How do you test power?

stolla

User
I have a question which has been bugging me quite a bit, i'm an electronic noob but my head says to me the way most people use logging data to get kw, by volts x amp is not a reliable true indication of power. Reason this bothers me is that for instance a lower powered, less efficient motor may in fact pull more amps for instance in a heli when motor bogs down due to too high pitch etc, whereas a more powerful similar sized motor wont bog and most likely draw less amps. The data will show the less powerful motor has more kw which is not true. I have experienced this first hand in a heli with onboard logging when it bogged severely, drawing very high amps
So in my mind the only true indication will be to have equipment measuring thrust with the same setup. Or at the very least with the calculation of v x a use the exact size prop bench tested for similar motor comparisons.
Am i missing something here?
 
Actually measuring the Volts and the Ampere are very good indicators for the "true" Power.

But what you probably did was to compare two Motors with 2 seperate conditions or even ESC's. The way the ESC works is essential. The efficency can very several percent.

So if you want to compare 2 different motors you need the same conditions, same ESC's.
 

Julez

User
A very good test is a setup where you can measure the torque of a motor.
This can be done easily by a movable L-shaped arm on which a motor is mounted, which presses down a small scale at the other end.
motor_test_stand2.jpg
Like this, only rotate the motor 90° so that the shaft points towards you.
With the "weight" the scale shows and the lenght of the arm, one can calculate the torque. When you check the motor speed as well, you can multiply it with the torque, and get the output power.
Compare this to the input power, and you get the efficiency at that given situation.
 
Oben Unten