Help finding correct size pump for current compressor motor

[DezRanger]

If anyone read this thread http://www.garagejournal.com/forum/showthread.php?t=201317 I ended up getting it for free, long story short wired up motor and it kicked on no problems...so i spent $70 buying new pressure switch, belt, pulley and some wire...hook it all up and pump makes terrible noise (pretty sure broken rod) still built pressure (twin cylinder) but i shut it off after about a min...its will under warranty but getting run around for not having original receipt. Since this is just a home compressor not in a professional setting i was looking at getting a HF pump.

145 psi 3hp
http://www.harborfreight.com/145-psi-3-hp-twin-cylinder-air-compressor-pump-67697.html#

or

145psi 5hp
http://www.harborfreight.com/air-tools/compressor-pumps/145-psi-5-hp-twin-cylinder-air-compressor-pump-67698.html

The current motor i have is 240v 15amp 3.2hp (so it says) RPM 3450

can i switch pulleys on motor to run the bigger pump at the correct RPM or does the motor just not have enought umpff? my pressure switch is also only 135psi so i wouldn't be going all the way to 145

 

[cinco]

My first thought was that it'd be worth it to get the 5HP pump and under-drive it for reduced noise. The problem is, they don't bother to tell us what the diameter of its pulley is, so we can't figure out how fast it'd be running with your current motor and pulley. It's also already rated pretty slow at 655 RPM for 5HP. To get from 3450 to 655 requires a ratio of 5.25:1 so with a 3" motor pulley (bordering on too small...) you'd be looking at a pump pulley over 15" diameter. With a 4" pulley you'd be looking at 21" on the pump and that's nuts. And that doesn't take into account the necessary under-drive.

The 3HP pump is probably already set up for your motor speed (what size pulley do you have on your motor?) and regardless would be much easier to deal with at a ~3:1 ratio. It's likely that at most you'd have to change your motor pulley to hit the rated 1050 RPM at the pump, but with a fast motor and a fast pump the thing is going to sound like it's falling down a flight of stairs when it's running. Neither option is great, but at least the 3HP pump will work without scavenging pulleys off of a... steam tractor? Treadle-power lathe? Where DO you get enormous pulleys? Anyway, you might do some more looking around before settling for one thing or another. It'd be nice to find something rated at ~1000 RPM for 5HP and then under-drive it to ~600, but pay attention to pulley sizes so you know what's going to happen.

 

[Provincial]

Go to HF and look at the compressors on the floor. Check out the motor RPM and HP to see if one has about a 3HP motor at 3450 RPM. Then see if they sell that pump separately. The 3HP pump you listed probably is matched to a 3450 RPM motor. If you can't find one that matches, keep looking at other places.

There are pumps designed to work with 3450 RPM motors, most of which are in the lower-price (and quality) range. Speedaire made lots of pumps that run in that speed/HP range. Better compressors run slower, but are more expensive.

Don't give up on the old pump until you do an autopsy. Often they sound worse than they are! A bad valve can make a lot of noise.

 

[zkling]

I'd pull that original pump apart and see what exactly the problem is before going out and purchasing a HF pump. Never know, it could be something simple with the old pump. They are very simple to tear down.

 

[DezRanger]

thanks for the replies. the pump specs are..

3hp
Rated Air Pressure 145 PSI
Maximum Speed 1050 RPM
Air Delivery @ 1050 RPM 11 SCFM @ 40 PSI 10 SCFM @ 90 PSI
Pump Pulley 12-1/2" diameter with single V-groove

5hp
Rated Air Pressure 145 PSI
Maximum Speed 1050 RPM
Air Delivery @ 1050 RPM 17.3 SCFM @ 40 PSI 15.2 SCFM @ 90 PSI
Pump Pulley 14-1/2" diameter with double V-groove


On my current motor its a 3" diameter pulley so for the 3hp to get it at 1050 I'd need 3.8" for the 5hp 4.4"

I got these numbers (using 5hp for example) by multiplying the Pump Pulley diameter (14.5") times the pump working RPM (1050) which equals 15,225,then divide by the motor RPM (3450). This will determine the motor pulley
diameter (in inches) needed to run the unit.

 

[liquidh8]

hope you get it figured out, I too have thought about getting the 5hp pump for my old compressor with a 3 hp motor.

good luck, keep us posted on your fix.