Air Compressor bogs down

[Regular Guy]

Hi Everyone!

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).

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

Help please.

 

[AC Power]

QUESTION: Upon the second incident where motor is laboring at 30psi, stop at 70psi: What stopped the motor? The motor's overload tripped? Or did it just quit at 70psi on its own as it always has, despite the insufficient psi of 120?

PRELIMINARY TRBL SUSPECT: A motor will always try to provide rated HP, regardless of voltage drop, or lack of induction within its windings. It accomplishes this by drawing more current. Current produces heat, and increased heat kill windings. The problem compounds until a dead fault occurs. Let me know what the answer is to the question above, and I'll go from there. THX

 

[Regular Guy]

Thank you for your in-put, AC! The compressor motor does not quit at 70psi but it just goes on chugging and sounding labored without increasing the tank's pressure. I have not let the motor go many minutes before shutting it off, once it was clear that the pressure was not increasing further than 70 psi, because the compressor sounded like it was laboring so hard. Any and all help appreciated!

 

[AC Power]

Ok, Thx for that. The "bogging / laboring" sound may be coming from the pump, and not the motor. Remove the belt from the motor to pump. Start the motor by turning ON the compressor, it is expected the motor will sound fine without any mechanical load.

1) You have a single stage reciprocating compressor. Ensure you've sufficient oil, that is, it is level with the top throat of the fill port.
2) Remove the air intake filter, try pumping up to 120psi again. Any change?
3) This is a little more complex, but it can be done. Do this is 0psi in the tank. Disconnect the air line that serves as the intake to the compressor head. This line serves as the "supply air" to the compressor. Turn on the compressor (belt is back on). Put your thumb over the intake (from the compressor). If you feel your thumb being drawn into the line, you've sufficient suction, or air intake. Did your thumb get drawn in? Now, do the suction (intake line) back together as it was originally. Turn compressor ON. Let it build to it's present max of 70psi, turn off compressor. Remove the intake line again as you did before, turn ON compressor, put thumb on the suction. Anything? Thumb drawn in?

Let me know.

 

[Regular Guy]

Thank you again, AC!

It is a single-stage compressor, as you say. If I leave the petcock open, even though the belt is still on, the motor sounds fine. So you are correct that it sounds fine with the belt removed too (and with no mechanical load).

There is enough oil (visible through the sight glass), which is full synthetic and changed earlier this year.

I removed the air intake filter and it did not change the fact that the compressor began laboring (bogging down) at about 70psi and stopped increasing the tank's pressure.

I will have be back to you about #3. However, since the pressure builds up in the tank t about 60-70 psi, I expect to confirm that my thumb is drawn into the intake when the pressure is below 70 psi (since how else could the pressure build up to that level?) What happens at the point when the compressor bogs down, I will have to find out, however, logically, it seems the intake would stop sucking -- otherwise where does that air go, since it does not go to increase the air in the tank.

Thanks! I will be back to you!

 

[Regular Guy]

Update: Last night I did this test: I took off the air intake filter. I put 60psi of pressure into the tank and then shut the compressor off. I then opened the intake check valves with my finger. No air came out. This is my thinking: if the output check valve(s) between the compressor and the tank were bad (leaking) then when there is pressure in the tank (with the compressor off) and I manually open the intake check valves, shouldn't the tank's pressure leak out the intake check valves? This is because, if the output check valve(s) were bad, then the tank's pressure would go back from the tank into the compressor and would then go out the intake check valves when those valves are opened (and the compressor is off). Does this experiment seem to rule out the output check valve(s) being bad? Your thoughts, AC and everyone?

 

[AC Power]

Yes, you'd be correct in your understanding of check valves. However, I was leaving the worst case scenario for the last most diagnosis check, that is, the "suction of the thumb" test in my earliest-most post. If you feel no suction at 70psi with your thumb on the intake pipe, the heads in the compressor worn.

Here's a vehicle equivalent to this: If the cylinders in a vehicle engine are worn, the vehicle will burn oil. Additionally, in the power stroke (of a 4 stroke engine), some of the expanding gases are escaping into the crankcase, reducing torque/power to the transmission, leading to a reduction of power for the vehicle. The vehicle is being starved of potential power that the engine could deliver if there were no internal losses.

Compressor scenario: As the pressure in the tank increases, any internal wear of the cylinder heads of your recip comp, or worn intake valves, or worn/damaged intake valve seats, will result in the same thing as the vehicle illustration above. Any wear in the compressor head (as a whole body) will become more-and-more apparent as the pressure builds. Eventually, the rate of lost/escaping air results in insufficient air output to the tank, 70psi back-pressure (from the tank) can't be overcome by the compressors handicapped output. Compressor still operates as the psi setpoint has not been reached, and will continue to run. The motor bogs down as the figurative "combustion chamber" is not at/near atmospheric pressure on it's figurative "compression" stroke, rather, its at/near 70psi. Motor is driving a compressor that is working against itself.

I've a feeling your compressor's recip head is dead.

recip = reciprocating
comp = compressor

Any help? Did you do the thumb test?

 

[Regular Guy]

Thank you very much, AC! Although my compressor receives regular light use, it has never been subject to heavy, prolonged use. However, I understand that fact does not eliminate the possibility of worn valves and other internal parts being worn out in this 16 year old compressor. However, I have always kept the oil level correct.

Yes, I did that "thumb test" and the air intake does continue to strongly suck in my thumb.

UPDATE: On Saturday, being "at wit's end" about this compressor bogging down, I let the compressor continue to run when it reached the 70psi bogging down point. Previously, when it bogged down at this pressure point, it stopped increasing pressure in the tank and sounded so alarming (loud, low, and slow) that I stopped the compressor, because I feared that it certainly sounded like it would be ruined if I left it run and it was not increasing in pressure anyway so it seemed it would do no good to leave it run.

Well, on Saturday, after several minutes running like this (alarming sound, no increase in pressure), the compressor slowly began increasing pressure and eventually went to 120psi, although slower than it used to do. Not only that, strangely, more than half of the 75 minutes in which it was running Saturday (this was my annual day of heaviest use, for my irrigation system blowout) it switched to a realatively normal sound and speed, alternating with the alarming loud, low, slow sound. I don't know why.

AC, your further ideas (and everyone's) are gratefully appreciated!

 

[AC Power]

In the electrical trbl/service industry, we this a "ghost" problem, that is, when we (as electrician service techs) show up to the site, the problem disappears, similar to a "ghost". While frustrated, as is the client too, we both would've liked the problem to remain, that way, we have something to diagnose.

I guess, at this point, I would check for belt slippage. It could/may of been that when the belt heated up from extended motor operation, it's elasticity produced more "grab" to its drive&driven pulley/sheave.

Otherwise, I don't know anymore. If you really want to keep this compressor, i would suggest you buy an ammeter. It determines the ampacity of a load. This way, you can see if the sounds coming from the motor are a mechanical stress that, in turn, produces an increase in AC current to the motor, which in turn, can kill the motor if prolonged. If you hear an awful sound, but the motor is NOT drawing excessive current (too far over its nameplate FLA or amp rating), the motor won't burn out. This does not rule out a worn recip head though.