Summary (also see photos of fan and motor below)

Update 2:

I bought a capacitor for small dollars and now the fan works again, perfectly.

Update 1: Capacitor was indeed broken (see photo at end)

  1. Belt and upper pulley fell off 40" attic fan

  2. Attic fan motor turns when not connected to anything

  3. Fan blades turn easily by hand

  4. When I put the pulley and belt together the motor does not turn the fan at all.

    What should I try?

Here are some more details.

My large (40" diameter) attic fan has worked flawlessly for years, on a thermostat. Recently we have not heard it.

I climbed into the low ceiling attic and found the 18" belt on the floor and the 7" top pulley on the floor as well.

The motor turned when the I manually triggered thermostat.

I put the pulley back on the shaft and tightened it in place with the set screw. Then I put the belt back on the pulleys and triggered the thermostat. I replaced the belt with my bare hands, so it cannot be very tight.

Surprise: the motor tries to turn (jerks in place) but does not turn.

The fan blade itself is easy to turn by hand.

Why did the pulley fall off and what should I try now to get it running again?

If you need any more info, tell me what you need.

Thanks. Fan



  • Idle thought -- what's the tension on the belt look like? Jul 26, 2017 at 1:22
  • @ThreePhaseEel I replaced the belt with my bare hands, so it cannot be very tight.
    – Yehuda_NYC
    Jul 26, 2017 at 1:23
  • Is the belt slipping by chance? What type of motor is turning it? Jul 26, 2017 at 1:23
  • @ThreePhaseEel The belt is not slipping. The motor shaft jerks just a little and then stops moving. If I take off the belt, the motor sins happily.
    – Yehuda_NYC
    Jul 26, 2017 at 1:25
  • @ThreePhaseEel. It is a little 110 v motor. It turns fast with no load. I can try to take a picture of the label on the motor.
    – Yehuda_NYC
    Jul 26, 2017 at 1:27

1 Answer 1


Check the capacitor

Since the motor starts and runs no-load, it's not the motor windings themselves, and the capacitor isn't open or shorted most likely. I'd check it anyway though, as it's likely way out of spec, which'd explain the behavior you're seeing (starts unloaded, but fails to start up under load) and the motor is a Permanent Split Capacitor (PSC) type, so there's no start switch involved here.

  • 1
    It is indeed the can in the photo of the motor -- there will be printing on it giving the value and voltage rating (the motor catalog says to use a 10M/370V cap which I suspect means 10µF/370V), and yes it's fairly easy to swap for that matter Jul 26, 2017 at 2:27
  • 1
    @ThreePhaseEel Just a thought, still possibly "a single" winding? With No load it could have enough to get it started. (would expect ti to spin erratically). The CAP is pretty common too.
    – noybman
    Jul 26, 2017 at 2:57
  • 1
    @noybman -- yeah, a good start and a bad run winding could cause this, I'd be more inclined to suspect the cap though Jul 26, 2017 at 3:17
  • 1
    I wonder if that strangely colored patch on the floor directly beneath the capacitor is a result of leaked electrolyte ...
    – brhans
    Jul 26, 2017 at 11:22
  • 1
    @Yehuda_NYC -- the capacitor is not polarized (it can't be since it has AC on both sides of it). Dunno how the capacitor failure and the pulley problem are related, though...maybe the motor failure caused the pulley to shake loose? Jul 27, 2017 at 11:26

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