
Jul 08, 2006, 03:01 PM  

The easiest and best is to just download Drivecalc 3 from www.drivecalc.de . Measure I, V and rpm for noload and a few props and Drivecalc will correlate this data and give you everything you need.
I used to use the motor equation: rpm=Kv(Vm  RmIm). Rearanged to rpm/Vm = RmKv(Im/Vm) + Kv. You then plot rpm/Vm vs. Im/Vm. This will yield a straight line with a slope = RmKv (so Rm=slope/Kv) and yintercept = Kv. But I think Drivecalc does a slightly better job of it. 
Jul 08, 2006, 03:03 PM  
Joined May 2003
768 Posts

Measure the following on your motor under test:
Io at approximately the voltage you plan on running the motor. No prop. Just the motor. Take the measurement in a few seconds of starting the motor. V, I and RPM for two different props. When you make this measurement, try to ensure the following: 1) The values of I aren't too close together. Ideally you'd want them to be 30% or more apart for about the same voltage 2) The prop is being operated at an RPM well below it's max RPM. Let's call the V, I and RPM measurements V1, I1 and RPM1 for the first prop, and V2, I2, and RPM2 for the second prop. Your motor constants are as follows: Io = Io, just as you measured above. Rm = (RPM2 * V1  RPM1 * V2) / (RPM2 * I1  RPM1 * I2) Kv = RPM1 / (V1  Rm * I1) That's it. www.peakeff.com will do this for you here and overlay prop data. 
Jul 08, 2006, 07:40 PM  
Joined May 2003
768 Posts

Quote:
I definitely see the value of direct measurements if you are developing a motor, but once you are interested in knowing what it does under load it's not as accurate as the indirect measurements in my observations. 

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