Any air compressor gurus here?

[Tony]

I reccently rigged an older 325 Quincy. Stuck a new 7.5hp motor, new pressure switch, and all new guages. The pressure switch isnt working properly and Ive walked in the shop twice to hear the pop offs dumping air and way over 200psi. on the tank. It will cycle fine for the first few times from a cold startup but after it builds some heat the contacts dont kickout on the high limit.

The pressure switch is acting as the starter and is wired directly between the source and the motor. The contacts in the switch arc pretty hard when they do separate, and have remained stuck together a few times to the point of having to pry them appart with a screw driver. I put another new pressure switch on and its doing the same thing.

Any suggestions? I'd really like to get this thing fixed before I walk in and find shards of air tank stuck in the side of my boat.:ack2:

 

[rchevelle71]

Sorry I cant help, just bought a new pressure switch for mine, mine was acting the opposite, wouldnt contact. Maybe the motor has too much current draw for the switches you are using, welding the contacts??

 

[Chris]

You must have a magnetic starter on anything that large. You'll see some of the very light duty 5hp units running without one but they go thru contacts. With a 7.5 you're grossly overloading that switch. Plus, you're running it basically unprotected.

 

[Chris]

I probably have several new one's at the shop. Is it single or 3 phase?

Don't worry- your compressor head will stall before the tank will pop. But then the motor will catch fire since you don't have protection on it. How's your insurance

 

[Boss460]

If you have to pry contacts apart they are welding themselves together and can't release. Get a heavier contactor. The contactor also doesn't serve as the release. There has to be a separate relay releasing the contacts. The contactor will have a coil that serves to pull the contacts together. Something has to supply power to that coil and serve as the release.

 

[Tony]

Thanks guys. Ive got a starter on the way.

 

[Madpoodle]

Once you have a starter on it, see if the pressure switch is setup for constant run. If you use a lot of air, the constant run will unload the unit when it reaches et pressure, but keep the motor and compressor turning. On demand, it loads the compressor and builds air again.. 7.5 should have that option..

 

[Ms PatriYacht]

Tony if the starter you ordered does not fix your problem give Ian a call, in his spare time he has been refurbishing and selling air compressors so he knows a lot about them. He is also selling phase converters that convert 3 phase to single phase. He finds brand new or almost new motors and then builds a sturdy electrical box, so far he has done several as large a 25 hp and as small as 5 hp

 

[Tony]

Once you have a starter on it, see if the pressure switch is setup for constant run. If you use a lot of air, the constant run will unload the unit when it reaches et pressure, but keep the motor and compressor turning. On demand, it loads the compressor and builds air again.. 7.5 should have that option..

Some of my monster compressors at work are setup that way. In a workshop application it would definately be benneficial for something like a blast cabinet or a DA sander.


Donna, I'l keep that in mind.

 

[Boss252]

The is a pressure valve on the tank that may be sticking. It is in the tank where the large copper tube comes out of the tank and goes into the compressor head. These do go bad or gets dirty. Drain the tank and take the tubing off the tank and take off the valve and inspect it. It is an easy fix. Just did one last Sunday.

 

[MacGyver]

A 7.5HP electric motor draws 40A single phase or 22A three phase. The National Electrical Code requires that Overload Protection be installed if the motor doesn't have any integral Overload Protection. A magnetic motor starter with properly sized thermal overload unit's (based on the motors full load current listed on it's nameplate) is definitely required.

 

[Tony]

The is a pressure valve on the tank that may be sticking. It is in the tank where the large copper tube comes out of the tank and goes into the compressor head. These do go bad or gets dirty. Drain the tank and take the tubing off the tank and take off the valve and inspect it. It is an easy fix. Just did one last Sunday.

I was wiring the starter tonight and found a remote 1/8" ish copper tube stuffed under the motor/head mounting plate that ran back into where the head discharges into the tank. It had a pencil lead sized plunger plumbed into the end of it. I went and dug the old pressure switch out of the trash and sure enough, that line at one time ran up to the pressure switch where there was a "foot" built into the contacter bar of the pressure switch that pushed the tiny plunger in the line down when the contacts opened. The new pressure switch isnt made the same way. Is this tiny little plunger made used for a contunuous run setup or something different?

At this point I'm convinced that my local go to compressor guy is a complete bufoon.

 

[Chris]

That's the unloader. When the pressure switch reaches max pressure it trips open and the lever depresses a little valve. That dumps the pressure between the second-stage exhaust valve and the compressor's foot valve and the line that runs between them. This is so the pump gets a rotation or two without being loaded against tank pressure. It reduces the starting load of the motor substantially- especially if the pump stopped at the bottom of its second stage's stroke.

Most Quincy heads have either a hydraulic or a governor unloader. I have a 325 in my garage at home and several old one's down at the shop. if you need parts or a picture, I probably have it.