General ESC Theory Discussion

[BrianG]

Wow, more responses than I would have thought!

So, it looks like the two braking methods are shunting all windings to ground (or to each other - same thing really), or to apply the back-EMF pulse to the battery to help recharge the cells somewhat. I can understand both braking theories, but don't agree on how they are implemented. Regenerative braking works fine for NiMH (or lead-acid in things like forklifts, cars, etc) or maybe even A123 cells, but lipos are more sensitive to overvoltage. It would be nice to have the ESC smart enough to automatically disable regenerative braking if a lipo mode and/or a LVC value selected since those settings are usually set only for lipos.

@BP-Revo: Yeah, I know that an FET will pop if subjected to too much current. What I was trying to get across is that one FET won't "current-hog" when paralleled like bipolar transistors do because of the way they conduct.

Originally Posted by sikeston34n

...One statement on the group of 36 FETS. That would be 2/3 the total in use at any given time. 24 would be energized at once. Two legs out of the 3 are always hot under power.

Well, this is something I'm not sure about. In a 36 FET ESC, there are 12 FETs per phase. I think we can agree on that. If in fact these ESCs are set up in an H bridge config, 6 of those would go from a phase to battery voltage, the other 6 would go from a phase to ground. This is so the ESC can reverse the polarity for reverse. At any one time, only two phases are "on"; one phase is connected to +V through its 6 "battery" FETs, and the other phase is connected to ground through its 6 "ground" FETs. So, current flows though 12 FETs in all, but through two sets of 6 FETs effectively in series. Wouldn't this mean that only 1/3 of the FETs are on at any one time?

@Serum: Yeah, I remember some discussion about this a while back, but couldn't find it using the search. It's either probably buried in an unrelated thread or my poor search phrase selection. I have no idea about the frequency for starting vs running, so I can't comment on that.

Programming/firmware DOES make a big difference in how an ESC operates. A while back, I posted a thread comparing how a MM works vs a Quark.

Originally Posted by Serum

There is another thing that affects the heat of the esc Brian;

the quality of the sign-wave. If this is an incorrect signal, with saw-tooth effect on high mark of the signal, this results into heat as well. The FETT tries to follow the steering signal (which is obviously amplified as well)

I guess I'm not quite following what you mean by the "saw-tooth" part of the signal. Do you mean the current/voltage spikes from driving an inductive load?

Hopefully GriffinRU pipes up here at some point...

[sikeston34m]

On the 1/3, 2/3rd's total fet count thing. You're absolutely correct Brian. For some reason, I completely forgot about reverse. Sorry, Brainfart! LOL

[zeropointbug]

Well, Artur says if the voltage coming off the motor from hard enough braking is high enough, and high enough to jump some 'diode' in the FET, then it charges the battery, I don't know if all of the energy at that point does, or if just the 'excess' does?

But I think it is worth noting, when I had my truck geared for roughly 50 mph... in my eagle tree data from that run I could see (if I was not mistaken) a blip of voltage spike on the battery (something like 0.25 volts) after a long speed straight, and after the voltage had immediately rose back up to no load... and it wasn't from applying throttle either. Maybe it was nothing, maybe it was a regenerative action? I don't know.

I was running 7s2p A123, 1512/3D (1700kv) and geared 18/51 with badlands (ballooning at that speed bad)... and this was with my 'super Quark'.