Computer fans are basically brushless motors with blades attached to them in order to move air. They are common-place inside a computer and can be constructed with different technologies. In this tutorial we will be examining one of the components found in fans that is the subject of a lot of confusion, the bearing.
You probably have seen it written several times that the fans used in PCs are “brushless,” but what does that mean? Let’s study first how a motor with brushes works.
Motors with brushes are a piece of metal with a copper wire wrapped around it. This piece of metal is movable and has a shaft attached to it, so whenever it moves, the shaft also moves, spinning whatever is attached to the shaft.
When energized, this piece of metal is transformed into an electromagnet, generating a magnetic field with two poles (labeled north and south). Around the motor there are two big pieces of curved magnets, one producing a north magnetic field and the other producing a south magnetic field. When the magnetic field generated by the electromagnet is the same as the magnetic field generated by the stationary magnet, the motor will move, because identically labeled magnetic poles repel each other. Also, the magnetic field presented by the opposite magnet will be the opposite, attracting the electromagnet. We illustrate this situation as the “first step” in Figure 1.
However, after the motor moves 180° pushed by the repelling magnetic forces, it will stop, because the system will now be equalized: since the motor spun, now the magnetic fields generated by the motor will be of the opposite type of the magnetic fields generated by the stationary magnets, making the motor to be attracted by the stationary magnets instead of being repelled.
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Figure 1: How a motor with brushes works
For the motor to rotate again, the direction of the current applied to the wire must be reversed. By reversing the polarity, the magnetic fields generated by the motor will also be reversed, creating again the same situation we had in the first step, moving the motor 180° on more time. Then it will stop again.
So, for the motor keep continuously rotating, we need a mechanism that will automatically reverse the polarity of the wire right after the motor spins 180°. The easiest and cheapest way to accomplish this is by using a pair of brushes. The ends of the wire that is wrapped around the motor are attached to two contacts (also called “commutators”) and two brushes touch these contacts. The brushes are always set to the same polarity, but since the contacts will move together with the motor the polarity will be reversed whenever the motor spins 180°. We illustrate this in Figure 2.
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Figure 2: Brush system
Of course this is an oversimplified explanation, but it should be enough for you to understand the principle behind motors with brushes.
The motors used in computer fans, however, don’t work this way, because motors with brushes are noisy, produce sparks whenever the contacts touch the brushes, wear very fast because of the contact between the commutators and the brushes, and are not very reliable at high speeds.