US2781496A - Coil system employing at least one highfrequency coil having a premagnetised rod-shaped core - Google Patents

Description

Feb. 12, 1957 F. J. M. LATHOUWERS ,7

COIL SYSTEM EMPLOYING AT LEAST ONE HIGH-FREQUENCY COIL HAVING A PREMAGNETISED ROD-SHAPED CORE Filed Dec. 12, 1952 N Ll N 7 3 S g a i Fig. 4

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FRANCISCUS JOHANNES MARIA LATHOUWERS AGENT United States Patent COIL SYSTEM EMPLOYIN G AT LEAST ONE HIGH- FREQUENCY COIL HAVING A PREMAGNET- ISED ROD-SHAPED CORE Franciscus Johannes Maria Lathouwers, Eindhoven, N etherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application December 12, 1952, Serial No. 325,503

Claims priority, application Netherlands December 29, 1951 Claims. (Cl. 336-110) The object of the invention is to provide a coil system of cheap construction suitable for mass production and including at least one high-frequency coil, the inductance of which is required to be capable of being adjusted once.

There are many coil constructions of this kind most of them based on variation of the eifective permeability (reluctance) of the core as a result of changing its effective section or its length or the size of the air-gap provided in the magnetic circuit, etc. The end to be achieved by all these constructions is to satisfy a number of opposed requirements, namely low price, satisfactory coil quality, small size and a comparatively large adjusting range, each construction being naturally a compromise between the requirements.

The invention achieves a very favourable compromise by using premagnetisation for controlling the inductance, by means of a permanent magnet made of a material of high electrical resistance arranged adjacent at least one of the ends of the coil core to obtain adjustable premagnetisation, a sliding member being provided to vary the magnetisation of the coil core produced by the magnet, the inductance changing always in the same sense when displacing said member in one direction within its sliding range. The sliding member may be the magnet itself.

The invention will be described with reference to the accompanying drawings, in which Figs. 1, 3 and 4 show various forms of a coil system according to the invention.

Figs. 2 and 5 are graphs showing the operation of the coil illustrated in Fig. l.

The high-frequency coil shown in Fig. 1 comprises a bar-shaped core 1 of ferromagnetic material on which a coil 3 is arranged. This coil is fixed, for example by means of wax or lacquer 2, inside an insulating sleeve 5 provided with an internal screw thread. Adapted to be screwed in the sleeve 5 is a tubular axially magnetised permanent magnet 7 which in some adjusting positions embraces the core 1 and effects adjustable magnetisation of the core 1.

To prevent losses in the magnet 7-which is arranged in the high-frequency field of the coilit is made of high-resistance material (the specific resistance being at least 10 ohms-cm.) having low hysteresis losses, for example ceramic material primarily composed of non-cubic crystals of polyoxides of iron and one of the metals barium, strontium, lead and if required calcium. The core 1 is preferably made up of a cubic mixed crystal material of oxides of iron and at least two other metals which combines low losses and high permeability (approximately 400) with comparatively low saturation, known as ferrites.

Fig. 2 shows the inductance L in microhenries of a coil of the above described type as a function of the distance a in mms. between the coil winding 3 and and the magnet 7. The length b of the part of the core 1 projecting from the winding 3, which in some adjusting positions 7 2,781,496 Patented Feb. 12, 1957 ice is embraced by the magnet, was 6 mms., the length 0 of the magnet 10 mms., and that of the winding also 10 mms. The thickness of the core 1 was 1.6 mms.

As may be seen from this graph the inductance L has a comparatively high value in the position in which the magnet 7 engages the winding 3-in which a maximum saturation of the core may be expected-and initially decreases as the magnet moves further away from the winding 3. At a distance a of approximately 2.5 mms., a minimum value P is reached and the inductance then increases to a point S, at which it substantially has the value obtaining without premagnetisation being provided.

This behaviour is to be explained by assuming that at a distance a of from 0 to 2.5 mms. the magnet 7 is shortcircuited to a greater or less extent by the core 1 so that the external field of the magnet is considerably reduced. Beyond the point of minimum value P this effect is apparently reduced or even eliminated and inductance increases with the distance a.

Both the range OP and the range P'S (P' and S are the abscissae of the points P and S) may be used to adjust the inductance of the coil, since shifting the magnet within these ranges in one determined direction causes the inductance to change invariably in the same sense, that is to say that it either increases or decreases. The entire area OS for example is of no use for the said purpose. The area PS' yields the greatest variation, a variation of approximately 46% being enabled with the above described coil.

If the region P'S' is used the part (b) of the core projecting from the coil winding 3 may be considerably shorter than the magnet. The core may even be short enough for the magnet never to embrace it, only the part RS' of the region PS being thus used. In this event the magnet may be bored for part of its length only or may not be bored at all but in the latter case the variation of the curve of Fig. 2 is less regular. Fig. 3 shows an alternative embodiment in which the coil winding 11 is wound in one layer and the magnet 13 has a comparatively wide bore enabling it to be moved over the winding 11.

Fig. 4 shows a modification in which the short-circuiting of the magnet by the core 1 does not occur, but in this case the magnet 19 must be magnetised radially, that is to say that for example the southpole (S) is produced on the internal wall of the bore and the northpole (N) on the cylindrical external surface of the magnet.

The above described construction permits the manufacture of very small and cheap coils of reasonably satis factory coil quality (w=angular frequency, r=loss resistance). The latter property is largely due to the coil winding being wound directly on the core, but that is to say without any intermediate space, the high permeability of the core being thus fully utilised. Winding directly on the core is permitted on the one hand by the high electric resistance of the cubic core material, a separate coil sleeve being unnecessary owing to this resistance, and on the other by the fact that the inductance is adjusted without moving the core in relation to the winding.

In Fig. 2 the curve 21 indicates the variation of the coil quality Q as a function of the control (distance a); as may be seen from this curve the Q changes little by the control.

A further advantage of the above described constructionsin which the core cannot move in relation to the windingconsists in that the coil winding together with the core may be enclosed within a thin water repellent shroud, for example, of plastic.

As mentioned hereinbefore, it is highly important that the hysteresis losses in the magnet material should be low and in view thereof it is desirable that the material should satisfy a particular requirement which will be explained with. reference to" Fig. 5.

This figure shows part of the magnetisation curve (BHcurve) of a usual form of magnet steel (curve I) and of one of a ceramic permanent magnetic material for use in a coil system according to the invention (curve II). The intersections Br and Br with the Y-axis are the associated values of the remanent induction and the intersections EH01 and EH02 with the X-axis are the values of the coercive field strength.

In superposing a weak alternating field on the permanent field of the steel magnet the latter field changes for example according to the loop 41 in Fig. 5; the area of this loop is a measure of the hysteresis losses. The corresponding loop 43 of the curve II has shrunk substantially to a line 43 which is coincident with the curve II and hence the hysteresis losses are very low.

The shape of the hysteresis loop and the correlated value of the hysteresis losses are found to be determined by the BrZBHc ratio It has been found that the losses are sufiiciently low if the BI'IBHc ratio is less than 4. With ceramic permanent magnetic materials a value of this ratio may be achieved which slightly exceeds unity and the electrical resistance may be high. Such material is therefore particularly adapted for use in the device according to the invention.

What I claim is:

1. A high frequency coil system comprising a substantially bar-shaped ferromagnetic core, a'coil winding on a portion of the length of said core whereby an end of the core projects a substantial distance from the corresponding end of the coil, said coil and core being fixed relative to one another, and a hollow, low-loss, highresistance permanent magnet coaxially aligned with said coil and core at the ends thereof and producing a magnetic field which premagnetizes said core, said magnet being axially movable over a predetermined range of distances relative to said core, including distances at which said projecting end of the core enters within said hollow magnet, for varying the premagnetization of said core and thereby varying the inductance of said coil, said range of distances also having values at which the inductance of the coil always changes in the same sense when the magnet is moved in one direction.

2. A coil system as claimed in claim 1 wherein the magnet is magnetized in the axial direction.

A coil system as claimed in claim 1 wherein the magnet is magnetized in the radial direction.

4. A coil system as claimed in claim 1 wherein the core is constituted of cubic mixed crystal ferrites, the magnet is primarily composed of non-cubic crystals of a composite oxide of iron and at least one of the metals barium, strontium and lead, and the coil is wound directly on the core.

5, A coil system as set forth in claim 4 wherein the coil and core are enclosed in a water-repellent insulating sheath.

References Cited in the file of this patent UNITED STATES PATENTS 2,000,378 Deisch May 7, 1935 2,413,201 Tillman Dec. 24, 1946 2,438,770 Tillman Mar. 30, 1948 2,503,155 Harvey Apr. 4, 1950

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