Field Weakening

I dont see any dedicated tread to this topic. Would it be out if hand to start this , and discuss what everyone has been testing and doing in this area? I have personally used it now for some time, and have been quite happy with the results.

Feedback welcome!



with vesc and other controllers I’ve found that field weakening can cause problems when trying to brake quickly. it can get sketchy

What kind of issues. I have had no issues at all. Quite seamless IMO.


how much current over do u have

40 amps staring at 75% duty cycle, ramp time of 1500ms and a Qfactor of 5.5


I don’t know anything about actually using it besides that I’ve heard it gets horribly inefficient the faster you go, and we don’t have range to spare in our esk8 usually

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It would be nice if field weakening was a remote toggleable setting, like a ‘hold for boost’ button or a racing gear


that is so high :hot_face:

What Grin has to say about field weakening on their controller:

I remember reading tho that if you disconnect, short, or burn out a winding with field weakening active all the “extra” current has nowhere to go and you get a motor lock-up. Can’t find the reference for that tho. On an ebike that’s no big deal, but if there’s any chance if that happening on an esk8 then to me the risk aint worth a slightly higher speed at a lower efficiency. No thanks I’ll just upgrade if I need more speed.


If the motor is spinning faster than its no-load speed on a given battery and there is some malfunction which causes the field weakening to stop, then it will likely brake uncontrollably down to top-speed (the no-load speed).

It won’t necessarily “lock-up”, but it would probably feel like a lock-up on an esk8. Once it slows to approximately top-speed at the battery voltage, it should begin spinning freely again, assuming no shorted windings or broken FETs. But it will probably slow to top-speed very quickly as the braking current will be practically unrestricted. However, with this unrestricted current it’s also very possible the FETs themselves could fail (via excessive body diode current) which could cause even more braking, way down past top-speed.

It could also cause a BMS shutoff if there is a BMS in the circuit (which isn’t bypassed) that could cause other effects.

Most of these details don’t really matter too much if the esk8 rider was already ejected from the vehicle in the first 200 milliseconds of the failure when the brakes were slammed on.

All in all, any kind of ESC failure while over top no-load speed seems incredibly hairy and dangerous. Esk8 are already dangerous though. But it just seems like a much better idea to use a higher voltage battery instead of field weakening. There are probably certain narrow circumstances when field weakening would make sense in certain situations for esk8.

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Gradual ramp up time as its for boost only. 1.5 sec, is softer on electonics. Q factor of 5.5 makes it almost imperceptible on entry and exit.


@b264 I agree that , given all the circumstances you outlined, it could induce a catastrophic failure. That is why , in my opinion, great care should be taken in all directions; correct BMS setup/choice, max pack discharge rates, conservative VESC setup ( as we all like to push past the limits mostly all the time ), etc.

And overall, we are not running at max duty cycle all the time, and I dare say, nowhere near all the time.

The vesc setup paradigm changes when you want to take FW into the mix, so IMO, you need to revisit all the config to account for that.

FW gives lower KV motors a bit of a boost for those moments when you need it. Been running it for 3/4 months without any issue with over 700km ridden.


Still running FW?

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What firmware are you running?

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Thank you, gonna update and give it a shot.

What esc and bat combo you have?

12s6p Molicel p42a with vesc 6.6