[Joel Koontz]

I am trying to install an electric fan in my garage so I can use a bay as a temp paint booth, and I also plan to use it to move hot air out of the garage during summer heat.

About 2 years ago, I removed a large (it is eiither 120,000 or 140,000 BTU, I forget which, originally used in a large farm house) hot air oil furnace from the shop. It was working fine, but was old and inefficient so I replaced it.

I pulled the large squirel cage fan off of it and mounted it on the wall and hooked it up. To my surprise it would just hum and not start the fan turning. If I helped it, it would run, but it obviously has a problem. It had a 1/4 motor on it. I took it to a local electric motor shop and they are going to check it out. If it has to be replaced, will cost well over $100 for a similar replacement motor.l

I have a 2 horse electric motor that was formerly used on an air comperssor and seems to still be working fine. I think it is the same RPM as the motor from the fan and I could bolt it up fairly easily.

If I use the 2 horse motor, it obviously will not be working very hard. Will it still use 8 times as much electricity as the 1/4 HP motor, or will the electrical useage be based upon the load that the motor encounters?

I don't mind using a little bit more to be able to use the "free motor", but I would be reluctant to use it if it will use 8X as much.

Any idea how a squirel cage fan reacts to faster speed. With the higher HP motor, I could easily use a bigger pulley on the motor to run the fan faster. I assume that there is a speed at which it is most efficient (probably at/near the OEM speed) but I wounder if I would get a significant increase in CFM my turning it approx 25-50% faster than stock?

The fan housing is 12" wide and about 19" in diameter. Any idea how many CFM it may flow.

Thanks, Joel

PS: If this won't work I may need to use the Mark VIII Fan off my car.

[george kujanski]

Assuming both motors are rated at the same speed, the large motor will draw only slightly more than the smaller one at the same speed, same fan, due to possibly more internal friction.

Don't know how the blower will operate at a higher speed, but you can get an idea by going to a fan mfgr's website and compare their specs.

Your smaller motor may not be starting if you don't have the start capacitor connected.

[Joel Koontz]

George, thanks for the reply.

Thats what I was hoping to hear regarding energy consumption by the bigger motor.

No capacitor is what I suspected when the motor would not start, but I looked at all the components in the old furnace and did not find any capacitors. Thats when I took it to the electric motor repair shop. The guy there, said this motor did not use a capacitor. He said it was probably a problem with the "starting switch". He said that this is an internal part of the motor and said it has a set of points in it, similar to points in a car. He said he may be able to file the points to get it to work.

[george kujanski]

Yep, some induction motors use an internal centrifugal switch that closes at low speeds and once it starts up, opens. The switch puts the windings into a different configuration for improved starting torque.

George

[66Mutt]

The 2 HP motor will likely draw more current as George stated just because it is larger. But it will also operate less efficiently if it is loaded lightly. The power factor will lower. I don't know about power factor for these small motors, but I suspect they may only be in the 80%'s range to start with. It could drop down to 50% on under low loads. But for the total current that you will be drawing, I would not be concerned with the lower efficiencies.
Doug

[Joel Koontz]

Thanks for the replies.

I got the 1/4 horse motor back (a wire was off of the starter switch) and it now operates like it originally did. Which as it turns out, may not be too good for what I want to do.

I hooked it up and it did not pull as much air as I would like. It has a nice, adjustable pully on it that can adjust from about 1.5" to 3.25". It was close to the 1.5" setting when I pulled it out of the furnace. I tried increasing the diameter and this definately helped move more air, but I noticed that the motor was heating up. I moved it back to slightly larger than "original" and it still got too hot and would cycle on and off (I assume thermal switch) Seemed to get warm, but kept running at orginal pully size so basically, using the orginal motor I can't increase the fan speed at all, but I sure would like to get some more airflow out of the fan.

I then hooked up the 2 HP motor(was petty easy to bolt up). Before installing, I noticed that it is not the same RPM as the 1/4 horse motor. The 2HP is 3450 RPM and the 1/4 HP is 1725 RPM. I tried the 2HP with a 1.5" pully. It has a different shaft size than the 1/4HP so I had to use different pulley.

The 2 horse motor would very quickly get up to speed (it has 2 capacitors on it) then slow down and after a couple seconds, "restart" for an instant, and the cycle would repeat. On about the 4th restart it tripped the breaker. Tried it 3 times with same results.

I suspect that may be starting on the capacitors, but for some reason, not continuing on the normal run cycle.

When I ran it at no load (only for a few seconds) it seemed to run normally. The 2HP motor has set around for at least 20 years. I think it worked OK when I stoped using it, but don't remember for sure. It may have had a problem when I quite using it years ago. Any idea what might be causing the problem and is it likely to be something fairly cheap to fix, or should I look for another motor if I want to turn the fan faster?

I am seriously considering trying to rig up a Mark VIII Fan. I have a 40 amp charger and a spare battery. Would this be a big energy hog or would this perhaps work OK?

[66Mutt]

What is the amp rating of the breaker on your circuit? A 15 amp breaker may not be enough. Maybe you are riding a fine line and the old motor may have tight bearings.
2 HP x 746 watts/HP = 1492 watts
1492 watts/120 = 12.4 amps
With a motor power factor of 0.85, you could be looking at pulling 14.6 amps or higher. But with losses and 20 year old bearings,

I also think that you could be drawing quite a bit more power. You did not state what pulley diameter you ended up using on the 2HP motor. The BHP goes up with the cube of the change in speed. Therefore, all else equal, when the fan speed is doubled, the horsepower draw is (2)^3 or an 8 fold increase. So a 1/4 load could be pulling 2 hp when the speed is doubled. If the pulley on the 2 hp motor is larger than the 1.5" pulley, then the hp draw would increase even more.
Doug

[Joel Koontz]

Doug, thanks for the replies.

I didn't realize that "The BHP goes up with the cube of the change in speed."

That certainly explains why the 1/4HP motor did not want to turn the fan much faster.

I only tried the 2HP motor with a 1.5" pulley.

The bearings don't seem overly tight. Spins freely when spun by hand.

It's a 20 amp circuit, but I did have an 8' florescent light on when I tried the 2HP motor.

[dancolkate]

Originally Posted by 66Mutt

What is the amp rating of the breaker on your circuit? A 15 amp breaker may not be enough. Maybe you are riding a fine line and the old motor may have tight bearings.
2 HP x 746 watts/HP = 1492 watts
1492 watts/120 = 12.4 amps
With a motor power factor of 0.85, you could be looking at pulling 14.6 amps or higher. But with losses and 20 year old bearings,

I also think that you could be drawing quite a bit more power. You did not state what pulley diameter you ended up using on the 2HP motor. The BHP goes up with the cube of the change in speed. Therefore, all else equal, when the fan speed is doubled, the horsepower draw is (2)^3 or an 8 fold increase. So a 1/4 load could be pulling 2 hp when the speed is doubled. If the pulley on the 2 hp motor is larger than the 1.5" pulley, then the hp draw would increase even more.
Doug

This is right-on. But if you want to know what either motor is actually drawing, put an amp probe on them.