Transcript CYCLE OF MAGNETISATION

CYCLE OF MAGNETISATION &
HYSTERESIS LOSS
TOPICS TO BE DISCUSSED
MAGNETIC INDUCTION(B)
 MAGNETISING FIELD(H)
 INTENSITY OF MAGNETISATION(I)
 CYCLE OF MAGNETISATION
 ENERGY DISSIPATION DUE TO
HYSTERESIS
 CALCULATION OF HYSTERESIS LOSS
 IMPORTANCE OF HYSTERESIS CURVES
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INTRODUCTION:
The proper selection of a magnetic material for
various purposes involves the knowledge of their
magnetic properties.
Magnetic permeability and magnetic susceptibility
play important role in determining the properties
of the substance. Both of these are determined
from graphical curves obtained between B and H
and between I and H resp. These graphs called
magnetization curves are obtained experimentally.
Magnetic induction or
Magnetic flux density(B)
The number of lines of
induction crossing per unit
area normally through the
magnetic material is
measure of magnetic
induction.
Its SI unit is tesla(T) or
weber/meter2 (wbm-2 )
MAGNETISING FIELD(H):
It is that magnetic field which produces
magnetism in a magnetic substance placed
inside it.
Its strength is known as magnetic intensity. It
is defined as the force experienced by a unit
north pole placed at a point .
Its units are ampere/meter (A/m) or J/Tm3 or
N/Tm2
Intensity of magnetization(I)
It is measure of degree of magnetization
of a specimen .
Quantitatively, it is defined as the
magnetic moment developed per unit
volume of the specimen when placed in a
magnetising field.
Its SI units are A/m or N/Wb.
CYCLE OF
MAGNETISATION
1) Retentivity: The magnetisation OF
left in the specimen when the
magnetising field H is reduced to zero.
2) Coercivity: The magnetic intensity
H(OG) required to reduce the magnetic
flux density B to zero.
3) Hysteresis: The lagging of magnetic
flux density B (or intensity of
magnetization I )
behind the magnetizing field
(magnetic intensity)H.
4)Hysteresis loop: It is a closed
B-H curve (EFGIJKE)or I-H
curve .
Energy dissipation due to hysteresis
When the magnetizing field is applied on an
unmagnetised material , the molecular magnets align
themselves parallel to the field. Work is done by the
magnetizing field against the mutual attractive forces.
When the field is switched off, the material
retains some magnetization . So the energy supplied
to the material during magnetization is not fully
recovered on removing the magnetic intensity. Hence
negative energy has to be applied to bring I to zero.
The balance energy retained in the material is
converted into heat and is finally lost to the
surroundings. It is known as hysteresis loss . It is
equal to the area of B-H loop.
CALCULATION OF HYSTERESIS
LOSS
To determine the hysteresis loss, consider an iron bar
over which a wire is wound having n turns.
 When the current in the wire changes, the magnetic
flux linked with it also changes and an induced e.m.f. is
produced in the wire which according to Lenz’s law
opposes the change in current.
 The induced e.m.f. is given by
e=-ndφ/dt
[-ve sign shows that
=-nd(Ba)/dt
direction of induced e.m.f.
=-nadB/dt
opposes the cause which
produces it.
The power consumed is given by
P=ei =na(dB/dt)I

But
i=Hl/n
Therefore ei = al H dB /dt
Energy spent in time dt is given by
dW= alHdB
Therefore ,net work done for a complete cycle of
magnetisation is then given by
W = al∫HdB
But HdB = area of B-H loop
Hence work done per cycle of magnetisation =al*area
of B-H loop
Work done per unit volume per cycle of
magnetisation=area of B-H loop.
This loss of energy is dissipated in the form of heat
and is known as hysteresis loss.
The study of hysteresis curves
gives the information about
following properties of a
magnetic substance.
(i)Susceptibility(χ):It is
measure of the ease with which a
substance can be magnetised.
χ =I/H
Its value depends only on nature of
material. It has no units.
It is greater for soft materials than for
hard materials.
(ii)Permeability(µ)
It is the degree to which
the magnetic lines of force can penetrate a material.
µ=B/H
Its SI units are Tm/A or NA
It is greater for soft materials than for hard materials.
(iii) Retentivity : It is greater for soft materials than for
hard materials.
(iv) Coercivity: It is less for soft materials than for hard
.
materials
(v) hysteresis loss: It is less for soft materials than for
hard materials.
Importance of hysteresis
curves: Knowing susceptibility, permeability
and hysteresis loss for different magnetic
materials their suitable selection can be made
for practical and industrial applications e.g.
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for making electromagnets
for making permanent magnets
for making transformer cores
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