Which material is used for transformer cores due to high permeability and low retentivity?

• A. Aluminum
• B. Copper
• C. Permanent magnets
• D. Soft iron

Answer: (D)

The key idea is using a core material that easily carries magnetic flux yet doesn’t stay magnetized when the exciting current reverses. This requires high magnetic permeability to guide flux with low reluctance, and low retentivity (or coercivity) so the material doesn’t retain magnetization after the field changes. Soft iron fits this perfectly: it becomes magnetized readily but can be demagnetized with a small reverse field, giving high permeability and minimal residual flux. Aluminum and copper are non-magnetic and don’t provide a good magnetic path, so they’re unsuitable for cores. Permanent magnets have high retentivity and would hold magnetization, causing ongoing losses and poor flux reversal. Hence soft iron is the correct choice.