What is ageing of a core?

Gradual rise in hysteresis loss and fall in permeability over time at working temperature

Increase in lamination thickness

What mainly causes ageing of an iron core?

Carbon (and N, O) impurities precipitating as carbide/nitride particles at working temperature

Mechanical vibration loosening the laminations

Rise of resistivity with temperature

Which is an advantage of adding silicon to iron?

Higher resistivity, so lower eddy current loss

Which is a disadvantage of adding silicon above about 4-4.5 %?

The steel becomes hard and brittle, making punching difficult and wearing tools

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2079 Bhadra · 5 marks
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Ageing of core and silicon addition

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.

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

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.

✓ Answer checked by Claude Sonnet 5.5

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