---
title: "Hard vs soft magnetic materials for relays"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:hard-soft-relay"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hard-soft-relay
site: BCT Engineering
---

# Hard vs soft magnetic materials for relays

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2081 Bhadra, 3+3 marks
- Video: [Magnetic Materials Used in Electrical Machines, Instruments and Relays](https://www.youtube.com/watch?v=ZK5q2tE6VfA) — EEE - E Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/hard-soft-relay
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

Differentiate between hard and soft magnetic materials and suggest which type of magnetic material would you choose for electromagnetic relay and why?

## Short answer

Soft magnetic materials have low coercivity, high permeability, and narrow hysteresis loop, making them easy to magnetize and demagnetize. For a relay, soft iron or permalloy is chosen because it provides strong flux with low current and releases quickly when de-energized, preventing contact sticking.

## Answer

#### Hard vs soft magnetic materials

| Property | Soft magnetic | Hard magnetic |
|---|---|---|
| Hysteresis loop | Narrow, tall | Wide, fat |
| Coercivity $H_c$ | Low (< 10³ A/m) | High (> 10⁴ A/m) |
| Retentivity | Low remanence after field off | High |
| Permeability | High | Low |
| Hysteresis loss | Small | Large |
| Domain walls | Move easily (pure, few defects) | Pinned by defects and fine particles |
| Magnetization/demagnetization | Easy | Difficult |
| Examples | Soft iron, Si-steel, permalloy, soft ferrite | Alnico, NdFeB, SmCo, hard ferrite |
| Uses | Transformer cores, motors, relays | Permanent magnets, speakers, meters |

```text
     B                      B
     ^  soft                ^   hard
    _|_                 ____|____
   / |/                /    |   /
  /  /------> H      -/-----+--/--> H
 /  /|               /      | /
                    /_______|/
```

#### Material for an electromagnetic relay
A **soft magnetic material** (soft iron, low-carbon steel or Ni-Fe permalloy) is chosen for the relay core, yoke and armature, because:
- **High permeability**: a small coil current produces enough flux to pull the armature; coil power is low.
- **Low coercivity and low remanence**: when the coil is switched off, the core loses its magnetism at once, so the armature is released quickly by the spring and does not stick.
- **Low hysteresis loss**: suits relays that operate frequently or on AC.
- **High saturation flux density**: gives strong pull from a small core.

A hard material would keep its magnetism after the coil is de-energized, so the contacts would not open reliably.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Compared with soft materials, hard materials have:
   A. Wide loop, high Hc
   B. Narrow loop, low Hc
   C. Higher permeability
   D. Lower retentivity
   Answer: A. Hard: wide fat loop. (Key point: Loops)

2. Hard materials have domain walls that are:
   A. Absent
   B. Pinned by defects and fine particles
   C. Easily moved
   D. Thick
   Answer: B. Pinning makes magnetization difficult. (Key point: Domain walls)

3. Relay core material:
   A. Soft magnetic (soft iron, permalloy)
   B. Hard magnetic (Alnico)
   C. NdFeB
   D. Diamagnetic
   Answer: A. Soft needs low current and releases. (Key point: Relay choice)

4. A hard material would fail in a relay because:
   A. Keeps magnetism, so armature may not release
   B. Low Bs
   C. High loss only
   D. Low coercivity
   Answer: A. Contacts wouldn't open reliably. (Key point: Why not hard)

## Related in Electrical Engineering Material

- [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
- [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
- [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
- [Critical current density, critical field, soft vs hard](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Acritical-density-soft-hard) — Electrical Engineering Material, chapter 5
- [Applications of soft magnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Asoft-magnetic-applications) — Electrical Engineering Material, chapter 4, asked 2×
- [Properties and uses of soft magnetic materials](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Asoft-magnetic-properties-uses) — Electrical Engineering Material, chapter 4

## 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
- [Soft versus hard magnetic materials](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asoft-hard-magnetic-materials) — Electric Machine Design, chapter 1, asked 5×
- [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
- [Static vs electromagnetic relays; components](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-vs-electromagnetic-relays) — Switchgear and Protection, chapter 8, asked 4×