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Little fun experiment today : we're gonna make spin this fan motor as fast as possible!
Originally, this engine was built on two balls bearing, but we've taken off one of them to reduce the friction. What we have here is a circular magnet, which has 2 north and 2 south poles. We're going to make it spin around this handmade coil, by using a square voltage. We're going to increase the magnet's speed by gradually increasing the frequency. Roughly at first, but very precisely afterwards.
At a low frequency, the coil's impedance is very low, which forces us to limit the current by adding a resistor. We're roughly at 5 amps. Once the frequency gets higher, we'll finally be able to open fire. We'll start with 80 Hertz, which equals to 20 revolutions per second. It's basically the most speed we can generate with our fingers.
Thanks to our system and according to our calculations, we could reach around 100.000 RPM. To give you an idea, it'll spin at one thounound six hundred revolutions per second, which is very impressive. First try ! We're slowly increasing the frequency with our 100k potentiometer.
To increase the rotor's kinetic energy consistently we have to increase the frequency more and more slowly. But when we move consistently the knob of the potentiometer the frequency increases faster and faster. Therefore we need to be very careful and precise while moving the knob to avoid the motor from stalling.
A few stalls, but it's possible to synchronize again at a lower frequency when the rotor is decelerating. At this speed, the standard potentiometer is not precise enough to make sure we're not to abrupt, making it harder to avoid stalls.
It's time to use the precise potentiometer (trimmer) and make a few rotations. From now, the impedance of the coil allows us to take off the resistance (no more current limitation). Now that there's no limitation anymore, we're undoing the revolutions stored in the trimmer to use its accuracy later.
Now that the power is higher (96W, 8A under 12V), we can put the 100K potentiometer on 0 ohm. It all comes down to this moment, with the precise potentiometer. Already 1000 revolutions per second, already hard to believe. As the frequency increases, the voltage is limited by the coil's impedance so motor then consumes less energy : the efficiency is increasing. The speed limit will be reached when the energy provided at every moment will just have to compensate the friction. Imagine the noise it makes, considering the camera is 5 m away from it!
Rotation speed : 1700 revs per second.
Kinetic energy : 740 Joules.
Speed at the edge of the rotor : 800 km/h (500 mph).
Stalling, the deceleration is pretty quick because the system isn't turned off. Free deceleration test from 100.000 RPM
Next step one day… : breaking the sound barrier (1200 km/h) on-the-spot !
