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.
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
On Sat, 14 Jul 2007 06:14:24 -0700, firstname.lastname@example.org wrote:
You might have it there. I was surprised how much wind was blowing in
the attic when I crawled up there to look at the thing. It sits right
under a ridge vent. I got it to run the right way by starting the fan
in the right direction with a stick and then turning it on.
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
"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
"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.
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.
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-
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