Removal of ageing and its effects

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 %).