Hi Guys! Needless to say this forum rocks! I have been working on an oil cooled direct drive build for the last few months. Since cooling these motors seems to be the limiting factor and failure mode for an overloaded motor the thinking was that I would be able to run a much smaller motor at a significantly higher power by running it submerged in dielectric oil. This would provide excellent heat transfer directly from the surface of the motor windings and conduct that heat out to an aluminum housing with a large surface area. The rotation of the motor would cause oil to circulate providing epic heat transfer and great lubrication for bearings and shaft seals.
Here’s the hardware I am starting out with:
-Alien Power Systems 5065 90KV brushless outrunner (small I know, read on)
-Alien 150A speed controller
-2x turnigy 20,000mah 6s batteries in series
- ebay “magic carpet” foil (cheap but I wouldnt reccomend it)
- beater foam board that will be replaced once I know I can make the thruster work
All the plastic stuff is currently printed in PLA and PLA+ using a Creality CR-10
So here’s where I am at. I was planning to start off with a small prop and work my way up to a larger and larger prop until the motor was loaded up to around 3500W-4000W. I was under the impression that I could just keep increasing the load on the motor until I either had enough power to ride, or the just motor burnt up. However, after trying 4 different props and don’t seem to be getting the power I need and cant get the motor to draw more than about 2000W.
I started by printing off the 3 blade version of the parametric prop and the duct that pacificmeister posted. It only drew about 1300 Watts and the board would move through my pool (without a rider) at maybe 4-5 mph. Need more power!
I then Printed off a 4 blade version of that prop. It only drew about 1600W and would move just a bit faster. Still need more power.
Next I bolted up a solas prop for a 4-6hp outboard engine thinking if this doesnt pull some current I dont know what will. That prop pulled just under 2000W and would just get the board up on the foil (without a rider). Maybe 8mph.
Now I am starting to second guess my theory about being able to just load up the motor with bigger and bigger props until it either works or goes up in smoke. So as a last ditch effort to pull some serious current through the motor i printed off a four blade, 8inch diameter version of pacificmeisters prop. I rev up the motor out of the water then confidently drop the board into my green ass, freezing cold pool expecting to see big numbers on the power meter. I hit the throttle and the motor just shutters for a few seconds and then shuts off. I guess the ESC is sensing that the motor isnt rotating like it should be and cuts off. If I release the throttle and then reapply it. It shutters again for a few seconds then turns off yet again. If I give it just a tiny bit of throttle the prop turns fine but I think the big prop just requires more torque than the motor is capable of producing.
I am confused at why I wasnt able to pull more current with the same props that Pacificmeister is using. The motor must not be turning at high enough rpm right? at 12s and 90kv it should be spinning at just under 4000rpm (no load). Maybe it is hitting it’s max rpm before the prop is spinning fast enough to generate a significant load on the motor? What prop RPM is the @pacificmeister setup turning? I know I am really pushing my luck running such a small motor but I would at least like to load that motor to failure before giving in and buying something bigger. Any thoughts?