# Help finding torque of BLDC motors at specified RPM

Hello,

I have been trying to understand this YouTube video by Skyentific. Can someone explain how he got the values for Power and Torque in the following chart? I tried to understand which given values he used to and what equations. Aerodrive and ODrive are the ones I am curious about but could not follow his math.

Thank you!

For the Aerodrive SK3 4250, they seem to have the results for power when coupled to different props:

The average of these seems only slightly higher than the value he used. Normally, Power = Voltage x Current, and Hobbyking lists the â€śPowerâ€ť at 1350W, which must be the stall since the â€śMax currentâ€ť is 57A and the max voltage is 19V.

Knowing the power, you can use the KV value (rpm per Volt) to know at what RPM the motor should rotate when supplied at a specific voltage (he wrote 350 x 20 under â€śrpmâ€ť, though on the manufacturerâ€™s site the KV is given as 500, so not sure where the 350 came from, especially since the manufacturer also lists the maximum voltage at 19V, and he wrote 20. Is there a different sub-model with those specs? Not sure. Given that you can estimate the rpm, and can use the power value from the information above, you can estimate the torque.

Since thereâ€™s not a lot of information on the manufacturerâ€™s site,

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It was in fact a sub-model, here is the link:

Hobby king lists these motors at 5s. Depending one what cell voltage you use you get (53.7)=18.5 and (54.2)=21 so maybe he took a sort of average battery voltage value?

Side Note: The two power equations are Electrical_Power=CurrentVoltage and Mechanical_Power=TorqueRPM*2pi/60.
Then does Mechanical_Power = Effeciency * Electrical_Power correct?

I have tried some of the math you mentioned but I still cannot get the torque value he put (In the comments he says it is max torque). I also looked at maxon motors which are very well detailed.

Here is a link to a 400Watt maxon motor which is very detailed. However, I have no clue how in the name they rate it as 400Watts based on itâ€™s specs. Might give on my quest here unfortunately.

Good questions honestlyâ€¦ I have not had my morning coffee yet. The site doesnâ€™t have much in terms of specs
KV = 350 rpm/V
Power = 1190W
Current = 53A (max)
Voltage = 20V (max)

In the rpm column he wrote 350 x 20, meaning he chose 20V as the voltage, so working backwards using P=IV with the max current, the power would be 1060W, which is different than his 866W. Using the same equation, his current would be 43.3A, which would mean a lower power using a lower voltage. Perhaps this is something he measured? He has the tools after all.

The reduction ratio to 60rpm is correct at 116.7 (350x20/60).

Torque = 1.181 using the equation he presented (didnâ€™t verify if itâ€™s correct) which also works out (use 350x20 as the rpm and 866 as power), and multiply this by the gear ratio to get the last column value of 138.

So, the question is really where he obtained the current draw at 20V, and I suspect that was measured, since I donâ€™t see any performance curve for that motor, at least on the manufacturerâ€™s site.

Maxon manufactures some of the best, highest quality BLDC motors on the planet, used in space exploration etc. Theyâ€™re also some of the best documented and supported. Their price (new as opposed to second hand or bulk) corresponds with this.

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@cbenson I did the math, it finally works out! Converting Kv to Kt gives a value of 0.0273 for the torque constant in Nm/amp. Then multiplying this by the 43.3A you presumedâ€¦ gives 1.18Nm ! ! ! Thank you so much.

So apparently he created his own rig to measure stall torque, but then trusted the manufacturerâ€™s max RPM number. Strange.

I was hoping this video would clarify how to perfectly convert/use manufacturerâ€™s numbers. Seems the lesson here is that one can only use an educated guess when choosing a hobby motor instead of doing serious analysis before owning it. If only these real values were compiled somewhere for hobby level BLDCs by someone.

Thanks again!

If you see his other videos, he tests torque quite often, so most likely that value was based on measurement (cannot be certain without asking him though). The lesson is that if a manufacturer does not provide a performance curve with the same setup as yours, you can use only so much of the data they provide, try referring to basic equations, and might need to test. In this case, a KT value (torque constant) would have helped too.

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There is Drivecalc.de
A tool I used to use frequently when I was busy rewinding BLDC stators.

Itâ€™s also a database for different props, surely worth a look.

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@o_lampe Thanks Iâ€™ll take a look! How many Kv does that top left motor have?

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It only seems to have one incorrectly wound copper wire at the top, but thatâ€™s an incredibly large hollow shaft though. Perhaps the artwork is intended to provide additional â€śFlower powerâ€ť.

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HarHar, itâ€™s just there for size comparisonâ€¦

the original motor (top right) is actually a switched reluctance motor from an ooold tapedeck.
Revox/Studer IIRC
I added the magnets and figured out my own winding concept.
A milestone in my career as electric propulsion engineer.

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