Fan speeds are controlled by switching capacitors of different sizes in
series with the supply line. If using a fan control other than the internal
pull chain control, one should leave the fan mounted one on "high".
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I recently installed three ceiling fans in a warehouse space. Of the
three units, one of them failed shortly after installation (the
installer didn't know what he was doing and blew it up), and someone
else replaced the wall controller with another unit. The replacement
wall controller had a 4 position slider (Off / Low / Med / Hi* ) like
the original, but I don't think it's exactly the same internally
because in the "Off" position, the fan is still moving at a very slow
rate (2-4 cycles per minute). Plus the slider goes the wrong way, but
that's just cosmetic.

The fans have a single hot wire going to them (and a neutral coming
back), so there's no fancy tach/feedback stuff going on here, just
supply power manipulation.

I'd like to replace the failed controller with the exact same model by
the same mfg (Rhine Electronic, Model UC9020, apparently Taiwanese) but
their website is nonfunctional and I can't even figure out who their
distributor in the US might be. So now I'm looking into at least
getting a controller that works the same.

I took the failed controller apart and it is built around two large
capacitors, labeled 8.0 uF and 12.0 uF (actually they're rated "uFJ",
whatever that means). There are a few resistors, the 4 position slider
switch and then that's it. I assume that this serves to cut the AC
waveform or drop the voltage somehow.

If I go buy a replacement fan controller, what should I be looking for?
Are there different types? (e.g. voltage limiting? current limiting?)
All the failed controller label says is the current rating (1.5A).
Obviously it is possible to get the wrong kind, because that's what
I've got now.