US1721785A - Electric conductor with artificially increased self-inductance - Google Patents

July 23, 1929. u. MEYER 1,721,785

ELECTRIC CONDUCTOR WITH ARTIFICIALLY INCREASED SELF INDUCTANCE Filed Nov. 17 1925 Petented July 23, 1929. l

PATENT OFFICE.

ULI'IIAS MEYER, F GOLOGNE-MULHEIM, GERMANY.

ELECTRIC CONDUCTOR WITH ARTIFIGIALLY INCREASED SELF-INDUCTANCE.

Application filed November 17, 1925, Serial No. 69,697, and in Germany November 22, 1924.

It is well known that the prejudicial influence of the capacity can be decreased by artificially increasing the self-inductance. Two different methods are in practice for this purpose. According to Pupin the increase in the self-inductance is attained by the insertion of coilsin the line and according to .Krarup a uniform increase in the self-inductance is attained by wrapping round the conductor a material of high permeability.

Both methods have their drawbacks. In Pupins method the interrupted distribution of the self-inductance leads vto undesirable consequences while in Krarups method the 5 increase in the self-inductance is possible only to a limited extent. Both methods have the drawback that the resistance is increased,

which increase is probably unavoidable.

There is a further method of increasing the self-inductance which may be considered to lie between the two methods above referred to. According to this third method the self- 7 inductance is continuously distributed and can be increased to any desired value. If a 5 wire offerro-magnetic material a (see the drawing) is wrapped with a tape b serving as a conductor for the current a coil is obtained which has a very great length and a very small diameter and constitutes the line with artificially increased self-inductance. The

properties of such a line are very much dependent upon the angle of inclination according to which the tape is wound helically around the core. It is advisable to utilise the 5 space as'comnletelv as possible bv wrapping the tape so that the windings shall lie verv close to one another. In this case the width of the tape determines the angle of inclination of the helix. It is' immaterial forthe following dis ussion whether the tape is or is the greater is the number of turns of the coil;

accordingly the self-inductance per unit length of line rapidly increases when the angle of inclination decreases. However, there is at the same time an increase in the resistance. The smaller the angle of inclination, the longer becomes the path traversed by the current and the smaller becomes the sectional area of the said path. According to the present invention it is of utmost importance for the manufacture of the conductor to choose the correct angle of inclination, i. e. to make it so that the gain obtained by increasing the self-inductance in any case exceeds the loss caused by the increase in resistance.

Let a be the permeability of the ferro-magnetic material, 0' the conductivity of the outer covering, 9 the angle of inclination of the helix, r, the radius of the ferro-magnetic material which corresponds to the inner radius of the conducting covering, 7', the outer radius of this covering and a, and a, the angles of inclination for the radii r, and 1', and thus tgal 2'51'1 21"" For the resistance R and the self-inductance L we then get the following approximating formulae i sin (1 sin a, 092 sin (1 Sin a,

cos a L n 9,0! 10 H/Kn' As the main point is that the conductor shall have an attenuation as low as possible, the angle of inclination must be so determined by taking into consideration the other constants of the conductor that the attenuation becomes a minimum. A certain clue may generally be obtained by proceeding as follows. If the formula valid under certain conditions for the attenuation constant is taken as a basis and the second term therein which increases with the root of the selflayers.

inductance is neglected, then the problem is simplified and only the position of the minimum of 5% has to be determined. By using the last formulae we find the condition for the angle of inclination to be tg a If all the circumstances are taken into consideration we find that in order to obtain the best form of conductor the angle of inclination has to be between 30 and 70.

If the conducting tape has a great thickness, the current distribution within the tape is unfavourable, since the path of current is longer in the outer layers and therefore the stream lines of the current threads are compressed inwards. In order to avoid the increase in resistance which is thereby caused it is advisable in the case of thick conductors, to subdivide the tape and to arrange several layers which are connected in parallel one above the other. However, this object is at tained only if the subdivision does in fact produce a more uniform current distribution. For this purpose it is necessary to adjust the angles of inclination of the individual layers in such a manner that the current density is as equal as possible in all layers. As shownby the formulae this is very nearly attained if the angles of inclination and not the thickness of the tape are made nearly equal in all the The insulation of the individual layers from one another and similarly the insulation of the windings from one another need to be only very small, as the potential difi'erence between adjacent points thereof is very small. a

The advantages of this kind of conductor viz. simple form of the ferro-magnetic material, and consequently an easy subdivision thereof by using bundles of thin wires, and easy thermic treatment-for improving the magnetic properties, can be fully attained only by using the correct angle of inclination according to the present invention.

In order to reduce also the hysteresis losses which become important even when the eddy-current losses are small, it is advisable to use a material in which the permeability depends upon the strength of the current as little as possible, since according to a theory of Jordan the hysteresis losses areproportional to this increase in permeability with the current.

A further improvement in this kind of conductors can be attained if besides the inner core an outer covering of ferro-magnetic material is employed. By this means the self-inductance is further increased and in addition thereto a good path is provided for the magnetic lines of force, so that the efiect of the magnetic field outwardly is weakened. Preferably this outer covering is also subdivided into thin wires in order to reduce the eddy current losses. If, as in the case of the core, these wires are laid parallel to the axis of the conductor they must be secured in position and this may be attained for instance, by winding them helically around the conductor, the direction of the winding being opposite to the direction of winding of the actual conductor.

The accompanying drawings illustrate two forms of construction according to the invention. In Figs. 1 and 2, a is the core of magnetic material and b is the copper conductor in theform of a tape, which is wound around the said core. In Fig. 2 a is an outer layer of magnetic material.

What I claim is v 1. A conducter wi'th artificially, increased self-inductance comprising a core of ferromagnetic material and a flattened conductor intended to carry the current, which con ductor is wound helically around the said core with windings lying very close to one another and with an angle of inclination which is not less than 30 and not greater than 70 for reducing to a minimum the attenuation of the main conductor with respect to the main frequency that comes into question.

2. A conductor with artificially increased self-inductance comprising a core of ferromagnetic material and a flattened conductor intended to carry the current which conductor is wound helically around the said core with windings lying very close to one another and with an angle of inclination which is not less than 30 and not greater than 70 for reducing to a minimum the attenuation of the main conductor with re spect to the main frequency that comes into question, and an outer covering of ferromagnetic material consisting of wires or tapes wound parallel to one another around the helically wound conductor.

In testimony whereof I have affixed my signature.

ULFILAS WYER.