I think a 50% de-rate is too much, more like 20-30% if any.
WEG make some that are non-derate-able, just a bit higher cost.
I have several 2HP VFD's running 2hp motors with no problem.
With a higher end model you can probably get away with a 5HP, which is usually the highest 1ph input model for most makes.
Sounds like you are just intending to run at 60hz?
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.