Friday, 27 April 2012

More on Magnetic materials

There are Three types of SOFT MAGNETIC MATERIALS:-
  • Solid Core Material :- These are parts of magnetic circuit carrying steady flux as core etc. Made of Soft iron, relay steel;.
  • Electrical sheet and strip
  • Special Purpose Alloys
Solid Core Materials

  • Iron, Low Carbon, Si Steel  ( Carbon is harmful it raises ciercive force & hystersis loss. Silicon is also harmful but with oxygen it becomes good)
  • Low Carbon Steel ( Steel has low resistivity and high edddy current losses)
  • Cast Iron :- Low relative permeability.
  • Gray Carbon:- Magnetically inferior to wrought iron or steel.
  • Cast Steel:- Used for those portions of magnetised circuits which carry steady flux and need superior mechanical quantities.
  • Soft Steel:-now widely used in place of cast steel.
  • Ferro Cobalt:-It has its saturation flux density 10 % higher than that of pure iron. Cost is High. Use is limited to pole pieces where a huge value of induction ( flux density) is desired.

Sheet Steel:-
Electrical Sheet Steel ( Non-Oriented):- Iron with low Carbon was used earlier. It was prone to ageing. Use of Silicon Steel, this is no issue. Advantages of adding Silicon ( More age, less hysterisis loss, increased resistivity, less eddy current loss). Disadvantages ( loss of permeability at higher flux densities & lower ductility). Hence Maxm Si = 5%
Electrical sheet steel can be Hot Rolled or Cold Rollled. In rotating parts we use steels of low Si content and are called "Dynamo Grade Steel" ( since these have to work at high flux densities).
4 to 5 % Si steel = "Transformer grade Steel"( also called high resistance steel (HRS))


CRGOS:- Saturates at a later stage . Constituent crystals, crystalline axis, cold reductions, annealing. Rolling direction = direction of high permeability.Reduces Iron loss.

Special Purpose Alloys:- SPA , HULOB.
They have high flux permeability at very low flux densities. Ni ( 60 to 90%) + Chr + Mb.
MUMMETAL :- Low permeability but high eddy currrent losses. Some Cu too.
PERMALLOY:High Ni (80%) permalloy ( high initial & maxm permeability , Mb + Cr , High resistivity) Low Ni (50%) Permalloy ( low permeability, high resistivity)
SUPERPERMALLOY:- High Purity, Relative permeability 100,000
PERMINVAR:- Permability independent of field strength.
PERMEANDER:-Capable of producing high flux density 49 % Cobalt 2 %, Vn 49 %

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