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Hysteresis Loss


The lag or delay of a magnetic material known commonly as Magnetic
Hysteresis, relates to the magnetization properties of a material
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Magnetic Hysteresis Loop



Starting with an un-magnetized core both B and H will be at zero, point 0 on the
magnetization curve
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

Any further increase will have no effect on the value of B, and the point on the
graph where the flux density reaches its limit is called Magnetic Saturation
...




This is because there is a limit to the amount of flux density that can be generated
by the core as all the domains in the iron are perfectly aligned
...




However, the magnetic flux does not completely disappear as the electromagnetic
core material still retains some of its magnetism even when the current has
stopped flowing in the coil
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

Now if the magnetizing current in the coil is reduced to zero, the magnetic field
circulating around the core also reduces to zero
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

The magnetizing force which must be applied to null the residual flux density is
called a “Coercive Force”
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

An increase in this reverse current causes the core to be magnetised in the
opposite direction and increasing this magnetization current further will cause the
core to reach its saturation point but in the opposite direction, point d on the
curve
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If the magnetizing current is reduced again to
zero the residual magnetism present in the core will be equal to the previous value
but in reverse at point e
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

Then the B-H curve follows the path of a-b-c-d-e-f-a as the magnetizing current
flowing through the coil alternates between a positive and negative value such as
the cycle of an AC voltage
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

Since a coercive force must be applied to overcome this residual magnetism, work
must be done in closing the hysteresis loop with the energy being used being
dissipated as heat in the magnetic material
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Magnetic Hysteresis Loops for Soft and Hard Materials



Magnetic Hysteresis results in the dissipation of wasted energy in the form of heat
with the energy wasted being in proportion to the area of the magnetic hysteresis
loop
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Eddy current loss


A rectangular core of magnetic material is shown along with the exciting coil
wrapped around it
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

An emf will be induced in the path abcd following Faraday’s law
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

Therefore a circulating current i

eddy

will result
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

It is important to note here that to calculate induced voltage in the path, the
value of flux to be taken is the flux enclosed by the path i
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, φmax=Bmax×
area of the loop abcd



The magnitude of the eddy current will be limited by the path resistance, R

path


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

To calculate the total eddy current loss in the material we have to add all the
power losses of different eddy paths covering the whole cross section
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

It is obvious if the cross sectional area of the eddy path is reduced then eddy
voltage induced too will be reduced (E ∞ area), hence eddy loss will be
eddy

less
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

Solid block of iron should not be used to construct the core when exciting
current will be ac
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

Each lamination is given a varnish coating for providing necessary insulation
between the plates
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

Eddy current loss per unit volume of the material can be found from the below
equation:



P



Thus eddy current loss per unit volume of the material directly depends upon
the square of the frequency, flux density and thickness of the plate
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2 2 2
eddy

=kB

e max

fτ



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