#### LohringMiller

##### Well-Known Member

- Joined
- Jan 25, 2017

- Messages
- 831

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 Brushless Motor Kv - Measure a Motor's Kv - RCexplained 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