I was asked about the "real" KV of the TenShock CZ11240 motor rated at 2680 KV. I took the measurements with the motors installed in two boats by spinning the drive shaft. One had a .150" shaft in a Teflon tube and the other had a .188" shaft in a brass tube. I was also interested in any difference in drag between the two shaft sizes. First I used the drill method (see namba.com/content/library/propwash/2022/april/26/). My drill turned the motors a little over 1000 rpm. I got a KV of 2523 on one motor and 2493 on the other; close to the same.

I then hooked up a 4S battery to each motor through Castle Hydra X ESC. I then ran them at full throttle for a short time until the current draw settled at a constant value. The voltage in each case was 15.1 volts. The battery was at its stored voltage. The current draws were surprisingly similar for both shaft sizes. The .150" shaft registered 12.4 amps while the .188" shaft registered 11.9 amps. The data logged rpm was 41,370 on the .150" shaft and 40,997 on the .180" shaft. That gives a KV of 2740 and 2715; again very close. Correcting for the internal resistance of 5 ohms gives 2678 and 2655. Those are very close to the manufacturers rated values; well within the accuracy of my measurements. See https://www.radiocontrolinfo.com/bru...e-a-motors-kv/ for details on how to calculate this.

The point is that the drill method rpm is a lot lower than the unloaded rpm. This gives values that are different. I haven't compared enough motors using both methods to see if lower rpm gives lower values of KV.

Lohring Miller