AN INSULATION DEVICE FOR ELECTRICAL COILS AND TRANSFORMER WINDINGS

Abstract: An insulation device for electrical coils and transformer windings with a substantially rectangular winding cross section comprises an insulation layer (5, 6) made from a foil material which after being formed to a tubular shape corresponding to the outer dimension of the coil or winding (1, 2) is deformed to a cross-sectional profile (7a) corresponding to the cross section of the winding.

Description: AN INSULATION DEVICE FOR ELECTRICAL COILS AND TRANS¬ FORMER WINDINGS. * T The invention relates to electrical coils and transformer windings with a substantially rectangular winding cross-section and comprising a layer of flex¬ ible foil extending on the internal side and the end 5 faces of the coil or winding. Such insulation devices are in particular used in transformer primary and/or secondary windings to perform an insulation barrier between the mains volt¬ age, e.g. 220V, and the user-touchable components of 10 electrical appliances, and the electrical energy autho¬ rities make specific demands on the insulation proper¬ ties for different application purposes. For the insulation between the primary and secondary windings of the transformer a creepage dist- 15 ance of at least 6 to 8 mm between any part of the primary winding and any part of the secondary winding is thus frequently required, together with a minimum thickness of the insulation barrier which in depend¬ ence on the use may vary between 0.4 and 1.0 mm. In 20 cases where an insulation barrier is composed of a number of thinner layers it is normally required that at least one layer can withstand a prescribed test voltage. To comply with said requirements the following 25 methods have traditionally been practised. 1. Arranging the windings in coaxial alignment, e.g. by using coil forms divided into compartments. This method results in a too high stray inductance and is not applicable in connection with high-frequency 30 pulse transformers (switch-mode transformers). 2. Coaxially placing the windings one over the other with an intermediary three-layer insulation, the necessary creepage distance being obtained by winding on a smaller width and completing with edging tape or plastic blocks and by use of insulated supply lines. Due to the unfavourable ratio between effective copper cross-section and insulation cross-section this method is in particular inappropriate in smaller transformers, in which a high stray inductance also arises. 3. Wrapping or bandaging the individual windings in a three-layer tape. In particular when using round field spools this method is troublesome and gives rise to a vast productional unreliability since, due to the incomplete overlapping of the taping, considerable local variations or holes may occur in the insulation thickness. 4. Using special coil forms allotted to a con- fined concrete product application or a corresponding particular design of the coil structure supplemented with encapsulation technique. From an economic point of view such methods are merely relevant in connection with series productions on a larger scale that may justify the comparatively high costs of tools etc. From GB 2 125 227 A and GB 2 125 228 A it is known to insulate concentric primary and secondary windings in a transformer from one another by means of a layer of extremely stretchable insulation tape which after taping the primary winding placed in a coil form is wound on its external side and is stretched so that its longitudinal edges are made to cover the end walls of the coil form up over and around their peripheral edges. The stretching of the tape entails, however, an uncontrollable reduction of the thickness of the insu¬ lation layer and the method is not appropriate for coil forms with a non-circular cross-section, e.g. square forms or coil forms with terminal blocks. This method further necessitates a completing embedment. On this background it is the object of the in¬ vention to provide a new design of an insulation device of the type concerned which may be used both in core- less coils and transformers independent of the trans¬ former type and which without encapsulation may also be used with and without the use of a coil form while pre- serving a favourable ratio between the effective copper area and the insulated winding cross-section and with an effective control to meet the requirements stipul¬ ated by authorties. With a view to obtain this, an insulation device according to the invention is characterized in that the insulation layer is produced from a foil material which after being shaped to a tubular form corresponding to the external dimension of the actual winding is deformed to a substantially U-shaped cross-sectional profile corresponding to the winding cross-section. The U-shaped cross-sectional profile of the insulation device has according to an embodiment of the invention end portions with such a width that they project so much beyond the intended winding, thickness that they by the deformation effected after the winding may be bent in over the external side of the coil or the winding. Insulation devices produced according to the invention may in principle be used both for coreless coils and for any winding on a transformer of the type concerned, i.e. the primary winding or the secondary winding alone or both of them, and it may as well be used for each of the primary or secondary windings in a transformer with a number of primary and/or secondary windings. The obtained insulation barrier is appropriate in connection with windings disposed in standard as well as in special coil forms but may further be used with no coil form in that the winding is effected on a special mandrel and is supported by the encircling insulation barrier. With a composition of the insulation material determined in consideration of the actually valid requirements stipulated by the authorities a high degree of security is obtained as to permanently complying with demands on creepage distances and mini¬ mum insulation thickness. The main advantage obtained by the invention is in particular an improved distribution between effect¬ ive copper area and insulated winding cross-section, which offers the possibility of reducing the trans¬ former dimensions or of lower operating temperatures. Owing to the possibility of increasing the effective coil width an improved coupling between the windings is obtained as a substantial advantage for the trans¬ former, which is in particular of importance as regards high-frequency pulse transformers of the switch-mode type. The invention further provides for a production¬ al relief because it is possible to a higher degree to avoid insulated supply lines through sufficient encap¬ sulation of the individual windings. In an embodiment of the invention appropriate for practical purposes, the insulation material con¬ sists of a shrinkable material and the deformation to said U-shaped profile is effected by shrinking under thermal stress or other physical stress. Such shrink- able material may be available in the form of an extruded tubular blank (shrink-flex) or in the form of tape. The invention will now be explained in detail with reference to the schemtical drawings, in which Fig. 1 as an embodiment of the invention illus¬ trates a transformer with concentric primary and secon¬ dary windings, Figs 2 to 4 illustrate the manufacture of the insulation device according to the invention, Figs 5 and 6 illustrate the forming of an insul¬ ation layer in a tubular form in connection with a cir¬ cular and a square coil form, and Figs 7 to 11 show various examples of coil and transformer windings with an insulation device accord¬ ing to the invention. In the transformer illustrated in Fig. 1 the primary winding 1 and the secondary winding 2 are disposed coaxially one over the other in a common coil form 3 so that the two windings in their full length overlap each other. The coil form 3 surrounds on its side an iron core 4. Each of the windings 1 and 2 consists in a known manner of a considerable number of turns of lacquered copper wire wound in such a manner that each of the windings is imparted a mainly rectangular wind¬ ing or copper cross-section. In the illustrated example the primary winding 1 as well as the secondary winding 2 are provided with an insulation device according to the invention in the form of insulating layers 5 and 6 completely covering in the illustrated Example the internal side of each winding facing the core 4, and the end faces of the winding, and are further bent in over and cover at least part of the external side of the winding. As it appears from Figs 2 to 4 the insulation layer is obtained in that a layer of foil material is firstly shaped to a tubular form 7 corresponding to the outer dimension of the end walls 8 of the coil form 3 or, if no coil form is used, the outer dimen¬ sion of the coil itself or the winding. The foil layer 7 shaped to a tubular form is subsequently deformed to a substantially U-shaped cross-sectional profile 7a as illustrated in Fig. 3, corresponding to the coil form 3 or, if no coil form is used, to the winding cross-section of the coil. The U-shaped cross-sectional profile is provided with end portions 7b, 7c with such a radial width that they project so much beyond the intended winding width that after winding they may be bent in over the external side of the coil or winding and cover a substantial part of or possibly the entire of this side. Producing the insulation layer with the tubular form 7 prior to deforming it to the U-shaped cross- sectional profile may be effected by cutting from a blank extruded in a tubular form a suitable insulation material or, as illustrated in Figs 5 and 6, by any other method that is more appropriate for a practical production in that the coil form 10, 10' is wound with a webshaped insulation material 9, 9' to constitute the tubular form 7. In an embodiment appropriate in practical use the insulation material is a shrinkable material and the deformation to the U-shaped cross-sectional profile is effected by a shrinking process, preferably by controlled thermal stress. As illustrated in Figs 7 to 11, there is by the use of insulation devices according to the invention in connection with transformers as illustrated in Fig. 1, a considerably free choice as regards designing and arranging the insulation device. In the embodiment illustrated in Fig. 7 which corresponds to the embodiment illustrated in Fig. 1, insulation devices 11 and 12 are thus arranged both around the primary winding 13 and the secondary wind¬ ing 14 in such a manner that they cover the internal sides and end faces of both windings and further cover part of the external sides of the windings. Figs 8 and 9 show designs in which only the primary winding 13' and the secondary winding 14', respectively, have an insulation device 11' and 12', respectively, designed in the same manner as in Fig. 7. Fig. 10 illustrates an embodiment in which an insulation device 15 around a primary winding 16 completely encapsulates the winding also on its external side. Finally, Fig. 11 illustrates an insulation device 17 encapsulating a single coil winding 18. Figs 12 and 13 illustrate alternative embodi¬ ments forming for each of the illustrated primary and secondary windings 19, 19' and 20, 20', respective¬ ly, two separate insulation layers each from their respective foil material shaped to a tubular form as described in the foregoing and afterwards deformed to a substantially L-shaped cross-section covering part of the internal side of the winding and one end face thereof and of which an end portion is folded in over part of the external side of the winding. in the embodiment in Fig. 12 there are in this manner for each of the windings 19 and 20 provided two separate insulation devices 21, 22 and 23, 24, respectively, covering only end portions of the wind¬ ings whereas, in the embodiment in Fig. 13 there are for each of the windings 10' and 20' provided two separate insulation devices 25, 26 and 27, 28 overlapping each other on the internal side of the winding.

Claims: P A T E N T C L A I M S 1. An insulation device for electrical coils and transformer windings with a substantially rectangular winding cross-section and comprising a layer of flex¬ ible foil extending on the internal side and the end faces of the coil or winding, characterized in that the insulation layer (5, 6) is produced from a foil materi¬ al which after being shaped to a tubular form (7) corresponding to the external dimension of the actual winding (1, 2) is deformed to a cross-sectional profile (7a) corresponding to the inner side and the end faces of the winding cross-section. 2. An insulation device according to claim 1, characterized in that said cross-sectional profile is substantially U-shaped to provide an insulation layer covering the internal side and end faces of the wind¬ ing. 3. An insulation device according to claim 1, characterized in that two insulation layers formed to U-shape are deformed each to their substantially L- shaped cross-sectional profile to constitute insulation layers covering individually part of the internal side and one end surface of the winding. 4. An insulation device according to claims 1, 2 or 3, characterized in that said cross-sectional pro- file (7a) extends so much beyond the intended winding thickness that, by a deformation effected after taping, they may be bent in over the external side of the coil or the winding (1, 2). 5. An insulation device according to one of the preceding claims, in which the coil or winding are dis¬ posed in a common coil form (3) encapsulating a ferro¬ magnetic core (4) , characterized in that said cross- sectional profile (7a) is adapted to the free winding cross-section of the coil form (3) . 6. An insulation device according to one of the preceding claims, characterized in that the insulation material in its basic condition prior to the deforma¬ tion to said cross-sectional profile is formed as an extruded blank. 7. An insulation device according to one of the preceding claims, characterized in that the insulation material is a shrinkable material and that the deforma¬ tion to said cross-sectional profile is effected by shrinking. 8. An insulation device according to any of claims 1 to 6, characterized in that the forming to the tubular shape (7) corresponding to the outer dimension of the actual winding is effected by flexible expansion of a tubular blank having a basic cross-section corres¬ ponding to the inner diameter of the winding, following which said cross-sectional profile is formed by revers¬ ible, flexible deformation back to the basic condition. 9. An insulation device according to one or more of the preceding claims, characterized in that the insulation layer in its basic condition is formed with the prescribed thickness from a series of thinner layers.

Inventors: CHRISTENSEN NIELS OVERGAARD, CHRISTENSEN, NIELS, OVERGAARD
Applicants: FLUX TRANSFORMERTEKNIK AS, FLUX TRANSFORMERTEKNIK A/S
Application year: 1993
Main classification: H01F41
IPC codes: H01F27, H01F41