Air Compressor Problem - Motor Stalls

If it's a DC motor, maybe it needs new brushes?

I agree to check the brushes, if it's not a brushless motor. Does the unloader bleed the entire tank off? If so, then the check valve is probably bad. It could also be that your shutoff set point has self adjusted back. Say your fill point is at 95 lbs, and your shutoff self adjusts down to around 95 lbs, it can cause it to continuously turn on and off. I had a compressor that used to do this to me for a while, until I put a little locktight on the adjustment screw. It could be just a worn out pressure switch as well. Maybe the spring has become weak in it.

Bad pressure switch. The unloader is part of the pressure switch so if it is being activated, the pressure switch is 'saying' that the pressure is sufficient. Maybe it is just mis-adjusted.

Check the adjusting screws, and reset the cutoff back to 120 PSI. Confirm the cuton is at 95 PSI.

Check the line voltage while opperating at the motor. I can do that stall with a long extension cord.

greg

Is it oiless ?If so, check the bearing.

greg

Compressor binding up.

Do you know for sure that your gauge is correct? Maybe the compressor and pressure switch are fine.

It's an AC induction motor, so no brushes. Could it be a capacitor issue?

It's a combination check/unloader valve. I cleaned and lubricated it, and it appears to be working fine. The tank holds pressure - the unloader just vents the tube between the tank and the compressor.

Good thought, but the pressure switch seems to be functioning well. I am getting around the problem by turning down the pressure below the point where the motor stalls. But, that's only about 90 psi, which is not as much as I'd like. Plus, the problem seems to be getting worse.

Thanks!

Actually, it's a mechanical unloader (combination check valve/unloader, which I have already cleaned and lubricated.) It seems to be working well.

The pressure swicth seems to be working fine. If I turn down the pressure adjustment, I can get the contactor to turn off the motor before it stalls, but that's only 90-95 psi or so. If I adjust it higher, the motor is unable to turn the compressor if the pressure is more than 95-100psi. It's definitely the motor stalling, not the pressure switch opening.

Thanks!

Line voltage is OK, about 123 volts. It drops to around 121 volts when the motor stalls. Voltage drop across the pressure switch is minimal (less than a volt.) I'm not using an extension cord.

The compressor is oil filled, and has oil in it (although it is rather old oil, come to think of it.) But, I can easily turn the compressor over by hand and it feels like it's moving freely, with no excess friction or binding.

Thanks!

I can turn it easily by hand, so it doesn't appear to be binding. However, I don't have an easy way to check it when it's at full load. Any suggestions?

Thanks!

Current draw. 1/3 horse shouldn't draw more than 7 amps at 120 volts or 3.5 amps at 240 volts.

Well, it's been working for a long time, but over the past few months has been exhibiting the stalling issue.

I guess the guage could be wrong, but I kind of know what 120psi "feels like" and it's not getting up to what it used to, either by the guage or by feel.

The compressor speed does not slow down normally, it just starts to sound a bit different under load.

greg

Nameplace current rating is 6.0 amps (I'm running on 120 volts.)

I measured about 5 amps at startup (0 psi) rising to close to 6 when it stalls. When it stops turning, the current actually drops back closer to 5 amps, then increases as the motor starts turning again.

The motor is also hot to the touch after pumping all the way up. I can hold my hand on it for maybe 6 or 8 seconds. I don't know if that's different from how it used to be, though.

Thanks!

It didn't used to, that's for sure. Now, it slows all the way to 0. :(

Can you disconnect the motor from the pump and run just the motor to measure its no load amps? And to see if it heats up with no load?

Assuming it's a 120V/240V motor connected for 120, can you check to see if both run windings are actually connected? Perhaps a nut is working loose on the terminal plate inside the motor.

Alan

No load, it's drawing about 1.5 amps. It gets warm, but not as hot as it was under load.