My son just built an electric skateboard, FOCBOC TENKA contol box 36V 5.2 firmware. I have a decent understanding of electronics but don’t yet understand how this thing works. As a starting point he has the ppm set at 30%, even so it does 17mph. So, as I understand, the handheld remote control puts out PPM, this is converted to, I think PWM to drive the motors. But I have looked at the PWM with a scope and it is just such a dirty signal I don’t really know what it is. I really thought RC brushless motors would have a nice sinewave voltage appear on each of the 3 legs, although, I’m not sure what my scope reference should be. I have built and drove motors with PWM. But, these are 3 phase motors, so it would seem like the controller would output 3 phase ac to the motor and the peak voltage would just get higher as the controller is set for more and more power. Can you either give me a nice explanation or a website that can explain this to me.
The part you’re referring to, the PWM signal is actually whats transmitted from your remote to the receiver then to the esc and that lets the esc know what throttle setting to use and how fast to drive the motor(s).
The way your esc drives the motors is this, each motor phase gets “tickled” at around 12khz in turn. Thats the best way I know how to describe it. Sending a constant current to each motor phase would quickly saturate the windings and create excess heat. Just guessing at the 12khz because it could be anywhere from 8khz to 32khz depending on the esc and settings used. So while the motor phase is rapidly being energized and de-energized via those 12khz pulses the magnetic field around each stator pole remains constant as it doesnt have enough time to collapse.
The other thing to keep in mind is that brushless outrunners are 3 phase DC. Even though there is back EMF that is produced when each phase is turned off and the electromagnetic field collapses and that energy again turns back into current which is typically just sent around in circles inside the esc in a freewheel circuit, that reverse current is not utilized to drive the motor. Confusing, I know, and with a bit of searching you can find lots of discussion about that topic on the Rc forums.
Please keep in mind I’m not an electrical engineer and that was just a fast and dirty explanation. It also does not cover ESCs equipped with brake regen since I’m not too familiar with those. Hope this helps.
