Air Compressor Motor Problems

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As I was in the middle of 2 hour media blasting session Friday night I noticed my compressor hadn't kicked in to pump up the pressure (it's in an adjacent room so I faintly here it cycling).  It's set to kick-in when the pressure dips to 110PSI and when I walked up the compressor the gauge was already down to 90PSI.  Flipped the pressure switch manually and nothing.  Went to the electric panel and sure enough the circuit breaker had popped.  Reset the breaker, walk over to the compressor, turn it back on and the motor doesn't turn, just makes a loud hum ( and I mean loud). Quickly turn it off and think...OK...maybe the centrif switch isn't engaging,  turn it back on and cautiously give the flywheel a spin...same result, no motor spin, just hum.  OK...so maybe the start cap went bad.  Unplugged the compressor and tested the caps (there are two in series) and darn it they both test good.  Uh oh, eliminating the easy stuff is leading me to a bad place. Man, I don't want t believe the motor could have cooked .But before I go down that roadI I do need help in identifying these other appendages on the motor and see if they could be contributing to my problemThey're on the opposite side of the start caps.  Any ideas? And if so any testing advice?Here's the motor nameplate infoAny advice would be appreciated.Trublue
Reply:Although you have a much larger compressor than I do, I had the same problem. I also had one at work do exactly the same thing as well. Generally, an electric motor either works or it doesn't, they generally won't "run on 5 cylinders", or diminished capacity. Turned out in my case that for both compressors, the check valve was toast. The check valve prevents the air pressure from the tank pushing back onto the pump piston, holding it down at 125 or whatever PSI your tank is running at. Check valve is on the tank at the bottom of the copper or Alu tube that feeds from the pump. The motor doesn't have the gusto to push past that initial load. The part is under $10 at HF (at least the 2 that I bought). The humming of the motor says that it's working and wants to run, but can't get past the pressure and that's what is tripping your breaker, remember that the starting of the motor is when it draws the highest amperage. For the time being, you will have to let the tank pressure drop to the point where the motor can push past the pressure, in my case both compressors started at around 60 PSI. Take off the belt that's driving the pump and run the motor just to make sure that it is working properly. I doubt that your pressure switch is faulty, it is kicking in at the correct pressure. Eliminating one issue at a time should help you pinpoint the issue with relative ease. Let us know how this turns out, hopefully this is the answer you seek, IMHO.  Last edited by bearston; 04-07-2013 at 08:24 PM.When a welder tells you to "stick it", what do they really mean?"In a time of universal deceit, telling the truth is a revolutionary act." - George Orwell
Reply:Maybe its not the electric motor? If the pump froze, it will pop the breaker and hum the very same way. Remove the belt and see if the wheel turns.
Reply:@ Deve/Bearston - Thanks for the replies but the check valve is new and the motor and pump can be spun manually.  From my past experience with AC motors the hum menas that the caps are bad, the centrif switch is bad or the start windings are toast.  But i'm open to others advice.Still realy curious as to what those three "thingys" are in the picture.Trublue
Reply:Pull the belt, spin the motor and see if it starts. Sounds like start windings are toast, or cent switch, or start cap. Your other capacitors are run capacitors.
Reply:Sounds like a problem with the starter winding, more specifically it's not engaging due to poor contacts or a worn centrifugal switch mechanism. I'd pull the motor apart if it still does it with no physical load.Welding/Fab Pics: www.UtahWeld.com
Reply:Originally Posted by Imtrublue@ Deve/Bearston - Still realy curious as to what those three "thingys" are in the picture.Trublue
Reply:You may have a problem with a start switch inside the motor too.Miller thunderbolt 250Decastar 135ERecovering tool-o-holic ESAB OAI have been interested or involved in Electrical, Fire Alarm, Auto, Marine, Welding, Electronics ETC to name a just a few. So YES you can own too many tools.
Reply:All good suggetions...but....there is no load presently...I can spin the motor and the compressor flywheel freely and I tried spinning the motor with the power on ( and it humming) and the motor did not engage, which in my experience suggests that the centif switch is not the problem.  The picture of the "thingys" are not the start caps.  The start caps are the traditional round caps that you would normally expect to see, I just didn't include a picture of them but they are just left of the center of the motor from the "thingys".  I did test the start caps with a tester that has a cap test function and they tested within 5% of there posted rating (747MFD/120v).There is 220v at the recep as it is shared with my dryer (and I've run the dryer since then).I think "Walker" might have hit the nail on the head.  I was wondering if they were run caps.  All the markings are worn off so I can't get any ID from them, so I don't know what to test for.  I suppose I could take the motor off and take it down to a motor shop but labor rates are pretty high in my part of the woods.  Might be worth it to buy a new Baldor for $375-400 and be done with it.TrublueLast edited by Imtrublue; 04-08-2013 at 01:09 AM.
Reply:Originally Posted by ImtrublueI can spin the motor and the compressor flywheel freely and I tried spinning the motor with the power on ( and it humming) and the motor did not engage, which in my experience suggests that the centif switch is not the problem.  Trublue
Reply:Originally Posted by ImtrublueI can spin the motor and the compressor flywheel freely and I tried spinning the motor with the power on ( and it humming) and the motor did not engage, which in my experience suggests that the centif switch is not the problem.  Trublue
Reply:Air compressors are like welders they have a duty cycle - should not be started morethan 6 times an hour - Due to a rush of high amps when starting they wont hold upThe main thing is not to panic or get excited Bobcat 250, X-Treme 12VS,  MM211Meltabo, Milwaukee,Porter Cable,Dewalt,MakitaVictor O/A, Ingersoll-RandEvolution Rage2, 40 amp PlasmaLincoln 225 AC/DC
Reply:Originally Posted by RenagadeAir compressors are like welders they have a duty cycle - should not be started morethan 6 times an hour - Due to a rush of high amps when starting they wont hold up
Reply:Those things are caps. You can check a cap with a multi meter,  can find out if they are shorted or open. Cap wires removed, using the ohm scale put the leads on the terminals for a few seconds, then reverse the leads if the cap is working you will see the meter needle deflect up and then drift back . This test shows the cap to be working as in charging and discharging.If shorted placing the leads on the terminals needle will deflect up and hold and switching the leads it will again do the same... cap is shortedIf open placing the leads on the terminals the needle will do nothing,  and switching the leads the same, needle dose nothing no movement... cap is openGregMagazines have issues, everything else has problems
Reply:WHEW HOO!!!!  Success   Thanks for all the advice.I opened up the motor and found the wire from the start cap to the centrifugal switch had been severed (no idea how that could have happened, must have been damaged by the previous owner).  Had to replace the wire and resolder to the existing 90 spade clips on both ends.  Cleaned the centrifugal contacts while it was open. Put everything back together and she started right up and is purring like a kitten.Stopped at the compressor store at lunch today and bought a new pilot valve.  The original was not working correctly and I had disabled it last week, which may have contributed to stressing the damaged wire.  So now my compressor loads at 90PSI and unloads at 115PSI and the motor continues to run, instead of cycling off and on when I'm using the media blaster. Again...thanks for all your suggestions and advice.Trublue
Reply:Glad you got it fixedIf it's not to much trouble could you post a picture of the part you changed - on the pumpsi have worked on the pilot valve only unloaded the compressor when power was shut offthey were mounted in the front cover of the pump ( frame end cover ) and worked off of a set of weights mounted on crankshaft.The part that allowes the pump to continue to run at full pressure is what we call a discharge unloader or constant speed control Just like to see how it compared to our's. Like everthing else things change over the years. We no longer supply pilot vlaves in our small units, it's done now with an unloading pressure switch.        THANKS And once again glad you got it fixedThe main thing is not to panic or get excited Bobcat 250, X-Treme 12VS,  MM211Meltabo, Milwaukee,Porter Cable,Dewalt,MakitaVictor O/A, Ingersoll-RandEvolution Rage2, 40 amp PlasmaLincoln 225 AC/DC
Reply:Now that's better ................. easy fix, those pesky wires breaking like that.Magazines have issues, everything else has problems
Reply:@ RenegadeHere's what the unloader pilot valve look like.  This was the only model the store had in stock at the time. It would be right at home on a gas compressor to unloade the pump heads in order to pull the motor starter.It's attached via the black air line to a T fitting and to each pump cylinder.As I mentioned previously my pump uses this unloader valve to hold open the intake valves after reaching the set presuure and allows the motor to continue to run, thus avoiding the stop/start cycling.
Reply:Originally Posted by ImtrublueAll good suggetions...but....there is no load presently...I can spin the motor and the compressor flywheel freely and I tried spinning the motor with the power on ( and it humming) and the motor did not engage, which in my experience suggests that the centif switch is not the problem.  Trublue
Reply:Thanks for the pictures - That valve is what we know as a csc valve - constant speed controlwhich allows the pump to reach a set pressure then valve opens and lets air go to the unloaders in the heads OR you can pull the little lever on end upto were it is parallel with valve, this will allow the compressor to keep pumping tillthe pressure switch shuts it off.  But the way yours is piped up, you can turn the little ball valve off and do the same thing The little unloaders in the head look like a fork if you could see the bottom sidedepending on the number of inlet valves on the valve plate it could have 2 to 4 fingers Thanks again for the pictures, always enjoy seeing how The other brands do theirsThe main thing is not to panic or get excited Bobcat 250, X-Treme 12VS,  MM211Meltabo, Milwaukee,Porter Cable,Dewalt,MakitaVictor O/A, Ingersoll-RandEvolution Rage2, 40 amp PlasmaLincoln 225 AC/DC
Reply:Just curious, what advantage do you think you are gaining by letting the motor run constantly? If you are sandblasting the compressor is likely to run constantly anyhow. You certainly would not want to just let the motor run all day while working in the shop only using air intermittently, so it seems like it would be a big nuisance.
Reply:Their would be no advntage in that - About the only time that the constant speed control would be used is if the pump is kicking on and off more than 6 times an hour - Then you would need it to run on to help cool the motor down so it dont over heatThe main thing is not to panic or get excited Bobcat 250, X-Treme 12VS,  MM211Meltabo, Milwaukee,Porter Cable,Dewalt,MakitaVictor O/A, Ingersoll-RandEvolution Rage2, 40 amp PlasmaLincoln 225 AC/DC