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Magnetic field reduction screening system for a magnetic field source used in industrial applications

Authors:
Hector Beltran at Jaume I University
Hector Beltran
Vicente Fuster at Polytechnic University of Valencia
Vicente Fuster
M García
M García
M García
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Abstract and Figures

This paper presents the description of the design and solution given as magnetic screen for a 50Hz industrial application, combining different materials to obtain the optimum reduction of the field. Important ideas are presented on the magnetic field behaviour, the response of the different materials subjected to magnetic fields, the effects and variations (in the shape and intensity of the field), introduced by the screen, and also, the differences in these influences produced by the screen as a function of their material properties, dimensions or positions. Keeping the magnetic field within a certain region of the space without disturbing the field in the other regions is not an easy task. That is why simulation and real measurements have to be combined. With the digital model, a large number of simulations are carried out modifying the screen step by step to obtain the optimal field reduction. The final measurements have validated the improvements performed by the screen.
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Magnetic field reduction screening system for a magnetic field source
used in industrial applications
H.Beltran, V.Fuster, M.García
Institut de Tecnologia Elèctrica.
Avda. Juan de la Cierva, 24 – Parc Tecnològic Paterna
46980 Paterna (Valencia). Spain
Phone: +34961366670 Fax: +34961366680 E-mail: Hector.Beltran@itenergia.com
Vicente.Fuster@itenergia.com, Marta.Garcia@itenergia.com
Abstract. This paper presents the description of the design
and solution given as magnetic screen for a 50Hz industrial
application, combining different materials to obtain the
optimum reduction of the field. Important ideas are presented
on the magnetic field behaviour, the response of the different
materials subjected to magnetic fields, the effects and variations
(in the shape and intensity of the field), introduced by the
screen, and also, the differences in these influences produced by
the screen as a function of their material properties, dimensions
or positions. Keeping the magnetic field within a certain region
of the space without disturbing the field in the other regions is
not an easy task. That is why simulation and real measurements
have to be combined. With the digital model, a large number of
simulations are carried out modifying the screen step by step to
obtain the optimal field reduction. The final measurements have
validated the improvements performed by the screen.
Keywords
Magnetic shielding, low frequency electromagnetic
fields, human exposure, screen parameters, industrial
application.
1. Introduction
During the last years, electromagnetic fields have been a
permanent point of conflict due to the growing awareness
of the health risks by the general public. Although lots of
biomedical studies have been carried out on this domain,
none of them has been able to establish a clear relation
confirming electromagnetic fields as a cause of any kind
of illness. However, it is clear there has to be limits in
order to control possible exposures. European authorities
have regulated these aspects fixing limits for the radiated
emissions produced by different kind of electromagnetic
sources [1]. These limits vary as a function of the
frequency and, for the case of the industrial applications
(50Hz) they are:
• For occupational exposure: 500µT
• For general public exposure: 100 µT
The application introduced in this paper deals with the
magnetic screening performed in an industrial process
trying to reduce the magnetic field emitted by one of the
components below the established limits.
2. Problem description
Placed in an industrial environment, as a part of the
productive process, an industrial coil is used to
demagnetize the workpieces being fabricated which
could keep a remanent magnetic fields on its inside as a
result of the productive process manipulation and
transformation. These fields existing in the workpieces
are not useful at all, resulting even damaging for the
correct functioning of the pieces once they are installed
and have to start working. Therefore, they have to be
eliminated.
Figure 1 - Coil with workpiece and magnetic flux lines.
Demagnetization is the reduction of a magnetic field
from its maximum magnetization intensity to almost
zero, achieved by a repeated polarity reversal at a given
frequency.
For demagnetization, the amplitude of an applied
alternating field must be continuously reduced as shown
in figure 2. The initial demagnetization field strength
must be at least equal to the magnetization field strength
existing in the sample. The reduction of field strength
within the workpiece can be achieved electrically by
reducing the magnetic field progressively while it
circulates through the coil, or mechanically by slowly
withdrawing the workpiece from the field of a constantly
energized demagnetization coil. The figure below can
give an idea of the procedure.
Figure 2 - Demagnetization process.
The dimensions of the coil which has been shielded are:
• Length = 300mm.
• Internal diameter = 450mm.
• External diameter = 500mm.
The coil is connected to the low voltage electric network
with a 400V supply establishing a value for the field
strength on its centre around 10kA/m. The elevated field
strength implies the existence of high field levels not
only in the centre of the coil but in all the surroundings.
These levels are to be checked and reduced in those
zones considered necessary. This will be done through
the installation of a magnetic screen which has as main
requirement, apart form shielding the magnetic flux in
the outside, not to alter the field strength in the centre of
the coil since it could damage the functioning of the coil,
therefore preventing the goal it has in the industrial
process, which is, the removal of remanent magnetic field
inside the workpieces.
3. Procedure
A. Initial measurements.
These measurements allow determining the initial
situation under normal conditions, in situ. For the
registration of the magnetic field values a total number of
13 points were controlled. The representation of the
magnetic field in these points could give an idea of the
shape and aspect of the field. The scheme of
measurements is displayed in figure 3. All the
measurements were submitted at the coil’s axe height,
since they were included like that in the same plane, and
the one where the values of magnetic field are maximum
referred to the centre of the coil.
Figure 3 - Map of measurements around the coil.
The values registered are (values of magnetic field
density in µT and position in meters being the (0,0)
reference point the centre of the coil):
Table 1 - Values of magnetic field density without screen.
These initial measurements made it possible to
characterize the field created by the coil. Once this field
was known, a digital model was established. The
magnetic field created by this model has the aspect
depicted in figure 4 and was developed to correspond
exactly with that created by the real coil. This was
obtained by the comparison of magnetic field values in
the 13 points controlled in situ and the field simulated by
the model in those 13 points.
Figure 4 - 3D view of the digital model with the magnetic field
flux lines generated by the coil.
The values of magnetic field are useful not only to
develop the model but also to have an idea of the initial
situation. With this knowledge it is easier to project the
type of screen which is going to be necessary in order to
fix the magnetic field in the surroundings under the
limits.
B. Design of the screen.
In view of the magnetic behaviour of the different
materials ([3]-[4]) iron and aluminium were selected as
the optimal materials to employ in the construction of the
screen. Another important decision to take was the
thickness of the plates to install [5], from ([2],[6]), 2mm
were selected for both aluminium and iron, trying with
this size to optimize the field reduction and the structure
weight. Apart from that, the shape for the screen as well
as the position had to be chosen, other studies ([2],[7])
helped to decide it should be placed as close to the coil as
possible, trying to enclose it inside the screen. There
were space limitations due to the industrial environment
where the demagnetizing coil was place, though the
width of the screen could not be large.
The first option was to introduce 2 iron plates, one on
each side of the coil, to analyze the absorption level of
magnetic field they were able to perform. The election of
iron located on the sides of the coil where the field is
parallel to the surface of the plate is because of its good
behaviour in that position [2]. The aspect of the field with
this first screen was:
Figure 5 – Flux lines distributions with the first screen.
Results from this redistribution of the magnetic field
were not successful, even increasing the width of the iron
from the initial 0.5m to nearly 2m the reduction was not
enough.
The second option was then the introduction of a second
iron plate on each lateral. Different simulations were
performed, varying the width of the plates as well as the
distance between them. The optimal distance was
concluded to be 5cm. With this new disposition, a great
reduction was obtained on the x direction but it was still
poor in the y direction. The aspect of the field
distribution can be observed in figure 6.
Figure 6 - Flux lines distributions with a second screen.
It was decided, instead of incrementing the lateral
volume of the screen, and due to problems of space, to
install other parts of the screen in the y direction. So,
aluminium was used to close the screen around the coil
placing 2 wings of this material, with an angle of 60º, on
each of the exterior iron plates. The resulting structure
was as follows:
Figure 7 - Section of the final design of the screen surrounding
the coil.
Various angles for the wings were simulated. Equally,
fixing them to the interior iron plate was tried too. Form
all the possibilities, the previous distribution was found
to be the best. This solution adopted as definitive creates
a distortion of the magnetic field as is visualized below.
Figure 8 - Magnetic flux lines simulated with the coil shielded.
Once simulated and the designed values obtained under
the limits and with a certain security margin, the screen
was constructed and installed.
C. Final measurements.
After the design, construction and installation of the
screen, new measurements were performed in order to
check the efficiency of the shielding and verify the
calculations and simulations carried out during the
design. The results are summarized in table 2. Once again
the magnetic field density is expressed in µT and the
position in meters having as the (0,0) reference the centre
of the coil.
Table 2 - Values of magnetic field density with screen.
By comparing tables 1 and 2, it is clearly observed that
the values of magnetic field have registered a big
reduction in all the point, except point number 9. This is
due to the fact that the screen reduces the field absorbing
magnetic flux lines and confining this energy into the
material, but also deflects the unabsorbed flux lines. This
phenomenon makes it possible to increase the magnetic
field in some regions of the space due to the
concentration of magnetic lines of higher field intensity.
This is the case of point 9 as has been seen during the
design. It is located close of the axe of the coil were
magnetic field lines are concentred.
Apart from that, the goal of the reduction of magnetic
flux density under the limits has been accomplished for
all the points. None of them rests above 100µT which
was the requirement. The percentage of reduction varies
from the 88% obtained in point 1 to the 20% of point 11.
The smaller the field was at the beginning, the smaller
the reduction obtained. Anyway, for the rest of the points
the reductions are all important but different, due to the
deformation registered by the screen.
4. Conclusions
The whole design of a magnetic screen has been
performed throughout measurements and simulations.
The magnetic field reduction achieved by means of the
installation of the screen goes beyond the 88% in the
most critical points referred to the initial situation. The
combination of two kinds of materials as well as the good
selection of the relative position has been fundamental in
order to obtain such a large reduction.
References
[1] Directiva 2004/40/CE del parlamento europeo y del
consejo de 29 de abril de 2004 sobre las disposiciones
mínimas de seguridad y de salud relativas a la exposición
de los trabajadores a los riesgos derivados de los agentes
físicos (campos electromagnéticos)
[2] “Estudio y diseño de sistemas de apantallamiento para
centros de transformación frente a los campos
electromagnéticos”. Internal document published by
UNION FENOSA in collaboration with the Instituto de
Tecnología Eléctrica. January 2005.
[3] L.Hasselgren, J.Luomi, “Geometrical Aspects of Magnetic
Shielding at Extremely Low Frequencies”, IEEE
transaction on electromagnetic compatibility, Vol.37, N.3,
August 1995.
[4] O.Bottauscio, M.Chiampi, D. Chiarabaglio, F. Fiorillo, L.
Rocchino, and M. Zucca, “Role of magnetic materials in
power frequency shielding:numerical analysis and
experiments,”. Proc.Inst.Elect.Eng., vol.148,pp. 104-110,
Mar.2001.
[5] U.Adriano, O.Bottauscio, M.Zucca, “Material Efficiency
in Magnetic Shielding at low and intermediate frequency”,
IEEE transaction on magnetics. Vol.39. N.5, September
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[6] H.Beltran, V.Fuster, A.Quijano, "Optimal Shielding
Thickness of Low Frequency Magnetic Fields" XII
International Symposium on Electromagnetic Fields, in
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(Spain) 2005.
[7] E.Salinas, “Conductive and ferromagnetic screening of
50Hz magnetic fields from a three phase system busbars”
Journal of Magnetism and Magnetic Materials. 226-230
(2001) 1239-1241.
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  • Apr 2001
  • IEE Proc Generat Transm Distrib
Shielding of magnetic fields at power frequencies has been investigated by computation and experiment. Different types of low-cost soft magnetic laminations (low-carbon steel, nonoriented Fe-Si, grain-oriented FeSi) and different shield geometries (plane, U-shaped and box-like) have been considered. The shielding factors of model screens have been measured, together with the magnetic flux density in the sheets employed, under field levels typical of industrial environment (~100 μT). Computations have been carried out by means of a hybrid finite-element/boundary element model, which result in good agreement with measurements. It is shown that grain-oriented laminations provide the highest shielding factor, which is reinforced in the plane shield configuration by the addition of an aluminium plate. It is predicted that the screening performance of the U-shaped and box-like shields can be appreciably impaired by the presence of air gaps at the corner joints
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Geometrical Aspects of Magnetic Shielding at Extremely Low-frequencies
Article
  • Sep 1995
The magnetic shielding effectiveness for closed and open shield structures is studied at extremely low frequencies. Analytical solutions are used for simple geometries, while more complex structures are evaluated using a finite-element method. Both highly conductive and ferromagnetic materials are studied, and their different shielding behavior is shown. Ferromagnetic shields give good results for small and closed shields and they also give a large field attenuation at close range to the source for open shield geometries. Highly conductive materials, on the other hand, are found to be suitable for large shield sizes. The attenuation is, however, reduced in the close vicinity of the source. Comparisons of numerical results with analytical calculations and measurements confirmed the high accuracy of the finite-element model
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Optimal Shielding Thickness of Low Frequency Magnetic Fields
  • Jan 2005
  • H Beltran
  • V Fuster
  • A Quijano
H.Beltran, V.Fuster, A.Quijano, "Optimal Shielding Thickness of Low Frequency Magnetic Fields" XII International Symposium on Electromagnetic Fields, in mechatronics, electric and electronic engineering. Baiona (Spain) 2005.