Determining the "best" max. RPM when choosing KV/voltage?

[lizard]

After reading Vedders very nice explanation in this link:
http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard/

There is still one big question to me: How to determine the "right" or "best" max. RPM for highest overall efficiency when choosing KV and voltage.

Vedder says, that the non-copper losses start to become significant around 60K ERPM, which would be around 9k RPM for a 14-pole motor, 20k RPM for a 6-pole or 30k RPM for a 4-pole motor:

Now we know that copper losses are proportional to the square of the torque produced by the motor, and at low RPM and high load they are dominant. As RPM increases, other losses start to add up exponentially. In my experience, these losses start to get significant around 60k electrical RPM, which for a 14-pole motor is about 8570 mechanical rpm (most 50mm+ outrunners have 14 poles, some unusual ones have 18). Because of the square relation, it is desirable to run at as high speed and low torque as possible as long as we stay below 8.6k RPM. To express the square relation in some numbers, having double the RPM and half the torque at a certain power output will cause four times less losses. The lesson from this is that: make sure the top speed you design the skateboard for is at around 8.6k rpm on the motor if you are using an 50mm-60mm outrunner.

However, these numbers have been determined for 50-60mm 14-pole outrunners. Is that a "universal" number? And if not, how can one determine it for 4- or 6-pole inrunners?

Going by the "60k ERPM rule", that would mean, that 6-pole motors should be run with about 20k RPM max. But there are 6-pole motors sold for 1/8 scale RC-cars (Tenshock) which have KV-Values that make them spin around 30k, just like 4-poles. However, they all run pretty hot when compared to any other 4-pole, even the cheaper not-so-efficient ones. Just a coincidence? Or is Vedders "60k ERPM rule" universally right? But why on earth then would Tenshock sell motors with those way too high kv values? I mean, they build motors, wouldn't they know this? You even cannot get lower KV-values from them.

Then I tried a TPPower 6-pole motor myself (with around 35k RPM max.) and it also ran pretty hot. Which again makes Vedders "60k ERPM rule" look valid. However, there are a lot of boating guys using 6-Pole TPPower motors well above 35k RPM, some even up to 50k RPM. Which again raises the question for me: Why? Maybe they just don't realize the inefficiency because they have watercooling and short runtimes anyway so the temps do not get out-of-control too fast for them? On the other hand, just like with then Tenshock motors, I cannot really believe that they're all doing it wrong.


Is there maybe some experimental way to determine this? Like letting the motor spin my drillpress (to create some load) and then revving it to different RPMs and logging currents and temperatures?

 

[Miles]

60,000 erpm that's 1kHz.

Seems a reasonable limit for 0.2mm thick laminations.

Bear in mind that eddy current losses go up as the square of the lamination thickness and it's eddy current losses that will dominate the parasitic losses at these frequencies....

Peak efficiency won't actually fall if you run at higher frequencies. It will just move to beyond the usable range of the motor (thermal limits).

You can enter a few simple measurements into this spreadsheet to get an idea of how the losses vary.
https://endless-sphere.com/forums/viewtopic.php?f=30&t=65757