Jamming a fan from rotating

Discussion in 'General Electronics Chat' started by paultwang, Apr 25, 2006.

  1. paultwang

    Thread Starter Well-Known Member

    Mar 8, 2006
    What are the effects of jamming a fan from rotating, besides that it doesn't move air?
    i.e. what happens to current, power dissipation, and what side effects. I am concerned with brushless DC fans, computer fans.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Several things come to mind, none of them good.

    1. Number One I would say is that whatever the fan was cooling is not getting the airflow it needs thus impacting the reliability of the item the cooling fan was intended to cool.
    2. One important beneficiary of the cooling air is the fan itself so the fan is no longer receiving its cooling air impacting the reliability of the fan.
    3. I think that when the fan blade is not turning on a brushless DC motor fan, the drive to the fan motor coil is disable or at least reduced. So I think that power dissipation will not increase significantly.

    I look forward to seeing what others have to say on this subject.

  3. Gadget

    Distinguished Member

    Jan 10, 2006
    Well, having seen a couple of jammed CPU cooling fans turn into something that Salvidor Dali would have been proud of, and being fairly sure that it wasn't the heatsink temperature alone that caused it..(as apart from a siezed fan, these machines continued working for months until their next service)...
    I have also seen a couple that were also jammed (normally because of bad cable dressing) that looked fine, but were also shot. Unsure if it was the internal circuitry, or the motor coils in those cases, as it is just easier to replace em.
    I know by measuring current draw that the current increases significantly when you stop them with your finger.... I'd say that they eventually Cook themselves......

    OK, in the interests of Science, I measured 4 different DC brushless fans.
    Fan 1 (CPU cooler) 120mA running, 200mA stalled
    Fan 2 (CPU cooler) 130mA running, 300mA stalled..(but then drops to zero, pulsing back to 300mA -- OverCurrent protection possibly..?
    Fan 3 (CPU cooler) 100mA running, 150mA stalled
    Fan 4 (Case cooler) 180mA running, 300mA stalled

    With the exception of Fan number 2, I would guess the others would eventually cook if left in a stalled state.
  4. SparkMan

    New Member

    Apr 27, 2006
    It's been a while since my motor theory class, so I am not fully sure of this answer:

    Besides the effect on whatever the fan is supposed to cool, what happens to the fan itself -

    DC Motors are a different animal than AC, So this might be more correct for AC motors. But basically in a motor voltage is applied to a coil of wire, and it comes in many configurations, but the two main ones are the coil is either the warped around the base (non-moving part), or the moving spindle (I beleive its called a stator).

    The voltage creates a magnet field, and there are once again 2 more possibilites on the next part: Some motors use magnets, and the magnet fields produced by the electrified windings (Electromagnetic Fields) react with the permanant magnet fields, thus forcing the stator to spin. Others use 2 electric fields, one created with the base windings, and one created with the stator, in opposing forces.

    My teacher had a theory he called the "happy coil". Basically, when a voltage is first applied to a coil winding, the coil tries to oppose it. When a motor starts spinning, this initial opposition is commonly reffered as "Counter Electromagnetic Force", or CEMF. As the motor picks up speed, CEMF decreases. When CEMF is the lowest, the coil is "happy"

    When there is a lot of CEMF, current draw is higher, so when the motor is first powered up, Current draw is highest, and when its running full speed, current is lowest.

    Now, when you jam a motor, you are increasing CEMF to the max, so current increases the same.

    This, if jammed long enough, can and/or will burn up the motor.

    Another lesson in electronics - if you don't use your learned knowledge, you'll forget it. This motor theory is what I can remember, I may be worng on some things, but that's basiclly the deal with motors.

    CEMF is a pain with motors. Big huge motors have starters and caps to try to resolve this issue. If you ever noticed your house lights dim when the Furnace kicks on, you start a power tool, washer, etc, its CEMF that does that.
  5. Grant


    Mar 5, 2006
    Hello SparkMan,
    The motor frame is the stator (as in stationary) and the moving central component is the rotor (as in rotating).
  6. paultwang

    Thread Starter Well-Known Member

    Mar 8, 2006
    I guess I'd better unplug the fan rather than jamming it.