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Double fan cooling

Discussion in 'Electronic Repair' started by N Cook, Sep 23, 2007.

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  1. N Cook

    N Cook Guest

    2 of the same fan blowing air in over the heatsink but there is a noise like
    a variable direction wind or slightly gusty wind as though one fan has a
    bearing problem.
    It would seem one fan is fighting the other to some extent, significant ? or
    just one of those things ? .
    I was thinking of putting a dividing baffle between the 2.

    Disconnecting each in turn and they run perfectly normally with no change of
    sound. I may dig out a strobe if i can remember where it is and see if the
    blade speed does vary in the double active case.
  2. Beat oscillation. Basically, the normal fan physical noise pitch of the
    two fans are _almost_ the same. What you're hearing is the difference between
    the two. (it may be less than 1Hz, because the fans will probably be quite
    closely matched in noise and speed).

    It doesn't cause any harm other than you getting fed up with the noise and
    smashing it to bits.

    To "fix" the problem, you could try to physically isolate the two fans from
    each other (or the common chassis) by using some sponge or rubber or some
    such, to help prevent fan vibration getting into the chassis or other fan (if
    you have them physically stuck together).
  3. N Cook

    N Cook Guest

    I should have said these are 3inch, 12V dc fans rather than shaded pole
    motors so unlikely to match speed that closely. Both are mounted rigidly, no
    grommets, to steel chassis and separated by 1.2 inch , the nearest parts of
    the heatsink fins are only 3/8 inch from the fan surrounds.
    Placing a piece of card between the 2 fans seems to stop the interference
    but not desirable in the case of failure of one fan perhaps.
  4. AJ

    AJ Guest

    Have you tried reversing one at the mount, using one fan to direct the air
    in one direction and the other one directing air the other direction? S.A.
    having them both direct air physically away from the heat sink or have one
    direct air towards the heat sink and the other removing air from the same
    area. Sounds like you are getting a vibration doppler effect due to the
    physical operations of both fans in a limited area.
  5. Jim Yanik

    Jim Yanik Guest

    sounds like a case of the heterodyning fans!
  6. N Cook

    N Cook Guest

    Cor a googlewhack , can I claim the prize.

    Both fans are 1/2 inch apart, 3/8 inch from the nearest part of the heatsink
    but heatsink channels an inch deep.
    Trying a piece of card divider in that 3/8 ins made no difference but the
    airflows could still mix in those channels.
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