Compressor, CFM Increase, Pullies?

[Derek Arita]

So, I have a 60 gal, 3hp, 11cfm compressor, that serves all my current needs. That said, I may sometime require higher cfm. Can I increase the cfm simply by replacing the motor pulley with a larger pulley or would it require a motor change as well? Oh...I would imagine it would also require a change or adjustment of the cutoff regulator?

[Ron Selzer]

If you increase the pulley size on the motor, then you probably will need to change motor size. Depends on how large of an increase in pulley size and how much if any reserve there is in the existing motor. If there are heaters/overloads in the existing starter then they would need to be upsized. Wire size could/would need to be upsized all depending on how much the amp draw goes up.
"require a change or adjustment of the cutoff regulator" no change needed

[andy bessette]

... Can I increase the cfm simply by replacing the motor pulley with a larger pulley...

No.

Add a second, small compressor to increase CFM.

[Derek Arita]

I guess you can't believe anything you see on YT. I saw folks install that larger pulley...not a lot larger though. I knew it couldn't be that simple, although the greater cfm model to mine, seems to only have a larger motor.

[Art Mann]

Think about it. If a manufacturer could get more out of a compressor by simply installing a different pulley, they would already have done it. The pulley is chosen to obtain the maximum output for that much horsepower without overtaxing the motor. You can probably get more performance temporarily by changing out the pulley but you would kill your motor fairly quickly.

[Alex Zeller]

If you do go with a larger pulley you will increase the speed of the pump. Chances are you will damage it. Compressors have the motor sized to the pump's volume and the speed at which it can run at. Chances are the model up from yours also has a larger pump.

[johnny means]

I once towed a 12' enclosed trailer filled with materials and equipment from Delaware to Iowa via the Allegheny Mountains hitched to my Toyota Avalon. I got there, no problems. Dropped the trailer there and left with nothing but a trunk full of tools. Made it back as for as Illinois and my tranny disintegrates. Moral of the story, it'll work, until it don't work.

[Richard Coers]

Great way to burn up the motor and or compressor. It's been engineered for a specific duty cycle and performance. Changing that will certainly change the life expectancy.

[Derek Arita]

Well, just looking at the specs on both machines...w 5HP stage pump is the same on both. Only parts that are really different are Pressure Switch Kit and Motor Pulley. Hmmm...

[Doug Garson]

Are the motor specs the same too? Note that the flow varies directly with speed while pressure varies by the power of 2 (IE squared) and power by the power of 3 (IE cubed).

https://www.engineeringtoolbox.com/docs/documents/196/fan-affinity-laws.png

[Edward Dyas]

So, I have a 60 gal, 3hp, 11cfm compressor, that serves all my current needs. That said, I may sometime require higher cfm. Can I increase the cfm simply by replacing the motor pulley with a larger pulley or would it require a motor change as well? Oh...I would imagine it would also require a change or adjustment of the cutoff regulator?I wouldn't alter the pulleys on the compressor. The larger you increase the pulley on the motor the more stress you put on the motor. Then it's designed to run at a certain RPM and if you put a larger pulley on the motor and it ran it would run too fast. If you someday need more cfm I would either get a second compressor or a larger one.

[Doug Garson]

I wouldn't alter the pulleys on the compressor. The larger you increase the pulley on the motor the more stress you put on the motor. Then it's designed to run at a certain RPM and if you put a larger pulley on the motor and it ran it would run too fast. If you someday need more cfm I would either get a second compressor or a larger one.
No the motor won't run faster the compressor pump (or fan) will run faster. The motor will pull more amps and may overload and fail. Motors run at synchronous speed and actually slow down when overloaded.

[Derek Arita]

So if I change to a 5 hp motor, with the proper pulley, then I should be OK? I mean, that's what the higher cfm model has.

[Doug Garson]

When you say the specs for the 5HP pump is the same as your pump, what specs are you looking at? If both machines have the same RPM motors and the 5HP unit has a larger pulley then the pump on that unit must run at a higher RPM.

[Mike Henderson]

Motors run at synchronous speed and actually slow down when overloaded.

Just a correction - Induction motors are not synchronous motors. They always have some "slip" (the difference between the hertz and the RPM). As the motor is loaded (any loading, not just overloaded) the slip increases, which means the RPM decreases. The specification of the RPM of a two pole 60hz motor is usually 3450RPM which means that at full load, the motor will run at (approximately) that RPM. When loaded lightly, it will run at a higher RPM. When loaded beyond the specification of the motor it will run at an RPM less than 3450. The change in RPM is smooth (up to a point), meaning that if you add a small increment of load, you get a small decrease in RPM.

If you load it enough the RPMs "crash" towards zero (the motor stalls). When you overload a motor, the motor is trying to create more HP than it was designed for. HP is torque times RPM (times a constant) so as you ask it to create more and more HP, the RPMs go down. If the motor cannot generate more torque to counter the slowdown in the RPMs the HP goes down. Once you reach that point, the motor stalls. Most motors can actually produce a bit more HP than the nameplate but they will be drawing excess current, which leads to excess heat. If continued, the motor fails from the heat.

Even when completely unloaded, an induction motor will not run at 3600RPM. An induction motor requires some slip to operate.

There are synchronous motors and those have an energised rotor - meaning that there are brushes and current is provided to the rotor. In an induction motor there are no brushes and the current in the rotor is "induced" by the difference between the hertz and the RPM.

Mike

[David Kumm]

Compressor pumps may or may not be rated to run within a range of rpms. If the same pump is used with different size motors to provide different cfm, you might be able to swap out but changing pulleys, motor, and starter electrics might cost more than it is worth. Quincy pumps were made to run from 600-1200 rpm but they were made a million times better than most aluminum head hobby level units. If your particular pump is also used on a larger motor at a higher rpm you might make it work. I'd sell it and find a used larger compressor with a larger tank though. More cfm benefits from a larger tank to cool the air and drop out the moisture into the tank. Used industrial quality compressors are pretty cheap and rebuilding them is pretty easy if needed. Dave

[andy bessette]

MH--good post.

[Doug Garson]

Just a correction - Induction motors are not synchronous motors. They always have some "slip" (the difference between the hertz and the RPM). As the motor is loaded (any loading, not just overloaded) the slip increases, which means the RPM decreases. The specification of the RPM of a two pole 60hz motor is usually 3450RPM which means that at full load, the motor will run at (approximately) that RPM. When loaded lightly, it will run at a higher RPM. When loaded beyond the specification of the motor it will run at an RPM less than 3450. The change in RPM is smooth (up to a point), meaning that if you add a small increment of load, you get a small decrease in RPM.

If you load it enough the RPMs "crash" towards zero (the motor stalls). When you overload a motor, the motor is trying to create more HP than it was designed for. HP is torque times RPM (times a constant) so as you ask it to create more and more HP, the RPMs go down. If the motor cannot generate more torque to counter the slowdown in the RPMs the HP goes down. Once you reach that point, the motor stalls. Most motors can actually produce a bit more HP than the nameplate but they will be drawing excess current, which leads to excess heat. If continued, the motor fails from the heat.

Even when completely unloaded, an induction motor will not run at 3600RPM. An induction motor requires some slip to operate.

There are synchronous motors and those have an energised rotor - meaning that there are brushes and current is provided to the rotor. In an induction motor there are no brushes and the current in the rotor is "induced" by the difference between the hertz and the RPM.

Mike
Agreed, my terminology was not precise, my point was that putting a bigger pulley on a motor won't speed up the motor. Your explanation is more detailed and better than mine.

[Derek Arita]

From what I see on the specs, they have the same Pump, but different Motors and Pulleys, which I would expect.

[Bill Dufour]

If you increase pump rpm you have to lower cutout pressure. Most are set at 150 PSi or higher. For most folks 100-120 PSi is more then enough.
The real way to figure out cutout pressure is to run the bigger pulley until the amp draw is right at FLA. Read the pressure gauge and the set the cutout pressure a little below that pressure.
Bill D.
3-5Hp is the range where a two stage piston pump will be more efficient and worth the extra cost in energy savings. 10-15HP is the point for a rotary screw compressor and it's maintenance cost. A screw machine has to be run often enough and long enough to dry out the oil. that expensive oil has to be changed several times a year regardless of use.
A piston pump is a better choice for those who do not run it for continuously for hours everyday
Bill D

[Delwyn Ching]

1. You may strain/burn the motor with at added torque/load of the larger pulley.

2. You may damage the pump with the increased rpm.