GB2119175A

GB2119175A - Molded coil structure

Info

Publication number: GB2119175A
Application number: GB08211230A
Authority: GB
Inventors: Hirofumi Kubo; Toshiyuki Fujimori
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1982-04-19
Filing date: 1982-04-19
Publication date: 1983-11-09
Also published as: GB2119175B

Abstract

An economical and light-weight molded coil structure is disclosed, in which sheet-form electrical insulator layers (15', 16') containing a thermosetting resin are wrapped around the inner and outer peripheries respectively of a coil (3) to function as a casting mold, and end-enclosing resin layers (17, 18) are applied to the both end faces respectively of the coil not covered with the sheet-form insulator layers. A gas, e.g. SF6, O2. or air, may be charged into the coil and sealed therein. The insulating layers 15', 16' may be fibrous glass or polyester impregnated with epoxy resin, and the layers 17, 18 may also be epoxy resin. <IMAGE>

Description

SPECIFICATION Molded coil structure This invention relates to a molded coil structure in a resin-molded transformer.

A resin-molded transformer employing a thermosetting resin for the electrical insulation of its coils has various merits including ease of maintenance and inspection, high reliability of insulation and incombusibility, and, therefore, there is an ever-increasing demand for such a transformer. However, conventional resin-molded transformers have various defects owing to the manufacturing process in which coils are positioned within a mold, and, after casting a thermosetting resin into the mold while evacuating the interior of the mold, applying heat to set the resin under heat thereby obtaining a molded transformer structure.

Figure 1 shows one form of a prior art molded coil structure, and Figs. 2a and 2b show, by way of example, how such a molded coil structure is manufactured. For a better understanding of the present invention, the prior art manufacturing process will be described with reference to Figs. 1, 2a and 2b.

Reference numerals 1, 2 and 3 designate a completed molded coil structure, a bobbin, and a coil wound in cylindrical form around the bobbin 2, respectively. An inner shell plate 4 of cylindrical shape, an outer shell plate 5 of also cylindrical shape, an upper end plate 6 and a lower end plate 7 constitute a resin casting mold, and packings 8 maintain a gas-tight seal between the shell plates and the end plates. The coil 3 is disposed in a predetermined position within the mold of above construction by the aid of spacers 9 and is then subjected to pre-drying in that state.

Then, while evacuating the interior of the mold by a vacuum pump, a thermosetting resin 10 is cast into the mold. Subsequently, the mold is transferred into a heating furnace to set the resin 10, under heat, and, after setting the resin and allowing the mold to cool down to the atmospheric temperature, the resin block enclosing the coil 3 therein is separated from the mold to complete the molded coil structure.

However, the prior art molded coil structure manufactured by the process above described has the following defects: (a) A large-scale casting equipment including a vacuum tank, a vacuum pump and a mixing vessel is required, and, therefore, the cost of initial equipment investment is large.

(b) Many molds must be prepared to deal with a variety of kinds of coils, and many man-hours are also required for the assembling of the molds and separation of products from the molds.

(c) One bobbin is required for each of the coils since the resin is cast to mold the coil mounted on the bobbin. Further, due to the fact that the coil is molded in the state mounted on the bobbin, transfer of heat into the resin is obstructed during the step of setting of the resin under heat, and the resin is not always uniformly set, resulting in a source of occurrence of cracks in the resin.

(d) For the molding of each individual coil, one bobbin and one mold are occupied from the step of casting of the resin to the step of setting of the resin, resulting in poor efficiency of utilization of the bobbins and molds.

Further, in the molded coil structure produced by the above-described manufacturing process in which the resin is cast into the mold while evacuating the interior of the mold, the resin ought to be sufficiently uniformly impregnated between the conductors of the coil. Actually, however, voids of vacuum are formed between the conductors of the coil because the resin cast into the mold has a viscosity although it may not be so high.

Figure 3 shows such voids formed in the molded coil structure. In Fig. 3, reference numerals 10, 11, 1 2 and 1 3 designate the resin cast into the mold, the conductors of the coil 3 buried in the resin 10, layer insulators interposed between the conductor layers, and voids of vacuum formed between the conductors, respectively. Although the resin 10 is set by application of heat thereto in the state shown in Fig. 3, the internal pressure of the voids 1 3 is still maintained at a vacuum of about 1 Torr. The corona starting voltage is generally defined by the Paschen's law.According to this Paschen's law, the corona starting voltage is lowest at such a pressure level of the voids 1 3. Accordingly, corona discharge tends to occur between the conductors of different phases in the molded coil structure including voids of vacuum as described above. In order to eliminate the tendency of occurrence of corona discharge, it is required to increase the spacing between the conductors of different phases or to split the coil into a plurality of segments thereby suppressing the level of voltage acting between the conductors of different phases. Such a requirement has increased the number of molded-coil manufacturing steps and has also increased the dimensions and weight of the molded coil structure, resulting inevitably in an increased manufacturing cost.

It is therefore an object of the present invention to provide an economical and lightweight molded coil structure which can be manufactured without requiring a large-scale manufacturing equipment.

Another object of the present invention is to provide a molded coil structure of good quality which eliminates substantially the possibility of occurrence of cracks in the resin.

Still another object of the present invention is to provide a molded coil structure of small size exhibiting an excellent corona discharge suppression effect.

The molded coil structure according to the present invention is featured by the fact that sheet-form layers of an electrical insulator im impregnated with a resin are wrapped around the inner and outer peripheries respectively of a coil to provide a mold, and a resin is coated only on the end faces of the coil not covered with the sheet-form insulator layers.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings, in which: Figure 1 is a perspective view of a prior art resin-molded coil structure; Figures 2a and 2b are a partly sectional elevation view and a sectional view taken along the line A-A in Fig. 2a respectively, of the resin-molded coil structure to illustrate how the molded coil structure is produced by a prior art manufacturing process; Figure 3 is an enlarged view of part of the coil in Fig. 2a; Figure 4 is a partly sectional elevation view of an embodiment of the resin-molded coil structure of the present invention to illustrate the state after the step of winding of a coil between sheet-form electrical insulator layers;; Figure 5 is a partly sectional elevation view illustrating the step of pre-drying; Figure 6 is a partly sectional elevation view illustrating the step of applying a putty of a resin to each of the end faces of the coil not covered with the electrical insulator layers, after withdrawal of the bobbin; Figure 7 is a partly sectional elevation view of another embodiment of the molded coil structure according to the rpesent invention; and Figure 8 is a graph showing the relation between the pressure of voids in the coil and the corona starting voltage.

An embodiment of the molded coil structure according to the present invention will now be described in detail with reference to Figs. 4 to 6. In figs. 4 to 6, like reference numerals are used to designate like parts appearing in Fiefs.

2a, 2b and 3. Manufacture of a molded coil structure including a single coil will be described, by way of example.

Referring first to Fig. 4, a coil bobbin 2 is mounted on the spindle 14 of a winding machine 103. The bobbin 2 may be formed of a metal so that it may be re-used. A layer of an electrical insulator 1 5 is wrapped around the outer periphery of the bobbin 2.

This insulator layer 1 5 is such that a sheet or tape of a fibrous electical insualting material such as fibrous glass or a fibrous polyester is impregnated with a semi-set epoxy resin or like resin, and the insulator 1 5 is wrapped around the bobbin 2 mounted on the spindle 14 of the winding machine 103 after coating a mold-separating agent on the bobbin 2.

Then, while rotating the bobbin 2 mounted on the spindle 14 of the winding machine 103, conductors and layer insulators are alternately wound around the outer periphery of the insulator layer 1 5 to form a coil 3.

After the formation of the coil 3, a sheetform or tape-form layer of an electrical insulator 16, which may be the same as the insulator 1 5 above described, is wrapped around the outer periphery of the coil 3. The length L of these insulator layers 1 5 and 1 6 is selected to be larger than the length I of the coil 3 so that they protrude a short distance AL beyond the both end faces of the coil 3.

Subsequently, the assembly of the thus formed coil 3 and the insulator layers 15, 1 6 is removed together with the bobbin 2 from the spindle 14 of the winding machine 103 and is transferred into a heating furnace to be subjected to pre-drying. By the step of pre- - drying, the coil 3 is dehydrated, and, at the same time, the resin impregnated in the insulator layers 1 5 and 1 6 is completely set under heat. Fig. 5 shows the state of the assembly in the step of pre-drying.

After the step of pre-drying, the bobbin 2 is withdrawn. The bobbin 2 can be withdrawn without causing deformation of the coil 3 since, at this time, the insulator layers 1 5 and 1 6 have a sufficient rigidity and are sufficiently bonded to the coil 3 due to the complete setting of the resin impregnated therein.

Then, as shown in Fig. 6, end-enclosing putties 1 7 and 1 8 of a resin such as an epoxy resin are applied by means such as a brush to the both end faces respectively of the coil 3 whose inner and outer peripheries are enclosed by the completely-set insulator layers 15' and 16'. The surfaces of the end-enclosing resin putties 1 7 and 1 8 are finished to be smooth and flat.

The application of the end-enclosing resin putties 1 7 and 1 8 can be done in the atmospheric air. For example, this step includes applying the end-enclosing resin putty 1 7 to one of the end faces of the coil 3, transferring the assembly into a heating furnace to set the resin putty 17, inverting the assembly and applying the end-enclosing resin putty 1 8 to the other end face of the coil 3, transferring the assembly into the heating furnace again to set the resin putty 18, and allowing the assembly to gradually cool down to complete the molded coil structure. In this example, one of the end-enclosing resin putties is set first, and, then, the other is set. This manner of setting under heat is effective especially when the viscosity of the end-enclosing resin putties is considerably low or when the endenclosing resin is in liquid form instead of the putty form. Even when the end-enclosing resin in liquid form is applied in the atmospheric air, it would not permeate into the coil 3. The use of an end-enclosing resin in putty form having a high viscosity is advantageous in that, after withdrawal of the bobbin 2, the end-enclosing resin putties applied simultaneously to the both end faces of the coil 3 can be simultaneously set under heat.In this case, the period of time required for the application of the end-enclosing resin can be greatly shortened since the thickness of the resin layer is small, and the period of time required for setting the resin under heat can also be greatly shortened since heat is satisfactorily uniformly transferred into the resin to uniformly set the resin. In Figs. 5 and 6, a pair of leads 1 9 are shown leading out from one of the end faces of the coil 3.

In the molded coil structure 1 manufactured according to the present invention, the interior of the coil 3 is maintained substantially at the atmospheric pressure.

It will be understood that the above-described embodiment of the present invention is advantageous over the prior art molded coil structure in that the mold of prior art form as well as a large-scale casting equipment is unnecessary, and all the steps including the step of resin application can be performed in the atmospheric air. In addition, the embodiment is advantageous in that the utilization efficiency of the bobbin can be enhanced since the bobbin used for the winding of the coil can be withdrawn from the coil after the step off pre-drying of the assembly, so that it can be re-used. Accordingly, the number of manufacturing steps can be greatly decreased to improve the mass-productivity of the molded coil structure, and the elimination of the necessity of a large-scale manufacturing equipment can reduce the manufacturing cost of the molded coil structure.Further, because of the small thickness of the resin layers enclosing the end faces of the coil, heat can be substantially uniformly transferred into the resin in the step of setting under heat thereby uniformly setting the resin under heat, so that a molded coil structure of good quality substantially free from cracks can be produced.

Further, due to the fact that the resin does not permeate into the coil, the weight of the molded coil structure can be made lighter by the corresponding amount than the prior art one, and the ease of conveyance of the molded coil structure can attain a corresponding reduction of the production cost of the molded coil structure.

Fig. 7 shows the right-hand half only of another embodiment of the present invention.

In Fig. 7, the coil 3 shown in Figs. 4 to 6 provides a primary coil, and a secondary coil 3a is wound around the primary coil 3. In Fig.

7, the same reference numerals are used to designate the same parts appearing in Figs. 4 to 6. A sheet-form electrical insulator layer 29 is wrapped around the outer periphery of the electrical insulator layer 1 6 with a duct 25 interposed therebetween, and another sheetform electrical insulator layer 30 is wrapped around the outer periphery of the coil 3a. The material of these insulator layers 29 and 30 is the same as that of the insulator layers 1 5 and 1 6. End-enclosing resin putties 22 and 23 are applied to the both end faces respectively of the coil 3a, and pipes 24 of a heatresistive material provide means for communication between the interior and the exterior of the coils 3 and 3a.

A process for manufacturing this second embodiment of the present invention will now be described.

As described already with reference to Fig.

4, the insulator layer 1 5 is wrapped around the bobbin 2, and, then, the primary coil 3 is wound around the outer periphery of the insulator layer 1 5. In this step, the heatresistive communication pipes 24 are inserted at one end thereof into the primary coil 3 to protrude at the other end thereof to the exterior from one of the end faces of the primary coil 3, so that they act as means for communication between the interior and the exterior of the primary coil 3. Then, the secondary coil 3a is wound around the primary coil 3 with the insulator layer 16, duct 25 and insulator layer 29 interposed in the above order therebetween.In this step, the heatresistive communication pipes 24 are inserted at one end thereof into the secondary coil 3a to protrude at the other end thereof to the exterior from one of the end faces of the secondary coil 3a, so that they act as means for communication between the interior and the exterior of the secondary coil 3a, as in the case of the primary coil 3. Subsequently, as in the case of the first embodiment, the assembly is removed from the winding machine and is transferred into the heating furnace to be subjected to the step of pre-drying for dehydrating the coils 3, 3a and completely setting the insulators 15, 16, 29, 30. Then the end-enclosing resin putties 17, 1 8 and 22, 23 are applied to the end faces of the coils 3 and 3a respectively to provide the molded coil structure shown in Fig. 7.The heat-resistive communication pipes 24 may be buried in the end-enclosing resin putties in the step of application of the putties. The molded coil structure is then transferred into the heating furnace again to set the end-enclosing resin putties applied to the end faces of the coils 3 and 3a, and is then allowed to gradually cool down to the atmospheric temperature to complete the manufacture of the molded coil structure. The end-enclosing resin putties can be applied in an atmosphere of atmospheric pressure as in the case of the first embodiment. A gas exhibiting an excellent corona discharge suppression effect, that is, a gas effective for substantially inhibiting occurrence of corona discharge is charged through the pipes 24 into the coils 3 and 3a of the completed molded coil structure, and, then, the openings of the pipes 24 are closed gastight.The gas may be oxygen or sulfur hexafluoride (SF6), or it may be air at the atmospheric pressure. The gas pressure is preferably equal to or higher than the atmospheric pressure. One of the pipes 24 extending into each of the coils 3 and 3a at one of the end faces is used for charging the gas, and the other is used for discharging the gas. In the step of setting, under heat, the coil endenclosing resin putties in the furnace, the gas charged into the coils will make thermal expansion. However, the expanding portion of the gas can be vented to the exterior through the pipes 24, and no pressure is imparted to the coil end-enclosing resin putties being set under heat, so that the adverse effect such as development of cranks in the resin can be avoided.

Fig. 8 is a graph illustrating how the corona starting voltage tends to vary relative to the ambient pressure according to the Paschen's law. It will be readily seen from Fig. 8 that the corona starting voltage is low when the pressure of voids is about 0. 1 Torr to 1.0 Torr, while it becomes progressively higher as the pressure increases progressively up to the atmospheric pressure of 760 Torr, At the pressure of 760 Torr, the corona discharge is difficult to occur, and the corona discharge suppression effect is improved up to the level where it is three to four times as high as that in the former pressure range.Fig. 8 is based on the results of measurement under the condition of maintaining constant the spacing between the corona discharge electrodes, and the vertical and horizontal axes represent the corona starting voltage and the pressure or voids developing the corona discharge, respectively.

According to the second embodiment of the present invention, therefore, the corona discharge suppression effect can be greatly improved by maintaining the internal pressure of the individual coils at about the atmospheric pressure, and the voltage acting between the layers of the conductors of the coils can be set at a high level. This means that the dimensions of the individual coils can be made smaller than those of the prior art ones, provided that the capacities of the individual coils are the same as those of the prior art ones. Although a gas is externally supplied into the individual coils in the second embodiment, the end-enclosing resin may include a substance which emits the desired gas (the gas exhibiting the satisfactory corona discharge suppression effect) when the resin is heated in the step of heating or when the molded coil structure is heated in use. Further, although the coil or coils of cylindrical shape are illustrated by way of example, it is apparent that the present invention is equally effectively applicable to coils of disc shape.

Claims

1. A molded coil structure comprising a coil of cylindrical shape formed by winding turns of a conductor around a bobbin, electrical insulator layers in sheet form wrapped around the inner and outer peripheries respectively of said coil, said layers being made by impregnating a fibrous electrical insulating material with a resin, and end-enclosing resin layers covering the end faces of said coil not covered with said sheet-form insulator layers, a gas being charged and sealed in said coil.

2. A molded coil structure as claimed in Claim 1, wherein said sheet-form insulator layers are wrapped around the inner and outer peripheries respectively of said coil so as to protrude a short distance beyond the both end faces of said coil, and said end-enclosing resin layers are filled in the spaces defined between said sheet-form insulator layers and the both end faces of said coil respectively.

3. A molded coil structure as claimed in Claim 1 or 2, wherein said end-enclosing resin layers are provided by setting under heat a resin in putty form.

4. A molded coil structure as claimed in Claim 1, 2 or 3, wherein said end-enclosing resin layers include means for supplying a gas into said coil, and said gas is charged and sealed at a predetermined pressure in said coil through said gas supplying means.

5. A molded coil structure as claimed in Claim 4, wherein said gas supplying means is a pipe of a heat-resistive material.

6. A molded coil structure as claimed in Claim 4, wherein said gas supplying means is a substance which emits said gas when heated.

7. A molded coil structure constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figs. 4 to 8 of the accompanying drawings.