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Apr 24, 2021, 02:54 PM
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EMax CF2805 rewind questions


I decided to try winding this motor to be more useful on 3s, as its 2800ish Kv wasn't much use unless I bought some 2s packs. I used 26AWG wire and was able to fit 18 wraps per tooth dLRK with a ∆ finish, but my Kv calculation using an oscilloscope, Dremel, and an online calculator linked somewhere in the sticky post suggested an improbable value (somewhere in the sub-1k range). By my estimation, it's closer to Kv=1800 or thereabouts, so I'm not sure what went wrong with the measurements/calculations.

I'm curious what others' recommendations are for power output on 3s, as it's original specs have it pushing only ~100W on 2s and a <5sec static test (just for fun) with an 8x4E APC had it close to 250W without frying. No-load RPM is about 22k, and with a 6x4E APC I saw about 16k RPM, which is lower than the 75% I saw recommended in a couple of places here, but after running that 6x4 for a bit on the bench, the coils barely felt warm.

Maybe a better question: what temperature (at the outer windings) is your limit for this motor? The wire I used is rated for 300° F, but I know the magnets won't like that heat. Props will pull less power in flight than on the bench; as I'm not interested in top speed shenaniganery, should I worry less about static power and heat, and instead just be conservative with the throttle during flight?

Separately, does anyone have advice for a clean >2-layer winding pattern that still has transitions at the bottom of the slot? I can only fit two layers at the bottom, but there's more room at the end of the teeth and I'd like to put another few wraps on the next wind I do.
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Apr 24, 2021, 02:56 PM
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The finished stator, ends notwithstanding


18 wraps per tooth because it looked about right with 26AWG, and ∆ termination because it seemed appropriate for initial testing.
Apr 27, 2021, 11:49 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by semininja
... but my Kv calculation using an oscilloscope, Dremel, and an online calculator linked somewhere in the sticky post suggested an improbable value ...
Other methods
www.bavaria-direct.co.za/constants/#Kv

All three phase voltages were the same in generator mode?
Did not mix up starts and ends of phases when hooking up in delta?
No chugging sound when driving motor with all phases shorted?

Current and power drawn are proportional to Kv cubed
Expensive realistic and extreme silly examples, hopefully instructive as well.


Quote:
Originally Posted by semininja
... what temperature (at the outer windings) is your limit for this motor? The wire I used is rated for 300° F, but I know the magnets won't like that heat. ...
Assume 80°Celsius, unless manufacturer says you can go higher.
See column max.operational temperature in this table:
www.supermagnete.nl/eng/physical-magnet-data (Europe)
www.kjmagnetics.com (USA) has a similar Neodymium magnet table on their site, somewhere.


Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
Apr 28, 2021, 12:20 AM
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Skylar's Avatar
Quote:
Originally Posted by semininja
... does anyone have advice for a clean >2-layer winding pattern that still has transitions at the bottom of the slot? I can only fit two layers at the bottom, but there's more room at the end of the teeth and I'd like to put another few wraps on the next wind I do.
Just to confirm that it's OK to have more layers of wire. It will help efficiency if you fill up available slot space.


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