A box fan is a portable fan used to cool off rooms and circulate air through buildings. The classic box fan is a box with a fan blade in it and a three-speed selector switch on it. It can be stuck in the window to pull in air directly from outside. There are also box fans with two fans inside, capable of blowing air into a room with one fan and out with the other for better circulation.
Box Fan Basics
Nearly all box fans are driven by induction motors. Electricity comes into the house as alternating current (AC). The current switches direction 60 times every second, going from negative to positive. When a moving electric current is run into a coil, it creates a moving magnetic field. This magnetic field induces a current and a magnetic field in anything conductive nearby. It is this induction that powers the motor.
The induction motor inside a box fan has several coils of wire around a free-spinning metal rotor. As alternating current flows into the wires, it sets up a moving magnetic field around the rotor. This makes the rotor take on its own magnetic field. The two magnetic fields repel each other, pushing the rotor around. The number of coils, and the speed with which the AC changes direction, control how fast the rotor turns. AC fans have a speed controller called an inverter, which changes the rate at which the field changes directions. The higher the speed, the faster the current changes.
The Fan Blades
Table fans move air by deflecting it forward with their angled blades. As the fan blade slides by the still air molecules, it gives them a push, sending them out along the surface of the blade into the room. When the fan blade passes, it leaves a vacuum. It has shoved some of the air forward, and now there is no air where it was. This vacuum pulls more air from behind the fan, creating a continuous stream blowing into the room.