Single phase 120VAC compressor motor problem

I have a small air compressor (actually 2 identical) Has been working great. Had it tucked away behind my Jig Borer but needed it outside before a friend gave me another Identical one. (it runs OK) Before putting it back behind the machine I wanted to extend the drain and add a valve so it was easier to drain any water that accumulated. Removed the plug, maybe a cup of water drained and I let it sit having another compressor until moisture had a chance to dry. In addition I removed the oil drain plug and drained the oil. (didn't seem very dirty) Replaced the oil with 30 non detergent oil. Didn't get a chance to hide it behind the Jig Borer yet but needed some air. It's a small compressor on wheels, handle removed to fit the space behind the jig borer with possibly a 15 gallon tank. Motor about 1/3HP (didn't check) Capacitor start (single Cap.) 120VAC single phase. Started well ran quite other than the typical puttering of the valve. I know it shut down one or 2 times and restarted. while I was near the compressor it attempted to start again but failed, it attempted, rotated compressor pulley about 1/4 turn and stopped. I quickly unplugged it. Compressor pulley about 10" motor pulley 2", semi new belt in that it was changed a while ago and in excellent condition nor is the belt banjo string tight plus it was running as adjusted for a while when belt was replaced. Wanting to check it out a day or 2 later I plugged it back in, started as it always does so I continued a lathe project so as to be near the compressor when it shut down. When it did stop I lowered air pressure with air nozzle until it attempted to restart, it didn't and made the same approx 1/4 turn of compressor pulley. I unplugged it, when attempting to rotate the compressor by hand it turned easy until on compression stroke then turned hard. I made no changes to the pressure switch and it is mounted directly to the tank, compressor outlet is also connected directly to the tank, no means of unloading provided. Power plug was in wall mounted outlet with no other load connected, I assume voltage is 120VAC at the outlet though I didn't check but no reason why it should be low.
Any ideas as to the problem? Capacitor getting weak? Approx 8' power cord getting old?? Looks good, no cracks in the rubber coating. Outlet valve leaking putting pressure on piston?

Its easy to test the capacitor if you have an ohmmeter. Discharge the capacitor before testing. Be great if your digital meter had a capacitor tester, but if not just put it on ohms and it should climb. Should not be a steady low value.

The other issues is if the start switch is stuck. Assuming this is an old one that has a mechanical switch. You can tap on it with a hammer to see if it will free up to engage (temporary fix). You can also test by pulling the belt off and spinning the shaft by hand to see if it will start (do it safely).

My Quincy 5hp 2 stage is about 30 years old. 12+ years ago it started not wanting to start (would hum). I took it apart and cleaned the start switch and has been great since. On my old small compressor, I burned up capacitors on a regular basis.

If you turn it by hand and it hits compression, it appears not to have a functioning startup pressure relief.

The question is whether it is supposed to have one. Some smaller units do not.

It is very obvious that it had no type of unloading. Compressor is piped (Copper tubing) directly to the middle of the tank and pressure switch also mounted direct near the end of the tank. No connection between the 2. Starts fine when tank is empty. Has no pressure gauge but I have a few and will check restart pressure then try to start at lower pressure but as I've said it worked great before without issues. I must say when attempting to rotate the compressor pulley when the motor couldn't restart and I pulled the plug was quite hard with very little rotation, maybe I should try rotating the motor pulley where I'll get a 5-1 advantage. Odd that is restarted a few times but failed on the 3rd or 4th attempt. I'm wondering if a leaking valve will keep the cylinder at tank pressure so that piston is forced down requiring restart up-stroke and full pressure no compression and attempting to force a full cylinder to over come tank pressure will not not allow motor to come up to speed being there is a 5-1 pulley advantage. Might be greater than 5-1, I'll check it.

yeah, leak-back was something that I thought of also. Could really jam the motor, since the start torque varies on those from something over normal torque to, with others, less than normal torque.

Once it is turning, torque may get better. So leak-back might catch it in a bad spot. The valve may have something that "sometimes" lets it leak back, or it may always leak, but vary as to where it catches the piston.

I know intake is a reed valve, out is probably reed too. Possibly some dirt got wedged in the valve and it's leaking back, I assume when motor is shut off by the pressure switch the compressor will stop on the compression stroke, any leak back will probably push the piston down so at restart there is no light load for the small motor to get up to speed. What I should do is run it until it shuts down and watch to see if compressor pulley slowly rotates backwards. I did check motor to compressor ratio, it's about 5-3/4 to 1 almost 6 to one but not quite. Empty tank start is quick motor has no problem.

Check the tank-check for leakage, swap the motor capacitor to rule that out. Small compressors rely on leakage through the pump valves for unloading. If the tank check leaks then they will never unload. Running very high duty cycles with them can also be problematic. (Need at least 5~20 or so seconds for them to leak down - instant restarts do not allow for that.)

Worst case you could always slap a beefier motor on there on the cheap from Craigslist, et. al. or swap the pressure switch for one with an unloader valve.

I don't see a check valve but was thinking there should be one. Copper tube is connected to another fitting fitting connected to the tank, that might be the check valve. I might have goten dirt in it when I turned the compressor upside-down to remove the drain valve though water was drained prior to flipping the unit. I replaced the valve with 90° elbow. 8" of pipe and a better valve so when tucked behind my Jig Borer I can easily drain any water. Compressor didn't quick start, at best only air being used might have been in a hose fitting leak, motor must have been off 15 to 20 minutes before restart attempt. A thought just came to mind, what would happen if compressor crankcase pressurized via blow-by, what I thought was compression stroke could have been intake, compressor is small single cylinder. Must be some kind of vent someplace for the crankcase. Compressor does not nor ever did leak oil.

Compressor pumps usually have breather holes of some fashion somewhere just like engines to alleviate the effects of blow-by and thermal expansion which would otherwise ruin the bearing seals, lock up the pump, etc.

Tank checks are called such because they are stealthily hidden right inside the tank inlet. You sure you didn't simply miss it? Most compressors have them because the reed valves in the pump don't form a perfect seal when the unit is not running.

Thanks for the photo Sparky, I think the same type valve is screwed into the tank and the copper outlet tubing fitting screwed into it. I'll remove that fitting and see what it is. I know there needs to be a crankcase vent not sure where on this compressor it is. Oil filler plug is mounted very low so case can't be overfilled, I don't think the pipe plug has a hole in it like most gearboxes.

Hi Froneck,
I think a good way of unloading small compressors is to connect a pneumatic electric solenoid valve that is Normally Open to the pipe between the compressor discharge valve and the receiver check valve and have the coil connected to the motor terminals. This means when motor turns OFF Valve is open to atmosphere.
Never does the compressor start against a pressure above atmospheric.
These pneumatic valves are quite cheap.

Jim

[strikethrough]If you end up using a VFD, you would want to connect such a valve to one of the relay outputs and use a signal such as 'running at set speed' - this means it will be off (open) while accelerating, too.[/strikethrough]

Wrong thread...

on the higher end models they have a built in unloader to the pressure switch from the air intake to the tank from the pump at the check valve.

Removed the compressor outlet copper tube. Yup, check valve is leaking. I removed it from the tank. It can be disassembled in that it's screwed together. Cleaned every thing inside well but it still seems to be leaking.

New check valve and away you go!

Hi Froneck,
If it is a metal to metal seat in the check valve it probably needs to be lapped in.
I think when the unloading valve is actuated by the pressure switch when the compressor motor is shutdown by anything other than the pressure switch, well the pressure is still against the discharge valve so the motor may not want to restart.

Jim

I found a similar valve on ebay, it's not like the one sparky has in the photo,My pressure switch has no means to unload, both compressor and switch connected directly to tank. check valve has provisions to unload tube connecting compressor to tank.

Up-Date

As per my last post I purchased a valve on ebay, looks exactly the same, thread size on both sides is also the same. Been busy but other day decided to install it. All went well, threaded on the compression fitting nuts by hand to about 90% or so on each end by hand so as not to cross thread. Started compressor, while it was running went back to lathe only few feet away. Compressor seem to have been running for long time. Checking I found the compression fitting on both sides leaking bad! Even on the compressor side where nothing was changed. Thought they were loose so I attempted to tighten, got tighter but still leaking less on compressor side so went to tank side. As I tightened compressor or something developed a knock like sound. Thinking pressure switch may have failed I pulled the plug. I removed the copper tubing with the compression fitting and see nothing wrong! Leak seemed to be high pressure so I'm wondering if air was not allowed to go into tank and only the tubing had very high pressure. No knock when started without the tube attached. Checked the valve, it seems to work as it should, I can't remove piston, lower end has check seal held on with push-on retaining ring. It has spring keeping it closed but at the most compression spring is a pound or 2. I didn't find a spring in the original but do see what failed, the push-on ring is missing and so is the seal.
Any ideas as to what can be wrong?