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Engineers read a high class as a badge of durability. It is not. The class describes how long the insulation survives at a given hot-spot temperature, not how hot the winding will run in your enclosure, at your duty cycle, in July.
Two machines can carry the same class and live entirely different lives. One is designed with a wide gap between rated temperature rise and what the insulation can take. The other sits right against that limit because the frame was too small. Same label, different decade.
The judgment you can apply this week: stop reading case temperature as winding temperature. On a typical enclosed frame the copper runs 15 to 30 K hotter than the housing. A case at 70 C in a hot plant means the winding is near 100 C, hot spot above that. That gap is the margin you give away when you add load or slow a fan.
Insulation rarely fails loudly. It fails as brittle varnish and one shorted turn, months later, on a shift that had nothing to do with the decision. Choosing a class is easy. Spending that headroom deliberately is the engineering part.
Two machines can carry the same class and live entirely different lives. One is designed with a wide gap between rated temperature rise and what the insulation can take. The other sits right against that limit because the frame was too small. Same label, different decade.
The judgment you can apply this week: stop reading case temperature as winding temperature. On a typical enclosed frame the copper runs 15 to 30 K hotter than the housing. A case at 70 C in a hot plant means the winding is near 100 C, hot spot above that. That gap is the margin you give away when you add load or slow a fan.
Insulation rarely fails loudly. It fails as brittle varnish and one shorted turn, months later, on a shift that had nothing to do with the decision. Choosing a class is easy. Spending that headroom deliberately is the engineering part.
