NO780893L - MAGNETIC CORE FOR INDUCTION COILS AND PROCEDURE FOR ITS MANUFACTURE - Google Patents

Description

The present invention relates to a magnetic core for induction coils and a method for its production

score.

When manufacturing induction coils for use in the tone frequency range and of the type used in the dividing filter for high-

speakers, it is common to use cores of ferromagnetic material.

The most commonly used values of self-inductances for this purpose have sizes from 0.1 to 10 millihenry and a resistance n<£ 1 ohm, so that the copper consumption in air coils from 1 mH and upwards is very significant.

Magnetic cores reduce this copper consumption to 15 - 30% of the copper consumption in air coils. It has been common to use coils wound on lamellar cores of transformer tin in the lowest frequency range, but with the ever-increasing demands placed on correct sound reproduction, it is now preferred to use cores of ferrite, especially Manganese/Zinc ferrite, which is a dksyd material with spinei structure.

This essentially avoids all signal distortion, as the permeability of the ferrite material varies less with the current load of the coil than the permeability of the iron material.

The ferrite material, however, becomes magnetically saturated at low field strength and can in this way cause distortion of powerful musical sections, if special attention is not paid to this condition.

It is thus necessary to avoid closed magnetic circuits when using ferrite. The factor by which the air coil's self-induction is increased when using the relevant core material must not exceed approx. 5.

Cores of ferrite are expensive due to materially complicated production, which includes the following processes: weighing, grinding, mixing, sintering, re-grinding, mixing, pressing, and final sintering at high temperature.

Cores produced fa. v~~\iron powder was introduced already in radio's infancy, i.a. in use in the intermediate frequency transformers of the superpetrodyne receivers and has also been used in the tone frequency range, e.g. in the so-called pupin coils.

Such iron powder is most often mixed with thermoplastic material or thermosetting insulating material, and the cores are produced by pressing. Attempts have been made to make these powder cores usable in the high frequency range. This means that the grain size of the powder must be below approx. 10 pm, as larger grains cause eddy current losses at high frequencies. This known technique is most often based on carbonyl iron consisting of similar, spherical particles with a diameter of 5-6 µm and iron powder produced by reducing iron oxide with hydrogen and with a similar particle size of less than 3 µm. The carbon content of these products has usually been approx. 1%.

Any application of magnetic material in powder form, with or without pressing, entails a strong reduction in permeability due to the spaces introduced into the material and which are filled with air or other magnetically passive substances.

It has not been common to achieve values for the initial permeability/a^ of more than approx. 20 in such powder kernels.

Ferrite materials used in divider coils most often have

u±> 1000.

According to the invention, the magnetic core material is produced by vigorous pressing of iron powder with a large dispersion in grain size. since half of the material is divided into grain sizes above 50 pm with a maximum grain size of 200 pm.

The spread can be achieved by mixing standard products with different grain sizes. A typical material for use in the frequency range 25 Hz - 2 5 KHz can have 80% of the material quantity divided between grain sizes from 40 to 200 pm and 20% under 40 pm.

As a result of the greatly varying grain size, it is possible at a press pressure of approx. 5 tonnes/cm 2 and achieve tight packing of the grains and a kj is neve ktutf^TnTng s: up to approx. 7, as the fine particles fill the spaces between the coarse ones.

It is also possible to use greater or lesser pressing pressure, preferably in the region of 2-10 tons/cm 2. The cohesion of the material can be achieved by molecular forces alone, but if greater mechanical strength is desired, binders can also be added, which fill out the material's finest pores.

For this purpose, it can e.g. aluminum hydroxide or bentonite is used, preferably in colloidal form.

Iron powder with a maximum grain size below approx. 30 pm can hardly be processed with pressing techniques according to the invention, as they do not flow easily enough into the mold even under vibration. If the grain size is similar, only small cohesive forces are achieved.

It is appropriate to add a lubricant to the mixture, whereby it is achieved that the grains pass each other more easily, so that a denser structure can be achieved at a given pressure.

The above-mentioned core weight supplement of approx. 7 is only achieved

when using a lubricant.

The lubricant can e.g. be molybdenum disulphide, wax, stearic acid or a stearate and can optionally be supplied in dissolved form by wetting powder with the solution, as well as drying and mixing in a rotating drum. Naturally, a powdered lubricant can also be used. The quantity share can be 0.2 - 1.5%.

A suitable material for use according to the invention can be so-called "sponge iron", which can be obtained with a carbon content below 0.02%. It is therefore a matter of very bare iron. Materials of this kind are used i.a. for the production of alloys.

A low carbon content means low hysteresis losses in the material

and consequently less signal distortion in the coils.

If, for certain purposes, greater mechanical strength is desired in the core, in addition to the already mentioned addition of colloidal binders, an addition of approx. 1% phenolic resin. This material is hardened by passage through a tunnel oven after pressing, and it is therefore not necessary to leave the core blank in the press mold during curing, as is the case with bakelite pressing.

It was expected that a core produced in the indicated manner

would be uégriét. at high frequency due to the content of (<g>rbj/e iron grains), and that in the tone frequency range it would exhibit properties that were between the properties of the corresponding

show iron lamellar cores and fer£ite cores.

It is therefore surprising that the manufactured cores have technical data that are almost identical to corresponding data for good ferrite material, when it comes to rod cores for use in the tone frequency range.

Coils with rod cores manufactured according to the invention

has constant self-induction and iron loss below 5% in heat

frequency range from 25 Hz to 2 5 kHz. This has to do with the material's low carbon content.

The factor by which the air coil's self-induction is increased when the core is inserted, also called apparent permeability, is 90 - 95% for rod cores according to the invention

of the apparent permeability that can be achieved by using a ferrite core of the same dimensions and the same coil. This is surprising in view of the fact that u for the ferrite material measured in a ring core is over 1000, while the corresponding value for the core material according to the invention is = approx.60.

The impedance varies very little with the coil current.

The sum of the harmonic current components through a pilot coil of 0.87 mH and 0.3 ohm loaded with very close sinusoidal voltage at frequencies of lQO, 500, 1000, 5000 and 10,000 Hz and at currents varying from 0.25 to 5 A, is below 1%, and a comparison with the same air coil after inserting a ferri±core with the same dimensions and with p.^ ^> 1000 showed almost identical data, when it is disregarded that at the current strength of 5 Amps and the frequencies 100 and 500 Hz, a significant signal distortion occurs in the ferrite coil due to saturation.

For investigation of the new material's stability it was

X attempted to discharge a capacitor of 1000 uF charged to

500 Volts/through the aforementioned pilot coil of 0.87 mH. This influence changed either coil's self-inductance or loss angle.

The specific resistance of the material is approx. 0.2 ohm.cm, which

is less than', f br" common" pul.yerkj erns, but far more than in compact iron. For use as rod cores in coils used in the frequency range 25 Hz - 25 kHz, the new material, despite its low value for p^, is just as suitable as ferrite, and the fact that the material can actually withstand higher loads without saturation will constitute a safeguard against distortion of powerful music episodes.

In addition, it has been shown that the material in question can be used in coils for radio noise reduction. In this application, it does not have an unfortunate effect that the coil in question has large losses at high frequencies, and the manufacturing method of the core enables a particularly cheap manufacture of coils with low gain numbers completely surrounded by magnetic material.

For noise reduction, hollow cores have often been used which are attached to the windings by feeding an insulated wire back and forth through openings in the core, which is difficult and requires a lot of manual work.

With the use of the new material, one or more cores can first be formed, which can easily be applied to wire winding and then inserted into a pressing tool and pressed into material of the same type, so that the winding in the finished product is surrounded by magnetic material.

It will often be appropriate to use this technique

which results in all magnetic circuits being closed. The press-in method can also be used for tone frequency coils, and in this case it is a particular advantage of the present magnetic material that it can withstand high field strength without saturation occurring.

Claims (8)

1. Magnetic core for induction coils and produced from iron powder by pressing, characterized by the fact that there is a large spread in the powder's grain size, with more than half of the material quantity distributed among grain sizes above 50 pm with a maximum grain size of 200 pm. 2. Magnetic core as stated in claim 1, characterized by approx. 80% of the iron powder is distributed among grain sizes from 40 to 200 pm and approx. 20% are below 40 um. 3. Magnetic core as specified in claim 1 or 2, characterized in that the core is made of iron powder with a carbon content of 0.02%. 4. Process for the production of magnetic cores as stated in claim 1, 2 or 3, characterized in that the core is produced by a single pressing process with pressure from 2 to 10-:.1 ton/cm after adding a lubricant to the powder in a proportion of 0.2 to 1.5%. 5. Method as stated in claim 4, characterized in that a binder is added which glues the iron grains together while filling the material's finest pores. 6. Method as specified in claim 5, characterized in that, in addition to colloidal binders that do not change when heated, approx. 1% phenolic resin, after which the manufactured core blanks are finished after pressing by heating in an oven to the resin's hardening temperature. 7. Induction coil for radio noise suppression produced using magnetic cores designed or produced as specified in claims 1-6, c a r a c t e r i s e r; t in that one or more cores are first produced which are applied to a winding of insulated wire and then pressed into magnetic material of the same type used in the cores. 8. Induction coil generator with a core of magnetic material for use in the frequency range from 2 5 Hz to 2 5 kHz, characterized in that the core is extended or produced as specified in claims 1-7.