Using a significantly smaller VFD is not likely to work, since VFDs have circuitry to monitor the feedback from the motor, in terms of current absorbed, phase shift, etc. A big motor has characteristics significantly different from a smaller one and, most likely, the VFD would interpret the feedback from a larger motor as a fault situation.
Paolo
That is certainly possible. However, the typical VFD is designed as a general
purpose drive. As such it does not know a lot about the motor that it is driving.
It cannot make assumptions about the electrical characteristics of the motor.
A general purpose VFD has to be able to drive all kinds of motors: a motor
that matches the HP rating, a motor much smaller than the HP rating, a modern
motor, and an antique motor. All of these examples will have very different
characteristics. A general purpose VFD motor has to drive them all.
The case where the VFD is driving a motor larger than it's rated HP will
probably not be that different. All that the VFD needs to do to protect
itself is to insure that the current flow does not exceed the rating of
it's electronics. It really does not need to worry about the specified
HP of the motor, only the actual HP load at the moment.
That said, you may be right, it may not work. VFD's are not that expensive
these days and getting cheaper as time goes on. If it were me I would
get a 5 HP VFD and give it a try. If it does not work the VFD could be
used elsewhere.