Shielding and measurement of magnetic fields

Transcription

1 Shielding and measurement of magnetic fields Workshop Solution Showcase Dr. Swen Graubner, SEKELS GmbH Berlin

2 Basic shielding principles Influences: Alloy Geometry Ideal shielding Magnetic field measurement

3 Maxwell- equations (in matter): D = ρ B = 0 H = J + D t E + B t = 0

4 Maxwell- equations (in matter): D = ρ The electric field starts and ends at electrical charges! + B = 0 The magnetic field is a source-free field: no start no end. Magnetic field lines are always closed!

5 Consequences for the shielding of electrical fields: Electrical charges in the shielding can move on the surface and follow the electric field. Alloys with high electrical conductivity of advantage (Cu, Al, Au) Consequences for magnetic shieldings: Two working principles exist: Field guidance around a protected area Field weakening by eddy current induction

6 Consequences for the shielding of electrical fields: Electrical charges in the shielding can move on the surface and follow the electric field. Alloys with high electrical conductivity of advantage (Cu, Al, Au) Consequences for magnetic shieldings: Two working principles exist: Field guidance around a protected area Field weakening by eddy current induction Magnetic insulators do not exist! There is no material that can block magnetic fields, any material at least conducts as good as vacuum (superconductivity excluded).

7 Working principles of magnetic shieldings: Primary magnetic field Flux guidance Shielding mesh Induced current Compensating magnetic field Field weakening by eddy current induction Only works for f>>0!!!

8 Important parameters for magnetic alloys: Permeability greed for magnetic field lines of the material Determines how much field is lost by stray field and how much is guided through the material. High permeable alloys can clean up very well permeability Secant slope

9 Important parameters for magnetic alloys: Permeability greed for magnetic field lines of the material Determines how much field is lost by stray field and how much is guided through the material. High permeable alloys can clean up very well permeability Secant slope

10 Important parameters for magnetic alloys: Permeability greed for magnetic field lines of the material Determines how much field is lost by stray field and how much is guided through the material. High permeable alloys can clean up very well permeability Secant slope

11 Important parameters for magnetic alloys: Saturation magnetization a measure of how much magnetic flux can be kept in a material Could be understood as some kind of holding capability Higher saturation magnetization allows space-efficient magnetic parts magnetization for H saturation magnetization Zoomed out!

12 Important parameters for magnetic alloys: Coercivity magnetic memory After a field source vanishes it determines how much counter wise field is needed to remove the rest of magnetization. Low-coercive materials show lower losses in dynamic processes (f>0) coercivity Strength of counter wise magnetic field

13 Important parameters for magnetic alloys: Coercivity magnetic memory After a field source vanishes it determines how much counter wise field is needed to remove the rest of magnetization. Low-coercive materials show lower losses in dynamic processes (f>0) Influences remanence remanence coercivity Strength of counter wise magnetic field

14 Parameters influence each other!! Permeability Shape Saturation behavior Remanence

15 Parameters influence each other!! Permeability Shape Saturation behavior Remanence

16 Shape influence Assumptions: Magnetic conductivity in material Magnetic conductivity in air 1 Material not saturated limited magnetic length: Sheered magnetic circuit Stray fields occur Hysteresis loop changes Magnetic field is guided without stray field losses Magnetic length of ring Raw material properties can be measured From:

17 Shape influence µ eff = µ eff =300 µ eff =20 Round hysteresis loop High permeability Material saturates fast Linearized hysteresis Low permeability Slow saturation

18 Influence of shielding shape: Four field sources

19 Influence of shielding shape: Low permeability (soft iron) High permeability (MUMETALL ) Sheet: Box:

20 Conclusions: Sheared magnetic shieldings have a bad effective permeability Good shielding only possible in closed geometry High-permeability material consisting of holes makes no sense! Low frequency shielding needs bulk One reason against porous or resin-bound magnetic shieldings. inner air gap reduces shielding effect significantly

21 Alloy Composition µ 4 (static) H C [A/m] B S [T] T C [ C] Density [g/cm 3 ] MUMETALL 80 % NiFe , ,7 CRYOPERM % NiFe * * * 430 8,7 PERMENORM 5000 H2 50 % NiFe , ,25 Soft iron 99,9 % Fe , ,86 Silicon-Eisen 97 % Fe , ,65 VACOFLUX % CoFe , ,12 VITROVAC 6025 X Ca. 80 % Co , ,86 VITROLAM 477 P8* Ca. 80% Fe ,5 1, * * Compound material mad of VITROPERM 800, PET-Foil and adhesives

22 Frequency Field strength Alloy Shielding (DC) Shielding (<500 Hz) Shielding (>500 Hz) Shielding (low strength) shielding (medium strength) Magnetic yoke (high strength) MUMETALL CRYOPERM PERMENORM 5000 H Soft Iron Silicon-Eisen VACOFLUX VITROVAC 6025 X VITROLAM 477 P Rule of thumb (qualitatively): Sum up the + and - for field strength and frequency to gain a hint which ally might fit!

23 The ideal shielding: Closed shape No holes, no gaps Several layers in defined distances High permeability alloy µ rel Low coercivity alloy H C High saturation magnetization B S Low spec. electrical Resistivity Thermal annealing necessary eierlegende Wollmilchsau egg-laying whool-milk pig From:

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25 Guidelines in general National and international guidelines not completely in agreement Long-time evaluation uncertain International: In Germany: 26. BlmSchV: BGV B11 : ICNIRP (not law) general public safety critical limits for workers

26 How do guidelines look like?

27 Basic data of MFA-110 Hardware Detector head Cross-section100 cm² Amplifier box Dimensions 280 x 220 x 50 mm³ Total weight 5 kg Detector weight 0,5 kg Aluminum casing 2 x USB, D-Sub (Sensor), BNC (EXT) Velcro connection of Notebook HP EliteBook Revolve 810 G1 Tablet (important: SSD-harddisk) (optional) Software M-STREAM Compatible to Windows 7, 8 (64 bit), LabView-based Properties Frequency range 10 Hz to 400 khz B-H- field range 6 µt 1.5 T at 1 Hz 70 nt 20 mt at 100 Hz 50 nt 15 mt at 1 khz (depending on detector head)

28 Working principle of the MFA-110 Measurement data are detected by detector head (induction coil principle) No filtering in the detector head Data are streamed to notebook with high transfer rate Data is stored at SSD Analysis after measurement After measurement change of analysis criteria possible Post-processing of old data possible Sniffer mode

29 Measurement at local transformer station in industrial estate

30 SEKELS your specialist for Magnetic Shielding Magnetic Measurements Magnetic applications

31 Thank you for your attention!! Any questions?? SEKELS GmbH Dieselstr Ober-Moerlen Deutschland Tel Fax mail@sekels.de Your technical contact: Dr. Swen Graubner Tel: SGraubner@sekels.de Hall 1.1 Booth F24A All statements, information and data given in this presentation are believed to be accurate and reliable, but are presented without guarantee, warranty or responsibility of any kind, expressed or implied on our part. Published by SEKELS GmbH, Germany. All rights reserved. Any distribution, re-utilization or publication of the data, pictures and diagrams need the authorization of SEKELS GmbH in written form.