Patent Publication Number: US-2022223328-A1

Title: Choke for a Multi-Conductor System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of German Application No. 102021100394.6, filed on Jan. 12, 2021, which application is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a choke for a multi-conductor system. 
     BACKGROUND 
     Current compensated chokes are usually connected between a power grid and an electrical device supplied with current by the power grid. Current-compensated chokes of the type mentioned are preferably used for attenuating asymmetrical interference currents fed into the power supply grid, for example, by a frequency converter. In general, every device connected to a power supply grid has repercussions on the power supply grid. In the case of high-frequency repercussion, this is referred to as radio interference. 
     Current-compensated chokes provide high inductance values for common-mode circuits. They are therefore also referred to as common-mode chokes. However, to establish electromagnetic compatibility (EMC) of an electrical device, inductance is required in most cases even for symmetric currents. Since symmetrical load currents are not compensated in the above-mentioned current-compensated chokes, only small inductance values for symmetrical load currents can be realized with current-compensated chokes. A stray inductance of the common-mode chokes is often used as a differential inductance. If the stray inductance of the common-mode choke is not sufficient, a further electrical component has to be used for each phase or conductor. The further electrical component is usually a differential-mode choke. The differential-mode choke is also known as a symmetrical choke or storage choke. 
       FIG. 9  shows a combined common-mode-differential-mode choke according to the state of the art, which combines the functionalities of a common-mode choke and a differential-mode choke in one component. The combined choke comprises, for example, a toroidal core assembly with a toroidal core  90  and a bar arrangement  91 . In the example shown, the toroidal core  90  is provided on two opposite sides with one winding  92 ,  93  each for the forward line and for the return line with the same number of turns. The bar arrangement  91  serves to increase the stray inductance of the toroidal core  90  by a pre-definable proportion. The stray inductance is thus higher than that of the same toroidal core without a bar arrangement. The bar comprises, for example, a ferrite rod. The ferrite rod is attached to the core  90  for this purpose after the windings  92 ,  93  have been applied, for example by means of bonding. 
     However, such chokes are costly to manufacture and have poorer saturation properties or reduced linearity compared with a common-mode choke without a bar arrangement. 
     SUMMARY 
     Embodiments provide a choke for a multi-conductor system which can be manufactured inexpensively, is suitable for filtering out common-mode interference and at the same time enables sufficient suppression of differential-mode interference, in particular in applications with currents above 10 A. Further embodiments provide a choke for a multi-conductor system which enables sufficient suppression of differential-mode interference, in particular in applications with currents above 10 A. 
     According to a first aspect, embodiments are characterized by a choke for a multi-conductor system comprising a core assembly and, for each conductor of the multi-conductor system, a winding block with a pre-determined number of turns. The core assembly comprises at least one stacking unit comprising a closed magnetic ring and a separation unit. The separation unit is disposed on a first surface of the ring and comprises a closed non-magnetic separation ring or a separation ring having a magnetic permeability which is smaller, in particular considerably smaller, than the magnetic permeability of the closed magnetic ring. As an example, the magnetic permeability of the separation ring is at most 1/10 of the magnetic permeability of the closed magnetic ring. The core assembly has an inner opening enclosed by the ring of the at least one stacking unit and by the separation unit of the at least one stacking unit. The windings of the respective winding blocks extend through the inner opening of the core assembly and enclose the ring and the separation unit of the at least one stacking unit. The closed ring is formed as a closed magnetic toroidal core. 
     As an example, the separation ring is formed as a plastic part. The plastic part can be hard, i.e., non-elastic so that the height of the separation ring and a distance between closed magnetic rings separated by the separation rings, if applicable, are well-defined. As an example, the plastic part may be an injection-molded plastic part. 
     Advantageously, the separation unit with the respective winding blocks each forms an air coil (μr=1) or a coil having a core with low permeability which acts as a differential-mode inductance. The differential-mode inductances of the choke thus each comprise a stray inductance and the inductances of the respective air coils. 
     According to a second aspect, embodiments are characterized by a choke for a multi-conductor system comprising a core assembly and, for each conductor of the multi-conductor system, a winding block with a predetermined number of turns. The core assembly comprises at least one stacking unit comprising a closed ring and a separation unit. The separation unit comprises a separation segment for each conductor. The separation segments are arranged on the first surface of the ring in a pre-determined spaced-apart relationship along the circumference of the ring such that there is a gap between each of the adjacent separation segments. The core assembly includes an inner opening enclosed by the ring of the at least one stacking unit and at least partially enclosed by the separation unit of the at least one stacking unit. The winding blocks are arranged in correspondence with the separation segment, such that the windings of the respective winding blocks extend through the inner opening of the core assembly and enclose the ring and the corresponding separation segment. The separation segments comprise a magnetic material or consist of a magnetic material and/or the closed ring is formed as a closed magnetic toroid. 
     If the ring or rings of the choke according to the second aspect are configured magnetically, the choke acts as a combined current-compensated differential-mode and common-mode choke whose differential-mode inductance is highly saturation resistant, i.e., exhibits high linearity. The ring or rings of the choke can also be slightly magnetic. “Slightly magnetic” can, in particular, mean here and in the following that the magnetic permeability is between 1 and 20 and/or that the magnetic permeability is considerably smaller than the magnetic permeability of other components of the choke. As an example, the magnetic permeability of the ring or rings is considerably smaller than the magnetic permeability of the separation segments. As an example, the magnetic permeability of the ring or the rings is at most 1/10 of the magnetic permeability of the separation segments. 
     If the ring or rings of the choke in accordance with the second aspect are non-magnetic, the choke acts as a differential-mode choke whose differential-mode inductance is very saturation resistant, i.e. has a high linearity. 
     The separation segments separated by the gaps generate the required differential-mode inductances. The separation segments are, for example, bonded to the ring, taped and/or held via joining parts. The gaps between the separation segments allow a differential-mode inductance to be very saturation resistant, that is, to have high linearity, and the combined differential-mode and common-mode choke for a multi-conductor system can be designed for high currents and still have a very compact design. 
     The first surface of the ring of the choke according to the first and second aspects is preferably arranged perpendicular to a longitudinal axis of the choke. The magnetic rings of the chokes according to the first aspect and the second aspect generate the required common-mode inductance. 
     The core assembly of the choke according to the first and the second aspect enables a very compact construction. The core assembly can be easily wound using known winding techniques. One winding block per conductor or phase is simultaneously a winding for the common-mode component and the differential-mode component. This reduces the amount of material required for a winding component and the series resistance of the winding blocks. Advantageously, a combined differential- and common-mode choke for a multi-conductor system can thus be provided in a very compact design. 
     In an advantageous embodiment according to the first and second aspects, the winding blocks each comprise an equal number of turns and/or an equal winding direction. This has the advantage that good current compensation for common-mode disturbances can be achieved. 
     In a further advantageous embodiment according to the first and second aspects, the winding blocks are arranged along a circumferential line of the ring such that respective distances between immediately adjacent winding blocks are equal or at least approximately equal, i.e. within usual manufacturing tolerances. The symmetrical arrangement of the winding blocks makes it possible to achieve good current compensation for common-mode interference. In particular, in a circular ring, the winding blocks are arranged at a distance of 360°/M, where M is the number of conductors of the multi-conductor system. For example, in a 3-wire system, the winding blocks are arranged at angles of 0°, 120° and 240°, respectively, and in a 4-wire system at angles of 0°, 90°, 180° and 270°. 
     In a further advantageous embodiment according to the second aspect, respective distances between adjacent separation segments are equal. In particular, the separation segments of the at least one stacking unit may have an equal size. This has the advantage that the symmetry properties for the current-compensating effect can be further improved and the choke can be manufactured simply and inexpensively. In particular, in a circular ring, the separation segments are spaced 360°/M apart, where M is the number of conductors in the multi-conductor system. For example, in a 3-wire system, the separation segments are arranged at angles of 0°, 120° and 240°, respectively, and in a 4-wire system at angles of 0°, 90°, 180° and 270°. 
     In a further advantageous embodiment according to the second aspect, the winding blocks each have only turns in the region of the respective corresponding separation segment. Thus, there are no windings of the winding blocks in the region of the gaps. The winding blocks thus lie in a defined position above the separation segments. This results in larger and clearly definable stray inductances. 
     In a further advantageous embodiment according to the second aspect, a non-magnetic filling material is arranged in at least part of the gaps. The filling material can also be slightly magnetic. As an example, the magnetic permeability of the filling material may be at most 1/10 of the magnetic permeability of the separation segments. As an example, the magnetic permeability of the filling material is between 1 and 20. 
     In a further advantageous embodiment according to the second aspect, the filling material is also arranged outside the gaps such that it forms separating bars separating the winding blocks. This has the advantage that the windings can be held in a defined manner above the separation segments. The separating bars simultaneously define clearances and creepage distances for safe insulation between the windings. 
     It is also possible that in an embodiment according to the first aspect, one or more separating bars for separating the winding blocks are arranged at the separation ring. The separating bars can be formed from the same material as the separation ring. The separating bars can be fixed to the separation ring, for example by bonding, or are formed as an integral part of the separation ring. 
     In a further advantageous embodiment according to the second aspect, an air gap is provided between the respective filling material and the adjacently arranged separation segment. Alternatively or additionally, a joining part comprising the filling material is arranged in the gaps and comprises one or more air slots. The air gap or the air slots are preferably designed in such a way that air or another fluid can flow through them easily and thus act as a cooling channel. In particular, the filling material may be arranged in the gaps in a non-positive-locking manner. 
     In a further advantageous embodiment according to the first and second aspects, the ring of the at least one stacking unit comprises a single closed ferromagnetic circular toroidal core element or a stack of a plurality of closed ferromagnetic circular toroidal core elements. 
     In a further advantageous embodiment according to the second aspect, at least one of the separation segments of the at least one stacking unit comprises a section or sector of a toroidal core, which comprises a single closed ferromagnetic circular toroidal core element or a stack of a plurality of closed ferromagnetic circular toroidal core elements. Advantageously for ease of fabrication, all of the separation segments of the core assembly are of the same configuration. 
     In a further advantageous embodiment according to the second aspect, at least one of the separation segments of the at least one stack assembly comprises a single I-core or a plurality of I-cores stacked parallel to the first surface or standing vertically on the first surface. 
     In another advantageous embodiment according to the first and second aspects, the core assembly comprises a plurality of the stacking units arranged along a longitudinal axis of the choke, and the windings of the winding blocks each enclose all rings and separation units of the stacking units together. Here, the separation units may be of different or the same design with respect to the rings and/or separation units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are explained below with reference to the schematic drawings. However, no references to scale are shown. The invention is not limited here to the embodiments shown. 
         FIG. 1 a    shows a side view of an exemplary core assembly of a first embodiment of a choke for a multi-conductor system; 
         FIG. 1 b    shows a side view of the first embodiment of the choke; 
         FIG. 1 c    shows a top view of the core assembly according to the first embodiment of the choke; 
         FIG. 1 d    shows a simplified electrical equivalent circuit diagram of the choke according to the first embodiment; 
         FIG. 2 a    shows a side view of an exemplary core assembly of a second embodiment of the choke for a multi-conductor system; 
         FIG. 2 b    shows a top view of the core assembly according to the second embodiment of the choke for a multi-conductor system; 
         FIG. 2 c    shows a side view of the second embodiment of the choke according to the second embodiment; 
         FIG. 2 d    shows a simplified electrical equivalent circuit diagram of the choke according to the second embodiment; 
         FIG. 3 a    shows a side view of an exemplary core assembly of a third embodiment of the choke for a multi-conductor system; 
         FIG. 3 b    shows a top view of the core assembly according to the third embodiment of the choke for a multi-conductor system; 
         FIG. 3 c    shows a side view of the choke according to the third embodiment; 
         FIG. 4  shows a side view of a core assembly of a fourth embodiment of the choke for the multi-conductor system; 
         FIG. 5  shows a side view of a core assembly of a fifth embodiment of the choke for the multi-conductor system; 
         FIG. 6  shows a top view of a stacking unit of a sixth embodiment of the choke for a multi-conductor system; 
         FIG. 7 a    shows a top view of a stacking unit of a seventh embodiment of the choke for a multi-conductor system; 
         FIG. 7 b    shows a side view of the core assembly according to the seventh embodiment; 
         FIG. 8 a    shows a course of the common-mode inductance of the choke  1  according to the third embodiment depending on an operating current; 
         FIG. 8 b    shows a curve of the differential-mode inductance of the choke according to the third embodiment as a function of the operating current; and 
         FIG. 9  shows a combined common-mode-differential-mode choke according to the prior art. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1 a    shows a side view of an exemplary core assembly of a first embodiment of a choke for a multi-conductor system. 
     The core assembly  10  comprises at least one stacking unit  12 . The at least one stacking unit  12  comprises a closed ring  14  and a separation unit  16 . Optionally, the core assembly comprises a further ring  15 . 
     The closed ring  14  and the further ring  15  each comprise a closed magnetic toroidal core or are each formed as a closed magnetic toroidal core. 
     The closed ring  14  here comprises, for example, a single closed magnetic toroidal core element  18  or a stack of a plurality of closed magnetic toroidal core elements  18  (see also  FIG. 2 a   ). 
     A surface that the toroidal core elements  18  each enclose may be rectangular or elliptical, for example. In particular, the surface may be circular. 
     The toroidal core element or elements  18  are formed, for example, as toroids having a rectangular cross-section or a square cross-section. The toroidal core element or elements  18  have, for example, a height of approximately 10 mm to 15 mm, an inner diameter of 50 mm to 60 mm and an outer diameter of 80 mmm to 90 mm. 
     The toroidal core element or elements  18  comprise a ferromagnetic material or consist of a ferromagnetic material. For example, the toroidal core element or elements  18  comprise ferrite and/or iron powder and/or a ferrimagnetic ceramic material and/or Sendust. 
     In particular, the ring core elements  18  of a ring  14  may comprise different or the same materials and/or have different or the same magnetic permeabilities. 
     The ring  14  according to the first embodiment comprises, for example, four closed toroidal ferrite core elements. 
     The separation unit  16  of the at least one stacking unit  12  is arranged on a first surface  20  of the ring  14 . The first surface  20  of the ring  14  preferably extends perpendicular to a longitudinal axis L of the ring  14 . The separation unit  16  is, for example, bonded to the ring  14 . 
     In the embodiment shown in  FIG. 1 a   , the separation unit  16  exemplarily comprises a closed non-magnetic separation ring. The separation ring may comprise a single ring element or may comprise a plurality of stacked ring elements comprising a non-magnetic material or comprising a non-magnetic material. The separation ring can also be formed of a slightly magnetic material. 
     The height of the separation ring can be chosen such that the differential mode inductance is at least 1.5 times the differential mode inductance of a choke without separation unit but otherwise identically constructed. As an example, the differential mode inductance is at least twice as large. As an example, the height of the separation unit is several millimetres, e.g., at least 5 mm. As an example, the separation unit  16  has a similar height as the ring  14 . As an example, the height of the separation unit  16  may be at least half the height of the ring  14  and at most twice the height of the ring  14 . 
       FIG. 1 b    shows a side view of the first embodiment of the choke  1 . The choke  1  has a winding block  22  with a predetermined number of turns for each conductor of the multi-conductor system. 
     The core assembly  10  comprises an inner opening  24 , the inner opening  24  being enclosed by the ring  14  and by the separation unit  16  of the at least one stacking unit  12 . 
     The windings of respective winding blocks  22  extend through the inner opening  24  of the core assembly  10  and enclose the ring  14  and the separation unit  16  of the at least one stacking unit  12 . 
     The winding blocks  22  may comprise a single electrically conductive conductor or a plurality of electrical conductors connected in parallel. 
     The choke  1  according to the first embodiment is configured, for example, as a current-compensated choke. This is achieved, for example, among other things, by the winding blocks  22  each having an equal number of turns and an equal winding direction. 
     The winding blocks  22  are preferably applied to the core assembly  10  in a symmetrical manner, so that a desired high degree of compensation of common-mode interference signals is achieved when the arrangement has a symmetrical geometrical configuration and due to characteristics of the ring  14  with respect to stray inductances. 
     Preferably, the winding blocks  22  are arranged along a circumferential line of the ring  14  such that respective distances between immediately adjacent winding blocks  22  are equal within normal manufacturing tolerances. 
     Furthermore, other characteristics of the winding blocks  22 , such as the distances between individual turns, and the like can be set up as symmetrically as possible for all winding blocks  22 . Furthermore, the winding blocks each comprise, for example, an equal number of conductors, and the conductors of the respective winding blocks  22  comprise, for example, an identical electrically conductive material or consist of the same electrically conductive material. 
     The choke  1  of the first embodiment may comprise separating bars for separating the winding blocks  22 . As an example, the separating bars may be fixed to the separation ring or may be an integral part of the separation ring. Depending on the position of the separation ring, the separating bars may protrude laterally, in an axial direction or both laterally and in an axial direction from the separating bar. The separating bars can be formed as the separating bars in  FIGS. 2 a , 3 a    and all other embodiments. All further characteristics of the separating bars disclosed for the other embodiments may be present in separating bars of the first embodiment. 
       FIG. 1 c    shows a top view of the core assembly  10  according to the first embodiment of the choke  1 . In particular,  FIG. 1 c    shows the course of the magnetic field lines in the ring  14 . 
     This example shows the choke  1  for a 3-conductor system or a 3-phase system. 
     For example, the choke  1  comprises three identical winding blocks  22  symmetrically arranged on the core assembly  10 . The magnetic fields induced by the currents in the three-phase windings add up in the toroidal core. The following therefore applies 
     
       
         
           
             
               
                 
                   
                     
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     Here, Hcore represents the magnetic field in the toroidal core that couples into all three winding blocks, le is the effective length of the magnetic path of the toroidal core, and n is the number of turns in each winding block  22 . Due to the finite permeability of the magnetic core material, a small portion of the magnetic field generated by the conductor currents iA, iB, and iC is leaked by the toroidal core into the air, which is indicated by dashed lines in  FIG. 1 c    and denoted by HLeak. The leakage flux of one winding is characterized by the fact that it is not interdinked with the other windings. The stray fields of current-compensated windings leaves the core essentially in the gaps between the windings. 
       FIG. 1 d    shows a simplified electrical equivalent circuit diagram of the choke  1  according to the first embodiment. 
     In the equivalent circuit diagram, the resistors RCU take into account the ohmic losses of the winding blocks. The resistors RFE take into account the magnetic losses of the toroidal core and Cp the coupling capacitances within the winding blocks. 
     Advantageously, the separation unit  16  forms an air coil (μr=1) with each of the winding blocks  22 , which acts as a differential-mode inductance. The differential-mode inductances LDM of the choke  1  thus comprise in each case, in addition to a stray inductance, and the inductances of the respective air coils. 
     For the choke  1  according to the first embodiment, this results in the following inductances (for typically LCM&gt;&gt;LDM): 
     
       
         
           
             
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     where Ae is the inner cross-section of the toroidal cores  14 ,  15 . Lstray is the stray inductance. The inductance of the air coil Lair coil depends on an air coil shape, which can be round, oval or rectangular, for example. 
       FIG. 2 a    shows a side view of an exemplary core assembly  10  of a second embodiment of a choke  1  for a multi-conductor system. 
     The core assembly  10  comprises at least one stacking unit  12 . The at least one stacking unit  12  comprises a closed ring  14 , and a separation unit  16 . 
     The closed ring  14  is formed, for example, as a closed magnetic toroidal core. 
     The ring  14  is formed, for example, as the ring  14  described in the first embodiment. 
     The separation unit  16  comprises a separation segment  26  for each conductor. The separation segments  26  are arranged on the first surface  20  of the ring  14  at pre-determined distances along the circumferential line of the ring  14 , such that there is a gap  28  between each of the adjacent separation segments. 
     The separation segments  26  preferably comprise a ferromagnetic material or consist of a ferromagnetic material. For example, they comprise ferrite and/or iron powder and/or a ferrimagnetic ceramic material and/or Sendust. 
     Alternatively, it is possible that the separation segments  26  are non-magnetic and made of a material having a relative permeability of μr=1. It is also possible that the separation segments  26  are slightly magnetic. As an example, the separation segments  26  may be formed as plastic parts. The plastic parts may be non-elastic. 
       FIG. 2 b    shows a top view of the core assembly  10  according to the second embodiment of the choke  1 . 
     The separation segments  26  of the at least one stacking assembly  12  comprise, for example, a section or sector of a toroidal core, which comprises a single closed ferromagnetic circular toroidal core element or a stack of a plurality of closed ferromagnetic circular toroidal core elements. In particular, the same toroidal core elements that are used for the ring  14  can be used to manufacture the separation segments. Here, a size of the toroidal core elements can be chosen to be the same or different. 
     In the example shown, the separation segments  26  are all of the same design, but may also be of different design, in particular in terms of shape and/or material. 
     For example, a non-magnetic filling material is arranged in the gaps  28 . The filling material can also be arranged outside the gaps in such a way that it forms separating bars which separate the winding blocks (see also  FIG. 2 a   ). 
       FIG. 2 c    shows a side view of the second embodiment of the choke  1 . The choke  1  comprises a winding block  22  with a pre-determined number of turns for each conductor of the multi-conductor system. 
     The separation segments  26  are arranged to correspond to the winding blocks  22 , such that the turns of the respective winding blocks  22  extend through the inner opening  24  of the core assembly and enclose the ring  14  and the corresponding separation segment  26 . In particular, the winding blocks  22  have windings only in the region of the respective corresponding separation segment  26 . Regions in which the gaps  28  are located are preferably free of windings. 
     For example, the winding blocks  22  are formed like the winding blocks  22  described in the first embodiment. 
     Preferably, the winding blocks are designed to provide optimal current compensation for common-mode currents. 
     In particular, the separation segments  26  are arranged such that the respective distances between adjacent separation segments  26  are equal. 
     The separation segments  26  are, for example, bonded or taped to the ring  14  and/or are held in place by joining parts. 
       FIG. 2 d    shows a simplified electrical equivalent circuit diagram of the choke  1  according to the second embodiment. In contrast to the choke according to the first embodiment, in this equivalent circuit diagram the resistor Rfe 2  represents the separation unit  16  with the separation segments  26  made of ferromagnetic material with μ*Rfe 2 &gt;&gt;1. In relation to the choke  1 , the resistor Rfe 2  exhibits poorer saturation behaviour with simultaneously higher differential inductance than the choke  1  according to the first embodiment. Depending on application requirements, for example, a choke  1  according to the first embodiment or a choke according to the second embodiment can be selected. 
     In the equivalent circuit, the resistors RCU take into account the ohmic losses of the winding blocks. The resistors RFE 1  and RFE 2  take into account the magnetic losses of the toroidal core and the magnetic separation segments, respectively, and Cp takes into account the coupling capacitances within the winding blocks. 
     In a further embodiment of the core assembly  10  or choke  1  shown in  FIGS. 2 a  to 2 c   , the ring  14  comprises, for example, a non-magnetic material and only the separation segments comprise a magnetic material. In this case, the choke  1  behaves as a differential-mode choke. The separation segments  26  may be bonded to the ring  14  and/or to each other. Alternatively, it is possible that all elements of the core assembly  10  are arranged a plastic trough or plastic cup. 
       FIGS. 3 a  and 3 b    show a side view and a top view, respectively, of an exemplary core assembly  10  of a third embodiment of the choke  1  for a multi-conductor system.  FIG. 3 c    shows a side view of the choke  1  according to the third embodiment. 
     The core assembly  10  comprises at least one stacking unit  12 . The at least one stacking unit  12  comprises a closed ring  14  and a separation unit  16 . 
     Further, the core assembly  10  comprises, for example, another ring  15  and the core assembly  10  is symmetrically configured along its longitudinal axis L. 
     The separation unit  16  of the at least one stacking unit  12  is configured, for example, as the separation unit  16  described in the second embodiment. 
     The ring  14  of the at least one stacking unit  12  is configured, for example, like the ring  14  described in the first embodiment. 
     The winding blocks  22  of the choke  1  (shown in  FIG. 3 c   , not shown in  FIGS. 3 a  and 3 b   ) are formed and arranged, for example, like the winding blocks  22  described in the second embodiment. 
     A non-magnetic filling material  30  is arranged in the gaps  28 , for example. The filling material  30  may also be arranged outside the gaps  28  in such a way that it forms separating bars separating the winding blocks  22 . 
     Optionally, an air gap  32  may be located between the respective filling material  30  and the adjacently arranged separation segment  26 . Alternatively, a joining part may be disposed in the gaps  28  comprising the filling material  30  and having one or more air slots. This allows air circulation and thus cooling of the core assembly. 
     For the choke  1  according to the third embodiment, the equivalent circuit diagram shown in  FIG. 2 d    can also be used to describe the electrical and magnetic properties. 
       FIG. 4  shows a side view of a core assembly  10  of a fourth embodiment of the choke  1  for a multi-conductor system. 
     The core assembly  10  shown in  FIG. 4  has a plurality of stacking units  12  arranged along the longitudinal axes L of the core assembly. For example, in the embodiment shown in  FIG. 4 , the stacking units  12  are each of the same design. The core assembly  10  shown in  FIG. 4  has, for example, a further ring  15  which may be of the same or different design as the rings  14  of the stacking units. 
     For example, the separation units  16  of the stacking units  12  are formed like the separation unit  16  described in the second embodiment or like the separation unit  16  described in the first embodiment. 
     The rings  14  of the stacking units  12  are formed, for example, like the ring  14  described in the first embodiment. 
     The winding blocks of the choke (not shown in  FIGS. 3 a  and 3 b   ) are formed, for example, like the winding blocks described in the first or second embodiment. 
       FIG. 5  shows a side view of a core assembly  10  of a fifth embodiment of the choke  1  for the multi-conductor system. 
     The core assembly  10  shown in  FIG. 5  comprises a plurality of stacking units  12  arranged along the longitudinal axes L of the core assembly  10 . In the embodiment shown in  FIG. 5 , the stacking units  12  are configured, for example, partially identically and partially differently. 
       FIG. 6  shows a top view of a stacking unit  12  of a sixth embodiment of the choke  1  for a multi-conductor system. 
     The stacking unit  12  shown in  FIG. 6  comprises at least one separation segment  26  comprising a single I-core or a plurality of I-cores arranged parallel to the first surface  20  in a stacked manner. Preferably, however, all separation segments  26  of the separation unit  16  are of the same configuration. 
       FIG. 7 a    shows a top view of a stacking unit  12  of a seventh embodiment of the choke  1  for a multi-conductor system. 
     The stacking unit  12  shown in  FIG. 7 a    comprises at least one separation segment  26  comprising a single I-core or a plurality of I-cores arranged perpendicularly on the first surface  20 . Preferably, however, all separation segments  26  of the separation unit  16  are of the same design. 
       FIG. 7 b    shows a corresponding side view of the core assembly  10  according to the seventh embodiment. In an optional embodiment, the core assembly has a further ring  15 , which is formed, for example, as a magnetic toroidal core. 
     In particular, the stacking units  12  according to the sixth and seventh embodiments can also be used as stacking units  12  for the chokes  1  according to the first to fifth embodiments. 
       FIGS. 8 a  and 8 b    show a course of the common-mode inductance and the differential-mode inductance, respectively, of a choke  1  according to the third embodiment as a function of the current. 
     The nominal common-mode inductance of the choke is 1.1 mH. The current has a frequency of 10 kHz. The common-mode inductance of the choke is reduced to about 80% of the nominal common-mode inductance when the current is increased from 0 A to 200 A. 
     The choke simultaneously provides a differential-mode inductance of 13 μH. The differential-mode inductance remains almost constant over the entire measuring range from 0 A to 180 A. 
     In the prior art current compensated choke as shown in  FIG. 9 , the possibility of increasing the stray inductance is very limited due to the limited design space and the differential-mode inductance is on average 1% of the common-mode inductance. If the winding space is limited and parallel windings are used, the value of the differential inductance is about 0.5% of the common-mode inductance. 
     In the case of the choke, an increase in the differential-mode inductance by a factor of 2.4 could be achieved for a choke  1  according to the second embodiment and the third embodiment. Also with chokes according to the first and all further embodiments, a considerable increase of the differential-mode inductance may be achieved. As an example, the differential-mode inductance may be 1.5 times or 2 times the differential-mode inductance of a choke without separation unit but otherwise identically constructed. This can depend, in particular, from the height of the respective separation unit. As an example, in all embodiments, a height of the separation unit may be at least 5 mm. 
     The invention described herein is not limited by the description based on the embodiments. Rather, the invention encompasses any new feature as well as any combination of features, which in particular includes any combination of features in the claims, even if that feature or combination itself is not explicitly stated in the claims or embodiments.