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

# Ageing, its minimization and silicon addition

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

- Asked in: 2071 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/ageing-minimize-silicon-merits
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What is ageing? How it can be minimized? Enlist the merits and demerits of addition of silicon with iron.

## Short answer

Ageing is the gradual increase in core loss from carbon precipitation. Minimization uses decarburised steel, silicon addition, annealing, and pre-ageing. Silicon merits: higher resistivity, lower eddy and hysteresis losses, no ageing. Demerits: brittleness, lower saturation flux density, reduced thermal conductivity, higher cost.

## 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.

#### Minimising ageing
1. Use steel with very low carbon content (decarburised steel).
2. Add **silicon**, which makes the steel practically non-ageing.
3. Anneal the laminations after punching, in a decarburising atmosphere.
4. Artificially age (heat-treat) the steel before use, and keep working temperatures low.

#### Merits and demerits of adding silicon
| Merits | Demerits |
|---|---|
| Higher resistivity, so less eddy loss | Brittle above about 4–4.5 % |
| Less hysteresis loss | Difficult to punch; tools wear faster |
| Ageing eliminated | Lower saturation flux density |
| Higher permeability, less magnetostriction | Lower thermal conductivity; higher cost |

Transformer steel has about 4–4.5 % Si. Rotating-machine steel has 0.5–3.5 % Si.

## Check yourself

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

1. How do the precipitated particles raise the core loss?
   A. They pin domain walls, widening the hysteresis loop
   B. They lower the resistivity and raise eddy loss only
   C. They increase permeability
   D. They shrink the lamination thickness
   Answer: A. Pinned domain walls widen the B-H loop, so hysteresis loss grows, up to 50-100 % over years. (Key point: Effect on loop)

2. Which is a way to minimise ageing?
   A. Use high-carbon steel
   B. Add silicon and anneal laminations in decarburising atmosphere
   C. Operate the core hot
   D. Remove insulation
   Answer: B. Silicon, low carbon, annealing and pre-ageing all reduce ageing. (Key point: Minimising 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)

## 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×
- [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×
- [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×

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