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Originally Posted by gpetilli
We did intend for the RS775 to reach max RPM when the MiniCIM was at 1/2 max RPM...
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775-18 free RPM is 13000. With 20:1 reduction that's
650 RPM.
MiniCIM free RPM is 6200. Half of that is 3100 RPM. With 6:1 reduction that's
517 RPM. So you're a bit off with your gearing.
Quote:
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That's an interesting way to look at it. Since I don't yet have a spreadsheet to dynamically calculate operating points, I used JVN table - which does include voltage droop (which looked like about 7.5V).
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I was just taking the numbers you posted to see where they would lead. The motor calculator I posted
here makes this easy to do.
Here are the results using 7.5V instead of 12V:
At 7.5V, a 775-18 motor draws
45 amps at 1407 RPM
At 1407/20*6 = 422 RPM, a MiniCIM draws
48 amps at 7.5V
See example calculation attached.
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I do have some serious unanswered questions with this configuration. If we put the ESC in coast does that sufficiently break the generator current path such that it does not significantly load the driving motor? I don't think anyone has tried coasting the motor after it exceeds free RPM but instead continue to drive at full voltage. If we back-drive the RS775 to twice it's free RPM (coast or driven) will the generator blow the ESC?
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Good questions. Maybe the engineers from VAX and CTRE can answer for their respective motor controllers.
There's also the question of the 775-18 spinning at 13000*2 = 26000 RPM. Can its rotor, fan, and the bearings handle this? And even if the the motor leads are open-circuit (zero generated current), how much load does the fan put on the MiniCIM at that speed, through a 20:1 gearbox?