Fan motor runs backwards

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

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,

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.

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

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.

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

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

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.

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)

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.

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 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.

Sounds like a cap problem to me.

Bob

Sounds like a cap problem to me.

Bob

Daaaaaaah, yah think?

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.

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