Vertical 300hp Well Pump Motor
- Thread starter adamscb
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Might as well. V/Hz control is just the basics of what a VFD can do. From there you get into various forms of "Vector Control" that improve torque control and response to changes in loads, but on a centrifugal pump that does little to nothing to enhance your experience. The down side however is that for Vector Control to function, it must be "tuned" to the motor. That's not too difficult either now days, but if you ever someday have to replace the motor, that means the drive must be tuned again to the new motor. If there is nobody available who knows how to do that when it happens (because Murphy's Law says it will happen at 3:00AM on Christmas Eve), then your pump is down until that person is available. Using V/Hz means that issue is not there, so any time the motor is swapped, the drive doesn't care. I always recommend using V/Hz on centrifugal loads for that reason alone.
Just for the record, not ALL pumps that are not positive displacement are necessarily centrifugal. Axial flow (propeller) and "mixed flow" pumps started into a filled system can display a mixture of CT and VT characteristics depending on the speed and head conditions, so the drives should often be sized as CT to be safe. But yours is a vertical turbine pump, so VT sizing should be fine.
doesn't matter much
2 basic equations, learn these
pump HP = (gpm x 8.34 x ft head)/(60 x 550 x pump eff)
= (gpm x ft hd)/(3957 x eff)
T = HP x 5252/rpm
8.34 lb per gal
60 sec/min
550 ft-lb/sec
5252 = 60 x 550/(2Pi)
typ eff 55-65%
positive displacement
flow proportional to speed x displacement ie, constant for a fixed speed
flow ~ constant with head, pumps the same at low or high head
what changes is torque, ie, variable, more force to lift the same mass higher
centrifugal
flow increases as head decreases
the curve is raised/lowered based on speed
if head decreases, flow increases, HP/torque increase
variable torque
the result is similar for both
by varying speed you are 'sliding' the pump curve up/down the system head curve (static + friction)
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Since this was my first foray into VFD's larger than 30hp, I went ahead and sized it for a Heavy Duty rating of 300 hp, Normal Duty rating of 350 hp, just to err on the side of caution.
Is it generally safe to say that if the load is constant torque, then size VFD according to Heavy Duty rating, and if the load is variable torque, then sizing according to Normal Duty rating is acceptable?
you want to size it for fla of the motor, not hp
hd basically derates power but allows greater overload
std duty is fine for most properly sized pumps
if starting stopping under load hd may be prudent
keep in mind many pumps have a sf >1.15 so you may want to set ol's higher, ie, heavy duty
hd basically derates power but allows greater overload
std duty is fine for most properly sized pumps
if starting stopping under load hd may be prudent
keep in mind many pumps have a sf >1.15 so you may want to set ol's higher, ie, heavy duty
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You will find that most (if not all) motor mfrs will tell you that if a motor is run from a VFD, the SF becomes 1.0.
kwired
Electron manager
- Location
- NE Nebraska
Makes sense. Many just throw a drive on an existing application for various reasons and never consider if the motor might already been running into any service factor load levels.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
At 300HP, the cost difference is significant, I likely would have gone ahead with ND, but there is nothing wrong with HD if you can afford it.
The difference is, as Ingineur said, in the Overload Capability of the power components inside the drive. In a VT load profile, the slower the motor runs, the LESS load there is on the motor, so the current requirement drops, meaning the components inside of the drive have LESS thermal stress. Even at full speed, a properly sized motor cannot be overloaded when matched to the flow requirements. Flow = load, so the only way to "overload" a centrifugal pump is to have more flow, and if properly sized, the only way for that to happen is if a pipe breaks and you go 'open channel flow". That's not something you WANT to have happen, so there is no need to have the VFD capable of it. So then if you have a properly sized VFD (meaning sized per the MOTOR NAMEPLATE amps), it cannot be overloaded when running a centrifugal pump even at full speed, so there is no need to build-in the capability for that in the transistor selection.
Sizing it for HD on a centrifugal load therefor becomes overkill, but not harmful to anything other than budgets. So to your question, if it is NOT specifically a Variable Torque load like a centrifugal pump or centrifugal fan, it is always going to need HD sizing. However according to the last US Census (which if you didn't know, tracks this sort of stuff), 70% of all AC induction motors used are used on centrifugal PUMPS alone, so I'd hazard a guess what when you add in centrifugal fans, it's close to 90%. That then becomes the definition of "Normal" duty.
ptonsparky
Tom
- Occupation
- EC - retired
I don’t remember an irrigation motor that isn’t running SFA.
kwired
Electron manager
- Location
- NE Nebraska
I can't say I have seen a lot drawing more then FLA on nameplate, but seems they are seldom drawing less then ~90-95% of FLA. They want to get every bit of HP out of them it seems. If you have a need for 85 HP then they go ahead and change sprinkler package to get closer to 100 since they are buying a 100 HP motor anyway.
mike_kilroy
Senior Member
- Location
- United States
We sell the VFD as "CT" or "VT", not "heavy Duty" or "Normal Duty." Not being familiar with MV drives, I cannot comment on those terms, but suspect they mean same thing.
Just to tie together what most replies have said here already, CT VFDs have 200% percent or more PEAK AMPS capability, while VT VFDs have only 100-110% typical PEAK amps. THIS is why VT drives are rated "higher HP" than CT drives. Nothing else.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Superfluous posts removed. Please stay on topic.
drktmplr12
Senior Member
- Location
- South Florida
- Occupation
- Electrical Engineer
If you go with heavy duty rated VFD, please consider that 430.122(A) requires circuit conductors be sized not less than 125% of the rated input current to the VFD. If you are reconnecting existing conductors they might not have adequate capacity.
Is the motor inverter duty rated? My guess would be no since it was originally across-the-line. The motor might not be compatible with a VFD without other considerations.
More of a problem in vertical pumps as opposed to horizontal, the VFD exposes the parts inside the motor to high-frequency currents which travel to ground through the shaft and bearings. This causes pitting leading to premature failure. One solution involves installing an alternative path to ground for the stray currents. There is a lot of information floating out there, good and bad, in regards to this topic, and I do not consider myself an expert.
Please consider a dv/dt output filter if the distance from the VFD to motor is longer than a couple hundred feet. Even better, discuss with VFD manufacturer and they can tell you if dv/dt is recommended.
You might benefit from having a conversation with the motor manufacturer and pump manufacturer and explain to them what you are doing. Express a concern regarding premature bearing failure due to VFD. They might offer some insights or recommendations.
Is the motor inverter duty rated? My guess would be no since it was originally across-the-line. The motor might not be compatible with a VFD without other considerations.
More of a problem in vertical pumps as opposed to horizontal, the VFD exposes the parts inside the motor to high-frequency currents which travel to ground through the shaft and bearings. This causes pitting leading to premature failure. One solution involves installing an alternative path to ground for the stray currents. There is a lot of information floating out there, good and bad, in regards to this topic, and I do not consider myself an expert.
Please consider a dv/dt output filter if the distance from the VFD to motor is longer than a couple hundred feet. Even better, discuss with VFD manufacturer and they can tell you if dv/dt is recommended.
You might benefit from having a conversation with the motor manufacturer and pump manufacturer and explain to them what you are doing. Express a concern regarding premature bearing failure due to VFD. They might offer some insights or recommendations.
The motor is in fact inverter-duty rated. I did add load reactors, but a dv/dt filter may be the way to go here, just to be on the safer side.
This 300 hp pump is by far the largest motor served by this transformer, I'd say it's 80 percent if not higher of the total load. The transformer is 2500kVA. The other remaining loads include small motors, a 480v panel, and some 120/240v panels. I did include line reactors in the design.
the xfmr must be lightly loaded
motor ~ 280 kva if fully loaded, ~10% of xfmr capacity
what is the xfmr config? delta:wye,, etc ???
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