Motor locking fins, works well after a few hours?

Discussion in 'General Electronics Chat' started by ElectricMagician, May 19, 2014.

  1. ElectricMagician

    Thread Starter Member

    Jul 26, 2012
    57
    0
    Hello everybody,

    I have a malfunctioning fan/motor (A.C. compressor fan). When the unit is turned on, this motor either:

    - Runs perfectly (when left for a few hours)
    - "Locks" the fins into place, making it very hard to even manually rotate the fan with a screwdriver, etc. The motor produces a buzzing sound. Once it's turned off, this lock is gone.

    Also, sometimes when this happens, the fan is actually moving extremely slowly.

    My understanding is that this is a motor problem, but the part that confuses me is that it works again if left turned off for a few hours.


    Is there an explanation for this?

    Thanks in advance.
     
  2. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Bad bushings (a cheap kind of bearing). Heat makes them swell up and seize. If they had any oil, they wouldn't get so hot.
     
    Last edited: May 19, 2014
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  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    i have had a few fan motors lock up like this, and the bushings were so worn that the armature was contacting the field, like putting on the brakes. unless the windings were seriously over heated, new bushings might fix it. probably easier to get a new fan though.
     
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  4. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    That's a rare event, but true.
    Once, I saw a fan motor where the center shaft had wallowed sideways through the aluminum end plate until the rotor hit the steel of the field windings. (Nobody is going to sell me the end plate for a fan motor.) Anyway...they usually just seize up and you buy a new motor. I've been through all the stages of grief trying to save them, and you can save them if they are from the 1950's, but modern motors have had computer aided quality removal. I don't dare oil one and run because I know they won't last 3 days after the bushings get that hot.
     
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  5. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    I had my home central AC unit compressor fan quit after a few years. It had a sleeve bearing failure. On the motor was the statement that the bearings should be oiled once a year. The kicker was that the motor was buried inside the unit so you would have to remove the motor in order to lube it. So to save a few bucks the AC manufacturer used a cheaper sleeve motor instead of using a permanently lubed one. I replaced it with a sealed ball-bearing motor that was lubed for life (or for at least a reasonable period of time).
     
  6. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Entirely reasonable solution. Sometimes, you can even replace the bearings if you have enough sense to press the new ones on using the inner race to push against.
     
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