60 gal air compressor bogs down

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DesertRider

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One final thought, please, after you fine the problem, come back and let us know what it was... we all are interested in the solution or we wouldn’t be here.
 

RegularGuy

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Thanks, Blue Jay and Desert Rider! Desert, it was running fine until this problem surfaced. The oil is half way up the sight glass. I will report back about replacing the capacitors.
 

RegularGuy

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Hi Everyone!

Update: I replaced both the run capacitor and the start capacitor and it did not help. Any ideas guys?

Original post below:

I have a 2005, 60 gal., single stage, 7ph (peak), belt drive, Sanborn air compressor which is wired directly to the breaker box by a short piece of romex wire. It has performed well for 16 years until recently, receiving regular but modest use. All parts are original. Model LA7006056.

In recent months it performed fine at start up but then the motor slows down and has a lower, labored sound when the pressure is roughly 60psi, although the compressor did still go to its usual maximum (120psi or so) but perhaps more slowly than when the compressor was newer. Then last Saturday, it began sounding labored when the pressure built up to 30psi or so, and did not go beyond about 70 psi, although it continued to run while sounding labored.

If I open the tank’s petcock at the bottom of the tank, it runs fine and continues pumping air into the tank (which then leaks out the open petcock).

Help please.
 

oldognewtrick

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Are all the connections clean? Did you check them to be tight? Just throwing thoughts out...
 

kok328

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Have you held an amp meter on it from start to bog down point?
We would expect a spike at startup and then for it to level off until setpoint is achieved.
We should expect an increase in amperage when it goes into bog mode.
At this point I suspect internal valve issues. Might consider googling or checking owners manual on how to adjust/replace.
 

RegularGuy

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Thanks, KOK, for your thoughts. But I am not quite following you, since I am not as handy as you are. I have a common electrical tester. What are you suggesting I do, and how? I have thoroughly read the owners manual. Specifically what issues with what internal valve please? Thank you in advance!
 

Hamberg

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Hopefully a stupid question but have you checked (and or changed) the oil recently? Much like a car engine these have piston(s), ring(s), head gaskets, etc and are lubricated much the same. If the unit runs ok without a load (petcock open) I would look at the compressor unit itself - 15 year old compressor is not that old (mine was 37 before I replaced) but they do need to be maintained (especially if not used daily)
 

RegularGuy

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Thank you for your reply, Hamberg! I did change the oil (full synthetic) last spring and it is half way up the sight glass, as the owner's manual specifies. I give the compressor light regular use, e.g., a half hour per month. Further thoughts, everyone?
 

kok328

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Thanks, KOK, for your thoughts. But I am not quite following you, since I am not as handy as you are. I have a common electrical tester. What are you suggesting I do, and how? I have thoroughly read the owners manual. Specifically what issues with what internal valve please? Thank you in advance!
These valves.
Compressor Valves
 

bud16415

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I think your motor has just got weak over time. My compressor motor went out last year and I have a smaller backup I was getting by with. The neighbor had a compressor he was junking and it had a motor one size down from mine and I took the motor and put on mine. It worked great up to 80 PSI and then started bogging down. Not enough HP is all. I set my regulator down and used it that way for all summer and I found another used motor with the same rating as my original. I swapped it out and bingo 100PSI no problem.



Motors don’t always die. They can get sluggish over time bearings and internal fits and such.

I think you have been thru everything else at this point.
 

BuzzLOL

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Update: I replaced both the run capacitor and the start capacitor and it did not help. Any ideas guys?
That's why we suggest testing parts before just replacing them... if it has a belt, it may be loose/slipping...
If it has a centrifugal starting switch inside the motor, it may have dirty electrical contacts... or as Bud suggests, the electrical wiring in the motor may be old and shorting out...
 

kok328

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That's why we suggest testing parts before just replacing them... if it has a belt, it may be loose/slipping...
If it has a centrifugal starting switch inside the motor, it may have dirty electrical contacts... or as Bud suggests, the electrical wiring in the motor may be old and shorting out...
no switch, it has capacitors. no short or you'd be tripping breakers.
take some amp readings and see what the motor is doing.
I think the valves are bad. At bog down point your asking the motor to keep up with a wide open valve leak while at the same time achieving higher pressure. The air pressure is pushing backwards against the compressor.
 

bud16415

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I’m not an expert but the pistons are always pushing against the pressure in the tank when trying to make the pressure go higher I think. When the high-pressure setting is reached the motor stops and the valve holds the pressure back so the piston doesn’t have to and so the motor when the low-pressure is hit the motor doesn’t have to start against full load. On the down stroke the inlet valve opens and on the up stroke the pressure valve is forced open by the air being compressed.



If the inlet valve were broke and stuck open you would just suck and blow air and not build pressure, if the pressure valve were stuck open the tank pressure would flow backwards on the down stroke and keep the inlet closed and you would just keep recompressing the same air.

It is easy enough to pull the head off and take a look at the valves. Some are just little spring steel flappers and some are like a puck with a flat spring behind it.
 

BuzzLOL

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no switch, it has capacitors. no short or you'd be tripping breakers.
take some amp readings and see what the motor is doing.
I think the valves are bad. At bog down point your asking the motor to keep up with a wide open valve leak while at the same time achieving higher pressure. The air pressure is pushing backwards against the compressor.
Depends on where the short is between the windings if it blows a fuse and the valves don't work like that...
 

bud16415

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Depends on where the short is between the windings if it blows a fuse and the valves don't work like that...
I agree when I first started in the factory one of my first jobs was a coil winder. With enameled motor wire it gets nicks if you are not careful and burn thru from heat and aging. There may be 1000 turns of wire and if there is a short turn to turn it might just pass a tiny fraction of the current and the turn to turn voltage drop is really little voltage. Sometimes even layer to layer drops are not great enough to cause a meltdown. When you get a short to frame or something then its going to melt or trip something like a breaker.

It could be old age. I now I have a dozen old motors under my bench I have picked up here and there or pulled them when junking something. They all run but I can tell some of them are no longer 100%.
 

RegularGuy

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Thanks for your in-put, Buzz. The belt works smoothly and it not loose but has the expected amount of resistance to turning which shows that it is moving the compressor when I turn the flywheel with my hand.

KOK, I have an ordinary electrical tester. What amp readings are you suggesting (where to put the tester probes and what readings)?

KOK, I wondered about a bad check valve too. So this is the test I performed: I turned on the compressor and allowed 60psi to build up. I then shut off the compressor and listened and felt around the compressor. I could neither see nor hear air escaping. If there is a bad check valve against which the compressor must fight, wouldn't the air continue escaping from this valve after the compressor is turned off?

Bud, thank you too. But let me think out loud with you please: if the air intake valve were stuck open, wouldn't this prevent the compressor from building up to 70psi? It does build up pressure to 60psi like it should.

Regarding the check valve between the compressor and the tank, if that one were bad, wouldn't that valve continue to allow air to escape when the tank at 60psi in it and I shut off the compressor? When the tank has 60psi in it and I shut off the compressor, everything is silent. The tank keeps its pressure and there is no air escaping that I can hear or feel. Doesn't this tell us that the check valve is working?

Bud, regarding your suggestion that the motor windings might be experiencing micro-electrical-shorts: is there a test to determine that? As Buzz said above, I hate to simply replace a motor I don't know is the problem.

Thanks, Guys! Further thoughts?
 

bud16415

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I agree that if you are making pressure and holding pressure the compressor and valves are working properly. It is IMO as the pressure builds the HP the motor has to produce goes up and your symptoms sounds exactly like what I had when I replaced my motor with a smaller motor and the load exceeded what the motor could produce.



If your motor has some internal weakening going on there are ways to test for that but they are not going to be tools you would have around like a megger or a load device that would measure HP. Small motor repair shops are around and some have testing equipment and many have used motors for sale or refurbished motors.



I needed a motor once in a blizzard and I called a guy and he said he had a used motor but his driveway had 2’ of snow in it. I said no problem I have a ¾ pickup with a plow on it. When I got there I dropped the blade and cleared a track down to his place. He told me if I plow it wider when I leave the motor would be free.

We will never know if it is the motor or not unless you take and get it tested or try a new motor. What are the specs on the motor? What type of compressor is it? What type of usage does it get?
 

RegularGuy

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The motor is: 3.1 RHP, 7 Peak HP, 3450 RPM, 56 Frame, 208/220 Volts, Single Phase, 5/8" Shaft

The compressor is a single stage, belt drive Sanborn 60 gal compressor.

The compressor is about 16 years old and gets regular light usage, e.g., 1/2 to 1 hour of usage per month.
 

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