Overhead fan turning backwards.

So a decade ago I installed overhead fans in the house. I noticed something:
When running, if you click it off, they go in that direction seemingly forever, indicating very good bearings inside the motor, well balanced, etc. The thing that is puzzling me is when it slows down, and is going very slow indeed at the last, there is a backwards movement of about one cm, then stops.

Now if I just spin the blade by hand a bit, that does not happen. Any idea what forces are involved that would cause that backlash only if it was running and turned off?

Originally posted by sonhouse
So a decade ago I installed overhead fans in the house. I noticed something:
When running, if you click it off, they go in that direction seemingly forever, indicating very good bearings inside the motor, well balanced, etc. The thing that is puzzling me is when it slows down, and is going very slow indeed at the last, there is a backwards movement of abou ...[text shortened]... a what forces are involved that would cause that backlash only if it was running and turned off?

My guess is it must be related to electricity(it only happens after power is consumed by the motor)and I don't recall any law of motion that accounts for that behavior.

Originally posted by joe shmo
My guess is it must be related to electricity(it only happens after power is consumed by the motor)and I don't recall any law of motion that accounts for that behavior.

That would be called back emf and that is a possibility I didn't consider. I was thinking maybe the air flow on the vanes somehow reversed temporarily.
I don't think it could be back emf because the blades turn for something like a full minute after power is shut off and back emf would occur right at turn off.

The motor could be acting like a generator but if there is no load such as a resistor in line with the motor there would be no absorption of energy. I think with power cut by hard wire, the rotor wiring is totally open, there would be no flow of current only a short term voltage build up in the windings but no place for the current to flow into except for capacitance to ground or some such and that would last only for milliseconds after the rotors generated some kind of current flow. So that would also have been dissipated right after power was hard wire cut.

Originally posted by sonhouse
That would be called back emf and that is a possibility I didn't consider. I was thinking maybe the air flow on the vanes somehow reversed temporarily.
I don't think it could be back emf because the blades turn for something like a full minute after power is shut off and back emf would occur right at turn off.

The motor could be acting like a generator ...[text shortened]... of current flow. So that would also have been dissipated right after power was hard wire cut.

I'm no expert, but I but I don't see the end result coming from fluid mechanics. I mean the pressure differentials, vortices, cirrculation, ect... that apply to the powered situation all equally apply to the non-powered situation. So, I'm sticking to my guns(albiet a pee shooter) and saying its related to the motor/electricity. Capacitors and Inductors with respect to voltage and current in discharging phases are governed by time dependent equations.

Originally posted by joe shmo
I'm no expert, but I but I don't see the end result coming from fluid mechanics. I mean the pressure differentials, vortices, cirrculation, ect... that apply to the powered situation all equally apply to the non-powered situation. So, I'm sticking to my guns(albiet a pee shooter) and saying its related to the motor/electricity. Capacitors and Inductors w ...[text shortened]... espect to voltage and current in discharging phases are governed by time dependent equations.

or it could be that its an optical illusion, did you actaully make measurement of this counter-rotation or did it simply appear to be counter-rotating?

Originally posted by joe shmo
or it could be that its an optical illusion, did you actaully make measurement of this counter-rotation or did it simply appear to be counter-rotating?

It was not illusion, I saw it go backwards about 1 cm. The thing about EMF and magnetic stuff is by the time it is going so slow, all charges would be dissipated way before it stops, you can't get much generator action from a motor turning 1 rpm.

This motion may be related to the inverse square law of a flux capacitor. 🙂

Some motors have a start and or run capacator wich stores energy, but why the backwards turn???..... or something to do with gravity perhaps??

Originally posted by patauro
Some motors have a start and or run capacator wich stores energy, but why the backwards turn???..... or something to do with gravity perhaps??

My current theory, so to speak, is there is a backwards flow of air like you say, due to gravity, a little reaction of the air going down the blades one last time, like a rubber band pressure kind of thing, stored pneumatic energy. That's the only thing I can think of that would account for that movement.

Originally posted by sonhouse
My current theory, so to speak, is there is a backwards flow of air like you say, due to gravity, a little reaction of the air going down the blades one last time, like a rubber band pressure kind of thing, stored pneumatic energy. That's the only thing I can think of that would account for that movement.

Is there a flex or something attaching the fan motor to the ceiling?

When the fan motor turns the fan it tries to turn itself in the opposite direction to the motion of the fan, this twists the flex slightly in the opposite direction. Whilst the fan is running the flex will be held in tension and will only relax, turning a bit, when the fan stops. I think this will turn the fan motor in the direction of the fan a little bit, possibly turning the fan the other way?

When you turn the fan manually the force is transmitted through the fan rather than the motor, because of gearing between the two the flex might not get twisted in the same direction, or by the same amount. This would then behave differently on stopping.

If all this is feasible, then the experiment needed is to observe a mark on the flex or something directly attached to it, to see how it behaves.

Originally posted by iamatiger
Is there a flex or something attaching the fan motor to the ceiling?

When the fan motor turns the fan it tries to turn itself in the opposite direction to the motion of the fan, this twists the flex slightly in the opposite direction. Whilst the fan is running the flex will be held in tension and will only relax, turning a bit, when the fan stops. I [i] ...[text shortened]... ed is to observe a mark on the flex or something directly attached to it, to see how it behaves.

I'm not buying it, the fan is in free spin in both cases. Once the power is cut, the motor cannot apply a torque, if the motor is not applying a torque the "flex" will immediately return to a state of minimum strain( ie its not going to wait for the fan to stop spinning to return)

Also, any torques that could be applied by the rotating fan on the "flex"(in free or powered spin) would be directly porportional to the angular acceleration of the the turning fan (ie when the fan has zero angular acceleration, the angle of twist on the flex is zero).

Originally posted by iamatiger
Is there a flex or something attaching the fan motor to the ceiling?

When the fan motor turns the fan it tries to turn itself in the opposite direction to the motion of the fan, this twists the flex slightly in the opposite direction. Whilst the fan is running the flex will be held in tension and will only relax, turning a bit, when the fan stops. I [i] ...[text shortened]... ed is to observe a mark on the flex or something directly attached to it, to see how it behaves.

That might be a viable theory, but one thing: the motor is a direct drive, no gears.
If there was a very sensitive pneumatic flow meter, maybe I could prove my air movement conjecture. The thing is, I don't see any other motive force available, since at the time of reversal the blades are going much less than one RPM. I don't know how to visualize air moving that slow though.

I don't know a lot about emf so I can't explain it, but I think it would be really fun to call the company where you bought the overhead fan and ask them about; give them a hard time if they can't explain it.

Originally posted by sonhouse
My current theory, so to speak, is there is a backwards flow of air like you say, due to gravity, a little reaction of the air going down the blades one last time, like a rubber band pressure kind of thing, stored pneumatic energy. That's the only thing I can think of that would account for that movement.

How exactly does your theory descriminate between the two cases?

Originally posted by sonhouse
So a decade ago I installed overhead fans in the house. I noticed something:
When running, if you click it off, they go in that direction seemingly forever, indicating very good bearings inside the motor, well balanced, etc. The thing that is puzzling me is when it slows down, and is going very slow indeed at the last, there is a backwards movement of abou ...[text shortened]... a what forces are involved that would cause that backlash only if it was running and turned off?

I think its the magnetic field inside the motor. The windings are spooled around a iron which shouldn't be magnetic. The rotating iron is magnetic. If the rotor stops in between two of these irons it turns back.
This theory doesn't explain why its only happening if you're cutting power.