Expecting to much.

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Ordered a centrifigal pump and put a vfd on it at customers request. They had a gasoline powered version but it was smokey and noisy inside the shop. We ordered same brand. Pump motor is rated at 11.6 amps with instruction to not restrict flow of water in or out. Pump pulls well over 14. We had to drop the freq to about 55 to prevent overload. Is it to much to ask that equipment perform as suggested?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Is the flow rate higher than it should be? Look at all of the specs as to pipe sizing, length, suction lift, discharge lift and things like that. Are they all within spec?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Also make sure the nominal voltage setting of the VFD matches the motor. If the VFD output voltage at speed is too low the motor will draw too much current.

Tapatalk!
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Is the flow rate higher than it should be? Look at all of the specs as to pipe sizing, length, suction lift, discharge lift and things like that. Are they all within spec?
Graph on GPM at 0 head shows about 219. I would consider that open discharge. We might have 6 ft of lift through 30 ft of 2" flex hose. 55 hz gives us 125.

Now the voltage might play into it a bit. 208 vs 240. Never worried much about that issue before.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Ordered a centrifigal pump and put a vfd on it at customers request. They had a gasoline powered version but it was smokey and noisy inside the shop. We ordered same brand. Pump motor is rated at 11.6 amps with instruction to not restrict flow of water in or out. Pump pulls well over 14. We had to drop the freq to about 55 to prevent overload. Is it to much to ask that equipment perform as suggested?
No. But don't expect the mfg to give any extra.

Is the flow rate higher than it should be? Look at all of the specs as to pipe sizing, length, suction lift, discharge lift and things like that. Are they all within spec?
Yes. If the install is not exactly per spec, see above:(

... Now the voltage might play into it a bit. 208 vs 240. Never worried much about that issue before.
Problem

Graph on GPM at 0 head shows about 219. I would consider that open discharge. We might have 6 ft of lift through 30 ft of 2" flex hose. 55 hz gives us 125. ....
I don't have anything really bright. Troubleshooting recalcitrant pump installations is never easy.
However, here is some physics about why the install is not performing.

Pump affinity laws:
Volume Capacity

q1 / q2 = (n1 / n2) (1a)
Head or Pressure

dp1 / dp2 = (n1 / n2)2 (2a)
Power

P1 / P2 = (n1 / n2)3 (3a)
q = volume flow capacity (m3/s, gpm, cfm, ..)
n = wheel velocity - revolution per minute - (rpm)
dp =
head or pressure (m, ft, Pa, psi, ..)

(assumes the wheel diameter doesn't change)

Decreasing the rpm 8% (60Hz to 55Hz) drops the power to 78%. Which appears to be exactly what you are seeing.

You don't have a lot of options:

Check the pump data. Possibly there are other pump models referenced. The difference between them will be the wheel diameter. Maybe they inadvertently put the 15HP wheel on the pump and a 15Hp motor. Power goes up with the cube of the wheel diameter - doesn't take much change.
P1 / P2 = (n1 / n2)3 (d1 / d2)3
d = wheel diameter


Model number on the pump (or a call to the mfg) should tell this. (Not likely - but i have seen this a couple of times.

And, as noted, the voltage of course. That's not difficult - just money. And you look pretty dumb if that isn't it.

The mfg restriction on outflow is odd. Most centrifugals don't mind higher outlet pressures, just reduces the flow rate following the pump curve. Some don't like shut-off head - makes them heat up. Generally speaking, high NPSH coupled with low outlet restriction causes the pump to run off the end of the curve and require high driver power. Might consider restricting the outlet, pushing up the discharge pressure. I would expect the current (power) to go down.

Generally, restricting the suction is never good. Just makes the pump prone to cavitate.

Good luck.

ice
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Input voltage to drive (which is not really a factor) or output voltage or V/f setting of the drive (which could, I believe be an issue if misconfigured)?

Tapatalk!
That is what I reviewed briefly today. I have not checked the actual output voltage. We do have the parameter set for pump/fan and the options to the V/f setting of the drive is only available for Custom configuration.

Obviously, Custom is not my forte.

The customer is happy enough, but that doesn't mean I have to be. To a point.
 

Cow

Senior Member
Location
Eastern Oregon
I run into this all the time at a large farm I work at. This farm has pump salesman sell them pumps that can't be ran at full speed, only to be used on drives. For example, they've got 4 identical pumps I can only run up to 47 hz before they hit the nameplate current.:roll:

Just today they borrowed a 125hp VFD driven pump to replace another 125hp that was used across the line. I started this previously VFD driven pump across the line and it was pulling 185-190 amps on a motor with a 145 amp nameplate. I choked off the pump discharge to about 60% closed with a hand valve and that brought the amps down to the nameplate rating just to keep them going through the night. I'm just amazed they make pumps without enough ampacity "cushion" that they can't be ran across the line without an overcurrent situation.

Tomorrow they plan on pulling the pump and swapping impellers with the old one.
 
Pumps are frequently manufactured with overloading impeller selections. That is, the pump is selected to produce say 400 GPM @ 100' of TDH with a 15 HP motor. If you lower the head so that the flow increases, the shaft HP increases to 19 HP. We sell a lot of pumps as part of the systems we design and sell and we always use only non-overloading selections. But people who make their money only on pump sales frequently offer selections that will overload because the smaller motor is less expensive giving them a competitive advantage over the non-overloading selections.

It is odd to say "don't restrict the outlet". Discharge head (restriction) is part of pump operation. There is a MCSF (minimum continuous stable flow) for any pump design but that's just one end of its operating envelope.

w piper
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Too much restriction in, for example, a submersible pump would restrict the cooling water flow to the point that the motor would overheat.
Other pump designs could still depend on water flow through the pump section to cool the motor.
Even though the power would go down with restricted flow a sealed motor could still theoretically overheat.
But the only other place I have seen rules on flow restriction has been with positive displacement pumps. Maybe the language just crept over?
Cavitation?

Tapatalk!
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
pumps are relatively application dependent. when they are selected properly for the actual operating conditions, it is almost certain that it will have issues if you change the operating conditions. that is just the way it is.

it is bad mojo IMO to size a pump and motor so it can run at worst case it might see if you move it to the other end of the plant to a different application. that is just lazy engineering and not real cost effective.
 

kwired

Electron manager
Location
NE Nebraska
Ordered a centrifigal pump and put a vfd on it at customers request. They had a gasoline powered version but it was smokey and noisy inside the shop. We ordered same brand. Pump motor is rated at 11.6 amps with instruction to not restrict flow of water in or out. Pump pulls well over 14. We had to drop the freq to about 55 to prevent overload. Is it to much to ask that equipment perform as suggested?
Did you by chance have any performance measurements on the gasoline powered unit to compare how it was loaded? Are they intentionally sizing the electric motor to run into it's service factor?
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Brewery Engineering Plant Technician - HVACR Electrical and Mechanical Systems
Centrifugal Pumps

Centrifugal Pumps

Sounds like a mismatch on correct horsepower for the duty and RPM.
When I have seen this in the past, I cut RPMs on the VFD in order to get some bearings if possible.
RPM is a real clencher on pumps and can cause current to go through the roof.
Horsepower on this type of pump goes DOWN when choked back on the discharge side as with forward curved centrifugal fans.
If the pump is over kill for the duty however it will scream madly at any rate of throttling.....never settling down at full RPM.

all the best

SC
 

kwired

Electron manager
Location
NE Nebraska
Did you by chance have any performance measurements on the gasoline powered unit to compare how it was loaded? Are they intentionally sizing the electric motor to run into it's service factor?
Just their say so on the time of filling. Motor is SF of 1.
Another thing, what is the RPM of the gasoline motor at it's normal full load rating, that very well may be the key here, something tells me it is going to be maybe 2500 - 3000 instead of 3450-3500, and that is why they recommend a VFD is same pump is powered by an electric motor.
 
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