VFD/Motor speed reduction
VFD/Motor speed reduction
(OP)
Hello all! We have a situation where a 200HP, 480v motor controlled by VFD needs to have it's max speed limited to 54Hz. This is a fan motor (variable HP and Torque), so as I understand, if speed is reduced to 90%, the HP is reduced to (90%*90%*90%) = 72.9%, and Torque to 81%. But this is what the load (fan) will demand. The question is, would the motor have any problem meeting this demand? Would I have to look at motor curves to determine that? Thanks for the help!





RE: VFD/Motor speed reduction
RE: VFD/Motor speed reduction
The demand upon the motor by the LOAD will drop by the cube of the speed change in a centrifugal machine, so the FAN will demand 73% less HP from the motor and therefore the motor now has even MORE capacity than the fan needs at that speed.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: VFD/Motor speed reduction
RE: VFD/Motor speed reduction
Just for my learning...(considering you answered to OP already)
90% speed = 90 % capability (guess is meant for power?) assumes there is linear ratio; so is this because it is assumed scalar control which has V/F = constant ?
If correct, then what about VFD where vector control can be applied, I am guessing right to say that this would allow to change the ratio, thereby changing the picture of 90% speed = 90 % capability ?
Thanks
RE: VFD/Motor speed reduction
RE: VFD/Motor speed reduction
RE: VFD/Motor speed reduction
Oh, if only it was so simple.
That is reasonably true of the in-phase component of the line current, assuming losses are low, but totally neglects the quadrature magnetising current which can be 1/2 - 2/3 of the line current in some motors.