---
title: "Advantages and disadvantages of adding silicon"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:silicon-iron-advantages"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/silicon-iron-advantages
site: BCT Engineering
---

# Advantages and disadvantages of adding silicon

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

- Asked in: 2078 Bhadra, 5 marks; 2075 Asoj, 4 marks; 2071 Shrawan, 5 marks (asked 3 times)
- 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/silicon-iron-advantages
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What are the advantages and disadvantages of adding silicon with iron?

## Short answer

Silicon alloying raises resistivity to cut eddy current loss, lowers hysteresis loss, eliminates ageing, improves permeability, and reduces magnetostriction. It makes steel brittle above ~4%, lowers saturation flux density, reduces thermal conductivity, and increases cost, limiting Si to 0.5–3.5% in motors.

## Answer

Silicon (about 0.5–4.5 % by weight) is added to iron to make the **silicon steel** used in transformer and machine cores. It improves the magnetic properties but spoils the mechanical properties.

#### Advantages
1. **Higher resistivity.** Resistivity rises from about 0.1 µΩ·m (pure iron) to about 0.6 µΩ·m (4 % Si). Since $P_e \propto \dfrac{B_m^2 f^2 t^2}{\rho}$, **eddy current loss** falls sharply.
2. **Lower hysteresis loss.** The coercive force falls and the hysteresis loop becomes narrower.
3. **No ageing.** Silicon combines with or removes carbon and oxygen, so carbide precipitation, and with it the rise in loss over time, is avoided.
4. **Higher permeability** at low and medium flux densities, so magnetising current is less.
5. **Less magnetostriction**, so transformer hum is lower.

#### Disadvantages
1. **Brittleness.** Above about 4–4.5 % Si the steel becomes hard and brittle, so it cannot be rolled thin or punched easily.
2. **Tool wear.** Dies and punches wear quickly, which raises manufacturing cost.
3. **Lower saturation flux density.** Silicon is non-magnetic, so it reduces the iron content. The core must work at a lower $B_m$, or be larger.
4. **Lower thermal conductivity**, so heat leaves the core less easily.
5. **Higher cost** of the material.

```text
 loss,          brittleness
 rho  ^ resistivity     /
      |      .-------  /
      |   .-'         /
      | .'  core loss/
      |\_           /
      |   ~~--.__  /
      +-----------+----> % Si
      0   1   2   3   4   5
     usual: 0.5-3.5 % (motors)
            4-4.5 % (transformers)
```

#### Practical choice
| Use | Silicon content | Reason |
|---|---|---|
| Transformer cores | 4–4.5 % (CRGO about 3 %) | Low loss is vital; strips need little punching |
| Rotating machines | 0.5–3.5 % | Teeth must be punched; high tooth flux density needed |

## Check yourself

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

1. Which is an advantage of adding silicon to iron?
   A. Higher saturation flux density
   B. Higher resistivity, so lower eddy current loss
   C. Easier punching
   D. Lower cost
   Answer: B. Resistivity rises from about 0.1 to 0.6 µΩ·m at 4 % Si, cutting eddy loss. (Key point: Merits of silicon)

2. Which is a disadvantage of adding silicon above about 4-4.5 %?
   A. Core loss rises sharply
   B. Ageing returns
   C. The steel becomes hard and brittle, making punching difficult and wearing tools
   D. Resistivity falls
   Answer: C. High silicon makes steel brittle, so rolling and punching are hard and dies wear quickly. (Key point: Demerits of silicon)

3. Why does silicon lower the saturation flux density?
   A. Silicon is non-magnetic and reduces the iron content
   B. Silicon raises resistivity
   C. Silicon adds carbon
   D. Silicon makes laminations thinner
   Answer: A. Being non-magnetic, silicon dilutes the iron, so the core must work at a lower Bm or be larger. (Key point: Saturation drawback)

## Related in Electric Machine Design

- [Ageing of core and silicon addition](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aageing-silicon-merits) — Electric Machine Design, chapter 1, asked 3×
- [Insulating material requirements and silicon steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-silicon) — Electric Machine Design, chapter 1, asked 2×
- [Ageing, its minimization and silicon addition](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aageing-minimize-silicon-merits) — Electric Machine Design, chapter 1