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Air compressor trouble


 
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Greg K
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Joined: 04 Jan 2012
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Location: Nebraska

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PostPosted: Tue Mar 26, 2013 4:06 pm    Post subject: Air compressor trouble Reply to specific post Reply with quote

A friend of mine bought a used air compressor, a 100 gallon 10 horse one. The previous owner rebulit the pump and said it worked fine. I hooked it up and turned it on and everything worked fine. The next week I got a call that it would trip the breaker unless it was left to bleed the tank down to 50 lbs. I put an amp meter on it and it draws 196 amps for several seconds then trips the breaker. I figured it wasn"t releasing head pressure, but that checked out fine. Since the motor would surge while trying to reach full speed I though that maybe the run capacitor or the internal run switch was acting up. Replaced the capacitors and checked the switch, no change. Finally I pulled the pressure relief valve out of the actual pump itself, which is located just before the line hooks to the pump to go to the tank and the check valve. Without this 1/4" check valve in it it works fine, just whistles really loud. So I shut it off, and replaced the valve and the problem showed back up. Thinking that maybe the check valve was not opening up against the tank pressure the friend disassembled it and didn"t find anything wrong. Next, thinking that maybe the new line from the pump to the tank was somehow too small he replaced it with a larger hose(3/4" to 1"). No change in the problem, the motor tries to come up to speed, but surges twice like it cannot quite overcome some hurdle and trips the breaker out.

Any ideas? I"m fresh out of ideas myself short of switching the pulleys out for something a little easier on the motor.
 
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Dusty MI
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PostPosted: Wed Mar 27, 2013 5:28 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote


Look for the unloader valve, usually at the pressure switch.
 
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John T
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PostPosted: Wed Mar 27, 2013 6:04 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

Sounds like you're already well familiar with all this but just for the record in case someone isn't:::: The "check valve" allows air flow FROM the compressor TO the receiver tank but NOT out of the tank back to the pump (Its NOT a head pressure unloader),,,,,,The "pressure relief valve" pops off if pressure is too high,,,,,,The "Unloader valve" is supposed to bleed off pump head pressure allowing for an easy unloaded start up but NOT of course, bleed off tank pressure.

So I would suspect a bad unloader buttttttttttt you stated "I figured it wasn"t releasing head pressure, but that checked out fine."""

If its NOT a pump or unloader problem next Id suspect a bad start capacitor (ifffffffff its a single phase split phase capacitor start motor but 10 HP is awful big????) if the motor is a capacitor start motor but sounds like you replaced a capacitor, was that the start (if single phase cap start) or run capacitor (If your motor is so equipped)

If its NOT an unloader problem or a capacitor problem, ya know if normal (can be 5 x running current) surge start up trips a regular circuit breaker sized to protect the feeders, the Code as I best recall allows the use of a higher amp rated breaker which allows a motor to start!!!!! The motors thermal overload protection device protects the motor while a short will still trip the circuit breaker and protect the feeders. Or if its a fused unit you can use dual element time delay (Class K5 if I recall) fuses, they will allow start up,,,,

Im rusty as an old nail having been too long retired as an EE but my friend the Ol Dusty man there as well as several other fine electricians techs and other engineers and sparkies here might add to or correct this

Ol John T and all
 
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Greg K
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PostPosted: Wed Mar 27, 2013 3:14 pm    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

Sorry I will clarify myself a bit here. It is a 10 hp single phase motor, nameplate rating of 42 amps. All of the capacitors, both start and run, have been replaced. The unloader valve is working fine. I was checking this by opening the pressure relief valve, and in the process of this I stumbled on the fact it will start if it has somewhere for the air to go besides the tank. The check valve appears to be working fine. The breaker that it is tripping is a 60 amp breaker. The shed is fed with #4 URD on a 70 amp breaker, which is too big by the way I read the code. The #4 feeder is about 80" long. It is possible that there is not enough service to the building, but I was wondering if I was missing some other possibility.
 
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markiz41
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PostPosted: Thu Mar 28, 2013 2:41 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

Could be something as simple as the motor pulley and pump flywheel alignment or the belt tension is too high. Check it out with a straight edge and check for proper tension too. Also, can you spin the flywheel by hand easily? The crank should have some endplay. Or like you said, it may just need a smaller pulley.
 
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John T
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PostPosted: Thu Mar 28, 2013 5:14 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

Since allllllllll else is fine according to your info (I'm taking you at your word), Id be tempted to try a higher amp rated breaker (that feeds the compressor), just high enough to allow it to start...Best I recall you can use like 175% of the size that protects the feeders so a 70 amp breaker would be close to NEC limits (If motor branch circuit wiring feed wires are rated 40 amps). Normally the branch circuit overload protection device is sized to limit feeder current to the wires ampacity (like if the wires ampacity is 40 amps you use a 40 amp breaker) HOWEVER a branch circuit used to feed a motor can use a breaker somewhat higher rated then normal......AS BEST I RECALL NO WARRANTY

NOTE AGAIN Im rusty on the NEC so check with it and local authority and more current professional electricians and engineers is my advice.. I'm workin with an oldddddddddd brain here remember lol

John T
 
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Russ from MN
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PostPosted: Thu Mar 28, 2013 7:21 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

You possibly need more volume between the compressor and the check valve. The compressor is so efficient that it is building up too much pressure before it is up to running speed. Put a longer line with a few coils between the compressor and the check valve, will cool the air some too!
 
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Ken Macfarlane
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PostPosted: Thu Mar 28, 2013 11:05 am    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

It sure sounds like your motor is trying to start against pressure. If it can start below 50 psi like you say and run up to 175 or whatever its set too your tank check valve is leaking or the unloader is bad.

If your motor doesn't have thermal overloads, be careful repeated trying it and letting the breaker trip it out, you will likely burn out some windings or cook the insulation. I have a 10 hp single phase on my bandsaw mill and I have to start it unloaded then throw the clutch or it pops my 60 amp breaker. Even unloaded I can see my meter hit 200 amps so I'm sure it goes higher for a bit. I used to run it on 50 feet of #4 SOW, its now on 100 ft of #6 SOW which is too small but watching the ammeter it only peaks out a tiny fraction of the time operating, normally at only 20 amps or so.
 
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XRogerX
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PostPosted: Thu Mar 28, 2013 1:57 pm    Post subject: Re: Air compressor trouble Reply to specific post Reply with quote

If I knew what pump it was I could give you a better diagnosis, but usually, the pressure relief valve on the pump itself is to relieve excess pressure between stages. If it pops, you have HP valve problems. Some people will swap it out for a full pressure valve (200psi or so) to mask the symptoms of failed valves, but all you are doing is forcing the 1st stage to do all the work, the second stage is just along for the ride. Do you actually hear pressure bleeding off after it stops? I have seen blow off lines completely filled with carbon on worn machines using car oil as the lubricant. The fact that it runs when you pull the valve is a sign that pressure is being trapped somewhere, along with the fact that it will start when it bleeds down to 50psi. You might put a pipe nipple and a pressure gauge in there to see what pressure you are getting. A picture would help greatly.
 
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