Eli5: Why do fans rotate only in one direction?

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Eli5: Why do fans rotate only in one direction?

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The blades of the fan are designed to push air in 1 direction with the standard rotation direction.

Ceiling fans mostly have the option to reverse that redirection for summer and winter settings.

The blades are shaped in a way to pull air from one direction and push it in another direction. That’s why fans on a computer are designed to go one way – the fans on the front pull cooler air from the outside room and push it into the case; the fans in the back pull that hotter air from inside the case and push it out into the room.

Some fans *do* rotate in both directions. Ceiling fans usually have a “summer setting” and a “winter setting” – set it to one direction and it will fan you with cooler air; set it to the other direction and it pulls the colder air out away from you.

Switch the poles of your wiring and they’ll go the other way! But actually, probably don’t as you might get electrocuted.

My window fan does have a button that Reverses direction to blow.

ALL makes and models of Ceiling fans also have a switch to reverse direction, either pulling air up or pushing down.

Many cheap upright or box fans simply don’t have or need this feature

The blades of a fan require careful design. It is likely that to make the air blow better in one direction the fan becomes worse at blowing air in the opposite direction. for this reason, they do not bother giving you the option to spin them backward.

Look at the blades of your fan. They aren’t flat like the earth, they are curved. This curvature allows them to push air out when rotating. When you flip the direction, the curvature will push the air in the opposite direction.

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.