[OP]
Contributor
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?
Contributor
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
Contributor
No.Originally Posted by Derek Arita
Add a second, small compressor to increase CFM.
"Anything seems possible when you don't know what you're doing."
[OP]
Contributor
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.
Member
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.
Contributor
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.
Contributor
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.
Contributor
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.
[OP]
Contributor
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...
Contributor
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).
Last edited by Doug Garson; 11-18-2019 at 5:50 PM. Reason: added fan law info
Member
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.
Contributor
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.
[OP]
Contributor
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.
Contributor
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.
Contributor
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
Last edited by Mike Henderson; 11-18-2019 at 9:02 PM.
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