Silicon content: transformer versus rotating machine steel

Adding silicon lowers core loss, but it also makes steel brittle and reduces its saturation flux density. The best silicon content therefore depends on whether low loss or mechanical and magnetic strength matters more. Transformers use about 4–4.5 % Si; rotating machines use about 0.5–3.5 % Si.

Why transformers use higher silicon content

1. The core is energised all the time (24 hours for distribution transformers), so iron loss decides all-day efficiency. High silicon raises resistivity (about 0.6 µΩ·m), which cuts eddy loss, and also lowers hysteresis loss.
2. Simple shape. Transformer laminations are plain rectangular strips that are sheared, not punched with teeth and slots. The brittleness of high-silicon steel does little harm.
3. The core is static, with no centrifugal or vibration forces, so high mechanical strength is not needed.
4. Moderate flux density (1.1–1.7 T) is used in the core, so the lower saturation of high-silicon steel is acceptable.

Why rotating machines use lower silicon content

1. Punching: stator and rotor laminations have many narrow teeth and slots punched with dies. High-silicon steel is brittle, cracks at the teeth and wears the dies quickly.
2. High flux density in teeth: teeth carry about 1.6–2.1 T. Low-silicon steel has a higher saturation flux density and needs fewer ampere-turns.
3. Mechanical strength: rotor laminations must withstand centrifugal forces, vibration and torque, so tougher steel is needed.
4. Air gap dominates the magnetising current in a rotating machine. The small loss benefit of high silicon is less important than in a transformer.
5. Low frequency in the rotor (slip frequency in induction motors) means rotor iron loss is small anyway.