---
title: "Ageing of core and silicon addition"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:ageing-silicon-merits"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/ageing-silicon-merits
site: BCT Engineering
---

# Ageing of core and silicon addition

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

- Asked in: 2079 Bhadra, 5 marks; 2074 Asoj, 5 marks; 2082 Chaitra (new course), 4 marks (asked 3 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/ageing-silicon-merits
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What is ageing of core of electrical machine? What are the advantages and disadvantages of adding silicon with iron?

## Short answer

Ageing is the gradual rise in core hysteresis loss due to carbon precipitation, avoided by adding silicon. Silicon increases resistivity (reduces eddy loss), eliminates ageing, and raises permeability. Disadvantages: brittleness above ~4% Si, lower saturation flux density, reduced thermal conductivity.

## Answer

**Ageing of core** is the gradual increase in the hysteresis loss (and the fall in permeability) of the iron core over time, when the machine runs for a long period at its normal working temperature.

- Its cause is impurities, mainly **carbon** (also nitrogen and oxygen), dissolved in the iron. At working temperatures (about 60–100 °C) these slowly come out of solution as fine particles of iron carbide/nitride.
- The particles pin the domain walls. This widens the hysteresis loop, so the core loss can rise by up to 50–100 % over a few years. Efficiency falls and temperature rise increases.

Ageing is avoided by using low-carbon steel, by alloying with silicon, and by annealing the laminations.

#### Advantages and disadvantages of adding silicon to iron
| Advantages of adding silicon | Disadvantages of adding silicon |
|---|---|
| Resistivity rises (about 0.1 → 0.6 µΩ·m at 4 % Si), so **eddy current loss** falls | Steel becomes **hard and brittle** above about 4–4.5 % Si; punching and rolling are difficult |
| **Hysteresis loss** is reduced | Tools (dies, punches) wear quickly |
| **Ageing** is practically eliminated | **Saturation flux density** falls, so working $B_m$ is lower |
| Permeability at low and medium flux densities increases | Thermal conductivity falls |
| Magnetostriction is reduced, so hum is lower | Steel costs more |

Because of these opposing effects, silicon content is a compromise:
- **Transformers:** about 4–4.5 % Si (or about 3 % in CRGO). Laminations are simple strips, so brittleness matters less, and low loss matters most.
- **Rotating machines:** about 0.5–3.5 % Si (dynamo grade). Teeth and slots must be punched, the teeth must carry high flux density, and the laminations need mechanical strength.

## Check yourself

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

1. What is ageing of a core?
   A. Gradual rise in hysteresis loss and fall in permeability over time at working temperature
   B. Sudden loss of insulation
   C. Fall of resistivity with age
   D. Increase in lamination thickness
   Answer: A. Ageing is the slow rise in hysteresis loss as the machine runs warm for a long time. (Key point: Definition of ageing)

2. What mainly causes ageing of an iron core?
   A. Mechanical vibration loosening the laminations
   B. Carbon (and N, O) impurities precipitating as carbide/nitride particles at working temperature
   C. Rusting of lamination surfaces
   D. Rise of resistivity with temperature
   Answer: B. Dissolved carbon slowly comes out of solution as fine particles at about 60-100 °C. (Key point: Cause of ageing)

3. 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)

4. 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)

## Related in Electric Machine Design

- [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
- [Advantages and disadvantages of adding silicon](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asilicon-iron-advantages) — Electric Machine Design, chapter 1, asked 3×
- [Ageing of core and its minimization](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aageing-core-minimization) — Electric Machine Design, chapter 1, asked 2×
- [Silicon content: transformer versus rotating machine steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asilicon-content-transformer-rotating) — Electric Machine Design, chapter 1
- [Temperature gradient in an iron core](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atemperature-gradient-iron-core) — Electric Machine Design, chapter 2
- [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×

## Related in other subjects

- [Iron core current for 1 T air-gap flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Airon-core-air-gap-1-tesla) — Electrical Machine, chapter 1, asked 2×
- [Number of turns of iron-core inductor](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Ainductor-turns-iron-core-500mh) — Basic Electrical Engineering, chapter 4
- [Magnetization characteristic of iron core with AC excitation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Amagnetization-characteristic-ac) — Electrical Machines I, chapter 1
- [No-load current components, iron loss and core flux](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ano-load-current-10a-0p3pf-500turns) — Electrical Machines I, chapter 2