On 2009-04-05, John wrote:

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

[ ... ]

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.

[ ... ]

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.

Except that you would need a 480V input power and a 480V VFD to
drive a 240 V motor at double RPM with full torque. A VFD being run
from 240 V just does not have the extra voltage to add as you increase
the frequency beyond the standard 60 Hz. That is why you get constant
torque from nameplate RPM on down -- because the VFD *can* reduce the
voltage to maintain the same current. But it *can't* increase the
voltage as you go above 60 Hz. It just isn't there to work with.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---