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Gotta love a turbo!
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Discussion Starter · #1 ·
Hi All,

I've had a 7.5HP / 26 CFM North Star compressor for over 20 years on a 40A breaker, I did replace the start cap a few years ago. We just got a Generac whole-home 24kw system installed and found a possible compressor issue. The installer thought the gen might possibly start and run this unit but when it cycled on, it sucked the gen down to stall and kicked it offline. He had an inductive amp meter on the line and it showed pulling 183 amps! The motor tag shows it should pull 31 amps (FLA) with start code "H" which should be an additional 7-ish amps for startup for a total of 38 amps. I'm not sure if it's the unloader or possibly my wiring. After reaching pressure, it kicks off with a loud pshhhhh of bleed-off. I wired this myself back in the late 90's, younger and confident, with little to no internet for reference. Now I'm having doubts because of reading about "floating ground" which I have no idea how to check for. This compressor has never popped a breaker of tripped the reset button...

Would a wiring problem (ie: floating ground) cause this huge amp draw?

Could a stuck unloader cause this much draw?


Russ

Note: After finding the manual. troubleshooting points to the unloader and/or discharge check valve. I think the pshhhh I'm hearing is actually the relief valve popping. The gen tech is coming back tomorrow to change the break-in oil, think I'll bleed the tank to zero and try it again. There are no parts available for this compressor, so I'll either need a new pump ($950) or a whole new unit.
 

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First, if your compressor works fine on the power from the utility company, then there is nothing wrong with your pump and probably nothing wrong with your motor.

There are two hot wires coming in supplying 120 volts on each wire.

Then there is a ground wire and a "neutral" wire. I never understood what a "neutral " wire was. Its actually a ground that goes back to the utility company.

Then there is the "ground" wire which you hook to a rod driven into the earth.

I'm not an electrical guy, but I wired all my house (with direction from a friend), and then later wired alll my out buildings. I have a large pole barn, an attached garage, and two out buildings. I have a total of 4 large compressors wired up. They have all worked fine so far.


If you go to the Hamb Journal and join, you can contact a guy called Mad Mike (with a space) . Good guy and he knows everything about electricity.
 

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Gotta love a turbo!
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Discussion Starter · #3 ·
Well, it's not the unloader. With the compressor off and system pressurized, I can manually activate the relief valve on the discharge tube and it's empty. So, I'll have to have some type of trip switch installed that kills the compressor and has to be manually reset.

Russ
 

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I'd still want to know why the emergency power generator won't run the compressor ? If it is happy running on a 40 amp breaker normally, then the generator should be able to run it. Nothing in a home pulls 183 amps ......including your air compressor.
 

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the 'Duracell Project'
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24k gen is 100A, it should start and run anything you have
while compressor motors do surge quite a bit higher when starting, 183A is 6X fla
i'd pull the motor and have it checked out by a shop before the shop wiring burns up
40A breaker is probably wired with #8 wire that is only good for 50A
i'd change that 40A breaker out too, it should trip instantly at 183A
 

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Gotta love a turbo!
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Discussion Starter · #6 ·
First off, it's rated 24kw on LPG, but 21kw (80A) on natural gas. Second, the sub-panel breaker is actually a 50A but is fused closed, so compressor protection is relegated to the 60A main breaker for the sub-panel which has never tripped. And my entire main panel was replaced in 2017 with all new stuff. All #6 AWG from the main thru the sub, to the compressor junction box. Third, the tech restated the load he saw as 130A and my system was already drawing 30A when the compressor started, so 100A of "in-rush" current. Still not enough to keep the generator from overload.

The tech installed a load-shed set to "off" for now. I'll prolly sell this old monster and buy a new 3 1/2 HP that will run on gen power. I used to be pretty active building rods, but all I use air for now is adjusting tires and cleaning up...

Russ
 

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the 'Duracell Project'
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I used to be pretty active building rods, but all I use air for now is adjusting tires and cleaning up...
in that case, why bother? would you ever need to fire up the compressor on the gen?
 

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Hates: Liver. Loves: Diesel
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I can't imagine this is the problem, but the only way a load can operate fine on a 40A breaker, but draw 180A on another source is if the other source is undervolted. But goodness... that would mean the voltage to the compressor from the genny would have to be awful... like 60v per line.

Still, couldn't hurt to test what you're getting at the breaker on genny power.

Also... could be that you're getting 120v to one leg and 0v to the other leg due to how the genny was wired. That would explain the draw. Something wired for 240 but only getting 120 would draw a lot. Keep in mind, your 40A 240v breaker is 40A for each line, because at any given time with AC current, the flow is 40A in each direction. One leg that isn't powered will make your compressor try to operate on double the amperage.
 

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Hates: Liver. Loves: Diesel
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Fire up the genny. Test voltage between the two "out" screws on the breaker. 240v (or close to it) is good. 120v is bad. Then test between one screw and the common bar (where all the whites or coppers converge) and the same to the other screw. If one of them reads 120v and the other reads 0v, that's your issue. The genny is only powering one line in the box.

Line to line should be 240v
Any Line to ground/white should be 120v
 

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Gotta love a turbo!
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Discussion Starter · #10 ·
in that case, why bother? would you ever need to fire up the compressor on the gen?
LOL, guessing you don't live in hurricane alley. Depending on the amount of destruction, tire repairs, nail guns, etc are pretty handy to have...

Russ
 
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