I am trying to calculate Ke, Kt and resistance of the motor.
The specifications is given by:
|Gear ratio:|131:1|
|Free-run speed @ 12V:|80 rpm|
|Free-run current @ 12V:|300 mA|
|Stall current @ 12V:|5000 mA|
|Stall torque @ 12V:|250 oz·in|
According to the FAQ it is stated that, “The electrical resistance of the motor can be approximated by dividing the rated voltage by the stall current (at the rated voltage). The electromotive force constant (Ke) can be approximated by dividing the rated voltage by the free-run speed (at the rated voltage). To approximate the motor torque constant (Kt), you can divide the stall torque by the stall current.”.
electrical resistance of the motor = 12v/5A = 2.4
electromotive force constant (Ke)= 12V/80RPM = 0.15
motor torque constant (Kt) = 1.765Nm/5A = 0.35308
I would like to ask if my caluclation is correct? Next i would like to ask do I need to take into consideration of the gear ratio 131.25 for the calculations of Ke and Kt? Is it okay if I don’t take in the gear ratio of 131.25?
Your calculations look correct (though you forgot the units). The values you calculated apply to the gearmotor as a whole. You only need to take into account the gearbox if you plan on using the motor without the gearbox.
Hi, I noticed that you took the stall torque of your motor as 1.765Nm, but the datasheet says that the stall torque is 45 Kgcm, about 4.412993Nm.
How did you obtain the 1.765Nm value?
I attach a link to the motor specs, I’m not aware if the specifications have changed since then.
The Pololu page states the “stall extrapolation”, which is further described in this footnote:
Stalling is likely to damage the gearmotor. Stall parameters come from a theoretical extrapolation of performance at loads far from stall. As the motor heats up, as happens as it approaches an actual stall, the stall torque and current decrease.
The post you are referring to is from 2018, when our estimates of the stall torques were based mostly on conservative theoretical calculations (it is difficult to empirically test stall current since that is an operating point at which the motor can destroy itself in multiple ways, and the parameters themselves change as the motor rapidly heats up). This was also before the current datasheets existed.
Since then, we have constructed more sophisticated testing setups for our motors and conducted thousands of tests across the different gear ratios and motor types, which has resulted in much more accurate estimates for the stall torques. You can see more about that in this blog post:
