15 March 2020

Magnetic Hysteresis Interview Question Answer - 1


1
What do you mean by the magnetic hysteresis?


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.


2
What do you mean by the residual flux density?


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.


3
Explain the term : Coercive force


Coercive force
  • The demagnetizing field strength required to wipe off residual magnetism in the magnetic material is called as coercive force.


4
What is area of hysteresis loop indicates?


  • 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.


5
Describe the significance of rectangular shape of hysteresis loop.


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.


6
Which material is used for magnetic memory in the computers? Why


The ferrite is used for magnetic memory in the computers.

7
Why hysteresis loss occurs in the magnetic material?


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.


8
How hysteresis loss can be reduced?


The hysteresis loss can be reducing by using materials which have low hysteresis co – efficient.

9
Which are the parameters affect the hysteresis losses?


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.

10
What is steinmetz hysteresis co – efficient?


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 )


11
Whether the B – H curve of magnetic material is linear? Why?


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 )

12
Whether the B – H curve of non – magnetic material is linear? Why?


The relative permeability of non – magnetic material is constant therefore the B – H curve of the non – magnetic material is linear.

13
Explain : Eddy current loss


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.


14
How eddy current loss can be minimized?


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.


15
What is the direction of eddy current in the core material?


The direction of the eddy current is always perpendicular to the direction of the magnetic flux.

16
Why the value of eddy current loss is very large in the solid core material?


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.

17
State the typical applications of eddy current.


Applications of eddy current
  • Induction heating
  • Damping torque in the PMMC instruments
  • Electrical braking in induction instruments


18
What do you mean by the screening effect?


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.


19
State the Fleming’s left hand rule


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.


20
On which parameter the force of attraction or repulsion depends when two current carrying conductors are placed in parallel?


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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