--- title: "Transformer grade versus dynamo grade steel" subject: "Electric Machine Design" chapter: 1 key: "semester_5/electric_machine_design:transformer-vs-dynamo-steel" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/transformer-vs-dynamo-steel site: BCT Engineering --- # Transformer grade versus dynamo grade steel Electric Machine Design · Chapter 1: Materials used in electrical equipment - Asked in: 2075 Asoj, 4 marks - Video: [#CRGO Vs #CRNGO | Which one is better? | #eicore #toroidalcore](https://www.youtube.com/watch?v=G5cFXr8mTP4) — ElectroGyan - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/transformer-vs-dynamo-steel - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1 ## Question Differentiate between transformer grade and dynamo grade steel. ## Short answer Transformer grade steel has high silicon (~3-4.5%), high resistivity, grain-oriented (CRGO), very low loss, used for static transformer cores. Dynamo grade steel has lower silicon (~0.5-3%), lower resistivity, non-oriented, tougher and easier to punch, used for rotating machine cores needing high saturation and mechanical strength. Their thicknesses are 0.23–0.35 mm vs 0.5–0.65 mm. ## Answer Electrical sheet steels are graded by silicon content and loss. **Transformer grade** steel is high-silicon, low-loss steel for static cores. **Dynamo grade** steel is low or medium silicon steel for rotating machines. | Point | Transformer grade steel | Dynamo grade steel | |---|---|---| | Silicon content | High: about 4–4.5 % (CRGO about 3 %) | Low/medium: about 0.5–3 % | | Resistivity | High (about 0.5–0.6 µΩ·m) | Lower (about 0.2–0.4 µΩ·m) | | Core loss | Low | Higher | | Grain orientation | Usually grain oriented (CRGO) | Non-oriented (hot rolled or CRNO) | | Mechanical property | Hard and brittle | Tougher; easy to punch | | Saturation flux density | Lower | Higher (teeth need high $B$) | | Thickness | 0.23–0.35 mm | 0.5–0.65 mm | | Flux in service | Alternating, in one fixed direction | Rotating/varying direction; high $B$ in teeth | | Use | Power and distribution transformers, reactors | Stators and rotors of induction motors, alternators, DC armatures | #### Reason for the difference - A transformer core is made of simple strips with almost no punching, and it is energised continuously. Low loss therefore matters most, and high silicon and grain orientation are used. - A rotating machine core needs many slots and teeth punched accurately, needs mechanical strength against centrifugal forces, and carries high tooth flux density in all directions. A less brittle, non-oriented steel with higher saturation (dynamo grade) is therefore chosen. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly compares transformer grade and dynamo grade steel? A. Both have the same silicon but differ in thickness only B. Dynamo grade has higher silicon and lower loss C. Transformer grade has low silicon and higher loss D. Transformer grade has high silicon and lower core loss Answer: D. Transformer grade is high-silicon, low-loss steel; dynamo grade has low or medium silicon and higher loss. (Key point: Silicon and loss) 2. Which pairing of grain structure and use is correct? A. Dynamo grade: grain oriented, used in transformers B. Transformer grade: non-oriented, used in motors C. Transformer grade: usually grain oriented (CRGO); dynamo grade: non-oriented D. Both are grain oriented Answer: C. Transformer flux stays in one direction so CRGO suits it; rotating machines have varying flux direction so non-oriented steel is used. (Key point: Grain orientation) 3. Why is dynamo grade steel preferred for stator and rotor laminations? A. It is thinner than transformer steel B. It is tougher and easy to punch, with higher saturation flux density C. It is harder and more brittle, so it resists wear D. It has the highest resistivity Answer: B. Rotating machine laminations need many punched teeth and high tooth flux density, so a tougher, higher-saturation steel is used. 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