A 75 hp across-the-line starter fire pump trips the secondary overload in the utility pole mounted transformer. We did four to five starts with no problem. Then transformer trips. All electrical inside is ok, no breakers are tripping. Could there be a bad "spot" in the motor windings. Other service on transformer is low, the transformer's not seeing a lot of load. any suggestions about what could be happening or what the next troubleshooting step should be. Utility monitored startup and running of pump and didn't see any issues on their end.
Motor troubleshooting help
- Thread starter DW98
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How long between start, stop, restart? Fire pump system installs are designed to start and run with minimal chance of tripping under any condition including locked rotor. Starting and stopping back to back approaches a lock rotor condition compared to simply running the pump. What rating are the transfomers? Are these integral overloads?
Other than questioning these, I'd say your first troubleshooting step is to hook up a recorder, repeat exercising, then evaluate.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
How close together were your starts?
What do you mean by "secondary overload in the utility pole mounted transformer"? Did a fuse blow on the cutout? If not, then the only other thing I can think of was a primary overload / cutout device. Did it look like this?
If so, those things are just like a circuit breaker, they have a time-trip curve and if you are starting a motor multiple times too rapidly, you heat them up and they trip. They are rate 42A on the primary side, but without knowing your utility primary voltage and the transformer size feeding your fire pump, it's impossible to say what these would see. But in my experience, I've seen utilities try to get away with transformers that are in my opinion, under sized. It's their prerogative because they own it and have to bear the cost to replace it if the constant over heating degrades the insulation. But sometimes it does interfere with a project start-up when their under sizing causes issues like this. For example if the transformer is too small, it saturates under starting load and heats up way more than it should. You start in rapid succession and the transformer can't cool down in between, so the 4th time, it's just too much for it. I had a similar problem at a pump station with 3 x 100HP Soft Starter controlled pumps in it, the utility fed it with a 225kVA pad mount transformer (said so right on the side of it). We kept clearing the primary cutout fuses when trying to start and test the third pump, they kept sending someone out to just replace the fuses, but it was a big waste of time for my crew. It took 3 days and 6 meetings to convince the utility engineer to put in a larger transformer (which did solve the problem).
We ran full test with utility company monitoring at the meter lugs. showed nothing out of the ordinary, showed normal voltage, the inrush current, expected amperages. other start stops were probably about 5 minutes apart. we discussed having the utility set up monitoring again to capture the event, but I wondering if anyone suspects the motor and that we should get it checked first. I'm not familiar with what testing would or can be done in the field
I'm leaning towards undersized transformers. If the motor was defective it would have likely showed up on monitoring... but the doubt in my head right now is whether the POCO was monitoring at the time the overloads tripped.
zog
Senior Member
- Location
- Charlotte, NC
What were your megger/DAR/PI meadings? Winding resistance?
we haven't made any motor measurements, megger, winding, etc. it may be best to go ahead and get the utility monitor back on and see what happens when the trip occurs, if the info is the same as when the motor starts, it may point to the poco xfmr.
thanks for all your responses and help.
thanks for all your responses and help.
zog
Senior Member
- Location
- Charlotte, NC
Thought you said "All electrical inside is OK"?
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