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Fan motor runs backwards Bobo 07-13-2007
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Posted by Mark Lloyd on July 14, 2007, 10:12 am
On Fri, 13 Jul 2007 23:38:13 GMT, "Mike Dobony"

>
>>
>>> What's up with a third horse whole house fan motor that has begun to
>>> occasionally run backwards. This started happening about the same time
>>> the switch went bad. I put in an identical switch and didn't mess with
>>> the motor wiring. It's AC, 120.
>>> Thanks
>>>
>> The Fan motor capacitor has two terminals each terminal connected
>> to a motor winding and one also connected to the Active. Switching
>> the Active to the other terminal will reverse the rotation of the motor
>>
>
>But how can this happen occasionally?
>

I remember finding an old rotating Christmas-tree stand that would
start in either direction (randomly). This seems to be the natural
condition of some motors.
--
Mark Lloyd
http://notstupid.laughingsquid.com

"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy

AppliancePartsPros.com, Inc.
Posted by on July 14, 2007, 12:55 pm
"I remember finding an old rotating Christmas-tree stand that would
start in either direction (randomly). This seems to be the natural
condition of some motors."

That type is a syncro-timer motor. There is a cog or detent that gets
sticky or worn that allows it to run in the opposite direction.
When it starts in the direction that is not desired...it hits the arm
or cog...and bounces back to the correct direction.


Posted by Dave Martindale on July 17, 2007, 1:55 am
pheeh.zero@gmail.com writes:

>That type is a syncro-timer motor. There is a cog or detent that gets
>sticky or worn that allows it to run in the opposite direction.
>When it starts in the direction that is not desired...it hits the arm
>or cog...and bounces back to the correct direction.

Microwave oven turntable drive motors are also synchronous motors which
run in either direction. But the oven doesn't care which direction the
turntable goes, so the motor doesn't need the reversing mechanism.
Sometimes the turntable goes one way, sometimes it turns the other way.

        Dave

Posted by Red on July 14, 2007, 9:17 am
> What's up with a third horse whole house fan motor that has begun to
> occasionally run backwards. This started happening about the same time
> the switch went bad. I put in an identical switch and didn't mess with
> the motor wiring. It's AC, 120.
> Thanks

Which brings up a question I've always wondered about: why do most
microwave oven turntables randomly reverse direction at turn-on?

Red


Posted by on July 14, 2007, 9:45 am
>
> > What's up with a third horse whole house fan motor that has begun to
> > occasionally run backwards. This started happening about the same time
> > the switch went bad. I put in an identical switch and didn't mess with
> > the motor wiring. It's AC, 120.
> > Thanks
>
> Which brings up a question I've always wondered about: why do most
> microwave oven turntables randomly reverse direction at turn-on?
>
> Red

This is from memory...so bare-in-mind. A shaded-pole motor uses a
small induction current with very slight torque. These can start in
the opposite direction in small factors (size or sise for the english-
types 8^p)


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