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

# Ageing of core and its minimization

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

- Asked in: 2082 Baishakh, 4 marks; 2076 Asoj, 5 marks (asked 2 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/ageing-core-minimization
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What is ageing of core of electrical machine? How it can be minimized?

## Short answer

Ageing is the gradual increase in hysteresis loss and permeability drop over time due to precipitation of impurities like carbon. It can be minimized by using low-carbon decarburised steel, adding silicon (making silicon steel non-ageing), annealing laminations in decarburising atmosphere, and artificial pre-ageing.

## 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.

#### Effects
- Iron loss increases over time, so efficiency falls.
- Temperature rise increases and insulation life is shortened.
- No-load (magnetising) current rises.

#### How ageing is minimised
1. **Use low-carbon steel.** Keep carbon below about 0.005 % by decarburising during manufacture, so there is little to precipitate.
2. **Add silicon** (about 0.5–4.5 %). Silicon reduces the solubility of carbon and helps remove it, so silicon steel is practically **non-ageing**. This is the main remedy.
3. **Anneal** the laminations after punching or cutting, in a decarburising atmosphere (wet hydrogen). This removes residual carbon and relieves stresses.
4. **Artificial ageing (stabilising):** heat the steel to about 100–150 °C before use, so any precipitation takes place before the core goes into service.
5. **Keep the working temperature low** through good cooling and ventilation, because precipitation speeds up with temperature.
6. Use modern **CRGO** or other silicon steels, which do not age.

## Check yourself

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

1. 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)

2. How does low-carbon steel help against ageing?
   A. It adds silicon
   B. Keeping carbon below about 0.005 % leaves little to precipitate
   C. It raises operating temperature
   D. It thickens laminations
   Answer: B. Decarburised steel has little carbon to form carbide particles. (Key point: Low carbon)

3. Which pair of measures also reduces ageing?
   A. Pre-heating steel to 100-150 °C before use and keeping working temperature low
   B. Overloading the core and removing cooling
   C. Adding carbon and annealing
   D. Using thicker laminations
   Answer: A. Artificial ageing brings precipitation forward before service; low temperature slows it. (Key point: Artificial ageing and cooling)

## 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
- [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×
- [Removal of ageing and its effects](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aageing-removal-effects) — Electric Machine Design, chapter 1
- [Six-stepped core area and turns (1000 kVA)](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asix-stepped-core-1000kva) — Electric Machine Design, chapter 3

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