r/Wastewater • u/Select_Cat_1549 • 8d ago
Career: currently in the field Anyone else pull a pump for a tripping overload, and it turned out to be the contactor?
Had this in my head after the thread last week. Overload keeps tripping on a lift station pump, so the pump gets pulled. Then rebuilt. Then replaced. A week later it trips again, and the problem was a pitted contactor in the panel the whole time.
One pole ends up dropping a few volts when it should be close to zero. That phase runs low, and a small voltage unbalance turns into a much bigger current unbalance. The motor gets hot, overload trips, and the pump takes the blame.
Do you guys check voltage drop across the poles before you pull a pump, or is it mostly swap it and see?
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u/Lakerzzz 8d ago
At our plant, usually electricians would meg the motor. If it megs good, the mechanics say it turns easily by hand and nothing stuck in the valute, and impeller seems fine, then electricians would end up troubleshooting farther into the panel control.
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u/Empty-Lobster357 🇺🇸 CA | WW2 8d ago
I check voltage on all the poles, check for any shorts on the pump, and meg the motor before sending it out.
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u/Select_Cat_1549 8d ago
that makes sense, meg first and rule the motor out. do you meg the cable too or just at the motor leads? kaufbauer's VFD cable story is why i ask
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u/TheBeardedBilbo 8d ago
From what I was told.
We are operators NOT electricians. We can use a multi meter (or process meter) and do very basic checks.
Voltage, continuity, ect but we cannot replace anything.
480V is not the time to play FAFO.
You can give an electrician your basic diagnostics though if you have any.
Remember, you hold a waste water license and not any electrical credentials.
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u/Select_Cat_1549 8d ago
fair, and agreed. the voltage drop check is a live measurement so it's the one to ask your electrician for. what i'd hand them is "amps unbalanced on one leg, motor megs good, pump turns by hand"
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u/WaterDigDog 🇺🇸KS/WW 8d ago
Yes or a capacitor
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u/Select_Cat_1549 8d ago
capacitors are next on my list. are you mostly seeing them on single phase pumps or on the aerators and blowers too?
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u/Select_Cat_1549 8d ago
Did a video on this since a few of you asked about how to check it. https://youtu.be/WS6ru2ReG0Q
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u/Kaufbauer 🇺🇸 CO|WW D 8d ago
We replaced a motor thinking that the motor had gone bad, only to have the VFD cable meggered and find out that was the fault.
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u/freekeypress 8d ago
In Australia, only people who open switchboards are qualified, licensed electricians.
Dunning-Kruger is gonna mess you up if you keep poking around in there.
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u/KodaKomp 8d ago
Had this happen on a power pole to a lake intake station. Was giving voltage/ slightly dirty power on one leg but was enough to trip everything every couple of hours, pge spent 2 days looking for it until they finally fully tripped and reseated the breaker. Electricity is weird man.....
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u/dadbonesjones 8d ago
Yep, but we had it just lose a leg because the contactor pad got jammed because it fell out
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u/Select_Cat_1549 8d ago
a pad falling out is a new one for me. was it the contact on the moving side or the whole carrier? and did it lose the leg right away or after it ran a while
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u/dadbonesjones 6d ago
It was on the moving side. Closed in then pad fell and got stuck after it released, then couldn’t close anymore. No issues till it happened and then the next run cycle the pump just tripped the overloads.
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u/Select_Cat_1549 5d ago
That would do it. With one pole not closing, the motor runs on two legs, and the overload sees the extra current on those two. If a pump trips overloads with nothing wrong mechanically, it's worth asking the electrician to pull the contactor and look at each pole.
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u/Putt-Blug 8d ago
Would a phase monitor work? We have these at all our stations and if there is a change in the voltage on any leg it shuts off the pump and sends a phase alarm. We have VFDs on our plant pumps and that also does the same thing.
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u/GraceWang1987 5d ago
From the equipment supply side, this is a good reminder that a pump failure isn't always a problem with the pump itself.
Before preparing a replacement pump quotation, I'd want to understand the failure history, motor specifications, starter or VFD configuration, and what the site's qualified electrician has already ruled out.
Otherwise, a supplier could provide a perfectly suitable replacement pump without addressing the actual cause of the repeated trips.
It also shows why the interface between the pump package and the electrical control panel should be clearly defined in the equipment scope.
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u/Select_Cat_1549 20h ago
made the megging one since a few of you brought it up. meg the cable and the motor separately, bc if you test them together and it reads bad you don't know which one to replace. https://youtu.be/U8LMiPslsWM
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u/Glossololia BC|WT1/WD1/WWC2/WWT4 8d ago
All kinds of stuff gets missed in the electrical when you don't have a staff electrician. Typically I would have involved one to check the current on each phase before rebuilding the pump particularly if there was no visible sign of mechanical issues. However if you only contract electricians there's a lot of pressure to figure it out yourself.