Ageing of core and its minimization

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.