Interpreting the Graphs From Nomura

Owned by Micah Black
Sept 14, 2020
2 min read

Note that this is how I interpreted the graphs, and may be wrong - if you think you have a better or more correct interpretation, please update this page and let me know!

https://www.reddit.com/r/solarracing/comments/9ul6vs/mitsuba_motor_efficiency_plot/

Here’s an excel sheet that I made with the values from the chart:

 

Power

The power column in the chart is the power delivered to the road through the torque * speed formula.

The Phase Power (or motor power) will be calculated as the (back_emf * phase current), which should be roughly equal to the speed * torque (minus motor losses in the core, windings, etc.)

Current

I have assumed the current to be the bus current, since the test is conducted with the motor controller.

Efficiency

This calculation I believe to be the Motor Power (torque * speed) divided by the controller power (bus voltage * bus current), since the test is done with the complete motor controller and motor combination.

Rotational Speed

The maximum rotational speed on the graph is the No-Load rotational speed. This can be used to calculate the Kv constant, as the back-emf should be equal to the bus voltage at this point (since we cannot force current through, supply voltage = back_emf)

Data Collection / Test Sequence

Given that the current is all nice numbers and on the X-axis, I believe they limited the bus current of the motor controller, and then loaded the dynamometer until the current limit was reached. This current limit would happen when the total phase voltage (back_emf + IR and inductive votlage) would be equal to the supply voltage. The back_emf is determined by the Kv constant and the rotational speed. The difference between that voltage and the supply voltage would give the amount of headroom that we have to push current through the coils.

For the no-load operating point, my guess would be that they removed the dynamometer and just ran the motor as fast as they could.