Carbon wrapped rotor motor…

[Crissa]

I think Elon said or implied that carbon wrapped motors had better efficiency. Why?

Because the motor tolerances don't change as much as it heats or spins. Expansion/contraction changes where the electromagnetic fields interact.

-Crissa

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[ajdelange]

The other part of "why?" which I forgot in my last post is that the carbon wrapping controls rotor expansion with temperature and load so that the rotor/stator gap can be smaller. The flux that can be developed for a given current is inversely proportional to that gap width so: smaller gap --> more flux --> more torque ---> more power all at the same current and same loss --> more efficient. Elon specifically mentioned this in his presentation.