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Posted by William Underhill on July 13, 2007, 11:58 pm
Don Young wrote:
>> 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?
>>>
>> Googling I've found one reference to a motor running backwards
>> sometimes as the capacitor failed, I haven't found even one more
>> mention of this though,
>>
> I believe an intermittent or failing starting capacitor has been known to
> cause intermittent reverse starting of lightly loaded motors. There seems to
> be some validity to the theoretical explanation of how this can happen. I
> would certainly try replacing the capacitor.
Hm - not sure. Depends on the exact type of capacitor motor. Capacitor
start only shouldn't run at all if the cap is bad, as there's no phase
shift in the armature windings. A cap-start, cap-run with a bad start
cap, now... that could very well be a reverse start, depending on how
much phase shift you wind up getting on the main field with the run cap.
I would strongly recommend having the motor checked over by a electrical
shop with motor specialisation - I'm going on fuzzily remembered motor
theory here.
Yours aye,
W. Underhill
--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from
poltroons who fear to make a choice!" R.A. Heinlein, "Double Star"
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