What the heck?

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CONTROL FREQ

Member
Location
OHIO
First time asking, be easy on me... So I was working my normal routine at the plant, got a call from the boss to come to our new building. Apparently the painters kept tripping breakers...

1) 3 paint pumps---3 dedicated cicuits.
2) nameplate voltage 115, FLA 15.5
3) 20 amp breakers, #12 THHN everything wired correctly
4) pumps ran fine pumping water (paint material was obviously too thick) BUT
5) HERES THE KICKER! when the pumps were plugged directly into the receptacles, there was an instantaneous trip (like a dead short)... BUT when plugged into a 100 foot extension cord they all started up fine and ran pulling ~6.5 A... I've been scratching my head and talking to myself ever since.

Using an AC line splitter I did see an initial spike around 60 Amps for a split seconds when plugged in directly, AND through the extension cord. But one way tripped the other way did not. PS the painters told me that when the pumps were plugged into the extension cords and running, they seemed to be at lower RPMs and pressure than normal.

JUST WHEN YOU THINK YOU KNOW IT ALL... something like this happens.
ANY THOUGHTS? They would be much appreciated.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110515-1009 EDT

Quite likely the initial peak inrush current without the extension cord was sufficient to get into the instantaneous trip point of the breaker.

The impedance of the extension, virtually all resistance with little inductive effect, was sufficient to lower the peak, but not extend the somewhat reduced high current sufficiently to trip the the I^2*T part of the trip curve.

.
 

CONTROL FREQ

Member
Location
OHIO
First, thanks for the replies. You guys are VERY intelligent, I've been reading this forum for a long time... my first inclination was to do the voltage drop thing---but as i was going for my calculator, I thought to myself... isn't this thinking kinda "bassakwards?" with lower voltage, shouldn't the amperage peak EVEN higher? I can NOT wrap my brain around this thing, I usually figure it out after a day or so. But not this time, I feel like the answer is RIGHT in front of me but still... something I neglected to mention---I noticed the pumps had a power cord that was like a power drill probably 16-3 maybe 14, but still... the ext. cord was 12-3. I am convinced my answer lies in my description, guess i've been doing it so long I overlook the obvious? somebody slap me...
 

hurk27

Senior Member
Much will depend upon type of motors on the paint pumps.

If the motors have brushes then they are a common AC/DC motor that will operate reduce voltage and not pull higher amps except at start up or if they are over loaded, and as was said resistance is current limiting so the extension cord acted like a ballast resistor to help limit the inrush, the problem is finding the balance of limiting the inrush and at the same time providing enough voltage to do the job?

Now if the motor is of the cap start AC type then the more voltage drop the more slippage, the more current, again the extension cord will limit the current but but it would cause the motor to stall at a certain point.
 
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wiigelec

Member
Location
Red Desert
5) HERES THE KICKER! when the pumps were plugged directly into the receptacles, there was an instantaneous trip (like a dead short)... BUT when plugged into a 100 foot extension cord they all started up fine and ran pulling ~6.5 A... I've been scratching my head and talking to myself ever since.
Did this happen only once or was it consistently repeatable?

As an aside, doesn't OSHA require this cord connected equipment to be GFI protected?
 

CONTROL FREQ

Member
Location
OHIO
What THE heck? WHERES CHALIE B WHEN I NEED HIM???!!!!!LOL!

What THE heck? WHERES CHALIE B WHEN I NEED HIM???!!!!!LOL!

Got sick of thinkin about it, went to the bldg, opened up the cat head on the motor... AC line comes in---goes through a 25A circuit protector w/reset button (much like a surge suppressor)--then, goes to a bridge rectifier?---then into the motor.... all this is small and installed IN the cathead. wierd yes, but doesn't appear to be WRONG? IMO... I just don't like things I can't explain---until I can, THEN I LOVE THEM!;)
 

kbsparky

Senior Member
Location
Delmarva, USA
Had a similar problem with a customer and his air compressor. He had recently moved from an old shop into a new one that we had wired. Since the outlets were relatively close to the panel, anywhere he plugged that thing in, it would trip out the breaker.

I told him to plug it into a 100 foot extension cord, and then call me back with the results.

He was able to operate the compressor normally for the rest of the day. That cord limited the amount of available inrush current so that the breaker would not trip on startup. Blew his mind, though!!

At the end of the day, I installed a 240 Volt outlet for his compressor, and rewired it for 240 Volt operation. Problem solved! :)
 

CONTROL FREQ

Member
Location
OHIO
Thanks guys. It makes a little more sense to me. After it was mentioned, the receptacles they were plugging into WERE within 10 feet of the panel... must have been what you guys said. First i've ever run into this. Usually, problems i've seen involved to LONG of a circuit not too SHORT!!! Threw me for a loop. THANKS FOR THE HELP!
 
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