---
title: "Silicon content: transformer versus rotating machine steel"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:silicon-content-transformer-rotating"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/silicon-content-transformer-rotating
site: BCT Engineering
---

# Silicon content: transformer versus rotating machine steel

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

- Asked in: 2069 Asar, 5 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/silicon-content-transformer-rotating
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Explain why sheet steel possessing higher silicon content is used in transformer while the sheet steels used in rotating machine has a lower silicon content.

## Short answer

Transformers use high-silicon steel (4–4.5%) to reduce iron loss because the core is continuously energized, laminations are simple strips not punched, and no mechanical stress; lower saturation is acceptable. Rotating machines use lower Si (0.5–3.5%) because high Si makes steel too brittle for punching teeth and slots, needs higher saturation flux density, and must withstand centrifugal forces.

## Answer

Adding silicon lowers core loss, but it also makes steel brittle and reduces its saturation flux density. The best silicon content therefore depends on whether low loss or mechanical and magnetic strength matters more. Transformers use about **4–4.5 % Si**; rotating machines use about **0.5–3.5 % Si**.

#### Why transformers use higher silicon content
1. **The core is energised all the time** (24 hours for distribution transformers), so iron loss decides all-day efficiency. High silicon raises resistivity (about 0.6 µΩ·m), which cuts eddy loss, and also lowers hysteresis loss.
2. **Simple shape.** Transformer laminations are plain rectangular strips that are sheared, not punched with teeth and slots. The brittleness of high-silicon steel does little harm.
3. **The core is static**, with no centrifugal or vibration forces, so high mechanical strength is not needed.
4. **Moderate flux density** (1.1–1.7 T) is used in the core, so the lower saturation of high-silicon steel is acceptable.

#### Why rotating machines use lower silicon content
1. **Punching:** stator and rotor laminations have many narrow teeth and slots punched with dies. High-silicon steel is brittle, cracks at the teeth and wears the dies quickly.
2. **High flux density in teeth:** teeth carry about 1.6–2.1 T. Low-silicon steel has a higher saturation flux density and needs fewer ampere-turns.
3. **Mechanical strength:** rotor laminations must withstand centrifugal forces, vibration and torque, so tougher steel is needed.
4. **Air gap dominates** the magnetising current in a rotating machine. The small loss benefit of high silicon is less important than in a transformer.
5. **Low frequency in the rotor** (slip frequency in induction motors) means rotor iron loss is small anyway.

| Factor | Transformer | Rotating machine |
|---|---|---|
| Si content | 4–4.5 % | 0.5–3.5 % |
| Main need | Lowest loss | Strength, punchability, high tooth B |
| Lamination shape | Simple strips | Teeth and slots punched |
| Mechanical stress | Nil | Centrifugal, vibration |

## Check yourself

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

1. What are the effects of adding silicon to steel?
   A. Loss falls but steel becomes brittle and saturation flux density falls
   B. Saturation rises and loss falls
   C. No effect on resistivity
   D. Loss rises and steel becomes tougher
   Answer: A. Silicon raises resistivity and lowers loss, but makes steel brittle and reduces saturation flux density. (Key point: Effect of silicon)

2. Why can a transformer use high-silicon (4-4.5 %) steel?
   A. It has many slots to be punched
   B. It carries centrifugal forces
   C. It runs at 2 T in the teeth
   D. Its core is energised continuously, has simple strip shape, is static and runs at moderate flux density
   Answer: D. Low loss matters most for an always-energised core, while brittleness and low saturation do little harm. (Key point: Transformer reason)

3. Why is high-silicon steel unsuitable for stator and rotor laminations?
   A. It cannot carry flux
   B. It has too low a resistivity
   C. It is brittle, cracks at the narrow teeth and wears punching dies
   D. It has very high saturation flux density
   Answer: C. Rotating machine laminations have many teeth and slots punched with dies, which brittle steel cannot take. (Key point: Punching)

4. Why is lower-silicon steel used for rotating machine teeth?
   A. Teeth carry only about 0.5 T
   B. Teeth carry about 1.6-2.1 T and low-silicon steel has higher saturation, needing fewer ampere-turns
   C. Low silicon gives lower saturation
   D. Rotor iron loss is large
   Answer: B. Teeth work at high flux density, where higher saturation steel needs less mmf. (Key point: Tooth flux density)

## Related in Electric Machine Design

- [Transformer grade versus dynamo grade steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atransformer-vs-dynamo-steel) — Electric Machine Design, chapter 1
- [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×
- [CRGO versus hot rolled steel laminations](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Acrgo-vs-hot-rolled) — Electric Machine Design, chapter 1, asked 3×
- [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×
- [Power versus distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Apower-vs-distribution-transformer) — Electric Machine Design, chapter 3, asked 3×
- [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×

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