12volt computer fan - want to reverse spin direction

Discussion in 'The Projects Forum' started by Offtwice, Oct 22, 2012.

  1. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    I have 4 12 volt computer fans (that I am not using for a computer) that I want to reverse the spin direction on. I understand there will be limitations due to the capacity of the fans themselves not designed to spin in alternate direction..........but can it be done?

    There are three wires, hot-ground-RPM. When I reverse the hot-ground.......the blades don't spin at all..........so I am assuming something internally is keeping me from trying this. Can I removed or bypass this restriction? It seems like this should be achievable..........but I am not experienced at this........and before I butcher a fan (trial and error) I figured I would ask the question here...........to see if someone smarter than me could provide some insight.

    Thanks!

    Offtwice
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Can you turn them over? Many fans are made to be (mechanically) reversible. The fan blades are likely foils that will have lower performance going the wrong direction.
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    I don't think you can reverse the rotation since its probably BLDC and the designers of the fan/blades had a specific direction and aerodynamics in mind.
     
    Last edited: Oct 22, 2012
  4. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    Unfortunately - the housings that I fabricated and chromed will not facilitate - flipping them over.
     
  5. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    I believe I could reinforce the blades to handle reverse direction - but am wondering if the wiring can be changed to get the fan to spin the
    Opposite direction.
     
  6. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    If I submit a picture of the board can anyone tell me where to solder the ground and hot leads to make this work - or to identify what piece to bypass to get the result I want?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    It's not a matter of reversing the polarity; that could in fact damage the fan.

    Those BLDC (Brush-less Direct Current) fan motors use Hall-effect sensors triggered by the relative positions of the magnets to energize the windings in a sequence that causes rotation in a specific direction. You might get it to run backwards by reversing the leads to all of the electromagnetic windings.

    The aerodynamics of the fan blades is another matter; they were designed to be very efficient when running in ONE direction. If you try to rotate them in reverse, the performance will be dismal; you will only get a fraction of the CFM rating through it.

    You would be much better off to either re-do the enclosure, or buy fans that are appropriate for your application. It's really not worth your time and effort to try to run them in reverse rotation.
     
  8. olphart

    New Member

    Sep 22, 2012
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    Howdy, JohnInTX is spot on, small DC fans are one polarity, no changing that. I don't quite see how they can't be flipped, they're usually built symmetrical. Gotta find the way to flip'em. <<<)))
     
  9. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    Well - thanks for the advice - going to tackle this another way - found replacement blades that I can mount on the housing ;)
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Oh please oh please.. tell us the reason you can't just turn the whole fan around 180 degrees. All these fans have a directional arrow on the housing showing airflow direction..
    Did you glue it to something instead of simply using the mounting holes?
     
    shortbus likes this.
  11. wayneh

    Expert

    Sep 9, 2010
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    I'd also like to hear why this is a concern. The fans should have been placed to blow into their heat sinks, if that's what they're being used for. Was this a boo boo from the beginning, that now needs to be fixed?
     
  12. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    you'll also find that mechanically, the blades, when run, settle into a support bearing. Reversing them would likely cause short life.
     
  13. kc5tpa

    Member

    Sep 21, 2012
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    Agree, rebuilding not an option, and blade direction/CFM massive factor. What are you planning on using this to cool? If it is something of critical design or delicate balance of temperature needed, forgo this route and find something else to work.
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Sorry to argue but fans should never be used to blow into a low volume or complex restricted cavity like a heatsink!

    They should be used to suck air from the heatsink as the fan will move a lot more air generating a negative pressure at the restriction than trying to generate a positive pressure at the restriction (which will just cause the fan to cavitate).
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Nope, that's backwards. You want to drive air INTO the heatsink. This ensures that the heat-removing medium - the air - is at a higher density in the vicinity of the sink and thus able to support higher conduction and convection rates from the sink into the medium. Turned around, at the extreme you would have a vacuum on the heat sink and leave no mechanism to remove heat except by radiation. Of course you would never actually achieve a vacuum but the effect of reduced pressure is to reduce the rates of conduction and convection.

    I know there is endless argument about this all over the internet. The argument persists because the difference - for most fans and sinks - is small. The typical fan is not able to put much pressure or vacuum on the typical sink, whichever way it is oriented. It's more about CFM in that case. The studies where both configurations are compared usually - not always - end up supporting my statement above. The studies are complicated by ambient conditions. For instance the sink fans need to work together with the cabinet fans so the heat actually leaves the cabinet.
     
  16. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I understand your reasoning but you are wrong. The air density is irrelevant as the density difference might be less than a couple of percent, BUT trying to blow into a complex or restrictive area with a rotary fan there will be cavitation and practically no flow, which is a massive loss of cooling!

    You know it is cavitating and not moving air as the fan RPM will go up, which is one reason the kids think it's "better" on their CPU, which is understandable I guess.

    I'm not that interested in what the net noobs are arguing about, I was properly trained in industry on heat management in electronics cabinets in cases where it really matters and they (and you too sorry to say) are simply wrong. Never use a rotary fan to blow into a restrictive space, especially something like a PC heatsink with very little volume.

    I've fixed a few PCs where some noob at the PC shop tried to make the fan blow on the heatsink, and they all gave significantly lower CPU temps and lower fan RPM after I reversed the fan.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Sooo, you're saying Intel, AMD, and the rest of the computer industry is doing it wrong? The industry standard is to have the fan blow INto the heatsink, not pull through it. The noobs argue about reversing this and occasionally find a situation where it helps, probably because of a particular case airflow situation, but the industry pros all adhere to the standard.

    Air density is not irrelevant, and the side-to-side difference is not that small. What do you think causes the cavitation you mentioned? Elevated pressure on the exhaust (as seen blowing onto a heatsink) and ambient pressure on the intake will not result in cavitation anyway. For that you need low pressure at the intake, like blocking the suction on a vacuum cleaner or trying to pull air through a heatsink. Plugging the exit of a vacuum cleaner does not speed up the fan.
     
  18. Offtwice

    Thread Starter New Member

    Oct 22, 2012
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    These fans arent mounted in a computer........I am using them in a golf cart..........it was my mistake that I fabricated chrome trim rings to mount them before I tested them to ensure they were blowing and not ......... well sucking.

    I can assure all of the respondents here (and thank you btw) that I cannot flip the fans over nor use alternative type fans (as the fabrication of chrome housing are completed and were expensive the first time) ..........and you have given me accurate responses that replacement blades and reversing polarity are not viable solutions.

    So..........I am leaving them as they are.............lol, they look great but provide absolutely no benefit to the passengers. :confused::confused:

    I am in good spirits about it, despite being very disappointed.:D:D

    Thanks for all the responses!!!!
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Sorry we could not have been of more help! :)

    I'm not sure Intel or AMD even make fans and heatsinks? The make semiconductors and the fans and sinks are probably bought cheap from China etc like most stuff. And I'm not sure where you got the "industry standard" from, my PC is genuine intel with a reasonable quality MB and the fan came properly configured as suck. However I have seen lots of incorrectly intalled fans which were easily flipped over.

    Rotary fans don't blow and suck equally. Same as with centrifugal blowers etc the air has mass, and it is natural for the blades of the fan to throw the air out and away from the fan, very efficiently if there is no obstruction. So when the outlet of the fan is to open air the fan will move quite a lot of air and is effective at generating a negative pressure at the inlet, giving you best case air movement when the inlet is the restriction and not the exhaust. That negative pressure at the inlet is still established very well even if the inlet is restrictive.

    If the fan is attempting to blow into a restriced area (like into a PC heatsink with little clearance) the air can't be thrown outward and away efficiently and instead creates pressure waves and the fan ends up pushing those pressure waves around in a circle in the cavity and moving much less air in total. The pressure difference in that zone compared to ambient pressure will not be as great as when the fan is sucking. And as with cavitation the fan RPM rises as it is not moving as much air (just pushing it around in a circle) and is experiencing less load.

    I'm not going to go on arguing with you here as the OP has wound up his thread. :) Maybe if you want to argue this fan/restriction behaviour further you could post a thread in the physics forum?
     
  20. t06afre

    AAC Fanatic!

    May 11, 2009
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    I WILL DONATE $5 MILLION to a charity of your choice. If You Release College Records. No just joking:p. But heat management is something I need to read up on. Do you have any references on this topic
     
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