How is this Possible? Nutone Model 800 fart fan suddenly runs backwards!

[ctdahle]

As far as I know, this is impossible, but it has happened.

Today I tore down the fart fan in the family bathroom for its annual cleaning.

I took off the ceiling escutcheon and ran it through the dishwasher.

I removed the fan motor and I scraped, wiped, dusted and blew away all the caked on grotus, then added a drop of oil at each end of the motor and reinstalled it. I have done this annually for the last 14 years.

After reinstalling the fan, I turned it on just to make sure I'd plugged it in. I was greated by a blast of hot dusty air as the fan sucked hot air out of the attic and blew it into the bathroom.

It is a pretty small (.8) amp AC motor. It spins up nice and quiet, just like always, but it is turning backwards. The fan motor has a bracket that can only bolt on one way, and the motor/bracket assembly can only go into the ceiling mount one way, so it is simply not possible that I have installed the motor "upside down" or "backward".

While I "know" that switching polarity cannot change the direction of rotation of an AC motor, it is possible to reverse the plug on this motor, so I tried that. As expected, this did not change anything.

Can any of you suggest ANY way that an AC motor can spontaneously start running in the wrong direction?

The only thing I can think of is that we did have a power failure about a month back which caused the protection circuits of several of our electronic toys to trip, but I can't see how that would "reverse" an AC motor.

[RegUS_PatOff]

can you post a picture of the Motor ?

[ctdahle]

Yeah, I think so, by attaching the files. I cut them down to 800

I'm attaching a picture of the motor sitting on my workbench. As you can see there is a mounting bracket that makes it impossible to install the motor incorrectly.

The second picture shows the motor properly installed in the cieling housing. The bracket has to be down and the fan blades on top of the motor. A lock out tab prevents the motor from being installed upside down.

Looks like I am allowed to upload one more picture.

This third one is the motor from what should be "out". You may be able to see the curvature of the fan blade showing that the fan should turn clockwise so that the rounded edges of the blades are the leading edges and push air out of the bathroom and into the vent housing and down the vent pipe. Instead, it turns counter clockwise, the squared off edges lead and the fan is pulling air from the attic vent pipe into the room.

I swear this thing has been working properly since I've owned the house and I have pulled it out for cleaning at least a dozen times.

[RegUS_PatOff]

was the Bracket removed from the Motor ?

[ctdahle]

No, not this time, but I have removed it before. The motor is old enough that it does not have sealed bearings and the first few times I cleaned the fan, I removed the impeller blade and mounting bracket bracket to get at the front and rear oil ports. A few years ago I found a nifty little oil syringe that allowed me to reach the front and rear oil ports without removing the impeller and the mounting bracket, so I haven't removed the bracket lately.

Importantly, if the bracket is removed, it has to go back on exactly the way it came off or it won't fit. If you look at the full sized view of the first picture above, you will see a relief stamped into the bracket. This relief lines up with an oil port on the motor housing. If the bracket is removed, it has to be re-mounted exactly the same way or it just wont fit back together.

Also, on the right side of the first picture, you will see that on the bracket arm, there is a spring clip with a tab that points toward the bracket arm. This tab slides into the mounting grommet at the end of the bracket, it prevents the bracket from being hung incorrectly on the mounting pins in the fan housing. In other words, there is only one way for the fan blade to attach to the motor, only one way for the motor to attach to the motor bracket, and only one way that the motor bracket can be placed within the fan housing.

While it is _possible_ to bend the tab out of the way and install the fan motor upside down, doing so causes the fan to protrude 4-5 inches into the room and prevents the installation of the escutcheon, so there is simply no way that I am somehow installing the motor "wrong".

The motor is running backwards and while I cannot determine when it started running backwards, it was definitely blowing humid and stinky air OUT of the bathroom when I finished cleaning it last fall, and through the winter, and it is definitely pulling dirt and dust IN from the roof vent now.

[ctdahle]

Seriously Guys, I understand that you think I'm nuts, but the motor is spinning backwards after probably 30 years of spinning forwards.

The fan is in the kids bathroom that I do not use, so it may have been doing this for awhile, but there has to be a reason.

I was a kitchen and bath contractor before I started teaching physics, so I did check all of the normal things. There are no obstructions, the louvers are not an issue.

Even if it were mechanically possible to install the motor "wrong" so that it blows the other way, the power cord would then have to run through the fan blade.

The darn thing is spinning the wrong direction, there has to be a reason, and there has to be a way to fix it. The alternative is to believe in faeries and leprechauns.

[RegUS_PatOff]

Try this:

Give the Fan Blade a good spin in the desired direction.

While the Fan Blade is still spinning, apply power..

What happens ?

[ctdahle]

Thank for staying with me on this. I did try spinning it the "right way". It quickly comes to a stop and then starts running the wrong way.

I took it apart last night and there are no switches, jumpers or anything that could be flipped or switched or bumped to change motor direction. It's just a basic AC induction motor.

I'm supposing I could pull out the stator coils and flip them over, but I'd have to hack a new hole in the case for the power cord and the cooling vent holes would be blocked.

I'm thinking about making a new impeller blade from 1/8" aircraft plywood, setting the blades to the opposite pitch direction, but getting something like that to be true and balanced would be a pretty major endeavour.

I've also thought about making a new mounting bracket that would put the motor "downstream", that is further into the vent housing so that it moves air the right way, but this would also present a routing problem for the wiring

Really, these are harebrained solutions...the whole thing is flippin' nuts.

The Nutone people think I'm batshit crazy (and they want $105 bucks for a new motor).

[Cactus Bob]

these motors can reverse . Most of the time it has to do with water getting on the windings , the winding then shorts and the motor reverses

the motor should run hotter than it used to and may even make a burning smell and overheat

I would not mess with it , these fans can and have burned down houses before

YOU KNOW THAT IT'S RUNNING THE WRONG WAY AND THAT'S NOT RIGHT,do you really want to chance some one will leave it on and you loose your house over $100.00 ?

Please replace it or unplug it .........PLEASE .....

[Scottthewolf]

I second Cactus Bob's suggestion.

[philipwrussell]

No dout about it motor must be replaced. Maybe you can cross referance with a cheaper mpotor from say graingers using motor size measurments and rotation etc.

[ctdahle]

Well I'll take that as a logical explanation. I was not aware that an AC inductor motor could run at all, let alone run backwards if the stator coils were shorted.

To rest your minds, it's not plugged in or even installed except in the controlled environment of the test stand on my work bench, and only allowed to run under "adult supervision". But it is not heating up at all. I let it run for 2 hours yesterday in the test stand. The case was barely warm to the touch, and it's only drawing .79 amps...right on spec.

Still, there is logic in thinking that a bath fan motor, after 30 years, may have been exposed to enough moisture to have broken down the enamel on the stator windings. Certainly the mounting bracket is rusty, as is the fan housing. And we did have a power failure/lightning storm about a month back that killed the cable tv box, and some other gizmos. Maybe that sent a spike down the line that melted the enamel and allowed a short in the stator coils.

I'm NOT going to attempt to rewind it, even though there is this nice big spool of enameled copper wire sitting here in front of me...

Phillip, you read my mind.

[certified tech group 51]

The induction motor uses 'electromagnetic induction' for operation, ( not 'magnetic induction ' ) The rotor has it's own magnetic field after the stator creates it's own magnetic field first....... Could it be possible that the rotor could be partially magnetized to create a reverse direction, I.E. when the stators field is first created..... Talking milli-seconds here ...... Any more talk like this hurts my head... ..