VFD and motor RPM

You need to call the motor manufacturer and ask them. I'm pretty sure the answer will be no. I think 20% over nameplate would be fine, but 100% is probably pushing it too far.

More likely have more than 5hp at over double the speed. Not 10, but mabee 7?

It will definitely be less than 5 HP due to losses at higher frequencies. Call the motor mfr to find out max safe speed. No need to speculate or outguess them. Yu will know the answer on Monday.

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. )

Your applicati> Kind of a follow up to my pump RPM question.

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.

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

Well ... a lot of VFDs will only go to double the line frequency (120 Hz from 60 Hz) so you could go no faster than 2400 RPM on the motor. Some will go to 400 Hz -- if you tweak some limiting variables in the setup parameters. You would need to go to 180 hz to get 3600 RPM.

Your torque will fall above the design frequency of the motor, as the VFD can't increase the output voltage above the input voltage level, and the required voltage is a function of RPM. As a result, you probably won't get 5 HP out of the motor at that speed.

Since it is likely that the same motor series is made with the same rotor for 3600 RPM and slower -- with only the field structure (poles) changing, I consider it likely (though not *certain*) that the rotor can survive 3600 RPM -- but no faster. Certainly don't stand in line with the possible parts of an exploding rotor for at least the first half hour of operation.

Enjoy, DoN.