Air Compressor Not Starting Properly.

[bborr01]

Hello.

I have a question about my 5HP Sullair 2 stage compressor. I have owned it since about 1980 and it has performed flawlessly until recently. The last few times I turned it on it started slowly. Once it came up to speed it ran like it always did but the next time I started it up it started even slower. I don't want to burn up the motor, which is a Baldor 5HP, 1725rpm brute of a motor.

I suspect it is a bad starter capacitor. I pulled the cover with the capacitors in it off and the caps look fine, no leakage or anything. It has 3 capacitors on it.

Is there a simple way to check them with an ohm meter? I recall checking some other smaller condensers with a meter by reversing the leads and seeing if the meter slowly drops.

Any ideas would be much appreciated.

Thanks,
Brian

[boslab]

It's trying to start under load, there's an unload valve to vent the cylynder, there should be a nylon pipe from the back of the non return on the tank to the starter, when the starter gets pushed or pulled it vents the line from the cylynders to the tank to release the air pressure in the pot, sounds like it needs adjusting
Like this
http://www.about-air-compressors.com...oo-noisy-.html
Mark

[bborr01]

Hi Mark,

The unloader valve seems to be working fine. When it shuts off I can hear it venting off. I think it has some kind of centrifugal unloader that vents by the crankshaft on the opposite end of the shaft from where the drive pulley is. Even with the tank at zero pressure it starts slow.

Brian

[JMS6449]

The unloader can work fine but the check valve at the tank may not be seating and leaking. I had this same problem with a Quincy 25hp compressor.

[bborr01]

It seems like if the tank is empty it wouldn't matter if the check valve was stuck or not. It has an 80 gallon tank so it takes a while for it to build pressure even when it is running right.

Brian

The unloader can work fine but the check valve at the tank may not be seating and leaking. I had this same problem with a Quincy 25hp compressor.

[Black_Moons]

No, reliable tests can not be done with an ohm meter.

That said, you could try replacing the start cap (Should be the highest uF cap of the bunch?), they are not too expensive and these days its more then likely they will die some day, having a spare on hand if its not the start cap would be a good idea.

Alternatively, photograph the connections/label them, and take the caps down to a motor rebuild shop, they should be able to test them.

[flylo]

I'm sure I have a 5hp motor you can use to see if that's the problem. Or maybe time for that 100 hp upgrade you've been thinking about

[bborr01]

Looks like I found the problem. I decided to pull the capacitors out of their enclosure and get them tested or find a way to test them myself. When I pulled out the first one, a 216-259 mfd start cap, one of the terminals fell off. It looks like there wasn't much bright brass where it broke off so it has probably been going bad for a while.

Thanks for the help guys.
Brian

[bborr01]

Update: I installed the new capacitor and my compressor starts fine now. The new capacitor was a little smaller, about an inch shorter and a little smaller diameter than the original one. I'm wondering if the new capacitors are more efficient and will there be any extra stress on the motor.

Anybody have an opinion on capacitor size?

Thanks,
Brian

[sarge41]

Brian: next time you need a new volt/ohm meter, get one that also checks capacitance. Costs a little more, but very handy. I have a Fluke 12B, but there are others these days. More common and cheaper than they used to be. A word of caution, make sure that each capacitor is fully discharged before testing with meter. This is done by jumping across the terminals of the capacitor with a resistor for a few moments. Good luck.

Sarge

[bborr01]

The cap that I put in is brand new and starts the compressor fine but I am just wondering if the smaller size of the new cap means anything. I'd hate to fry a big heavy Baldor motor by using a cap that isn't up to the job. It is rated the same as the old one though.

I have a Fluke meter, I think it is a model 88 as I recall. I don't recall seeing anything about cap testing with it.

Brian

Brian: next time you need a new volt/ohm meter, get one that also checks capacitance. Costs a little more, but very handy. I have a Fluke 12B, but there are others these days. More common and cheaper than they used to be. A word of caution, make sure that each capacitor is fully discharged before testing with meter. This is done by jumping across the terminals of the capacitor with a resistor for a few moments. Good luck.

Sarge

[J Tiers]

The capacitor will start it OK as long as it is the correct value, which it will be.

Yes, they have become more efficient space-wise, and even more efficient in terms of losses and expected lifetime. It's the capacitor that will take the abuse, not the motor. Worry if the motor begins to take a long time to start again.