--- title: "Soft versus hard magnetic materials" subject: "Electric Machine Design" chapter: 1 key: "semester_5/electric_machine_design:soft-hard-magnetic-materials" source: https://www.bctengineer.com/q/semester_5/electric_machine_design/soft-hard-magnetic-materials site: BCT Engineering --- # Soft versus hard magnetic materials Electric Machine Design · Chapter 1: Materials used in electrical equipment - Asked in: 2075 Chaitra, 5 marks; 2073 Shrawan, 4 marks; 2073 Chaitra, 5 marks; 2069 Asar, 5 marks; 2082 Chaitra (new course), 4 marks (asked 5 times) - Video: [Soft & Hard Magnetic Materials | Ferromagnetic Materials | Electrical Machine Design! #electrical](https://www.youtube.com/watch?v=7MDOT6C1uj8) — Prasant Decodes - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/soft-hard-magnetic-materials - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1 ## Question Differentiate between soft and hard magnetic materials on the basis of B-H curve with sufficient examples. ## Short answer Soft magnetic materials have narrow B-H loops, low coercivity, high permeability (e.g., silicon steel, permalloy) for flux-carrying cores. Hard magnetic materials show wide loops, high coercivity, large remanence (e.g., Alnico, NdFeB) for permanent magnets. ## Answer Magnetic materials are divided into **soft** and **hard** materials according to the shape of their B-H (hysteresis) loop. Soft materials are easy to magnetise and demagnetise; hard materials keep their magnetism once magnetised. #### B-H curves ```text Soft material Hard material B B | __..-- __|______ | / / | / -----|/-----> H -----+---|----+---> H /| / | / --'' | /_____|__/ | | narrow loop, small Hc wide loop, large Hc, Br ``` - A **soft** material has a tall, narrow loop: small coercive force $H_c$, high permeability, small loop area. - A **hard** material has a short, fat loop: large coercive force $H_c$, large remanence $B_r$ and large loop area (large $(BH)_{max}$, energy product). #### Comparison | Point | Soft magnetic material | Hard magnetic material | |---|---|---| | Hysteresis loop | Narrow, steep | Wide, nearly rectangular | | Coercivity $H_c$ | Low (below about 1 kA/m) | High (10 kA/m to over 1000 kA/m) | | Permeability | Very high | Low | | Retentivity | Low to moderate | High | | Hysteresis loss | Very small (small loop area) | Large | | Energy product $(BH)_{max}$ | Negligible | High | | Ease of magnetisation | Magnetised and demagnetised easily | Hard to magnetise, hard to demagnetise | | Use | AC flux paths: cores of machines and transformers | Permanent magnets | #### Examples - **Soft:** pure (Armco) iron, low-carbon steel, silicon steel (hot rolled and CRGO), nickel-iron alloys (permalloy, mumetal), soft ferrites (Mn-Zn, Ni-Zn) and amorphous metal. Used in transformer cores, armature and stator cores, relays, chokes and high-frequency cores. - **Hard:** carbon steel, tungsten steel, cobalt steel, Alnico, barium/strontium ferrite, samarium-cobalt and neodymium-iron-boron (NdFeB). Used in permanent-magnet DC motors, PM synchronous machines, measuring instruments, loudspeakers and magnetos. Machine cores carry alternating or rotating flux, so the loop is traced many times per second. This is why they need soft materials with a small loop area. A permanent magnet must keep its flux against demagnetising fields, so it needs a hard material. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do the B-H loops of soft and hard materials differ? A. Soft: wide loop; hard: narrow loop B. Soft: tall narrow loop with small Hc; hard: fat loop with large Hc and Br C. Both have the same loop but different scales D. Soft: no loop at all Answer: B. Soft materials have narrow loops with small coercive force; hard ones have wide loops with large Hc and Br. (Key point: B-H loop shapes) 2. Which list contains only hard magnetic materials? A. Silicon steel, permalloy, soft ferrite B. Alnico, samarium-cobalt, NdFeB C. Armco iron, mumetal, amorphous metal D. CRGO, Armco iron, Alnico Answer: B. Alnico, SmCo and NdFeB are permanent-magnet (hard) materials; the others are soft. (Key point: Examples) 3. Why are machine cores made of soft material? A. Flux alternates, so the loop is traced many times per second and a small loop area keeps hysteresis loss low B. They must retain flux against demagnetising fields C. They need a large energy product D. Soft materials have low permeability Answer: A. Alternating flux repeats the loop each cycle, so small loop area means small loss. (Key point: Machine cores need soft) ## Related in Electric Machine Design - [Soft magnetic materials and commercial applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asoft-magnetic-applications) — Electric Machine Design, chapter 1, asked 2× ## Related in other subjects - [Hysteresis loop energy, hard and soft magnetic materials](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ahysteresis-area-energy-hard-soft) — Electrical Machines I, chapter 1 - [Hard vs soft magnetic materials for relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-relay) — Electrical Engineering Material, chapter 4 - [Hard vs soft magnetic materials by hysteresis](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-soft-hysteresis) — Electrical Engineering Material, chapter 4 - [Hard magnets and soft high-frequency materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ahard-permanent-soft-high-frequency) — Electrical Engineering Material, chapter 4