by "Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
Published 12/30/2017 at 17:34
by "Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
Published 12/30/2017 at 17:34
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If a three-phase AC electric motor is rated at 4.3 amps, what would be the draw on each individual leg? Seriously, no bullshit, please.
"diplodicus forgot his password" (diplodicusforgothispassword)
12/30/2017 at 17:55, STARS: 0
It depends on motor load percentage. Because 3ph motors are typically controlled by variable frequency drives. But I’m fairly certain whatever your load is all 3 phases carry the same amount of current. It’s just that the waveform is such that ph1 peaks first then ph2 is offset and ph3 is offset from ph2.
"Stephenson Valve Gear" (stephensonvalvegear)
12/30/2017 at 17:57, STARS: 1
If a 3-phase motor has the amperage listed as 4.3 amps on the nameplate, then that would be the current on each leg at full load.
So, take the rated amps and mulitply by 3 (to get the current on all 3 phases), then multiply by the rated voltage, then multiply by the rated efficiency, divide by 746 and that should give you the horsepower of the motor. Don’t forget, you may need to divide the voltage by 1.73 depending on if it is given as Phase-to-phase, and depending on the configuration of the motor.
"Aremmes" (aremmes)
12/30/2017 at 18:11, STARS: 0
IANAEE, but from what I recall the current on each phase is I ÷ sqrt(3). So for your example, 4.3 A ÷ sqrt(3) = 4.3 A ÷ 1.732 = 2.4 A.
"TheTurbochargedSquirrel" (thatsquirrel)
12/30/2017 at 18:41, STARS: 0
They are typically rated for current on each phase.
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
12/30/2017 at 18:51, STARS: 0
So here’s the situation, and the reason for my terseness in my original post:
My brother-in-law is superintending the building of a bridge to carry traffic across a channel in an environmentally sensitive marsh. The bridge columns are concrete and are wrapped in some sort of carbon fiber wrapping that needs to cure for 72 hours. There are submersible electric pumps that have been happily keeping the area drained for months. Last night, for reasons unknown, the pumps began tripping breakers. I own a multimeter with an amperage-reading claw, and I took the meter to the jobsite because my brother-in-law didn’t have one. He’s not an electrician...
The motors are drawing closer to 6 amps on each leg currently, which seems like clearly too much. Just why, we do not know.
Thanks for the reply. Any further thoughts?
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
12/30/2017 at 18:51, STARS: 0
Thank you.
http://oppositelock.kinja.com/1821672816
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
12/30/2017 at 18:52, STARS: 0
Thank you.
http://oppositelock.kinja.com/1821672816
So if anything, he should be drawing a good deal less than 4.2 A, and not the 6 A that he is currently measuring.
"TheTurbochargedSquirrel" (thatsquirrel)
12/30/2017 at 18:55, STARS: 0
Sounds like something might be over loaded and possibly stalling out.
"merged-5876237249235911857-hrw8uc" (merged-5876237249235911857-hrw8uc)
12/30/2017 at 19:12, STARS: 0
Possibly the bearings or impeller section wearing out and causing the motor to pull more amperage to keep spinning? What’s the typical lifespan of the pumps? And has the liquid they are pumping changed in any way, more viscous, or more grit in it? Might just be worn out pumps, or thicker liquid that is asking more of the pumps?
"PyramidHat" (pyramidhat)
12/30/2017 at 19:21, STARS: 2
Could the pumps have ingested something that’s loading the motors (mud, weeds)?
I used to work at a chem factory and we had amp meters on the agitator motors...when the amp draw started climbing, we knew the batch was starting to thicken and seize...
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
12/30/2017 at 19:54, STARS: 0
Those are all good questions.
"Stephenson Valve Gear" (stephensonvalvegear)
12/30/2017 at 19:58, STARS: 1
I’ll echo the rest of these responses... if the overload is relatively equal on all 3 phases, then it is likely a mechanical issue or crud in the pump that is causing the motor to slow down and draw excessive current. An electrical issue will usually manifest as excessive current on just one phase (or possibly two, depending on circumstances) whereas mechanical drag will load all 3 phases evenly.
"Stephenson Valve Gear" (stephensonvalvegear)
12/30/2017 at 20:05, STARS: 0
One more thing... is the voltage where it should be? Low voltage - specifically once it falls below 90% of nominal - can cause the current to increase at an exponential rate on an induction motor that is under load.
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
12/30/2017 at 22:28, STARS: 0
It turns out that he had the pumps wired backwards. They moved the water, but drew more amperage. Crisis over, but expensive goofup.
"Stephenson Valve Gear" (stephensonvalvegear)
12/31/2017 at 14:28, STARS: 0
I can see that... excessive drag caused by spinning the impellers (I assume they are impeller pumps) the wrong direction. At least that was probably the easiest/cheapest fix compared to the other possible causes.
"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura)
01/01/2018 at 00:46, STARS: 0
Running perfectly now.