Calculating Fan Motor Speed

[Richard D]

I nhave a vintage Coolair 8 blade belt driven whole house fan, 48" dia, 20" pulley. I want to use it as an outdoor box-type shop/patio fan.
Pictures here:
http://www.garagejournal.com/forum/showthread.php?t=306644
The original sales bulletin from 1941 shows 405 R.P.M. with a 1-1/2 H.P. motor, but no mention of the motor R.P.M. I have a 3/4 H.P. 1745 R.P.M. motor on there with a small( about 2") pulley. I don't know the blade R.P.M., but it doesn't move a lot of air. My 36" 6 blade moves way more, with a 1/3 H.P. 1745. Would a 3/4 H.P. 3450 with a 2-1/2" pulley be a wise choice? I found one for $99.

 

[pseudorealityx]

I'm not sure what you're asking for...

 

[wayne55]

1745 x 2 / 20 = 175 fan rpm

3450 x 2.5 / 20 = 431 fan rpm

 

[APEowner]

The fan speed is a simple ratio. With a 20" driven pulley and a 2" drive pulley (20/2 = 10) the fan turns 174.5 RPM with a 1745 RPM motor (1745/10 = 174.5). If you use the 3450 RPM motor with the 2 1/2 pulley then the fan will turn 431.5 RPM (20/2.5 = 8, 3450/8 =431.5).

There are two problems with that however, The first is that with the numerically lower ratio and the low HP motor, compared to what the manufacture specified originally, you may need to had start the fan to get it turning. The other is that spinning fans faster than they're designed to can reduce their efficiency and sometimes be dangerous.

What you really need to move as much air as possible with that fan is a 1 1/2 HP 3450 RPM motor with a 2 3/8 drive pulley

Here's a link to a handy calculator so you don't need to do any math http://www.blocklayer.com/Pulley-BeltEng.aspx

 

[Richard D]

Thanks, I'll look for a 1-1/2 3450.

 

[DonPowers]

If you speed up the fan, you will still need the horse power to get the flow without overloading the motor.

You are going to want 317 RPM for the 3/4 HP motor.
A 1.75" pulley will give you 302 RPM at the fan.

 

[404]

The power needed may go up as the square of the rpm change. Suggest measuring the motor current on what ever set up you choose, to make sure motor is not over amps.

 

[Richard D]

What you really need to move as much air as possible with that fan is a 1 1/2 HP 3450 RPM motor with a 2 3/8 drive pulley

Thanks, I took your advice. Pictures of the fan:
http://www.garagejournal.com/forum/showthread.php?t=306644
"Update: I put a 1-1/2 H.P., 230V, 3450 R.P.M. motor on there, this thing is a beast!"

 

[Freejack]

Just a quick note to add to the comments about pulley ratios: Fan and blowers follow an affinity law, meaning that HP increases exponentially as RPM increase, so doubling the RPM of a fan does not mean in needs just double the HP...

Jake

 

[Richard D]

An update. Yesterday I ordered a 1.5 H.P., 3450 3 phase motor/VFD combo. This thing puts out crazy air; I think with variable speed it will be nice. I'll keep an eye on motor temps.