JPH0150090B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a molded transformer with a shield that is safe even if a human body comes into contact with the surface of the molded coil during energization.

So-called molded transformers, in which the entire coil is covered with an insulating resin layer, have excellent insulation properties, are easy to maintain, and have no risk of secondary damage in the event of a fire, so they are used in power distribution where reliability and disaster prevention are important. They are rapidly becoming popular in power grids, but unlike oil-immersed transformers, there is a high possibility that the human body will come into contact with the surface of the molded coil, which is exposed to the air while power is being applied.

FIG. 1 is a diagram showing an example of a molded transformer used in a power distribution system, where 1 is an iron core and 2 is a molded coil combined with the iron core. FIG. 2 is a perspective view of a conventional molded coil, and FIG. 3 is a sectional view taken along the line A-A'. As shown in this figure, the molded coil 2 is constructed by covering a bare coil 3 with an insulating resin layer 4. , line terminal 5, and tap terminal 6 are completely insulated. However, in such a conventional molded transformer, as shown in the equivalent circuit in Fig. 4, in addition to the capacitance C 1 due to the insulating resin layer 4 between the coil conductor and the ground, there is also a capacitance C ₁ between the coil conductor and the ground. There is a capacitance C ₂ between earth. Therefore, when the molded transformer is energized, the surface of the resin layer has a fairly high potential, and if a human body touches the surface of the resin layer in this state, there is a risk of electric shock. Conventionally, as a countermeasure, danger markings have been placed on the surface of the molded coil, but this cannot be said to be a foolproof measure.

It was already proposed in Publication of Utility Model Publication No. 40-9227 to provide a grounded shield layer using metal foil, wire mesh, conductive paper, conductive paint, etc. on the surface of the molded coil of a high voltage transformer to prevent electric shock. There are, but
In order to apply such a shield to a general mode transformer, there are several issues that need to be resolved, such as the adhesion of the film used as the shield layer with the resin layer, the long-term stability and reliability of the film, and the ease of film formation. There are many problems.

Therefore, an object of the present invention is to form a shield layer on the surface of a molded coil that has good adhesion with the resin layer and is stable and highly reliable over a long period of time, thereby ensuring the safety of the molded transformer. be.

In order to achieve the above object, the present invention provides a grounded shield layer made of a sprayed aluminum or zinc coating on the surface of the resin layer of the molded coil, except for the area around the terminal, and the thickness of the coating is increased from 20 to 20 mm.
The thickness was selected to be 200 μm.

Thermal spraying of aluminum or zinc is an established technology that can easily form a dense, chemically stable metal film on the resin layer surface of molded coils used in transformers of various capacities, making it possible to control the film thickness. is also free.

When a metal is thermally sprayed onto the surface of a resin layer, one of the causes of a decrease in the adhesion of the thermally sprayed coating is the difference in the coefficient of linear expansion between the thermally sprayed coating and the resin layer ^. This problem can be solved by using a casting resin (epoxy resin) that has a coefficient of linear expansion close to that of zinc (3.3×10 ^-5 /℃) or zinc (linear expansion rate of ^3.3 ×10 -5 /℃).

The present inventors have focused on the fact that the thickness of the coating is an important factor in improving and stabilizing the adhesion of the above-mentioned metal coating by thermal spraying, and conducted experiments with various coating thicknesses. As a result, we found the optimal range of film thickness. In other words, both aluminum and zinc form a stable oxide film on their surfaces and are known to have good corrosion resistance, but if the film is thinner than 20 μm, most of the film will oxidize while the transformer is left unused or in operation. The electrical resistance increases, the surface potential increases, and the shielding effect disappears. On the other hand, in the process of cooling the film immediately after thermal spraying, the surface of the film in contact with the air drops in temperature faster, and the surface in contact with the resin layer drops in temperature slower. Stress that tends to separate from the resin layer is generated inside. This tendency becomes more pronounced as the film becomes thicker,
If the thickness is 200 μm or more, the adhesive strength decreases rapidly and it becomes easy to peel off. For the above reasons, the film thickness should be 20~
A suitable thickness is 200 μm.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a perspective view of a molded coil 2' according to the present invention, FIG. 6 is a cross-sectional view taken along line B-B', and FIG. 7 is an equivalent circuit diagram. Parts corresponding to those in FIGS. and show.

The surface of the insulating resin layer 4 covering the bare coil 3 of the molded coil 2', except for the areas around the line terminals 5 and tap terminals 6, is sprayed with aluminum or zinc to form a shield consisting of a sprayed coating with a thickness of 20 to 200 μm. A layer 7 (shown with diagonal lines in FIG. 5) is formed. The thermal spraying method may be either arc or gas combustion method. Thermal spray coating may be applied to the entire surface of the molded coil except for the terminals, but spraying can be omitted near the center of the inner circumference of the molded coil, which is out of reach of human hands during use. Workability is also good.

The molded coil 2' provided with the shield layer 7 is combined with the iron core 1 in the same manner as shown in FIG. 1 to constitute a molded transformer, and the ground terminal 8 provided on the shield layer 7 is connected to the earth wire. Therefore, even if a human body touches the surface of this shielded molded coil during energization, there will be no electric shock. That is, as shown in FIG. 7, the ground capacitance (C ₂ in FIG. 4) on the surface of the molded coil disappears, and the ground side potential of the capacitance C ₁ due to the insulating resin layer 4 becomes zero potential. Electric shock at the terminals can be prevented by covering the terminals of the molded coil 2', which cannot be shielded, with another protective cover.

Figure 8 shows a resin made of epoxy resin (for example, Hitachi's New MT Resin, linear expansion coefficient 26×10 ^-5 /℃, thermal conductivity 0.3kcal/mh℃) used as a casting material for molded coils. The results are shown in which the adhesion strength was measured by thermally spraying aluminum and zinc onto the surface of the layer and varying the thickness of the coating. Here, the adhesive strength was expressed as the tensile force per unit area when a tensile force was applied to another specimen adhered to the surface of the thermal sprayed coating and the coating was peeled off. As seen in this figure, both aluminum and zinc showed almost the same tendency, and it was confirmed that stable adhesive strength could be obtained in the film thickness range of 20 to 200 μm.

Figure 9 is an explanatory diagram of the peeling phenomenon of the thermal sprayed coating that occurs when the coating is thickened, where a is the state immediately after thermal spraying, and b
shows the state after cooling, where peeling of the film occurred due to the difference in shrinkage speed on both sides of the film.

As explained above, according to the present invention, the surface of the resin layer of the molded coil is made of a thermal sprayed coating of aluminum or zinc that has good adhesion to the resin layer and is stable and reliable over a long period of time, except for the area around the terminal portion. Since a grounding shield layer is provided, there is no risk of electric shock even if a human body comes into contact with the surface of this shielded molded coil during energization, and to easily provide molded transformers of various capacities that ensure safety. Can be done.

[Brief explanation of drawings]

Figures 1a and b are front and side views showing an example of a molded transformer, Figure 2 is a perspective view of a conventional molded coil, Figure 3 is its A-A' cross-sectional view, and Figure 4 is a cross-sectional view of the conventional molded coil.
Figure 5 is a perspective view of the molded coil according to the present invention, Figure 6 is a sectional view taken along line B-B', Figure 7 is an equivalent circuit diagram, and Figure 8 is the thickness of the sprayed coating. A diagram showing the relationship between and adhesive strength, Figure 9 a, b
FIG. 2 is an explanatory diagram of a peeling phenomenon of a thermally sprayed coating. 1: Iron core, 2: Molded coil without shield, 2': Molded coil with shield, 3: Plain coil, 4: Insulating resin layer, 5: Line terminal, 6:
Tap terminal, 7: Shield layer, 8: Ground terminal.

Claims (1)

[Claims] 1 In a molded transformer in which a molded coil covered with an insulating resin layer is combined with an iron core, the insulating resin layer having a linear expansion coefficient of 2.3×10 ^-5 /℃ to 3.3× ^{10 -5} /℃ and the periphery of the terminal part are a grounding shield layer made of a sprayed aluminum or zinc coating with a thickness of 20 to 200 μm formed on the surface of the resin layer excluding the resin layer.