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

# Removal of ageing and its effects

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

- Asked in: 2071 Shrawan, 5 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/ageing-removal-effects
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What is ageing? How was it removed? What were the advantages and disadvantages after removal of ageing problem?

## Short answer

Ageing is the time-dependent increase in hysteresis loss from carbon precipitation. Removed by adding silicon and using decarburised annealed steel. Advantages: constant low loss, higher resistivity, no ageing, steady efficiency. Disadvantages: brittleness, hard punching, 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.

#### How ageing was removed
The ageing problem was removed by:
1. Adding **silicon** (about 0.5–4.5 %) to iron. Silicon steel is non-ageing because silicon helps remove carbon and oxygen and stops carbide precipitation.
2. Making very low-carbon steel by **decarburising** and **annealing** the laminations in wet hydrogen after cutting.
3. Pre-ageing (artificial ageing) the steel by heat treatment before use.

#### Advantages after removing ageing (silicon steel)
- Core loss stays constant over the life of the machine, so efficiency does not fall.
- Resistivity is higher, so eddy loss is lower; hysteresis loss is also lower.
- Permeability is higher and magnetising current lower.
- Temperature rise stays steady, so insulation life is longer.
- Magnetostriction and noise are reduced.

#### Disadvantages
- Silicon steel is **brittle** and hard, so punching slots and teeth is difficult and tools wear quickly.
- **Saturation flux density** is lower, so the core must work at a lower $B_m$ (a larger core).
- Thermal conductivity is lower.
- The material and its processing (annealing, decarburising) cost more.

For this reason, transformers use high silicon (4–4.5 %) and rotating machines use lower silicon (0.5–3.5 %).

## Check yourself

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

1. Why does adding silicon practically eliminate ageing?
   A. It makes the steel harder
   B. It helps remove carbon and oxygen and stops carbide precipitation
   C. It lowers the working temperature to below 60 °C
   D. It raises saturation flux density
   Answer: B. Silicon reduces carbon solubility and removes carbon/oxygen, so no carbide particles form. (Key point: Silicon cures ageing)

2. What is an advantage after ageing is removed?
   A. Core loss stays constant over life, so efficiency does not fall
   B. Higher saturation flux density
   C. Easier punching
   D. Lower cost
   Answer: A. Without ageing the loss does not rise, so temperature rise and insulation life stay steady. (Key point: Constant loss)

3. 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)

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