Fan motor runs backwards

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

Reply to
Bobo
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Hmm, Maybe switch is intermittent. There is no guarantee new switch is 100%.

Reply to
Tony Hwang

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

Reply to
Telstra

But how can this happen occasionally?

Reply to
Mike Dobony

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,

Reply to
Bobo

That is a possibility, but the explanation given the circumstances does not fit. Switching the active to the other terminal will reverse rotation permanently, not occasionally.

Reply to
Mike Dobony

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.

Don Young

Reply to
Don Young

But he's stopped talking about switching the actived to the other terminal.

His words just above yours are "as a capacitor failed".

That might be intermittent.

Reply to
mm

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

Reply to
William Underhill

Okay, poorly worded question. How would a switched connection cause occasional reversal? Is that better?

Reply to
Mike Dobony

Thought. Let's say the start cap is intermittent or failed and there is a thermal draft rotating the blades. Applying power...the motor will continue in that direction.

Reply to
pheeh.zero

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

Red

Reply to
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)

Reply to
pheeh.zero

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.

Reply to
Mark Lloyd

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.

Reply to
Butzmark

"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.

Reply to
pheeh.zero

Sounds like a cap problem to me.

Bob

Reply to
Bob F

Sounds like a cap problem to me.

Bob

Daaaaaaah, yah think?

Reply to
pheeh.zero

It may also be the centrifugal start switch is not making contact for the start winding. It can be replace by a solid-state device if faulty. Or replace/clean the contacts.

Reply to
pheeh.zero

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

Reply to
Dave Martindale

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