Air Compressor Will Not Turn Off

[ricpad]

I have a 7.5hp air compressor w/ a magnetic starter. The compressor won't turn off and constantly runs to the point which the safety valve releases at 200psi. At first I thought it was the pressure switch but even when there is no power to the pressure switch it still runs. So correct me if I'm wrong but when there is power to the pressure switch and the pressure is below the set PSI the contacts in the switch connect and send power to the magnetic starter coil (contactor?). The coil then closes the contacts to form a bridge between the 240V input power on one side to the output of power to the starter motor. If ther is no power to the pressure switch would that mean the coil is bad and is constant connection with the power in side to the power out side? When I checked with the power off and a continuity tester both ends of the coil were in continuity. Shouldn't there be no continuity if the coil is not activated by the pressure switch? Any ideas on what can be causing this situation?



Thanks

 

[driz]

Sounds like you need a new pressure switch. Try sanding the contacts first though. They do wear .


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[Garett]

magnetic starter stuck closed? disconnect the pressure switch to test

 

[Jagmandave]

Hang on Driz, he said even with no power to the pressure switch it runs, right?

Sounds to me like the contacts have bridged, I would pull the contactor apart and see if the contacts are stuck.....

Ricpad I think your assessment is correct, just make sure there's no power on the line side when you go to work on it - shut off the breaker or open a disconnect and meter it to make sure before you go in there.

 

[RacerX]

Contacts have welded themselves together. Might have bad caps in mag starter also.

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[ricpad]

Hang on Driz, he said even with no power to the pressure switch it runs, right?

Sounds to me like the contacts have bridged, I would pull the contactor apart and see if the contacts are stuck.....

Ricpad I think your assessment is correct, just make sure there's no power on the line side when you go to work on it - shut off the breaker or open a disconnect and meter it to make sure before you go in there.

Thanks...for the input. I will give that a look. When you say bridged do you mean like they got "welded together"? If so could they be separated or is it just best to go with a new contactor if that is the case?

 

[ricpad]

Contacts have welded themselves together. Might have bad caps in mag starter also.

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Thank you ….I will check those possibilities

 

[BarryWells]

Can you CALL the mfg'r ?

 

[Jagmandave]

Some contactors you can buy replacement contacts for, others you have to replace the whole damn thing....

 

[TuxThePenguin]

Can you CALL the mfg'r ?

To many people, there is literally zero possible benefit to doing that unless it's under warranty.

Don't get why people constantly suggest that. Manufacturers don't support products forever, they typically overcharge for fixes, and it's extremely rare to get technical help over the phone, and even if you manage to, it will probably be after a significant amount of time waiting on hold and talking to clueless people first.

A manufacturer has no more ability to diagnose a bad pressure switch or contactor than we do.

 

[MacMcMacmac]

I've seen 5hp compressors weld starters shut even when they were rated for 5hp at the correct voltage. Some are just junk. Stick to good name brand electrical gear. Although even that is no guarantee these days. Black contacts are also not necessarily a sign of damage. More than one manual specifically warned not to file blackened contacts since they contained silver and were perfectly fine as is. Nasty pitting is another matter and is a good sign the contactor is not up to the job.

 

[Iron Beaver]

I agree about the contactor. I personally use Eaton brand contactors, and try to make sure they are rated at least 1.5x the nameplate HP of the motor, preferably 2x

 

[nadogail]

IMHO, your motor starter is being held closed; perhaps by a stuck pressure switch.

 

[The Cobbler]

I'm in the stuck contactor camp . points have welded closed

 

[ricpad]

Those of you who suspected the contacts welded together hit the nail on the head...or should I say the spot on the weld. One leg was totally welded the other leg had welded and separated but had a big glob of melted metal on the end which was in constant contact. The contactor is rated at FLA 40 the specs on the motor are 208-230v 32.8-31.4 amps 39.3-37.0 SF amps. Would a contactor rated at 50 amps be a better choice. The magnetic starter and 40 amp contactor are what came packaged with the unit.

 

[ricpad]

I've been searching for replacement contactors online and they all seem to have lug connectors on both ends. The one I need has lug connectors on one end and strap connectors on the other. When I run the part number it comes up as the entire contactor and starter. What is this tab called ? Maybe I can purchase the tab to connect the strap and just remove the lugs on one end and replace them with this connector.

 

[JABgj]

Seems you can buy the whole unit, but is very spendy.

https://www.grainger.com/product/SQUARE-D-Definite-Purpose-Magnetic-11W198?

Call this vendor and see if they have the parts you need using their part number for the unit. Good luck.

 

[driz]

Look around. When mine went up on my 220 v CH the company sold them for $65. I literally replaced it with a unit off a well pump for around $20 . Mines been getting hell beat out of it for many years now like that. If it fits and has enough rating it should do the deed.


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[Phantomd]

While not always the cheapest I like to buy those sorts of things from McMaster.

https://www.mcmaster.com/contactors...rrent-relays/number-of-circuits-controlled~3/

 

[seber]

You have a 7 1/2 hp motor on a contactor rated for just that. In my experience that contactor should have been just a bit larger. They tend to push the ratings.

 

[matt_i]

I think you can do better than a Square D DP = definite purpose contactor. Not the finest device that was ever built.

Not saying a Square D won't work with a higher end series but I can't tell you which one.

These days I'd look for an IEC contactor with equivalent HP or if you really want to go big look for a NEMA -size starter...there's a table somewhere. NEMA is very very conservative and should last several generations in a residential shop. It might however drive a different sized enclosure (!)

Automation Direct is probably the best price point for a brand new one.

Just match up the coil voltage and the rated HP when ordering.

As far as your lug dilemma I would just strip wires and land multiple wires under the same screw terminal. Do a pull test to make sure that different AWGs are captured...usually its the smallest size that doesn't quite get trapped.

 

[ricpad]

Thanks to all for your input and recommendations.

 

[dagofast]

If you are looking for a more reliable motor contractor than Square D, I'd highly recommend Allen Bradley.