Correct answer: D — High permeability
Permeability and the magnetic circuit
A transformer core carries alternating magnetic flux linking the primary and secondary windings. The core material should have high magnetic permeability, so it offers a relatively easy path for this flux. In a simplified magnetic-circuit model:
Magnetic reluctance ℜ = l/(μA)
Φ = NI/ℜ
Here l is magnetic-path length, μ is permeability, A is core cross-sectional area and NI is magnetomotive force. Increasing μ reduces reluctance. For a specified flux, less magnetomotive force is then needed, so the magnetizing component of current is lower.
Why this property matters in a transformer
Applied voltage and frequency largely determine the required alternating flux. High permeability allows that flux to be established without an unnecessarily large excitation current. Suitable electrical steel also has low hysteresis loss, while its silicon content increases electrical resistivity and thin insulated laminations restrict eddy currents.
Permeability should not be confused with retentivity, which describes the flux remaining after the magnetizing field is removed. Soft magnetic materials are often discussed using several related properties, but their remanence depends on the material and hysteresis loop. The supplied paper specifically keys high permeability. Near saturation, incremental permeability falls and excitation current rises sharply.
