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CRGO versus hot rolled steel laminations
Short answer
CRGO laminations have grains aligned along rolling direction, enabling higher flux density (1.5–1.75 T vs 1.2–1.4 T), lower specific loss (0.9–1.3 W/kg vs 2.5–3.5 W/kg), higher permeability, thinner gauge, better stacking factor, and less noise, requiring mitred joints and annealing.
Explain the advantages of cold rolled grain oriented steel laminations over hot rolled steel laminations in transformer.
Cold rolled grain oriented (CRGO) steel is silicon steel (about 3 % Si) rolled cold and then annealed so that most crystals (grains) have their easy direction of magnetisation along the rolling direction. Hot rolled silicon steel has grains oriented at random, so its properties are the same in all directions but poorer.
In a transformer the flux in the limbs and yokes flows along a fixed direction. The core strips are cut so that this direction is the rolling direction, and the full benefit of CRGO is then obtained.
Advantages of CRGO over hot rolled laminations
| Point | CRGO steel | Hot rolled steel |
|---|---|---|
| Working flux density | 1.5–1.75 T | 1.2–1.4 T |
| Specific core loss (at 1.5 T, 50 Hz) | About 0.9–1.3 W/kg | About 2.5–3.5 W/kg |
| Permeability (rolling direction) | Very high | Moderate |
| Magnetising current | Very low | Higher |
| Lamination thickness | 0.23–0.35 mm | 0.35–0.5 mm |
| Stacking factor | About 0.95–0.97 (smooth surface, thin coating) | About 0.90–0.92 |
| Magnetostriction / noise | Lower | Higher |
| Ageing | None | Slight |
Resulting benefits
- A higher flux density means a smaller core section for the same voltage per turn. Less core and less copper are needed, and the transformer is smaller and lighter.
- Lower iron loss and magnetising current give better efficiency, especially for distribution transformers that are energised all day.
- Thinner laminations with a good stacking factor (carlite/phosphate coating) further reduce eddy loss.
- Less noise and lower no-load current.
Precautions
- Along the cross (transverse) direction the properties are poor. The corner joints must therefore be mitred at 45° so that the flux stays mostly along the grain.
- CRGO is sensitive to mechanical stress. Strips are annealed again after cutting, and bolt holes are avoided, so cores are clamped with bands or resin instead.
- CRGO costs more, but the saving in core size and losses more than pays for this.
✓ Answer checked by Claude Sonnet 5.5
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