Eli5: Why do fans rotate only in one direction?
In: Other
One interpretation of the question is why fans are designed to spin in one direction. The blades of the fan are designed to efficiently push air in only one direction. If you were to spin the blades in the opposite direction, the fan would make a lot of noise but move only a small amount of air. Fan blades that would be designed to work in either direction do not work very well, its better to have them designed to push air only one way.
The other interpretation is HOW fans spin in only one direction: AC fans like the ones you plug into the wall at home have a special starter coil in the motor that creates a very brief force on the blades when the motor is powered on. This gets the blades spinning in the right direction, and the blades are then powered by the main coils of the motor.
DC fans like the ones inside a computer can often be made to run in the wrong direction by reversing the voltage powering the motor. The fans spin in the right direction only if they are wired into their power supply correctly.
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