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Motor kV ratings being off, how much is tolerable?


and if you want accurate Kv… chuck your motors up to a drill motor and measure volts produced per RPM

mucho-combo-more accurate Kv assessment…

a werd to the wise…


Or even better spin it with another BLDC motor

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why better? and imagineer how eggzakery this would look incomparison to being chucked up in a drill-motor???

sure you “could” make that work… but a drill motor has a chuck… easy peasy hook-up… and usually they have a rated RPM if you don’t happen to have an optical tach… voltage is easy-peasy to obtain…

I’m just saying this method “could” work but introduces complexity…

but I don’t mind if peeps skin their own cats their own way… but peeps been doin’ it with drill motors for a long-long time…


oh the bldc motor was to avoid the inaccuracy of the drill RPM measurment.

I’m sure a drill works just fine, maybe on a bench power supply for constant voltage would be good.

with an optical tach there is no need for constant voltage supply…

in fact… for science I charged up my cordless, spun it up… hit the optical tach and it was so close to the rated 3600 rpm… i don’t even bother with the optical tachometer any more, full charge and pull the trigger…

3-maybe 6 times… once or twice on each phase… average the results and I have a reliable Kv measurements in 5 mins…


with the drill press in the shop… I set the RPM…


This is good stuff. I have an optical tach anyway, going to measure the drill and do this.


I’m stoked you see the logic… please report back your findings for science!

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I have an 18V cordless with a brushless motor that should be able to do 2100RPM. I have to check the corded hammer drill but that thing is old. Pretty sure it will spin faster though. I’ll have time for this tomorrow. Plus I need to get a white Back to the Future Doc coat first.

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if you have some steam-punk goggles… that would help the science…

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True, true. I’ll go amazon on those.


I have 3 pairs and a @ducktaperules stovepipe hat that i wear for science… if it’s the great unknown, I have the magic-eightball app and a merlin hat that I wear with steam-punk goggles…

it seems to help the science

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For the truly out-of-the-observable-universe stuff, I’ll use my St. Patrick’s day hat with attached red beard. Science, bitch!


Go forth in the name of science… also… look above for @Pedrodemio’s suggestion… subtleties matter…

and please report back for science… the more knowledge base we gain… the stronger we become…

Yeah, this are my goggles. Made them myself, even hand stitched the leather.

They really help in situations like this.


almost essential

Those are absolutely and universally beautiful. Nice work!


Ok, done some testing on BLDC. First I’d like to mention that apparently I’m not good at tweaking BLDC because the Flipdick keeps throwing DRV errors. Second, it’s my first time tweaking BLDC and it’s not as straightforward as FOC. I haven’t been able to get to the same ERPM as on FOC without DRV errors. Obviously, it’s me and not the FSESC. I have whipped out the optical tacho only to find the batteries dead. Getting new batteries tomorrow. Reading up on BLDC settings.

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for science fully charge your cordless… and do some quick pulls to a stable voltage…assume 2100rpm and see what that gives you for Kv… you can always redo it or opto-tach the drill motor later…


Manufacturers specify a ± 10% tolerance on the Kv rating.
Here’s how to calculate the Kv from the measured flux linkage


Dude, that is gold. Thanks!

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