The present invention relates to a magnetic circuit de¬ vice, provided with a current coil and comprising a current coil wound from a strip of magnetic sheet and a separate magnetic circuit component for passing a magnetic field generated by the current coil from one end of the magnetic longitudinal axis (winding axis) of said coil to its opposite end, the current coil made of a magnetic material forming a part of the magnetic circuit.

An advantage gained by this type of structure is that a coil, used in a conventional ballast or some other magnetic circuit and made e.g. of copper, can be com¬ pletely eliminated or the amount of copper can be sub¬ stantially reduced. Thus, the material costs will be reduced.

It is prior known to manufacture the coil of a ballast or a transformer from a strip of magnetic sheet, the current and magnetic field both travelling in the same sheet material (NO Patent Specification No. 77209 and DOS Publication 2 306 761). In this case, the current and the magnetic field travel in a direct¬ ion substantially perpendicular to each other.

A drawback in the prior art structures is a consider¬ able stray flux, caused by a plurality of joints be¬ tween the components of a magnetic circuit outside the coil. A major stray flux produces a radial magnetic field which, due to a wide coil strip, generates with alternating current a high eddy current in the coil and, thus, increases dissipations. Another drawback in the prior art structures is that the density of a

magnetic flux within the end zone of a coil changes, which is why the cross-sectional area of a magnetic circuit shall not be exploited to the optimum.

An object of the invention is to eliminate these draw¬ backs by passing a magnetic flux from one end of a coil to its other end, so that the number of joints in the magnetic circuit components outside the coil will be as few as possible while the density of a mag¬ netic flux can be optimized to be equal over the en¬ tire end zone of a coil.

According to the invention, this object is achieved in a manner that said magnetic circuit component con¬ sists - seen in a section in the travelling direction of a magnetic flux - of up to two elements, at least one which is U-shaped, and that an air gap between the coil end and said magnetic circuit component changes in the direction of the coil radius.

The most typical embodiments of the invention will now be described in more detail with reference made to the accompanying drawings, in which

fig. 1 shows a first embodiment of a magnetic circuit device of the invention in side and plan view.

Figs. 2 and 3 illustrate different coil designs seen in the direction of the winding axis.

Fig. 4a shows a coil in the direction of the winding axis and figs. 4b and 4c show different em¬ bodiments of the coil of fig. 4a in side view.

Fig. 5a shows a device according to one embodiment of the invention in a section parallel to the

winding axis (sections A-A in figs. 5b and 5c) and figs. 5b and 5c illustrate different alter¬ native designs of the device shown in fig. 5a.

Figs. 6a and 6b show in sections parallel to the wind¬ ing axis (section B-B in figs. 6c and 6d) dif¬ ferent embodiments of a magnetic circuit com¬ ponent outside the coil and figs. 6c and 6d il¬ lustrate different alternative designs of the devices shown in figs. 6a and 6b in the direct¬ ion of the winding axis.

Fig. 7a shows in a section parallel to the winding axis (section C-C in figs. 7b and 7c) one embodiment of a magnetic circuit component outside the coil and figs. 7b and 7c illustrate different alter¬ native designs of the device shown in fig. 7a in the direction of the winding axis.

Fig. 8 illustrates the changing of an air gap by means of dimensioning and design of a magnetic circuit outside the coil.

Fig. 9 shows still another embodiment of a device of the invention in side view.

Figs. 10 and 11 show in a cross-section different embodi¬ ments of a coil strip used in a device of the invention.

The component indicated by reference numeral 1 in fig. 1 is a coil wound from a strip of magnetic material. The width (dimension A) of this strip is equal to the coil length in the direction of winding axis. In this exam¬ ple, the coil is wound into the shape of a circle. A

suitable material can be e.g. pure iron, which has relatively low specific resistance and good magnetic properties. Outside the coil are located magnetic circuit components 2 , whose function is to close a magnetic circuit extending through coil 1. If, as indicated in the figure, a current I is floving through the coil along said strip, said current will produce a magnetic flux . It will be noted that both magnet¬ ic field and current travel in the same material at coil 1. The turns of a wound strip of sheet material are of course insulated from each other and from mag¬ netic circuit components 2. The insulation gap be¬ tween coil 1 and its external magnetic circuit com¬ ponents 2 is indicated in the figure with dimensions K. If magnetic circuit components 2 are composed of sheet laminates, the structure shown in the figure functions as a ballast on alternating or direct current, A strip of sheet material used in the coil must be some magnetic material and, in order to reduce current

2 losses (= 1 x R) , a low specific resistance is pre¬ ferred. In order to reduce winding resistance R, the magnetic strip can, if necessary, be plated with an electrically highly conductive coating, such as an aluminium or copper coating (see fig. 10). The coat¬ ing can be made by electroplating, vaporization or spraying. The coil can be manufactured also by wind¬ ing an aluminium or copper strip at the same time as a magnetic coil strip.

Since, in a structure of the invention, both ends of a coil must be electrically insulated from the rest of the magnetic circuit, both ends of said coil are usually provided with air gaps, in which are generally inserted members of some insulating material. There¬ fore, this construction is best suitable for such mag-

netic circuits in which an air gap is necessary, such as ballasts.

As the width of a coil strip is equal to the coil length in the direction of winding axis, there will only be two air gaps, which in practice leads to an air gap of sufficiently small size and the size there¬ of is readily controllable.

In some cases, a large air gap is required and then it may be preferable to divide a coil in two or more successive coil segments with an air gap therebetween (fig. 9b) . In this case, the coil segments are elec¬ trically connected in parallel or in series or they can be coupled to different circuits. In the case of fig. 9b, the magnetic metal strips may be wound at the same time as the common insulant strip, whose width is equal to the total width of both coils (see fig. 11b) .

The magnetic circuit components required for passing a magnetic field from one end of a coil to its other end can be made of some massive material, pieces of a sheet material or a rolled-up strip of sheet mater¬ ial, a powdered core material or ferrite or a com¬ bination thereof. A suitable material is primarily determined by the frequency of an electric current connected to the device.

In order to make magnetic field strength equal through¬ out an air gap 3, the size of said air gap is dimen¬ sioned in a manner that the gap size changes in the direction of the coil radius. For example, with the coil structures shown in fig. 4, this is achieved by having the width of a coil-forming strip of sheet

material change in the direction of the coil radius.

According to fig. 8, this has been resolved in a manner that, the surface of an external magnetic circuit, which is set against the end of a coil, will be so inclined that a desired equal flux density is obtained.

In the case of fig. 5, a magnetic circuit outside the coil is provided by placing alongside the coil two segments of a magnetic circuit of U-shaped cross-section, the ends of such U-branches coming against each other at the ends of a coil. In the case of fig. 5b, such magnetic circuit segments are shaped as half-cylinders closed at both ends. The magnetic circuit may also be composed of more than two segments, as seen in a sect¬ ion transverse relative to the direction of a flux. In the case of fig. 5c, the magnetic circuit is of a square shape in plan view but, if consisting of more segments, it may be in a polygonal shape. The magnet¬ ic circuit shown in figs. 5a and 5c can also be pro¬ duced as a single circular element, as illustrated in figs. 5d and 5e.

In the case of fig. 6, a magnetic circuit outside the coil differs from fig. 5 in that the coil end zone is provided with U-shaped segments 4. Such ϋ-segments may be circular (fig. 6d) , square (fig. 6e) or, as a compromise, polygonal.

In the embodiment of fig. 7, a magnetic circuit outside the coil comprises segments of L-shaped cross-section, placed outside the coil in a manner that one L-branch extends along the coil end and the other L-branch along the coil side. Thus, air gaps 3 can be readily adjusted. Even in this case, the magnetic circuit can

be circular seen in the direction of winding axis (fig. 7b), square (fig. 7c) or, as a compromise, polygonal. In fig. 7c, dimension h may be substantially smaller than dimension D.

If such structures are used as a transformer, one coil can function as a magnetizing (primary) coil and the other (secondary) may be just a segment of a magnetic circuit that penetrates a magnetic field produced by the primary.

Fig. 10 illustrates different alternative cross-sections for a coil strip. Layer Fe consists of a magnetic material and layer Cu of a non-magnetic, but electric¬ ally conductive material. These layers may consist of separate strips that have been wound at the same time (fig. 10a). Layers Fe and Cu can also be laid on top of each other by electroplating, vaporization or spray¬ ing (figs. 10b and 10c).

In the case of fig. 11, an insulant strip 7 is coated by electroplating, vaporization or spraying one or a plurality of magnetic material strips Fe. This can of course be provided with an electrically conductive layer, as indicated in connection with fig. 10.

In the case of fig. 11c, two magnetic strips are in¬ sulated from each other and wound at the same time. Thus, there will be two coils that can be connected parallel in series or coupled to separate circuits. This structure can also be used to provide a high capacitance between the coils. This capacitance can be utilized e.g. in interference-elimination ballasts. In the case of fig. 11d, one of the strips consists

of a magnetic material (Fe) and the other of a non¬ magnetic material (Cu) . In both figures 11c and 11d, the number of strips (coils) can be more than two.