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Old December 6th, 2017, 10:28 AM   #1
lesmar96
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deep well pump tripping on low current fault

One of our customers is a municipial water authority. They have several deep well pumps. The one well pump is on a vfd to slow it down because they put a new pump in and it pumped way too fast.

They also have a plc that monitors the amperage of each of the well pumps and will fault out if the pump current gets too high or too low. Each winter, we have the issue where the plc will start throwing a fault for low current on the pump with the vfd. It will work fine all summer long, but on some of the cold days and weekends, it will trip out sometimes a couple times a day.

What we are trying to figure out is what would cause the pump to pull low amperage when it is cold. Naturally, we would assume it could be running out of water, but why does that change when it is cold?

From the vfd to the pump is approx. 500' but we have filtering etc. This is basically the only issue we have with this. Any suggestions what we should check or upgrade?
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Old December 6th, 2017, 10:45 AM   #2
bright676
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Too cold, Ice?
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Old December 6th, 2017, 10:54 AM   #3
lesmar96
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Quote:
Originally Posted by bright676 View Post
Too cold, Ice?
How would ice cause it to trip on low current? I figured if there was too much ice and the pump would lock up, we would have high current? The well is too deep for the water to be frozen.
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Old December 6th, 2017, 11:03 AM   #4
JohnCalderwood
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Is the impeller coming loose when it is cold?

Less load on the motor shaft....
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Old December 6th, 2017, 11:13 AM   #5
Rob...
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Impeller is just spinning surrounded by ice. Not drawing any load other than that necessary to spin the motor.

Or the pipe work has frozen, dependent on the type of pump, it may be happy sitting there pumping against a dead head rather than up to a 500' head. Although high lift pumps I've worked on need a head and somewhere to pump to otherwise they will trip on over current. Charging the lines with water was always fun.

If it was cold water there you would expect slightly higher current readings as water gets denser. (Then less dense as it crystallizes)
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Old December 6th, 2017, 11:19 AM   #6
alive15
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I = V/R, as it gets colder, resistance increase, therefore current decreases.

This is a complete guess.
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Old December 6th, 2017, 11:24 AM   #7
mendonsy
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Lower ambient temperature = higher efficiency = less current draw
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Old December 6th, 2017, 11:34 AM   #8
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I = V/R, lower voltage, so lower current; resistance stays the same ??
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Old December 6th, 2017, 11:57 AM   #9
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You need to be logging the normal current and the lowest current so that you can see how bad the problem is. If it is borderline OK then you might need to adjust your current trip level. If it is a sudden current drop then you are looking at other problems including wiring issues (copper shrinks as it gets cold). You need data to find and solve problems, otherwise it is all just guesswork.
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Old December 6th, 2017, 01:16 PM   #10
GaryS
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Ice really. The frost line in Pa. is about 18" (deep well pump) I m guessing at least 100' or more. I know a lot that are much deeper then that. the water temp will be about 50 year round.
monitor and chart the motor current. less amps means less load, less volume or less pressure. is it possible that for winter they open a bleeder valve to keep some water flowing over the winter to keep the above ground pipes from freezing lowering the static pressure on the system
the water table could be dropping over the winter and caveating the pump but with all the rain we have had in Pa this year I really don't think the water table dropped this year I know placed that it's higher then normal.
how are you measuring the motor current ?
also Pa. hasn't had any days cold enough to freeze yet I don't expect it to jet that cold until about mid Jan.

Last edited by GaryS; December 6th, 2017 at 01:19 PM.
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Old December 6th, 2017, 01:53 PM   #11
lesmar96
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Yes, Gary, this pump is around 120', I believe. I do not think ice or water temp is the issue.

We are monitoring the current with a analog CT reading to the PLC. I have double-checked the accuracy of this and the CT seems to be reading accurately. A graph of motor current would be very helpful for troubleshooting. I will see about adding that to the program.
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Old December 6th, 2017, 01:59 PM   #12
jraef
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In a centrifugal pump, load = flow. No flow = no load, no load = low current. The entire PURPOSE of monitoring for low current is in fact to detect when the flow is blocked. The icing could be happening anywhere in the pipe system.
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Old December 6th, 2017, 02:05 PM   #13
kekrahulik
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Can't see well water changing temp more than a smidge.

Do you have any data on pressure and flow. My guess is that something is happening to drop the backpressure on the pump (some one opened a valve somewhere etc) and with it, the amps drop.

How long does it need to be low amps before the alarm is set?
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Old December 6th, 2017, 02:45 PM   #14
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One thing I should have caught the first time.
you can't use a CT on the motor leads of a motor driven from a VFD to monitor the current the will always give you bogus readings CT's are designed to work with 60hz sign wave neither of these are produced in the output of a VFD.
if you want to monitor the motor current us the analog output for the VFD it will be much better then you will ever get with a CT.
I still don't see what the time of the year has to do with the load on the pump. there has to be something outside of the pump causing the flow to change
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Old December 6th, 2017, 03:27 PM   #15
g.mccormick
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Is it as simple as water demand/usage decreases in the winter time? Less demand means less flow means less current? And I also agree that standard CT may not work well on vfd. Using analog out from vfd would be better choice
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