The caps reduce noise traveling up and off the motor wires. This causes interference. I don't believe the 2.4 ghz boards are affected by this because of the caps in between the positive and negative contacts on the board. . I can only speculate that PN doesn't include them on their motors because they know this already seeing as they've made they're own 2.4 board...
This is true too. If you are next to sensitive equipment (AM radio), you may hear a click in the audio if your motor is close enough to the antenna. May even hear a high-pitch whine kind of like a poorly installed car stereo where you can hear the alternator through the audio.
As a matter of fact, this 'back EMF' produced by the motor is causing problems with my dynamometer project. I am experiencing the main CPU reset itself in the even that I coil the motor wiring and place that coil close to the wiring to the tachometer input. The tachometer wiring is coupling in the noise and delivering it right to an input on the CPU board. The spike interferes with VCC of the CPU and actually disrupts the supply, and induces a power cycle effect like turning the power switch off then on really fast.
I will need to be careful how I route the power cabling with respect to the tachometer cabling to prevent this. It's amazing how much noise motors can produce.
The motor caps 'could possibly' be just a hoop that Kyosho had to jump through to satisfy the FCC too, because any electronic device has to conform to rules to ensure that it only can produce a certain level of interference to other electronic devices. Without the caps, the mini-z may float into a category of interference that would make it illegal to mass produce.
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