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I know it from my professor that for a general transformer like this picture if the core was a solid material its area would too big. Thus the core loss increases. To prevent this we use metal insulated sheets. thus the area become less and hence the core loss decreases.
(source: electrical-engineering-portal.com)
My question is that how does the area decrease? I can't be clear. I can visually see that the area is nearly same in both cases.
The reason behind using laminated cores is minimising eddy currents and therefore heat losses. If solid iron core was used, the current could be induced and flow in three degrees of freedom.
However, because the core consists of laminated sheets, such as E-I, M or C sheets, current cannot create a loop because the circumference around the coils is always split into two or more elements (no current can flow between E and I). And because sheets in the sheet stack are insulated from each other, no current can flow 'sideways'.
Still, some current is induced along the elements. To minimize this, high resistivity steel is used.
The area of laminated core is higher because of the added thickness of insulation (oxide/laquer) between individual layers of iron.
A solid core is a shorted turn (due to induced eddy currents) but, because the core is made of iron, this shorted turn isn't a massively great super-conductor hence core losses appear resistive (as implied by the name "losses").
To minimize eddy currents, laminations are used that are insulated from each other - now there is no longer a shorted turn and the cross sectional area of each laminate is tiny compared to the whole core cross section.
If the core was made from ferrite (a really poor conductor due to how the material is granulated and held together in a type of epoxy resin), the cross sectional area would never be regarded as "too big" because, if you analyzed it as separate isolated tiny pieces of ferrite, the eddy current losses would be minute except as frequency gets really high.
Same mechanism - laminations are OK for AC power frequencies and do exactly the same job as the insulated granules of ferrite material at much higher frequencies. Ideally, a core would have great magnetic permeability and zero conductivity.
Visually, the cross sectional area looks the same of course but not in terms of eddy currents.
