EMI filter

Abstract

This invention provides improved or alternative EMI filter circuits and methods for filtering electromagnetic interference (EMI) within the circuits. [Solution] The EMI filter has a first core pillar 1, a second core pillar 2, and a third core pillar 3 extending in parallel directions to form a closed magnetic circuit, the ends of each core pillar being connected by first and second bridging legs 6, 7. A first coil 9, which is part of a DC power line conductor, is arranged around the first core pillar and / or the first bridging leg, and a second coil is arranged around the third core pillar and / or the second bridging leg. The EMI filter further comprises first and second compensation coils 25, 26, the first compensation coil being arranged around the first core pillar and / or the first bridging leg, and the second compensation coil being arranged around the third core pillar and / or the second bridging leg, in order to reduce the magnetic flux generated in the second core pillar by the first and second coils.

Description

Technical Field

[0001] The present invention generally relates to electromagnetic interference (EMI) filters and EMI filter circuits, as well as processes for filtering EMI interference.

[0002] Chinese Patent Specification No. 115938747 discloses an EMI filter for use in a filter circuit of an electric compressor controller. The EMI filter has a core formed from two E-shaped core portions arranged in an opposing configuration. Thus, the core has two parallel pillars joined at the upper and lower ends by a bridging portion. The core is provided with two winding coils for supplying current to and from the compressor motor, and the winding coils are arranged on the two pillars. To suppress differential mode (DM) interference, it is known to arrange an intermediate pillar between the two outer pillars, whereby the magnetic flux generated by DM interference in the two winding coils can use a common path through the intermediate pillar. However, this results in saturation of the intermediate pillar when DM current is present. In the embodiment shown in Chinese Patent Specification No. 115938747, a gap is introduced into the intermediate pillar. This gap reduces the sensitivity to saturation of the magnetic core material.

[0003] An object of the present invention is to provide an improved or alternative EMI filter circuit and a method for filtering electromagnetic interference (EMI) within the circuit.

[0004] This is achieved, in a first aspect of the present invention, by providing an EMI filter with a filter core having a compensating winding.

[0005] The core forms a closed magnetic circuit and has a first core pillar, a second core pillar, and a third core pillar extending in parallel directions, the second core pillar being positioned between the first and third core pillars. The ends of the first core pillar and the ends of the second core pillar are connected by a first bridging leg, and the ends of the second core pillar and the ends of the third core pillar are connected by a second bridging leg. The first core pillar, the second core pillar, and the first bridging leg define a first opening, and the second core pillar, the third core pillar, and the second bridging leg define a second opening.

[0006] A first coil is arranged around a first core pillar and / or a first bridging leg, and the first coil is part of a first DC power line conductor for connection to a DC power source and load. A second coil is arranged around a third core pillar and / or a second bridging leg, and the second coil is part of a second DC power line conductor for connection to a DC power source and load. The EMI filter further comprises a compensating winding for connection to a power source and a compensating current controller for controlling the current supply to the compensating winding. The compensating winding is arranged on the core and comprises a first compensating coil and a second compensating coil, wherein the first compensating coil is arranged around the first core pillar and / or the first bridging leg, and the second compensating coil is arranged around the third core pillar and / or the second bridging leg, in order to reduce the magnetic flux generated in the second core pillar by inducing a magnetic flux density in the second core pillar in the opposite direction to the direction of the magnetic flux density generated in the second core pillar by the DC current supplied through the first and second coils.

[0007] DC power lines can be subject to direct mode interference, which is noise or undesirable signals carried on two power lines but in opposite directions. This noise can occur when using high-frequency switching devices in power supply circuits, such as when using a motor inverter. The first coil on the EMI filter core is part of the DC power line and forms an inductor to suppress DM interference. Similarly, the second coil on the EMI filter core is part of the DC power line and forms an inductor to suppress DM interference.

[0008] The first and second coils are wound around the core such that DC currents, i.e., currents to and from the DC power supply, flow in opposite directions through the respective first and second openings. Thus, differential-mode noise induces opposing circumferential magnetic fluxes within the core around the first and second openings, respectively. These fluxes are summed within the second core pillar, making it susceptible to saturation from high DC current values ​​in the first and second DC power line conductors. While it is possible to introduce a gap in the second pillar of the core to reduce magnetic flux density and susceptibility to saturation, this also reduces the core's inductance characteristics. Instead of an air gap, it is proposed to use a compensating winding positioned on the core to induce a magnetic flux density in the second core pillar in the opposite direction to the magnetic flux density generated in the second core pillar by the DC current supplied through the first and second coils. Thus, the magnetic flux generated in the second core pillar is reduced, thereby making it less susceptible to magnetic saturation. This method ensures the maximum DM inductance value of the core, regardless of the DC current flowing through it.

[0009] Furthermore, in the configuration according to the present invention, the EMI filter can suppress both common-mode interference and differential-mode interference, thereby generating a magnetic flux in the core that is summed up when common-mode interference occurs when noise appears in phase on both power lines. The common-mode noise generates a flow of magnetic flux in the circuit across the outer circumference of the core at the first core pillar and the third core pillar and the bridging legs. Thus, the energy from the common-mode interference is stored in the core so that the noise in the power lines is attenuated and the noise does not propagate further through the circuit. Consequently, the core and coil have an inductive impedance that attenuates common-mode noise and is not affected by the DC current value.

[0010] The core is preferably gapless, which means forming a closed magnetic circuit without air gaps. The core is preferably made from a material having or having uniform permeability. Making the core gapless does not degrade the inductance characteristics of the core.

[0011] The core may comprise two or more core portions. These portions can be connected to each other in a gapless manner to form a closed magnetic circuit. In a preferred embodiment, the core is an EE core. However, other configurations can also be used to form a core with three pillars, such as an EI or TU type core. The cross-section of the second core pillar can be rectangular or circular.

[0012] In a preferred embodiment, one end of the first coil is connected to a first terminal of a DC power supply. Similarly, one end of the second coil is connected to a second terminal of a DC power supply.

[0013] The first coil and the second coil are positioned in the power circuit before and after the load, respectively.

[0014] In a preferred embodiment, the first and second compensating coils are connected in series, with the first compensating coil positioned around one of the first pillars or first bridging legs. The second compensating coil is positioned around one of the third pillars or second bridging legs. This prevents core saturation of the CM signal in the circuit across the outer circumference of the core. The first and second compensating coils 26 are connected in series and wound in opposite directions so that the flow of magnetic flux is guided across the outer circumference of the core in opposite circumferential directions and in the same direction through the second core pillars. In another embodiment, the coils can be connected in parallel.

[0015] The compensating current controller includes an open-loop compensation algorithm that can supply a compensating current to the compensation winding in response to the DC current in the first and / or second coil. Alternatively, the compensating current can be made dependent on at least two of the phase currents of the inverter or electric motor. For example, at least two, preferably all, of the phase currents can be supplied as input to the compensating current controller. The current in the DC power line can also be measured.

[0016] The first coil comprises multiple windings of a conductor having a rectangular cross-section, as part of a DC power line. Similarly, the second coil may comprise multiple windings of a conductor having a rectangular cross-section.

[0017] The EMI filter can preferably be part of the motor drive.

[0018] The motor drive comprises a circuit in which a first coil is connected to a DC power supply and located in the circuit upstream of the load, particularly the motor; a second coil is connected to a DC power supply and located in the circuit downstream of the load; and a compensating winding forms part of a compensating circuit comprising a compensating current controller for supplying current to the compensating winding. A DC power supply or current source is connected to the compensating winding.

[0019] DC power line conductors are connected to the inputs of a motor inverter, which is configured to convert DC inputs into three or more AC phases for driving an electric motor.

[0020] As described above, the compensating current controller includes an open-loop compensation algorithm that can supply a compensating current to the compensating winding in accordance with at least two phase currents of the AC phases of the inverter, preferably all phase currents. Alternatively, the measured currents in at least two phases may be used as inputs for estimating the DC power line current and / or for controlling the current in the compensating winding.

[0021] In a preferred embodiment, the DC power supply is configured to supply a voltage of at least 48V between a first DC power line conductor away from the DC power supply and a second DC power line conductor leading to the DC power supply.

[0022] A second aspect of the present invention provides a method for filtering electromagnetic interference (EMI) in a power supply circuit. The method is as follows: - A step of preparing a core that forms a closed magnetic circuit, wherein the core has a first core pillar, a second core pillar, and a third core pillar extending in parallel directions, and the second core pillar is positioned between the first core pillar and the third core pillar. - The end of the first core pillar and the end of the second core pillar are connected by the first bridge pier, and the end of the second core pillar and the end of the third core pillar are connected by the second bridge pier, -The first core pillar, the second core pillar, and the first bridging leg define the first opening, - The second core pillar, the third core pillar, and the second bridging leg define the second opening. Steps and - A step of providing a first coil around a first core pillar and / or a first bridging leg, wherein the first coil is part of a first DC power line conductor for connection to a DC power source and load, - A step of providing a second coil around the third core pillar and / or the second bridging leg portion, wherein the second coil is part of a second DC power line conductor for connecting to a DC power supply and a load. - A step of preparing a compensation circuit including a compensation winding, wherein the compensation winding is disposed on a core and includes a first compensation coil and a second compensation coil, the first compensation coil is disposed around the first core pillar and / or the first bridging leg portion, and the second compensation coil is disposed around the third core pillar and / or the second bridging leg portion. - A step of controlling a compensation current in the compensation winding so that a magnetic flux density in the second core pillar is generated in a direction opposite to a magnetic flux density generated by the first coil and the second coil. Including.

[0023] Any feature disclosed as part of the first aspect of the present invention can, in the second aspect of the present invention, be followed alone, or in combination, or in any arrangement or permutation of any one or more of the recited elements.

[0024] Here, embodiments will be described by way of example only with reference to the accompanying drawings.

Brief Description of the Drawings

[0025] [Figure 1] A schematic diagram of an EMI filter according to the present invention is shown. [Figure 2] A schematic diagram of an EMI filter according to the present invention in a motor drive is shown. [Figure 3] A perspective view of a part of an EMI filter according to an embodiment of the present invention is shown. [Figure 4] An exploded view of a part of an EMI filter according to an embodiment of the present invention is shown. [Figure 5] A schematic diagram of an EMI filter according to an embodiment of the present invention is shown. [Figure 6] A table is shown.

[0026] Detailed explanation of the diagram Figure 1 shows a schematic diagram of the EMI filter 4 according to the present invention. The EMI filter 4 is a first inductor L DM1 The first coil 9 and the second inductor L form a coil DM2 The first coil 9 and the second coil 22 are provided on the core 5 as shown in Figure 3. The core 5 forms a closed magnetic circuit and has a first core pillar 1, a second core pillar 2, and a third core pillar 3 extending in parallel directions. The second core pillar 2 is positioned between the first core pillar 1 and the third core pillar 3. The end of the first core pillar 1 and the end of the second core pillar 2 are connected by a first bridging leg 6, and the end of the second core pillar 2 and the end of the third core pillar 3 are connected by a second bridging leg 7. The first core pillar 1, the second core pillar 2, and the first bridging leg 6 define a first opening 8, and the second core pillar 2, the third core pillar 3, and the second bridging leg 7 define a second opening 34.

[0027] The first coil 9 is positioned around the first core pillar 1. However, in other configurations, the first coil 9 may be positioned, for example, around the first bridging leg 6, i.e., around one of the bridging legs 6 at the upper or lower end of the core in Figure 3, or partially on the bridging leg 6 and partially on the first core pillar 1. The first coil 9 is part of the DC power line conductor 10 connected to the DC power supply 12 via the first terminal 21 and is also connected to the load 11. The second coil 22 is positioned around the third core pillar 3. However, in other configurations, the second coil 22 may be positioned, for example, around the second bridging leg 7, or partially on the bridging leg 7 and partially on the third core pillar 3. The second coil 22 is part of the return DC power line conductor 23 connected to the load 11 and also connected to the DC power supply 12 via the second terminal 24. The DC power supply 12 can be a battery or the output of an AC / DC converter. Therefore, the first coil 9 and the second coil 22 are positioned in the power circuits before and after the load 11, respectively.

[0028] The EMI filter 4 includes an across-the-line capacitor C connected to the first power line conductor 10 and the second power line conductor 23 to suppress differential mode noise. X In addition, two line bypass capacitors C are connected between the first and second power line conductors 10 and 23 and ground, respectively, to suppress common-mode noise. Y Furthermore, the capacitor provides a low-impedance path for noise to pass through, rather than continuing along the power line.

[0029] The EMI filter 4 further includes a compensation winding 14 connected to the power supply 15 and a compensation current controller 16 for controlling the current supply to the compensation winding 14.

[0030] In the embodiment shown in Figure 3, the compensating winding 14 is located in the core 5 around the first bridging leg 6 and the second bridging leg 7, but other configurations are possible in this case as well. In the embodiment shown in Figure 5, the compensating winding 14 is located in the first core pillar 1 and the third core pillar 3.

[0031] The compensating winding 14 is positioned to induce a magnetic flux density in the second core pillar 2 in a direction opposite to the direction of the magnetic flux density induced in the second core pillar 2 by the DC current supplied through the first coil 9 and the second coil 22. This reduces the magnetic flux generated within the second core pillar 2, thereby making the second core pillar 2 less susceptible to magnetic saturation. This is schematically shown in Figure 5. The first coil 9 is,

number

number

[0032] DC power lines can also be subject to direct mode interference, which is noise or undesirable signals carried on two power line conductors 10 and 23 but in opposite directions. This noise can occur when using high-frequency switching devices in power supply circuits, such as when using a motor inverter.

[0033] The first coil 9 on the EMI filter core 5 is part of the DC power line conductor 10 and forms an inductor for suppressing DM noise. Similarly, the second coil 22 on the EMI filter core 5 is part of the return DC power line conductor 23 and forms an inductor for suppressing DM noise. Thus, the same core 5 can be used for both common-mode and differential-mode noise attenuation.

[0034] The direction of the magnetic flux generated in the core by the first coil 9 and the second coil as a result of DC current being supplied to the load 11 is indicated by arrow 20. The direction of the magnetic flux generated by the first coil 9 and the second coil 22 as a result of DM noise may be the same as the direction indicated by arrow 20. In Figure 5, it can be seen that the magnetic flux generated by DM noise in the first coil 9 and the second coil 22 is summed in the second core pillar 2. This DM noise can lead to undesirable magnetic saturation of the second pillar 2.

[0035] To counteract this, the compensation winding 14 in this embodiment is provided by a first compensation coil 25 located on the first pillar 1 and a second compensation coil 26 located on the third pillar 3, although other configurations of the compensation winding are also conceivable. In the embodiments shown in Figures 3 to 5, the first compensation coil 25 and the second compensation coil 26 are connected in series and wound in opposite directions so that the magnetic flux flow is directed in opposite circumferential directions around the outside of the core 5 and in the same direction through the second core pillar 2. By using two coils 25, 26 connected in series, core saturation of the CM signal in the circuitry around the outside of the core can be prevented.

[0036] The compensating winding 14 is connected to the compensating current controller 16 and the power supply 15, which supply DC current to the compensating winding 14. As can be seen in Figure 5, the compensating winding 14 generates a magnetic flux indicated by arrow 17 in the opposite direction to the magnetic flux 19 generated by supplying DC current to the load 11. By providing the compensating winding 14 in the core 5, positioned to counteract the magnetic flux density generated by the DC current in the power line coil 9, magnetic saturation of the second core pillar 2 is avoided. This technique ensures the maximum DM inductance value of the core regardless of the DC current value flowing through the core 5.

[0037] In the embodiment shown in the figure, the core 5 is gapless, which means that the core 5 forms a closed magnetic circuit without gaps. The core is formed of a magnetic material, preferably ferrite, iron powder, or amorphous metal. By making the core gapless, the inductance characteristics of the core 5 are not reduced. Referring now to Figure 4, which shows an exploded view of the EMI filter core 5, the core 5 consists of two core parts that form an EE-type core, since both parts have an E-shape. These parts can be connected to each other in a gapless manner to form a closed magnetic circuit. However, other configurations, such as an EI-type or TU-type core, can also be used to form a core with three pillars. In the embodiment, the cross-sections of the first core pillar 1, the second core pillar 2, and the third core pillar 3 are rectangular, but other cross-sections, such as circular, can also be provided. The first core pillar 1 and the third core pillar 3 can also be curved and merge with their respective bridging legs 6, 7 to form a D-shape with the second core pillar.

[0038] The EMI filter 4 can, for example, be part of a motor drive 31 comprising a drive circuit 28, as schematically shown in Figure 2, where a first coil 9 is connected to a DC power supply 12 and located in the circuit 28 upstream of the motor 30. A second coil 22 is connected to the DC power supply 12 and located in the circuit downstream of the motor 30. A compensation winding 14 forms part of a compensation circuit 13 comprising a compensation current controller 16 for supplying current to the compensation winding 14. A DC power supply (not shown) is connected to the compensation winding 14. The DC power line conductor 10 is connected to the input 29 of a motor inverter 32 configured to convert the DC input to three AC phases 33 using a switching device 36, such as a MOSFET, for driving the electric motor 30. In other embodiments, more than three phases can be used.

[0039] The compensating current controller 16 includes an open-loop compensation algorithm that can supply a compensating current to the compensating winding 14 in accordance with the DC current of the first coil 9 or the second coil 22. Alternatively, the compensating current in the compensating winding 14 can be made dependent on the phase current of the AC phase 33 of the inverter 32. Alternatively, the measured current in at least two of the phases 33 may be used as an input for estimating the current in the DC power line conductor 10 and / or as an input for controlling the current in the compensating winding 14. In either case, a suitable current sensor is provided. It is also possible to measure the current in the DC power line.

[0040] The DC power supply 12 is configured to supply a voltage of at least 48V between the DC power line conductor 10 away from the DC power supply and the DC power line conductor 23 leading to the DC power supply. The first coil 9, as part of the DC power line conductor 10, comprises a plurality of windings 27 of a conductor having a rectangular cross-section. Similarly, the second coil 22 may comprise a plurality of windings 27 of a conductor having a rectangular cross-section. The number of windings is preferably 1 to 10, such that the cross-section of the coil conductor in the provided space can carry a current sufficient for high-power applications. A larger number of windings may be provided with respect to the compensating winding 14, which carries a lower current.

[0041] Figure 6 is a table showing the reduction of EMI noise at different DC currents passing through the core 5. The magnetomotive force is an indicator of the current in the first coil passing through the core and depends on the number of windings. It can be seen that when compensation is active, i.e., when a controlled current is supplied through the compensation winding 14, the amplitude of noise that can be transmitted through the power line conductor 10 is significantly reduced.

[0042] The present invention also provides a method for filtering electromagnetic interference (EMI) in a power circuit. The method includes providing such an EMI filter 4 and controlling a compensation current in a compensation winding 14 such that the magnetic flux density in the second core pillar 2 is generated in the opposite direction to the magnetic flux density generated by the first coil 9 and the second coil 22 in the second core pillar 2. [Explanation of symbols]

[0043] 1. First core pillar 2. Second core pillar 3. The third core pillar 4 EMI filter 5 cores 6. First bridge pier 7. Second bridge pier 8. First opening 9. The first coil 10 First power line conductor 11 Load 12 DC power supply 13 Compensation circuit 14. Compensation winding 15 Power supply 16. Compensation Current Controller 17 Magnetic flux density 18 Compensation current magnetic flux direction 19 DC magnetic flux direction 20 Magnetic flux density 21 First terminal 22 The second coil 23 Second power line conductor 24 Second terminal 25 First compensation coil 26. Second compensation coil 27 Coil winding section 28 circuits 29 Inverter Input 30 motors 31 Motor Drive 32 Inverters 33 Motor Phases 34 Second opening 35 Common Mode Arrow 36 switches

Claims

1. EMI filter (4), The device comprises a core (5) that forms a closed magnetic circuit, the core (5) having a first core pillar (1), a second core pillar (2), and a third core pillar (3) extending in parallel directions, the second core pillar (2) being positioned between the first core pillar (1) and the third core pillar (3), The end of the first core pillar (1) and the end of the second core pillar (2) are connected by the first bridging leg (6), and the end of the second core pillar (2) and the end of the third core pillar (3) are connected by the second bridging leg (7), The first core pillar (1), the second core pillar (2), and the first bridging leg (6) define the first opening (8), The second core pillar (2), the third core pillar (3), and the second bridging leg (7) define the second opening (34), A first coil (9) is arranged around the first core pillar (1) and / or the first bridging leg (6), and the first coil (9) is part of a first DC power line conductor (10) for connecting to a DC power supply (12) and a load (11). A second coil (22) is positioned around the third core pillar (3) and / or the second bridging leg (7), and the second coil (22) is part of a second DC power line conductor (23) for connecting to a DC power supply (12) and a load (11). The EMI filter (4) further comprises a compensating winding (14) for connection to a power supply (15) and a compensating current controller (16) for controlling the supply of current to the compensating winding (14). The compensation winding (14) is arranged on the core (5) and comprises a first compensation coil (25) and a second compensation coil (26), and the first compensation coil is arranged around the first core pillar (1) and / or the first bridging leg (6), and the second compensation coil (26) is arranged around the third core pillar (3) and / or the second bridging leg (7), in order to reduce the magnetic flux generated in the second core pillar by inducing a magnetic flux density (17) in the second core pillar (2) in a direction (18) opposite to the direction (19) of the magnetic flux density (20) generated in the second core pillar (2) by the DC current supplied through the first coil (9) and the second coil (22), EMI filter (4).

2. The EMI filter (4) according to claim 1, wherein the core (5) is gapless and forms a closed magnetic circuit from a material having uniform magnetic permeability.

3. The EMI filter (4) according to claim 1 or 2, wherein one end of the first coil (9) is connected to a first terminal (21) of a DC power supply (12), and one end of the second coil (22) is connected to a second terminal (24) of the DC power supply (12).

4. The EMI filter (4) according to any one of claims 1 to 3, wherein the first compensation coil (25) and the second compensation coil (26) are connected in series.

5. The EMI filter (4) according to any one of claims 1 to 3, wherein the first compensation coil (25) and the second compensation coil (26) are connected in parallel.

6. The EMI filter (4) according to any one of claims 1 to 5, wherein the first coil (9) comprises a plurality of winding portions (27) of a conductor having a rectangular cross-section.

7. The EMI filter (4) according to any one of claims 1 to 6, wherein the compensation current controller (16) comprises an open-loop compensation algorithm that provides a compensation current to the compensation winding (14) in accordance with the DC current in the first coil (9) and / or the second coil (22).

8. The EMI filter (4) according to any one of claims 1 to 7, wherein the core (5) comprises two or more core portions, in particular an EE, EI, or TU core.

9. A motor drive (31) comprising a circuit having an EMI filter (4) according to any one of claims 1 to 8.

10. The motor drive (31) according to claim 9, comprising a circuit (28), wherein the first coil (9) is connected to a DC power supply (12) and is located in the circuit (28) upstream of the load (11), the second coil (22) is connected to the DC power supply (12) and is located in the circuit (28) downstream of the load (11), and the compensating winding (14) forms part of a compensation circuit (13) comprising a compensating current controller (16) for supplying current to the compensating winding (14).

11. The motor drive (31) according to claim 9 or 10, wherein the first DC power line conductor (10) is connected to the input (29) of a motor inverter (32) configured to convert a DC input into three or more AC phases (33) for driving an electric motor (30).

12. The motor drive (31) according to any one of claims 9 to 11, wherein the compensating current controller (16) comprises an open-loop compensation algorithm that provides a compensating current to the compensating winding (14) in accordance with at least two, preferably all, phase currents of the AC phases (33) of the inverter (32).

13. The motor drive (31) according to any one of claims 9 to 12, wherein the DC power supply (12) is configured to supply a voltage of at least 48V across the first DC power line conductor (10) and the second DC power line conductor (23).

14. In a method for filtering electromagnetic interference (EMI) in a power circuit, A step of preparing a core (5) that forms a closed magnetic circuit, wherein the core (5) has a first core pillar (1), a second core pillar (2), and a third core pillar (3) extending in parallel directions, and the second core pillar (2) is positioned between the first core pillar (1) and the third core pillar (3). The end of the first core pillar (1) and the end of the second core pillar (2) are connected by the first bridging leg (6), and the end of the second core pillar (2) and the end of the third core pillar (3) are connected by the second bridging leg (7), The first core pillar (1), the second core pillar (2), and the first bridging leg (6) define the first opening (8), The second core pillar (2), the third core pillar (3), and the second bridging leg (7) define the second opening (34). Steps and Steps include providing a first coil (9) around the first core pillar (1) and / or the first bridging leg (6), wherein the first coil (9) is part of a first DC power line conductor (10) for connecting to a DC power source (12) and a load (11), Steps include providing a second coil (22) around the third core pillar (3) and / or the second bridging leg (7), wherein the second coil (22) is part of a second DC power line conductor (23) for connecting to a DC power source (12) and a load (11), A step of preparing a compensation circuit (13) comprising a compensation winding (14), wherein the compensation winding (14) is arranged on the core (5) and comprises a first compensation coil (25) and a second compensation coil (26), wherein the first compensation coil is arranged around the first core pillar (1) and / or the first bridging leg (6), and the second compensation coil (26) is arranged around the third core pillar (3) and / or the second bridging leg (7), The steps include controlling the compensation current in the compensation winding (14) such that the magnetic flux density (17) in the second core pillar (2) is generated in the opposite direction to the magnetic flux density (20) generated by the first coil (9) and the second coil (22), A method that includes this.

15. The method according to claim 13, wherein the core (5) is gapless and forms a closed magnetic circuit from a material having uniform magnetic permeability, one end of the first coil (9) is connected to a first terminal (21) of a DC power supply (12), and one end of the second coil (22), which is part of a DC power line conductor (23), is connected to a second terminal (24) of the DC power supply (12).

Images