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Compressor tripping thermal overload

S

strutaeng

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bochnak said:
Same specs on the SB705. I'm getting those specs, I have timed the fill and calculated the CFM.

"Cheaper" pumps at 5hp do get 15-16 CFM. Most of these are reed. I wanted discs.

My motor is 1725.

Eaton replied back and they use a 7.8" pulley (so 800ish pump rpm) and their motors pull 24A. I don't know what motor they are using.

I will try to get amperage on lower PSI. I vapor blast at 60-70psi so I can modify unloader to unload at 80psi or something much lower.
Click to expand...

Ok, you are right, seems like SB PL-735 or similar delivers around 17 CFM @ 175 psi.

Man, piecing together a compressor does get expensive. I know your pain.

At this point you are at a loss no matter what you do. I would say try to keep it at a working range of around 120 psi. I don't know how much you plan to use it, but assuming everything is working properly, a smaller pulley for your motor is least expensive.

I would go that route...
 
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strutaeng

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bochnak said:
7.75 motor
16.75 pump

I don't know eaton's min rpm, however the SB705 literature states 540rpm with 3hp motor.
Click to expand...

So you are spinning the pump around 800 RPM...Doesn't seem too high, but you must know what the specs are.

Too bad compressor manufacturers don't publish pump curves, like water pumps. That way it would be better to match motors to desired CFM. I'm sure there are out there somewhere.

Now, what are you using to measure amps? A clamp-on meter? Can you test it on another motor to make sure you are getting accurate readings. Although seems unlikely if overloads are tripping.

At first I was going to suggest double checking overloads to make sure they are sized properly. There are some tables out there for this.
 
A

American Locomotive

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As someone mentioned, I'd check the capacitors inside the motor. It is likely to have one or two run capacitors in addition to the start capacitor. If the run capacitor is out of spec, it can cause the motor to work harder than it normally would.

Check the belt tension, over-tight belts can cause a lot of extra load. Also check how easy the pump is to turn by hand. Your 15A unloaded seems sort of high to me, and makes me think there might be something else at play here.

Zeke said:
I'd focus more on the 175 PSI. That's ridiculous. At that pressure I assume it's a 2-stage pump or that's even more ridiculous. If you need more air, volume is the answer, not higher pressure.
Click to expand...
175 PSI is a pretty normal pressure for 2 stage compressors. You're supposed to have a regulator, so you can maintain 90-120 PSI while the tank pressure moves around.
 
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sberry

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The people that design and build all these 2 stage comps for decades that set them from the factory must be stupid. They should really put some intrusions in the manuals for people that really know better so they can change the pressure settings on them the first thing.
While they are at it could include some in case someone puts a 5 hp pump on a unit came with a 3.
 
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bochnak

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PROBLEM FIXED!

Run capacitors were shot. Replaced the (2) 15uf capacitors.

Motor also sounds smoother. There was a hum or buzz with old caps.

Went from 15A to 9A in unload mode (intake valves open)

Went from 26A or so to 19.1A at 110psi

Now have 17.9A or so at 100psi

Now have 21.1A at 150psi

Also new cut in/cut off is 110-150, no reason to go higher.

I setup my unloader valve for 90-110 for blasting. Should keep the motor and pump happy.

Ordered caps amazon prime same day for $25. More blasting to do tomorrow.

Motor voltage is 240V when running.

I was able to manually move the centrifugal switch to see if contacts were dropping out the start caps and winding. Works fine.


Here is a crappy phone video of it running and showing the amperage etc...:

https://photos.app.goo.gl/k54DYtWzj6C3riEw9


Matt
 
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Bert_

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Good to see the solution. The OL was actually doing it's job.

Not saying the OP was going to do it but many people go straight to putting in a bigger heater or cranking up the overload right away. In this case that would have fried the motor.
 
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bochnak

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BTW, the overload is a Furnas M40, rated for 19.7-22.6A.

Should I consider installing a new heater given that this one has tripped several times? Do these wear out?
 
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sberry

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Glad to see its working and the actual problem found. Way better than the endless speculation and re engineering. Leave it alone unless it has another problem.
 
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NitroGarage

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I have the same exact compressor, I just rebuilt the top end after scoring it online for scrap price because it pumped up too slow, had a blown head gasket between the low and HP cylinders. Would run and shut off, tripped the OL. Amp reading was 32-33 amps running. Found a new "dome top" run capacitor on top of the motor that shouldn't have been domed :ROFLMAO:. New one inbound. Hope the previous owner wasn't running it for too long like that, but the wire nuts used at the motor side were half melted so I think it likely was for a bit or they had a loose connection there. Wild adventures with equipment!
 
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