sysrq said:
"Never use thermal compounds and thermal pads together."
https://blog.arctic.ac/blog/2015/07/23/thermal-pads-or-thermal-paste/
Not sure if always true.
Forgot to take into account typical BLDC motor efficiency (85%), so the requered power handling capacity of the heat pipe would be only around 50-60W for a 350W motor. That would mean that wrapping one 5 mm pipe around the motor and attaching it with thermal epoxy in to the round corner next to the controller should be enough. This way there isn't any need to attach anything to the black low thermal mass external cover.
Another low key solution I can think of would be tying around the motor some 2-5mm thick OFHC copper L bends depending on measurements in between the screws around in conjunction with thermal pads using a jubilee clip. The other ends would press against the round protrusion of the external cover above the motor terminal plate making it possible to put some pressure on the 2 mm thick thermal pads while tightening the cower screws.
So, I don't know for sure, but this is what I am thinking...Usually thermal grease has way high Wm/K thermal transfer rating than the pads...so I think when you use both, the thermal grease is usually touching whatever you are mounting, and then the thermal pad.... I think if the thermal grease has a higher Wm/K rating than the pad, it makes it not work as good because it is pulling the heat from the pad, if that makes sense.... but the way we have it configured...the thermal paste is on the outside of it all, away from what we are trying to cool, so it is continuing to pull heat out, and transferring it to the motor housing cover... either way it is needed to lube up the sides, and it definitely is working well, so that is what I know for sure...
BTW I use 12 Wm/K GP extreme thermal pads... and GP extreme thermal grease too...they both seem to have the highest Wm/K rating for a readily available retail/product.
Just more detail, since im slow with the pics...
qty: 4 - GP extreme 40x80mmx3mm - one gets cut longways in half, and wrapped around middle of motor, then the next two stay whole and wrap around the motor. the ends dont touch - they dont fully encapsulate the motor, which is fine, because one side of the motor does not touch the motor housing side - it faces the controller. this is also where I install a temperature sensor.. the last one gets cut into strips and lays around the circular edge in the motor housing cover.
qty: 2 - GP extreme 40x80x2mm - one lays flat in the middle of the circle in the motor housing cover, the other is chopped into strips, and stacked on the 3mm strips around the edge of the motor housing cover, to make it 5mm.
qty: 1 - gp extreme thermal grease - luuuuuuuuube up the sides (doesn't take much)
total cost - approx $120 + shipping!!! YIKES!!!
...but still makes it a new motor IMO... worth every penny...I am open to something else, but this is super quick and simple... and works very good...