14
\$\begingroup\$

I've got a PC fan here (I've peeled off a sticker).

enter image description here

And although this one doesn't come with the usual 3 or 4 wire connectors, it has enough electronics in it to only operate when power + is on red, and - is black but/and it won't operate when power is connected the other way round. I don't know anything about how these (brushless?) PC fans are controlled, but I assume one of the basic/first steps in the circuitry is to make sure polarity is right. Right?

So, now, is it possible to reverse the motor with a simple hack, or is is not possible without heavy modification of the controller or stuff...

Ideas how to trick it into spinning the other way round?

(Notes:

  • Sorry for not providing the exact model of the fan. Let's assume it's pretty generic.

  • Also, I know the fan blades are designed to spin this way round, and reversing it would mean having a less optimal fan.

  • I can't just flip the housing. It's a long story why, but that's not an option.

  • I couldn't get the casing open or the fan blades off, it's all pretty sturdy)

Update:

Taking all the feedback I had so far into account, I first tried to get that darn thing open (no success) and then had a closer look from the outside:

enter image description here

What we see here from the side is (4 legs) a hall sensor, right?

That means/would mean: it's the "sensored type" of fan, meaning that even breaking it open and swapping motor cables would have no effect (no cables btw., the motor seems soldered to the controller board).

As I have a hard time deciding on the accepted answer, I think I have to check Olin's as he was the first to point that out, although pericynthion was first with an answer.

\$\endgroup\$
12
  • 8
    \$\begingroup\$ why would you want to reverse it? because the physical blades of the fan arn't reversible and won't work properly running backwards anyway. If you can't just flip the entire housing then this fan simply won't work in the other direction. \$\endgroup\$
    – JamesRyan
    Commented May 12, 2016 at 16:22
  • 3
    \$\begingroup\$ To open it, remove that black plug that's in the picture, and then use pliers to remove the c clip that holds the fan shaft in place. Like arstechnica.com/civis/viewtopic.php?f=7&t=1301035 \$\endgroup\$
    – Passerby
    Commented May 12, 2016 at 17:11
  • 8
    \$\begingroup\$ Why can't you just turn the fan around 180 degrees, so that it blows air in the oppssite direction, without actually reversing the motor direction? \$\endgroup\$
    – Criticizing Israel not allowed
    Commented May 12, 2016 at 20:54
  • 10
    \$\begingroup\$ Something definitely smells fishy here. These types of fans normally have a symmetrical housing by design, so that they can be installed either way around. The only explanation I can think of for why the OP can't just flip the fan around is that they superglued it to their PC case or something equally crazy. \$\endgroup\$
    – Ilmari Karonen
    Commented May 13, 2016 at 8:39
  • 2
    \$\begingroup\$ Sigh... OK, I'm trying to shed light on the why: I don't use the outer housing, it's not installed in a PC or similar, it's mounted near the motor's axis, and as I can't bolt down the rotor side (obviously) the non-moving part is where the thing is held - but it blows into the direction of where it's held now, and I need it the other way round. ;) makes a tremendous amount of sense, right?! \$\endgroup\$
    – isync
    Commented May 13, 2016 at 23:21

7 Answers 7

Reset to default
37
\$\begingroup\$

Reversing the direction of rotation will be difficult. Since you only supply power, there is a controller in the fan that senses rotor position and commutates the motor accordingly. If you can get in there, you can probably reverse the direction by flipping two of the three sensors and two of the three windings. That won't be easy to do since these are all nicely integrated onto a small board.

Supplying negative power won't work, just fry the electronics.

However, you can still "reverse" the fan by simply installing it backwards.

\$\endgroup\$
3
  • \$\begingroup\$ Mh. You're second to suggest flipping cables. But as you already pointed out "getting in there" seems/seemed difficult. But I get the notion from answers it's the only way, ultimately. And only if it's the "simpler" type of controller, one without sensors. "Installing backwards" is sadly not an option (see notes). \$\endgroup\$
    – isync
    Commented May 12, 2016 at 15:50
  • 6
    \$\begingroup\$ I agree, even if this particular fan can't be flipped there are many available which have virtually identical installation form factors for either 'blow' or suck' and this is likely to be cheaper and easier than trying to hack the one you have. If you need to change direction on the fly you could always mount two in series and opposed and switch between them electronically. \$\endgroup\$
    – Chris Johns
    Commented May 12, 2016 at 21:25
  • \$\begingroup\$ Yes. Doable, but, unless outsourced to someone who will do it for a single hamburger, you're better off buying one. \$\endgroup\$
    – kaay
    Commented May 13, 2016 at 9:01
15
\$\begingroup\$

Most PC fans are two phase with 4 pole stators, PM rotor and a single Hall sensor. I think I've seen 3-phase but not for more than 15 years.

Eg. (from here)

enter image description here

You may be able to just reverse the two coil wires to make this work. There is more than one way of ensuring that the two-phase motor (which otherwise would be happy to spin in either direction) starts in the correct direction.

It's also possible a small magnetic field would bias the sensor so it would spin in the opposite direction, but modern hall driver chips use edge detection and won't be fooled by that.

\$\endgroup\$
8
\$\begingroup\$

The circuit is a three phase inverter. Ultimately there will be three wires going from the PCB into the motor windings. Swap two of those three.

\$\endgroup\$
4
  • \$\begingroup\$ Thought about this (modifying the wires between controller and actual motor) - as I couldn't get the case open - that was not an option (so far). But your answer implies: it's not possible from the outside, right? \$\endgroup\$
    – isync
    Commented May 12, 2016 at 15:48
  • 6
    \$\begingroup\$ This only works if the driver is using the "sensorless" method of position sensing. If there are position sensors, like Hall effect, then the driver will get very confused when the sensors and the coils don't match. It will probably just sit there and vibrate unless the corresponding two sensors are flipped too. \$\endgroup\$
    – Olin Lathrop
    Commented May 12, 2016 at 15:50
  • \$\begingroup\$ Ah, ok. That's what Olin is referring to. Mh.. \$\endgroup\$
    – isync
    Commented May 12, 2016 at 15:51
  • 5
    \$\begingroup\$ While I don't doubt that what you state is correct, I think it would improve your answer considerably if you add a sentence or two on why you conclude that the circuit is a three phase inverter, particularly given that we don't know the specific model fan involved in the question. \$\endgroup\$
    – user
    Commented May 12, 2016 at 16:29
6
\$\begingroup\$

No. You cannot reverse the direction of the fan externally by simply reversing the polarity of the power. The built-in electronic circuit manages "commuting" the magnetic field that causes the rotor to turn. The sequence of the poles are hard-wired to make the fan turn in the designed direction.

\$\endgroup\$
3
\$\begingroup\$

Also possible: some cheaper fans do actually run "backwards" if underpowered. The controller chip isn't supposed to do this but its actually an anti-stall mechanism so that it at least spins. Increase the voltage or current and it reverts to normal operation again.

\$\endgroup\$
1
\$\begingroup\$

Apart from the fan blades only working efficiently in forward direction, and centrifugal ones only pushing air in the forward direction, computer fan windings are not 3 phase. It is a single phase 4 pole winding, with the pole pieces shaped out slightly towards the acw side to decide the direction, and only one sensor to commutate it (see Spehro's picture). I don't know what effect it will have if you reverse the winding connections, but the fan will be inefficient even if it does work.

\$\endgroup\$
0
\$\begingroup\$

A Centrifugal fan may not reverse flow when rotating in the other direction. It just becomes a poorer fan. This depends on rotor geometry. You can lookup "centrifugal pump" on wikipedia. Regards, Wim Hubers

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.