ORIGINAL: rascal35
How is a 3s making it hotter compared to the 2 cell? I dont understand. ...
Motorcurrent wants to go up
squared with voltage (keeping everything else the same: battery, controller, timing, motor, prop).
So, going from 2 to 3 cells will give an increase in motorcurrent by factor 2.2! 120% more. And losses in the (any) copperwire go up
squared with current. Therefore, copperlosses (=heat produced) increase with voltage⁴ (power 4). In your case that would be by a factor (3/2)⁴ = 5! 400% more heat to get rid off!
And that's from copperlosses only. Then there are also the magnetic hysteris losses in the iron, they go up linearly with voltage (voltage = Kv* rpm), and to top it off, the eddy current losses in the iron go up squared with voltage.
If motor gets to hot, (Neodymium) magnets will die, resulting in higher Kv -> higher current -> higher temperature -> deader magnets -> higher Kv -> higher current ... In short: temperature runaway.
Eddy currents explained:
http://en.wikipedia.org/wiki/Eddy_current
Eddy current demos/videos:
http://www.rcgroups.com/forums/showthread.php?t=465019
Vriendelijke groeten
Ron