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Electric motor and pulley sizes

A

Anonymous-0

Well-known Member
I'm trying to piece together an air compressor and trying to use a 1750 rpm motor where a 3450 was. The 3450 had a 4 inch pulley so do you go to an 8 inch pulley for the 1750 and get the same speed?? Or just use the 4 inch and the 1750 motor and be a little slower on recovery. thanks chris
 
I don't have the old motor but the tag on the compressor says 7hp peak but new compressor like this one says 3rhp thinking running horsepower?? I'm just trying a 2 hp farm duty Dayton for laughs. chris
 
I would try a 6" pulley, should make the actual rpm of the compressor lower than original, so you'll lose some capacity. But you have a 2h.p. motor where you had 3h.p. before so you will need to reduce the load a li8ttle. The other thing to try & maybe first is to use the existing 4" pulley and belt. You will lose some capacity, but it may be ok for your purpose and it won't cost you anything. Just a thought.
 
Speed X Diameter = Speed X Diameter

I was working behind auto parts sales counter one day and had a pulley/speed problem to deal with. I did NOT know answer and nobody else really knew either. I "vaguely" remembered that there was an easy way to figure this out, so I called my class instructor from many years ago...apologized for "sleeping in his class" one day...and he gave me the answer shown above that will work for farm type projects.

Now as far as HP questions....I don't have a freaking clue!!

Rick
 
If you use the same pulley it will take longer to build up pressure. Each revolution of the compressor pulley will displace the same amount of air(measured in standard cubic feet at given psi, not per minute, as one will clearly be slower). The question at issue is how fast will it reach desired pressure? If you use a larger drive pulley to develop a faster speed, you will sacrifice torque. I don't know what your power requirements are, yet, if the replacement motor is a smaller rating, it may not support the reduced torque.
 
7HP peak was likely a 2HP or 3HP continuous HP motor.
Is there CFM and psi data on the compressor?
iirc it takes 2HP to move 10cfm at 95psi.
You would be safe with a 3HP farm or compressor duty motor spinning a 8" pulley.
 
(quoted from post at 20:03:39 10/16/11) I'm trying to piece together an air compressor and trying to use a 1750 rpm motor where a 3450 was. The 3450 had a 4 inch pulley so do you go to an 8 inch pulley for the 1750 and get the same speed?? Or just use the 4 inch and the 1750 motor and be a little slower on recovery. thanks chris
Click to expand...

We cant answer your question without knowing what the other pulley size is. You cant just double the pulley size and expect the speed to double without knowing what the other pulley size is because the other pulley size affects things.

I think the only way doing a simple pulley doubling with a half speed motor will work is if the compressor pulley is the exact same size as the motor pulley you started with. Any other time, you are dealing with ratios.

We know your origional motor size is running a 4 inch pulley with a 7hp peak/3hp running motor spinning at 3450 and you want to put on a 2 hp 1750 motor. If you give us the compressor pulley size we can figure out the pulley size you want and also guess what will work best with a smaller hp motor.
 
Matthis, is the model #5KCR49TN8058T? Is it capacitor start, capacitor run? Do you have the name plate data for the air pump, including pressure and scfm's, what will you set the pressure switch at and will you run the compressor at 115v or 230v? I don't absolutely need the last two to tell you which size pulleys to use. They only indicate the load that will be placed on a 2hp motor. Remember that each increase of speed is a decrease of torque and that your replacement motor is less torque and speed.
 
(quoted from post at 23:15:38 10/16/11)
(quoted from post at 20:03:39 10/16/11) I'm trying to piece together an air compressor and trying to use a 1750 rpm motor where a 3450 was. The 3450 had a 4 inch pulley so do you go to an 8 inch pulley for the 1750 and get the same speed?? Or just use the 4 inch and the 1750 motor and be a little slower on recovery. thanks chris
Click to expand...

We cant answer your question without knowing what the other pulley size is. You cant just double the pulley size and expect the speed to double without knowing what the other pulley size is because the other pulley size affects things.

I think the only way doing a simple pulley doubling with a half speed motor will work is if the compressor pulley is the exact same size as the motor pulley you started with. Any other time, you are dealing with ratios.

We know your origional motor size is running a 4 inch pulley with a 7hp peak/3hp running motor spinning at 3450 and you want to put on a 2 hp 1750 motor. If you give us the compressor pulley size we can figure out the pulley size you want and also guess what will work best with a smaller hp motor.
Click to expand...

If the pump pulley is still the stock factory size. And you know the factory motor spun at 3450 with a 4" pulley. You have to be joking when telling us a 8" pulley on a 1740 rpm motor will change the pump rpms.
 
40 years ago, I tried little experiment like this with a box fan. Increased pulley size only 1 inch and it would not run without overheating. Just a lousy FAN. You have engineers for a reason. Dave
 
Centrifugal fan power demand increases with the cube root of rpm.
Double the speed and it takes 8 times more power to drive. Triple the speed and it takes 27 times more power to drive.
Basic highschool physics .
 
Well I tried the compressor tonight with the 1725 rpm 1 1/2 hp motor with the 4 inch pulley. Started well took around 10 minutes to fill a 60 tank. Also drained tank down to see if it would restart and did well as dry start. The info that's left on the compressor 7hp peak, single stage 60 gal, 12.3CFM@40 and 10.3CFM@90. I'm running the motor on 220 volts. The compressor pulley is 12 inches in diameter. The chart that was shown by one guy actually works out that I need an 8 inch pulley, might try a 6 inch, since it would fit the sheild better. chris
 
No model that I find or name plate. Just a cheaper farm/fleet store special. Don't know much about motors but it has two capacitors/covers on the motor, and its a farm duty continuous. 220v sorry chris
 
Forgot to mention that the motor wasn't warm to the touch, but it was only 40 degrees when I ran it. Also I called a industrial supplier I buy parts from and he also came up with an 8 inch pulley, I think I'll be safe with a 6. I also tried a little hand held tach, old AC Delco, and was right at 1725rpm on the motor but compressor was maybe around 500rpm, that's where the tach starts so guessing. You guys are great with all the info, thanks chris
 
Use a clip on ammeter and measure the motor current.
If using only a 1-1/2HP instead of a 2HP. The motor will be maxed out at approx 95psi and overloaded at 135psi.
 
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