VFD's are a complicated subject. Many lower power ( 2HP or less) are designed
to run off single phase. If so they may give full rated power even running
from single phase, check the specs. I have some 10 HP VFD's that are single
phase capable but must be de-rated to 5 HP (it's in the spec's).
In your case with a sander you have a better situation. A VFD running a pump
may run at full power all of the time so power limits should be checked carefully.
Your sander probably uses a fraction of the full power most of the time. Starting
is probably very easy for a sander, no heavy load. This is an easy job for a VFD.
VFD's have circuitry watching the load current all of the time. If you cause an
overload it will shut down without harm. You could probably use a 1 HP VFD with
no problems. When you use your sander and exceed the 1 HP limit it will just
shut down, no harm done.
So, you could probably buy a 2 HP unit and not worry about de-rating it. The
2/3 number comes from a simple calculation relating to rotary converters and does
not necessarily apply to VFD's (check your spec's).
I run 2 HP machines off 2 HP VFD's without problems. They are mills, lathes,
and surface grinders. All machines that don't use their full power unless
I do really heavy work. When I exceed their limit they shut down, no problem.
They really are an ideal solution. Just remember, no switches or fuses between
the motor and the VFD.
Mostly, check the spec's.