How to determine the Kv of a motor
Method 1
(Power drill method)  quick and dirty method
1. Drive the motor with a power drill.
2. Measure RPM and voltage generated.
3. Kv = RPM / voltage
(also see Method 3)
        
Method
2 (Running the motor without a load)
This method
is not very accurate with brushless motors, because different ESC's will
give slightly different results.
1. Run the motor without load at full throttle
at various input
voltages and measure the RPM and controller input voltage.
2.
You can measure the noload RPM by marking two stripes
on the bell with an indelible pen. Then shine a torch on the
rotating bell and tach the reflected light.
3.
To find Kv either:
a) Graph RPM vs Voltage, the slope of the line is Kv
b) For each voltage divide RPM by voltage, and
average your results.
Theoretically, a) is more accurate.
     
  
Method
3 (Colin Dedman's formula)  this is my
favourite method
1. Drive the motor with a drill press.
2. Measure RPM and voltage
generated (by one phase).
3. Kv = RPM /
(volts x 1.41 x 0.95)
     
  
Method
4 (Louis Fourdan's formula) 1.
Run the motor noload at full throttle
at voltage V for a RPM in the good range.
2. Measure the noload RPM and current (Io). 3.
Find the internal resistance of the motor in ohms. It can be measured
using the Kelvin (4wire) method or taken from the specifications supplied by the
manufacturer (some risk of error).
4. Kv = RPM /
(volts  amps x resistance).
5. It's better
to measure at 2 different voltages and average the results.
