Computers and other electronics produce heat, and they need to be cooled. Most electronics depend on fans or some other type of blower to move air and cool the system. This cooling could be in the form of direct heat removal, as blowing air over a hot component (such as a heatsink), or ambient conditioning such as removing hot air from a closed case and introducing fresh cooler air in its place.
Regardless of why we use fans, many people don’t really know how the fan blades move. How a fan moves its blades is actually quite simple, and its basic principle relies on electromagnetism.
This article doesn’t go over how a fan moves air, the difference in air pressure, efficiency of aerodynamics or any of that stuff. We’re talking about the scientific principles that actually cause the fan blades to spin around a central hub.
Basic Principles
Everyone who’s graduated elementary school knows about magnets. Every magnet has a North and South pole; like poles repel each other and opposite poles attract each other. This principle is used to create motion in an electric motor, and a PC fan is at its heart a very simple electric motor.
Instead of using two physical magnets, most fan designs create a magnetic field created by a several electromagnets, which act against a permanent magnet embedded in the fan blade hub. Using electromagnets allow the fan can be turned on and off, and fan speed can be controlled by varying the voltage.
For the uninitiated, electromagnets are a series of metal coils (usually copper) that produce a magnetic field when electricity is run through it. The positive polarity of the current produces one pole, and the negative polarity produces the other. The pole produced is determined by the direction the wire is coiled, and which way the electricity is flowing.
Permanent magnets always have a magnetic field, regardless of any electricity applied. I won’t get into the whole physics of how electricity and magnetism are really the same thing, but I’m sure you could look it up if you wanted to.
The blade speed is controlled either by a small circuit board or amount of copper in the coils. If every fan had the same direct voltage and the exact same coil size then every fan would have the exact same rotation speed.
On the next page we discuss the scientific principles that actually make the fan blades move…
Keeping a fan going is like pushing a merry-go-round at a playground. The merry-go-round requires a “push” at a specific timed interval, else it will either stall or start slowing down due to friction. In a complex electric motor, the coils in the fan hub change polarity (the flow of electrons) at certain intervals, thus reversing the north and south poles. This change is pretty much the same thing as the kid pushing the merry-go-round (but now you have four kids pushing it, as most PC fans use four coils).
In a complex electric motor, this change of polarity is caused by an electrical component called a “brush”. Brush can be a misleading term, as in this context it means “to rub against” rather than having anything to do with cleaning. An electrical brush is a springy metal or carbon piece that rub against the contacts, changing the polarity as the brush moves.
Nearly every PC fan made today is “brushless”, and manufacturers like to tout this feature as some major engineering feat. While a brushless design is quieter, it also makes the motor extremely simple, which reduces manufacturing cost. Instead of each coil switching polarity, a brushless design utilizes two positive and two negative coils, set 90° from each other, which propels the permanent magnet in the fan blade in one direction.
Have you noticed that sometimes if you stop a fan and it won’t start all by itself (requiring you to push it with your finger)? If you reduce the voltage then the magnetic field weakens, causing the the push to be smaller, which causes the fan to spin more slowly or stall. A fan becomes stalled when the permanent magnet in the hub gets caught between the positive and negative fields produced by the electromagnet.
Some fans have LED lighting, and LEDs have a specific tolerance to electricity. If it gets too little electricity then they can either dim or start to pulse, depending on the situation. Some fans separate out the LED and fan leads so that the fan speed can be reduced without pulsing, and even better fans use LEDs with higher tolerances so they won’t change with the fan speed.
There are other part of PC case fans that affect quality, noise, and airflow. Different manufacturers will try and tell you that Fan A is better than Fan B because it uses more ball bearings, is bearingless, brushless, or the like, but the basic principle of how the fan blades move is the same.
The differences in most PC fans is the size and rotation speed, which has the most effect on noise and airflow. A 120mm fan will move more air than an 80mm fan at the same rotation speed. Because of this, larger fans can reduce their rotation speed to be quieter, and still move enough air for its designated purpose.
PC fan manufacturers are constantly trying to “sell” their fans by showing off some new aerodynamic design or unique feature. Some of the more interesting features are UV-reactive plastic, LED readouts (like the Thermaltake iFlash fan reviewed here), integrated silencers and LED lighting, which are purely for aesthetics.
I hope this guide helps you to understand more about how the humble PC case fan does its job.