively 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.

The core 10, bobbin 12 and 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. 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 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 compound is General Electric silicon RTV flexible moulding compound used with Betaone curing agent in the ratio 10:1. The use of such a semi-rigid material gives a degree of resistance to stress resulting from thermal expansion of all transformer components with which it is in contact. No heating is required in order to cure this insulating material.

The dimensions of the layer 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 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.

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

Claims (13)

1. An electrical transformer comprising a ferromagnetic core, an insulating bobbin having a tubular portion which surrounds the core and annular end walls which extend radially outwardly from each end of the tubular portion, and first and second coils disposed concentrically around the tubular portion and between the end walls of the bobbin; the first and second coils being separated by one or more layers of extensible insulating tape, each layer of tape extending around the circumferential surface of the innercoil and radially outwardly to the circumferential edge of at least one end wall of the bobbin so that the tape forms a continuous layer of insulating material overthe surface of the inner coil and the said end wall or walls.

2. A transformer according to claim 1 , in which the coils are separated by a single layer of tape which extends over the circumferential surface of the inner coil and radially outwardly to the circumferential edges of both end walls of the bobbin.

3. A transformer according to claim 1 or 2, in which the tape can be extended to at least 1.5 times its unextended length.

4. A transformer according to any preceding claim in which the tape is polyisobutylene tape.

5. An electrical transformer according to any preceding claim in which the core has a plurality of limbs arranged to define at least one substantially closed loop and in which the tubular portion of the bobbin surrounds a limb of the core; the first and second coils being 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.

6. A transformer according to claim 5 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.

7. A transformer according to claim 6, in which the thickness of the layer of insulating material is not less than 2 millimetres.

8. A transformer according to claim 6, in which the thickness of the layer of insulating compound is not greater than 3 millimetres.

9. A transformer according to any of claims 5 to 8, 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.

10. A transformer according to any of claims 5 to 9, in which the curable insulating compound is semi-rigid when cured.

11. A transformer according to claim 10, in which the cured insulating compound is substantially less rigid than the bobbin.

12. An electrical transformer substantially as hereinbefore described with reference to Figures 3 and 4 of the drawings.

13. An electrical transformer substantially as hereinbefore described with reference to Figure 5 of the drawings.