Suggested motor for large fan

[tarbellb]

I'm out of my league when it comes to electrical applications, looking for some help.

We are hooking up a large exhaust fan, and need to size the appropriate motor for it.

32" cast iron 6 blade fan w/ 4" pulley

Connected via a belt (60"?)

120 v / 20 amp breaker

Everything sounds freely, there is no reason to think we have any drivetrain issues.

We tested a ½hp 1800 rpm, and it tripped the breaker almost instantly.

Hooked up a 1 ⅓hp 1150 rpm motor w/ 4" pulley and it ran fine for 10+ mins. Motor housing did get pretty warm to the touch though?

Anybody know what the correct motor size / rpm /pulley we should be running?

 

[oldironfarmer]

It's just one of those things that is not as easy as it seems it should be. Fan speed as well as air flow rate affect the horsepower required. If you knew the rated CFM you could calculate the horsepower required.

You are almost there by trial and error, though. If you can get an ammeter, measure the amperage the 1-1/2 hp motor is pulling and compare that to the nameplate rating on the motor. A motor only consumes the amperage required for the load, not for it's rated size. A 2 hp motor running a 1/2 hp load will only pull a little more amperage than a 1/2 hp motor running the same load.

So, if you are running the fan with the 1-1/2 hp motor and the amperage is lower than nameplate, you should be ok to operate it as is. If it is at or slightly above nameplate rating, put a larger sheave on the fan to make it run slower. See how much the amperage goes down. A 4-1/2 inch sheave will make a big difference, even more with a 5 inch sheave on the fan. Of course you can also slow the fan by putting a smaller sheave on the motor.

Once you have the motor amperage under nameplate, you can determine whether you have enough air flow for the job you are trying to do. If you need more fan speed you need a bigger motor.

Without an ammeter you are totally guessing. Remember it is the fan speed that makes the big difference here, since you already have the fan.

 

[nh_yota]

Might need to put a smaller pulley on the motor.

 

[tarbellb]

Wow great answer Old Iron!

I should note that we are maxed out on the pulley side of the blade. It's in a tube.

The 1 ⅓ hp motor was pushing a lot of air. Maybe to much, but we will be putting a filter over the opening.

Also, we wired it to what I thought was correct but the fan was blowing in instead of sucking out. Can I reverse the hot and neutral to reverse the rotation?

 

[larry_g]

https://www.grainger.com/category/b...-refrigeration/ecatalog/N-ylbZ1z0o2uhZ1z0o2uc

The above graingers site has some fans and if you look at the individual fan specs it will give a HP range, speed range, and a cfm range. From this you should be able to make a good guess at what HP it will take to turn your fan at a given speed. Graingers has many more fans if this does not match what you have. You should be able to gather enough data points to extrapolate the information you need.

lg
no neat sig line

 

[bsaint]

How much does the fan blade assembly (fans and rotor) weigh?

 

[sberry]

No, hot and neutral do not reverse it.

 

[oldironfarmer]

If the motor is reversible there should be a wiring diagram on the motor nameplate. I assume it is 110 or 220 and not three phase. Adding a filter may or may not increase the horsepower required. Since you are maxed out on the fan sheave, a smaller one on the motor is the same thing.

But get an ammeter.

 

[Guster]

Can I reverse the hot and neutral to reverse the rotation?

No. Simply put:
-Single phase you normally change the orientation of the start winding in relation to the run winding. Still basically swapping two wires/connections but not as simple as swapping the supply as on DC motors.
-Three phase you tend to change the orientation of one winding in relation to the others.

Many newer motors have a terminal block and diagram for changing motor direction inside the terminal box. Older ones may be marked with terminal identification like u1/2,v1/2, w1/2 or a1/2,z1/2 etc. and you may be able to work out how to swap windings.

Be aware that some are not reversable and some that are don't indicate it. If you are not able to identify windings and not comfortable with poking around then I suggest you find someone who does.

 

[oldironfarmer]

No. Simply put:
-Single phase you normally change the orientation of the start winding in relation to the run winding. Still basically swapping two wires/connections but not as simple as swapping the supply as on DC motors.
-Three phase you tend to change the orientation of one winding in relation to the others.

Many newer motors have a terminal block and diagram for changing motor direction inside the terminal box. Older ones may be marked with terminal identification like u1/2,v1/2, w1/2 or a1/2,z1/2 etc. and you may be able to work out how to swap windings.

Be aware that some are not reversable and some that are don't indicate it. If you are not able to identify windings and not comfortable with poking around then I suggest you find someone who does.

Guster: Do motors run backwards down under?

 

[MEngineer]

Just a thought, but if you can't rewire the motor, can you re-mount it in the opposite direction from where it is facing now to have it turn the fan the other way?

 

[Guster]

Guster: Do motors run backwards down under?

Ours always run forwards, whether it is clockwise or anticlockwise.

Though I am curious if there is in fact enough polar difference(>90degrees) between your parts of Oklahoma and Auckland that our forwards would appear backwards relative to you(will have to do the maths sometime)

 

[tarbellb]

Thanks guys, I will take a look at the plate and see if it gives me a diagram for reversing.

I know just enough to get into trouble, thats why I have the fire extinguisher close by. JK, kind of.

I cant easily just flip the motor, there isnt enough clearance. Plus there is a nice adjustable mounting plate for belt tension. Oh, and its my personal motor and dont want to sacrifice it to this.

 

[tarbellb]

How much does the fan blade assembly (fans and rotor) weigh?

Its up in a duct, dont want to pull it out if I dont have to.

But if I were guessing, I think its cast iron, 6 blades ..... 30-40lbs?

 

[pancho400cid]

Wow - that's a cool set up. Some good comments above.

As for rotation, looks to me like the rotation was intended to be CCW as viewed from below the fan (as in the pic). It would then force air from the room out the tunnel toward the daylight in the pic, which is what you want apparently.

Just looking, I would guess the power was originally well above the motors you mentioned.

Fan driving power is mainly a function of RPM and back pressure. The inertia etc. is generally a very small fraction of the HP needed, but I'm more used to centrifugal blowers. That axial style probably takes some oompf just to get it to operating speed even in a vacuum. The more RPM the fan turns, the more air it moves and the more power it takes - up until the fan stalls or something comes apart (fairly spectacularly in your case - so don't go there).

The more back pressure in the system, the less air the fan moves and the less driving HP it takes. That one is a little counter-intuitive, as people tend to think more pressure = more HP. Looks like you are pretty much discharging right out the cover so not much back pressure.

Is there a nameplate on the fan or anywhere that gives the design speed? Don't exceed that in your experiments.

I agree with info above as far as reading the current on the 1.33 HP motor to see how it compares with the rated full load current - but if you are rotating backward the power needed is lower than if you are rotating correctly.

One last hint is the mounting base. It may be sized for a particular NEMA motor frame. If you had the nameplate fan RPM and the NEMA frame size of the motor you could start getting into the ballpark the original designers were in.

 

[tarbellb]

Damn, I love this place.

Thanks all for the tips, advice, and things to watch out for!

There is a 1.5HP / 3450 rpm motor available on the cheap locally, I wonder what size pulley I would have to mount to bring the rpms down.

When running the 1.33HP motor w/ 4" pulley @1150 rpm, it was moving a lot of air.

 

[Craptain]

Damn, I love this place.

Thanks all for the tips, advice, and things to watch out for!

There is a 1.5HP / 3450 rpm motor available on the cheap locally, I wonder what size pulley I would have to mount to bring the rpms down.

When running the 1.33HP motor w/ 4" pulley @1150 rpm, it was moving a lot of air.

I would be looking at a lower speed motor (1725). High speed = low torque and vice versa. For your use I think the higher torque would be better, especially starting torque. You already seem to be close on hp rating so that is what I would be looking for.

Sent from my LG-D801 using Tapatalk

 

[oldironfarmer]

I'm assuming the 1-1/3 hp motor is not reversible? I didn't see where you may have indicated that.

Also, did you check amps on the 1-1/3?

Sheave size is a direct ratio. You have 4" on motor and fan now, so fan is turning at motor speed, 1150 for the 1-1/3. If you put a 2" sheave on a 3750 motor and 4" on the fan, the fan will turn at 1875 rpm. That's a 63% increase, and axial fans power goes up with the cube of the speed, so trying to run that fan at 1875 will use more than four times the power at 1150 rpm.

Practically, you don't want less that a 2" sheave. The fan you have may have had a low speed three phase motor. If you fabricate a new motor mount to enable you to reverse your motor you could include a jack shaft to reduce the fan speed further: 3" on motor, 4" on jack shaft input (idler shaft) and 2-1/2" on jack shaft output. Reduction in speed about 50%, fan speed 539 rpm and the horsepower goes way down (10% of 1150 rpm horsepower). It will still move a lot of air at 500 rpm.

I used to have a 3 ft by 3 ft squirrel cage fan I drove with a 1/2 horse motor very slowly. You could see the blades go by, but it threw a lot of air across the shop.

I know it's more information than you want, but if you don't grasp the basics you can't power the fan.