The net reduction in effective core area due to the lamination and insulation is approximately:

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ALP CBT 2 Electrician Previous Paper: Held on 22 Jan 2019 Shift 3

Answer 1

Laminated iron cores are used to reduce the eddy current losses in the transformer, because the laminations are insulated from each other.
The process of lamination involves dividing the core into thin layers held together by insulating materials such as Varnish, Impregnated paper, etc.
Due to lamination effective cross-section area of each layer reduces in the order of 10% and hence the effective resistance increases.
As effective resistance increases, the eddy current losses will get decrease.