Motor Insulation Life Calculator (Class F)

Exponentially: each 10 K above the 155 °C reference halves expected life, and each 10 K below roughly doubles it (the Montsinger 10-degree rule used in IEC 60085 thermal classification — Class F insulation is rated 155 °C). A sustained 20 K overshoot quarters the life — which is why a 'small' cooling problem is never small over the years.

Measure the rise: every 10 K of extra winding temperature halves insulation life. A motor whose fins are caked enough to run 20 K hot ages 4× faster — a 15-year winding becomes a 4-year winding while still 'working fine'. An IR gun on the frame (add the frame-to-winding offset, roughly 10–20 K for TEFC) at every PM, logged against ambient, catches it for free.

It is the engineering linearisation of the Arrhenius equation over the normal operating window, adopted by the relevant standards because it matches test data well within ±20–30 °C of the reference. Far outside that window (or where a different failure mechanism takes over, e.g. mechanical wear) the rule loses authority — treat extreme extrapolations as indicative only.

The hot-spot (the hottest point of the insulation/material), because chemistry happens at the hottest spot. If you can only measure surface or ambient, add the typical rise for your equipment class — datasheets usually state hot-spot rise over ambient at rated load — and remember intermittent peaks age the asset during the peak hours, not the average.