Temperature and life of insulating materials

Temperature is the most important factor deciding the life of insulating materials, and so the life of the machine. A machine's rating is in fact limited by the temperature its insulation can safely withstand.

Why temperature matters

• At high temperature the organic materials in insulation (paper, cotton, varnish, resins) undergo slow chemical changes: oxidation, loss of volatile matter and depolymerisation.
• The insulation becomes dry, brittle and cracked. Its mechanical strength falls first; the vibration and electromagnetic forces of normal service then crack it, and its dielectric strength is lost. Breakdown follows.
• The rate of these chemical reactions follows the Arrhenius law, so it rises rapidly with temperature:

$$L = A\, e^{B/T}$$

where $L$ is life, $T$ is absolute temperature, and $A$, $B$ are constants of the material.

Montsinger's rule (10-degree rule)

A practical form of the Arrhenius law: the life of insulation is roughly halved for every 8–10 °C rise in operating temperature above its rated limit.

$$L = L_0 \times 2^{-(\theta-\theta_0)/10}$$

For example, class A insulation running at 115 °C instead of 105 °C would last only about half as long.

Consequences for design

• Insulation is grouped into thermal classes (Y 90, A 105, E 120, B 130, F 155, H 180, C >180 °C). Each class's limit gives an economic life of about 20–25 years.
• The design must keep the hot-spot temperature (not the average) below the class limit. Permissible temperature rise = class limit − ambient (40 °C) − hot-spot margin.
• Short overloads are allowed only while the temperature stays within limits. Thermal time constants decide how long these overloads can last.