EMI filter

Abstract

An EMI filter including a core forming a closed magnetic circuit, the core having first, second and third stems extending in parallel directions, the second stem being disposed between the first and third stems. The ends of the stem are connected by first and second bridge legs. A first coil, which is part of the first DC power line conductor, is disposed around the first stem and / or the first bridge leg. A second coil, which is part of a DC power line conductor, is disposed around the third stem and / or the second bridge leg. The compensation winding is arranged on the core and comprises a first compensation coil and a second compensation coil, the first compensation coil is arranged around the first core column and / or the first bridge leg, and the second compensation coil is arranged around the third core column and / or the second bridge leg for inducing a magnetic flux density in the second core column in the following direction. The direction is opposite to the direction of the magnetic flux density generated in the second stem by the direct current supplied through the first coil and through the second coil for reducing the resulting magnetic flux in the second stem.

Description

Technical Field

[0001] This invention generally relates to electromagnetic interference (EMI) filters and EMI filter circuits, as well as methods for filtering out EMI interference. Background Technology

[0002] CN115938747 discloses an EMI filter in a filter circuit for an electric compressor controller. The EMI filter has a core formed by two E-shaped core components arranged in a relative configuration. Thus, the core has two parallel posts joined at the top and bottom by a bridging component. The core is provided with two winding coils for supplying current to and receiving current from the compressor motor, the winding coils being arranged on the two posts. To suppress differential-mode (DM) interference, it is known to have an intermediate post arranged between the two outer posts, such that the magnetic flux generated by DM interference in the two winding coils can use a common path through the intermediate post. However, this can cause the intermediate post to saturate when a DM current is present. In the embodiment shown in CN115938747, a gap is introduced in the intermediate post. This gap reduces the sensitivity of the core material to saturation. Summary of the Invention

[0003] The purpose of this invention is to provide an improved or alternative EMI filter circuit and a method for filtering electromagnetic interference (EMI) in the circuit.

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

[0005] The core forms a closed magnetic circuit and has a first core post, a second core post, and a third core post extending in a parallel direction, wherein the second core post is arranged between the first core post and the third core post. The ends of the first core post and the second core post are connected by a first bridging leg, and the ends of the second core post and the third core post are connected by a second bridging leg. The first core post, the second core post, and the first bridging leg define a first opening, and the second core post, the third core post, and the second bridging leg define a second opening.

[0006] A first coil is arranged around the first core post and / or the first bridging leg, the first coil being part of a first DC power line conductor for connection to a DC power supply and a load. A second coil is arranged around the third core post and / or the second bridging leg, the second coil being part of a second DC power line conductor for connection to a DC power supply and a load. The EMI filter also includes a compensation winding for connection to the power supply, and a compensation current controller for controlling the current supply to the compensation winding. The compensation winding is arranged on the core and includes a first compensation coil and a second compensation coil, the first compensation coil being arranged around the first core post and / or the first bridging leg, and the second compensation coil being arranged around the third core post and / or the second bridging leg, for inducing a magnetic flux density in the second core post in a direction opposite to the direction of the magnetic flux density generated in the second core post by a DC current supplied through the first coil and the second coil, for reducing the resulting magnetic flux in the second core post.

[0007] DC power lines can be susceptible to differential-mode interference, which is noise or unwanted signals carried on two power lines in opposite directions. This noise can occur, for example, when using high-frequency switching devices in power circuits, such as motor inverters. The first coil on the EMI filter core is part of the DC power line, forming 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 the DC current, i.e., the current traveling to and from the DC power supply, flows in opposite directions through the corresponding first and second openings. Therefore, differential-mode noise induces magnetic flux in the core around the first and second openings in opposite circumferential directions. This magnetic flux accumulates in the second core post, making it susceptible to saturation due to high DC current values ​​in the first and second DC power line conductors. A gap can be introduced into the second post of the core to reduce the magnetic flux density and saturation sensitivity; however, this also reduces the inductance characteristics of the core. Instead of an air gap, it is proposed to use a compensating winding arranged on the core to induce a magnetic flux density in the second core post in a direction opposite to the direction of the magnetic flux density generated in the second core post by the DC current supplied through the first and second coils. Therefore, the resulting magnetic flux in the second core post is reduced, making the second core post less prone to magnetic saturation. This method ensures the maximum DM inductance value of the core, regardless of the DC current flowing through the core.

[0009] Furthermore, in the configuration according to the invention, the EMI filter is capable of suppressing both common-mode and differential-mode interference, wherein the common-mode interference generated when noise occurs in phase in both power lines produces a superimposed magnetic flux in the core. The common-mode noise generates magnetic flux flow in the loop surrounding the core in the first and third core posts and the bridging leg. Therefore, the energy from the common-mode interference is stored in the core, causing the noise in the power lines to be attenuated and preventing further propagation through the circuit. Thus, the core and coil have an inductive impedance that attenuates common-mode noise, unaffected by the DC current value.

[0010] The core is preferably gapless, meaning it forms a closed magnetic circuit without any air gaps. The core preferably has uniform permeability, or is made of a material with uniform permeability. By making the core gapless, the inductive properties of the core are not reduced.

[0011] The core may include two or more core components. These components may be joined together in a gapless manner to form a closed magnetic circuit. In a preferred embodiment, the core is an EE core. However, other configurations may also be used to form a core with three posts, such as an EI or TU type core. The cross-section of the second core post may be rectangular or circular.

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

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

[0014] In a preferred embodiment, a first compensation coil and a second compensation coil are connected in series, with the first compensation coil arranged around one of the first bridging legs or a first post. The second compensation coil is arranged around one of the second bridging legs or a third post. This prevents core saturation of the CM signal in the circuit surrounding the core. The first and second compensation coils 26 are connected in series and wound in opposite directions, such that magnetic flux flows in opposite circumferential directions around the core, but in the same direction when passing through the second core post. In an alternative embodiment, the coils may be connected in parallel.

[0015] The compensation current controller includes an open-loop compensation algorithm that provides a compensation current in the compensation winding based on the DC current in the first and / or second coils. Alternatively, the compensation current may depend on at least two phase currents from the inverter or motor phase currents. For example, at least two phase currents, preferably all phase currents, can be provided as inputs to the compensation current controller. The current in the DC power line can also be measured.

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

[0017] EMI filters can be advantageously integrated into motor drives.

[0018] The motor driver includes a circuit in which a first coil is connected to a DC power supply and is positioned upstream of a load (particularly a motor) in the circuit, and a second coil is connected to a DC power supply and positioned downstream of the load in the circuit. A compensation winding forms part of a compensation circuit, which includes a compensation current controller for supplying current to the compensation winding. A DC power supply or current source is connected to the compensation winding.

[0019] The DC power line conductor is connected to the input of a motor inverter, which is configured to convert the DC input into three or more AC phases for driving a motor.

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

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

[0022] In a second aspect of the invention, a method for filtering electromagnetic interference (EMI) in a power circuit is provided. The method includes: - Provides a core forming a closed magnetic circuit, the core having a first core post, a second core post, and a third core post extending in parallel directions, wherein the second core post is disposed between the first core post and the third core post. The ends of the first and second core posts are connected by a first bridging leg, and the ends of the second and third core posts are connected by a second bridging leg. - The first core post, the second core post, and the first bridging leg define the first opening, and - The second core post, the third core post, and the second bridging leg define the second opening. - Provide a first coil surrounding a first core post and / or a first or bridging leg, the first coil being part of a first DC power line conductor for connection to a DC power supply and a load. - Provide a second coil surrounding the third core post and / or the second bridging leg, the second coil being part of a second DC power line conductor for connection to a DC power supply and load. - Provides a compensation circuit including a compensation winding disposed on the core and including a first compensation coil and a second compensation coil, the first compensation coil being disposed around a first core post and / or a first bridging leg, and the second compensation coil being disposed around a third core post and / or a second bridging leg. - Control the compensation current in the compensation winding such that a magnetic flux density in the second core is generated in a direction opposite to the magnetic flux density generated in the second core by the first coil and the second coil.

[0023] Any feature disclosed as part of the first aspect of the invention may exist alone or in combination in the second aspect of the invention, or follow any arrangement or arrangement of any one or more of the elements. Attached Figure Description

[0024] The embodiments will now be described by way of example only with reference to the accompanying drawings, wherein: 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 driver is shown. Figure 3 A perspective view of a portion of an EMI filter according to an embodiment of the present invention is shown. Figure 4 An exploded view of a portion 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. Detailed Implementation

[0025] Figure 1 A schematic diagram of an EMI filter 4 according to the present invention is shown. The EMI filter 4 includes forming a first inductor L. DM1 The first coil 9 and the second inductor L DM2 The second coil 22. (As shown) Figure 3As shown, a first coil 9 and a second coil 22 are disposed on a core 5. The core 5 forms a closed magnetic circuit and has a first core post 1, a second core post 2, and a third core post 3 extending in a parallel direction. The second core post 2 is disposed between the first core post 1 and the third core post 3. The ends of the first core post 1 and the second core post 2 are connected by a first bridging leg 6, and the ends of the second core post 2 and the third core post 3 are connected by a second bridging leg 7. The first core post 1, the second core post 2, and the first bridging leg 6 define a first opening 8, while the second core post 2, the third core post 3, and the second bridging leg 7 define a second opening 34.

[0026] The first coil 9 is arranged around the first core post 1. However, the first coil 9 can be arranged in other configurations, such as around the first bridging leg 6, meaning around... Figure 3 The first coil 9 is located at one of the bridging legs 6 at the top or bottom of the core, or partially on the bridging leg 6 and partially on the first core post 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 also connected to the load 11. The second coil 22 is arranged around the third core post 3. However, the second coil 22 can also be arranged in other configurations, such as around the second bridging leg 7, or partially on the bridging leg 7 and partially on the third core post 3. The second coil 22 is part of the return DC power line conductor 23 connected to the load 11 and connected to the DC power supply 12 via the second terminal 24. The DC power supply 12 can be the output of a battery or an AC / DC converter. Therefore, the first coil 9 and the second coil 22 are arranged in the power circuit before and after the load 11, respectively.

[0027] The EMI filter 4 also includes a cross-line capacitor C connected to the first power line conductor 10 and the second power line conductor 23 for suppressing differential mode noise. X And two line bypass capacitors C connected between the first power line conductor 10 and the second power line conductor 23 and ground, respectively, for suppressing common-mode noise. Y The capacitor provides a low-impedance path for noise, causing it to continue through the capacitor rather than along the power line.

[0028] The EMI filter 4 also 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.

[0029] exist Figure 3 In the illustrated embodiment, the compensation winding 14 is arranged on the core 5 around the first bridging leg 6 and the second bridging leg 7; however, other configurations are also possible. Figure 5 In one embodiment, the compensation winding 14 is disposed on the first core post 1 and the third core post 3.

[0030] The compensating winding 14 is arranged to induce a magnetic flux density in the second core 2 in a direction opposite to the direction of the magnetic flux density induced in the second core 2 by the direct current supplied through the first coil 9 and the second coil 22. This reduces the resulting magnetic flux in the second core 2, making it less prone to magnetic saturation. Figure 5 The diagram is schematically shown. The first coil 9 is arranged such that the current is used to... The first current direction, indicated by ⊙, flows through the conductor passing through the first opening 8, and the second coil 22 is arranged such that the current flows through the conductor passing through the second opening 24 in a second current direction, opposite to the first direction. The first coil 9 and the second coil 22 are arranged in a circuit that may be affected by common-mode noise and differential-mode noise. Common-mode noise is conducted in the same direction in the conductors 10 and 23 passing through the first opening 8 and the second opening 34. Due to the common-mode noise, magnetic flux flows through the core 5 as indicated by the common-mode arrow 35. The magnetic flux flows in the loop surrounding the core 5 in the first core post 1 and the third core post 3, as well as the bridging legs 6 and 7. Therefore, the energy from the common-mode interference is stored in the core 5, so that the noise in the power line is attenuated and the noise does not propagate further through the power circuit 28. The core 5 and the coils 9 and 22 therefore have inductive impedance that attenuates common-mode noise, which is unaffected by the DC current value.

[0031] DC power lines can also be affected by differential-mode interference, which is noise or unwanted signal carried on two power lines 10 and 23 but in opposite directions. This noise can occur, for example, when high-frequency switching devices are used in power circuits, such as when using a motor inverter.

[0032] The first coil 9 on the EMI filter core 5 is part of the DC power line 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 23 and forms an inductor for suppressing DM noise. Therefore, the same core 5 can be used for both common-mode noise attenuation and differential-mode noise attenuation.

[0033] The direction of the magnetic flux generated in the core by the first coil 9 and the second coil as a result of supplying DC current 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 can also be the same as the direction shown by arrow 20. Figure 5 It can be seen that the magnetic flux generated by the DM noise in the first coil 9 and the second coil 22 is superimposed in the second core 2. This DM noise can lead to undesirable magnetic saturation of the second core 2.

[0034] To counteract this, the compensating winding 14 in this embodiment is provided by a first compensating coil 25 arranged on the first post 1 and a second compensating coil 26 arranged on the third post 3; however, other configurations of the compensating winding are also possible. Figures 3 to 5 In the illustrated embodiment, the first compensation coil 25 and the second compensation coil 26 are connected in series and wound in opposite directions, such that the magnetic flux flows in opposite circumferential directions around the outside of the core 5, but in the same direction when passing through the second core post 2. Using the two coils 25 and 26 connected in series prevents core saturation of the CM signal in the circuit surrounding the core.

[0035] The compensation winding 14 is connected to the compensation current controller 16 and the power supply 15, which provides DC current to the compensation winding 14. Figure 5 As can be seen, the compensating winding 14 generates a magnetic flux as indicated by arrow 17, the direction of which is opposite to the direction of the magnetic flux 19 generated by the DC current supplied to the load 11. By providing the compensating winding 14 for the core 5, which is arranged to cancel the magnetic flux density generated by the DC current in the power line coil 9, the second core post 2 is prevented from magnetic saturation. Regardless of the value of the DC current flowing through the core 5, this method ensures the maximum DM inductance value of the core.

[0036] In the embodiment shown in the figure, core 5 is gapless, meaning that core 5 forms a closed magnetic circuit without any air 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 core 5 are not reduced. Now refer to Figure 4 The diagram shows an exploded view of an EMI filter core 5, which consists of two core components forming an EE-type core, as both components are E-shaped. These components can be joined together in a gapless manner to form a closed magnetic circuit. However, other configurations can also be used to form a core with three posts, such as an EI-type or TU-type core. In the illustrated embodiment, the first post 1, the second post 2, and the third post 3 have rectangular cross-sections; however, other cross-sections, such as circular ones, can also be provided. The first post 1 and the third post 3 can also be curved, merging with corresponding bridging legs 6, 7 to form a D-shape with the second post.

[0037] For example, the EMI filter 4 can be part of a motor driver 31 that includes a drive circuit 28, wherein the first coil 9 is connected to the DC power supply 12 and is arranged upstream of the motor 30 in the circuit 28, as shown below. Figure 2The diagram is schematically shown. A second coil 22 is connected to a DC power supply 12 and is arranged in the circuit downstream of the motor 30. A compensation winding 14 forms part of a compensation circuit 13, which includes 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. A DC power line conductor 10 is connected to the input 29 of a motor inverter 32, which is configured to convert the DC input into three AC phases 33 via a switching device 36 (e.g., a MOSFET) for driving the motor 30. In other embodiments, more than three phases may be used.

[0038] The compensation current controller 16 includes an open-loop compensation algorithm that provides a compensation current in the compensation winding 14 based on the DC current in the first coil 9 or the second coil 22. Alternatively, the compensation current in the compensation winding 14 may depend on the phase current of the AC phase 33 of the inverter 32. Alternatively, the current measured in at least two phases of phase 33 may be used as input for estimating the current in the DC power line 10 and / or as input for controlling the current in the compensation winding 14. A suitable current sensor is provided in each case. The current in the DC power line may also be measured.

[0039] DC power supply 12 is configured to supply a voltage of at least 48V across DC power line conductor 10, which is located away from the DC power supply, and DC power line conductor 23, which is located towards the DC power supply. A first coil 9, as part of DC power line 10, includes a plurality of turns 27 of conductors having a rectangular cross-section. Similarly, a second coil 22 may include a plurality of turns 27 of conductors having a rectangular cross-section. The number of turns is preferably between one and ten turns 27, such that the cross-section of the coil conductors can carry sufficient current in the provided space for high-power applications. More turns may be provided for the compensating winding 14, wherein the compensating winding conductors carry a lower current.

[0040] Figure 6 This is a table showing the reduction in EMI noise under different DC currents passing through core 5. The magnetomotive force is a measure of the current passing through the core in the first coil and depends on the number of turns. It can be seen that when compensation is activated, i.e., when a controlled current is supplied through compensation winding 14, the noise amplitude that can be transmitted through power line conductor 10 is significantly reduced.

[0041] The present invention also provides a method for filtering electromagnetic interference (EMI) in a power circuit. The method includes providing an EMI filter 4 and controlling a compensation current in a compensation winding 14 such that a magnetic flux density in a second core 2 is generated in a direction opposite to the magnetic flux density generated in the second core 2 by a first coil 9 and a second coil 22.

[0042] List of reference numerals

Claims

1. An EMI filter (4), comprising: - Core (5), which forms a closed magnetic circuit, the core (5) having a first core post (1), a second core post (2) and a third core post (3) extending in a parallel direction, wherein the second core post (2) is arranged between the first core post (1) and the third core post (3). - The ends of the first core post (1) and the second core post (2) are connected by the first bridging leg (6), and the ends of the second core post (2) and the third core post (3) are connected by the second bridging leg (7). - The first core post (1), the second core post (2), and the first bridging leg (6) define the first opening (8), and - The second core post (2), the third core post (3), and the second bridging leg (7) define the second opening (34). -In this configuration, a first coil (9) is arranged around a first core post (1) and / or a first bridging leg (6), and the first coil (9) is part of a first DC power line conductor (10) for connection to a DC power supply (12) and a load (11). -In this configuration, the second coil (22) is arranged around the third core post (3) and / or the second bridging leg (7), and the second coil (9) is part of the second DC power line conductor (23) for connection to the DC power supply (12) and the load (11). The EMI filter (4) also includes a compensation winding (14) for connection to a power supply (15) and a compensation current controller (16) for controlling the current supply to the compensation winding (14). - A compensation winding (14) is arranged on the core (5) and includes a first compensation coil (25) and a second compensation coil (26), the first compensation coil being arranged around a first core post (1) and / or a first bridging leg (6), and the second compensation coil (26) being arranged around a third core post (3) and / or a second bridging leg (7), for inducing a magnetic flux density (17) in the second core post (2) in a direction (18) opposite to the direction (19) of the magnetic flux density (20) generated in the second core post (2) by a DC current supplied through the first coil (9) and the second coil (22), for reducing the resulting magnetic flux in the second core post (2).

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

3. The EMI filter (4) according to claim 1 or 2, wherein, The first coil (9) is connected at one end to the first terminal (21) of the DC power supply (12), and the second coil (22) is connected at one end to the 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 the preceding claims, wherein, The first coil (9) comprises multiple turns (27) of a conductor having a rectangular cross-section.

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

8. The EMI filter (4) according to any one of the preceding claims, wherein, The core (5) includes two or more core components, particularly EE, EI or TU cores.

9. A motor driver (31) comprising a circuit having an EMI filter (4) according to any one of the preceding claims.

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

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

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

13. The motor driver (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. A method for filtering electromagnetic interference (EMI) in a power circuit, comprising: - Provide a core (5) that forms a closed magnetic circuit, the core (5) having a first core post (1), a second core post (2) and a third core post (3) extending in a parallel direction, wherein the second core post (2) is arranged between the first core post (1) and the third core post (3). - The ends of the first core post (1) and the second core post (2) are connected by the first bridging leg (6), and the ends of the second core post (2) and the third core post (3) are connected by the second bridging leg (7). - The first core post (1), the second core post (2), and the first bridging leg (6) define the first opening (8), and - The second core post (2), the third core post (3), and the second bridging leg (7) define the second opening (34). - Provide a first coil (9) surrounding a first core post (1) and / or a first bridging leg (6), the first coil (9) being part of a first DC power line conductor (10) for connection to a DC power supply (12) and a load (11). - Provide a second coil (22) surrounding the third core post (3) and / or the second bridging leg (7), the second coil (9) being part of a second DC power line conductor (23) for connection to the DC power supply (12) and the load (11), - Provides a compensation circuit (13) including a compensation winding (14), the compensation winding (14) being arranged on a core (5) and including a first compensation coil (25) and a second compensation coil (26), the first compensation coil being arranged around a first core post (1) and / or a first bridging leg (6), and the second compensation coil (26) being arranged around a third core post (3) and / or a second bridging leg (7). - Control the compensation current in the compensation winding (14) so ​​that a magnetic flux density (17) is generated in the second core (2) in the direction opposite to the magnetic flux density (20) generated by the first coil (9) and the second coil (22).

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

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