The main problem is that the motor is being overloaded. As mentioned
already, induction motors don't get tired or exhibit lower power output with
age.

You don't state some of the most essential info.. namely, the brand name and
model number of the air compressor, and the size and type of air pump.
The complete info listed on the motor label should also be included.

We can't see it from where we are.

Forget the capacitor.. the motor is starting normally, so the cap isn't the
problem.

A mechanical failure in the motor may be contributing to the overload. The
motor bearings may be worn out, they're likely to be sleeve bearings in a
1/3 HP motor, and not replaceable, but all hope is not lost if the motor
shaft isn't scored.
FWIW, sleeve bearings can wear to the extent that the rotor begins to rub on
the stator segments (lots of friction and excess heat).
If the bearing surfaces aren't completely wiped out, re-orienting the end
bells of the motor may provide better bearing surfaces. If the bearings are
the sleeve type, and haven't been oiled regularly, it's likely they would be
worn or damaged.

I can't positively explain why the motor isn't already destroyed, or why the
circuit breaker isn't tripping.

It's very likely that the overloading of the motor is related to air
pressure from the tank resisting movement of the piston in the pump, if the
pump is a piston-type pump.
Normally on small air compressors, tank pressure is isolated from the piston
by a check valve at the tank fitting.
Additionally, (in a piston air pump) there are likely to be reed valves in
the head of the air pump that would separate the cylinder from the outlet
fitting (the tubing constitutes a manifold in air compressor nomenclaure, if
the unit doesn't have a separate part designated as a manifold).
If the air intake is restricted (or choked off from a clogged filter as many
small air compressors only have a piece of wool felt filters), this may
contribute to more load on the motor.

Many piston air pumps have oil lubrication in the pump's case. If the oil
becomes very dirty or the level is inadequate, bearings, the pison and
cylinder can be damaged, resulting in excess friction until a full failure
takes place.

It's possible that the motor's thermal protector is responding to high
current, and if this is the case in this particular situation, it's the
reason the circui breaker hasn't tripped, and likely saved the motor from
destruction.
It's fairly obvious that the restarting actions described are the result of
a self-resetting protective device.

With the power cord unplugged, and the belt removed from the air pump (if
it's a piston-type pump), the pumping action can be checked by placing a
finger over the outlet fitting and briskly turning the pump pulley by hand.
The pump's basic operations of intake and compress/ouput can be observed
with this fairly simple test.

Some of the same conditions included above may pertain to diaphram-type air
pumps, although many diaphram types are direct drive, where the motor and
pump are a single unit, which may make checking the pump's basic operation a
little more difficult.

--
Cheers,
WB
..............



"Sparks Fergusson" wrote in message
...

I'm having a problem with my 20 gallon, single stage electric air
compressor. It's acting like the motor is underpowered - it pumps up
to about 95 psi ok, but then the motor stalls. The unloader bleeds the
pressure off, the motor restarts, runs for a couple of seconds, and
then stops again...repeat...repeat...

snippage

Anyone have any ideas?

Thanks!