John,
Excellent description. You are correct, but I know of no VFD unit that
increases voltage with frequency. The effect is that current drops from
rated with frequency above rated frequency, but speed increases limiting
somewhat the drop in net power. Overheating of the motor occurs at frequency
below the motor rating without reducing the applied voltage. Again, I know
of no VFD that reduces voltage either.
Steve

"John" wrote in message
...

"Karl Townsend" wrote in message
anews.com...


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.

Ya, I'm not going to bother.




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.


I've run a 1800 RPM 7 1/2 HP at 3600 for 100s of hours. (CHNC spindle).
As no one seems to know, I'm just going to guess that it is better to
find an 1800 RPM motor - pretty sure that will work. These are fairly
easy to find.

Karl




As the frequency applied to the motor increases, the internal reactance of
the motor increases so then you need a higher voltage to push the same max
current through the windings as limited by the wire size. Now if you
maintain the current by increasing the voltage you get a lot more power
out of the motor until you reach a frequency where the internal magnetic
loses of the motor generate too much heat. The other limiting factors are
the size of the bearings and the output shaft. They are limited as the HP
they can handle. Theroretically, you can double the frequency double the
voltage and get twice the HP out of the motor.



John