I'm trying to model a PMDC motor using manufacturer data and curves. What seems like it should be relatively simple is not adding up, and I'm hoping someone can tell me where I'm going wrong.
I've got the no load speed and the rated speed / rated load torque from the manufacturer. From these two points I generate a linear load Torque vs. speed curve. Additionally, I have kt, R, Vrated and Irated. Without assuming necessarily that kb = kt, I calculate kb from the rated speed, rated current and resistance. kb calculated and the kt given by the manufacturer are verified to be quite close (in SI units).
The model includes a viscous friction term and coulomb friction term.
Steady-state equations are I = (V - kb*w)/R, and Tload = kt*I - b*w - c. w is speed (rad/sec), b is viscous friction coefficient, and c is the coulomb friction. Combining those gives: Tload = -[(kt*kb)/R + b]*w + (kt*V)/R - c. It seems like this would be straight-forward to fit to the w vs. Tload curve to get b and c. However, whenever I do that with the manufacturer values for the above, I get b is negative, which suggests that viscous friction decreases with speed, and the coulomb friction is excessively high. Is there something fundamental I'm doing wrong here?
Thanks
I've got the no load speed and the rated speed / rated load torque from the manufacturer. From these two points I generate a linear load Torque vs. speed curve. Additionally, I have kt, R, Vrated and Irated. Without assuming necessarily that kb = kt, I calculate kb from the rated speed, rated current and resistance. kb calculated and the kt given by the manufacturer are verified to be quite close (in SI units).
The model includes a viscous friction term and coulomb friction term.
Steady-state equations are I = (V - kb*w)/R, and Tload = kt*I - b*w - c. w is speed (rad/sec), b is viscous friction coefficient, and c is the coulomb friction. Combining those gives: Tload = -[(kt*kb)/R + b]*w + (kt*V)/R - c. It seems like this would be straight-forward to fit to the w vs. Tload curve to get b and c. However, whenever I do that with the manufacturer values for the above, I get b is negative, which suggests that viscous friction decreases with speed, and the coulomb friction is excessively high. Is there something fundamental I'm doing wrong here?
Thanks
