The present invention relates to an inductance coil having a core in the form of a closed loop, particularly of circular cylindrical shape, which core comprises a plurality of mutually superimposed layers of a continuo s strip of magnetic material. The invention also relates to a method of manufacturing the coil.

When manufacturing inductance coils having cores of magnetic material, normally iron, particular attention must be paid to the magnetic properties of the iron, since these vary greatly with the strength of the magnetizing field and the size of the air-gap, when one is provided. In many applications it is necessary to utilize an air- gap, since this reduces changes of the inductance with changes in current strength through the coil. Thus, it can be said that the air-gap is effective in extending the closed magnetic circuit. This takes place, however, with a multiplication factor which may reach from 100-1000, mean¬ ing that the size of the air-gap has a totally decisive influence of the characteristics of the inductance coil. In the case of, inter alia, toroidal inductance coils, which in many aspects are highly advantageous, it has been found very difficult to provide small air-gaps of a pre- cize, predetermined size, because of the elasticity of the magnetic material. The U.S patent specification 1 318 787 discribes a method of solving this problem, where¬ at the iron core is composed of a plurality of annular elements which are perforated axially by slots which are limited in a radial direction by at least one unbroken part, which serves to hold the annular element together. Efficient manufacture of iron cores today is effected by winding a continuous strip onto a mandrel until the de¬ sired core thickness is reached. If the method proposed in the aforementioned U.S. patent specification were to be applied, all turns in the core, perhaps with the exception of the last turn, v/ould be completely cut off. Because of

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the elasticity of the strip material, this would mean tha deformation forces exerted by the inner turns would act on the outer turn. Further, the possibility cannot be ex¬ cluded of certain displacements between the ends of the cut turns, which should mean that the width of the air-gap would vary sligthly from turn to turn.

Because there are no acceptable solutions for pro¬ ducing an exactly defined air-gap in annular iron cores i tended for high qualititive purposes, the cores today are normally in the form of so called "C"-cores, which are co bined pair-wise with an intermediate air-gap, or in the form of a combination of "E"-cores and "I"-cores, whereat an air-gap is normally formed between the centre leg of the E-core and the I-core. In order to produce a core for applications in which a precise toroidal shaped inductance coil is desired, it necessary first to imbed the core in a plastics material, and then to saw said core. As will be understood, this in volves an additional working step, which renders the core more expensive and limits the sphere of applicability of said method.

As a result of a multiplication factor of 100-1000, variation in the length of an air-gap has a great affect on the characteristics of the inductance coil. Thus, for example, a change in an air-gap of from 0,5-0,6 mm causes a 20% extension thereof, which as a result of the high multiplication factor results in an even greater change i the characteristics of the inductance coil. Because of this, it has been normally hitherto to use relatively lar gaps, where small differences do not produce similar larg effects when seen proportionally.

A prime object of the present invention is to provid an inductance coil of the kind mentioned in the introduc¬ tion, which can be provided with a narrow and well define air-gap.

This object is achieved in accordance with the inven¬ tion by providing the core with perforations or slots ex¬ tending radially through the core, which perforations or slots are limited axially in each layer of the core by at least one unbroken part which holds the layer together.

This means that the inner layers do not exert any force on¬ to the outer layer, since each layer includes at least one unbroken tongue of material, which holds respective layer together. Preferably, the area of the unbroken part or parts in each layer is such that said part or parts become saturated at those operational conditions under which the inductance coil is intended to work, since in this way they have the same effect as though they themselves also com¬ prised air. A method of manufacturing an inductance coil according to the invention, in which the core is formed by winding a plurality of mutually superimposed layers on a mandrel from a continuous strip of magnetic material^ is characterized in that the perforations or slots are produced by punching openings in the strip prior to or during the winding of said strip onto said mandrel, in a manner such that said openings when winding said strip are located opposite one another, or alternatively by a radially directed drilling of the core, or cutting said strip in some other way, when the core is. fully wound. When the perforations are created by drilling the core radially, the dimensions of the air-gap can be adjusted progressively while measuring the proper¬ ties of the core.

The strip can be punched while winding the same, by means of a punch tool controlled in dependence upon the winding length of the strip on the mandrel, or alternative¬ ly may be controlled by a master strip.

Exemplary embodiments of the invention will now be described in more detail with reference to the accompany- ing drawings.

Figure 1 is a plan view of a toroidal choke core according to the invention.

Figure 2 is a sectional view of the core shown in Figure 1, taken on the line II-II. Figure 3 shows part of a strip of magnetic material, used when manufacturing the choke core of Figure 1 and 2. Figure 4 illustrates an alternative embodiment of a strip intended for a magnetic core.

Figure 5 illustrates part of a further embodiment of a strip intended for a choke core.

Figure 6 is a sectional view of a toroidal choke hav ing an iron core and formed from a strip according to Figure 5.

The choke core illustrated in Figures 1 and 2 has bee obtained by winding a strip blank 1 of the Figure 3 embod ment about a mandrel, which when removed leaves a space 2 which can be used for the subsequent winding of the actua choke. For the purpose of providing the desired air-gap the strip 1 has been punched to provide pairs of slots 3, which when winding the strip onto said mandrel are locate opposite one another. The distances a, b and c thus corre spond to the periphery of successive strip turns. Located between the slots 3 of each pair is an unbroken part 4, which holds the strip together and enables said strip to be wound. Thus, each turn is held together with the aid o said unbroken parts, and hence there are no springback te dencies which could affect the size of the air-gap. Further, the inner turns will not cause any deforming for to act solely on the outer turn or turns. The strip I ¹ shown in Figure 4 differs from the stri 1 shown in Figure 3, in that said strip 1' is provided wi elongate, continuous slots 3 ¹ , whereat an unbroken part 4' is located at each end of respective slot. The part 4' fi the same function as the part 4 in the strip 1 showed in Figure 3. Preferably, said parts are given such dimension

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that they become saturated at those operating conditions under which the choke is intended to work. This means, that the unbroken parts also obtain air-gap properties. The slots shown in Figure 3 and 4 are only preferred embodi- ents, since said slots can be varied as desired, both with respect to the number and the shape. It is important, how¬ ever, that at least one unbroken part is obtained, for holding the strip of each turn together.

The slots may either be punched by means of a one piece punching tool prior to winding the strip, or may be punched while the strip is being wound, whereat the punch¬ ing of the strip is either controlled by the winding length or by means of a master strip. The choke core may also be wound from an unbroken strip, and the requisite air-gap formed subsequently by drilling, sawing or some other cutt¬ ing operation. In this case, the properties of the core may be continuously measured during the process of core manu¬ facture, so as to obtain the desired characteristics.

By means of the aforedescribed method, it is possible to produce choke cores with extremely small air-gaps, in the order of 0,5 mm, with great precision, e.g. with an accuracy of about 5/100 mm.

Figure 5 illustrates a part of a so-called wedge- shaped clipping from a sheet metal forming a strip 11, which is provided with slots 13 by punching, in accordance with the aforegoing, said slots being located opposite one another when the strip is wound onto a mandrel. As a re¬ sult of the wedge-shape of the strip, there is obtained when winding said strip onto a mandrel a core having a parallel-trapezoidal shaped cross-section, said core en¬ abling a choke to be wound with substantially planar upper and lower surfaces. When winding the choke, the number of winding layers will be greater within the choke, at 16, than at its outer surface at 17. Thus, the number of layers at the ends of the choke will decrease from the

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inner part 16 to the outer part 17, as indicated at 15. Such a choke shape enables, inter alia, said choke to be capsulated in a much simpler fashion, since the choke can, for example, be placed in a tin or the like, whereat only a very small space need be filled with fill¬ ing material. Further, such a shape provides good heat ^" dissipation properties, as a result of the planar mountin surfaces, v/hich act as cooling surfaces. If only one plan surface is required, the core can be given a ro boidal cross-section. The invention can also be modified in othe respects within the scope of the following claims.

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