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Ageing, its minimization and silicon addition

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.

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

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

MeritsDemerits
Higher resistivity, so less eddy lossBrittle above about 4–4.5 %
Less hysteresis lossDifficult to punch; tools wear faster
Ageing eliminatedLower saturation flux density
Higher permeability, less magnetostrictionLower thermal conductivity; higher cost

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

✓ Answer checked by Claude Sonnet 5.5

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