755 Over amping

[Slwagner73]

Gents:

Quick rundown.

Water pump, 75ft of head pressure on a 10” line, open at the discharge end to a pond. (Per customer).

755 75hp 480VAC 3ph.
75hp 230/460 motor, 170/85.1 amp, 1775 rpm.

I ran the setup wizard for this, did the usual rotation check, and then did the static tune. Every thing came out just fine, and the pump starts and runs like it should, amps match hz’s, everything looks good until you go above 40hz.
Then the amps really climb, going way over 110 amps at 50hz. At which point it faults on overcurrent. That’s kind of a no-brainer, it’s doing what it should.
Long story short, I have no idea what could cause this.
The pump is belt driven, the discharge valve is wide open, and the check valve “appears” to be functioning.

This pump ran fine at 30-35hz for a week, but now they want to ramp it up higher, and like I stated, you see the results.

Now I’m stumped.

[DarrylR]

Looks to me you may have an undersized drive. 75hp @480vac is rated for 96 amps. 100/125hp @480vac are good for 124/156 amps.

[g.mccormick]

Is the motor wired correctly for the voltage? I had similar issue before on a centrifugal pump. The VFD was 480 and configured for 480. The motor got wired for 230V instead of 460V. As the speed and power increased (centrifugal pump variable torque load) the current would go through the roof.

[g.mccormick]

Here is my thread from a few years ago from my issue. Sounds almost exactly the same as yours.

http://www.plctalk.net/qanda/showthread.php?t=125669

[jaden]

Centrifugal pumps work on the "Laws of Affinity", which means the Torque goes up as the square of the speed difference, and Horsepower goes up as the cube of speed variation. Ex., if you double the speed of a pump, the torque will be 4 times , and the HP eight times.

[jraef]

Originally Posted by jaden

Centrifugal pumps work on the "Laws of Affinity", which means the Torque goes up as the square of the speed difference, and Horsepower goes up as the cube of speed variation. Ex., if you double the speed of a pump, the torque will be 4 times , and the HP eight times.

To further this concept;
In a centrifugal pump, load = flow. So if the discharge is "wide open", it may be that the pump and motor were selected in such a way (by the mechanical engineer, if any) for a flow rate that was NOT supposed to ever be "wide open" on the discharge end. So as mentioned, the pump may be too small for any more flow than you can get at 40Hz. That's a mechanical issue, not an electrical one.

[Slwagner73]

Thank you all for your answers.
So, here is the update:


I checked the motor for correct wiring. And it is correct.
The drive is sized for the motor, i.e. HP is correct, Amps are correct, so I can't find any issue there.
The setup is the same as the two existing pumps, except they are on contactors, so the amp draw on them matches the nameplates of those moors. Ergo, I don't believe it's an engineering issue.
I am still at a loss...


Thanks:
Scott:

[Tom Jenkins]

You need to plot two curves together:

Start with the pump curve. Ideally, it should show performance at multiple speeds. On it plot the designer's system curve head vs. flow. his may be a horizontal line at 75 ft if the head was considered constant.

Then plot the actual system curve. This will be a parabola with a non-zero intercept. The intercept will be the actual static head. The parabola is the friction loss and Δh = k q²
You can use the actual head measured at any measured flow to calculate k.

h total = h static + Δh

Once you've done that you will have a handle on the actual hydraulic performance and the original design performance. Then you can eliminate or confirm the pump and hydraulic system as the source of your problems.

https://www.linkedin.com/pulse/apply...tzAFYldQ%3D%3D