Insulating materials: classes, requirements and properties

An insulating material is a substance of very high resistivity that stops current flowing between parts at different potentials. In machines it insulates turns, coils and phases from each other and from the core and frame.

Classification

(a) By state

• Solid: mica, paper, pressboard, cotton, glass fibre, asbestos, porcelain, rubber, PVC, polyester film, epoxy and varnishes.
• Liquid: mineral transformer oil, silicone fluid, synthetic esters.
• Gas: air, nitrogen, hydrogen, SF₆.

(b) By origin

• Organic (paper, cotton, resins, plastics): flexible, but suited to low temperature.
• Inorganic (mica, glass, porcelain, ceramics): suited to high temperature, but brittle.

(c) By thermal class (IS 1271 / IEC 60085)

Fundamental requirements of a good insulating material

1. High dielectric strength and high resistivity.
2. Low dielectric loss and low power factor.
3. Able to withstand high temperature without deterioration; non-inflammable.
4. Good thermal conductivity and low thermal expansion.
5. Good mechanical strength, flexibility and resistance to vibration.
6. Non-hygroscopic; resistant to oil, chemicals and ozone.
7. Stable with age; easy to work; cheap.

Electrical properties of insulating materials

1. Insulation resistance / resistivity. Volume resistivity (through the body) and surface resistivity (along the surface) should be very high. Both fall with moisture and temperature.
2. Dielectric strength. The maximum voltage gradient (kV/mm) the material withstands before breakdown. It depends on thickness, temperature, moisture and the duration of voltage.
3. Permittivity (dielectric constant $\varepsilon_r$). In composite insulation the stress divides inversely as $\varepsilon_r$, so materials with similar $\varepsilon_r$ are preferred. A low $\varepsilon_r$ is preferred for cables.
4. Dielectric loss and loss angle ($\tan\delta$). The power lost in the dielectric is $P = \omega C V^2 \tan\delta$. A low $\tan\delta$ is needed to avoid heating and thermal breakdown.
5. Surface tracking and arc resistance. The ability to resist forming a conducting carbon track along the surface under discharge and contamination.
6. Partial discharge resistance, which matters in high-voltage machine insulation (mica is excellent here).