Pressure Switch issues

by Steven
(Miami, Fl USA)

I inherited a SpeedAire Air Compressor from my grandfather. It was moved from it's original location at his house to mine. Nothing has been done to it, and it was working fine. When I plugged it in, it starts as normal but starts trying to shut off at about 40psi. The switch is rated at cut-on at 90 psi and shut-off at 120 psi. The motor tries to continue but the overheat mechanism shuts down the motor to keep from overheating. Had anything similar to this?

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Bill says...

Steven, as someone important once said, and I paraphrase as I can't remember who or the exact statement, "there is no try, there is only do or not do".

That's a long way from asking, what you mean when you say that your compressor tries to shut off. The switch either trips the motor off, or it doesn't.

If the pressure switch trips the motor off at 40 PSI and it's supposed to cut out at 120 PSI, my first inclination is to say it's time for a new pressure switch.

However, the overheat issue concerns me.

So first, does it shut off at 40 PSI or not, and what do you mean about it trying to, please?

Bill




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Pressure Switch issues

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Sep 30, 2010
 Reply to Bill - still pressure switch questions
by: Steven Howard

Bill,

The pump slows down a little prior to the pressure switch trying to cut off the motor.

So, it seems like it is trying to do its job and shut down the motor but seems to me that it doesn't quite do the dis-connect.

In other words, I think it disconnects but reconnects all happening in mili-seconds. I found a pressure switch on-line that is a replacement that I'm thinking about buying but wanted your thoughts first.

By the way the motor seems to run fine up until it goes through the on/off stages the pressure switch seems to be causing. I hope this clarifies somewhat. Or maybe I confused you. Let me know!

Steven

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The switch is supposed to turn the motor on a 90 PSI and off again at, i believe you said, 120 PSI. Regardless, the switch should not be reacting to pressure in the 40 PSI range at all.

The switch has an internal diaphragm that has a specific surface area. That surface area depresses or releases as the air pressure in the system changes. Ultimately, increased pressure on the diaphragm will generate enough force that it, against a spring, trips the switch to cut the power to the motor. That is the cut out pressure.

When you use air the pressure in the system drops. As it drops, that diaphragm relieves pressure on the internal spring, to the point where the spring is allowed to trip, and this allows power to flow to the motor and start the compression cycle. This is the low pressure cut in setting.

Neither of these are at 40 PSI, so your pressure switch shouldn't be doing anything at that pressure except allowing power to flow to the motor until the pressure trips the switch at the cut out pressure level.

Having said that, if it weren't for the motor over-heating, I would certainly replace the switch. You may have to anyway.

The motor overheats because it isn't getting enough amps through the supply line (too many things drawing power away from the compressor), or increasing load in the system prevents the motor from working efficiently, and it overheats and eventually, if this happens often enough, burns out.

I don't know why yours is lugging at 40 PSI, and why this should affect your switch.

With the plug out, and the tank emptied of air, I would removed the check valve and make sure it was working right, if it were my compressor.

Bill

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