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.

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.

1 Like

You look it up usually! For FRC, https://motors.vex.com/ 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 youâ€™re actually using.

To find motor data I use

https://www.reca.lc/motors

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

5 Likes

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 doesnâ€™t make a significant difference.

As far as kT being equal to 1/kV, Iâ€™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 thatâ€™s not always true. In any case, the two constants represent two different quantities and shouldnâ€™t be conflated even if they are numerically equal.

Source: I TA the motor physics course at my university

1 Like

Citing sources is good practice, because it allows your readers to reproduce results. Reproducible results are the solid core of science and technology â€“ the reason we distinguish between sciences that are â€śhardâ€ť 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:

3 Likes

Iâ€™ll second Hendershot and Miller as a good source. itâ€™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.

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.