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  #1  
Old 04-18-2008, 04:04 AM
chvyfan chvyfan is offline
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Default Bad run capacitor or ?

I have an older Campbell Hausfeld Air compressor (2hp) that I bought probably 20 years ago, when they were pretty good quality.

I used it pretty heavily early on restoring one of my first cars. For years I've had problems with it being able to start. As I flip the switch to turn it on, it tries to start, turning probably less than 2 revolutions, then stalling for a few seconds then pops the fuse. I always thought it was the motor getting weak. It acts as if the oil is REAL stiff and thick, and the motor doesn't have enough power to start it. My solution has been to drain the air compressor tank, and baby the starting. After about 10 - 20 times or so of trying to get it to start and having to switch it off before it blows the fuse, it will finally start and run. Once the motor is warmed up, it starts OK, as long as it isn't forever before it needs to start again.

After doing some research online this weekend, I thought it was actually the start capacitor. (This is my first experience with capacitors.)

I found the equivalent start capacitor at Grainger for $5 and replaced it, with high hopes it would sove my problem.

No such luck!

Now I'm wondering if it isn't the Run capacitor? I think I've found a replacement for this to, and am willing to invest the additional $10 to see if it works, if someone can give me a 50/50 chance.

I found the following text on the capacitor:
20 uf 370 vac 60 hz

I also found this on Graingers site:
Run Capacitor,20 MFD,370 VAC
Motor Run Capacitor, MFD Rating 20, Voltage 370, 60/50 Hz, Round, Dia 2 In, Height 2 7/8 In, Width 2 In, 1/4 In Male Terminals, Temp Range -40 to 69 C
http://www.grainger.com/Grainger/items/2GU23

Is this the same spec? It looks the same other than the 20 UF vs 20 MFD.

Does it sound like this could be my problem, and would you take the chance on getting another new part? I know it could also be a motor winding from what I've read, but since I can eventually get it to run, I'm thinking the winding must be good?

Any help appreciated!!!
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Old 04-18-2008, 04:15 AM
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thingmaker3 thingmaker3 is offline
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Can you tell if the centrifugal switch is working? Do you ever hear it click over when the motor gets up to speed?
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Old 04-18-2008, 10:32 AM
jpanhalt jpanhalt is offline
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The caps and motor are the first place I would look, just as you have. Another place is the pressure release valve. That valve is probably located where the tube from the cylinder head enters the tank. With the V-belt off the motor, does the motor start normally? If there is no V-belt, you might have to remove the pressure tube at one end for the same test. Based on what you describe, the pressure release would not be my first guess, but it is another factor to consider. In my experience, one gets less than a full revolution (i.e., it stops on compression) when the pressure release is bad. John
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Old 04-18-2008, 11:26 AM
chvyfan chvyfan is offline
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Thingmaker3 - I wasn't familiar with a centrifugal switch is, but looking at a quick wikipedia definition, I do think it is working. It does seem that once it gets to a certain speed, it gets some extra ummpphhh.

jpanhalt - I don't think it's the pressure relief value for 2 reasons: I hear a hiss once the motor shuts off and since I bleed the tank dry and it still struggles it wouldn't seem like the relief value. One test I should probalby do is to actually leave all the ports open when starting it too (not just zero pressure). I'll give that a shot this afternoon and post if it really fires up. I suspect it won't, just because even with zero pressure, but closed ports, it still struggles. It has a 20 gallon tank, so it would take several full revolutions before it really got any pressure at all.

Thanks!!!
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Old 04-18-2008, 12:07 PM
jpanhalt jpanhalt is offline
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I agree. The relief valve is not my first guess. I mentioned it, because I had an old C-H compressor and that valve was a constant problem. As an aside, I did get the unit 1/2 price, because the dealer heard the hissing and thought the tank had a leak. So much for knowing what you are selling! John

PS: If it is belt driven, I would try removing the belt. That will surely help pinpoint the problem.
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Old 04-18-2008, 03:03 PM
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After you remove the belt to see if the compressor is not seizing up. Turn the pulley by hand.

I would pull the end cap off the motor to see if the centrifugal switch contacts are not dirty/cruded up..and to look for other loose over heated wire connections.

Cap meters are nice if you have one, but if you get an ohm meter on your disconnected caps, short the terminals and check to see if they start out low ohms and rise to open/infinite over several seconds.

It won't give you an accurate measurement of value but will tell you if it is open/shorted.

Alternative crude test is with another known good cap of near equal capacitance can give you the number of seconds required to get to a certain resistance.

For instance, if one 20 uF cap takes 10 seconds to reach 10M ohms and another takes 30 seconds then one of them has changed value. Maybe both are wrong, A man with two watches does not know what time it is, however, if you compare to known good cap then a ballpark of sorts can be achieved.

Also these static tests do not account for stress of several hundred volts pounding the plates of the cap while in operation, which could break down the electrolytic .

cheers
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Last edited by pfofit; 04-18-2008 at 03:12 PM.
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Old 04-18-2008, 04:10 PM
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What is the condition of the power cord? It may need replacement. Starting loads will be quite heavy, near the capacity of the cord when it was new. As the power cords get older, used & abused, the stranded wire inside can break due to all of the flexing and stress. The connections on the plug itself can get corroded. The outlet you're using may also be corroded/pitted.

It's also possible that the contacts on the pressure switch have become fouled/pitted.

Monitoring the voltage on the motor side of the pressure switch during starting would verify whether they are good or not.
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Old 04-18-2008, 06:37 PM
techroomt techroomt is offline
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i would say a "belt off" test is not necessarily definitive. i just had a jointer motor with a bad start capacitor start fine under no load. if your motor is that old and under dusty conditions, i'd pull it, open it up, and blow it out. check out the centrifigul switch/plate and contacts as mentioned. replace both start and run caps, electrolytics go bad after those many years.
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Old 04-19-2008, 04:45 PM
chvyfan chvyfan is offline
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OK so I tried it with open ports - still hesitated, though it did start (realize though that the compressor kicks back on at 80 or 90 psi, so zero psi isn't adequate).

I have a direct drive compressor, so no luck starting it by hand, plus I think since I replaced that start capacitor, it wouldn't prove much (unless my new start capacitor was bad.

I think what I need to know first is if the run capacitor was bad, would it still run once up to speed? Again, if I really baby it, I can get it to start. I am sure the centrifugal switch is good - I can see a spark as it switches over once up to speed, and I can hear it click back on shut down.

Once it has run for 30 seconds, I can get it to start no problem (almost like I would say the motor, pump or oil has warmed a little). I'm almost certain the pump is good, as it has been doing this for years, and if it was bad it would have surely given out by now.

One hard part in diagnosing it is once it "warms up" even a little, I have to wait hours for it to "cool" down before testing again.

At this point, since it runs once up to speed, I'm almost assuming the run capacitor is good. I'll double check all the connections for a tight fit.

One other thing to note - the cord at the outlet does get warm to the touch. The thing I would say to counter that is the compressor hasn't been moved much. It's lived in two homes, and pretty much been in the same spot in both the whole time - ie not much flexing of the cord.

I also fully cleaned out the motor before my post - I had suspected worn bushings, but none in the motor. Cleaned it out really well at that time.
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Old 04-19-2008, 06:30 PM
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If the cord is getting warm, that's not good. That means you're dissipating power in the cord or plug or wall socket instead of in the compressor motor.
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