RoyJ wrote:
The usual rule of thumb for VFD's is that you have rated torque up to
the nameplate RPM (ie less than your 5 hp), and rated HP above nameplate
RPM (this is a function of the amount of power the motor can handle
without overheating. )

That's what I've heard (on RCM), but I don't recall seeing it in my VFD
documentation. And I've never understood it. Keeping the HP constant
above the rated RPM means that the torque/current is being
proportionally reduced. Why would you want to reduce the current above
the rated speed? It can't be to avoid overheating - that's just a
matter of current, not power. In fact, higher speeds would generally
mean more air flow & more cooling, allowing *more* current. Maybe it's
a matter of the VFD's limitations.

Your application would require running the motor at 180 hz and 3 times
the rated rpm. Pumps require pretty much full output for extended
periods of time. I doubt if the motor would take it for any reasonable
period. You could might get a test run in without too much grief.

Others have said "Check with the manufacturer" - I'll tell what they'll
say: "Don't do it, it's not rated for it". But you won't know if
they're just covering themselves or if they mean it. If you ask "Why",
you won't know whether you've gotten a straight answer to that, either.

I can see 2 effects of higher that rated RPM: centrifugal forces on the
rotor windings (do 3 ph motors have rotor windings?) and bearing
overheating. Now, many motors have a rated speed of 3600 rpm & I doubt
that there's much difference between their designs (rotor & bearings)
and 1200 rpm motors. I doubt that there would be enough cost savings to
justify different "technologies".

So, you gots to ax yourself "Do I feel lucky?". IOW, are you willing to
risk that motor to find out? I hope you do, 'cause I'd like to know.
So be sure to follow up if you do.

Awaiting your report,
Bob