fannoiseturbulence
I used to live in a house that had ceiling fans, which were great for the summer and so wonderfully silent when running.
Now however I've moved to a house that has no ceiling fans so I've bought a couple of standing fans. However, they're too noisy to use at night without disrupting sleep.
So my question is: what is it about the design of a ceiling fan that allows it to be so quiet?
(I'm in Australia, hence thinking about fans at this time of year)
In summertime, the ceiling fan blows air downwards and cools down your body using the wind chill effect.
In wintertime, if you have an active heating system at home, it will heat up the air in the room. Hotter air moves up and accumulate near the ceiling, colder air being down. A ceiling fan in reverse direction moves cold air up pushes hot air downwards to people's level. It can do this in forward direction but then you will have the wind chill effect again.
Why does the chill effect only occur in summertime mode ?
Consider this fan :
You can try it yourself: If you stand in front of the fan you will feel a strong air stream, if you stand behind you will feel a much weaker stream. This is because the fan collects air from all directions in the back and blows it in one direction in the front.
So, in wintertime mode, wind speed above the fan will be higher than that below, there will be a weak stream of air at your level so you will not feel the chill effect.
The overshoot behavior you noticed is called cogging and occurs when the magnet arrangement in the motor "catches" the rotating magnetic core of the motor during shutdown and jerks it back to one of the local strong spots in the field.
You can demonstrate this yourself by carefully rotating the fan blade around with your finger when the motor is off. You will notice there are certain rotation angles where the fan wants to come to rest and others which it wants to avoid. If the motor passes one of the preferred spots but fails to rotate far enough to "climb the hill" and snap forward into the next cog spot, the motor will very briefly rotate backwards a fraction of a turn and go "boing-oing-oing-oing" as it settles into that cog position.
Best Answer
The noise is generated to a large degree by the fan blades. The portable fans you mention typically provide much higher flow velocities than ceiling fans, and their blades obviously rotate much, much faster. Two factors are then responsible for their significant noise level:
In order to make the fans more quiet, one would want to both reduce the blade speed and install higher quality blades (with proper aerodynamic profiles, rather than just bent pieces of sheet metal).