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...and I thought I knew something about electricity!

Discussion in 'The Lounge' started by SqueamishOssifrage, Mar 3, 2021.

  1. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    My preamp is pure class A, which means it is effectively constantly running at full power, even if the modulated signal is less. Yes, I know that is an over-simplification, but it serves to illustrated why, after running for a few hours, it gets extremely hot. Because Cyprus can also get extremely hot, last spring I put a row of five small PC 12 volt 0.39 amp fans in a row across the cooling grid at the back of the top. There are lots of holes in the bottom of the chassis to allow air to circulate. The fans are powered by a 12 to 24 volt DC adjustable power supply, giving 5 amps at 18 volts, the minimum voltage without all stalling. I wired them up in series so I could keep the fan speed slow and quiet, rather than howling like a 747 on go-around power.

    It worked satisfactorily with the exception of fan 4, numbered from positive to negative, which stalled on lowering the voltage earlier than the others, thus limiting my ability to go for max quietness, as when stalled it acted as an inlet vent for the other fans, but I lived with it for the summer. Today I thought I would have another look at it.

    I tried various series/parallel configurations, but to no avail, as either that fan howled or the other fans stalled, so I just decided to put it back as it was. I took fan 4, the problem child, which was on wander-leads and rewired it back into the all-series loom, but to save time I just added it to the end of the current series as fan 5, +ve to -ve.

    It all worked perfectly. WTF?

    I rewired it back in the the fan 4 position - and it started stalling again. WTFingF?

    I have now put it back as fan 5 - working perfectly again!

    All the fans, which are good quality are exactly the same, bought at the same time out of the same box in the computer store's storeroom, and supposedly from the same manufacturers batch.

    I have no explanation for this behaviour - does anybody else?
  2. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    Just put it down to quantum theory or God
    Or fans might have a pecking order and you upset no 4.
    IvorETower and David Loxley like this.
  3. ascu75

    ascu75 Well-Known Member

    ? Only thing I can say is ‘wander leads’ did they make some extra resistance or am I talking crap as usual?
  4. David Loxley

    David Loxley Well-Known Member

    Forgive me, I know not of the background of Squeamish O. welcome to the field of electrical engineering.
    D. Lox. C.Eng. M.I.E.E.
    daft_biker likes this.
  5. dangie

    dangie Senior Knobhead

    Strangely enough neither have I.
    Basically because I haven't a clue as to what you're talking about :)
  6. Footloose

    Footloose Well-Known Member

    One of the fans is a snob? :rolleyes:
  7. SXH

    SXH Well-Known Member

    It's the runt of the litter?
  8. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    I'm impressed, but I would be even more impressed if you could tell me how a D.C. device can have a different behaviour depending on it's position in a serially connected loop. I have just rerun the experiment and the result is consistent with the earlier one.
  9. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    Not valid, as the wander leads were only used during the serial/parallel evaluation.
  10. SXH

    SXH Well-Known Member

    A less than optimal connection (with a marginally higher resistance) on a couple of the motors such that when the two are together, the resistance is high enough to cause a problem but when they are separated it's low enough for normal operation?
  11. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    I asked him about it, and he said he didn't do quantum mechanics as it was much too difficult.
    Learning and AndyTake2 like this.
  12. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    It must be something like that, but I am damned if I can see precisely how. Possibly due to inductance differences? Actually,that may be it. Back to my A level physics now, I think ! :eek::D
  13. David Loxley

    David Loxley Well-Known Member

    Five small d.c. fans each rated at 12V. supply. You connect them in series to a d.c. power supply set at 18V. If the motors are the same specification then each will have 3·6 across it. This would be a first check to see if they do in fact have that same value. Are they arranged to suck or blow? If these are as small as I might imagine then the manufacturing tolerances for the bearings and in particular the reluctance of the permanent magnet rotor and the magnetic gap between rotor and field pole pieces need only be marginal to be affected by the series arrangement given the well below specification operating voltage. Have you compared the volt drops across each motor when functioning 'correctly' with the volt drops when the 'odd-ball' stalls? Is the circuit current in the 'stalled' condition the same as that for the 'correct' connection?
    Funny stuff electricity
  14. ascu75

    ascu75 Well-Known Member

    Another idea just sprang to mind make that one number one in the set if it works forget it ever happened and all will be well with the world until lone burns out of the bearing fails and the noise drives you insane ( personal experience of that one)
  15. Footloose

    Footloose Well-Known Member

    I can't imagine why, but since the fan motors employ magnets, could this be due to some kind of magnetic polarity mismatch?
  16. Learning

    Learning Ethelred the Ill-Named

    Do you know what that means? No. neither do I.
    I am guessing some airflow reason but cannot be bothered to explain the obvious tests to prove that.
  17. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    I may have missed this, but do you own a basic amps, volts, ohms meter? (and know how to use it?)
  18. Bipolar

    Bipolar Well-Known Member

    I suspect the fans are creating ripple on top of the DC voltage and its effecting the
    circuitry that runs the electric motors. The fan that is stalling may have a component
    in its circuitry that is weaker than the rest.
  19. ascu75

    ascu75 Well-Known Member

    Oh go on I need to read something completely useless and irrelevant to me everyday I dare you to to bore me with an answer. Or you could be kind and ignore me completely o_O Please ignore me :confused::cool: :):);) Don
  20. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    I have three, a 'standard' model for household and automobile use, a high precision one, which also has a high AC frequency mode, and an old needle type with a single 1.5v battery in it which I use for measuring resistance of delicate items, like phono cartridges or tweeters, where I wouldn't want to connect something with a nine volt battery in it.
    Yesterday afternoon I set about investigating the anomaly, and although not fully explained I have cast some light on it, and it looks as if your conjecture is spot-on!

    In its modified line-up, 1, 2, 3, 5, 4 I measured the following voltages across each fan, with an input voltage that measured between 18.29 to 18.31. It needed a minimum voltage of 19.38 to start the fans. On a constant load, the power supply shows no voltage variations.

    fan 1: 3.65 > 3.67
    fan 2: 3.64 > 3.66
    fan 3: 3.67 > 3.69
    fan 4: 3.64 > 3.66
    fan 5: 3.64 > 3.66 -> problem child, old fan 4

    All was working as it should, with all fans stalling equally at 16.25 volts. I then returned the original, anomalous version, and reproduced the original problem. Before I had taken any voltages, I saw what was happening straight away. Fan 4 was running roughly on the 18 volt setting, observed by using a diode lamp, and when I dropped the voltage to 16 it stalled immediately. The other fans slowed, but didn't stall. The voltage on fan 4 had increased to a steady 3.88, and the voltage on all the other fans stopped fluctuating, settling around the middle of their previous range. Going back into 'normal' mode. I noticed that as I increased the voltage, the fluctuations reduced, stopping completely at 24 volts from the power supply.

    Because of latency in my meter, I can't really tell by how much the voltage was fluctuating (my analogue meter was also showing voltage swings) but it looks as if when the fan motors are run at low voltage their impedance starts to become unstable, and presumably fan 4 had enough 'stiction' that it could stall very easily. Why this should change by reconnecting fan 4 to fifth position in the loom, unless it was due to being two fluctuating impedance devices, and already being sensitive to stalling, I don't know.

    Anyway, last night I rewired all the fans in parallel and ran them overnight at the nominal 12 volts (they had been run very little before), and went to bed and dreamt about being on a 747 that had to abort a landing and go around again. :D Come this morning, all fans are working as planned down to18 volts, still with fluctuation but not as much as before. I can no longer reproduce the problem, regardless of the order in which string I the fans together.

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