--- title: "CRGO versus hot rolled steel laminations" subject: "Electric Machine Design" chapter: 1 key: "semester_5/electric_machine_design:crgo-vs-hot-rolled" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/crgo-vs-hot-rolled site: BCT Engineering --- # CRGO versus hot rolled steel laminations Electric Machine Design · Chapter 1: Materials used in electrical equipment - Asked in: 2080 Bhadra, 5 marks; 2078 Kartik, 5 marks; 2074 Chaitra, 4 marks (asked 3 times) - Video: [Lecture Video8_18EE55_Module-1_Cold Rolled Grain Oriented Steel_Gopinath K](https://www.youtube.com/watch?v=Dlm68yFheQo) — Sairam EE Video - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/crgo-vs-hot-rolled - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1 ## Question Explain the advantages of cold rolled grain oriented steel laminations over hot rolled steel laminations in transformer. ## 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. ## Answer **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 1. 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. 2. **Lower iron loss and magnetising current** give better efficiency, especially for distribution transformers that are energised all day. 3. **Thinner laminations** with a good stacking factor (carlite/phosphate coating) further reduce eddy loss. 4. 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What makes CRGO steel better in a transformer? A. Grains are random B. Grains have easy magnetisation along the rolling direction, which is the flux direction C. It has no silicon D. It is thicker Answer: B. Cold rolling and annealing align grains so flux along rolling direction sees high permeability and low loss. (Key point: Grain orientation) 2. What follows from the higher working flux density (1.5-1.75 T) of CRGO? A. Larger core B. Smaller core section and less copper, so a lighter transformer C. Higher noise D. Lower efficiency Answer: B. Higher B gives a smaller core section for the same voltage per turn. (Key point: Benefit of high B) 3. Why are CRGO corner joints mitred at 45°? A. To reduce cost B. To keep flux mostly along the grain, as cross-direction properties are poor C. To increase eddy loss D. To allow bolt holes Answer: B. Across the grain properties are poor, so mitring keeps flux along the rolling direction. 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