How long has it been since you pumped some air.
Maybe the cylinders are full of water. Take the spark plugs out and crank it over. Maybe the rings are stuck to the cylinder walls.
Maybe the Capacitor is incompacitated.

Mikie, how do you remove the spark plugs from an AIR COMPRESSOR????
Hut123, remove the belts to see if the motor will turn freely by hand, without power of course. Next, apply power to see if it will start with no load.(If it comes up to speed instantly and you hear a "click",it's OK. If it comes up to speed slowly and "clicks",there is a problem with the start circuit contacts). If the motor checks out OK, disconnect air line from comp. to tank, try turning compressor flywheel by hand. if it turns hard(some resistance will be felt due to compression), you probably have internal problems in compressor.If the comp. starts and runs with the air line disconnected, it could possibly be the check valve, but I doub't it if there's no pressure in the tank.Usually if the check valve is stuck,it will try to turn a little bit and then stall.
Hope this is understandable and helps,
Mike

Thanks, guys. I pumped air with it last week and noticed no problem. When the problem surfaced I released all pressure from the tank without any apparent effect. It's anchored to the wall to keep it from tipping so I'll have to undo that and turn it around to get to the belt side and go from there. Shoot, being without the compressor is about like walking around with only one shoe on.

I'll keep your posts handy when I get into it. Think I can safely eliminate spark plugs as the problem. :)

After further thought, if comp motor hums with no load, it could also be a capacitor as mikie suggested.
Mike

If a capacitor was shot, would it not stink, or have a burned out appearance ?
These units can be checked, I think ,using a good multi-meter.
If the unloader valve was sticking closed, then this could cause the motor not to turn (too much pressure to overcome).
One could drain the air from the tank and see what happens at very low pressure - I think it was a pressure valve that gave me trouble with the Sanborn over 10 years ago..

Starting caps fry pretty easily so check those first.
With the power disconnected see how hard it is to turn the pulley by hand to rule out mechanical problems. siezed -bad bearings etc.
The hum means some kind of power is there but are you getting both phases?

Check the plug contacts and make sure they are clean and not corroded. If they are corroded the contact resistance will be too high and not enough power will get to the motor to start under load.

Finally got a chance to look into the compressor problem. Once I got the cold, stiff plastic shroud off (it now looks as if a large rat had chewed around the edges) I found the motor and compressor turn freely. Removed the belt and the motor coasts normally to a stop after a quick spin - but it still won't start even without a load. I'll warm up some and go pull the motor and get it on the bench. Thanks for your help.

Sounds like a capacitor or start circuit contacts.Disconnect the two hot leads in the motor junction box(with the cord unplugged, of course),make sure they aren't touching anything and check voltage between them and from each one to ground. should read 240v between them and 120v from each one to ground.If any of these readings are below as stated, there is a problem somewhere between the motor junction box and the circuit breaker or fuse.
Please don't think I'm trying to treat you like a dummy, but ground is the white or green wire in motor junction box or any bare metal on motor frame if it's wired properly.If all this checks out, I'd recommend taking it to an elec motor shop.
Hope this helps and BE CAREFUL!!!!,
Mike

Check the circuit breaker at the panel. If it is wired to two individual breakers, one could be tripped. The crew at our shop discovered this after they had ordered a new motor and had the old one off.

Many times the start switch in the motor get stuck. Somnetimes a good rap while humming will dislodge it.
Also if you don't use the compressor a lot look for dirtdaubber and or wasp nest in the motor. We get several in for repair each spring with these problems.

are you getting both phases ???

ground is the white or green ???

First Single phase only has One phase...
Two phase is only used on ships...
Three phase in factorys...

Next white is never ground...
White is the return for the 110 side of the voltage flow...

Kevin, much residential wiring that I'm familiar with uses the white as ground for 230 volts. A lot of the older circuits use a black, red and white - there is no bare copper ground.
Then again you will find black and white and maybe a bare copper ground, in this case the white replaces the red.

I think the electrical service is ok as I've been using the compressor for a year or so - and I've double checked the fuses. Finally got the motor off and it's now at a repair shop (Wednesday). I'm awaiting word from them. I'll let you know what they find.

Word from the motor repair people is that it's totally smoked, fried, etc. Exact replacement (as close as they could match it) was $400+ and a same-size motor of different brand was about $275. I'm now looking for a used motor to put on the compressor. They did tell me that according to amperage drawn by the motor it was in actuality a 2 or 2.5 h.p. size - not 6 h.p., as stated on the compressor tank (hype, I suppose). Think I would buy a new compressor (60 gallon) before paying that amount of money for a new motor.

I would check at Harbor Freight - link below.
3hp $99 15 amp

Here is a link that might be useful: harbor freight 3 hp