The relative importance of the various
magnetic properties of a magnetic material varies from one
application to another. In general, properties of interest may
include normal induction, hysteresis, dc permeability, ac
permeability, core loss, and exciting power.
It should be noted that there are
various means of expressing ac permeability. The choice depends
primarily on the ultimate use. Techniques for the magnetic testing of
many magnetic materials are described in the ASTM standards.
The magnetic and electric circuits
employed in magnetic testing of a specimen are as free as possible
from any unfavorable design factors which would prevent the measured
magnetic data from being representative of the inherent magnetic
properties of the specimen.
The flux “direction” in the
specimen is normally specified, since most magnetic materials are
magnetically anisotropic. In most ac magnetic tests, the waveform of
the flux is required to be sinusoidal.
As a result of the existence of
unfavorable conditions, such as those listed and described below, the
performance of a magnetic material in a magnetic device can be
greatly deteriorated from that which would be expected from magnetic
testing of the material.
Allowances for these conditions, if
present, must be made during the design of the device if the
performance of the device is to be correctly predicted.
Leakage.
A principal difficulty in the design of
many magnetic circuits is due to the lack of a practicable material
which will act as an insulator with respect to magnetic flux. This
results in magnetic flux seldom being completely confined to the
desired magnetic circuit. Estimates of leakage flux for a particular
design may be made based on experience and/or experimentation.
Flux Direction.
Some magnetic materials have a very
pronounced directionality in their magnetic properties. Failure to
utilize these materials in their preferred directions results in
impaired magnetic properties.
Fabrication.
Stresses introduced into magnetic
materials by the various fabricating techniques often adversely
affect the magnetic properties of the materials. This occurs
particularly in materials having high permeability. Stresses may be
eliminated by a suitable stress-relief anneal after fabrication of
the material to final shape.
Joints.
Joints in an electromagnetic core may
cause a large increase in total excitation requirements. In some
cores operated on ac, core loss may also be increased.
Waveform.
When a sinusoidal voltage is applied to
an electromagnetic core, the resulting magnetic flux is not
necessarily sinusoidal in waveform, especially at high inductions.
Any harmonics in the flux waveform cause increases in core loss and
required excitation power.
Flux Distribution.
If the maximum and minimum lengths of
the magnetic path in an electromagnetic core differ too much, the
flux density may be appreciably greater at the inside of the core
structure than at the outside. For cores operated on ac, this can
cause the waveform of the flux at the extremes of the core structure
to be distorted even when the total flux waveform is sinusoidal.
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