1
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What do you mean by the magnetic hysteresis?
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Magnetic hysteresis
- The hysteresis means to lag behind.
- When a magnetic material is magnetized in the magnetic field, there is
always some magnetism after removing magnetizing force. Therefore the flux
density ( B ) in the magnetic material always lags behind the magnetizing
force ( H ).
- This phenomenon is known as magnetic hysteresis.
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2
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What do you mean by the residual flux density?
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Residual flux density
- Once the magnetic core is magnetized and if the magnetizing force is
removed, the core is not completely demagnetized but the flux density has
still some value.
- It is called as residual magnetism.
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3
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Explain the term : Coercive force
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Coercive force
- The demagnetizing field strength required to wipe off residual
magnetism in the magnetic material is called as coercive force.
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4
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What is area of hysteresis loop indicates?
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- The area of the hysteresis loop indicates the energy absorbed by the
core material to overcome friction involved in changing the alignment of
magnetic domains for complete cycle.
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5
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Describe the significance of rectangular shape of
hysteresis loop.
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Significance of the shape of
hysteresis loop
- The rectangular shape of the hysteresis loop suggests that the flux
density remains constant at the maximum value until magnetic field strength
is changed to its negative maximum value.
- The ferrite material has
rectangular type hysteresis characteristics.
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6
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Which material is used for magnetic memory in the
computers? Why
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The ferrite is used for magnetic memory in the computers.
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7
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Why hysteresis loss occurs in the magnetic material?
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Hysteresis loss
- When the magnetic material is magnetized, its domains are aligned in
the direction of the applied field.
- If the magnetic material is magnetized in the reverse direction its
domains try to align into reverse direction.
- The domains rotate in both directions after complete cycle of magnetic
field in this way.
- There is a loss in this process called as hysteresis loss.
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8
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How hysteresis loss can be reduced?
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The hysteresis loss can be reducing by using materials which have low
hysteresis co – efficient.
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9
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Which are the parameters affect the hysteresis
losses?
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The hysteresis loss per meter3 per cycle of magnetic
material depends upon ( a ) The maximum flux density established in the
material.( b ) Quality or type of magnetic material.
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10
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What is steinmetz hysteresis co – efficient?
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Steinmetz hysteresis co – efficient
- The hysteresis loss Wh
α Bmax joule/meter3/cycle
- Wh = η Bmax joule / meter3 / cycle ( Where η
= Steinmetz hysteresis co – efficient in joule / meter3 / cycle
and its value depends upon the type of magnetic material )
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11
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Whether the B – H curve of magnetic material is linear?
Why?
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B – H curve
- The relative permeability of magnetic
material is not constant but depends upon the flux density therefore the B –
H curve is not linear.
B = μH
B = μ0 μrH
μr = ( B / μ0H )
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12
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Whether the B – H curve of non – magnetic material
is linear? Why?
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The relative permeability of non – magnetic
material is constant therefore the B – H curve of the non – magnetic material
is linear.
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13
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Explain : Eddy current loss
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Eddy current loss
- The core material is made of concentric shells.
- As the iron is conducting material, each shell may be treated as a
closed coil.
- Therefore an emf is induced in each shell due to flux links with the
each shell.
- This emf causes a large current flow due to small resistance of the
core.
- This current causes losses in the core which is better known as eddy
current loss.
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14
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How eddy current loss can be minimized?
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Eddy current loss
- The eddy current loss can be minimized by using core of lamination having thickness varies from 0.45 mm to 0.5 mm in the electrical machines.
- Each lamination is insulated from each other by using thin coating of
oxide or varnish.
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15
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What is the direction of eddy current in the core
material?
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The direction of the eddy current is always perpendicular to the
direction of the magnetic flux.
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16
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Why the value of eddy current loss is very large in
the solid core material?
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The cross section of the solid core material is very large. Therefore
its resistance is very small and thus large eddy current flow in the solid
core material.
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17
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State the typical applications of eddy current.
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Applications of eddy current
- Induction heating
- Damping torque in the PMMC instruments
- Electrical braking in induction instruments
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18
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What do you mean by the screening effect?
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Screening effect
- The flux density at the centre of the core is lower than the outer
surface due to flow of eddy current in the core.
- It is known as screening effect.
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19
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State the Fleming’s left hand rule
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Fleming’s left hand rule
- Keep first finger, middle finger and thumb such that they are mutually
perpendicular.
- If the first finger shows the direction of the magnetic field, middle
finger shows the direction of the current, the thumb indicates the direction
of the force.
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20
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On which parameter the force of attraction or repulsion
depends when two current carrying conductors are placed in parallel?
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Force on parallel current carrying
conductors
- Force of attraction : If the direction of the currents in both
conductors is in the same direction.
- Force of repulsion : If the direction of the currents in both
conductors is in the reverse direction.
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