Don Young wrote:
"Bobo" wrote in message
...
On Fri, 13 Jul 2007 23:38:13 GMT, "Mike Dobony"
wrote:

"Telstra" wrote in message
...
"Bobo" wrote in message
...
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"