Power / Break Resistor Selection with loads


#1

Hey there,

When viewing http://docs.odriverobotics.com/ , there’s a subsection called “Do I really need a power resistor? What values to choose?”, it mentions:

When braking at max speed and with maximum motor current, the power that is dissipated in the power resistor can be calulated as: P_brake = V_emf * I_motor where V_emf = V_bus / motor_kv.

Unless I’m missing something, this equation doesn’t mention the affect of load / torque on the calculation. (On our 5 DoF Arm at Greppy, the motors on the arms at the base are subject to quite a big more load because they are supporting the rest of the arm.)

Is there an equation we can use that would take load into account?


#2

motor current is proportional to torque, so that’s the load here. Specifically, the torque of a motor is given by:
8.269933431 * I_motor / kv in Nm.

Actually that’s not the right formula, I fixed it in the documentation. It should be: where V_emf = motor_rpm / motor_kv. By the way that documentation is still a preview right now, so there may be a fair amount of incorrectness.


#3

Thanks for the insight, @madcowswe!

Do you know how this V_emf = motor_rpm / motor_kv changes, if there’s an additional mechanical load attached to the motor (in our case, extra joints of an arm)?


#4

The important changes to the power will be to do with I_motor due to the added torque.