Attic fan help

[IAN WALKER]

This may not be the correct forum for my thread, but I hope it is. I recently acquired a 48” attic fan that I intend to install in my barn. This is my problem. It was a bare fan. There isn’t a pulley on the fan shaft. Also, no motor. I have a couple of motors. I have a 1/2hp fan duty motor that draws 7 fla on 120v. I also have a 1hp general purpose motor that pulls 13 fla at 120v. Both are 1725 rpm. I have no idea how to size the pulleys, or if either of my motors would work. I’m not super concerned with what cfm is when I’m done since it is in a barn, but I do want it to move some air. I’m sure there is some formula that someone a lot more intelligent than myself knows. Any help would be greatly appreciated. Thanks

 

[Compositepro]

There is no way that you will have enough accurate data to be able to calculate an answer. Fan power increases with the cube of the RPM so there is a narrow speed range where your fan will operate well. You can only come close with a best guess (informed by looking at similar fans) and then use an variable pitch pulley for fine tuning. The time and cost of doing this will probably be more than a new complete fan.

 

[IAN WALKER]

Okay, that being said, does anyone have a 48” attic fan that happens to know pulley sizes and motor data? Or at least an educated guess to get me in the ballpark to start?

 

[itsmoked]

Ah. Well you're in a bit of luck. Once those type fans hit 48" they're kinda standard. Look up a couple Grainger or wherever and see what sixe motors they use and get a feel for gearing. Look at the pulley size ratios you can see.

You can come back with any links or pictures and we can help you come to a reasonable conclusion of your motors and likely pulley ratios.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

They seem to hover around 1/2 to 3/4hp. You may be in luck.

I'd find about a 10" to 12" pulley for the fan shaft and a 2" pulley for the motor shaft.
Hang it all together and run the motor with a clamp-on ammeter in hand. Check the amps. If they're less than the FLA listed on the motor name plate you're good to go. If it's 20% less than the FLA get an inch smaller fan pulley and recheck the current. If you're over the motor FLA you definitely need to increase the fan pulley or use a larger motor.

That's the only way to get the pulleys right***. Try them and ammeter the result.

If the draw is slightly too high an alternative is to choke down the fan hole a little. The more restriction to air flow the less power needed. That will essentially reduce the fan from a 48" down to something less.

*** short of getting the model number and asking the manufacturer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IAN WALKER]

Just wanted to update you guys. I went with a 12” pulley on the fan shaft and a 3” variable pitch motor pulley(that was the smallest I could get locally). I tried my 1/2hp motor first. I opened the motor pulley about 3 turns. It was running about 7.8 amps. The motor does have a 1.25 SF, so the 7.8 would have been okay, but I opted to open it 1 more turn. BAM! 7.1 amps. It moves a TON of air! So I’m all in for about $140, not counting the motor. Not bad.....
Thank you guys for your help.
Ian

 

[Compositepro]

Well it is refreshing to see a guy who gets stuff done! Good work.

 

[itsmoked]

Yay!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[chicopee]

As a passing note, you could have gone on line and check out Grainger website to determine the size(Hp) of the motor, air flow (CFM) and RPM for a 48" fan.