Good insulating material requirements and applications

An insulating material separates current-carrying parts from each other and from the iron and frame. In a machine it must also stand heat, vibration, moisture and oil for its whole life.

Fundamental requirements of a good insulating material

1. High resistivity and high dielectric strength, to withstand normal and surge voltages.
2. Low dielectric loss ($\tan\delta$) and stable permittivity.
3. High thermal stability: it must work at the temperature of its class (A, E, B, F, H) without charring or becoming brittle.
4. Good thermal conductivity, so the heat of the copper can pass through it.
5. Mechanical strength and flexibility, to withstand winding forces, vibration and short-circuit forces.
6. Non-hygroscopic, and resistant to oil, chemicals and ozone.
7. Not inflammable, with long life and low cost.

Application of insulating materials in machines

In short, solid insulation holds and separates the conductors, liquid insulation (oil) both insulates and cools, and gases such as air and hydrogen serve as insulation and coolant.