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Fan reverses rotation by it's self ?

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Davebones

Senior Member
Location
North Of Tampa Fl
We have a 120 volt bathroom exhaust fan that turns on and off with the switch for the lights . The other day one of the ladies claimed that it was blowing hot air into the bathroom . My boss and one of the other electrician's removed the cover on the roof to look at it . They claim that when they turn the disc off and then on the fan reverses rotation . They said they did this a few times on the roof and each time they turned it off and then on it changes rotation . I have not personally seen this but I wondered if anyone has ever seen something similar . I know a contractor electrician changed this out a few month's ago . I'm pretty sure this is a single phase 120 -208 volt fan .....
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Had that recently with an exhaust fan for a cooking hood - didn't always reverse but sometimes did. Most the time it failed to start altogether. Centrifugal switch that opens circuit to start capacitor after reaching certain speed failed. Most the time it would fail to start at all, but if there were enough of a draft to make it turn either direction while attempting to start that was enough to get it to start in that direction.
 
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Davebones

Senior Member
Location
North Of Tampa Fl
I do know we have negative pressure in the building so when the fans off , if its turning it would be in the wrong direction . I will at it myself when I get a chance .
 
Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
The start capacitor may be bad. If it's bad, it won't give the motor a kick in the right direction when starting. Significant back pressure can also overwhelm the kick. There should be a flapper over the duct to stop back flow.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Coppersmith said:
The start capacitor may be bad. If it's bad, it won't give the motor a kick in the right direction when starting. Significant back pressure can also overwhelm the kick. There should be a flapper over the duct to stop back flow.
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My incident it happened to be the centrifugal switch that had the problem, but yes a bad start capacitor would have been about same issue. This for capacitor start motor, if it has a run capacitor and the run cap is good, that should help get it going the right direction even if switch for start cap failed.
 
M

Michael15956

Senior Member
Location
NE Ohio
kwired said:
Had that recently with an exhaust fan for a cooking hood - didn't always reverse but sometimes did. Most the time it failed to start altogether. Centrifugal switch that opens circuit to start capacitor after reaching certain speed failed. Most the time it would fail to start at all, but if there were enough of a draft to make it turn either direction while attempting to start that was enough to get it to start in that direction.
Click to expand...

That centrifugal switch is an ingenious device, it's cool how it opens & closes the cap circuit with centripetal force and a spring!
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
rambojoe said:
wait till you see a dirt bike engine start in reverse... (well, a 2 stroke...)
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were you trying to ride the bike and went the wrong direction?:cool:

I think it was pretty common for some old two stroke engines to run either direction whether intended or not.
 
drcampbell

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Registered Professional Engineer
kwired said:
... I think it was pretty common for some old two stroke engines to run either direction whether intended or not.
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Not just old engines. Large marine Diesel engines are often two-stroke, and run equally well in either direction. They're direct drive and when you want to reverse the boat, just stop the engine and restart it the other way. This saves the weight, space, cost and energy-efficiency penalties of a gearbox.

And not just two-stroke engines, either. Many older four-stroke Diesel engines, with a mechanical injection pump and injection timing near TDC, will run the other way if something starts turning them the other way. It's a little alarming when this happens, as they suck air into the tailpipe and blow exhaust out the air cleaner, and the most-common time for this to happen is when climbing a steep hill with too heavy a load or in too high a gear. Let up on the pedal for a moment, the truck rolls downhill in reverse, its momentum turns the engine the other way, and the next thing you know, the engine is POWERING the truck downhill in reverse and there's exhaust smoke emanating from under the hood.
 
GeorgeB

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
I think emissions standards eliminated them, but many 2 stroke golf carts were like the marine diesels mentioned reversing the motor to reverse the cart.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
99.99999% of fart fans are Shaded Pole motors, so no start switch, no capacitor. The issue with them however is that the phase shift caused by the shading coil is very weak and IF the fan is already spinning when you energize it, it will keep spinning in that direction.

So the problem here most likely is that the exhaust duct does not have a back-draft flapper on it, or it is not working correctly, so the air is back flowing and spinning the fan backward ever so slightly BEFORE the switch is closed and therefore the fan will keep spinning that wrong direction when energized. Not an uncommon problem if there is no back-draft flapper.
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
But it is harder to explain a perfect reversal of direction on each start.
One hypothesis in that direction is that there is no starting winding or shaded pole in play, but instead there is a cogging effect of the main run poles from residual magnetism.. Each time the shaft will coast to a stop just past the "top dead center" point and be pulled in the opposite direction when voltage is applied, starting always in the opposite direction.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Jraef said:
99.99999% of fart fans are Shaded Pole motors, so no start switch, no capacitor. The issue with them however is that the phase shift caused by the shading coil is very weak and IF the fan is already spinning when you energize it, it will keep spinning in that direction.
Click to expand...
That reminds me of the time in college when before a physics lecture started someone had taken the clock in the back down and managed to get it running backwards. The professor got very confused as he took glances at the clock to see how things were progressing (or not). Then after a few chuckles from the students he realized he'd been had. ;)
It was an old clock so it probably had a synchronous motor with a shaded pole, and it was susceptible to getting reversed as Jraef mentioned.

Good call by Jraef on the OP's puzzling conundrum. :thumbsup:
 
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