Generator tripping breaker - confusing me

[Shootinok]

My old trusty RolAir compressor started tripping the breaker. It will fill the tank from empty, then when air drops below start pressure it trips the breaker.
I've replaced both capacitors and the unloader valve - still tripping the breaker.
This is a pretty old compressor but has been a real good workhorse.
The compressor turns easily, no restrictions found, the oil and filter are clean...

What's next ??

 

[Dave455]

I’d give you even money it’s the internal insulation on the motor breaking down.

It may not sound logical, but I’ve known a couple of machine tools have a similar issue. They start o.k. the first time, and are o.k. even as they heat up, but when you try to start hot, they trip.

You could confirm it by putting a megger across it, both cold and hot.

I don’t know about the U.S. but in the U.K. we tend to have RCD’s (residual current detectors) which are very sensitive. These are fine in theory, and very safe, but all these big old motors were designed to be protected by a fuse with a bit of fuse wire, and the insulation probably didn’t meet modern standards when new, and certainly not now it’s degraded for however long!

If you have a conventional circuit breaker you could try swapping it. In the U.K. again, we have “Type B” which are basically domestic breakers, and “Type C” which are industrial rated and will withstand a greater overcurrent. If you’re wiring a workshop with machine tools, not upgrading to Type C breakers invites persistent tripping!

Maybe someone familiar with U.S. electrical procedures could clarify? Trouble is, there could be a lot of other causes, and it’s difficult to diagnose without seeing it!

 

[kngelv]

How many amps is the compressor and what size breaker?

James

 

[Captain Spaulding]

I’d be thinking a bad cold start valve.

 

[Shootinok]

OK, so reading more threads for ideas I decided to try the pressure switch next (also helped that I already had one - no idea where it came from)
The pressure switch was the problem! This new one when it hit 120 psi caused the relief valve to blow off but the motor kept running. A little adjustment and now everything works again as it should.