Power / Break Resistor Selection with loads

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?

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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.

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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)?

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