--- title: "Requirements of transformer core material" subject: "Electric Machine Design" chapter: 1 key: "semester_5/electric_machine_design:transformer-core-material-requirements" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/transformer-core-material-requirements site: BCT Engineering --- # Requirements of transformer core material Electric Machine Design · Chapter 1: Materials used in electrical equipment - Asked in: 2072 Kartik, 5 marks - Video: [Transformer core materials](https://www.youtube.com/watch?v=hrYotAoAgaw) — saurabh goel - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/transformer-core-material-requirements - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1 ## Question What are the requirements of magnetic material for making transformer core? ## Short answer Transformer core material must have high permeability for low magnetising current, high saturation flux density to reduce core size, low hysteresis and eddy current losses, high resistivity, availability in thin sheets with insulating coating, high stacking factor, no ageing, low magnetostriction, and good mechanical workability. CRGO silicon steel meets these requirements and is standard for power transformers. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why should transformer core material have high permeability? A. To keep the magnetising (no-load) current small B. To increase the hysteresis loop area C. To make the core brittle D. To reduce eddy current loss Answer: A. High permeability means little mmf is needed to set up the flux, so the magnetising current is small. (Key point: High permeability) 2. What is the benefit of high saturation flux density? A. Eddy current loss becomes zero B. The core can be made thicker C. Noise is eliminated D. A higher working Bm can be used, reducing core area, turns and size Answer: D. A higher Bm lets the same flux go through a smaller core area, so the core is compact. (Key point: High saturation flux density) 3. Which pair of properties reduces hysteresis and eddy loss respectively? A. Narrow B-H loop; thick sheets B. Wide B-H loop; low resistivity C. Narrow B-H loop; high resistivity D. High magnetostriction; thick sheets Answer: C. A narrow loop (soft magnetic material) lowers hysteresis loss and high resistivity lowers eddy current loss. (Key point: Loss reduction) 4. Which material is the standard choice for power and distribution transformer cores? A. Dynamo grade hot rolled steel with 0.5 % Si B. CRGO silicon steel (about 3 % Si) C. Cast iron D. Ferrite Answer: B. CRGO has very low loss and high permeability along the rolling direction, working at 1.5-1.75 T. (Key point: Standard material) ## Related in Electric Machine Design - [Why CRGO is a superior magnetic material](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Acrgo-superior-material) — Electric Machine Design, chapter 1 - [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2× - [Core type versus shell type transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Acore-vs-shell-transformer) — Electric Machine Design, chapter 3, asked 3× - [Leakage reactance of core type transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aleakage-reactance-core-type) — Electric Machine Design, chapter 3, asked 3× - [Per unit resistance drop of core type transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aper-unit-resistance-drop) — Electric Machine Design, chapter 3 - [Design of transformer core](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atransformer-core-design) — Electric Machine Design, chapter 3 ## Related in other subjects - [Soft magnetic material, Faraday's laws](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asoft-magnetic-faraday-laws) — Electrical Machines, chapter 1 - [Transformer no-load and load operation, constant core flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aideal-transformer-noload-load-constant-flux) — Electrical Machines I, chapter 2, asked 5× - [Magnetic material for electromagnetic relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Arelay-magnetic-material) — Electrical Engineering Material, chapter 4 - [Requirements of a good pipe material](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arequirements-good-pipe-material) — Water Supply Engineering, chapter 8, asked 2×