All,
I'm working on installing a 300hp VFD for one of our vertical Well Pump motors...is this a constant torque or variable torque application? Please advise, thanks
All,
I'm working on installing a 300hp VFD for one of our vertical Well Pump motors...is this a constant torque or variable torque application? Please advise, thanks
is the pumping rate constant? head and gpm?
is the drive modulated or running at a constant speed once started?
we used vfd's, to limit inrush current, but the sewage pumps always ran at full speed
they were not much more expensive than a SS, but offered more flexibility
our utility REQUIRED anything over 25 HP to have a vfd (or SS), even for constant speed application
best to include the whole post rather than selectively edit and highlight to demean someone
Ingenieuris the pumping rate constant? head and gpm?
is the drive modulated or running at a constant speed once started?
If it is a centrifugal, as some of suspect, wouldnâ€™t it still be a VT application steady state speed or not?
Tom
TBLO
We are planning on varying the frequency on this drive, in the past we started it across-the-line and ran it at a full 60 Hz all the time. I'm in an industrial plant, if we used a VFD for everything above 25hp we'd be broke haha. I believe we're trying to change the pumping rate with this VFD.
constant torque at a fixed speed
variable if speed varies, I guess true for both centrifugal and PD pumps
T = Torque
P = Power
n = speed
subscript 1 and 2 denote speed change
affinity laws P1/P2 = (n1/n2)^3
T1 = P1 5252/n1 and T2 = P2 5252/n2 or 5252 = T2/P2 n2
substitute
T1 = P1/n1 T2/P2 n2 or T1/T2 = P1/P2 n2/n1
substitute
T1/T2 = (n1/n2)^3 n2/n1 = (n1/n2)^2
so torque changes with the square of the speed
if speed does not change torque is constant
let n1 = 1200, n2 = 1800, T1 = 100
T2 = T1 (n2/n1)^2 = 100 (1800/1200)^ = 225
the requirement applied if you took service at 480 vac or less (which we always did for remote pumping sites)
you probably take it much higher
you app is variable torque then
but honestly, iffy, say if you change the speed once a year, then fixed for long periods, versus a controller constant varying speed under changing head conditions
either way, safe with variable torque
Last edited by Ingenieur; 12-18-17 at 01:21 PM.
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