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Air Compressor RPM? Answered

I found an old cast iron 50 + year old 2 stage air compressor and I was wondering what RPM's I should run it at.  My dad says it is running much slower than he remembered and it is going 323 RPM's compared to the original gear ratio that would have run at 269 RPM's.  That should be dramatically faster, so could going too fast decrease performance, or something, or is there a reason the original motor may have slowed down? The RPM's are a complete guess based upon the 1725 RPM motor average.  The only reason I had to change pullies was because the original had FLAT BELTS (awesome) from back when people cared and build stuff right.  Now I have a v-belt on it and the compressor pulley wobbles by like 2 inches and the thing is out of line, but these shouldn't be slowing it down.
Basically I need to know what RPM's I should be running the compressor at.

P.S.  It can double as a vacuum  pump and it uses old fashion check valves!


The three fastest possible no-load speeds for a AC induction motor, with a power line frequency of 60 hertz, are:
3600 RPM (for a 2 pole motor)
1800 RPM (for a 4 pole motor)
1200 RPM (for a 6 pole motor)

Assuming a "slip" of about 4.2%, those numbers become:
3450 RPM (for a 2 pole motor)
1725 RPM (for a 4 pole motor)
1150 RPM (for a 6 pole motor)

Assuming the original motor was a different motor than the one you're using, it might have been a 2-pole, instead of a 4-pole.  In that case the motor speed should be 3450 RPM, not 1725 RPM.

If you are using the same motor as before, then I can't guess at a reason for the suspected slowness.

Also ask yourself if you are satisfied with how fast the compressor is pumping air for you? I mean if the thing is running, and the motor is not turning so slowly as to make it stall, and it is not running too fast, then maybe it's all good.

BTW,  the formula for determining induction motor speed, from power line frequency, and number of poles, is:

no-load speed (in rev/min) =2*(60 s/ min)*(frequency)/(number of poles)

And this page explains that a little bit:

It takes 25+ minutes to pump up, I am not satisfied at all. I spent a lot of money on this thing and I was expecting it to be really fast. compared to my little single stage one, but it isn't even close. Do you think it would be hard on anything to run at 600 RPM's, or should it be ok.

I am guessing it would be ok at 600 RPM.  I mean a car engine is basically an air pump(a single stage kind), and those run at a few thousand RPM. 

I mean you can sort of see where this goes.  If you run it twice as fast as you're running it now, it will take 12+ minutes to pump up, right? 

Also I am guessing that twice the current speed is the fastest it could have possibly run, based on my previous reasoning of 3600 RPM being the fastest possible speed for an AC induction motor.

If you want to try increasing the speed in increments smaller than just doubling it, uh... I'm trying to think how you would do that.  Maybe some successively smaller pulleys?  

I mean that would be the way to proceed if you're too chicken to try just doubling the speed, and I do not blame you if want to be cautious. Especially since you say you spent a lot of money on this compressor.

V belts are MUCH higher power rating than flat belts.

Any idea of the CFM rating of this compressor ? If you know that, and you can measure the displacement easily, you can work out the RPM


No such information included, I don't know if they cared back in this thing's day, the just assumed, it pumps in 5 minutes, ITS A FAST ONE

TBH, keep it well lubricated, and go for it.

Without more information, its going to be difficult for anyone to tell you what RPM it should run at. What I would suggest you do is get a service manual for it (if you haven't already). Failing that, I would take it in to someone who's knowledgable with older air compressors and have it fully inspected and serviced. I believe that an incorrect belt can affect its performance, as well as a number of other things (especially if it hasn't been used in a while, or maintained).

It was maintained great and stuff, but hasn't been run for 40+ years. I cleaned it all up and restored it as best I could. Any information on it is non-existent, the company that built it made custom compressors, in Chicago. It came to Colorado brand new and has been here ever since. The company that made it is all but lost among all reaches of the internet and around here nobody knows any more than I do about old compressors.

That's unfortunate. The reason I suggested the servicing was to check the filter(s), seals, valves and to ensure that internal parts are still well lubricated and moving freely. Even a well maintained compressor, sitting for 40+ years will dry out and may have a lot of dust inside etc. all which can affect how it runs now.

Being a custom compressor, the chances of finding a service manual is probably near impossible. However, there are many other sites that specialize in compressors, and posting photos and as much info as you can may yield more advice on what to do to get it running properly.