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Electric Fan, when Not Spinning, gets Hot?

  1. Mar 3, 2016 #1
    I have an electric fan that looks like this:

    Recently, when I flip the switch, the fan blades won't start spinning without any initial assistance. I have to manually push the blades to get them going. But this is not due to too much friction, the friction is the same.

    When I flip the switch, the motor starts humming, and the blades don't turn. I leave it like that and wait. When I return, the blades are still not moving, but the motor housing is freakin' hot.

    If the blades were given some assistance and carried on spinning, the housing would not heat up.

    Why does it heat up? What's the cause?
  2. jcsd
  3. Mar 3, 2016 #2
    In order to get a starting torque in single-phase asynchronous motors, you need two out-of-phase windings. As you only have one ac source, the phase shift is created by a number or ways: a capacitor (for example in some washing machine motors), shading coils, etc. Once the motor is started, the motor gets a torque because the "operation torque" of the two windings are not symmetrical and do not cancel each other.
    The probleme here is that you have current in the "main" winding, but for some reason not in the other. So the winding heats up and no starting torque is available until you push the blades manually.When the blades are spinning, the air flow cools down the motor.
    Check for example https://en.wikipedia.org/wiki/Shaded-pole_motor
  4. Mar 3, 2016 #3

    jim hardy

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    i had one doing same thing.
    A drop of 3-in-One oil on each motor bearing made it good as new.
    I think it was the bearing on blade end causing the trouble. It has to support weight of blade.
    Let us know.

    If you let it overheat it will destroy the windings. When you get that "burnt varnish smell" damage has begun.
  5. Mar 3, 2016 #4
    Much simpler and probably true.
  6. Mar 3, 2016 #5


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    Fan heaters sometimes have the same problem . Usually due to a coil of debris winding itself around the motor spindle in the gap between motor and fan . Mostly hair and clothing fibres .
  7. Mar 3, 2016 #6


    Staff: Mentor

  8. Mar 3, 2016 #7


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    For whatever reason the motor is stalled, ie. it is not rotating. So it is drawing more current (much more) than when it is rotating. So the resistive loss in the windings is much greater (P = I2R)
    Normally a rotating motor creates a back emf opposing the current and keeping it to a safe level.

    As for the cause/ cure of the stall: I have experienced the same as Jim and effected the same cure, but there was palpable friction. Worth a try anyhow.
    Failing that, Xareu has a likely culprit. Check out the capacitor.

  9. Mar 4, 2016 #8
    @xareu I'm not sure if its due to airflow that cools it, because the wind behind the fan feels much weaker than the wind coming from the front of the fan. (I'm also wondering why that is the case :wideeyed:)
  10. Mar 5, 2016 #9
    Well, as Merlin said, once the motor is running, current and thus losses are much lower, no need for a big airflow. Did you try oiling the bearings?
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