# Thread: Vertical 300hp Well Pump Motor

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.
doesn't matter as long as tuned well
a plc gives alot more flexibility

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just got word this is a centrifugal pump (so variable torque), is V/Hz still recommended?
Last edited by adamscb; 12-18-17 at 07:40 PM.

just got word this is a centrifugal pump, is V/Hz still recommended?
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.

just got word this is a centrifugal pump (so variable torque), is V/Hz still recommended?
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
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)
Last edited by Ingenieur; 12-18-17 at 08:29 PM.

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Originally Posted by Jraef
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.
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?

6. 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

7. Originally Posted by Ingenieur

keep in mind many pumps have a sf >1.15 so you may want to set ol's higher, ie, heavy duty
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.

8. Originally Posted by Jraef
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.
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.

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?
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.

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Good deal, thanks everyone for all the replies, this is really good stuff.

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