Self-made air cleaner noise?

[Phil Thien]

I built an air cleaner with a 1/4-HP three-speed blower on which I got a good deal.

The blower's motor makes a louder hum than I was expecting.

When under power, at any of the speeds, I feel the motor vibrating. As soon as I cut power to the motor, the hum stops immediately. During spin-down, there is just the noise of the moving air.

If I run the thing in high speed and cut the power, the unit coasts to a stop with nothing but the noise of the moving air. So it isn't a balance issue or anything, as the noise is only made under power.

The motor is mounted via vibration isolators. And I'm sure they're helping. But it is still noisy.

Any thoughts? Do your air cleaners made any sort of humming noise? Is this a "feature" of multi-speed motors?

[Thomas Hotchkin]

Phil,
Yes my self made air cleaner is noisy. It's made out of 3/4" birch ply with a single speed 1/3 HP squirrel cage motor assy. When I first started using it, I would use some type of heading protection. First mod was a light dimer switch/motor speed control, very small change in motor speed made very large change in noise/hum. Second mod was to hang box assy. from four hangers made from rubber strips. Air cleaner hangs about two inches below shop ceiling. Now in less I change motor speed I do not hear it running with normal shop back ground noise. ( Radio, hand sanding, hammer, hand saw, and the like). Then again my wife says I don't hear again thing anyway. Selective hearing? Tom

[John M Wilson]

I would suspect that your motor (when under power) is hitting some sort of harmonic frequency with either the box it is in, or the structure it is hanging from.

Harmonics are tricky, since a very small input vibration is amplified to an output all out of proportion with the input. So, a motor (or other vibration source) that appears to be running very smoothly creates an unexpected ruckus.

The (possibly only) good thing about harmonics is that if you can get even a small distance away from the exciting frequency (as Thomas did with his dimmer switch) you can often eliminate any vibration from wherever you are getting the harmonic noise from.

So, if it is a harmonic issue, you have two choices: change the frequency of the input (vary the motor speed) or change the frequency of the resonator (change the shape of the box, change the mounting to the ceiling, etc.)

In a previous life, I worked with the guys at the car factory that would chase down buzzes, squeaks, & rattles in the new cars. It's more art than science.

Good luck!

[glenn bradley]

You can check for cabinet resonance by clamping the cabinet. Without sophisticated equipment and we're just trying to see if the problem is cabinet resonance; one random placement is as good as another. If the noise changes, try another position and another till you get your best result (assuming this works). If you are able to find "sweet spots" that drop the noise level when reinforced, experiment with ways to make those spots more rigid; glue and screw a piece of scrap to the spot maybe(?). My shop made unit noise varies with filter density (resistance?). I found some filters that give me adequate filtering with reduced noise and bought a bunch. Good luck.

[Phil Thien]

Thomas, thanks for the real-world feedback. You've given me some things to think about.

John, I agree that the box is amplifying the sound. I made a mistake in going with a box that is roughly a cube, as I have four surfaces all the same size and all sympathetic to the same frequency. I can always use bracing to break the panels into difference sizes.

I'm just surprised that I can put my hand on the motor and feel as much vibration as I do.

[Steve DiMasi]

Just a thought: have you tried running it without a filter? That might help determine if it's related to that particular brand of filter or if it is a box stiffness issue.

[Phil Thien]

Just a thought: have you tried running it without a filter? That might help determine if it's related to that particular brand of filter or if it is a box stiffness issue.

Yep, with and without. All three speeds.

[Steve DiMasi]

It seems strange that it would have harmonics at all three speeds, with and without the filter. Maybe the squirrel cage is out of balance? I wonder if you could spin it, let it stop, mark the bottom of the cage and then see if it continually stops near the same place. If it does, you could add something for weight to the top.

[Phil Thien]

It seems strange that it would have harmonics at all three speeds, with and without the filter. Maybe the squirrel cage is out of balance? I wonder if you could spin it, let it stop, mark the bottom of the cage and then see if it continually stops near the same place. If it does, you could add something for weight to the top.

As soon as I cut power, the noise stops. It is ONLY when the unit is under power that it is noisy.

When I do cut power (including at the highest speed) it takes a significant time to spin down, and it never makes the noise on the way down.

If I had a way to spin the unit at-speed without another motor that was perfectly silent, the assembly would only make the noise of rushing air.

I suspect this particular motor I've got was designed in such a way that it hums a bit. Or I got a bad motor.

[Steve DiMasi]

Good point. I guess I didn't read your original post very well!

I'm stumped.

[Steve DiMasi]

One more thought - would mounting the motor with rubber isolators or foam help? The motor may still vibrate, but it might not transfer to the cabinet as much.

[Phil Thien]

One more thought - would mounting the motor with rubber isolators or foam help? The motor may still vibrate, but it might not transfer to the cabinet as much.

It came with real nice vibration mounts.

[David Cefai]

Does the hum sound like a 60Hz one. In that case it may be rapid (60Hz) expansion/contraction - either thermal or magnetic, or a component (winding, transformer...).

Like the hum an electric heater emanates, that's thermal, or a transformer that's magnetic + possibly thermal.

[Phil Thien]

Does the hum sound like a 60Hz one. In that case it may be rapid (60Hz) expansion/contraction - either thermal or magnetic, or a component (winding, transformer...).

Like the hum an electric heater emanates, that's thermal, or a transformer that's magnetic + possibly thermal.

If I had to guess, I'd say it was higher than 60-Hz.

I should have used isolation mounts between the blower and the enclosure.

[Greg Peterson]

Have you run the motor without the squirrel connected?

I had a Delta AP-100 that vibrated so bad a fluorescent tube fell out of a nearby fixture. I fashioned a mounting system that eliminated that problem. I have since replaced the Delta with a Jet unit, but I use the same mounting system. See below.

I suppose I could take it up a notch and use some rubber bushings, but frankly that would be addressing a non-existent problem for this unit.

You might also try placing something in each corner of your enclosure to eliminate or minimize standing waves. Or some kind of material that dampens (disrupts) sound waves.

[Phil Thien]

Have you run the motor without the squirrel connected?

No. But I did use divining rods designed to locate a noisy motor. They were able to detect this one.

Just kidding, I did run the motor by itself. It vibrates a bit. More so than the 1/4-HP motor on my furnace blower. I've come to the conclusions that: (1) The blower itself amplifies the noise. (2) The box further amplifies it.

[Greg Peterson]

What OS is the motor running? You know if it was running Linux or Mac OS X you wouldn't have this problem. Have you tried rebooting it? Have the latest drivers?

[Larry Edgerton]

Phil

Mine is quiet, and it is hung on threaded rod.

I did however wire on of the fans wrong, and it worked, but it sounded like you discribe. This is why I don't do wiring. I made my OMGA run backwards too! I consulted a shaman and he got me going right then......

[Tim Janssen]

I think you have bad motor, and since it only hums when power is applied to the motor I suspect the field laminations might be loose. I'm assuming that the hum is a 60 or 120 cycle hum? Open the motor up and see if you can find anything unusual.
Good luck.

Tim

[David Hostetler]

Phil,

For what it's worth, I didn't build mine, but rather bought it. I did cobble together one prior to buying my Grizzly, but had too many problems with bad motors... (long story...) On the one I was able to get to work, somewhat passably, it hummed like crazy like a harmonic vibration with the box. As an experiment, I laid down a heavy piece of rubber sheeting on the bottom inside of the box, and the hum went away... Maybe some sort of vibration damping material is in order for yours? (FWIW, I gave up on the DIY approach after too many locked up bearings on pull out motors / squirrel cages... I guess people around here don't replace A/C units until they are gasping their last breath...)

[Phil Thien]

I think you have bad motor, and since it only hums when power is applied to the motor I suspect the field laminations might be loose. I'm assuming that the hum is a 60 or 120 cycle hum? Open the motor up and see if you can find anything unusual.
Good luck.

Tim

I think you're right. I think I have a bad motor. The frequency does seem to be right at about 120-Hz.

I guess I didn't get such a good deal after all.

[Phil Thien]

Phil,

but had too many problems with bad motors...

I was able to compare the noise with a frequency generator and I'd say I'm hearing 120-Hz. I think I have a bad motor, too.

Oh well, back to the drawing board.

A gable fan made its way to my door. I have a feeling any level of filtering will kill the airflow. But no cost, so worth playing around with it.

[Alan Schaffter]

Phil,

I suspect you may have some sort of magnetic resonance caused by improper wiring, windings, or loose stator iron(s). Or it could be the magnetic pull is off center somehow and when that is combined with a bad bearing(s), causing the rotor to wobble only when the power is applied. Make sure the motor housing bolts are tight and there is no play in the sleeve bearings. If that doesn't work, open up the motor and make sure the stator irons and windings are fixed and can't vibrate. Here is a link to an excellent tutorial with photos of what's inside a multi-speed squirrel cage fan. (http://www.johndearmond.com/2008/10/01/dissection-of-a-furnace-fan-motor/) You can bypass the flame about trolls at the beginning (it is also an interesting read on its own merits since we've seen some of that here lately) and start at "Show 'n Tell" .

Good luck!

[Ryan Hellmer]

I wonder if you're overloading the motor. Does it get pretty warm after running for some time? The reason I ask is that those blowers are designed to run with a certain amount of static pressure, relieving them of that makes the fan move LOTS more air increasing the load on the motor substantially and perhaps leading to additional noise/vibration (this is particularly true with low pressure forward curve squirell cages). Have you tried limiting air flow by blocking part of the inlet/outlet? You may find that helps.

Ryan

[Phil Thien]

Thanks Alan, I'll look into the link and see if there is a way to silence this motor.

Ryan, the motor is rated for 3.75A (I think). When I run it on the lowest speed and have the filters in place, I'm drawing about 3.6A. I have tried blocking-off some of the inlet with a piece of hardboard and dropped to 3A. The amplitude dropped a little bit, but the noise was still there.

I did run the motor in free-air. It vibrates. I think I have to disassemble it and look for the cause.

[Alan Schaffter]

Thanks Alan, I'll look into the link and see if there is a way to silence this motor.

Ryan, the motor is rated for 3.75A (I think). When I run it on the lowest speed and have the filters in place, I'm drawing about 3.6A. I have tried blocking-off some of the inlet with a piece of hardboard and dropped to 3A. The amplitude dropped a little bit, but the noise was still there.

I did run the motor in free-air. It vibrates. I think I have to disassemble it and look for the cause.

One quote from my link above (bold and red added by me):

"You can also see how the “end turns” of the windings are laced tightly with lacing twine. This holds the windings in place and keeps them from vibrating, creating hum."

[Larry Edgerton]

I wonder if you're overloading the motor. Does it get pretty warm after running for some time? The reason I ask is that those blowers are designed to run with a certain amount of static pressure, relieving them of that makes the fan move LOTS more air increasing the load on the motor substantially and perhaps leading to additional noise/vibration (this is particularly true with low pressure forward curve squirell cages). Have you tried limiting air flow by blocking part of the inlet/outlet? You may find that helps.


Ryan
I have 8 20"x25"filters on mine and it does not vibrate. Of course mine is an octagon, which is a collection of pyramids when you think about it, so I am collecting all of that positive energy which is probably why mine runs so smooth.

Genius! Shear Genius!

Sometimes I just amaze even myself.......

Larry