We learned this the hard way this year.
We tried to use a FP to run a centrifigal fan out of a dewalt vaccuum. It functioned o.k. 1:1 but didn't give us the speed we needed. We geared it up 1:2 and after running 30 seconds wide opened the Fish smoked a bit and thermal-tripped. We thought we had a bad motor since there wasn't that much load on it (it was spinning at a REALLY good clip) so we wired up another and prompty toasted that.
That's how we learned that you don't run an FP at 100% of rated power if you can help it - even if it is not stalled and is still turning pretty darn fast.