US2905912A - Variable inductors - Google Patents

Description

Sept. 22, 1959 I GElSER 2,905,912

VARIABLE INDUCTORS Filed Oct 21, 1955 INVENTOR. DA V/D I G'E/SER H/S A romvsrs United States Patent VARIABLE INDUCTORS David T. Geiser, North Adams, Mass, assignor to Sprague Electric Company, North Adams, Mass, a corporation of Massachusetts Application October 21, 1955, Serial No. 542,061

1 Claim. (Cl. 336-73) This invention relates to toroidal inductors, and more particularly to toroidal inductors of the type whose inductance can be selectively varied.

Variable inductors are used in various types of radio and higher frequency applications. They may be used for intermediate-frequency transformers, medium waveband coils, short wave coils, radio-frequency chokes, and various higher frequency applications such as in television service, etc.

Q, or the Q factor, can be defined as two pi times the ratio of energy stored to energy dissipated per half cycle of sine wave excitation. It is indicative, therefore, of the operating efliciency of the coil. For optimum operating characteristics, a maximum Q should be obtained in a minimum volume or space occupied by the coil. Another important factor is economy of construction.

An object of this invention is to provide novel structures for variable inductors which are economical to manufacture and provide eflicient operating characteristics.

In accordance with this invention, a magnetic core of endless construction is wound with a flux inducing winding. A flux producing means is associated with said core for reacting with the flux induced by the winding to form a leakage field about the core.

An element of flux-controlling material is disposed adjacent the core. This element is movably mounted to permit it to be selectively inserted within the leakage field to provide means for varying the inductance of the winding.

The novel features of the present invention will become apparent to one skilled in the art from a reading of the following description, in conjunction with the accompanying drawings in which:

Fig. 1 is a somewhat diagrammatic representation of one embodiment of the present invention;

Fig. 2 is a somewhat diagrammatic representation of another embodiment of the present invention;

Fig. 3 is a somewhat diagrammatic representation of still another embodiment of the present invention;

Fig. 4 is a somewhat diagrammatic representation of a further embodiment of the present invention; and

Fig. 5 is a perspective View of a portion of another embodiment of the present invention.

Identical parts throughout the various figures are designated by the same reference characters, followed by suflixes in indicative sequence.

In Fig. 1 is shown a variable inductor 8. A doughnut or toroidal-shaped endless core of magnetic material is provided. The core is made of any magnetic material having a minimum permeability of approximately 2. It may be, for example, of powdered iron bonded together by a resin and pressed. Although doughnutshaped toroids are shown in the drawings, endless cores of any shape such as elliptical or angular may be utilized .to advantage. For higher frequencies, ferrites of the type described in U.S. Letters Patents No. 2,579,978,

No. 2,452,529 and No. 2,452,530 are particularly suita 2,905,912 Patented Sept. 22, 1959 able. The core 20 may also be made of insulated spirally wound foil or wire. For lower frequencies, ring-shaped laminations may be bonded or clamped together to form a core.

A number of turns of conductive wire 12 are looped or wound around a portion of the core 10 to form a winding 13. The core, for example, may be a toroid having an outer diameter of 0.60 inch, an inner diameter of 0.27 inch, a cross-sectional area of 0.023 square inch, and an initial permeability of 8. When the core is made of a ferrite having a high electrical resistivity, the wire need not be specially insulated from the core. The winding provided by the conductive wire 12 may be formed of a number of equally spaced loops of conductive wire. The wire may be bare or covered by insulation such as those of the type known as Formvar or Ceroc-T. Litz wire of the braided or unbraided type also may be used. Formvar is a vinyl acetal enamel and magnet Wire coated with this insulation is obtainable from the Acme Wire Co. of New Haven, Connecticut. For a description of the structure and properties of the Ceroc-T type wire, refer to U.S. Letters Patent No. 2,707,703. The winding may, for example, be made up of ten turns of No. 26 wire of the aforementioned types. The winding formed by wire 12 is connected across a source of alternating current 14 which may be R.F., U.H.F., etc.

A sheet-shorted turn 16 is disposed about another portion of the core 10. The sheet-shorted turn is made of a sleeve of conductive material. This material may be, for example, conductive foil stock. The sleeve may be formed by looping a sheet of foil about the core and then soldering the outer edge. A sheet material printed or plated with a conductive pattern may also be used for the sheet-shorted turn. This sheet-shorted turn can be insulated or uninsulated from the core whether the core is made of conductive material or non-conductive material.

A pair of protuberances or poles 18 and 20 can be provided on the inner portion of the core 10. These protuberances 18 and 20 are oppositely disposed across the central aperture 22 of the core 10. These poles 18 and 20 are disposed on an axis lying approximately between the winding 13 and the sheet-shorted turn 16. They provide a path to help leakage flux transverse the aperture 22.

A slug 24 is mounted within the aperture 22 by means permitting axial movement of the slug within the aperture. The slug 24 slides in a channel provided by protuberances 18 and 20. This slug is a flux-controlling member and therefore is made of either highly permeable material or highly conductive non-magnetic material. Insertion of the slug within the aperture therefore varies the effective permeability of the core. This in turn results in a change in the inductance imposed on the source of alternating potential 14 by the variable inductor shown in Fig. 1.

For a wide variation in inductance, the slug 24 may be divided into a highly permeable portion 26 and a nonmagnetic highly conductive portion 28. The highly permeable section may be, for example, soft iron and the non-magnetic portion 28 may be brass or copper.

There may also be provided a cap or disc-like piece which bridges across a face of the core to effect a further variation in inductance by being adjustably positioned with respect to the core. This cap can be of the same inductance-controlling materials indicated above for the slug 24, and may also be formed integral with the slug or used without the slug, if desired.

Portions 26 and 28 of slug 24 may be selectively positioned within the aperture 22 by sliding the slug 24 to and fro. Where a fine adjustment is desired, the slug and inner surfaces of the poles 18 and 20 may be cooperatrively threaded" to'permit'axial positioning of theslug by rotation thereof. ,Variable inductors embodying the structure shown in l have yielded a widerange as inrl'uctance variation atr'elatively high 'Q factors. Y

' In- Fig. 2is -'shown avariahle inductor 81 similar to the inductor 8 shown in Fig. l. The inductor 8a includes an end-lessor toroidal-core 30,- similar in composition to toroidal core lttof Fig. l. The core 30, however,-ineludes a"recessed"slot"32 within which "a close-wound or lumped winding 34*is contained. The-winding is made up of wirelZa of similar compositiontothat of' wire12 shown in Fig. 1. Protuberances l sw-and wa-are provided on the toroidal core'fit). These'protuberances are similar in structure-and function to protuberances 18 and 20 'shown-in Fig;1. Asheet-shortedturn 36 is'looped about anotherpor-tion of the core 3t). Thesheet-shorted turn 'is 'made 11pofsheet material of -a composition similar tothat-of the sheet-shorted turn' '1-6 -shown in Fig. -1. Sheet-shorted turn 3a6 is, therefore, made up of copper foil stock. For'a half-inch nominal-(average) diameter toroidalcore 30,-sheet-shorted turn 36- may be made of astrip of one mil copper -tape'%" wide. The-sides of the tape are slit to form-four parallel strips. he central portion of the tape remains intact 'toform a s-hor-ting band throughoutthe lengthof the resultant loop- The in-tact a permanent magnet material of the type known as Magnadur. Magnadur is a material which may be obtained from the Ferroxcube Corporation of Saugerties, New York. It is manufactured by mixing oxides having the approximate formula BaFe O The oxides are pressed or extruded into a suitable shape and sintered. This forms a hard, rather brittle substance having excellent permanentmagnet :qualities. .This permanent-magnet core half may be also formed of other permanent magnet materials, such .as Alnico. 'Ainico 1 maybe used; for example. It is a permanent magnetalloy whose average composition is as follows: iron62%, nickel-21%, aluminum-l2 and -cobalt-5%. A Winding 130 formed of wire 1 is provided on the core half 52. An inductance varying slug'24c is provided in the inner portion or aperture of the core 5% to vary the inductance in 4 a similar manner to that shown and described in conjunction -with l'F-ig. 1. Alternatively-theme of :apermeable tuningslug 24c willicause adivision of permanent magnet flux between .thecore isiliand the slug '24s, perrnitting ut'ilization of. somewhat lower frequency material in higher frequency applications while giving 'greateriadjustable inductance range.

portion of the'tape is placed within the central portion er thecore and the outer strips,--whic h rnay be designated as 36 ,86 36 ,-and'fifitrare-radially bent about the core to -form 'four' loops joined by theintact central portion. The -"-intactcentral portion of the turnis designated by reference character 38. The outerrnating edges of the looped strips 36 to '3 6 are conductively joined at their outer edge. They are-soldered, fonexample.

The. structure Sa-s-hown inZ-Fig. 2 provides a convenient method of forming-asheetashorted turn which-provides the-same -favorable operating characteristics which are provided by the structure 8 shown inFigjl. The close wound winding 34 provides-Qfactors which are relatively higher than those provided by even spaced winding I3,-'shown in Fig. 1. In close-winding the turns-may be wound at random ontopofeachother. The recess 32 provides means for winding 2. core withoutmaking the structure bulky as well as leaving the protruding inner portionsof the core=t0 act-as poles. Therecessing-of the sheet-shorted turn-is also seento have these advantages. Various inductance varying structures may -be,' therefore,

moved into close proximity with the core w-ithout being interfered with by the flux-inducing winding. :In Fig.s3-'showna variable inductor stmcture 8b, includingzasheet-shorted turn 40 =whichis painted upon thewreiitlb. Thelcoreltlb is made eta highly-resistive ferrite :of thettypeidescribed in conjunction wi-th Fig. 1. Protuberancesof the typedesignated by reference charactersii and220.in:-Eig. .1: may be optionally-provided.

"This .core ltlb is, .foriexample, :made of a fired equimolarimixture of: nickel ;oxide ('NiO) I and ferrous oxide Fe O These .mixing weights-are accurate within 3 of 1%. This material is prepared in the normal manner to form the/toroidal: core. Other.acceptableformulations for-the .core are .described.ini.previously mentioned Patents No. 2,579,978,'1No. 2,452,529 and hid/2,452,530.

:The: sheet-shorted turn .is formed of highly conductive material. A? silver :turn provides. extremelyhigh conductivity. A:si'lveriturn isapplied to theicoreltlb by painting approximately. half .of. the core with silver paint which is baked to harden. A winding .l'3b.is lprovided' of I wire "12b; whichxis similar in composition and function to that shownin Fig. l.

in-Fig. -4, avariable inductor structure 80 is shown. It is formed of a core'St) 'whichis made of two 'halves, 52

'aridl54. Thecore half 52 ismade of xa ferriteofsimilar constructionto'the core-of-Fig.'l. The core'half 54, however, ismade-ota magnetized-permanent magnet'material. The core 11215 54 maybe ma'de for example, of

Thisstructureprovldes :airelatively high amount of leakage flux between Ithe poles orprotuberances 18c and 20c. Movement of slug 24c, therefore, varies the-inductance over a wideirange. This inductance-variation is accomplished at.relatively high .Q factors over the entire range of a variation.

In .Fig. 5: iss'shown an inductance-varying .means leie which,i fortexample may beemployed to vary the inductance of 'a'toroidal core in a manner similar to that accomplishedibyportion 28 .of slug:24 shown in Fig. -1. -'lhis inductance-varying means .may'be .describedas a cage. ilt 'iszmade upuof two vanes 82and:84,;crossed atrightQangles. -These vanes aremadeof nonmagnetic highly conductive material. These vanes, for example, i areimade ofsheet brass. These vanes are-supported upon the.:enlarged 'headffifi :of a rod-88. The rod :88Qpro- 'vides:means forselectively inserting the cage within the aperturetof the core. 7

The sheet-shorted form of the present invention should .extendialongataleast about one-sixth of the-endless: path ilefined'tbythe toroid. .If'less inner circumferential-sheet .lengthibecomes.desirahle, itAis possible to formlseparate sheet loops:ofshighlyxonductive material bounding that portion of the toroid opposite the flux-inducing Winding. Somewhatflzetter Qsare :generally obtained'when the lturnsioflthe winding are closely wound, that isxconfined to only rabouttoneafourth of thetoroid pathor less.

More than one winding canlbe u'sed;.per core, as'for example/to:makeltwoewinding transformers suitable for use as intermediatefrequency transformers in frequencymodulation receivers .or AM-receiving :sets, :preferably .rite,.shows.-at 47.25 .megacyclestper secondan inductance -of 0.866-microhenry, a Q of 1 69, when tuned with a capacitance "of 13.25 micro-microfarads.

With a slug having a permeability of 12, the inductance can be raised to OJ984-microhenry, with'aQpf 1 39 and a'tuning capacitance of l1.-42-micro-microfarads. The further addition of a cap with a permeability of 6.8 increases the inductance 'tg 1.033, lowers the tuning capacitance tQ 11.03 micro microfarads, and does not change the Q. In general, it is preferred to operate at frequencies at least as high as 10 megacycles per second, using wires having an effective thickness corresponding to from No. 14 to about No. 30.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood the invention is not limited to the specific embodiments hereof except as defined in the appended claim.

I claim:

A variable inductor comprising a one-piece toroidal core of magnetic material having a central aperture, a winding looped about said core, means for applying an alternating current signal to said winding to induce magnetic flux in said core, a thin layer of silver paint baked about the surface of approximately half of the said core lhaving extremely high conductivity for reacting with said flux induced in said core by said winding to cause a leakage field of flux to traverse said central aperture a pair of protuberances oppositely disposed on the inner said aperture for widely varying the inductance of said winding.

References Cited in the file of this patent UNITED STATES PATENTS 2,395,881

Klemperer Mar. 5, 1946 2,547,793 Spoor Apr. 3, 1951 2,666,187 Ketcham Jan. 12, 1954 FOREIGN PATENTS 476,422 Canada Aug. 28, 1951

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