Electrical vs Mechanical ouput power.

[SevitRoy]

Hi there,

I was trying to make a comparison between the electrical and mechanical output power of a hebi x8-9. I know, that the electrical power (W) is the voltage (V) multiplied by the current (A) and the mechanical power is the effort (Nm) multiplied by the speed (Rad/sec).

For the electrical power, what values do I take to calculate it? Now I multiplied the bus voltage with the PWM value (hoping to get the winding voltage) this in turn is then multiplied with the winding current, to get the electrical power. This value however seems to be too high (repeated peaks of 90W). While the repeated peaks in the mechanical power are about 30W. Is there an obvious mistake I am making here?

Kind regards,

Roy

[daverolli]

Roy,

Your methods are correct. You can calculate the electrical power the way you did above, or also by directly multiplying the voltage and motorCurrent feedback since both are measured at the bus.

You may not be far off in your results. The peak efficiencies we see with the actuators are around 50%. This is because the brushless motor itself has a peak efficiency of about 70% efficient (and that's only in the med/high-speed range, and assuming cool winding temperatures). And then the geartrain is about 60-70% efficient in getting the motor torque to the output.

So a 30% efficiency for a general motion is probably about right. Overall, the electromechanical efficiency is pretty much entirely dependent on the motion profile. A high-speed motion will usually have better efficiency, and all motors at 0% 'efficient' at stall 🙃.

[SevitRoy]

Hi Dave,

Thanks to that reply, indeed it makes sense if you picture it that way...

Kind regards,

Roy