Sparks Fergusson wrote:

Alan Douglas wrote:

And all those replies ignore the fact the unloader is operating (at a
wrong, low pressure) which certainly doesn't point an evil finger at
the motor at all...


Hmmm, yes, I think that wasn't in the original post so I missed it.
That would account for gradual deterioration too.

The unloader is functioning properly, as far as I can tell. It's a
combination check valve/unloader. So, when the check valve is open, it
allows the high pressure air into the tank. When the check valve
closes, it vents the pump side to atmosphere.

So, then the motor stalls, the check valve closes and the unloader
part releases the pressure. When the presssure falls sufficiently, the
motor restarts and the cycle continues.


It shouldn't. The unloader is supposed to open when the motor first
starts, to let it come up to speed. Since the motor is stalling, you
either have a bad pressure sensor, or the run capacitor has lost some or
most of its capacitance. The pressure sensors deteriorate with age, and
the calibration goes to hell.

The capacitors have to pass a lot of current, and aren't made to last
forever. They are AC electrolytics to keep the size and cost within a
range you can afford to build and sell a product. A film capacitor
would be quite large, and can caost more then a new compressor.

Some compressors have two capacitors. One in the start circuit, and
the other in series with one of the run windings to provide the required
phase shift.


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