Motor Insulation Life Calculator (Class H)

Exponentially: each 10 K above the 180 °C reference halves expected life, and each 10 K below roughly doubles it (the Montsinger 10-degree rule per IEC 60085 — Class H insulation is rated 180 °C). A sustained 20 K overshoot quarters the life — which is why a 'small' cooling problem is never small over the years.

PWM harmonics typically add 5–15 K depending on switching frequency, cable length and whether the motor is inverter-rated (better slot insulation, sometimes bigger fans). By the 10-degree rule the midpoint (~10 K) costs half the insulation life — which is exactly why 'inverter-duty' motors with Class H insulation and Class F rise exist. If you retrofitted a VFD onto a standard motor, enter your measured winding temperature here and see what the retrofit is really costing.

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.