Hey guys. I'm planning on building a volumetric display by rotating a matrix of LEDs at ~1200 RPM. I've got most of it planned out, but of course this requires a motor. I'd like to go for brushless, as the display should be able to operate for long time periods without overheating or failing. Additionally brushless offers the advantage of low noise. I'm wondering if small drone motors could be powered via USB (max possible available power is ~18W) and if so, how this would be done.
This device is one I am considering. As a PSU rather than charger, it should be fairly safe, I'd think (also with a start capacitor for stall current).
It is the case that you do not need a LiPo to power a BLDC, even a low resistance drone BLDC. Done correctly you can use a low current higher voltage supply to provide a low voltage high current recirculating current inside the BLDC's coils. It is a bit complex though, you've got to provide PWM independently to each of the motor's 3 phases, you've got to ensure each phase is either taken to the higher supply voltage (briefly) during which time the rate at which current in the motor rises is limited by the motor's (fairly small) inductance, or is held low together, you don't have any phases floating (as a high impedance) when operating a BLDC this way. Then you need to have a means to measure the BLDC rotor's position, in terms of electrical angle, not just mechanical angle (the two are related by multiplying/dividing by the motor's pole pair count), and you vary the PWM values at each phase so they always corresponds to an angle 90 electircal degreees ahead of where the rotor is right now. Each phase of the motor is briefly taken high, where the inrushing current is limited by inductance, then sent to ground (remaining low impedance) at which point the current recirculates due to the inductance until it is time for you to take the motor's phase(s) high again. Look up about buck converter principles to get a flavour of how this works. You could run a BLDC off a USB supply (full 2A wall wart advisable, not a vulnerable low current computer's USB port) and with luck it will be drawing well below the full 2A most of the time (though it will have many amps recirulating within the motor's coils). I would pick a BLDC with a kV < 1500, this means that it would generate a back emf of about a volt for every 1500rpm of roation, which, so long as when spinning it isn't having to fight against much air resistance torque, would be conceivable for a 5V supply running at maybe 1A to provide enough power to the motor to both overcome that backEMF at 1200rpm (ish) and still have enough current in the motor coils to give enough torque to keep it moving. It would be easier from a 12V wall wart than a 5V USB one though. You may well have to build your own driver as typical commercial BLDC ESC units might not support this buck converter like mode of operation very well and might be quite dependent on having high current supply to run from.