KR20230063544A - Method for manufacturing magnetic core for transformer, air gap composition for the magnetic core and the magnetic core with air gap - Google Patents

Abstract

The present invention comprises the steps of sequentially stacking the core composition powder and the non-magnetic air gap composition powder on the lower punch to form the inner yoke of the magnetic core, the core composition powder on the lower punch and the inner yoke. A method for manufacturing a magnetic core for a transformer comprising the steps of forming an outer yoke spaced apart in parallel with the inner yoke and a leg perpendicular to the longitudinal direction of the outer yoke by stacking and pressing the leg by an upper punch, According to the present invention, an air gap can be formed in a magnetic core without assembly or material processing.

Description

Method for manufacturing magnetic core for transformer, air gap composition powder for magnetic core, and magnetic core for transformer with air gap formed

The present invention relates to a method for manufacturing a magnetic core for a transformer, a magnetic core having an air gap, and an air gap composition for manufacturing the magnetic core.

A transformer is a conversion device that changes the magnitude of voltage and current of alternating current by using electromagnetic induction, and is widely used from small electronic devices to large-scale substation facilities or power transmission facilities.

In general, a transformer has a structure in which a plurality of coils are wound around a magnetic core, and power is input to one coil and then power is output to the other coil.

The magnetic material constituting the core is saturated with magnetic properties from a certain load, and heat is generated, and the magnetic material ceases to function.

Accordingly, a technique for increasing the limit load is required, and a typical method is to arbitrarily create an airgap in a magnetic core as shown in FIG. 1 .

That is, as shown in FIGS. 2 and 3 , through the airgap, flux leakage is made so that saturation of the material is delayed.

Since the slope of the B-H curve is lowered by magnetic flux leakage, the effect of a change in magnetic characteristics (slope dB/dH or dB/dI) can be obtained even in a wider external magnetic field (H) or current load.

Prior art Japanese Patent Registration No. 5987847 for making such an air gap arbitrarily uses an adhesive between each separated magnetic structure, which is advantageous for vibration durability. However, since the number of assembly steps is large, workability is difficult, and since assembly must be performed at regular intervals, precision may be reduced.

And, in the case of prior art Korea Patent Publication No. 10-2019-0026323, a gap was formed by making the thick flesh part thin. In the case of this method, it is easy to manufacture a plate material such as a silicon steel plate. However, there is a risk of damage during vibration and assembly, and magnetic flux is concentrated in a thin portion and is easily saturated. In addition, it is very difficult to realize such thin skin in molding/sintering methods such as magnetic materials, which are representative materials of transformers.

In this way, the prior art has achieved its purpose by structurally imparting an airgap to the transformer/inductor core, but there is a limit to mass production due to poor manufacturing workability or risk of damage.

Matters described in the background art above are intended to aid understanding of the background of the invention, and may include matters other than those of the prior art already known to those skilled in the art.

Japanese Patent Registration No. 5987847 Korean Patent Publication No. 10-2019-0026323

The present invention has been made to solve the above problems, and the present invention provides a method for manufacturing a magnetic core for a transformer, an air gap composition for a magnetic core, and a magnetic core with an air gap for forming an air gap without assembling or material processing. Its purpose is to provide

A method for manufacturing a magnetic core for a transformer according to one aspect of the present invention includes forming an inner yoke of a magnetic core by sequentially stacking a core composition powder and a non-magnetic air gap composition powder on a lower punch, the lower Laminating the core composition powder on a punch and the inner yoke to form an outer yoke spaced apart in parallel with the inner yoke and a leg perpendicular to the longitudinal direction of the outer yoke and pressing the leg by an upper punch includes

Here, the forming of the inner yoke is characterized in that the air gap composition powder is laminated to a thickness of 1 mm or more.

Meanwhile, the air gap composition powder is characterized in that any one or a mixture of two or more of ZnO, BaTiO ₃ , and CuO.

Specifically, the air gap composition powder is characterized by including 75 wt% or more to 100 wt% or less of ZnO and 0 wt% or more to 30 wt% or less of CuO based on total wt%.

Alternatively, the air gap composition powder is characterized in that it includes more than 55 wt% of BaTiO ₃ to less than 75 wt% and more than 25 wt% of CuO to less than 45 wt% based on total wt%.

Furthermore, the core composition powder is characterized in that it comprises 30wt% or less of MnO, 20wt% or less of ZnO, and the balance of Fe ₂ O ₃ based on the total wt%.

And, the shrinkage rate of the air gap composition powder is 15% or more and 20% or less.

In the forming of the inner yoke, the core composition powder and the air gap composition powder are alternately laminated two or more times, respectively.

Next, a magnetic core for a transformer according to one aspect of the present invention includes a leg made of a magnetic material, an outer yoke made of a magnetic material bent from both ends of the leg, and an inner yoke bent from the middle part of the leg, It is characterized in that the yoke is integrally formed with a non-magnetic air gap layer.

And, the thickness of the air gap portion is characterized in that 1mm or more.

Meanwhile, the air gap part is formed by mixing any one or two or more of ZnO, BaTiO 3 , and CuO.

Specifically, the air gap portion is characterized in that it is formed by mixing 75 wt% or more to 100 wt% or less of ZnO and 0 wt% or more to 30 wt% or less of CuO based on the total wt%.

Alternatively, the air gap portion is characterized in that it is formed by mixing BaTiO ₃ in an amount of more than 55 wt% to less than 75 wt% and CuO in an amount of 25 wt% to 45 wt% based on the total wt%.

Furthermore, the magnetic material is characterized by including 30 wt% or less of MnO, 20 wt% or less of ZnO, and a balance of Fe ₂ O ₃ based on the total wt%.

Meanwhile, the air gap portion is characterized in that a plurality of layers are formed to be spaced apart from each other.

Next, the air gap composition for a magnetic core according to the present invention is characterized in that any one or a mixture of two or more of ZnO, BaTiO ₃ , and CuO is used.

Furthermore, it is characterized in that it comprises 75 wt% or more to 100 wt% or less of ZnO and 0 wt% or more to 30 wt% or less of CuO based on the total wt%.

Alternatively, BaTiO ₃ is characterized in that it comprises more than 55 wt% to 75 wt% or less and CuO 25 wt% to 45 wt% based on the total wt%.

According to the manufacturing method of the present invention, it is possible to integrally manufacture a magnetic core with an air gap without processing materials or assembling.

Therefore, manufacturing workability is improved and the risk of breakage is compensated to enable mass production.

Nevertheless, magnetic body saturation can be delayed through magnetic flux leakage.

1 shows a conventional magnetic core in which an air gap is formed.
2 and 3 show BH curves according to air gaps.
4 schematically illustrates a magnetic core according to an embodiment of the present invention.
5 to 12 sequentially illustrate a method for manufacturing a magnetic core according to an embodiment of the present invention.
13 shows the characteristics according to the content of the magnetic core composition.
14 illustrates a method of manufacturing a magnetic core composition.
15 schematically illustrates a pair of magnetic cores according to an embodiment of the present invention.
16 to 21 show experimental results according to each condition according to the embodiment of FIG. 15 .
22 to 24 show experimental results according to each condition by the magnetic core of another embodiment of the present invention.

In order to fully understand the present invention and the advantages in operation of the present invention and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the present invention and the contents described in the accompanying drawings.

In describing the preferred embodiments of the present invention, known techniques or repetitive descriptions that may unnecessarily obscure the subject matter of the present invention will be reduced or omitted.

4 schematically illustrates a magnetic core according to an embodiment of the present invention, and FIGS. 5 to 12 sequentially illustrate a method of manufacturing a magnetic core according to an embodiment of the present invention.

Hereinafter, a method for manufacturing a magnetic core for a transformer, an air gap composition for a magnetic core, and a magnetic core with an air gap according to an embodiment of the present invention will be described with reference to FIGS. 4 to 12 .

The magnetic core 101 of the present invention may be configured in a form in which, for example, a pair of E-type cores are combined as shown in FIG. 4 .

One magnetic core 101 includes a leg 110, an outer yoke 120 vertically bent from both ends of the leg 110, and an inner yoke 130 vertically bent from the middle portion of the leg 110, as shown in the drawing. ), and the outer yoke 120 and the inner yoke 130 may be parallel.

Such a magnetic core may be composed of a Mn ferrite base material, and in the present invention, as shown in the drawing, the air gap part 131 is integrally formed in layers on the inner yoke 130, and the air gap part 131 has a certain thickness. It has a plurality of layers may be formed spaced apart.

In this way, the magnetic core 101 integrally formed with the air gap portion is integrally manufactured by simultaneously casting different materials of the core composition constituting the magnetic core and the air gap composition forming the air gap portion.

As shown in FIG. 5, a powder input device (20, feed shoe) including a mold device and a core composition input unit 21 and an air gap composition input unit 22 for injecting the core composition and the air gap composition powder into the mold unit. The magnetic core of the present invention can be manufactured by using.

The core composition input unit 21 and the air gap composition input unit 22 are connected to the powder filling container, respectively, so that different types of powder can be input according to the control.

The mold device is made to fit the shape of the final molded body, and the core composition powder and the air gap composition powder are sequentially filled by the horizontal motion of the powder injector 20.

At this time, when the powder input device 20 supplies the powder, a part of the lower punch may move vertically so that the base material and the airgap formation can be well positioned in an accurate area.

That is, as shown in FIG. 5, in a state in which only the first lower punch 11 is lowered, as shown in FIG. 6, the powder input device 20 is located above the first lower punch 11 and passes through the core composition input unit 21. 1 Place the core composition powder 31 on the upper surface of the lower punch 11.

Then, as shown in FIG. 7 , the airgap composition 32 is injected and seated on the core composition seated through the airgap composition inlet 22 to form a layer.

As shown in FIG. 8, the core composition powder 33 of the next layer is introduced through the core composition inlet 21, and the air gap composition powder 34 of the next layer is introduced through the air gap composition inlet 22 as shown in FIG. is injected so that two air gap parts 131 as in the example are formed on the inner yoke 130.

In order to alternately inject the powder by the core composition inlet 21 and the air gap composition inlet 22, the powder injector 20 moves to the right and left in the horizontal direction and sequentially stacks them during the above operations. .

Here, a manufacturing method in which the air gap part is composed of two layers is exemplified, but it may be composed of one layer or three or more layers.

In addition, the plane formed by the air gap unit is preferably perpendicular to the longitudinal direction of the inner yoke 130 . That way, it is formed parallel to the magnetic flux path and generates magnetic leakage.

Then, as shown in FIG. 10, the third lower punch 13 is lowered while maintaining the positions of the second lower punch 12 and the fourth lower punch 14, and as shown in FIG. 11, the third lower punch 13 , The core composition input unit 21 moves on the upper surface of the second lower punch 12 and the upper surface of the previously formed air gap composition powder layer 34 to inject and deposit the core composition powder.

In this way, after introducing the powder in the shape of the magnetic core 101 as shown in FIG. 4, the upper punch 20 descends and applies pressure as shown in FIG. 12 to complete the final molded body.

Next, the composition and manufacturing method of the core composition will be described with reference to FIGS. 13 and 14 .

The magnetic core of the present invention may be a MnZn ferrite core, and includes Fe, Mn, and Zn as main components.

Assuming that the main components are composed of Fe ₂ O ₃ , MnO, and ZnO, and the total weight is 100 wt%, MnO is 30 wt% or less, ZnO is 20 wt% or less, and the rest may be composed of Fe ₂ O ₃ .

In addition to 100 wt% of the main component, additional additives such as Co ₂ O ₃ , CaO, SiO ₂ , NbO, Al ₂ O ₃ , CuO, etc. may be added at hundreds of ppm to thousands of ppm.

As shown in FIG. 14, the main components are Fe ₂ O ₃ , MnO, and ZnO as raw materials, and the powders of the main components are mixed well.

At this time, depending on how the main components are mixed, the main basic properties (high Bs, low loss, high permeability) of the material as shown in FIG. 13 are determined.

Each mixed raw material has a diameter of 1 ~ 5um, and when the mixed powder is subjected to a calcination process in an atmosphere of 800 ~ 1000 ℃, the components are homogenized.

Thereafter, through pulverization, the aggregation of the material subjected to the calcination process is disturbed to have an appropriate degree of sinterability. The particle diameter of the pulverized powder at this time has a diameter of 5 ~ 10um.

Next, in the secondary grinding process, the pulverized ferrite powder is dried, and then additional pulverization is performed by mixing additives and binders.

And, in order to improve the moldability of the manufactured powder, first, the granulation process of the granulation raw material is performed as a pretreatment.

The reason for this is that the binder added according to the needs of molding or subsequent processes, and the molding powder composed of aggregates with different moisture, size, shape, and density, etc., have many fine powders and lack fluidity, so they are not densely filled in the mold This is because it is not densified into a uniform microstructure and the density of the molded body is not uniform, so it is difficult to control the size of the final product and the physical properties of the final sintered body are deteriorated. The granulation process is therefore used to produce a satisfactory feedstock consisting of controlled agglomerates called granules.

Granules are made either directly by suitably adding and mixing a liquid or solution of a binder and other additives into an agitated powder or indirectly by spray drying a slurry.

If the spray drying process is properly controlled, it is possible to obtain relatively dense spherical granules with a size of 20 μm or more, which flow and fill well during compression molding.

Next, an appropriate amount of moisture is added to the dried granulated raw material for moisture control. Moisture destroys the granules during molding, enabling high densities with low molding pressure.

After adjusting the moisture, the molding pressure is 1 to 2 T/cm ² , and then molding is performed using a press using a molding pressure.

Maintain the maximum sintering temperature of 1,100 ~ 1,360 ℃ for 4 ~ 20 hours, and adjust the oxygen partial pressure during temperature increase / sintering.

Next, the air gap composition is described.

When the airgap composition is non-magnetic, it has a magnetic permeability of 1 μ level, which is the relative air permeability, with respect to a region occupied by the corresponding airgap part.

The non-magnetic airgap composition may be ZnO, BaTiO3, CuO or a complex oxide. Complex oxides can be carbonates, nitrates, hydroxides, metal compounds, and the like.

The airgap composition may consist of 100% of the entire composition by mixing one or two or more selected from the above-mentioned materials.

In addition, water and polyvinyl alcohol (PVA) may be added in the manufacturing process in an amount of 2 wt% or less in the entire composition.

An airgap composition having a sintering temperature higher than the sintering temperature of MnZn ferrite needs to raise the sintering temperature for overall sintering. At this time, since MnZn ferrite may be oversintered, the sintering temperature of the airgap formation is similar to or higher than the sintering temperature of MnZn ferrite, which is the base material. It is preferable to have a sintering temperature below.

In addition, MnZn ferrite is in a state of contact between the powder constituting the molded body and the powder during the sintering process, but there are many voids.

In general, a sintering phenomenon occurs as diffusion between atoms constituting a temperature of 700 ° C or higher occurs. Therefore, the parent material shrinks as much as the reduced air gap.

When configuring the airgap composition, the shrinkage rate should be similar to that of MnZn ferrite, which is the base material, so that distortion due to shrinkage does not occur during sintering after product molding, and simultaneous sintering is possible. Therefore, it is appropriate for the airgap composition to have a shrinkage rate of 15% or more and 20% or less after sintering.

For the examples, ZnO, BaTiO3, and CuO were prepared as non-magnetic airgap compositions.

These mixtures were calcined at 800° C. for 4 hours, and then wet-ground with a ball mill for 24 hours to obtain powder.

A slurry was formed by mixing 1wt% PVA binder therein, and a raw material for granulation was made through spray drying.

After that, it is molded into a toroidal with an outer diameter of 25 mm, an inner diameter of 15 mm, and a height of 5 mm at a pressure of 1.5 t/cm2. was carried out at 3°C/min.

When the temperature was raised, the oxygen partial pressure was 10%, and in the high temperature maintaining step, the oxygen partial pressure was 1%.

The shrinkage rate of the sintered airgap formation can be obtained by Equation 1.

Here, D1 is the diameter (mm) of the specimen before sintering, and D2 is the diameter (mm) of the specimen after sintering.

The composition of the examples in which the air gap composition has a similar shrinkage rate to that of the MnZn ferrite base material is as follows, and Table 1 summarizes the examples and comparative examples and the resulting shrinkage rates.

In the case of a mixture of ZnO and CuO, ZnO is preferably 75wt or more to 100wt% or less, and CuO is 0wt or more to 30wt% or less.

In the case of a mixture of BaTiO ₃ and CuO, it is preferable that BaTiO ₃ is more than 55 wt% and less than 75 wt%, and CuO is more than 25 wt% and less than 45 wt%.

Powder composition (wt%) Shrinkage (%) ZnO BaTiO ₃ CuO Comparative Example 1 0 0 100 2.1 Example 1 100 0 0 19.1 Example 2 95 0 5 17.9 Example 3 90 0 10 17.1 Example 4 85 0 15 16.2 Example 5 80 0 20 15.6 Example 6 75 0 25 15.0 Comparative Example 2 70 0 30 14.2 Comparative Example 3 0 100 0 5.8 Comparative Example 4 0 95 5 3.5 Comparative Example 5 0 90 10 2.5 Comparative Example 6 0 85 15 1.3 Comparative Example 7 0 80 20 0.4 Example 7 0 75 25 9.1 Example 8 0 70 30 8.0 Example 9 0 65 35 6.8 Example 10 0 60 40 5.9 Comparative Example 8 0 55 45 4.7

A magnetic core as shown in FIG. 15 was prepared for magnetic flux leakage and magnetic saturation experiments using the above examples.

The magnetic core of FIG. 15 is a combination of a pair of E-type magnetic cores 101 and 102 described above, and coils are wound in the inner space partitioned by the inner yoke 130, respectively.

15 and below, an air gap portion 131 is formed at the end of the inner yoke 130 of one E-type magnetic core, and the air gap portions of both magnetic cores contact each other to form one air gap portion. heard.

If there is no air gap, magnetic flux flows inside the magnetic core of the transformer. However, due to the thick air gap, leakage of magnetic flux occurs, and the size of the leakage also increases (fringe effect).

Accordingly, magnetic saturation is delayed for the same current (or magnetic field) and can be used at a high current (or magnetic field), but heat is generated in the leaked portion.

Referring to FIGS. 16 to 21, the degree of magnetic saturation delay due to the air gap part will be investigated. The applied current is 100A.

16 is a case without an air gap part, and the maximum magnetic flux density Bmax = 21.2 (T).

17 is a case where the thickness of the air gap part is 0.5 mm, and the maximum magnetic flux density Bmax = 0.672 (T).

18 shows the case where the thickness of the air gap part is 1.0 mm, and the maximum magnetic flux density Bmax = 0.433 (T).

19 is a case where the thickness of the air gap part is 2.0 mm, and the maximum magnetic flux density Bmax = 0.233 (T).

20 is a case where the thickness of the air gap part is 3.0 mm, and the maximum magnetic flux density Bmax = 0.162 (T).

21 is a case where the thickness of the air gap part is 4.0 mm, and the maximum magnetic flux density Bmax = 0.117 (T).

Through this, it can be seen that as the thickness of the air gap portion increases, the saturated portion (red is greater than 0.5T) in the transformer decreases.

This is due to the fringe effect caused by the air gap portion, and leakage magnetic flux may be generated to delay material saturation.

Table 2 below shows the results of calculating the maximum magnetic flux density according to different applied currents under the conditions of FIGS. 16 to 21 .

division Maximum magnetic flux density (T) according to the thickness of the air gap Number of air gap parts Current (A) 0mm 0.5mm 1mm 2mm 3mm 4mm One 10 2.12 0.067 0.043 0.022 0.016 0.012 20 4.24 0.134 0.086 0.044 0.032 0.024 30 6.36 0.201 0.129 0.066 0.048 0.036 40 8.48 0.268 0.172 0.088 0.064 0.048 50 10.6 0.335 0.215 0.11 0.08 0.06 60 12.72 0.402 0.258 0.132 0.096 0.072 70 14.84 0.469 0.301 0.154 0.112 0.084 80 16.96 0.536 0.344 0.176 0.128 0.096 90 19.08 0.603 0.387 0.198 0.144 0.108 100 21.2 0.672 0.433 0.223 0.162 0.117

Since MnZn ferrite is generally saturated at 0.5T, it can be seen that it is saturated when 80A or more is applied at 0.5mm and when there is no air gap part as a result of the analysis.

In summary, as the current increases, the maximum magnetic flux density increases proportionally.

And, as the airgap thickness increases, the maximum magnetic flux density decreases exponentially.

In order not to saturate the magnetic material, a certain thickness of the airgap portion must be formed, and more preferably 1 mm or more.

Next, FIGS. 22 to 24 are magnetic flux density maps formed in a coil by magnetic leakage around an airgap formation according to the number of airgap parts 131 . 22 shows one air gap part, FIG. 23 shows two, and FIG. 24 shows three air gap parts. In the experiment, the applied current is 100A and the total thickness is 3mm.

The total thickness of the air gap portion 131 is the same, but as the number increases, the degree of saturation of the surroundings is greatly reduced.

Since this is proportional to the degree of heat generation of the transformer, it can be said that the coil heat generation is significantly reduced as the number increases.

The larger the number of air gaps, the more advantageous it is to reduce coil heat generation, and is independent of the maximum magnetic flux density and saturation of MnZn ferrite, which is the base material of the transformer.

Conventionally, an airgap is formed by polishing some surfaces of MnZn ferrite, but it is impossible to form several airgap as described above. In the present invention, the airgap can be formed in several layers inside as described above, which also helps to significantly reduce transformer heat generation.

Although the present invention as described above has been described with reference to the illustrated drawings, it is not limited to the described embodiments, and it is common knowledge in the art that various modifications and variations can be made without departing from the spirit and scope of the present invention. It is self-evident to those who have Therefore, such modified examples or variations should be included in the claims of the present invention, and the scope of the present invention should be interpreted based on the appended claims.

102, 102: magnetic core
110: leg
120: outer yoke
130: inner yoke
131: air gap part

Claims (17)

Forming an inner yoke of a magnetic core by sequentially stacking a core composition powder and a non-magnetic air gap composition powder on a lower punch;
forming an outer yoke spaced apart in parallel with the inner yoke and a leg perpendicular to a longitudinal direction of the outer yoke by laminating the core composition powder on the lower punch and the inner yoke; and
pressing the leg with an upper punch;
The step of forming the inner yoke is characterized in that the air gap composition powder is laminated to a thickness of 1 mm or more.
Method for manufacturing a magnetic core for a transformer. The method of claim 1,
The air gap composition powder is characterized in that any one or a mixture of two or more of ZnO, BaTiO ₃ , CuO,
Method for manufacturing a magnetic core for a transformer. The method of claim 2,
Characterized in that the air gap composition powder contains 75 wt% or more to 100 wt% or less of ZnO and 0 wt% or more to 30 wt% or less of CuO based on the total wt%.
Method for manufacturing a magnetic core for a transformer. The method of claim 2,
Characterized in that the air gap composition powder contains more than 55 wt% of BaTiO ₃ and less than 75 wt% and less than 25 wt% of CuO and less than 45 wt% based on total wt%.
Method for manufacturing a magnetic core for a transformer. According to claim 3 or claim 4,
Characterized in that the core composition powder comprises 30 wt% or less of MnO, 20 wt% or less of ZnO and the balance of Fe ₂ O ₃ based on the total wt%.
Method for manufacturing a magnetic core for a transformer. The method of claim 5,
Characterized in that the shrinkage rate of the air gap composition powder is 15% or more and 20% or less,
Method for manufacturing a magnetic core for a transformer. The method of claim 1,
In the forming of the inner yoke, the core composition powder and the air gap composition powder are alternately laminated two or more times, respectively.
Method for manufacturing a magnetic core for a transformer. legs of magnetic material;
outer yokes made of magnetic material bent from both ends of the legs; and
Including an inner yoke bent from the middle portion of the leg,
Characterized in that the inner yoke is formed integrally with a non-magnetic air gap layer.
Magnetic cores for transformers. The method of claim 8,
Characterized in that the thickness of the air gap portion is 1 mm or more,
Magnetic cores for transformers. The method of claim 8,
Characterized in that the air gap part is formed by mixing any one or two or more of ZnO, BaTiO3, CuO,
Magnetic cores for transformers. The method of claim 10,
Characterized in that the air gap portion is formed by mixing 75 wt% or more to 100 wt% or less of ZnO and 0 wt% or more to 30 wt% or less of CuO based on the total wt%.
Magnetic cores for transformers. The method of claim 10,
Characterized in that the air gap portion is formed by mixing more than 55 wt% of BaTiO ₃ to 75 wt% or less and 25 wt% or more to 45 wt% or less of CuO based on the total wt%,
Magnetic cores for transformers. According to claim 11 or claim 12,
Characterized in that the magnetic material includes 30 wt% or less of MnO, 20 wt% or less of ZnO, and a balance of Fe ₂ O ₃ based on the total wt%.
Magnetic cores for transformers. The method of claim 8,
The air gap portion is characterized in that a plurality of layers are formed spaced apart from each other,
Method for manufacturing a magnetic core for a transformer. Characterized in that any one or a mixture of two or more of ZnO, BaTiO ₃ , CuO,
An air gap composition for a magnetic core. The method of claim 15
Characterized in that it comprises 75wt or more to 100 wt% or less of ZnO and 0wt% or more to 30wt% or less of CuO based on the total wt%,
An air gap composition for a magnetic core. The method of claim 15
Characterized in that it contains more than 55 wt% of BaTiO ₃ to 75 wt% or less and 25 wt% or more to 45 wt% or less of CuO based on the total wt%,
An air gap composition for a magnetic core.

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