Requirements of transformer core material

The core of a transformer provides a low-reluctance path for the mutual flux. Since it is magnetised continuously at supply frequency, the material must give low iron loss, low magnetising current and a compact core.

Requirements of transformer core material

1. High permeability, so the magnetising current (no-load current) is small.
2. High saturation flux density, so a higher working $B_m$ can be used. This reduces the core area, the turns and the size.
3. Low hysteresis loss: a narrow B-H loop (soft magnetic material).
4. High resistivity, to reduce eddy current loss.
5. Availability in thin sheets (0.23–0.35 mm) with a thin, strong insulating coating. This gives low eddy loss and a high stacking factor.
6. No ageing: the loss must not increase over time.
7. Low magnetostriction, for low noise (hum).
8. Mechanical properties: it can be cut and assembled without losing its magnetic quality, and is not too brittle.
9. Low cost and easy availability.

Material meeting these needs

• CRGO silicon steel (about 3 % Si, grain oriented) is the standard choice for power and distribution transformers. It has very low loss and high permeability along the rolling direction, and works at 1.5–1.75 T.
• Hot rolled silicon steel (4–4.5 % Si) was used earlier and is still used for small transformers.
• Amorphous metal is used in energy-efficient distribution transformers (very low no-load loss).
• Ferrites and nickel-iron alloys are used in high-frequency and instrument transformers.