ELECTRICAL ENGINEERING: April 2012

Friday, 27 April 2012

Electric Heating and Welding

Q) What is difference between types of electrical heating?
Ans) Resistance Heating is done at power freq. while Induction Heating and Dielectric Heating is done at high freq.

Q) Why adjustable AC voltage is used for Salt Bath Furnace?
Ans)Slat Bath Furnace ( No oxidation while hardening) is used for Steel Tools Hardening at 20 V, 3000 V AC ( DCcan cause electrolysis of NaCl solution). Since Resistance drops with temperature, Adjustable voltage is needed.

Q) Why Heating Materal/Element should have high sp. resistance?
Ans) So that short length of wire is enough.

Q)Why heating element should be free from oxidation?
Ans) Otherwise life will be short. ( Iron addition means oxidation)

Q) Why heating element should have low temperature coeff of resistance?
Ans) So that R does not change much with T.

Q) What are common heating materials?
Ans) Ni-Chr, Ni-Chr-Fe, Ni-Chr-Al, Ni-Cu.

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 %