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#60265 12/27/05 10:03 AM
Joined: Dec 2003
Posts: 524
... Here's an interesting phenomenemenen...
We were summoned to check out a "loud, noisy" rooftop downblast fan at the Diner we do alot of business in.. the thing was making an awful racket,..and we shut her down to dis-assemble the mushroom housing..
We found nothing loose or rattling inside so we put her back together again and I hit the switch,..and all that happened was this LOUD (the best way to describe it is like a hum) or more like an electronic buzzer at 100db.... the fan is a direct drive fan, and all the connections were good.. it seems that the negative air pressure at the roof penetration forces the fan to "free-wheel" in the opposite direction, and the start windings are shot.. is this the case??

.."if it ain't fixed,don't break a Licensed Electrician"
#60266 12/27/05 11:40 AM
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
I have heard a similar sound with three-phase motors, when I mis-connected the wires in the motor's box.
For a single-phase motor, it is possible that there is a problem with the starting winding. Another possibility is the motor was damaged by running at an under-voltage on one leg (sort of a form of single-phasing.)

I suggest opening the motor, and looking for damaged windings.

#60267 12/27/05 12:57 PM
Joined: Mar 2001
Posts: 333
If the motor has a start capacitor and it has gone bad, that could also cause the problem.


#60268 12/27/05 02:15 PM
Joined: May 2001
Posts: 376
If its three phase...
could be a winding gone bad(rewind or get a new motor)
Could also be simple as the blower wheel binding.
Could be that some one put pressure on the rotor shaft and gave a worn bearing the clearance needed to cause locked rotor on every startup.
Test the windings with a meter for proper Resistance.
Watch the blower/shaft on startup to see if moves vertically and locks.
If it's single phase check all above plus replace the capacitor if applicable(if the blower starts and runs in the direction you spin it then cap for sure)
Willing to bet its a mechanical problem if i noise was the original complaint but if not once you get it running check the blower wheel for loose fins
Also old blower wheels are hard to find so that may be a show stopper right there( better check it FIRST)

good luck and happy new year

edited because I'm illiterate

[This message has been edited by frank (edited 12-27-2005).]

#60269 12/27/05 02:43 PM
Joined: May 2001
Posts: 376
One more thing.Are they seriously forcing unfiltered air into a restaurant or is it an exhaust fan thats been running in reveres for years.Mushrooms are typically exhaust only.

#60270 12/27/05 04:15 PM
Joined: Oct 2004
Posts: 135
It's common to see make-up air fans installed in commercial kitchens. The exhausted air needs to be replaced. I would suggest, if after checking the start gear, including the centifugal switch mechanism (sometimes they stick due to grease build-up or just corrosion) that you may want to look into a 'anti-windmill brake'. This would stop reverse rotation caused by negative pressure or wind. They are available at HVAC supply houses, Johnstone carries them. I don't know about the noise you're talking about, that could be a lot of things.

#60271 12/27/05 05:47 PM
Joined: May 2001
Posts: 376
I think i read it wrong anyway.I think he meant blasting air on the roof top.I had pictured a typical exhaust fan with the mushroom 2 feet off a flat rooftop sucking in dust and any bacteria thats forming in the sun warmed rain puddles around it.You cant use any fan for an intake unless listed for that purpose and it must be filtered is the point i guess i was trying to make.

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