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Using our 3-Dimensional engineering and modeling software,
Amuneal's technical staff and experienced sales people work
with you to design the most cost-effective shielding solution
for your application.
Magnetic Shielding Theory
Magnetic shielding strategies that rely on the interactions
between magnetic fields and special high permeability materials
are called Passive Shielding Strategies. In order to understand
how passive shields operate, the following terms and parameters
must be defined.
Magnetic Field Strength (H)
The Magnetic Field Strength, called the "H" field,
describes the intensity of a magnetic field in free space*.
Field strength(H), measured in Oersteds (Oe), is a function
of the intensity of the magnetic source and the distance from
the source at which it's measured.
* at some distance away from its source
Magnetic Flux Density (B)
The Magnetic Flux Density, called the "B" field,
describes the concentration of magnetic lines of force in
a material. Flux density(B), in Gauss (G), measures the number
of magnetic lines of force that reside in a square centimeter
of material. The flux density depends on intensity of a magnetic
source, the distance of the material from the magnetic source,
and the material's attractiveness to the magnetic fields.
Material Permeability (µ)
Permeability, notated by the Greek symbol µ (mu), refers
to a material's ability to attract and conduct magnetic lines
of flux. The more conductive a material is to magnetic fields,
the higher its permeability. Mathematically, µ = B/H
which simply states that the permeability of a material can
be determined by measuring the magnetic field strength (H)
at a point in free space and then measuring the flux density
(B) at that point after the insertion of a material. The higher
the permeability of that material, the greater the concentration
of flux lines. Any difference in the resulting B field, all
other factors being equal, is due to the permeability of the
material. The greater the change the higher the permeability
of that material. (Note: Magnetic Shielding Materials are
chosen for their unusually high permeabilities.)
Saturation
Saturation is the limiting point of a material to conduct
additional magnetic lines of flux. Each permeable material
has a specific saturation point which is defined by the maximum
number of magnetic flux lines that can be conducted through
each square centimeter of that material. Once a material has
become saturated it no longer functions as a proper shield.
(Note: The saturation and permeability characteristics of
a material are inversely related, therefore the higher a material's
permeability, the lower its saturation point.)
Frequency
The frequency of a magnetic field, measured in cycles per
second (Hz), is the same as the operating frequency of the
field's source. For example, a 60Hz power line will create
a 60Hz magnetic field. Knowing the frequency of a magnetic
field is important in determining the proper composition and
thickness of material to be used in a given shield design.
Attenuation
Attenuation is a ratio used to measure the effectiveness of
a given shield. The ratio is expressed as the field strength
at a given point vs. the resulting field strength at the same
location with a magnetic shield in place. For example, a shield
which provides a reduction of 100 times has an attenuation
of 100:1. A customer's desired attenuation or field reduction
often defines the shielding objective.
Knowledge of these six basic parameters will help in understanding
the dynamic interactions between AC and DC magnetic fields
and their role in passive shielding strategies.
Design Considerations
Understanding the following parameters is essential to designing
quality Magnetic Shielding. They are intended as a guideline
to aid in your shielding design.
Geometry
Most magnetic shielding formulas and models are based on the
theoretical geometry of a sphere or an infinitely long cylinder.
As these shapes are usually not practical in the real world,
the actual geometry of a given shield must be considered to
determine its effectiveness as a shield. When doing calculations,
we must subjectively degrade values for the material's permeability
based on how much a given shield's geometry differs from that
of a sphere or infinitely long cylinder.
Shape
It is difficult for magnetic flux lines to turn 90°; therefore,
rounded shields, such as cylinders or boxes with rounded corners,
are better at redirecting lines of flux than square shields.
Similarly, gentle radii are better than sharp corners to contain
and redirect flux that is already entrapped. It is important
to keep the shape of a shield simple, always having a low
reluctance, or "path of least resistance," in mind.
Size
The smaller the effective radius of a shield, the better its
performance. Therefore, it should always be a goal to design
a shield that will envelop the component or space you are
attempting to shield as closely as possible. In addition,
because material consumption is a major cost component in
shield design, a smaller shield will usually yeild better
performance at a lower cost.
Continuity
It is necessary to insure magnetic continuity, or contact,
whenever a shield is constructed from two or more pieces.
This includes lids, covers, overlapping corners, seams and
doors. Continuity can be maintained either mechanically, using
friction and hardware, or through welding the material in
the case of corners or transitions. Maintaining continuity
between surfaces insures that the magnetic flux will be able
to continue along a low reluctance path, thus increasing shielding
performance.
Closure
Whenever possible, a shield should be closed on all sides.
This configuration, even if rectangular, most closely approximates
a sphere and creates a closed magnetic circuit. In addition,
complete closure provides shielding in all axes thus guaranteeing
the highest shielding performance. To accomplish this, removable
covers, lids and doors should be designed into a shield when
exceptional performance is required.
Length to Diameter Ratio and Openings
When you are unable to close one or both ends of a shield,
or if the shield must have holes, then close attention should
be paid to the diameter of those openings. As a rule of thumb,
magnetic fields can travel into any opening a distance equal
to five times the diameter of that opening. For shields with
open ends the ratio of the shield's diameter to its length
should be considered to improve performance. By increasing
the length of a shield while maintaining its diameter we approximate
an infinitely long cylinder. Achieving this type of configuration
will improve the shielding performance at a region of increasing
distance from the opening. Similarly, tubulations can be used
to protect shields with large holes and penetrations. The
length of the tubulation should be proportionate to the diameter
of the opening that it is protecting.
Multi-layer Shields
In many applications, a single-layer shield cannot provide
either the level of attenuation or saturation protection required.
In these cases, multi-layer or "nested" shields
are employed. Nesting two or more high permeability shields
within one another, utilizing air gaps provided by spacing
between them, results in excellent shielding factors. The
more demanding your shielding objective, the more layers you
may require.
When shields need to operate in very high magnetic field environments
such as those in close proximity to electric arc furnaces
or large superconducting magnets, the saturation of materials
is a concern. In these cases, nested shields constructed from
different compositions of materials is an option. The layer
closest to the highest field levels should be fabricated from
a lower permeability, high saturation material. This "buffer"
shield and the added attenuation due to the spacing of the
air gap should reduce the source field to a level where it
is safe to use high permeability materials without the fear
of saturation.
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