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Insulating material requirements and silicon steel

Short answer

Good insulating materials require high resistivity, dielectric strength, thermal stability, mechanical strength, low moisture absorption, and chemical resistance. Adding silicon to iron increases resistivity and reduces hysteresis and ageing, but makes the steel brittle, lowers saturation flux density, and increases cost.

What are the fundamental requirements of a good insulating materials? List out the merits and demerits of addition of silicon with iron with suitable diagram.

An insulating material has very high resistivity. It separates conductors from each other and from earthed parts, and must remain reliable under the electrical, thermal, mechanical and chemical stresses inside a machine.

Fundamental requirements of a good insulating material

Electrical

  1. High insulation resistance (resistivity) and high dielectric strength.
  2. Low dielectric loss (low tan⁡δ\tan\delta) and suitable permittivity.

Thermal 3. Able to work continuously at high temperature (a high thermal class) without losing its properties. 4. Good thermal conductivity, to pass heat to the iron, and low thermal expansion. 5. Non-inflammable.

Mechanical 6. Good mechanical strength, flexibility and resistance to vibration and abrasion.

Chemical and others 7. Non-hygroscopic (does not absorb moisture), and resistant to oil, acids, alkalis and ozone. 8. Does not deteriorate with age, easy to shape and apply, and cheap.

Merits and demerits of adding silicon to iron

Advantages of adding siliconDisadvantages of adding silicon
Resistivity rises (about 0.1 → 0.6 µΩ·m at 4 % Si), so eddy current loss fallsSteel becomes hard and brittle above about 4–4.5 % Si; punching and rolling are difficult
Hysteresis loss is reducedTools (dies, punches) wear quickly
Ageing is practically eliminatedSaturation flux density falls, so working BmB_m is lower
Permeability at low and medium flux densities increasesThermal conductivity falls
Magnetostriction is reduced, so hum is lowerSteel costs more
 loss,          brittleness
 rho  ^ resistivity     /
      |      .-------  /
      |   .-'         /
      | .'  core loss/
      |\_           /
      |   ~~--.__  /
      +-----------+----> % Si
      0   1   2   3   4   5
     usual: 0.5-3.5 % (motors)
            4-4.5 % (transformers)

The figure shows that resistivity rises and core loss falls as silicon increases. Brittleness, however, rises sharply beyond about 4 %. Practical limits are therefore about 4–4.5 % for transformer steel and 0.5–3.5 % for rotating-machine steel.

✓ Answer checked by Claude Sonnet 5.5

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