Calculate torque constant Kt

If I want to calculate the torque constant for any motor, should I do stall_torque / stall_current?
I was kinda confused by the Wikipedia page here: Motor constants - Wikipedia

Where it says Kt could also be equivalent to 1 / Kv.

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You look it up usually! For FRC, last two columns in this table. But yes - torque constant is usually just the ratio of amps to torque.

Units matter to the extent that, in your actual calculation, the dimensional analysis works out for the units you’re actually using.

To find motor data I use
This gives you a lot of data including kT and kV. Also this site allows you to compare motors in graphs too.
To find a torque constant you divide the torque by the current


To be super accurate, you want to divide stall torque by the difference between the stall current and free current

k_T = \frac{T_s}{I_s-I_f}

In the real world though, the free current is usually sufficiently small relative to the stall current that it doesn’t make a significant difference.

As far as kT being equal to 1/kV, I’ve never seen it defined that way. Usually 1/kV is called kE or kB (the back-EMF constant). kE and kT are often equal for an ideal motor when both are transformed into SI units, but for a real motor that’s not always true. In any case, the two constants represent two different quantities and shouldn’t be conflated even if they are numerically equal.

Source: I TA the motor physics course at my university

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Citing sources is good practice, because it allows your readers to reproduce results. Reproducible results are the solid core of science and technology – the reason we distinguish between sciences that are “hard” and others.

This is the source I generally cite for brushless motor analysis:

and here is the page on which the torque constant is defined:


I’ll second Hendershot and Miller as a good source. it’s probably slightly too much math for the average FRC student, but the ones who have taken E&M and calc will probably get the gist of it.

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