V-belt pulley diameter/compressor pump question

[Tim The Tool Man]

I have been rebuilding an old American Kellogg model 340TV compressor. It initially had a 7.5 hp 3-phase motor but since I only have single phase I've mounted 5 horse 1-phase motor as a replacement. This motor is a bit undersized for this model pump but it was all I could find at the time.

Anyhow I am having a brain freeze with this conversion. When I began the rebuild project over a year ago I had researched pulley diameters and determined that I needed to reduce the motor pulley from 7" to 4.5." The compressor pump pulley remains 18" in diameter. I reasoned that this would reduce the torque on the new motor sufficiently to allow me to use it without burning it up. Sure it would reduce the pump speed but that is acceptable to me.

Anyhow I recently restarted my rebuild and now I am having second thoughts about the smaller pulley diameter and I cannot locate my original research that lead me to purchase the new 4.5" pulley. I am now thinking that if I rode a ten-speed bike up a hill in high gear my legs would get real tired. Maybe I should be in 1st gear instead which means a larger diameter pulley.

Any thoughts or advice would be much appreciated. Thanks

 

[Jim Johnstone]

If you want to slow down the pump, you need the install a smaller pulley on the motor. Did you do any hunting around for details on that pump? Several pumps I've looked at in the past had manufacturer specs for what RPM to spin the pump for different size motors.

 

[larry_g]

http://www.aircompeq.com/ka.htm

Going from a 7.5 hp to a 5 hp is a 30% reduction in hp. So you need to slow the pump down by ~ 30%. Looking at the chart above you can run this pump at two speeds depending on the working pressure you set the system at. So you need to think about the pressure you want the cut out to be at, 100 psi or 175 psi or something in between. Bottom line is what the motor FLA is and the the amp draw you at at cutoff. You have the option of setting the system to run at 150psi and then if your exceeding FLA turn down the cutout pressure to bring in the amp draw to what you need. Me I would start at trying to run the pump in the 460 to 500 RPM range and then monitor the current and see if you are within window of acceptable current and operating pressure.

lg
no neat sig line

 

[nehog]

Just one comment. Some compressors do have a minimum RPM, something I didn't know at first. You can actually damage them by running them too slowly--they rely on a certain amount of splash lubrication and apparently if not run fast enough the oil won't splash all the way up the (underside) of the cylinder and pistons.

(I don't think this will be an issue with your setup however--just a FYI for the future.)

 

[Tim The Tool Man]

Thanks, this information helps! I found a couple websites that have calculators that helped me find a drive pulley that will get me in the 500 RPM range...

http://www.culvermotor.com/Engineering-Formulas/Pulley-and-RPM-Calculator.html
http://www.temecularodrun.com/ref/rpm_calc.asp

Looks like I need one of these in 5.25" diameter

American Kellogg's have a unique "Ring Flo" oil lubrication system that works fine at lower RPM so that is not a major concern. The centrifugal unloader system had me somewhat worried operating at a lower rpm but I found that it was relatively easy to modify to work at a lower rpm.