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Old 01-08-2020, 01:34 PM   #1
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Default Motor Start-up Inrush

I service a small village in N.E. Ohio where there are 29 sewer lift stations and received a request to troubleshoot one of the station that is reportedly tripping a 20 amp motor on start up on occasion. Motor supply is a 35 amp Square D, QO breaker, signal phase, 240 volts. Also this motor was recently rebuilt and installed and the last motor was also tripping the breaker occasionally on start-up.

So, my first thought was to just install a 40 amp breaker and that should provide more inrush current. But now since both motors are reportedly causing the same problem I'm wondering why.

Yes, it still could be the breaker is to small but can it be anything to do with the water pressure?

50-60% of the people in this village are snow birds and go south for the winter. So would that cause lower the water pressure on the motor impellers and cause higher inrush current for the motors?
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Old 01-08-2020, 01:40 PM   #2
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Quote:
Originally Posted by Roger123 View Post
I service a small village in N.E. Ohio where there are 29 sewer lift stations and received a request to troubleshoot one of the station that is reportedly tripping a 20 amp motor on start up on occasion. Motor supply is a 35 amp Square D, QO breaker, signal phase, 240 volts. Also this motor was recently rebuilt and installed and the last motor was also tripping the breaker occasionally on start-up.

So, my first thought was to just install a 40 amp breaker and that should provide more inrush current. But now since both motors are reportedly causing the same problem I'm wondering why.

Yes, it still could be the breaker is to small but can it be anything to do with the water pressure?

50-60% of the people in this village are snow birds and go south for the winter. So would that cause lower the water pressure on the motor impellers and cause higher inrush current for the motors?
Lower pressure would mean less current because the motor will get up to speed faster. I think you should probably check the startup draw, with the motor uncoupled if possible. Meg the windings and the feeder too. Then maybe just go to a larger breaker. 250% is the max.
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Old 01-08-2020, 05:35 PM   #3
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Check the capacitors and current sensing (start) relay. I just fixed a local one last fall. It was a 2 pump alternating system single phase input. Like a lot of single phase well pumps the caps and starting relay are mounted in the pump control panel. the pump would start but the start winding would not kick out or the start winding would not kick in on start up. Either way it was high amps and loud growoing noises.
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Old 01-08-2020, 07:51 PM   #4
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Breakers get weak so there's no surprise if the breaker is tripping early.

Tbh you need to put a amp meter on the motor and see whats really going on as anything else is basically a guess. If the breaker trips instantly you either have a short or a bad breaker, If the breaker holds in for 5 seconds plus you either have a motor starting problem (mechanical or electrical) or a weak breaker.

While you have a meter handy also check the voltage during start up as a voltage drop can compound the problem.
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Old 01-09-2020, 10:13 AM   #5
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Thanks for the info. but this happens occasionally on start-up not repeatedly. Why I asked about water pressure.

If the caps were bad it would happen repeatedly I would think.

As reported "motor runs and operates fine for days and then motor breaker trips." If so, that would be hundreds of start-ups and then maybe a trip in a week or so.

Scheduled to troubleshoot it tomorrow, Friday, so I'll see what I find.
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Old 01-09-2020, 10:16 AM   #6
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Breakers get weak so there's no surprise if the breaker is tripping early.
Tell me more about this.
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Old 01-09-2020, 10:46 AM   #7
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Is it a magnetic breaker?
Is the trip curve set correctly?

What make/model is it?
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Old 01-09-2020, 10:50 AM   #8
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If the breaker is borderline for the starting current, and the voltage was even a little lower, this could cause a trip. There could also be a loose connection or some component, overload relay, breaker, etc, that is near failure - to where sometimes it works and sometimes it does not.

These occasional problems are the worst to deal with.
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Old 01-09-2020, 10:54 AM   #9
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Tell me more about this.
Multiple trips can weaken a breaker.
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Old 01-09-2020, 11:02 AM   #10
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If this is a lift station, has anyone checked the pump? It could have something hung in it or bearings failing. Are these pumps submersible or shaft driven? If submersible, the seals could be failing. If shaft driven, these bearings can fail or the shaft can bind due to being bent or misaligned.

If, like most things that I work on, it is some combination of problems.

If the motor FLA is 20 amps, a 35 amp breaker is undersized. In that breaker class, I would recommend a 50 amp minimum breaker.
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Old 01-09-2020, 11:12 AM   #11
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As reported "motor runs and operates fine for days and then motor breaker trips." If so, that would be hundreds of start-ups and then maybe a trip in a week or so.



I have had pumps suck up something, then when they shut down it gets backwashed out so pump works fine until it sucks something up agaoin.
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Old 01-09-2020, 11:26 AM   #12
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Is it a sewage lift pump?

You keep mentioning water so it sounds like a fresh water pump and not an effluent pump.

You said it was rebuilt.
Was only the motor rebuilt or was the pump also rebuilt?

What is the size motor, voltage and control for this “Pump”?
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Old 01-09-2020, 01:24 PM   #13
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I had a similar situation in a set of sewage lift station pump controls at a lake where virtually everyone was only there on vacations. Turned out that when too many of them showed up at the same time and it was a hot day with everyone running their A/C, the service voltage dropped 20%, which made the lift pumps pull higher than normal current and trip the breakers. Had to be addressed by the utility, who repeatedly denied it until I hooked up a recording meter and sent them the charts.
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Old 01-09-2020, 06:45 PM   #14
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Any chance a check valve is not holding and the pump is turning backwards when it is energized?
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Old 01-09-2020, 07:28 PM   #15
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I had to deal with one today that had tripped the breaker. (3 phase)

Turned out to be a bad low level cut out float was causing the pump to start and stop rapidly until the thermal in the breaker decided it couldn't take it anymore and tripped. (beat the overloads in a race to trip).
When i got there the breaker/overloads had cooled down so we tested everything and ran a few pump down cycles and everything looked good so we deiced to hook up the data recorder and go to lunch.

As i walked off to get the recorder from the truck i heard 4 quick thumps of the starter then the breaker clicked. On testing we found a broken wire between the panel and pump (where it was secured) and when wiggled it would make and break.
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Old 01-09-2020, 07:34 PM   #16
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I would have replaced the breaker on the first site visit with a correctly sized replacement. QO breakers are inexpensive and we keep them on the truck. You really can't afford to NOT change the breaker just to eliminate it as a potential problem.

On a sidenote, I've seen so many incorrectly sized components, wiring, etc, I'm getting to the point where I verify everything before I replace something just because so many things are installed incorrectly it seems.

Just looked at a deal today at a rock crusher where they couldn't figure out how to lockout one of their 7.5HP conveyors without killing the 400 amp main disconnect. So they called me. Found someone had connected the starter directly to the 400 amp buss duct with #12's and no OCPD.

Yesterday I found a green bonding screw ran through the neutral lug on a subpanel fed with 4 wires. We were just there to repair a bent wind damaged mast and the aerial wire feeding the sub.

It's worth it to keep your eyes peeled to address these issues.

I'd definitely be looking into the correct breaker sizing for that 20 amp pump of yours.
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Old 01-09-2020, 07:37 PM   #17
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Quote:
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Tell me more about this.
Google how a magnetic thermal breaker works (some good you-tube videos)

Once you understand how the bi-metal strip works you will understand how constantly heating and cooling the strip eventually leads to it remaining slightly bent which means it takes less amps to cause it to trip.

If it has been tripped in the past that will also weaken the spring.

Easy to spot with a amp meter.
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Old 01-09-2020, 11:06 PM   #18
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Quote:
Originally Posted by gpop View Post
Google how a magnetic thermal breaker works (some good you-tube videos)

Once you understand how the bi-metal strip works you will understand how constantly heating and cooling the strip eventually leads to it remaining slightly bent which means it takes less amps to cause it to trip.

If it has been tripped in the past that will also weaken the spring.

Easy to spot with a amp meter.
I've had engineers argue this point with me, saying that it doesn't happen. But I have witnessed it too. I always point out to those that say it doesn't happen that per the UL489 testing standards for molded case breakers, they are not actually required to trip and reset more than 50 times (less for breakers over 1200A). That might seem like a lot, but not when it starts tripping every day.
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Old 01-10-2020, 07:47 AM   #19
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Quote:
Originally Posted by Cow View Post
I would have replaced the breaker on the first site visit with a correctly sized replacement.
Quote:
Originally Posted by varmit View Post
If the motor FLA is 20 amps, a 35 amp breaker is undersized. In that breaker class, I would recommend a 50 amp minimum breaker.
It's interesting it worked with that small of a breaker most of the time but I don't see any reason you'd not just bump it, there was nuisance tripping before and after the rebuild.

Quote:
Originally Posted by gpop View Post
Tbh you need to put a amp meter on the motor and see whats really going on as anything else is basically a guess.
I don't know if anyone has a Fluke 287 or 289, I know they have data logging, with an amp clamp would those be able to log inrush as events?
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Old 01-10-2020, 08:19 AM   #20
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Thanks guys, lot of good info. here.

Let you guys know what I find out today.
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