compressor motor tripping breaker

[a]

I have an older model electric motor driven compressor that causes my electrical circuit breaker to trip when the motor kicks in to "refill" a low pressurized tank. In other words, when the tank is empty and I plug it in, it starts and works fine, filling up the tank, then stops automatically when full. After use or when the tank's pressure becomes low, the motor attempts to start the compressor again, but is not able to. The motor hums for a short while, then trips the breaker.

The circuit is one a 15 amp breaker, the motor draws less than 11 amps. Any ideas?

[Kyle Boatright]

Have you really checked the motor draw? It can be substantially higher at start-up than in a steady state situation. I experienced breaker trips for years in the exact same situation with my compressor. My solution was to run a dedicated circuit to the shop for the compressor. I think I used a 25 or 30 amp breaker and sized the wire appropriately.

KB

[Greg O]

On all compressors there is some sort of unloader that releases the air pressure at the head or discharge of the compressor pump. My bet is your compressor's unloader has failed, and does not dump the air at shut off. Some units have this unloader tied in to the pressure switch, others use a valve at the tank that looks like a plain old fitting. Greg

[PrecisionMachinisT]

Ditto.

The function of the unloader being so the motor doesnt start under load--allows it to get up to speed before having to do any serious work, as motor startup draws a buncha amperage as is.

Oftentimes the unloader valve gets clogged with crud from the compressor oil and simply need cleaning in solvent of some type.

Usually a properly functioning unloader will release a small amount of air each time the pump cycles off and it makes a short hissing sound at this time.

[Charles Spitzer]

automatically

warning: thread hijack for another compressor question

i also have one that has a problem. it will go on, charge up the tank to 125 psi and shutoff correctly. however, the relief valve under the electronics will then open, vent to 75psi, close, and the motor will go on again.

there is another safety release valve screwed into the top of the tank. that one isn't the one that lets go.

is this also a failure in the unloader? there's a plug in the top that has 2 tubes coming out of it. one goes to the motor, one to the valve that's giving me the problem. does this plug have some sort of diaphragm that could be bad? the schematic i have of the compressor doesn't even have this part listed, so i don't know what it does.

thanks, charlie

[Greg O]

With the unloader that is controlled by the pressure switch, there is a one-way cheack valve in the compressor discharge line, before the line enters the tank. Your check valve is stuck open. The function of the check valve is to stop air from leaveing the tank, flowing back through the discharge line, when the unloader is working. Greg

[Charles Spitzer]

thanks. any idea where it could be, or if it can be fixed? what would it look like? there's no check valve on my parts list.

regards, charlie

[m Ransley]

Unloader or not a motor on a compressor can take 9x surge on an old worn motor- compressor. Test your surge laod, wires etc.

[Greg O]

What brand of compressor? My guess it would be right where the pump discharge line enters the tank. Greg

[PrecisionMachinisT]

message

Agreed.

Probly a brass plug looking thing screwed into the top of the tank, 1in pipe threads or so usually--the compresser generally delivers air to it through copper or aluminum tubing.

[dixie-chicks-fan]

It could be you have a weak breaker. Swap it with another one in the box (of the same amperage), or just buy another one and try. At most you'll be out the cost of the breaker, usually around $5, and you'll have a spare breaker. Motors consume huge amounts of power during startup. most breakers allow for that. Your breaker may just not be able to handle it. You could also have a weak/dying capacitor in the motor.

[DanG]

The problem is with the compressor.

It is either the unloader or the back check valve, more likely the back check (on some compressors the same piece performs both functions). This part is located where the compressor discharge line enters the tank on many units. Your compressor is trying to start against the pressure in the tank for which it was never designed.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

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compressor oil

amount of air

[Charlie Spitzer]

there is a plug screwed into the top of the tank on the side that isn't visible in this picture. however, that part isn't listed in the parts list that came with it, so i don't know if it's orderable. is this fixable, or jury-riggable with another check valve?