Vertical 300hp Well Pump Motor
- Thread starter adamscb
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kwired
Electron manager
- Location
- NE Nebraska
If it would be a positive displacement pump it could be constant torque, centrifugal pump would be variable torque.
Most likely variable. All the ones I've done have been. But note what kwired posted.
300hp VFD
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
Ingenieur
is the pumping rate constant? head and gpm?
is the drive modulated or running at a constant speed once started?
is the drive modulated or running at a constant speed once started?
ptonsparky
Tom
- Occupation
- EC - retired
If it is a centrifugal, as some of suspect, wouldn’t it still be a VT application steady state speed or not?
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
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:
The question was CT or VT. That has been answered.
Spare the irrelevant clutter. OK?
smoothops10
Member
- Location
- FL
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- EE
You should coordinate this with pump motor mfr and drive mfr. A lot of special requirements for deep submersibles with VFD. Notably ramp up/down times for porpoer cooling and OL protection.
femyers.com/~/media/websites/myers/sellsheets/48150.pdf
femyers.com/~/media/websites/myers/sellsheets/48150.pdf
Our service is 69kV. We're using a PID loop to constantly vary the speed under changing head pressure conditions.
Yes, but that would probably be of more significance for the motor.
A little tale. And something the OP ought to be aware of. I was called to a site where the motors on submersible pumps with VFDs were failing at the rate of about one a month. To keep it brief, the cause was excessive dv/dt. Rate of change of voltage. Over 3000V/us in some cases. A killer if not taken care of at the project design phase.
Output chokes was the fix. Not all VSD come with them.
I've taken note and installed both line and load reactors. I guess my question then is what control method would be best for this application, since we're varying speed based on plant header PSI, V/Hz or open loop? Since it's varying torque you'd think open loop right? From my understanding V/Hz is for constant torque applications.
Constant v/f is the way I would go. Reducing the v/f ought to reduce losses. I tried that on some 630kW pump drives. Test results were inconclusive and introduced other problems so we reverted to constant v/f open loop.
Speed control was from an external 4-20mA loop. We also did the PLC which generated that. But that's another story for another day.
v/f since you do not need low f high torque capability
since you have closed loop PID control the drift will not be an issue
we use open loop vector for tensioning winch applications
when you need max torque at <0.3 Hz
the motor is essentially stationary ~ +/- a few tenths Hz
One thing I do want to point out, is that the PID loop isn't set up inside the VFD, but rather in our PLC.
Yes. And that's how we usually arranged it - a number of drives and one common controller.
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