Overhead fan turning backwards.

While the fan is spinning fast but the circuit is cut, does the motor build up a charge because the fan is trying to push electrons through the circuit? Then when the fan stops, this charge can finally dissipate which gives a small current flow, turning the fan a bit the other way?

Originally posted by iamatiger
While the fan is spinning fast but the circuit is cut, does the motor build up a charge because the fan is trying to push electrons through the circuit? Then when the fan stops, this charge can finally dissipate which gives a small current flow, turning the fan a bit the other way?

no

Originally posted by iamatiger
While the fan is spinning fast but the circuit is cut, does the motor build up a charge because the fan is trying to push electrons through the circuit? Then when the fan stops, this charge can finally dissipate which gives a small current flow, turning the fan a bit the other way?

The reason Coquette says no is when the motor is turned off, it loses the ability to conduct electrons in the normal way, through completed circuits.

In that condition, there would be an electron charge and a short time of some of that charge going to ground through capacitance discharge of the motor windings but after that there would be just a small voltage built up as long as the motor is turning.

The problem for EMF to cause the reversal is such charges would be long gone when the blades are turning a couple of RPM and slowing down. The electron movement generated by the turning blades would be all dressed up and nowhere to go.

As an example, suppose we have an ordinary extension cord, say 100 feet long, just the double wire, with insulation of course. Both ends are not connected to anything. Then connect a battery to the two wires at one end.

For a brief moment, electrons will flow from the battery down the wire and at the speed of about 10 nanoseconds per foot, so about 1000 nanoseconds later (electrons in copper moves only about 10% of the speed of light, which is about 1 nanosecond per foot) said electrons meet the other end of the wire and find nowhere else to go.

In that 1000 nanoseconds of time, because there is a voltage gradient along the wire, there will be some transfer of charge across the insulation due to capacitance effects.

However, after the electrons have proven to themselves there can be no further movement or transfer across the insulating barrier, all electron movement stops and thereafter only maintains the DC charge of whatever voltage the battery has provided. Of course, if you short out the wires at the other end of the cable, electrons will move bigtime and you will have a massive current in the wire, depending on the internal resistance of the battery.

For instance, if it was a watch battery, there would be a rather high internal resistance and not much current could flow, limited by the small chemical charge available in that little button cell.

If it was a fully charged automobile battery, shortly after you established the connection at the long end of the cable, it would in short order start heating up and most likely start melting and burning the insulation because the internal resistance is very low in auto batteries and the chemical charge very large, thousands of watts available for a few minutes, more than enough to burn up the cable.

None of that can happen when the far end of the cable is disconnected, and it doesn't matter at that point if the battery is a button cell for a watch or a thousand auto batteries in parallel, the current flow across the insulating barrier through capacitance is only in proportion to the voltage of the battery and the distance between the wires and the type of insulation between the wires.

That capacitive current only lasts for nanoseconds or microseconds at best so in the fan motor, there would be not enough energy to start anything going in either direction. The blades of a regular ceiling fan are rather massive and it takes a fair amount of energy to cause some initial movement, which you can verify for yourself by just turning the blades by hand, you can feel the resistance to motion due to the blades mass. Way too heavy for transient effects of capacitive discharge to make any headway to move said blades.

does the backward movement also occur, when you switch on the energy for a very short time, which is exactly the same as when you push it?

Most fans have a switch to make them go the other way. If these have that, an obvious other experiment is to flick the switch and see if the fan does the same thing.

Originally posted by iamatiger
Most fans have a switch to make them go the other way. If these have that, an obvious other experiment is to flick the switch and see if the fan does the same thing.

I have done that but not to completely stop, I used the reverse switch to stop the blades actively when I was balancing the vanes, which is a bit trickier than I thought at first. I will try that. Although if it does not reverse I would think it would lend credence to the idea the small reversal was due to air movement. Blades reversed, means air is pushed upwards instead of downwards. I'll have to try it.

There is a simple answer to this complex problem: When the current is turned off to the fan the thermocouple connected to the hamster's buttocks also shuts down. Thus the hamster has no energy and is not able to convert ADP to ATP. The hamster slows down along with the wheel/cage he is running in. When the wheel hits the highest point in its travel it stops. Then gravity acts upon the exhausted hamster causing said hamster to rotate slightly COUNTER CLOCKWISE ( BACKWARDS) approx. 1 cm. Simple, very very simple.

Originally posted by smw6869
There is a simple answer to this complex problem: When the current is turned off to the fan the thermocouple connected to the hamster's buttocks also shuts down. Thus the hamster has no energy and is not able to convert ADP to ATP. The hamster slows down along with the wheel/cage he is running in. When the wheel hits the highest point in its travel it stops ...[text shortened]... mster to rotate slightly COUNTER CLOCKWISE ( BACKWARDS) approx. 1 cm. Simple, very very simple.

thats why was asking about the same thing happening when you just turn the fan by hand. in that case, you would rub the belly of the hamster, which causes the heat of the friction to be turned into a tiny amount of energy. Not using this for motoric actions, the excess is, as any excess of energy by common social theory, turned into rebellious energy. The individual undergoes a momentary flux of non-redundant counter-action. However, since the hamster has been bound securely in place, it becomes sick upon its obvious desperation: the cramps of meaninglessness intertwine with the Sartreist absurdity implied by the purpose of the social individual.

Hence, facing absolute postsemanticist feminism and its trial upon sexual identity, the hamster pukes.

Every action forces a reaction.

The fan turns backwards.

If in doubt, please refer to:
http://www.elsewhere.org/pomo/

If still in doubt, please press F5 again and again until the doubt is gone.

Originally posted by tharkesh
thats why was asking about the same thing happening when you just turn the fan by hand. in that case, you would rub the belly of the hamster, which causes the heat of the friction to be turned into a tiny amount of energy. Not using this for motoric actions, the excess is, as any excess of energy by common social theory, turned into rebellious energy. The i ...[text shortened]... sewhere.org/pomo/

If still in doubt, please press F5 again and again until the doubt is gone.

You're a Communist.....right?


GRANNY.

Some factors exist like: Heat builup of time causing any metal in the fan to shift or change causing the rollback, or even the grease in bearings becoming more malleable during the extensive use of "current-based usage" than just the simple manual push method. I think it could be attributed to time usage or heat buldup as much as anything.
I think maybe trying the "cold" fan that hasn't been used in some time, with a quick swith-flip to get it going, and then a "cold" manual push should tell us if that could be the case.

Still curious on the opposite direction thing though. How did that turn out?

Originally posted by MrVarnell
Some factors exist like: Heat builup of time causing any metal in the fan to shift or change causing the rollback, or even the grease in bearings becoming more malleable during the extensive use of "current-based usage" than just the simple manual push method. I think it could be attributed to time usage or heat buldup as much as anything.
I think maybe ...[text shortened]... the case.

Still curious on the opposite direction thing though. How did that turn out?

I have yet to do that, a lot of stuff going on in the house, freezing pipes (UGGH) stopped up sewer line, broken doors, I have been just a bit busy for the last month with BS projects like those!

Maybe I missed it, but...

I assume the fan is reversible so you can run as downdraft or updraft.

Question 1. Have you operated the fan in both directions with exactly the same result?
Question 2. Does the fan move backward every time that you stop it and let it come to a complete stop, or is it just sometimes?
Question 3. When the blades finally come to a complete stop are they always in the exact same position in relation to a fixed reference spot?

Originally posted by mwmiller
Maybe I missed it, but...

I assume the fan is reversible so you can run as downdraft or updraft.

Question 1. Have you operated the fan in both directions with exactly the same result?
Question 2. Does the fan move backward every time that you stop it and let it come to a complete stop, or is it just sometimes?
Question 3. When the blades finally ...[text shortened]... complete stop are they always in the exact same position in relation to a fixed reference spot?

What I'm thinking is that perhaps once the blades come to a complete stop, gravity is now coming more into play.

If the blades are not perfectly balanced and if the bearings of the motor are extremely good, a downward pull of gravity could make the blades rotate back a small amount until an equilibrium point is found.

If this is the case, the blades may not always move, as it would be dependent on how close to the equilibrium point the initial stopping point is.

Just a thought....

Originally posted by mwmiller
Maybe I missed it, but...

I assume the fan is reversible so you can run as downdraft or updraft.

Question 1. Have you operated the fan in both directions with exactly the same result?
Question 2. Does the fan move backward every time that you stop it and let it come to a complete stop, or is it just sometimes?
Question 3. When the blades finally ...[text shortened]... complete stop are they always in the exact same position in relation to a fixed reference spot?

I finally was able to try the reverse experiment, and it turned out it turned out to also reverse but not as much as the other direction. I think it's a result of air that was moving in one direction takes off in the other direction due to the fact it was 'climbing' up hill when the blades turn and then the air rolls back down the blades, causing a tiny torque, enough to move the blade backwards a mm or two. It looked like it moved about 3 mm in the forwards direction, that is to say when air is being forced down into the room, then rolls back about 3 mm when it stops but when in the opposite mode, pulling air up to the ceiling, it only moved backwards about half that of the forwards direction.

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 CD players have brushless DC direct drive motors. Air factors should be consistent and minor. I tried it with a CD on two of my players that have an openable door, which upon opening, turns off the motor. One of them, a Kloss 88CD, displayed no reverse motion after coming to rest. The other, a Sony Discman, displayed a slight pause and then a definite reverse motion about 3/4 of the time, up to about a half cm on the CD edge, and a slight pause and then a small forward motion the other 1/4 of the time.

I am wondering about the segmented rotors and stators wanting to stop as close to each other as possible if both have some magnetism. The presence of this magnetism might cause a "lumpy" or pulsed pattern of slowdown as the rotors pass by the stators. The final "lump" may, most of the time, bring the rotor to a halt just after it passes a stator, the attraction pulling it to a halt, and then the stator and rotor might move a little more toward each other (or into full alignment if frictional resistance is low enough). I think other comments have hinted at this. Why one player does it and the other doesn't I don't know. I did notice that the Kloss slows down faster. Perhaps its frictional elements dominate. There may be design or wear differences too, or differing residual magnetism.

Of course friction is the reason the motor slows down. There could also be a physical "lumpiness" in the structure of the motor, such that it has has a preferred or several preferred final rest positions around the circular "track" bearings. If there is some up and down motion as it rotates, it may preferentially come to a halt as it is rising, then roll back a little.

It remains puzzling why this would not be seen when you manually push the fan. When I manually push-start these two CD players they display the same behavior as when started via their motors.

Some LP turntables are direct drive, too, so that could be tried. Reportedly, PC fans are too. (Wikipedia)