# Danfoss vfd - irrigation problem



## farlsincharge (Dec 31, 2010)

New installation
Danfoss NXS vfd feeding 75 hp turbine pump sitting on a well fed by a reservoir.
Single Phase 480 input, 3 phase 480 output. Running off pressure transducer set at 40% or approximately 60 psi. 
Everything is good if you keep the upper frequency limit below about 51hz and the system pressure therefor remains below approx 50 psi.
Anything much above and the pump will suddenly slow down, start to buck, and the pressure rapidly fluctuates between 20 and 60 psi.

Tried playing with the gain, it just increases the frequency/slightly dampens the fluctuation.

I started to think maybe the well couldn't keep up and it was cavitating. But they claim it is fed with a 3' diameter inlet pipe, and in 9' of water.

ANY ideas? don't care how outlandish.


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## paulengr (Oct 8, 2017)

Run in manual. It might just be overloaded load torque or need to adjust PID gains because it is oscillating, probably decrease D or I. If it works fine in manual it’s not a load problem. Otherwise start there.

With any control loop if you can’t operate it in manual, it’s not likely to work at all.


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## JRaef (Mar 23, 2009)

Did you use at least a 150HP drive to deal with having the single phase input?


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## 460 Delta (May 9, 2018)

Just because it has plenty of water doesn't mean it's not cavitating. If you hear the gravel sound as you bump up the speed slowly, it's cavitating.


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## gpop (May 14, 2018)

A video with sound would be nice. 

First step is to see if its electrical or mechanical. If the hertz are stable but the flow/pressure are hunting then a plugged inlet screen, pressure relief valve or another mechanical fault could be to blame. (can be hard to tell unless you disable the pid)

If the pressure/flow is moving with the hertz then its most probably a electrical problem which means you have to figure out why the hertz are moving. 
Depending on the program it might be the pid but it more likely to be the drive overriding the pid in protection mode. (reducing hertz based on amps or buss voltage)

Pauls idea of running it in manual speed control would be the easiest way to narrow the problem down.


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## just the cowboy (Sep 4, 2013)

I'm on my way out to a well now to do just that. Run in manual to prove or disprove the well. 
Cowboy


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## Cow (Jan 16, 2008)

I've never had one do that.

I agree with all the suggestions above for things to check. I'd also be curious to know what the running amperage is during all of this?


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## SCR (Mar 24, 2019)

My money is on the pump is sucking air somewhere. Had one with exactly those symptoms though on a lot smaller pump.


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