---
title: "Why CRGO is a superior magnetic material"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:crgo-superior-material"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/crgo-superior-material
site: BCT Engineering
---

# Why CRGO is a superior magnetic material

Electric Machine Design · Chapter 1: Materials used in electrical equipment

- Asked in: 2070 Chaitra, 5 marks
- Video: [Why CRGO silicon steel is used in Transformer Core?](https://www.youtube.com/watch?v=v_5LG9TrvLo) — NO MORE ratta maar
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/crgo-superior-material
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Why CRGO is known as superior magnetic material?

## Short answer

CRGO steel is grain-oriented 3% Si steel with very high permeability along rolling direction, low core loss (0.9-1.3 W/kg at 1.5T), high working flux density (1.5-1.75T), thin laminations, high stacking factor, no ageing, and low magnetostriction, making it superior for transformer cores with unidirectional flux.

## Answer

**CRGO (cold rolled grain oriented) steel** is about 3 % silicon steel that is cold rolled and then annealed so the grains line up with their easy direction of magnetisation (the cube edge) along the rolling direction. This is called the Goss texture. Along this direction its magnetic properties are much better than those of ordinary hot rolled steel, so it is called a **superior magnetic material**.

#### Reasons
1. **Very high permeability along the rolling direction.** Only a small magnetising force is needed, so no-load current is very low.
2. **High working flux density:** 1.5–1.75 T, against 1.2–1.4 T for hot rolled steel. A smaller core area and fewer turns are needed for the same rating.
3. **Very low core loss:** about 0.9–1.3 W/kg at 1.5 T, 50 Hz, against roughly 2.5–3.5 W/kg for hot rolled steel. The loop is narrow, and the thin sheets have high resistivity.
4. **Thin laminations** (0.23–0.35 mm) with a smooth surface and a thin inorganic (carlite/phosphate) coating give a high **stacking factor** (about 0.95–0.97).
5. **No ageing**, so the loss stays constant over the transformer's life.
6. **Low magnetostriction**, so the transformer is quieter.

```text
 B ^         CRGO (rolling dir.)
   |      .------------
   |    /    hot rolled
   |   /  .------------
   |  / /
   | //
   +---------------------> H
 CRGO reaches high B at small H
```

#### Conditions for using it well
- The flux must flow **along the rolling direction**. Across the grain the properties are poor, so corner joints are **mitred at 45°**. This is why CRGO suits transformers (one-direction flux) better than rotating machines.
- The strips must be annealed again after cutting, and bolt holes avoided, because stress spoils the grain properties.

Because CRGO gives smaller, lighter, cooler and more efficient transformers, it is the standard core material for power and distribution transformers.

## Check yourself

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

1. What is the working flux density of CRGO compared with hot rolled steel?
   A. 1.5-1.75 T against 1.2-1.4 T
   B. 1.2-1.4 T against 1.5-1.75 T
   C. Same for both
   D. 0.5 T against 1.2 T
   Answer: A. CRGO works at a higher B, so a smaller core area and fewer turns suit the same rating. (Key point: Working flux density)

2. Why are corner joints in CRGO cores mitred at 45 degrees?
   A. To increase magnetostriction
   B. To reduce the cost
   C. To make the sheets easier to stack
   D. To keep the flux along the rolling direction, as properties across the grain are poor
   Answer: D. CRGO is good only along the rolling direction, so joints are mitred to let flux follow it. (Key point: Direction of flux)

3. Which is true of the core loss of CRGO?
   A. It is the same as hot rolled steel
   B. It is about 2.5-3.5 W/kg at 1.5 T, 50 Hz
   C. It is about 0.9-1.3 W/kg at 1.5 T, 50 Hz, much lower than hot rolled steel
   D. It rises with age
   Answer: C. The narrow loop and thin high-resistivity sheets give very low loss, and there is no ageing. (Key point: Core loss)

4. Why does CRGO suit transformers better than rotating machines?
   A. Rotating machines need very low silicon
   B. Its flux in a transformer is in one direction, whereas rotating machines have flux in changing directions
   C. It is too soft to punch
   D. It has a very low permeability
   Answer: B. CRGO properties are good only along the grain, which suits the fixed flux direction in a transformer. (Key point: Suitability)

## Related in Electric Machine Design

- [Requirements of transformer core material](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atransformer-core-material-requirements) — Electric Machine Design, chapter 1

## Related in other subjects

- [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
- [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
- [Classification of magnetic materials by magnetization](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aclassify-magnetic-materials) — Electrical Engineering Material, chapter 4, asked 6×
- [Strong magnetic field and critical current](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Acritical-current-strong-field) — Electrical Engineering Material, chapter 5, asked 5×