Motor parameters for 6V LP 25D motors

I’ve purchased 18 of your 25D motors for use in mechatronics class. Based on reviewing some of your prior forum posts, I’ve attempted to estimate the dynamic model properties. I wanted to confirm the inductance parameters as I’ve seen a few values on forums. I am interested in the 6V LP 25D models. #4826 and #4829.

I also attempted a SWAG at inertia by assuming 1/2 mass rotates as shaft of 0.9 cm radius (including gearing and rotor). It seemed like dividing that by G gave a more reasonable response.

Can you sanity check these values, and confirm the L value of 1mH for the 6V LP motors?

PDF version of spread sheet
6VLP motors.pdf (54.1 KB)

Spread sheet
6VLP motors.xlsx (11.5 KB)

I tweaked a few lines from original post.


The inductance of our 6V low-power (LP) 25D gearmotors is about 3.1 mH.

I think your methods for calculating each value are okay. There are a couple of things I would do differently, though their effect is minor and within what I would expect to be the margin of error for this anyway:

  • Your notes say that you calculate the motor resistance by dividing the rated voltage by the stall current, but the value in your cells is actually from V_rated/(i_stall - i_free). The free run current does not need to be factored into this value, but the difference is only ~0.03Ω.
  • Factoring (R_m *i_free) into the electric constant (back EMF constant) equation is also questionable, but the effect on the result is also fairly negligible.

I did not check your actual calculations too thoroughly, but there is at least one error. In your friction calculations you are using your back EMF value instead of the motor torque constant value.

- Patrick