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Is it possible for an AC compressor fan motor to run in reverse? It seems very odd to me that it could reverse polarity. But this is what I was told by a technician.

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    yes it's possible, depending somewhat on the type of motor, if the wiring is changd it can run in reverse.
    – Jasen
    Feb 5 at 3:09
  • Can't vouch for that motor in specific, but just about every electric drill on the market has a reverse mode, as do most ceiling fans. As do... Yes, a reversible AC motor is certainly possible. In fact the trick is getting one to reliably start in a specific direction; a basic design will happily spin either way.
    – keshlam
    Feb 5 at 7:46
  • AC motors can definitely run in reverse. Some of them also has a switch that changes its direction. I had an AC unit that had its fan running in the wrong direction and wouldn't properly cool the house. Went outside, flipped the switch, and fixed the problem.
    – Nelson
    Feb 5 at 8:27
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    Why the question? Are you trying to do something in particular? Feb 5 at 11:17
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    Get an opinion from another HVAC company. The motor may be good but some other issue may have occurred that affected the wiring.
    – RMDman
    Feb 5 at 13:12

3 Answers 3

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This depends too much on the type of motor for there to be any definitive answer. On a brushed / universal motor, reverse the field and armature relative to each other and the motor will reverse.

What is certainly true is a single-phase straight induction motor is very much like a 1-cylinder steam engine - it doesn't care at all which direction it spins, and once it's running, momentum will keep it running in that direction. It actually needs a "kick" to get cranking in the right direction. That happens via starter fields, shaded poles, or other schemes.

With a shaded pole, you're going to have trouble getting that to run in reverse. (unless you build in two shaded poles, and selectively open one of them). But with a starter field, it should be possible to reverse the field or use a different field to kick it in the other direction.

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Yes it is possible for that fan motor to run in reverse, and it is a routine failure mode -- but usually not a failure of the motor!

That motor is almost certainly a single-phase induction motor. This kind of motor relies on a capacitor - or an external push - to help it start running. When the capacitor is working it'll ensure the motor always spins the correct direction. When the capacitor is absent or is failed a slight breeze could be enough to get the fan spinning, and it can easily happen that the breeze gets the motor turning the wrong direction. You could even reach a stick in through the grille and push the fan in the correct direction, then turn on the power, to convince yourself that the motor can still run in the correct direction.

Moral of the story: test (or just replace) the capacitor.

Note that if the motor is powered on but nothing helps it to start spinning it'll sit there motionless and generate quite a lot of heat until its built-in thermal protection cuts the power. Eventually it'll cool down and try to run again. This cyclic overheating will cause damage and eventually ruin the motor, but given that it is still working at this point, might as well give it a new capacitor and see how much more life the motor has to give.

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Yes it is possible

Motors may have a reverse switch, and many will run in reverse if they are wired wrong at installation.

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