General ESC Theory Discussion

[BrianG]

@Aragon: I see what you are saying. However, a coil can deliver more voltage than what was put into it if the magnetic field is allowed to collapse faster than it was built up. Take any coil, apply a 9v battery on it, and then remove the battery. Depending on the coil rating (in milli-henries), a voltage MUCH MUCH higher can be generated. This is how the cow fences I've seen operate. Applied to R/C; depending on how fast you brake, the field can collapse faster than it was built up and can generate higher voltages than the battery.

@DrKnow65: I did an experiment a while back to test braking force of a motor with shorted phases. At low rpm (turning by hand), there IS resistance, but not seemingly enough to stop a vehicle. But spin that sucker as fast as we do (~30k rpm) and the braking force is high indeed! At 0 rpm, I would think that the ESC actually locks (or "pulse locks") the phases to keep the vehicle from rolling from a standstill since there is no braking force from a motor not spinning at all or very slowly.

Using an oscilloscope to measure 3 phase motors can be a bit tricky since there is no "common" point. Too bad wye motors don't have the common connection point accessible to the outside so you could use that as the common point. With delta motors, there is no common point at all other than what the ESC determines as common at any particular time. The best you could do is use CH1 on one phase and CH2 on another phase and use the delta function on the scope. This procedure is used for floating ground circuits or to measure signal across a specific device. Otherwise, using the scope's actual ground lead could short stuff out depending on the circuit design. Probably not a big deal for R/C use since the vehicle ground is totally isolated from the mains ground. Also, it might be hard to get a trigger since pulsewidth and even frequency may vary with throttle and load. Internal triggering uses the 60Hz wall frequency so that won't work. External trigger is what must be used for something like this. That said, you could still get a decent measurement on one phase using a dual-trace scope and just assume the other phases will be similar, just shifted in phase 120*.