mamba max 6900 limitations
 

I'm kinda dumb and don't that much about electronics in general, but I have a question about running a 6900kv on 3s. What's the main limiting factor? I know you're not supposed to, is it mainly because the motor itself can't handle the rpm or is because the speed controller can't handle the load and generate too much heat? I've noticed the sidewinder 5700 says 2s limit, so I'm thinking its an ESC limit, because the same motor can do 3s on the max. And the 7700 with 3s will put it in dangerous rpm territory. But what about the 6900, its kind of an in betweener.
:whistle: Warranty aside, the 6900 on 3s with a CC BEC does seem pretty stable and ballistically fast and the temps are surprisingly very low on the motor and within normal operating ranges on the esc. I have my punch control disabled and have taken steps to assure low operating temps.

Its the motor, at nominal voltage of 11.1v the 6900 at wot would be turning 76,590rpm. Freshly charged to 4.2v/cell, that 12.6v would turn the motor at 86,940rpm. I don't know what rpm they're rated for, think I've seen 60,000 so doubt they'll take those kind of rpm for very long.

Yes, but real world setting, do you think under load it is really seeing that kind of rpm? The voltage in the battery will drop as soon as you go. Perhaps if you were geared down it would run at high rpm, but in my case of doing all out speed runs, it takes some time to get it at high speeds and I don't think I've been able to top it out due to the car being too far away to want to keep going that fast. I'm starting to believe that without a ubec, the temps of the esc will get too hot too fast. The 5700 probably pulls less amps than the 6900 and if you combine that with higher voltage, the 6900 will probably put the esc at unsafe operating temperatures whereas the 5700 won't and keeps it within its recommended range.

Actually temps depend almost entirely on gearing -- if you use a low enough gear you can keep the 6900 and ESC pretty cool on 3S. The real issue is the bearings and the magnet, you can get a catastrophic magnet failure at above 75000RPM or so. You wouldn't normally see that RPM, but if you pull full throttle during a jump, for example, the motor will unload and hit full RPM pretty quickly.

That said, the newer magnets are handling around 100K RPM without any problems, but the bearings also can't take that RPM (even the Koyo CNC Japanese bearings on the newer CM36 motors) -- so to be safe, we make 60,000 the top allowable RPM.

when you go to a higher kv motor there is less wire involved with the windings and this decreases resisitance and this reduction causes a higher amp draw and that causes heat, I wouldnt use 3s above a 6000kv motor, above that go to a 2s even with a BEC and keep your gearing on the conservative side

Well I've already tasted the forbidden fruit, I don't think I can not run 3s ever again. I'm actually thinkin of getting another 3s. 2s is fine and great for general bashing about, but 3s is just plain wicked. If I toast the motor, I'll buy a new one, probably another 6900, it's really an awesome performer for my application.

Yay! Good hear man! It may sound like I'm beating on a dead horse, but get some cooling on that motor and ESC.

Hey there Patrick, does CC plan on making a sub-3600 kV motor with those 100,000 rpm magnets? If you do, I will buy at least 3.

Take a Tekin reline 632 (3200 kv) or 537 (3700 kv) they are based on the same type of design and work flawlessly with CC controler