Symptoms Of A Bad Or Failing Air Compressr 'run/in-rush Capacitor'

[countryguy]

Due to the failure of several old hand-me-down 20 Gal compressors, I found a local estate sale deal (steal) on a 100Gal PUMA. Date of mid-80s. Single stage, 21.6 CFM It's a 5HP motor 220Vac Upon inspection it did run, but when I got it home:

1) The unit is poping 30amp slow blow's like Candy? My other 5HP 220V 20gal runs on a 20Amp no issue. I'm wondering if the run Cap is dry or going bad. Any known symptoms? the compressor flywheel turns with minimal resistance. There is only 1 cap housing. I do not think there is a run cap, just a start cap actually. Possibly a dual cap in the housing? (start and run?). Not my wheelhouse there. What would you anticipate the start up Amps to be?

2) I dumped at least 5 of water from it when I got it home. Drain nut was clogged. Prob sitting 10 years that way! So from a tank & rust perspective, is there anything I can dump in or spray in to coat it? Running a Plasma table demands clean air.

TIA,
CG.

 

[JimDawson]

I wonder if the centrifugal switch is welded shut and not switching to the run windings? A 5 HP should draw around 25 Amps on 230 volts at full load. You might pull the motor down for an inspection and cleaning. Replacing the caps would not hurt.

An internal inspection with a camera might be in order. Look for serious degradation and pitting of the tank. As far as clean air for the plasma, good external air filtration and a refrigerated air dryer would be the ideal system. If that is not in the budget, then a good water separator and a couple filtration stages would be the next best.

 

[John Hasler]

Due to the failure of several old hand-me-down 20 Gal compressors, I found a local estate sale deal (steal) on a 100Gal PUMA. Date of mid-80s. Single stage, 21.6 CFM It's a 5HP motor 220Vac Upon inspection it did run, but when I got it home:

1) The unit is poping 30amp slow blow's like Candy? My other 5HP 220V 20gal runs on a 20Amp no issue. I'm wondering if the run Cap is dry or going bad. Any known symptoms? the compressor flywheel turns with minimal resistance. There is only 1 cap housing. I do not think there is a run cap, just a start cap actually. Possibly a dual cap in the housing? (start and run?). Not my wheelhouse there. What would you anticipate the start up Amps to be?

2) I dumped at least 5 of water from it when I got it home. Drain nut was clogged. Prob sitting 10 years that way! So from a tank & rust perspective, is there anything I can dump in or spray in to coat it? Running a Plasma table demands clean air.

TIA,
CG.

I'd want to see the motor nameplate, but most likely you need at least a 40 amp breaker (if you use fuses you might need to go as high as 50 or 60 amp time-delay) and #8 wire. Peak starting current could be as much as 75 amp. I suspect that your other compressor is 5 "consumer" hp.

 

[jim18655]

What's the motor name plate amp rating? The other compressor motor might be over rated HP wise. Some manufactures have been known to boost the HP on the compressor. The NEC Table 430.248 calls for a 28 amp branch circuit rating for 5 HP 240 volt motor. You might need a 40 amp circuit and 175% slow blow fuses for that motor.

 

[Nammar]

CG, While I'm not an electrician, my understanding of single phase electric motors is that they are wound with two windings, one being the running winding and one being the start winding. The start winding is connected to the input single phase wire through an internal centrifugal switch and a capacitor. The running winding is just connected straight to the input single phase wire. The capacitors function is to create a second phase, which is about 60 degrees out of phase with that of the incoming phase. It is the start windings that determine the direction of rotation of the electric motor, as the start winding phase will either be about 60 degrees ahead of the incoming phase or about 60 degrees lagging the incoming phase, depending on how you connect the start winding. When the external power supply is switched on, power goes to both windings and when the single phase motor comes up to speed, then the internal centrifugal switch opens and the electric motor runs on the running winding only. Momentum is what keeps the motor running and is why the start winding is disconnected after start-up.

With your electric motor pulling so many amps, it may just be that the internal start winding switch is not opening the start winding circuit, thereby causing a higher amperage draw than what the motor is supposed to draw while running.

Further, there should be an unloader on your compressor that prevents the compressor from compressing air until it has come up to speed, further preventing high start loading, especially when re-starting, to top up the air pressure in the cylinder.

With you having found a lot of water in the cylinder, it is strongly advisable to have the air receiver chemically cleaned internally and then hydraulically pressure tested to 1.5 times safe working pressure. Just a visual internal inspection may not be enough. I use phosphoric acid as a rust remover. I fill the air receiver with 4 gallons of phosphoric acid and then fill it with water and let it stand for about two days. After that I drain and flush the cylinder and look internally for deep pits. (If there are inspection ports.) If the internal condition is good, then I coat the internal parts of the cylinder with a paint, by just pouring in thinned down primer and rolling the tank. After that two coats of topcoat go in also by rolling the tank. You may want to pre-dry the tank using compressed air blowing through the receiver and between coats of paint.

Once you are happy with the internal condition of the receiver, you need to refill the receiver with water and hydraulically pressure test the cylinder to 1.5 times the safe working pressure of the air receiver, using a hydraulic water pump and test pressure gauge. This will give you the safety margin that you need to prevent catastrophic failure of the air receiver, while in service.

I've seen the results of a ruptured pressure vessel and they are not nice, to say the least. A local scuba club member went to the happy hunting ground refilling a diving cylinder. While a back yard repair shop or a home shop may not always be subject to government control, with regards to pressure vessels or lifting equipment, it is wise to treat all pressure vessels with a high degree of respect, as when they bite, they bite big. Geoffrey.

 

[countryguy]

Thanks for the write ups everyone. I hope to get a few hours in tonight to wire it up and get into the tank ports. Appreciate the noted on the bath and paint process.

 

[countryguy]

OK, rather late last night I was able to wire in the 50Amp breaker and get the unit cleaned up some. I have the motor badge image and a tank image. The motor is a bigg'n at 25Amps so I am sure my 30Amp fuses were a lost cause. I really did not check the motor plate. My fault! On the 50Amp she starts just fine but have not run to full PSI till I inspect the tank. I'll flush and open the lower inspection port this week. Still trying to get my first cuts on the CNC mill too! Ohh how the projects are stacking up LOL.

Nammar, Thanks for the great write up! I did see the cut-out happen after start up. Very cool now that I know what's going on there! Learning learning learning. Thanks!

For the CNC Plasma air quality setup I'll see what I can find for inexpensive filter items. Time to take a break and get thru the spring and home projects plus play w/ the Mill. ;-) Thanks everyone for the help! This 100gal is a bit scary knowing the damage it can cause. I'll check it over and test soon!
Thanks!
CG