On 2009-04-04, Karl Townsend wrote:
Kind of a follow up to my pump RPM question.

I found a 1200 RPM 5 hp motor and an old Hitachi SJ200 5hp. vfd in the spare
stockroom (er junkpile)

Any reason I can't spin this combo at 3600 RPM? Still have 5 horse?

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.

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